Key Control Techniques In Cascaded Multilevel STATCOM/BESS

Cascaded multilevel converters (CMC) can achieve high voltage ac output usingrelatively low rating switching devices. This feature made the CMCs are very suitable formodular design and mass production, and occupy an important role in the high-voltagelarge-capacity converters. The CMC is particularly popular in applications such as staticsynchronous compensator (STATCOM) and battery energy storage system (BESS), where acommonly connected dc source is no longer required. The CMC is considered as having apromising application prospect in the increasing penetration of renewable energygeneration. However, as new comer of the multilevel converter family, CMC still has lotsof unsolved problems. This research is originated from problems encountered in thedevelopment and operation of±10Mvar/10kV cascaded STATCOMs.Ability to work under unbalanced grid conditions is one of the key feature of gridconnected power converters, especially for power quality improvement devices such asSTATCOM and BESS. This research proposes a control method for star connectedcascaded STATCOMs under unbalanced grid voltage. By applying individual phase currenttracking, each phase leg (cluster) of the cascade STATCOM is treated as an individualconverter. Each cluster has its own phase current tracking loop and DC voltage feedbackloop. This control method generates necessary negative sequence current and positivesequence voltage automatically to rebalancing unbalanced power, along with unnecessaryand harmful zero sequence current. An optimal zero sequence current reference separationalgorithm is developed to remove the unwanted zero sequence current, by making a smallvariation of reactive power command. The proposed algorithm can decouple thethree-phase currents in the3wire system. Even in the star-connection, each cluster of theCMC can be controlled individually like in delta-connection. Therefore no extra meansneed to be taken when unbalance occurred. This method has been used in±10Mvar/10kVcascaded STATCOMs products.In addition to the control methods, another factor that influence the ability to workunder unbalanced grid conditions is the voltage stress on the converter. This research aim tofind the theoretical limit of difficult control methods for CMC, from the angle of voltagestress. By analyzing most commonly used control methods for CMC, this research build anevaluation model for unbalanced condition work ability, based on some geometrical principles. And the analytical relationships between voltage stress and degree of unbalancefor difficult control methods are also found.Grid-connected power electronics converters/inverters usually have certain amount ofdc component at their AC terminal. They are likely to inject unwanted direct current intothe power grid, unless a line-frequency transformer is employed. In this research this dcinjection problem is investigated. Analysis connects the dc injection with CurrentTransformer (CT), a current sensing element widely used in high voltage converters. Byintroducing CT’s model into the STATCOM’s model, this study found that using CT forcurrent feedback can cause large dc injection, if no extra means is taken. Expressions of thedc injection for different controllers were derived. Then, a dc injection elimination methodwas proposed. This method eliminates the dc component of the output current by buildingan indirect direct current feedback loop; therefore, it can prevent injecting the direct currenteven when it cannot be sensed. Experimental results taken on a25-level cascadedmultilevel STATCOM prototype verifies the analysis and the proposed method.Because of having multiple dc terminals, CMC is considered to be suitable for largescale battery energy storage system. In this research, a100MVA cascaded BESS is studiedand the main circuit parameters are designed. To solve the problem of dc reactive filterbeing too large, an Active Ripple Compensator (ARC) is proposed. The design shows byincreasing2.5%of the total power device capacity, the54dc inductors can be decreased to1/10. Analysis also shows that the ARC needs no dc source. And only one close loop withone parameter is all that needed for the ARC to reduce the current ripple and meantimemaintain its dc bus voltage. This is possible because a natural feedback mechanism isintroduced. A simulation is performed for this active ripple compensated cascaded BESSwith54sub systems in PSCAD/EMTDC. Simulation shows that this system can operateproperly even under unbalanced conditions.