Page 126 - Hybrid-Renewable Energy Systems in Microgrids

110 Hybrid-Renewable Energy Systems in Microgrids

and hence their outputs. Undesired performance such as voltage and current distortions,
power oscillations or uncontrollable amount of current and power injection may arise.
In the last few decades, many studies have been conducted to improve performance of
VSIs under unbalanced conditions using instantaneous power theory [2] and symmetrical
component theory. Despite their diverse objectives and designs, this topic can be regarded
as the generation of current references under fault conditions. This chapter firstly presents
the instantaneously power theory, which serves as the foundation of the current reference
generation. The control strategies of VSIs under unbalanced faults are reviewed next in a
general way in terms of two groups, namely flexible oscillating power control and flexible
positive- and negative-sequence power control. As the current flowing through converters
should be restricted in each phase, the current limitation method introduced by Ref. [3]
is reviewed for flexible positive- and negative-sequence power control. This method is
then further extended to flexible oscillating power control. Based on the same principle,
both methods calculate the maximum allowed power delivery under fault conditions to
ensure the current is restricted within the limit for each phase. For a voltage-oriented
current-controlled VSI, the inner current controller can be implemented in different refer-
ence frames. This chapter also provides a brief review on current controllers in terms of
dq-frame, αβ-frame, and abc-frame. For a microgrid with only VSI-interfaced DG units,
the islanded mode operation requires a different design of the control system to provide
voltage and frequency references. The basic control structures for islanded operation
are reviewed next based on Refs. [4,5]. To improve the short-circuit response, the con-
trol system is further modified with the capability of negative-sequence current control.
Finally, this chapter applies all the reviewed methods to a microgrid consisting of only
VSI-interfaced DG units, followed by the discussions on microgrid protection based on
the examination of short-circuit current through simulations.

2 Instantaneous power theory

If the instantaneous three-phase voltage and current measured at the point of common
T
T
coupling (PCC) are denoted by [v a v b v c ] and [i a i b i c ] , respectively, the Clark trans-
formation of three-phase quantities can be expressed by:
 v     v 
 0   1/ 21/2 1/ 2   a 
v vαvβ = 1/21/21/21−1/2−1  v α  =  1 − 1/2 − 1/ 2   v b  (7.1)
0
/203/23/2vavbvc   v   0 3 /2 3/ 2   v 

 β        c  
 i     i 
 0  2  1/ 21/2 1/ 2   a 
i iαiβ = 231/21/21/21−1/  i α  =  1 − 1/2 − 1/ 2   i  (7.2)
0
b
2−1/203/23/2iaibic    i β    3    0 3 /2 3/ 2    i c   
  