Environmental Engineering Reference
In-Depth Information
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 at each frequency according to the superposition theorem. Here, this is
applied to inverter systems.
For an inverter, whether it is connected to a load, a grid or both, the mathematical model
of the system can be illustrated as shown in Figure 21.1(a), where the inverter is modelled
as a voltage reference
v r with an output impedance Z o ( s ) and the load is modelled as the
combination of voltage sources and current sources. The terminal or output voltage is
v o = v o 1 + h = 2 v oh
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