Page 130 - Hybrid-Renewable Energy Systems in Microgrids

114 Hybrid-Renewable Energy Systems in Microgrids

grid-connected VSI is given in Fig. 7.1, where the current controllers are implemented
in synchronous reference frame. The operation of the inverter is regulated to inject a
certain amount of active and reactive power, whose references are usually generated by a
higher level controller. However, this way of controlling a VSI yields undesirable perfor-
mance under unbalanced faults. With Park transformation, the negative-sequence compo-
nents will appear in positive-sequence frame as signals oscillating at twice fundamental
frequency. Due to the limited bandwidth, proportional-integral (PI) controllers result in
imperfect tracking when the control signals accommodate such oscillations [6]. As an
example, the short-circuit response of the system shown in Fig. 7.1 subject to a phase A-B
fault with 0.1 Ω fault resistance is given in Fig. 7.2A, where the output current exhibit
significant distortions under the fault. To improve the short-circuit response of the VSI
under unbalanced faults, only positive-sequence signals are used in the control diagram
shown Fig. 7.1. The corresponding short-circuit response to the same fault is illustrated in
Fig. 7.2B. Even though the current is free of distortions, the peak current and the amount
of injected power are out of control due to uncontrolled negative-sequence current. There-
fore, a similar control loop is implemented for negative-sequence component, resulting a
control structure in Fig. 7.3, where only positive-sequence components are used in posi-
tive-sequence reference frame control loop and only negative-sequence components will
appear in negative-sequence reference frame control loop. With the help of instantaneous
power theory, the four current references can be generated in various ways to achieve
certain objectives. This makes the short- circuit response of the VSI significantly different
from each other under unbalanced faults.

Figure 7.1 Control structure of a grid-connected VSI.