Abstract
A new inter-cluster DC capacitor voltage balancing scheme for a delta connected modular multilevel cascaded converter (MMCC)-based static synchronous compensator (STATCOM) is presented. A detailed power flow analysis of applying negative sequence current (NSC) and zero-sequence current (ZSC) injection methods in addressing the issue of inter-cluster DC voltage imbalance under unbalance grid voltage is carried out. A control scheme is proposed which integrates both inter-cluster methods using a quantification factor QF. This is used to achieve the sharing of the inter-cluster active power between the NSC and ZSC injection methods. An accurate method of determining the quantification factor is also presented. The proposed method offers better sub-module DC capacitor voltage balancing and prevents converter overcurrent. The influence of unbalanced grid voltage on the delta connected MMCC-based STATCOM rating using this integrated cluster balancing technique is investigated. The control scheme is verified with a 5 kV 1.2MVA MMCC-STATCOM using 3-level bridge sub-modules, and the results show the advantages of the proposed method over other inter-cluster methods.
Highlights
Grid voltage unbalance can be caused by remote faults, uneven distribution of loads such as single-phase traction drives, open wye and delta transformer banks, asymmetric transmission impedances, and so on [1]
In [14,15,16,17], inter-cluster DC capacitor voltage balancing is achieved under an unbalanced load condition by injecting a zero-sequence current (ZSC) to circulate the three phases of the single delta bridge converter (SDBC)
This paper proposes a new control scheme for phasecluster voltage imbalance and overcurrent of deltaconnected modular multilevel-cascaded converter (MMCC)-Static synchronous compensator (STATCOM) operating under unbalanced voltage conditions
Summary
Grid voltage unbalance can be caused by remote faults, uneven distribution of loads such as single-phase traction drives, open wye and delta transformer banks, asymmetric transmission impedances, and so on [1]. In [14,15,16,17], inter-cluster DC capacitor voltage balancing is achieved under an unbalanced load condition by injecting a zero-sequence current (ZSC) to circulate the three phases of the single delta bridge converter (SDBC). This injection method can lead to currents exceeding their rated limit and damaging converter switches. As the overall average active and reactive power control uses the positive sequence current, the two remaining control freedoms of negative and zero sequence currents are employed to regulate the inter-cluster control.
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