H. YANG, B. H. ZHANG
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666
which low frequency and low voltage happen at the same
time.
5)The present UVLS and UFLS may not prevent is-
land from collapse under some situations(e.g., the per-
centage equals 35% or 40% ),in which high frequency
and low voltage happens at the same time, causing that
UFLS cannot act and UVLS is too late to shed insuffi-
cient amount of load.
5. Conclusions
The island frequency response is decided by both me-
chanical power and electromagnetic power. Island volt-
age affects electromagnetic power, which change the
frequency dynamic in island. Initial reactive-power defi-
cit and load characteristics are two key factors affecting
the frequency response dynamic through coupling of
voltage and frequency when initial active-power main-
tains unchanged.
When initial reactive-power or percentage of induction
motor in loads increases, the voltage decreases, leading
the decrease of total electromagnetic power. As the de-
crease of unbalanced active-power in island, the decrease
of frequency decline rate and the increase of frequency.
While initial reactive-power deficit or percentage of in-
duction motor in load exceeds a certain threshold, the
signal of unbalanced active-power in island would
change, which may cause high frequency despite of se-
vere active-power deficit in the island.
Ignoring the coupling of voltage and frequency dy-
namic would result in remarkable errors when analyzing
the frequency response of the splitting island with large
initial active-power deficit.
Moreover, the currently used UVLS and UFLS, whose
setup and performance are independent without consid-
ering the coupling of voltage and frequency, would cause
excessive when low voltage and low frequency happen at
the same time or insufficient control when initial reac-
tive-power deficit or percentage of induction motor in
load exceeds a certain threshold.
It is necessary to develop coordination control for vol-
tage and frequency on the basis of research on their cou-
pling effects.
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