Page 138 - Hybrid-Renewable Energy Systems in Microgrids

122 Hybrid-Renewable Energy Systems in Microgrids

Figure 7.5 Short circuit response of VSI with flexible positive- and negative-sequence
power control. (A) k p = 1, k q = 1. (B) k p = 0.5, k q = 1. (C) k p = 0, k q = 1. (Voltage: V; Current:
A; Power: kW/kvar).

where K and K are referred to proportional gain and integral gain, respectively. Based
i
p
on symmetrical component theory, a voltage vector can be resolved into positive-
sequence component rotating counter-clockwise and negative-sequence component
rotating clockwise (zero-sequence is ignored). If a set of unbalanced signals is trans-
formed into positive-sequence reference frame, the negative-sequence component
will be registered as AC signals oscillating at double the fundamental frequency and
vice versa if positive-sequence components are transformed into negative-sequence
reference frame. Because of this coupling between the two synchronous refer-
ence frames, PI controllers cannot give satisfactory performance under unbalanced