Research On The Transformer-isolated DSTATCOM And Its Control Technology

Distribution-level Static Reactive Compensator (DSTATCOM) is an effective device improving the distribution system power quality, through compensating the reactive power, negative sequence and harmonic current at the Point of Common Coupling. Nowadays, DSTATCOM has been the important research and developing direction in compensation of reactive power and control of power quality in the distribution system. This paper introduces the configuration, advantage, and basic compensation algorithms of the transformer-isolated DSTATCOM.This paper designs the parameters of a2.5Mvar transformer-isolated DSTATCOM Prototype, and the simulation modeling in PSCAD to preliminarily verify the effectiveness of the DSTATCOM and its parameters.The maximum zero sequence in the phase current under the action of optimal phasor scheme is calculated out, which is the theoretical basis for the design of the side column in the transformer. The maximum imbalance degree is also calculated out, which is proved by simulation result and analysis in planimetry.An optimal current compensation strategy applied in DSTATCOM is proposed which is named negative sequence current preferential compensation strategy. When the capacity of DSTATCOM is adequate to compensate the negative sequence current, but not enough to compensate the full load current, the strategy can compensate the full negative sequence and the most amount of the reactive power current. It gives priority to ensure three-phase symmetry in the power quality treatment of the distribution system. The strategy which does not require particular structure of the DSTATCOM can be applied in both triangle and star structure. The simulation result in PSCAD verifies the validity and accuracy of the proposed strategy.Aiming at the equivalent parallel circuit for the DSTATCOM configuration, this paper analyses the harmonic elimination principle and proposes the selection basis for the PWM scheme. CPS-SPWM and some SVM schemes are introduced and applied in the DSTATCOM modeling. It’s proved that CPS-SPWM is the most suitable PWM scheme, through comparing the current THD in different PWM schemes.According to the imbalance of the distribution system, this paper analyses the mechanism of DC fluctuation, and introduces a decoupled double synchronous reference frame phase-locked loop (DDSRF-PLL) which is proved the effectiveness in the simulation. It’s proved the control scheme of the DSTATCOM could avoid over current caused by the negative sequence current. These measures ensure a high compensation precision of the DSTATCOM applied in unbalance distribution system. At last, the debugging progress of a25kvar transformer-isolated DSTATCOM is introduced.