DC-link voltage balancing system and method for multilevel converters
A control system for a multilevel converter includes a differential mode current regulator, a neutral point (NP) controller and a PWM controller for generating switching pulses for the multilevel converter. The differential mode current regulator generates reference voltage command signals based on a difference between reference current command signals and actual current command signals, and the NP controller determines a modified neutral point current signal in response to a DC link voltage unbalance. The NP controller utilizes the modified neutral point current signal to generate a common mode reference voltage signal. The switching pulses are generated by the PWM controller based on the reference voltage command signals and the common mode reference voltage signal.
1. A control system for controlling a multilevel converter, the control system comprising:
a differential mode current regulator for generating reference voltage command signals based on a difference between reference current command signals and actual current command signals;
a neutral point (NP) controller for determining a modified neutral point current signal in response to a DC link voltage unbalance and using the modified neutral point current signal to generate a common mode reference voltage signal; and
a PWM controller for generating switching pulses for the multilevel converter based on the reference voltage command signals and the common mode reference voltage signal.
2. The control system of claim 1 , wherein the reference current command signals comprise d-q domain reference current command signals and the reference voltage command signals comprise d-q domain reference voltage command signals.
3. The control system of claim 2 further comprising a DC link voltage regulator for generating the d-domain reference current command signal based on a difference between a reference DC link voltage signal and a desired DC link voltage signal and a reactive power regulator for generating the q-domain reference current command signal based on a difference between a reference reactive power signal and a desired reactive power signal.
4. The control system of claim 2 comprising transformation matrices for transforming the d-q domain reference current command signals into a-b-c domain reference current command signals and d-q domain reference voltage command signals into a-b-c domain reference voltage command signals.
5. The control system of claim 2 , wherein the NP controller comprises a min-max classifier for determining instantaneous minimum, medium, and maximum voltage and current signals among reference current command signals and reference voltage command signals or among measured current command signals and reference voltage command signals.
6. The control system of claim 5 , wherein the NP controller comprises an NP current pre-calculator for generating a neutral point current signal based on the instantaneous minimum, medium, and maximum voltage and current signals.
7. The control system of claim 6 further comprising a NP limit calculator for determining minimum and maximum limits for the neutral point current signal and the common mode reference voltage signal.
8. The control system of claim 7 further comprising a waveform corrector for generating the modified neutral point current signal based on the minimum and maximum limits for the neutral point current signal and a DC control signal.
9. The control system of claim 8 , wherein the DC control signal is generated by a PI regulator based on a differential DC link voltage signal.
10. The control system of claim 8 , wherein the modified neutral point current signal is equal to the neutral point current signal when the DC control signal is zero.
11. The control system of claim 7 further comprising a dynamic common mode (CM) voltage calculator for generating the common mode reference voltage signal based on the modified neutral point current and minimum and maximum limits for the neutral point current signal and the common mode reference voltage signal.
12. The control system of claim 11 , wherein the dynamic CM voltage calculator comprises a look-up table for providing the CM reference voltage signal magnitude according to the modified current signal and a load power factor.
13. A method of controlling a multilevel converter comprising:
(a) generating reference voltage command signals based on a difference between reference current command signals and actual current command signals;
(b) in response to a voltage unbalance on a DC link of the multi-level converter,
(i) using the reference voltage command signals and reference current command signals for generating a modified neutral point current signal to compensate for the DC link voltage unbalance;
(ii) using the modified neutral point current signal for generating a common mode reference voltage signal; and
(iii) generating switching pulses for the multilevel converter based on summation of reference voltage command signals and the common mode reference voltage signal.
14. The method of claim 13 , wherein the reference current command signals comprise d-q domain reference current command signals and the reference voltage command signals comprise d-q domain reference voltage command signals.
15. The method of claim 14 , wherein the d-domain reference current command signal is based on a difference between a reference DC link voltage signal and a desired DC link voltage signal and the q-domain reference current command signal is based on a difference between a reference reactive power signal and a desired reactive power signal.
16. The method of claim 14 , wherein generating the modified neutral point current signal comprises modifying a neutral point current signal based on the minimum and maximum limits for the neutral point current signal and a DC control signal.
17. The method of claim 16 , wherein the neutral point current signal is generated based on instantaneous minimum, medium, and maximum reference current command signals and reference voltage command signals.
18. The method of claim 16 , wherein the DC control signal is generated by based on a differential DC link voltage signal.
19. The method of claim 13 , wherein the common mode reference voltage signal is generated based on a load power factor and the minimum and maximum limits for the neutral point current signal and the common mode reference voltage signal.
20. A multilevel converter comprising:
a plurality of phase legs each having at least two inner switching devices, at least two outer switching devices, at least two clamping diodes and a split DC link; and
a control system for controlling the multilevel converter comprising
a differential mode current regulator for generating reference voltage command signals based on a difference between reference current command signals and actual current command signals;
a neutral point (NP) controller for determining a modified neutral point current signal in response to a DC link voltage unbalance and using the modified neutral point current signal to generate a common mode reference voltage signal; and
a PWM controller for generating switching pulses for the multilevel converter based on the reference voltage command signals and the common mode reference voltage signal.