Frequency control and power balancing in disturbed power inverter system and method thereof
A system and method are disclosed for controlling DC to AC power converters (inverters) that are operated as voltage sources and are paralleled on the AC side. This method allows for controlling frequency while sharing real power. The disclosed method requires no communication between inverters for proper load sharing, thus enhancing reliability of a system of parallel inverters. The only signals required for control are the AC voltage and frequency and their reaction to the inverter output current. The present invention allows a multiplicity of inverters or active rectifiers to share power in relation to their share of the available power capacity of the system. This makes it possible to parallel DG sources with various response times while they are connected to either a utility grid or in a stand-alone power network with no communications required between inverters. The addition of communications to the system allows the dispatch of energy for economic reasons and to fine tune load balance. Communications can be used for systems status and maintenance but is not required for reliable short-term operation.
1. An electrical power control system comprising:
a first inverter;
an electrical power grid;
a first inductive member connected in series between said first inverter and said electrical power grid;
a first voltage signal line between said electrical power grid and said first inductive member;
a first current signal line between said first inductive member and said first inverter;
a phase locked loop including an input, an output and a transfer function providing a predetermined phase shift between 0.5 to 500 degrees per Hz during nearly steady operation, said phase locked loop input is connected to said first voltage signal line said phase locked loop including means for generating a sine wave corresponding to the desired fundamental value and phase at the output of said first inverter; and,
a first inverter control system having an a first input coupled to said phased lock loop output, a second input coupled to said first voltage signal line, and a third input coupled to said first current signal loop.
2. The electrical power control system of claim 1 wherein said transfer function is selected to provide a smaller phase shift when the output phase angle changes in less than 0.1 seconds.
3. The electrical power control system of claim 1 wherein the phase locked loop is a first order phase locked loop.
4. The electrical power control system of claim 1 wherein the phased locked loop is a second order phase locked loop.
5. The electrical power control system of claim 4 further comprising a phase shift electrically connected to said phase locked loop and said low pass filter.
6. The electrical power control system of claim 5 further comprising a summation circuit electrically coupled to said low pass filter and a phase detector.
7. The electrical power control system of claim 1 further comprising:
a second inverter electrically coupled to said electric power grid;
a second inductive member connected in series between said second inverter and said electrical power grid;
a second voltage signal line between said electrical power grid and said second inductive member;
a second current signal line between said second inductive member and said first inverter;
a second phase locked loop including an input, an output and a transfer function providing a predetermined phase shift, whereby said second phase locked loop input is connected to said second voltage signal line and said phase locked loop includes means for generating a sine wave corresponding to the desired fundamental value and phase at the output of said second inverter.
8. A method of controlling power output from a control system having an inductor and a load, comprising the steps of:
receiving a signal representative of voltage from an electrical grid on the load side of the inductor;
receiving a signal representative of voltage from an inverter coupled to the grid downstream from the inductor;
obtaining a phase error signal that represents the difference between the inverter and electrical grid voltages;
generating a frequency error signal that is a function of the phase error signal, wherein said phase error signal and frequency error signals are voltage signals;
adjusting the phase error signal via a feedback loop that includes a phase shift and low pass filter of the frequency error signal; and,
shifting the phase between 0.5 to 500 degrees per Hz during nearly steady operation.
9. The method of controlling power output from a control system of claim 8 comprising the step of adjusting the frequency of the inverter by the use of the frequency error signal.
10. The method of controlling power output from a control system of claim 9 comprising the step of filtering the phase shift signal from a phase-locked loop.
11. The method of controlling power output from a control system of claim 10 comprising the step of generating a filtered phase shifted signal and generating said frequency error signal as a function of said filtered phase shifted signal.
12. The method of controlling power output from a control system of claim 11 comprising the step of generating a sine output and a cosine signal from said filtered phase shifted signal.
13. The method of controlling power output from a control system of claim 12 comprising the step of receiving said sine and cosine signal in a control system.
14. The method of controlling power output from a control system of claim 13 comprising the step of altering the output of an inverter based on the sine and cosine signal.