CA1180755A - Method of operating a high voltage direct current transmission system - Google Patents
Method of operating a high voltage direct current transmission systemInfo
- Publication number
- CA1180755A CA1180755A CA000416224A CA416224A CA1180755A CA 1180755 A CA1180755 A CA 1180755A CA 000416224 A CA000416224 A CA 000416224A CA 416224 A CA416224 A CA 416224A CA 1180755 A CA1180755 A CA 1180755A
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- Prior art keywords
- station
- voltage
- current
- value
- rectifier
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
- H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
- H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/75—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means
- H02M7/757—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only
- H02M7/7575—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only for high voltage direct transmission link
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Inverter Devices (AREA)
- Supply And Distribution Of Alternating Current (AREA)
- Direct Current Feeding And Distribution (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A method of operating a high voltage DC transmission system with any number of transformer stations. Transformer stations which operate as rectifier stations operate with voltage regulation having a current dependent control input. In the range from 1 per unit to 1-.delta. per unit of rated voltage, a hard current limit is in effect, and in the range from 1-.delta. per unit to 1 - .delta. - .epsilon. per unit rated voltage, the current limiting value is reduced to a minimum current, where .delta. and .epsilon. are preselected fractions. Transformer stations which operate as inverter stations principally operate with current regulation and in the voltage range from 1 per unit to 1-.delta. per unit of rated voltage, the maximum desired current value is reduced from rated current value down to zero.
A method of operating a high voltage DC transmission system with any number of transformer stations. Transformer stations which operate as rectifier stations operate with voltage regulation having a current dependent control input. In the range from 1 per unit to 1-.delta. per unit of rated voltage, a hard current limit is in effect, and in the range from 1-.delta. per unit to 1 - .delta. - .epsilon. per unit rated voltage, the current limiting value is reduced to a minimum current, where .delta. and .epsilon. are preselected fractions. Transformer stations which operate as inverter stations principally operate with current regulation and in the voltage range from 1 per unit to 1-.delta. per unit of rated voltage, the maximum desired current value is reduced from rated current value down to zero.
Description
BACKGROUND 0~ THE INVENTION
_ _ .
The present invention relates to a method for transmitting power and in particular to a me-thod -for transmitting power between a plurality of locally separated transformer stations of a high voltage DC transmission system.
In prior art systems for high voltage direct current transmission (HGU) that include only two stations, a direct current transmission line or cable connects the stations together at their DC ends, while the AC end of each station is connected to the local three-phase mains. In such a "two-point"
HGU, one station takes energy from the local three-phase mains and feeds itJ
rectified, into the direct current line. At the other end of the line, the other station operates as an inverter which transforms the energy of the direct current into a three-phase current and feeds it to the local three-phase mains.
The operation of the inverter station is performed by a rectifier in inverter operation.
This rectifier in inverter Gperation is a critical element in the arrangement. When there are malfunctions in the connected three-phase mains, there may occur commutation errors, referred to as flipping, as a result of which the direct voltage across the terminals of the inverter station breaks down. In order to prevent excess current loads on the components of the inver-ter, particularly its current valves or controllable elements, the rectifier station is equipped with a regulating device which keeps its direct current constant at a settable value. In this way it is possible to keep the current in the transmission line connecting the rectifier and inverter stations at a given value even if the inverse inverter voltage breaks down and thus protect the inverter station against current overloads.
Another aspect to be considered in equipping the stations is the 7~ t3 reactive power to be produced by the stations from the three-phase mains. The reactive power received by the rectifier station is proportional to the magni-tude of the direct current produced thereby and increases with the size of the control, or conduction, angle of the valves, a maximum occurring at a control angle of 90, at which the direct voltage is appro~imately zero. Therefore, the inverter station is operated with the smallest possible control angle ~ at which stable operation is just assured. The quench angle y of the valves in the inverter station is also measured and regulated and is characteristic -for stable operation. The direct voltage of the transmission is set via the step setter of the inverter transformers. The rectifier station mwst have the smallest possible control angle ~ in that the idle, or no load, voltage of the rectifier station follows the step setters of the rectifier transformers in the sense of keeping the small control angle constant.
The control angle ~ is defined to be the electrical angle from the time when the anode-voltage of the valve becomes positive up to the firing instant. The control angle ~ is defined to be the electrical angle from the firing of the valve up to the point where the commutation voltage becomes negative. From this results ~+~ = 180. The quench angle ~, or as it is called the extinction angle, is the electrical angle from the valve current extinction point up to that point the commutation-voltage becomes negative.
Because the setting of the transformer step setters takes place relatively slowly, if a rapid increase in current is required, it is necessary that the inverter temporarily reduce its inverse voltage until the step setter of the rectifier has adjusted the idle voltage to the required higher value.
This is accomplished by equipping the rectifier in the inverter station with a current regulator which is limited in a dominating fashion by a quench angle regulator in the direction toward smaller control angles ~. If the current 5 . ~
regulator in the inverter station receives a current control input which is marginally smaller than the direc-t current Elowing in the transmission line, as determined by the current regulator of the rectifier station, then the inverter current regulator attempts to decrease the control angle ~ and thus reaches the limit defined by the quench angle regulator so that ultimately the operation is determined by the quench angle regulator. If, ilowever, the direct current in the transmission line becomes smaller than the given current control input of the inverter current regulator, because, for example, the current regulator in the rectifier station is unable to set a current corresponding to its current control input, even with a control angle of ~ = 0 (i.e. during a rapid inten-tional current increase or if one of a plurality of series connected partial current rectifiers is malfunctioning) then the inverter current rectifier is able to set a larger control angle, thus reducing the inverse voltage of the inverter station so that the current in the transmission line is kept to a value which is lower by the marginal value instead of dropping completely to zero.
For proper operation with such a "marginal current method" it is necessary for the current control input of the current regulator in the recti-fier station to always be greater by the marginal value, or at least greater in any case, than the current control input of the current regulator in the inverter station. In such a system, the station having the larger current regulator current control input always plays the part of the rectifier station and thus determines the direction of energy transmission. It is possible to avoid a reversal of direction in the energy transmission due to erroneous association of the current control input by employing control angle range limita-tions, or the malfunction can be limited to only a change in the transmitted power to zero. It is thus very important for the current control input 7~5 association to always be correct. Ihis is accomplished by appropriate long-distance signal transmission devices as disclosed in German Auslegeschrift 1,488,085.
The marginal current system has been fowld to be particularly satis-factory in two point ~IGU's because of its emergency operation characteristics.
These same characteristics have led to its use in a system employing more than two stations. The stations are divided into a rectifier group and an inverter group with respect to a real or fictitious common point in the DC network, and the rules of the marginal method are used for both groups. (See, for example, German Auslegeschrift 1,488,085.) The main requirement for multistation operation according to the marginal current method is that the sum of the current regulator current control inputs of all rectifier stations must always be greater than the sum of all such inputs of the inverter stations. In such a system the current in all stations is regulated except for the current in one station which reaches an operating state that determines the voltage for the DC system. This one station operates either as an inverter or a rectifier station. As an inverter station with quench angle regulation, it must be able to receive a current corresponding to its current control inputs and additionally the marginal value. As a recti-fier station it operates with a control angle of zero. Due to the possibility of overloading an inverter station, the margina] value must not be selected too large. It is then necessary to quickly and accurately follow up with all current control inputs when the power of such a station is to be changed. This requires a central load equalization, as described in German Patent No.
~1,523,555l, which requires a considerable amount of expense for remote reporting, or monitoring telecommunications.
If in the plurality of inverter stations, one station is included which has a smaller rated power than the others, the additional marginal cwr-rent in it may, under certain circumstances, lead to an overload, unless the rated power of that station is raised accordingly. Since it is desirable that such a station not become voltage determining for the DC system, special mea-sures must be taken to associate the voltage control variable of the DC system with a certain station.
One drawback of such a system is that the malfunction of a series-connected partial rectifier in one station leads to a corresponding voltage reduction and thus to a power reduction in the entire system. This state can be avoided by additional voltage limitation, as disclosed in German Auslegeschrift 1,588,067, which simultaneously reduces the requirements for telecommunication connections. Nevertheless, a central load equalization of the current control inputs is still required as in other marginal current systems.
To avoid the above-described difficulties of the marginal current method German Offenlegungsschrift 1,588,750 discloses a method which operates with voltage-dependent controlled current control inputs and does not require remote signal transmission for operation. All stations operate with regulated current The current regulators of the individual stations are thus given current/voltage characteristics of a natural type, i.e., the rectifier as the current source has a characteristic which descends with increasing current and the inverter as current user has a characteristic which ascends with increasing current. The points of intersection of the current/voltage characteristics indicate the stable operating point of the system. The drawback in this case is that the points of intersection, particularly those of the inverter charac-teristics, are points of intersection at a small angle (sliding intersections) and are thus not very precise. This can easily lead to overloads, particularly in an inverter station of low power compared to the other stations. Changes in load, moreover, can be accomplishcd only very slowly because they require a change in the transformer step setter.
