In AC power systems of this kind use is typically made of inverter units that controlled by pulse width modulation can supply a filtered output voltage to the load, in which both fundamental frequency and harmonics can be controlled.
AC power systems comprising a number of such inverter units connected in parallel are known in the art and may be used as redundant power supply systems in applications, where maintenance of electrical power supply is critical, such as electrical power supply in computer systems, aircraft, radar installations, hospital systems and similar applications. Typically, the redundancy of such systems will be provided by the use of two or more units for continued supply of the common load in situations of failure or fall-out of a unit, whereby power supply to the load is maintained by the remaining unit or units.
In other applications of AC power systems parallel connection of inverter units may be necessary to maintain a desired power supply capacity.
In U.S. Pat. No. 5,745,356 a parallel redundant power supply system without use of intra-unit signaling is disclosed, in which the information necessary for load sharing is derived solely from the output power level of each unit, whereby the output of a unit is phase-locked to the output of other units connected in parallel therewith. Power level samples are generated either from a DC input voltage from the DC input power source or the AC output voltage from AC output voltage generating means and used for determination of the AC output power by control of the AC output phase and frequency by a control signal determined by integration of a derivative of the output power level of the AC output voltage generating means as obtained from the power level samples.
In such a system distribution of load current harmonics between the individual inverter units is not possible.
In installations, however, requiring electrical power supply of non-linear loads, typically a load consisting entirely or partly of rectifiers, e.g. diode or thyristor rectifiers, such as computer aircraft and radar installations sharing of load current harmonics between inverter units is critical. A rectifier load causes distortion of the current supplied by an inverter and, in result, the load current will have a content of harmonics in addition to the fundamental frequency. If such current harmonics are not distributed evenly between the inverters there will be a risk of overloading of the individual inverter and, in result a reduction of its life time.
In U.S. Pat. No. 5,436,512 a method for active sharing of a harmonic load between inverters connected in parallel is disclosed. This method requires electrical connection between the inverter units in addition to their common connection to the load. Beyond adding complication and expense such a solution may create problems with respect to reliability and will not be suitable for use in a distributed system with individual inverters placed geographically apart from each other.
For single inverter units supplying an output voltage which becomes distorted to an unacceptable extent by load current harmonics various methods for active limitation or reduction of the distortion, e.g. by use of the dynamics of the inverter to compensate for the distortion, have been disclosed in the art. Thus, U.S. Pat. No. 5,001,619 and U.S. Pat. No. 5,377,092 discloses methods, by which amplitude and phase relationship for a single harmonic is determined by discrete Fourier transformation. A vector representing the amplitude and phase shift of the harmonic is used as input to an integrating control unit to produce an integrated reference quantity, which is transformed back to the time domain by inverse discrete Fourier transformation and added to the control reference for the inverter. Thereby, the inverter is controlled to compensate for a load current harmonic such that the output voltage is not significantly distorted, even by a non-linear load.
In U.S. Pat. No. 5,383,107 an inverter control unit is disclosed in which a complex objective function based upon harmonic content in a minimum number of the phase-to-phase AC voltages produced by the inverter, e.g. two out of the three phase-to-phase voltages, is determined to control the switches of the inverter to minimize the objective function and thus control the harmonic content in the inverter output voltages.
In general, these prior art methods for reduction of distortion cannot be applied to a system of several inverters connected in parallel. Thus, in compensation methods a fatal situation may arise, when two or more inverter units counteract each other, e.g. by an inverter unit generating a compensating voltage harmonic, which another inverter unit will try to remove. In result circulating current harmonics may flow from one inverter unit to the other and, not only from an inverter unit to the load as desired. Such circulating currents will cause increasing loss and a reduction of the overall capacity made available to the load.
On this background it is the object of the invention to provide a method and an AC power system of the kind set forth, which will provide active load sharing of current harmonics between inverters connected in parallel.
According to the invention this object is achieved by a method for independent load sharing of current harmonics between parallel inverter units in an AC power system without intra-unit control signal communication, each of said inverter units comprising a DC to AC converter means with associated control means for controlling switching times of said converter means and each being connected to a common load through a separate impedance, said method comprising for each of said units the steps of measuring at least a current in a power output line from the unit before said separate impedance, processing the measured current value for determination of amplitude and phase of a selected current harmonic, comparing the amplitude and phase of the selected current harmonic with amplitude and phase of a feed-back voltage or current from said power output line to produce an error signal and using said error signal for determination of a control signal component for the selected harmonic for incorporation into a control signal input to said control means for controlling the content of said harmonic in the voltage and/or current supplied by the inverter unit to the common load via said separate impedance.
The method according to the invention may be applied to control of one or more harmonics in the inverter output voltage by determination of a control signal component for each such harmonic, the control signal component being variable from zero to a value determined by the measured current harmonic.
For each inverter unit the control signal component may be determined by methods as disclosed in the art, e.g. in U.S. Pat. No. 5,001,619 and U.S. Pat. No. 5,377,092 mentioned above.
Thus, by the method according to the invention the reference signal for the inverter control unit is modified to adapt the selected output voltage harmonic to the current in a way so as to avoid circulating currents between individual inverter units and, provide equal sharing of load current harmonics between the individual inverter units.
Preferred implementations of the method are stated in dependent claims 2 to 8.
Further in accordance with the invention an AC power system is provided, comprising a number of inverter units connected in parallel with a common load without intra-unit control signal communication, each of said inverter units comprising a DC to AC converter means with associated control means for controlling switching times of said converter means and each being connected to the common load through a separate impedance, each of said inverter units further comprising means for measuring at least a current in a power output line from the unit before said separate impedance, means for processing the measured current value for determination of amplitude and phase of a selected current harmonic, means for comparing the amplitude and phase of the selected current harmonic with amplitude and phase of a feed-back voltage or current from said power output line to produce an error signal and means for determination from said error signal of a control signal component for the selected harmonic for incorporation into a control signal input to said control means for controlling the content of said harmonic in the voltage and/or current supplied by the inverter unit to the common load via said separate impedance.
Preferred and advantageous embodiments of such a power system are stated in dependent claim 10 to 14.
Application of the method and AC power system of the invention is not limited to any particular type or number of inverter units, nor to the number of output voltage phases or any particular method for determination and control of current and/or voltage harmonics.