Research On Zonal-voltage Control Of Smart Distribution System

Hierarchical voltage control is more widely used than the other voltage control methods, in which the zonal-voltage control layer is the backbone of improving the voltage quality, control speed and voltage stability. Current voltage control strategies, however, does not take advantage of the “centralism-distribution” power supply structure, which hinders the further increase of distributed generations(DGs) penetration level. Thus, an intensive research on zonal-voltage control of the smart distribution system is carried out in this article, which includes the zonal-voltage control strategy of the smart distribution system with fine dynamic performance and high static precision, and the combination schemes between the protection & emergency control and the local & global voltage control. The key points are showed as follows:(1) To achieving distributed and flexible voltage control in smart distribution system, this article proposes a design scheme of flexible DG(FDG) with the ability to control the positive and negative sequence voltage effectively, which is based on the analysis on the cause of voltage sag and voltage unbalance, as well as the voltage fluctuation with the integration of DGs. The FDG is composed of an input subsystem and an output subsystem. For the input subsystem, the nonlinear analysis method to ensure the output stability is studied taking the flywheel energy storage system(FESS) built in our laboratory as an example; for the output subsystem, the control method which consists of power generation and voltage control is proposed.(2) A zonal-voltage control strategy with FDGs at the core is proposed, which introduces the secondary control idea to FDGs’ local control and thus solving the problems of poor dynamic performance of zonal-voltage control and parallel operation of FDGs. With the modified negative/positive sequence voltage control, the strategy cancels the coupling of the line impedance with the negative/positive reactive power, achieving better reactive power distribution precision among multi-FDGs connected to the same point of common coupling.(3) A FDG-based zonal and coordinated voltage control(ZCVC) scheme under security and defense system of microgrid is proposed to solving the "point-surface" contradiction of voltage control and the security and defense control of microgrid. The proposed ZCVC consists of the dynamic voltage control and the adaptive voltage recovery control before and after the boundary of clearing the fault, which has vital practical significance in popularization and application of voltage control.(4) A two-stage zonal-voltage control(TSZVC) strategy with distributed static synchronous compensator(DSTATCOM) and DGs at the core is proposed for solving problems that the dynamic voltage control cannot be realized, as well as the interaction among the voltage control devices. In the primary state of control, only the DSTATCOM compensates the voltage with local information and the power flows of adjacent lines, avoiding the problems caused by using only local information that the upstream voltage beyond the upper limit or the downstream voltage beyond the lower limit, and improves the voltage quality of PCCs in the region. In the secondary state of control, the DSTATCOM shifts part of the reactive power demands to DGs, which aims to retain the sustainable voltage compensation ability and avoid the interaction among the voltage control devices.The proposed zonal-voltage control strategies for microgrid and ADN are integrated in the two dimensions, i.e. fast voltage control(FVC) and dynamic voltage control(DVC), which can easily be embedded into the existing hierarchical voltage control structure of the smart distribution system, thus forms a voltage control system from the top down with global voltage quality improvement.