Germa.n Auslegeschrift 2,518,910 describes a method for solving the problem of providing power to inverter sta-tions without mutual influence while avoiding central matching of the current control input. A current dependent voltage regulator is provided in the rectifier station with a current control input having a single, fixed current limitation which is higher, by a set marginal value, than the power capability of the station. A voltage dependent current regulator is provided in each of the inverter stations in which the maximum value of the current control input for the current regulator is deter-mined without detailed information of the calculating circuitry employed accor-ding to a current/current characteristic matched ~o the power capability of the rectifier station. By itself, the voltage of the mains provides information about the power capability of only a single rectifier station. This method cannot be transferred to a system including a plurality of rectifier stations.
Since it is not known which stations are in operation at any point in time it is not possible to draw a conclusion as to the power capability of any one rectifier station from the mains voltage.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of the above-mentioned type in which unequivocal operating points can be set relative to any number of rectifier and inverter stations, without complicated telecom-munications devices for central load matching, without the provision of a specialmarginal current for current limitation in connection with the current control input for the rectifier, and without the use of di.fferent current/
current characteristics for each rectifier station for the formation of the maximum value of the current control input for the inverter current regulators, 75~
which characteristics would be complicated to calculate.
The above and other objects are accomplished according to the in-vention wherein a method is provided for transmitting power between a plurality of locally separated transformer stations of a high voltage DC transmission system in which each station has a DC end and an AC end. The DC end of each station is connected to a respective one of a plurality of transmission lines connected in network having a common network point. Each station is connected at its AC end with a respective local three-phase network. Each station in-cludes a rectifier portion having a positive pole and a negative pole and a polarization reversal switch for selectively connecting the respective one of the transmission lines to its positive pole for operation of the station as a rectifier station or to its negative pole -for operation of the station as an inverter station. Each station is operated to maintain a direct current in its respective transmission line and a direct voltage at that pole which is connected to its respective transmission line. At least one transformer station is connected as a rectifier station and at least one transformer station is con-nected as an inverter station. The method includes controlling the rectifier portion of each station via a first regulating device to regulate the direct current in the respective transmission line to a desired value, and further controlling the rectifier portion of each station via a second regulating device which dominates the first regulating device to set the direct voltage at that pole of each station which is connected to the respective transmission line to a given limit value with reference to the common network point. In the at least one rectifier station the step of controlling to set the direct voltage includes reducing the given limit value linearly with the direct current until the given limit value reaches a rated direct voltage value of that rectifier station at a rated direct current value of that rectifier station~ and the step of controlling '755 to regulate the direct current includes adjusting the desired value of the direct current in clependence on the direct voltage valve of that rectifier station such that in a voltage range from 1 per unit of rated voltage value to l-c, per unit of rated voltage value the desired direct current value corres-ponds to the rated direct current value and in a voltage range from l-c) per unit of rated voltage value to 1- c, - ~ per unit of rated voltage value the desired direct current value is linearly reduced to a preset minimum value, where ~ and e are preselected fractions. In the at least one inverter station, the step of controlling to regulate the direct current includes limiting the value of desired directcurrent in linear dependence on the direct voltage value of that inverter station, starting from the rated current value of the inverter station at a direct voltage of 1 per unit of rated voltage value down to zero current at a direct voltage of l-c, per unit of rated voltage value.
The method thus provides advantageous measures for operating an HGU
network having any desired number of stations. The problems of dividing the load between rectifier stations of emergency operation during lack of power in the mains and the avoidance of overloads during malfunctions of individual rectifier stations are thus solved in a simple manner.
In each rectifier station, a current dependent voltage regulator is provided whose current control input depends on the current actually occurring in the station. There thus exists a fixed and a voltage dependent current limitation. The current limit in each of the inverter stations is linearly reduced during its formation to the range of the current limit at which the rectifier stations operate, without requiring a current/current characteristic and the setting of a marginal current. Rather, all of the characteristics for all rectifi.er stations and for all inverter stations, respectively, have the same per unit values, with the actual limit state of all rectifier stations .
7S5i being detectable in the voltage in the range from 1 to 1-~ per unit.
Ihe present invention can thus be used in an ~IGU system which com-prises a plurality of stations including, -Eor example, 10 such stations. The staiions may have different rated powers and may be divided into any number of rectifier and inverter stations so long as there is at least one station of each type. In such a network, if a malfunctioning station were to reduce its voltage by, for example, one half, steps must be taken to ensure that the other stations do not lose half their power due to the malfunctioning station.
The present invention is therefore based on the realization that multistation operation in a larger DC network with parallel connected stations can be per-formed sensibly only with a constant network voltage, and stations which are unable to hold their voltage must be disconnected. To allow for rapid reaction for intentional changes in load, all stations are operated with a constant direct idle voltage. Such intentional changes in load may occur, for example, when auxiliary power is fed into a three-phase network because the network has lost a generator. Thus, a larger control angle and higher reactance capability must be maintained Eor operation under partial load conditions than for opera-tion under full load conditions. In this manner the reactive power designed for full load operation in the form of capacitors, filter circuits and phase shifters connected at the three-phase end of the rectifier station exceeds the reactive power required by the rectifier station.
These conditions lead to an undesirable increase in the three-phase voltage, particularly in "weak" three-phase networks, i.e. those having a high internal resistance. It is therefore desirable to avoid such excess reactive power by either disconnecting compensation means or by intentionally enlarging the control angles to thereby increase the reactance power absorbed by the rectifier station in partial load operation. The operation of the rectifier _ g _ stations with constant direct idling voltage, in the sense of the present in-vention, therefore meets the demands oE the connected networks. Only at rated operation do the rectifier stations absorb the reactive power corresponding to the minimum control angle.
With respect to the inverter stations, stable inverter operation is assured by regulating -the quench angle at the rated operating point. Under par-tial load conditions, the quench angle is larger and thus inverter operation is more reliable. Accordingly, nothing is changed in the station design com-pared to those in prior art embodiments for multistation operation. In the arrangement according to the invention, the step setters at the transformers only serve the purpose of compensating the generally slower occurring changes in the voltage of the three-phase network so as to keep the idle voltage of the rectifier stations constant.
According to the invention, all rectifier stations are operated with voltage regulation and a current dependent control permits paralle] operation of these stations. The rectifier stations are additionally equipped with a current regulator which produces an absolute current limitation. All inverter stations are equipped with a current regulator which assures that the desired current is supplied. As long as none of the rectifier stations reach their absolute current limit which would reduce the direct voltage of the network to below rated value, each inverter station can freely set its current between 0 and rated value. In the range between the rated network voltage value ~1 per unit) and 1-~ per unit rated voltage, the desired current for each inverter station is reduced from 1 per unit at rated voltage to O at 1-~ per unit rated voltage. In this way, the law applicable for any type of multistation operation (i.e. that sum of the inverter station current control inputs must be less than or equal to the sum of the rectifier station current control inputs over the mains voltage as the in:Eormation value) is automatically adhered to without requiring any special telecommunications connection.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a DC transmission network comprising six transformer stations.
Figure 2 is a circuit schematic for one transformer station in accordance with the invention.
Figure 3 illustrates the regulating equipment of a rectifier station in accordance with the invention.
Figure 4 is a diagram showing the operating characteristics of a rectifier station equipped with regulating equipment in accordance with the invention.
Figure 5 illustrates the regulating equipment for an inverter station according to the invention.
Figure 6 is a diagram showing the operating characteristics o-f an inverter station equipped with regulating equipment according to the invention.
Figures 7a-7e are diagrams illustrating the operation of a network with five ~tations operated according to the invention.
Figures 8a-8e are diagrams illustrating the operation in another operating state of a network having five stations operating according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 shows six transformer stations 101-106 which are connected, ; respectively, to six three-phase networks 1-6. Each transformer station 101-106 includes a rectifier portion 11, 21 . . . 61, respectively, and a polarization switch 12, 22 . . . 62J respectively. Rectifier portions 11, 21 . . . 61 of each station 101-106, respectively, each have a current output pole marked "~".
