Study On Unbalanced Control Strategy Of Static Synchronous Compensator For Distribution Network And Its System Design

Distribution Static Synchronous Compensator abbreviated as DSTATCOM, is a hot subject for modern static reactive power compensation technology since it has several functions such as the bidirectional dynamic compensating to the power system, restraining harmonics, and compensating unbalanced loads. The traditional design and operational analysis of power electronic equipments were done under the balanced circumstances which were not agreeable with the practical situation, especially under the three-phase four-wire system. Therefore, study on DSTATCOM and its control technology under unbalanced system, plays an important role in the power quality improvement for distribution network. Thus, the theoretical and technological investigations on DSTATCOM were carried out aiming at solving the problem of unbalanced compensation in distribution system. The main results were summarized below.The mathematical model of three-phase four-wire DSTATCOM was established. The switch model and average model of three-phase four-wire three-level NPC inverter were formed from the abc coordinate. Then the average model in the abc coordinate was transformed into the one with dqo coordinate. Based on introducing the conception of "equivalent capacitance" to DC capacitor, the electric quantity of AC-link and DC-link were processed simultaneously. An averaged mathematical model of three-phase four-wire DSTATCOM based on per unit, independent of the main circuit topology, was established, and then the equivalent circuits were given. The introduction of per unit model provided a uniform method for the stability analysis and the parameter evaluation of the device, and solved the problems of different voltage classes and capabilities, thus made parameter comparisons easy.A new PWM modulation algorithm of three Dimensions multi-level Space Vector based on decomposing referential voltage was proposed. The method included the following steps, decomposing reference voltage into offset vector and 3-D two-level vector for the switching vector selection firstly, while introducing 3-D two-level space vector PWM algorithm to compose multi-level vector, and then reducing any numeracy level 3DSVPWM to two-level 3-D SVPWM.The effects of unbalanced distribution network on DSTATCOM operation and control were analyzed. The complex vector model and zero-sequence single-variable vector model of DSTATCOM in the unbalanced power system were established. The positive-negative sequence bisynchronous controller and zero-sequence synchronous controller were investigated to compensate the unbalanced 3-phase loads which accomplished several functions such as compensating to the negative-sequence current and the harmonics current of unbalanced loads, and controlling the null line current of 3-phase 4-line system. Meanwhile, an unbalanced voltage control strategy based on the regulation of positive and negative components, was proposed to solve the unbalanced voltage in the distribution.The own fault of DSTATCOM and the influence of power system fault on the device were analyzed, a pulse-block operation mode serving as anti-fault means under the emergency condition was proposed. To overcome the problem of the NPC inverter bridge arm, a nonstop to the short circuit breakdown was proposed using the anode reactance, the clamping circuit limit short-circuit current, and the damping short-circuit current vibration. This made the short-circuit current in IGBT surge current bearing capacity. Under the circumstance of unbalanced fault in the power system, the symmetrical modulation method carrying out fast control on the positive-sequence current was adopted to resist the impact on the device, while the unsymmetrical modulation method was used to increase the reactive power to the system under the safe condition.A 100-kVAR DSTATCOM prototype based on 3-D space vector modulation 3-lever NPC inverter was developed according to control strategies proposed in this paper. Emulation and operational experiments shown the multi-lever 3-D space vector PWM modulation algorithm had the advantages such as easy design and short calculation time. The unbalance control strategy of DSTATCOM fulfilled the compensation of unbalanced loads and harmonics, and reduced the asymmetry of the distribution networks. Therefore, the DSTATCOM achieved the expected purpose.