Study On Phasor Measurements And DC Model In Power System State Estimation

The power system state estimate (SE) gives system real-time status for power system's analysis and control. Both the analysis' veracity and the control's effect are influenced by the performance of the SE. The interconnection of power grids and the running of power market not only make the power system behavior become more complex and more difficult to anticipate, but also require that the analysis and decision be more accurate and the control be more effective. Then the SE should give exact state data in time and comprehensively. The SE can filter the measurement error by data processing, and gain the best estimation of the power system state. The quantity, quality and placement scheme of the measurements influence the SE performance.The traditional SE utilizes only the measurements of the SCADA-supervisory control and data acquisition system. The PMU-phasor measurement unit, has been placed in the power system expansively, and the PMU measurements have become another important data source. The PMU measurements have high precision, synchronous sampling, and short updating cycle, and can measure the node state directly. For too high cost for PMU placement, the network is not observable only with PMU. The question How to Utilize the PMU measurements in the SE effectively, must be faced and solved now.The direct current transmission (DC) technology has been used more and more extensively in the power system for its advantages in power transmission over long distance and interconnection of alternating current (AC) networks, and especially it will play an important role in the power transmission from west to east and union of the whole country networks in China. Then more and more AC/DC systems will come into being, but the traditional SE arithmetic is not fit for the system with DC model, so exploring a fast and stable SE arithmetic for AC/DC systems has important value for the theory research and practical applications.The paper studies the problem of application of PMU measurements in the SE and the SE arithmetic for AC/DC systems.The PMU can measure the state variables of PMU nodes, then the SE is not necessary if the network is observable only with PMU, but this can't be realized in a long time. The paper analyzes all kinds of application models of PMU measurements in SE, presents the feasible idea: utilizing both the PMU measurements and SCADAmeasurements in the nonlinear SE, improving the SE performance with PMU measurements characters, so points out the research direction and accelerates PMU measurements' application.The PMU voltage phasor measurements can be used easily, but it is difficult to use PMU current phasor measurements, especially the current phase measurements in the nonlinear SE directly. The paper presents two new modes to utilize current phasor measurements: transforming the branch current phasors to branch powers; transforming the branch current phasors to the correlative node's voltage phasors, then all PMU measurements can be utilized efficiently in the SE.The PMU can measure the state variables of PMU nodes, and the state variables of correlative nodes can be calculated with PMU measurements, so the local area is observable with PMU. The paper presents another model of PMU measurements application in SE: regarding the nodes' status values measured or calculated as SE results of these nodes in the SE operation process, the model can reduce the scale of estimator, improve the SE equations' numerical stability and convergence speed, however it is not necessary to be propitious to the estimation precision because of the PMU measurements error.The paper presents a double SE model for the first time, which includes both nonlinear and linear SE: at first, the nonlinear SE is done on the condition of taking nodes' status values measured or calculated by PMU as estimation results, then the linear SE is done with both nonlinear SE results and PMU measurements. The model can improve the SE equations' convergence speed and estimation precision simultaneously by PMU measurements. The model is the transition...