., ~L~ '755 The role of each station 101-106 as a rectifier (feeding energy into the DC
network) or as arl inverter ~delivering energy from the DC network) is fixed by virtue of its connection to the DC network by means of polarization switches 12, 22 . . . 62, respectively. In Figure 1 each station has only one pole to ground. In practice, however, a station will ordinarily have two poles to ground. The two poles would be interchanged by the polarization switch and the ground point would remain fixed. With the positions of polarization switches 12, 22 . . . 62 illustrated in Figure 1, stations 101, 102 and 103 operate as rectifiers and stations 104, 105 and 106 operate as inverters.
The transmission lines of the network are shown by their resistances RLl, RL2 . . . RL6 relative to a common network point P. The network may of course also be designed as a ring network or as a meshed network. In any case, however, the network configuration can be restructured to a star shape with corresponding resistance values.
Figure 2 shows a circuit schematic for a single station operated according to the invention. Figure 2 is generic to both rectifier and inverter stations. The rectifier portion includes two series connected partial recti-fiers composed of a Y connected transformer 10, a delta connected transformer 10' and controllable bridge circuits 13 and 13' connected in series with transformers 10 and 10', respectively. Bridge circuits 13 and 13' include current valves 43, such as SCR's, which are controlled by phase position signals produced by a control unit 15. Control unit 15 is responsive to the three-phase network voltage obtained via voltage converters 17, and a control voltage 62. Control voltage 62 is produced by a regulating device 16 and is proportional to a firing delay angle a for valves 43.
Details of a control unit, which may be employed as control unit 15, are described, for example, in an article by E. Rumpf and S. Ranade "Comparison of Suitable Control Systems for l-IVDC: Stations Connected to Weak AC Systems" in IEEE Transactions on Power, Apparatus and Systems, Vol. PAS-91, No. 2, pages 5~9 to 564.
An embodiment of a rectifier or an inverter station having trans-formers 10 and 10' and bridge circuits 13 and 13' is known, for example, from the book of E. Uhlmann "Power Transmission by Direct Current", Springer-Verlag, Berlin, Heidelberg, New York, 1975, pages 14 and 15.
A valve 43 mentioned above may be constructed as is known from a CIGRE-Report of the International Conference on Large High Voltage Electric Systems, 1982 Session - 1-9 September "Design And Testing of Thyristor Valves And Their Components for the IIVDC Back-To-Back Link Durnrohr in Austria" by P. Lips, G. Thiele, H. Rotting and K.H. Weck.
In order to secure a constant idle voltage at the valve side of the transformers 10 and 10' adjustable step setters and devices are adjusting these step setters have to be provided at least at one of the transformers. A device for adjusting step setters of a transformer is known through the German patent 21 09 763.
The design of regulating device 16 depends on whether the station is a rectifier or inverter station, which, as noted above, is fixed by the polarity of polarization switch 202, which in the position illustrated, corresponds to switches 12, 22 and 32 of rectifier stations 101, 102 and 103, respectively, in Figure 1.
Regulating device 16 is responsive to a DC measuring value 81 which is proportional to DC transmission line current Id. Measuring value 81 is obtained, for example, from a current converter 18 which is coupled between bridge 13 and polarization switch 202. A smoothing choke 14 is interposed be-tween converter 18 and switch 202. Regulating device 16 is further responsive t755 to a predetermined vol-tage 61 and a locally mcasured voltage value 91 which is obtained, Eor example, from a voltage divider 19 connected across the load and ground terminals of polarization switch 202.
Figure 3 is a block diagram of regulating device 16, according to the invention, as employed in one of rectifier stations 101-103. Control voltage 62 is obtained via a comparator 66 which selects the smaller of the outputs -produced by a voltage regulator amplifier 63 and a current regulator amplifier 64. Voltage regulator amplifier 63 regulates the voltage at common network point P. Current value 81 is combined with a signal proportional to line resis tance RL in a multiplier 632, to produce a signal proportional to line voltage drop Id RL. An amplifier having the gain factor RL could be used in place of multiplier 632. Line voltage drop Id . RL is then subtracted from locally measured voltage 91 to form a voltage vaiue 92 which is, in turn, subtracted ~rom voltage 61.
Voltage 61 is the desired voltage for voltage regulator amplifier 63 and is obtained from a potentiometer 631. The value of voltage 61 constitutes the idle voltage value. Depending on the amount of line current, a member 633, which may be a multiplier, provides a value s Id, where s corresponds to a given slope of the voltage characteristic of the voltage control and is, for example, 0.05, which value is subtracted from voltage 61.
Current regulator amplifier 64 has an input which is formed of cur-rent measuring value 81 adjusted as follows. A voltage corresponding to the maximum desired current value for the rectifier station is set at a potentiometer 641. Normally, the maximum current will be the rated current IdN. But if for operational reasons the station is able to carry only a smaller current, e.g.
Id, this can also be set. This current value constitutes the absolute current limit of the rectifier station. A comparator member 642, which emits at its output the smaller o its two inputs, limlts with respect to voltage the given desired current value. Based on IdN at a voltage 1-~ per unit of rated voltage (Udr~) the desired current value for the station is linearly reduced to 0 at a voltage of (1 - ~ - ~) . UdN, where ~ may be approximately 0.1 per unit of rated voltage ~UdN) and e may be approximately 0.05 per unit of rated voltage (UdN). This is accomplished by subtracting a voltage corresponding to (1 - ~ - c) . UdN from voltage value 92 and multiplying the result in multiplier 646 by 1/~. The output of multiplier 646 is passed through element 645 which is constructed to pass only positive signals. A comparator 643 compares the output of comparator 642 with a minimum desired DC value IrlIN and emits the larger one of these two input signals at its output 68. This sets a minimum DC
value IdMIN for the station, which is algebraically combined with current mea-suring value 81 to form an input 67 to current regulator amplifier 64.
The operating characteristic of the rectifier station produced by the above-described regulating devices, with respect to the common network point P, is shown in Figure 4. In this characteristic, the letter a identifies the part of the characteristic realized by the voltage regulator. The letter b identifies the part of the characteristic given by the absolute current limit IdN. The letter c identifies the part of the characteristic determined by the voltage dependent current limitation, and the letter d identifies the part of the characteristic determined by the minimum current regulation, IdMIN. Nor-mally, the station is intended to operate with voltage regulation, i.e. on the portion of thc characteristic marked with the letter a. The rated operating poin~ is at N. This point is common to the characteristic ~=~MIN at which one station operates by setting a minimum control angle, and it can be seen that the rectifier station according to the invention need not be dimensioned any dif-ferently than stations of prior art design. Characteristic portion a may be U~55 shiftcd ln the vertical dircction to, for examplc, the dotted line a', by adjust-ing desired voltage value 61 at potentiometer 631. In this way the power emitted by the station can be varied. At rated voltage UdN at network point P, characteristic portion a' p:roduces operating point N'. If the absolute current limit b is set at IdN this station would furnish higher currents up to the rated value if the network voltage were reduced to below its rated value.
Portion c of the characteristic serves to protect malfunctioning inverter stations against overloads. During inverter station malfunctions the direct voltage of the inverter station collapses. Due to portion c of the characteristic, the rectifier stations reduce their current to a minimum amount (characteristic portion d) so that the error current is limited. In a network including very many stations, it may be useful to operate only some of the rectifier stations with a finite minimum current and to set the minimum current to zero in the other stations. However, at least one of the rectifier stations must have a finite minimum current, so that after cases of malfunction, the network is charged up again and can continue operation as soon as the voltage level has been reestablished.
The regulating devices of an inverter station are shown in Figure 5.
Comparator 76 selects the greater value of the output value of a quench angle regulator amplifier 65, and the output value of a current regulator amplifier 64, to produce control voltage 62, which again is proportional to angle ~ of valves 43. Quench angle regulator amplifier 65 is constructed in a known manner for regulating deviation of the actual value ~actual from the desired value ~des of the quench angle of inverter valves 43. It is also possible to use one of the known quench angle control methods.
A quench angle regulator and a method for quench angle control is known, for example, from the United States Patent No. 4,212,055 ~Podlewski).
~l~V755 Input 75 to current regulator amplifier 64 i5 formed of current measuring input 81 from current converter 18, and a current control input 70 produced by comparator 744 which switches through the respectively smaller one of its input values to its output. An input value proportional to 1.0 per unit of rated current limits control input 70 to the rated current value of the inverter station. A voltage value 71 set at potentiometer 641 represents the desired current of the inverter station during operation. The output of element 645 limits current control input 70 in dependence on its input voltage in a range from IdN at UdN to 0 at ~ ) UdN. In order to maintain a constant voltage at common network point P, locally measured voltage 91 is added to a value formed of the measured current value 81 multiplied in multiplier 632 wi~h the value RL of the line resistance, to form voltage value 92. The voltage value corresponding to 1-~ per unit of rated voltage is subtracted from the voltage value 92 and the result is multiplied in multiplier 646 with a factor 1/~. Element 645 passes only positive values received from multiplier 646.
