Control of Power Inverters in Renewable Energy and Smart Grid Integration

21

Harmonic Droop Controller to Improve Voltage Quality

As discussed in Chapter 2, if the right amount of harmonic voltages is injected into the reference voltage of an inverter, then it is possible to cancel the harmonic voltages dropped on the output impedance due to the harmonic current components so that the THD of the output voltage is improved. In this chapter, this idea is developed further and a control strategy is presented. At first, the load and/or grid connected to an inverter is modelled as the combination of voltage sources and current sources at harmonic frequencies. As a result, the system can be analysed at each individual frequency, which avoids the difficulty in defining the reactive power for a system with different frequencies because it is now defined at each individual frequency. After developing a droop control strategy for systems delivering power to a constant current source, instead of a constant voltage source, a harmonic droop controller is developed so that the right amount of harmonic voltage is added to the inverter reference voltage. This forces the output voltage at the individual harmonic frequency close to zero and improves the THD of the output voltage considerably. Both simulation and experimental results are presented to demonstrate its capability of significantly improving the voltage THD.

21.1 Model of an Inverter System

It is widely known that a (linear) circuit having supplies/sinks with different frequencies can be analysed separately ...