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Current Limit Strategy of Voltage Controller of DeltaConnected
HBridge STATCOM under Unbalanced Voltage Drop
Gum Tae Son* and JungWook Park¢Ó 
Abstract 
This paper presents the current limit strategy of voltage controller of deltaconnected Hbridge
static synchronous compensator (STATCOM) under an unbalanced voltage fault event. When
phase to ground fault happens, the feasibility to heighten the magnitude of sagging phase voltage is
considered by using symmetric transformation method in deltastructure STATCOM. And the
efficiency to cover the maximum physical current limit of switching device is considered by using
vector analysis method that calculate the zero sequence current for balancing the cluster energy in delta
connected Hbridge STATCOM. The result is simple and obvious. Only positive sequence current has
to be used to support the unbalanced voltage sag. Although the relationship between combination of
the negative sequence voltage with current and zero sequence current is nonlinear, the more negative
sequence current is supplying, the larger zero sequence current is required. From the fullmodel
STATCOM system simulation, zero sequence current demand is identified according to a ratio of
positive and negative sequence compensating current. When only positive sequence current support
voltage sag, the least zero sequence current is needed. 
Keyword 
Cluster balancing control, Deltaconnected Hbridge STATCOM, Symmetric component
current, Unbalanced fault

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