The operating characteristics of an inverter station realized with the equipment according to Pigure 5 are shown in Figure 6. Part e of the char-acteristic is determined by quench angle regulator amplifier 65 which assures stable operation of the inverter station. Part f of the characteristic is pro-duced by current regulator amplifier 64 with a maximum control input of 1.0 per unit of rated current. When desired current value 71 is less than rated current, characteristic f' results at point IdREF. Part g of the characteristic is produced when the voltage dependent control input from element 645 is passed by comparator 744.
The rated design point N of the inverter station must be selected in such a way that the quench angle regulation at rated current occurs at voltage (l~s) UdN. This is necessary because if there is only one operational inverter s~
station in a network having a plurality of operational rectifier stations, it is still able to receive its rated current. [f the current is very low, the rectifier stations set a voltage near (l+s) UdN. The operating point of the inverter station with quench angle control is given by the point of intersection of the ~ = constant characteristic at (l+s) . UdN and must therefore lie at least at the current value IdN (design point N).
An example of numerical values is given with the p.u. values in Fig-ures 3 and 5. The p.u. values relate to either UdN or IdN and may be represent-ed in the control equipment by 10 V - 1 p.u.
The basic values are chosen:
s = 0.05 p.u.
~5 = 0.1 p.u.
= 0.05 p.u.
UdiO = 1.2 UdN IdN RL = 0-05 p.u.
= const = 18 MIN
The numerical values are related to the operating points of Figures 7b (recti-fier) and 7c (inverter), which are described in detail further down.
According to the invention, the value ~ is to be equal for all stations, and the values s and e are to be equal for all rectifier stations.
There are, then, two primary operating states for a network having any desired number of stations. In normal operation, all inverter stations operate with current regulation, characteristic part f, and all rectifier stations operate wi.th voltage regulation, characteristic part a. This state is shown in Figures 7a-7e for a network having two rectifier stations (GR 1, GR 2) and three inver-ter stations (WR 1, WR 2, WR 3). To simplify the explanation, the rated current of all stations is assumed to be selected the same. In Figures 7, the current control inputs of the inverter stations are set in such a way that their sum does not exceed the sum of the rectifier rated currents. WR 1 is set to 0.3 IdN, WR 2 is set to 0.5 IdN and WR 3 is set to 1 IdN.
The operating points are determined by the common voltage UB at net-work pOillt P at which the Kirchhof:E node rule d is also met at network point P. This occurs somewhat above UdN. The operating points Bl, B2 of the rectifier stations lie on the voltage regulation part of their characteristic, while the operating points BW 1, BW 2, BW 3 of the inver-ter stations lie on the current regulation part of their characteristic.
Figures 8a-8e show the operating state of the same network when the sum of the current control inputs given in the inverter stations exceeds the sum of the currents emitted by the rectifier stations. The ratedcurrentis given in each of the inverter stations. The rectifiers reach their current limiting regulation and thus the direct voltage is reduced. In the inverter stations, the effective current control input is reduced according to part g of the characteristic due to the voltage that has dropped below the rated value.
The operating points are again determined under the condition of equal voltage and meeting the node point rule ~I = 0, in network point P.
The rectifier stations operate with current regulation at operating points Bl, B2. The inverter station operating points BW 1, BW 2, BW 3 lie at
_ _ .
The present invention relates to a method for transmitting power and in particular to a me-thod -for transmitting power between a plurality of locally separated transformer stations of a high voltage DC transmission system.
In prior art systems for high voltage direct current transmission (HGU) that include only two stations, a direct current transmission line or cable connects the stations together at their DC ends, while the AC end of each station is connected to the local three-phase mains. In such a "two-point"
HGU, one station takes energy from the local three-phase mains and feeds itJ
rectified, into the direct current line. At the other end of the line, the other station operates as an inverter which transforms the energy of the direct current into a three-phase current and feeds it to the local three-phase mains.
The operation of the inverter station is performed by a rectifier in inverter operation.
This rectifier in inverter Gperation is a critical element in the arrangement. When there are malfunctions in the connected three-phase mains, there may occur commutation errors, referred to as flipping, as a result of which the direct voltage across the terminals of the inverter station breaks down. In order to prevent excess current loads on the components of the inver-ter, particularly its current valves or controllable elements, the rectifier station is equipped with a regulating device which keeps its direct current constant at a settable value. In this way it is possible to keep the current in the transmission line connecting the rectifier and inverter stations at a given value even if the inverse inverter voltage breaks down and thus protect the inverter station against current overloads.
Another aspect to be considered in equipping the stations is the 7~ t3 reactive power to be produced by the stations from the three-phase mains. The reactive power received by the rectifier station is proportional to the magni-tude of the direct current produced thereby and increases with the size of the control, or conduction, angle of the valves, a maximum occurring at a control angle of 90, at which the direct voltage is appro~imately zero. Therefore, the inverter station is operated with the smallest possible control angle ~ at which stable operation is just assured. The quench angle y of the valves in the inverter station is also measured and regulated and is characteristic -for stable operation. The direct voltage of the transmission is set via the step setter of the inverter transformers. The rectifier station mwst have the smallest possible control angle ~ in that the idle, or no load, voltage of the rectifier station follows the step setters of the rectifier transformers in the sense of keeping the small control angle constant.
The control angle ~ is defined to be the electrical angle from the time when the anode-voltage of the valve becomes positive up to the firing instant. The control angle ~ is defined to be the electrical angle from the firing of the valve up to the point where the commutation voltage becomes negative. From this results ~+~ = 180. The quench angle ~, or as it is called the extinction angle, is the electrical angle from the valve current extinction point up to that point the commutation-voltage becomes negative.
Because the setting of the transformer step setters takes place relatively slowly, if a rapid increase in current is required, it is necessary that the inverter temporarily reduce its inverse voltage until the step setter of the rectifier has adjusted the idle voltage to the required higher value.
This is accomplished by equipping the rectifier in the inverter station with a current regulator which is limited in a dominating fashion by a quench angle regulator in the direction toward smaller control angles ~. If the current 5 . ~
regulator in the inverter station receives a current control input which is marginally smaller than the direc-t current Elowing in the transmission line, as determined by the current regulator of the rectifier station, then the inverter current regulator attempts to decrease the control angle ~ and thus reaches the limit defined by the quench angle regulator so that ultimately the operation is determined by the quench angle regulator. If, ilowever, the direct current in the transmission line becomes smaller than the given current control input of the inverter current regulator, because, for example, the current regulator in the rectifier station is unable to set a current corresponding to its current control input, even with a control angle of ~ = 0 (i.e. during a rapid inten-tional current increase or if one of a plurality of series connected partial current rectifiers is malfunctioning) then the inverter current rectifier is able to set a larger control angle, thus reducing the inverse voltage of the inverter station so that the current in the transmission line is kept to a value which is lower by the marginal value instead of dropping completely to zero.
For proper operation with such a "marginal current method" it is necessary for the current control input of the current regulator in the recti-fier station to always be greater by the marginal value, or at least greater in any case, than the current control input of the current regulator in the inverter station. In such a system, the station having the larger current regulator current control input always plays the part of the rectifier station and thus determines the direction of energy transmission. It is possible to avoid a reversal of direction in the energy transmission due to erroneous association of the current control input by employing control angle range limita-tions, or the malfunction can be limited to only a change in the transmitted power to zero. It is thus very important for the current control input 7~5 association to always be correct. Ihis is accomplished by appropriate long-distance signal transmission devices as disclosed in German Auslegeschrift 1,488,085.
The marginal current system has been fowld to be particularly satis-factory in two point ~IGU's because of its emergency operation characteristics.
These same characteristics have led to its use in a system employing more than two stations. The stations are divided into a rectifier group and an inverter group with respect to a real or fictitious common point in the DC network, and the rules of the marginal method are used for both groups. (See, for example, German Auslegeschrift 1,488,085.) The main requirement for multistation operation according to the marginal current method is that the sum of the current regulator current control inputs of all rectifier stations must always be greater than the sum of all such inputs of the inverter stations. In such a system the current in all stations is regulated except for the current in one station which reaches an operating state that determines the voltage for the DC system. This one station operates either as an inverter or a rectifier station. As an inverter station with quench angle regulation, it must be able to receive a current corresponding to its current control inputs and additionally the marginal value. As a recti-fier station it operates with a control angle of zero. Due to the possibility of overloading an inverter station, the margina] value must not be selected too large. It is then necessary to quickly and accurately follow up with all current control inputs when the power of such a station is to be changed. This requires a central load equalization, as described in German Patent No.
