Abstract
Modular multilevel converter based unified power flow controller (MMC-UPFC) can provide flexible power regulation and plays an important role in the smart grid and future distribution networks. PI controller is widely adopted in the Unified power flow controller (UPFC) control. However, it is often difficult to figure out a pair of stable control parameters with good dynamic performance. A passive sliding mode control strategy of MMC-UPFC is proposed in this paper to achieve the advantages of both the passive controller and sliding mode controller. The proposed passive sliding mode control strategy has a fast response speed and is not sensitive to system parameter variation. Since unbalance grid voltage conditions are common in the modern distribution networks, an unbalanced grid treatment strategy is also presented with the series side of MMC-UPFC. Negative sequence components of the grid voltages are detected by the cross-decoupling method with second order generalized integrator and then compensated by the series side of MMC-UPFC. The series side converter controls the power flow and treats unbalanced grid conditions simultaneously. The efficiency of the general device is increased thus. A 27-level MMC-UPFC simulation system is built based on the Shanghai UPFC Project and validate the proposed approach.
Highlights
Modern flexible AC transmission systems (FACTS) devices are the key components in the smart grid and the future energy internet
The general power flow control performance of modular multilevel converter (MMC)-Unified power flow controller (UPFC) is given firstly
The unbalanced grid condition treatment strategy for distribution networks is proposed with the series side of MMC-UPFC
Summary
Modern flexible AC transmission systems (FACTS) devices are the key components in the smart grid and the future energy internet. The passive sliding mode control is proposed in the inner current loop of both the shunt and series sides of the MMC-UPFC instead of the traditional PI controller, which provides easier parameter design and better dynamic performance for the proposed MMCUPFC. The general treatment strategy is to provide negative sequence voltages by the series side of MMC-UPFC and compensate the negative components of the grid voltage so that the transmission line current can return to symmetry. The positive sequence components help to control the transmission line power while the negative sequence components are to be eliminated according to the unbalanced grid condition treatment strategy.
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