Page 142 - Hybrid-Renewable Energy Systems in Microgrids

126 Hybrid-Renewable Energy Systems in Microgrids

With the rapid development of digital devices, digital controllers such as predic-
tive deadbeat controller and hysteresis controller are also gaining popularity. Apart
from voltage-oriented current control, these controllers are widely applied in virtual
flux-oriented and direct power control scheme. The review of these control schemes is
beyond the scope of this chapter.

3.4 Converter current limitation
The current flowing through converters should be restricted due to the limited over-
load capability of semiconductors. Over-current in any phase may lead to block or
undesirable disconnection of the converter. For a three-phase fault, the current contrib-
uted by a VSI does not contain any negative-sequence component ideally. This makes
the design of current limitation straightforward. For example, the permitted maximum
current I max can be expressed by Eq. (7.38) in dq-frame:

Imax=id2+iq2 I max = i d 2 + i 2 q (7.38)

If reactive current injection takes the first priority under fault conditions, the per-
mitted value of i can determined by:
d
2
id=Imax2−iq2 i d = I max − i q 2 (7.39)

This way of limiting converter current is valid as long as no negative-sequence cur-
rent is injected during faults. However, current limitation becomes more complicated
when negative-sequence current is injected when using other control strategies and
the design of current limitation usually correlates with the deployed control strategy.
Based on the current ellipse theory [3], this section presents a generic current limita-
tion method for flexible oscillating power control and flexible positive- and negative-
sequence power control. It limits active and reactive power references carefully under
fault conditions so that the current in each individual phase is restricted within the limit.

3.4.1 Flexible positive- and negative-sequence power control
With flexible positive- and negative-sequence power control Eqs. (7.28) and (7.29),
the peak current value in each phase can be expressed by [3]:

2
2
+
,
2
Îm2=IpLcosγm−IqLsinγm +I Î m 2 = ( I cos γ − I sin γ ) ( I cos γ + I sin γ ) m ⊂ (ab c, , ) (7.40)
m
qL
qS
m
pL
pS
m
m
2
qScosγm+IpSsinγm , m⊂a,b,c
 k p 1− k   k p 1− k 
p
p
IpL=P*kpv++1− kpv− , IpS=P*kp I pL = P *  + + −  I , pS = P *  + − −  (7.41)
 v v   v v 
v+−1−kpv−