~1,523,555l, which requires a considerable amount of expense for remote reporting, or monitoring telecommunications.
If in the plurality of inverter stations, one station is included which has a smaller rated power than the others, the additional marginal cwr-rent in it may, under certain circumstances, lead to an overload, unless the rated power of that station is raised accordingly. Since it is desirable that such a station not become voltage determining for the DC system, special mea-sures must be taken to associate the voltage control variable of the DC system with a certain station.
One drawback of such a system is that the malfunction of a series-connected partial rectifier in one station leads to a corresponding voltage reduction and thus to a power reduction in the entire system. This state can be avoided by additional voltage limitation, as disclosed in German Auslegeschrift 1,588,067, which simultaneously reduces the requirements for telecommunication connections. Nevertheless, a central load equalization of the current control inputs is still required as in other marginal current systems.
To avoid the above-described difficulties of the marginal current method German Offenlegungsschrift 1,588,750 discloses a method which operates with voltage-dependent controlled current control inputs and does not require remote signal transmission for operation. All stations operate with regulated current The current regulators of the individual stations are thus given current/voltage characteristics of a natural type, i.e., the rectifier as the current source has a characteristic which descends with increasing current and the inverter as current user has a characteristic which ascends with increasing current. The points of intersection of the current/voltage characteristics indicate the stable operating point of the system. The drawback in this case is that the points of intersection, particularly those of the inverter charac-teristics, are points of intersection at a small angle (sliding intersections) and are thus not very precise. This can easily lead to overloads, particularly in an inverter station of low power compared to the other stations. Changes in load, moreover, can be accomplishcd only very slowly because they require a change in the transformer step setter.
Germa.n Auslegeschrift 2,518,910 describes a method for solving the problem of providing power to inverter sta-tions without mutual influence while avoiding central matching of the current control input. A current dependent voltage regulator is provided in the rectifier station with a current control input having a single, fixed current limitation which is higher, by a set marginal value, than the power capability of the station. A voltage dependent current regulator is provided in each of the inverter stations in which the maximum value of the current control input for the current regulator is deter-mined without detailed information of the calculating circuitry employed accor-ding to a current/current characteristic matched ~o the power capability of the rectifier station. By itself, the voltage of the mains provides information about the power capability of only a single rectifier station. This method cannot be transferred to a system including a plurality of rectifier stations.
Since it is not known which stations are in operation at any point in time it is not possible to draw a conclusion as to the power capability of any one rectifier station from the mains voltage.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of the above-mentioned type in which unequivocal operating points can be set relative to any number of rectifier and inverter stations, without complicated telecom-munications devices for central load matching, without the provision of a specialmarginal current for current limitation in connection with the current control input for the rectifier, and without the use of di.fferent current/
current characteristics for each rectifier station for the formation of the maximum value of the current control input for the inverter current regulators, 75~
which characteristics would be complicated to calculate.
The above and other objects are accomplished according to the in-vention wherein a method is provided for transmitting power between a plurality of locally separated transformer stations of a high voltage DC transmission system in which each station has a DC end and an AC end. The DC end of each station is connected to a respective one of a plurality of transmission lines connected in network having a common network point. Each station is connected at its AC end with a respective local three-phase network. Each station in-cludes a rectifier portion having a positive pole and a negative pole and a polarization reversal switch for selectively connecting the respective one of the transmission lines to its positive pole for operation of the station as a rectifier station or to its negative pole -for operation of the station as an inverter station. Each station is operated to maintain a direct current in its respective transmission line and a direct voltage at that pole which is connected to its respective transmission line. At least one transformer station is connected as a rectifier station and at least one transformer station is con-nected as an inverter station. The method includes controlling the rectifier portion of each station via a first regulating device to regulate the direct current in the respective transmission line to a desired value, and further controlling the rectifier portion of each station via a second regulating device which dominates the first regulating device to set the direct voltage at that pole of each station which is connected to the respective transmission line to a given limit value with reference to the common network point. In the at least one rectifier station the step of controlling to set the direct voltage includes reducing the given limit value linearly with the direct current until the given limit value reaches a rated direct voltage value of that rectifier station at a rated direct current value of that rectifier station~ and the step of controlling '755 to regulate the direct current includes adjusting the desired value of the direct current in clependence on the direct voltage valve of that rectifier station such that in a voltage range from 1 per unit of rated voltage value to l-c, per unit of rated voltage value the desired direct current value corres-ponds to the rated direct current value and in a voltage range from l-c) per unit of rated voltage value to 1- c, - ~ per unit of rated voltage value the desired direct current value is linearly reduced to a preset minimum value, where ~ and e are preselected fractions. In the at least one inverter station, the step of controlling to regulate the direct current includes limiting the value of desired directcurrent in linear dependence on the direct voltage value of that inverter station, starting from the rated current value of the inverter station at a direct voltage of 1 per unit of rated voltage value down to zero current at a direct voltage of l-c, per unit of rated voltage value.
The method thus provides advantageous measures for operating an HGU
network having any desired number of stations. The problems of dividing the load between rectifier stations of emergency operation during lack of power in the mains and the avoidance of overloads during malfunctions of individual rectifier stations are thus solved in a simple manner.
In each rectifier station, a current dependent voltage regulator is provided whose current control input depends on the current actually occurring in the station. There thus exists a fixed and a voltage dependent current limitation. The current limit in each of the inverter stations is linearly reduced during its formation to the range of the current limit at which the rectifier stations operate, without requiring a current/current characteristic and the setting of a marginal current. Rather, all of the characteristics for all rectifi.er stations and for all inverter stations, respectively, have the same per unit values, with the actual limit state of all rectifier stations .
7S5i being detectable in the voltage in the range from 1 to 1-~ per unit.
Ihe present invention can thus be used in an ~IGU system which com-prises a plurality of stations including, -Eor example, 10 such stations. The staiions may have different rated powers and may be divided into any number of rectifier and inverter stations so long as there is at least one station of each type. In such a network, if a malfunctioning station were to reduce its voltage by, for example, one half, steps must be taken to ensure that the other stations do not lose half their power due to the malfunctioning station.
The present invention is therefore based on the realization that multistation operation in a larger DC network with parallel connected stations can be per-formed sensibly only with a constant network voltage, and stations which are unable to hold their voltage must be disconnected. To allow for rapid reaction for intentional changes in load, all stations are operated with a constant direct idle voltage. Such intentional changes in load may occur, for example, when auxiliary power is fed into a three-phase network because the network has lost a generator. Thus, a larger control angle and higher reactance capability must be maintained Eor operation under partial load conditions than for opera-tion under full load conditions. In this manner the reactive power designed for full load operation in the form of capacitors, filter circuits and phase shifters connected at the three-phase end of the rectifier station exceeds the reactive power required by the rectifier station.
These conditions lead to an undesirable increase in the three-phase voltage, particularly in "weak" three-phase networks, i.e. those having a high internal resistance. It is therefore desirable to avoid such excess reactive power by either disconnecting compensation means or by intentionally enlarging the control angles to thereby increase the reactance power absorbed by the rectifier station in partial load operation. The operation of the rectifier _ g _ stations with constant direct idling voltage, in the sense of the present in-vention, therefore meets the demands oE the connected networks. Only at rated operation do the rectifier stations absorb the reactive power corresponding to the minimum control angle.
With respect to the inverter stations, stable inverter operation is assured by regulating -the quench angle at the rated operating point. Under par-tial load conditions, the quench angle is larger and thus inverter operation is more reliable. Accordingly, nothing is changed in the station design com-pared to those in prior art embodiments for multistation operation. In the arrangement according to the invention, the step setters at the transformers only serve the purpose of compensating the generally slower occurring changes in the voltage of the three-phase network so as to keep the idle voltage of the rectifier stations constant.
According to the invention, all rectifier stations are operated with voltage regulation and a current dependent control permits paralle] operation of these stations. The rectifier stations are additionally equipped with a current regulator which produces an absolute current limitation. All inverter stations are equipped with a current regulator which assures that the desired current is supplied. As long as none of the rectifier stations reach their absolute current limit which would reduce the direct voltage of the network to below rated value, each inverter station can freely set its current between 0 and rated value. In the range between the rated network voltage value ~1 per unit) and 1-~ per unit rated voltage, the desired current for each inverter station is reduced from 1 per unit at rated voltage to O at 1-~ per unit rated voltage. In this way, the law applicable for any type of multistation operation (i.e. that sum of the inverter station current control inputs must be less than or equal to the sum of the rectifier station current control inputs over the mains voltage as the in:Eormation value) is automatically adhered to without requiring any special telecommunications connection.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a DC transmission network comprising six transformer stations.
Figure 2 is a circuit schematic for one transformer station in accordance with the invention.
