1 2
the secondary winding of at least one of the phase shift
VOLTAGE AND HARMONIC CONTROL OF A ing transformers is a delta or a wye winding connected
MULTI-POLE INVERTER to a neutral line in order to simplify the quasi-harmonic
neutralization of the output. BACKGROUND OF THE INVENTION 5 Another timely approach directed towards solving This invention relates in general to multi-phase in- the problem of harmonic neutralization is with the imverter systems for AC generators which incorporate plementation of 12-pulse and 24-pulse inverters assemstatic electrical power conversion elements, and more bled from pairs of six-pulse inverters with strategic particularly, to multi-phase inverters which provide an placement of time-paired notching sets for the reversal AC output voltage which is controllable, independent 10 of the pole voltage. This approach places each notch of a DC input voltage, over a predetermined range. with a timing duration within a limited band near to the The invention is particularly applicable to aviation leading and trailing portions of a half-wave periodicity, electrical power generation requirements which rely on and it significantly decreases the original phase disstate of the art electronic features, such as variable placement of the pole voltages, while offering the simspeed constant frequency (VSCF) electrical power 15 plicity of a single output coupling transformer. This is generators with DC-link control, which are needed the subject of pending U.S. Ser. No. 07/612,647, filed aboard present and future military aircraft and for mul- Nov. 13,1990, entitled "Static Inverter", assigned to the ti-engine commercial aircraft as well. It will be appreci- same assignee as the present invention, and it is hereby ated that the invention has broader applications such as incorporated by reference into the present application wherever it is needed to provide output power voltage 20 ^ will ^ hereinafter referenced as the first incorporegulation from a static inverter without mandating the rate(j u.S. application for patent. The clear benefit to be controlling of the DC input voltage to the inverter. derived in notching the switching output of the invertEach VSCF generator unit aboard an aircraft in- ers> within this ieadjng md trailing portion timing eludes a multi-phase or three-phase power inverter range, is the affect on some of the most significant harpaired with a propulsion engine in order that each en- 25 ... ^ yj mt is for SQme of the practical gme can be used to help fulfill the requirement of sys- applications of the quasi.harmonic neutralized output tern redundancy to enhance operational safety This voha waveform te which ^ harmonics ^ reensures that the entire electneal power output needs for duced md/QT eliminated i e. the 5th and 7th harmonics the aircraft wiU be met ,n order to keep the^an-craft arg eliminated ^ the mh and 13th m signiflcantly operational. The VSCF system according to U.S Pat. 30 reduced
No. 4,554,501, entitled "Variable Speed Constant Fre- _ . . , . .... r
quency Power System With Auxiliary DC Output" is °ne °f thef Pnnci?al Pract?cal considerations for the
Assigned to the same assignee as the present invention, realization of ^tic inverter harmonic neutralization of
and it is hereby incorporated by reference into the pres- ^ dfenbed is that the AC output voltage is
ent application. Another variable speed generator is of 35 harmonically neutralized over a predetermined range
the type described in U.S. Pat. No. 4,851,723 entitled for wluch volta8e contro1 of the fundamental frequency
"Coolant Pump System for Variable Speed Genera- component is independent of the DC source voltage,
tors" which also is assigned to the present assignee, and Another problem related to the above concerns is that
since it also provides a proper environment for the the controlled notching of the multi-pole inverter cir
utilization of the presently intended multi-phase in- 40 cuit should reduce the harmomc content of the inverter
verter system it is likewise incorporated by reference as circuit to a filterable minimum of harmomc voltage
the second incorporated U.S. Patent. components immediately next adjacent to twelve times
The three-phase inverters that are used with DC-link the fundamental frequency, frequency conversion for aircraft power generation The solution should further concern the provision systems, operating at 400 Hertz and 20 to 50 Kilovolt- 45 that all remaining harmonic voltage components immeamps (KVA), are a subset of static inverters employing diately adjacent to odd number multiples of six times semiconductor switching devices which are used for the fundamental frequency are to be cancelled, while a many other applications. These include industrial drive, substantial decrease in the 11th and 13th harmonics is power conditioning, and the generation of controllable accomplished over the entire range of control, leading or lagging reactive current. In each of these 50 SUMMARY OF THE INVENTION applications, there is a common problem which is difficult to solve. It is the elimination and/or reduction to In accordance with the present invention, there is filterable minimum values of troublesome AC harmonic provided a new and improved multi-pole inverter appafrequencies which are whole number multiples of the ratus and method used for producing a harmonically fundamental frequency in hertz. One approach to solv- 55 neutralized AC output voltage at a constant frequency, ing the problem of harmonic neutralization is according over a predetermined controllable range, while being to U.S. Pat. No. 4,870,557, entitled "Simplified Quasi- controlled independently of a DC source voltage. Harmonic Neutralized High Power Inverters" which In accordance with another aspect of the invention, has been assigned to the same assignee as the present the multi-pole inverter is a six-pole, three-phase inverter invention. The above assigned patent is hereby incorpo- 60 system which is adapted to provide switched or rated by reference into the present application and will notched output voltages with the notches becoming be hereinafter referenced as the third incorporated U.S. optimized in their variable placement with respect to patent. This incorporated U.S. patent takes advantage preselected phase positions and variable pulse width or of the double fed 24-pulse inverter comprised of stan- duration, in order to reduce the harmonic content of the dard groups of inverters such as four six-pulse inverters 65 output voltage of the inverter circuit to a filterable connected to reduce the complexity of and to half the minimum of harmonic voltage components of a frenumber of normally required phase shifting transform- quency immediately next adjacent in whole number ers with open primary winding neutrals. Furthermore, multiples to twelve times the fundamental frequency,