Figure 3 illustrates the regulating equipment of a rectifier station in accordance with the invention.
Figure 4 is a diagram showing the operating characteristics of a rectifier station equipped with regulating equipment in accordance with the invention.
Figure 5 illustrates the regulating equipment for an inverter station according to the invention.
Figure 6 is a diagram showing the operating characteristics o-f an inverter station equipped with regulating equipment according to the invention.
Figures 7a-7e are diagrams illustrating the operation of a network with five ~tations operated according to the invention.
Figures 8a-8e are diagrams illustrating the operation in another operating state of a network having five stations operating according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 shows six transformer stations 101-106 which are connected, ; respectively, to six three-phase networks 1-6. Each transformer station 101-106 includes a rectifier portion 11, 21 . . . 61, respectively, and a polarization switch 12, 22 . . . 62J respectively. Rectifier portions 11, 21 . . . 61 of each station 101-106, respectively, each have a current output pole marked "~".
., ~L~ '755 The role of each station 101-106 as a rectifier (feeding energy into the DC
network) or as arl inverter ~delivering energy from the DC network) is fixed by virtue of its connection to the DC network by means of polarization switches 12, 22 . . . 62, respectively. In Figure 1 each station has only one pole to ground. In practice, however, a station will ordinarily have two poles to ground. The two poles would be interchanged by the polarization switch and the ground point would remain fixed. With the positions of polarization switches 12, 22 . . . 62 illustrated in Figure 1, stations 101, 102 and 103 operate as rectifiers and stations 104, 105 and 106 operate as inverters.
The transmission lines of the network are shown by their resistances RLl, RL2 . . . RL6 relative to a common network point P. The network may of course also be designed as a ring network or as a meshed network. In any case, however, the network configuration can be restructured to a star shape with corresponding resistance values.
Figure 2 shows a circuit schematic for a single station operated according to the invention. Figure 2 is generic to both rectifier and inverter stations. The rectifier portion includes two series connected partial recti-fiers composed of a Y connected transformer 10, a delta connected transformer 10' and controllable bridge circuits 13 and 13' connected in series with transformers 10 and 10', respectively. Bridge circuits 13 and 13' include current valves 43, such as SCR's, which are controlled by phase position signals produced by a control unit 15. Control unit 15 is responsive to the three-phase network voltage obtained via voltage converters 17, and a control voltage 62. Control voltage 62 is produced by a regulating device 16 and is proportional to a firing delay angle a for valves 43.
Details of a control unit, which may be employed as control unit 15, are described, for example, in an article by E. Rumpf and S. Ranade "Comparison of Suitable Control Systems for l-IVDC: Stations Connected to Weak AC Systems" in IEEE Transactions on Power, Apparatus and Systems, Vol. PAS-91, No. 2, pages 5~9 to 564.
An embodiment of a rectifier or an inverter station having trans-formers 10 and 10' and bridge circuits 13 and 13' is known, for example, from the book of E. Uhlmann "Power Transmission by Direct Current", Springer-Verlag, Berlin, Heidelberg, New York, 1975, pages 14 and 15.
A valve 43 mentioned above may be constructed as is known from a CIGRE-Report of the International Conference on Large High Voltage Electric Systems, 1982 Session - 1-9 September "Design And Testing of Thyristor Valves And Their Components for the IIVDC Back-To-Back Link Durnrohr in Austria" by P. Lips, G. Thiele, H. Rotting and K.H. Weck.
In order to secure a constant idle voltage at the valve side of the transformers 10 and 10' adjustable step setters and devices are adjusting these step setters have to be provided at least at one of the transformers. A device for adjusting step setters of a transformer is known through the German patent 21 09 763.
The design of regulating device 16 depends on whether the station is a rectifier or inverter station, which, as noted above, is fixed by the polarity of polarization switch 202, which in the position illustrated, corresponds to switches 12, 22 and 32 of rectifier stations 101, 102 and 103, respectively, in Figure 1.
Regulating device 16 is responsive to a DC measuring value 81 which is proportional to DC transmission line current Id. Measuring value 81 is obtained, for example, from a current converter 18 which is coupled between bridge 13 and polarization switch 202. A smoothing choke 14 is interposed be-tween converter 18 and switch 202. Regulating device 16 is further responsive t755 to a predetermined vol-tage 61 and a locally mcasured voltage value 91 which is obtained, Eor example, from a voltage divider 19 connected across the load and ground terminals of polarization switch 202.
Figure 3 is a block diagram of regulating device 16, according to the invention, as employed in one of rectifier stations 101-103. Control voltage 62 is obtained via a comparator 66 which selects the smaller of the outputs -produced by a voltage regulator amplifier 63 and a current regulator amplifier 64. Voltage regulator amplifier 63 regulates the voltage at common network point P. Current value 81 is combined with a signal proportional to line resis tance RL in a multiplier 632, to produce a signal proportional to line voltage drop Id RL. An amplifier having the gain factor RL could be used in place of multiplier 632. Line voltage drop Id . RL is then subtracted from locally measured voltage 91 to form a voltage vaiue 92 which is, in turn, subtracted ~rom voltage 61.
Voltage 61 is the desired voltage for voltage regulator amplifier 63 and is obtained from a potentiometer 631. The value of voltage 61 constitutes the idle voltage value. Depending on the amount of line current, a member 633, which may be a multiplier, provides a value s Id, where s corresponds to a given slope of the voltage characteristic of the voltage control and is, for example, 0.05, which value is subtracted from voltage 61.
Current regulator amplifier 64 has an input which is formed of cur-rent measuring value 81 adjusted as follows. A voltage corresponding to the maximum desired current value for the rectifier station is set at a potentiometer 641. Normally, the maximum current will be the rated current IdN. But if for operational reasons the station is able to carry only a smaller current, e.g.
Id, this can also be set. This current value constitutes the absolute current limit of the rectifier station. A comparator member 642, which emits at its output the smaller o its two inputs, limlts with respect to voltage the given desired current value. Based on IdN at a voltage 1-~ per unit of rated voltage (Udr~) the desired current value for the station is linearly reduced to 0 at a voltage of (1 - ~ - ~) . UdN, where ~ may be approximately 0.1 per unit of rated voltage ~UdN) and e may be approximately 0.05 per unit of rated voltage (UdN). This is accomplished by subtracting a voltage corresponding to (1 - ~ - c) . UdN from voltage value 92 and multiplying the result in multiplier 646 by 1/~. The output of multiplier 646 is passed through element 645 which is constructed to pass only positive signals. A comparator 643 compares the output of comparator 642 with a minimum desired DC value IrlIN and emits the larger one of these two input signals at its output 68. This sets a minimum DC
value IdMIN for the station, which is algebraically combined with current mea-suring value 81 to form an input 67 to current regulator amplifier 64.
The operating characteristic of the rectifier station produced by the above-described regulating devices, with respect to the common network point P, is shown in Figure 4. In this characteristic, the letter a identifies the part of the characteristic realized by the voltage regulator. The letter b identifies the part of the characteristic given by the absolute current limit IdN. The letter c identifies the part of the characteristic determined by the voltage dependent current limitation, and the letter d identifies the part of the characteristic determined by the minimum current regulation, IdMIN. Nor-mally, the station is intended to operate with voltage regulation, i.e. on the portion of thc characteristic marked with the letter a. The rated operating poin~ is at N. This point is common to the characteristic ~=~MIN at which one station operates by setting a minimum control angle, and it can be seen that the rectifier station according to the invention need not be dimensioned any dif-ferently than stations of prior art design. Characteristic portion a may be U~55 shiftcd ln the vertical dircction to, for examplc, the dotted line a', by adjust-ing desired voltage value 61 at potentiometer 631. In this way the power emitted by the station can be varied. At rated voltage UdN at network point P, characteristic portion a' p:roduces operating point N'. If the absolute current limit b is set at IdN this station would furnish higher currents up to the rated value if the network voltage were reduced to below its rated value.
Portion c of the characteristic serves to protect malfunctioning inverter stations against overloads. During inverter station malfunctions the direct voltage of the inverter station collapses. Due to portion c of the characteristic, the rectifier stations reduce their current to a minimum amount (characteristic portion d) so that the error current is limited. In a network including very many stations, it may be useful to operate only some of the rectifier stations with a finite minimum current and to set the minimum current to zero in the other stations. However, at least one of the rectifier stations must have a finite minimum current, so that after cases of malfunction, the network is charged up again and can continue operation as soon as the voltage level has been reestablished.
The regulating devices of an inverter station are shown in Figure 5.
Comparator 76 selects the greater value of the output value of a quench angle regulator amplifier 65, and the output value of a current regulator amplifier 64, to produce control voltage 62, which again is proportional to angle ~ of valves 43. Quench angle regulator amplifier 65 is constructed in a known manner for regulating deviation of the actual value ~actual from the desired value ~des of the quench angle of inverter valves 43. It is also possible to use one of the known quench angle control methods.
A quench angle regulator and a method for quench angle control is known, for example, from the United States Patent No. 4,212,055 ~Podlewski).
~l~V755 Input 75 to current regulator amplifier 64 i5 formed of current measuring input 81 from current converter 18, and a current control input 70 produced by comparator 744 which switches through the respectively smaller one of its input values to its output. An input value proportional to 1.0 per unit of rated current limits control input 70 to the rated current value of the inverter station. A voltage value 71 set at potentiometer 641 represents the desired current of the inverter station during operation. The output of element 645 limits current control input 70 in dependence on its input voltage in a range from IdN at UdN to 0 at ~ ) UdN. In order to maintain a constant voltage at common network point P, locally measured voltage 91 is added to a value formed of the measured current value 81 multiplied in multiplier 632 wi~h the value RL of the line resistance, to form voltage value 92. The voltage value corresponding to 1-~ per unit of rated voltage is subtracted from the voltage value 92 and the result is multiplied in multiplier 646 with a factor 1/~. Element 645 passes only positive values received from multiplier 646.
The operating characteristics of an inverter station realized with the equipment according to Pigure 5 are shown in Figure 6. Part e of the char-acteristic is determined by quench angle regulator amplifier 65 which assures stable operation of the inverter station. Part f of the characteristic is pro-duced by current regulator amplifier 64 with a maximum control input of 1.0 per unit of rated current. When desired current value 71 is less than rated current, characteristic f' results at point IdREF. Part g of the characteristic is produced when the voltage dependent control input from element 645 is passed by comparator 744.
The rated design point N of the inverter station must be selected in such a way that the quench angle regulation at rated current occurs at voltage (l~s) UdN. This is necessary because if there is only one operational inverter s~
station in a network having a plurality of operational rectifier stations, it is still able to receive its rated current. [f the current is very low, the rectifier stations set a voltage near (l+s) UdN. The operating point of the inverter station with quench angle control is given by the point of intersection of the ~ = constant characteristic at (l+s) . UdN and must therefore lie at least at the current value IdN (design point N).
An example of numerical values is given with the p.u. values in Fig-ures 3 and 5. The p.u. values relate to either UdN or IdN and may be represent-ed in the control equipment by 10 V - 1 p.u.
The basic values are chosen:
s = 0.05 p.u.
~5 = 0.1 p.u.
= 0.05 p.u.
UdiO = 1.2 UdN IdN RL = 0-05 p.u.
= const = 18 MIN
The numerical values are related to the operating points of Figures 7b (recti-fier) and 7c (inverter), which are described in detail further down.
According to the invention, the value ~ is to be equal for all stations, and the values s and e are to be equal for all rectifier stations.
There are, then, two primary operating states for a network having any desired number of stations. In normal operation, all inverter stations operate with current regulation, characteristic part f, and all rectifier stations operate wi.th voltage regulation, characteristic part a. This state is shown in Figures 7a-7e for a network having two rectifier stations (GR 1, GR 2) and three inver-ter stations (WR 1, WR 2, WR 3). To simplify the explanation, the rated current of all stations is assumed to be selected the same. In Figures 7, the current control inputs of the inverter stations are set in such a way that their sum does not exceed the sum of the rectifier rated currents. WR 1 is set to 0.3 IdN, WR 2 is set to 0.5 IdN and WR 3 is set to 1 IdN.
The operating points are determined by the common voltage UB at net-work pOillt P at which the Kirchhof:E node rule d is also met at network point P. This occurs somewhat above UdN. The operating points Bl, B2 of the rectifier stations lie on the voltage regulation part of their characteristic, while the operating points BW 1, BW 2, BW 3 of the inver-ter stations lie on the current regulation part of their characteristic.
Figures 8a-8e show the operating state of the same network when the sum of the current control inputs given in the inverter stations exceeds the sum of the currents emitted by the rectifier stations. The ratedcurrentis given in each of the inverter stations. The rectifiers reach their current limiting regulation and thus the direct voltage is reduced. In the inverter stations, the effective current control input is reduced according to part g of the characteristic due to the voltage that has dropped below the rated value.
The operating points are again determined under the condition of equal voltage and meeting the node point rule ~I = 0, in network point P.
The rectifier stations operate with current regulation at operating points Bl, B2. The inverter station operating points BW 1, BW 2, BW 3 lie at
2/3 IdN on part g of the characteristic. This shows that even with unreason-able current control inputs, operation of the network continues without requiring a telecommunications connection. The case illustrated by Figure 8 happens only in exceptional circumstances. Generally, a network will initially have avail-able sufficient rectifier power before the inverters take over their power.
The case of Figures 8 may occur, however, if a third rectifier station had previousl~ been in operation, resulting in a state s.imilar to Figures 7, which was then disconnected, :Eor example, due to a mal:Eunction.
It will be understood that the above description of the present invention is susceptible to various modi-Eications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equiva-lents of the appended claims.
The case of Figures 8 may occur, however, if a third rectifier station had previousl~ been in operation, resulting in a state s.imilar to Figures 7, which was then disconnected, :Eor example, due to a mal:Eunction.
It will be understood that the above description of the present invention is susceptible to various modi-Eications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equiva-lents of the appended claims.
Claims (4)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method for transmitting power between a plurality of locally separated transformer stations of a high voltage DC transmission system wherein each station has a DC end and an AC end, the DC end of each station is connect-ed to a respective one of a plurality of transmission lines connected in network having a common network point, and each station is connected at its AC end with a respective local three-phase network, each station including a rectifier portion having a positive pole and a negative pole and a polarization reversal switch for selectively connecting the respective one of the transmission lines to its positive pole for operation of the station as a rectifier station or to its negative pole for operation of the station as an inverter station, each station being operated to maintain a direct current in its respective trans-mission line and a direct voltage at that pole which is connected to its respec-tive transmission line, wherein at least one transformer station is connected as a rectifier station and at least one transformer station is connected as an inverter station, said method comprising:
(a) controlling the rectifier portion of each station via a first regulating device to regulate the direct current in the respective transmission line to a desired value; and (b) controlling the rectifier portion of each station via a second regulating device which dominates the first regulating device to set the direct voltage at that pole of each station which is connected to the respective transmission line to a given limit value with reference to the common network point; and wherein (c) in the at least one rectifier station (1) said step of controlling to set the direct voltage includes reducing the given limit value linearly with the direct current until the given limit value reaches a rated direct voltage value of that rectifier station at a rated direct current value of that rectifier station; and (2) said step of controlling to regulate the direct current in-cludes adjusting the desired value of the direct current in dependence on the direct voltage valve of that rectifier station such that in a voltage range from 1 per unit of rated voltage value to 1-.delta. per unit of rated voltage value the desired direct current value corresponds to the rated direct current value and in a voltage range from 1-.delta. per unit of rated voltage value to 1 - 3 - .epsilon. per unit of rated voltage value the desired direct current value is linearly reduced to a preset minimum value, where .delta. and .epsilon. are preselected fractions; and (d) in the at least one inverter station, said step of controlling to regulate the direct current includes limiting the value of desired direct current in linear dependence on the direct voltage value of that inverter station, starting from the rated current value of the inverter station at a direct voltage of 1 per unit of rated voltage value down to zero current at a direct voltage of 1-.delta. per unit of rated voltage value.
(a) controlling the rectifier portion of each station via a first regulating device to regulate the direct current in the respective transmission line to a desired value; and (b) controlling the rectifier portion of each station via a second regulating device which dominates the first regulating device to set the direct voltage at that pole of each station which is connected to the respective transmission line to a given limit value with reference to the common network point; and wherein (c) in the at least one rectifier station (1) said step of controlling to set the direct voltage includes reducing the given limit value linearly with the direct current until the given limit value reaches a rated direct voltage value of that rectifier station at a rated direct current value of that rectifier station; and (2) said step of controlling to regulate the direct current in-cludes adjusting the desired value of the direct current in dependence on the direct voltage valve of that rectifier station such that in a voltage range from 1 per unit of rated voltage value to 1-.delta. per unit of rated voltage value the desired direct current value corresponds to the rated direct current value and in a voltage range from 1-.delta. per unit of rated voltage value to 1 - 3 - .epsilon. per unit of rated voltage value the desired direct current value is linearly reduced to a preset minimum value, where .delta. and .epsilon. are preselected fractions; and (d) in the at least one inverter station, said step of controlling to regulate the direct current includes limiting the value of desired direct current in linear dependence on the direct voltage value of that inverter station, starting from the rated current value of the inverter station at a direct voltage of 1 per unit of rated voltage value down to zero current at a direct voltage of 1-.delta. per unit of rated voltage value.
2. A method as defined in claim 1 wherein the rectifier portion of each transformer station includes at least one transformer having adjustable step setters, and current valves connected to the at least one transformer, and said method includes producing a constant idle voltage at the valve side of the transformer via devices for adjusting the step setters in the at least one transformer.
3. A method as defined in claim 2 including connecting the valves together to form two partial rectifiers which are in turn connected together to form the positive and negative poles.
4. A method as defined in claim 1 wherein .delta. is preselected at 0.1 and .epsilon. is preselected at 0.05.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19823225285 DE3225285A1 (en) | 1982-07-03 | 1982-07-03 | METHOD FOR OPERATING A HIGH VOLTAGE DC TRANSFER SYSTEM WITH ANY MANY TRANSFORMER STATIONS |
| DEP3225285.4 | 1982-07-03 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1180755A true CA1180755A (en) | 1985-01-08 |
Family
ID=6167777
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000416224A Expired CA1180755A (en) | 1982-07-03 | 1982-11-24 | Method of operating a high voltage direct current transmission system |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US4459492A (en) |
| CA (1) | CA1180755A (en) |
| DE (1) | DE3225285A1 (en) |
| SE (1) | SE8303795L (en) |
| ZA (1) | ZA834835B (en) |
Families Citing this family (25)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4599519A (en) * | 1984-05-16 | 1986-07-08 | The United States Of America As Represented By The United States Department Of Energy | Superconducting magnetic energy storage for asynchronous electrical systems |
| US4788635A (en) * | 1987-02-04 | 1988-11-29 | Westinghouse Electric Corp. | Converter system combining a two-quadrant voltage-source rectifier and a four-quadrant voltage-source inverter, and a motor drive embodying the same |
| SE463953B (en) * | 1989-06-19 | 1991-02-11 | Asea Brown Boveri | INSTALLATION FOR DRAINING ELECTRIC POWER FROM A HIGH-SPEED DC POWER TRANSMISSION LINE |
| US5003453A (en) * | 1989-11-22 | 1991-03-26 | Tandem Computers Incorporated | Apparatus for a balanced three phase AC power supply in a computer having variable DC loads |
| SE9403486L (en) * | 1994-10-13 | 1996-04-14 | Asea Brown Boveri | Method and apparatus for controlling a plant for transmission of high voltage direct current |
| SE514826C2 (en) * | 1994-10-13 | 2001-04-30 | Abb Ab | Method and apparatus for controlling a series compensated inverter station |
| SE504522C2 (en) * | 1995-07-06 | 1997-02-24 | Asea Brown Boveri | Power transmission with high voltage direct current comprising more than two inverter stations |
| SE514920C2 (en) * | 1997-03-24 | 2001-05-14 | Abb Ab | Electric power plant with DC and AC networks including detecting unbalanced states of inverters |
| SE510353C2 (en) * | 1997-06-12 | 1999-05-17 | Asea Brown Boveri | Installation of high voltage direct current containing bidirectional controlled thyristors |
| US6157555A (en) * | 1999-11-09 | 2000-12-05 | International Business Machines Corporation | Current limiter responsive to changing parallel power supply capacity |
| US6975098B2 (en) * | 2002-01-31 | 2005-12-13 | Vlt, Inc. | Factorized power architecture with point of load sine amplitude converters |
| US20040125618A1 (en) * | 2002-12-26 | 2004-07-01 | Michael De Rooij | Multiple energy-source power converter system |
| DE102004016034A1 (en) * | 2004-03-30 | 2005-10-20 | Alstom Technology Ltd Baden | Electrical system for coupling a power supply network and a central DC voltage line and method for operating such a system |
| CN101341642B (en) | 2005-12-21 | 2010-12-29 | Abb技术有限公司 | High voltage DC system |
| CN101297455B (en) * | 2006-01-18 | 2011-05-11 | Abb技术有限公司 | A converter station |
| RU2396664C2 (en) * | 2006-01-18 | 2010-08-10 | Абб Текнолоджи Лтд. | Converting substation |
| CN101479910B (en) * | 2006-06-30 | 2012-01-18 | Abb技术有限公司 | Hvdc system and method to control a voltage source converter in a hvdc system |
| KR101247448B1 (en) * | 2008-12-17 | 2013-03-25 | 에이비비 테크놀로지 아게 | A method of upgrading a plant for transmitting electric power and such a plant |
| CN103814496B (en) * | 2012-11-06 | 2016-11-09 | Abb技术有限公司 | The transient state control method of VDCOL and VDCOL synthesizer thereof for tandem type MTDC system |
| CN103904673B (en) * | 2013-11-29 | 2017-05-10 | 南方电网科学研究院有限责任公司 | Method for optimizing PI parameter of high-voltage direct-current transmission constant-current controller |
| KR101555498B1 (en) * | 2013-12-30 | 2015-09-24 | 주식회사 효성 | Power supply for high voltage direct current controller |
| CN103972919B (en) * | 2014-05-14 | 2016-05-04 | 国网上海市电力公司 | A kind of friendship changes the choosing method of DC rated voltage after three utmost point direct currents |
| CN104362662B (en) * | 2014-11-26 | 2016-08-24 | 湖北工业大学 | A kind of LCC-VSC type Hybrid HVDC system topology and startup method |
| DE102016120149A1 (en) * | 2015-10-22 | 2017-04-27 | Ayman S. Abdel-Khalik | Method for converting high voltage AC lines to bipolar high voltage DC systems |
| CN112086989B (en) * | 2020-08-26 | 2022-07-01 | 东南大学 | Power electronic equipment control initialization method |
Family Cites Families (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1488085B2 (en) * | 1963-01-30 | 1970-12-17 | Allmänna Svenska Elektriska AB, Västeras (Schweden)j | Transmission system for high-voltage direct current |
| CA824267A (en) * | 1966-02-18 | 1969-09-30 | Licentia Patent-Verwaltungs-Gmbh | Current control of a system |
| DE1588067B1 (en) * | 1967-08-05 | 1971-01-28 | Bbc Brown Boveri & Cie | Control device of a high-voltage direct current transmission system for multi-point network operation |
| DE1588750B2 (en) * | 1967-08-16 | 1971-04-29 | Siemens AG, 1000 Berlin u 8000 München | METHOD AND DEVICE FOR TRANSMISSION OF DIRECT CURRENT |
| DE2109763C3 (en) * | 1971-02-25 | 1980-10-09 | Licentia Patent-Verwaltungs-Gmbh, 6000 Frankfurt | Device for voltage regulation in converter stations of a high-voltage direct current transmission system |
| JPS5523011B2 (en) * | 1974-12-06 | 1980-06-20 | ||
| DE2518910C2 (en) * | 1975-04-28 | 1977-03-31 | Siemens Ag | ARRANGEMENT FOR TRANSFERRING DIRECT CURRENT BETWEEN AT LEAST ONE RECTIFIER STATION AND SEVERAL INVERTER STATIONS |
| DE2736540C3 (en) * | 1977-08-11 | 1980-08-28 | Licentia Patent-Verwaltungs-Gmbh, 6000 Frankfurt | Method and device for adapting the control arrangement for the DC voltage of an inverter station |
| DE2904786C2 (en) * | 1979-02-08 | 1981-02-19 | Siemens Ag, 1000 Berlin Und 8000 Muenchen | Process for regulating inverters in parallel operation and circuit arrangements for carrying out the process |
| DE3029358A1 (en) * | 1980-08-01 | 1982-03-11 | Siemens AG, 1000 Berlin und 8000 München | DEVICE FOR BRIDGING SHORT-TERM NETWORK OUTPUTS FOR INTERMEDIATE VOLTAGE CONVERTERS |
-
1982
- 1982-07-03 DE DE19823225285 patent/DE3225285A1/en active Granted
- 1982-09-28 US US06/426,164 patent/US4459492A/en not_active Expired - Fee Related
- 1982-11-24 CA CA000416224A patent/CA1180755A/en not_active Expired
-
1983
- 1983-07-01 SE SE8303795A patent/SE8303795L/en not_active Application Discontinuation
- 1983-07-01 ZA ZA834835A patent/ZA834835B/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| DE3225285C2 (en) | 1987-02-05 |
| SE8303795L (en) | 1984-01-04 |
| ZA834835B (en) | 1984-05-30 |
| SE8303795D0 (en) | 1983-07-01 |
| DE3225285A1 (en) | 1984-01-05 |
| US4459492A (en) | 1984-07-10 |
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