Wide Area Measurement System (WAMS) Reliability And WAMS Based Voltage Stability

The WAMS can be used to conduct real time monitoring and control in dynamic system states and enhance the system security level because it utilizes the high precise synchronous clock on the earth such as GPS to build a unified space-time ordinate for the whole system. However, like any other physical system, the WAMS can fail while it can provide much better observability and controllability in power system operations. The consequence of WAMS failure is severe and could be a large blackout because of its wide-area impacts if its emergency control function does not work properly. It has been recognized that the reliability of WAMS must be quantitatively evaluated and assured while utilizing its beneficial features. In other words, the WAMS based protection and control scheme would be a"single leg"one if the risk of WAMS failure is ignored. The author has carried a wide range of literature reading, which indicates that the quantitative reliability evaluation of WAMS has not been addressed so far although considerable efforts have been devoted to other aspects of power system reliability assessment. For the first time, the dissertation proposed the WAMS reliability evaluation techinques and established relevant reliability models of WAMS.The WAMS itself is a complex system that is associated with both software and hardware and contains multiple devices (such as many PMUs) and special communication networks. The whole WAMS is first divided into several sub-systems including PMUs, PDC, local communication network, backbone communication network and control center. The Fault Tree Analysis (FTA) method is used to derive the general formula of evaluating WAMS system reliability. Then, the reliability models of all sub-systems are individually developed based on their features and their two-state equivalent Markov models are established using a variety of probabilistic reliability assessment techniques. Sensitivity analyses are also performed to identify key modules and parameters in each sub-system. Finally, the two-state equivalent Markov model of the whole WAMS is established using the two-state models of sub-systems and the general formula. This two-state model of the whole WAMS can be widely used to assess the effects on the system risk of various WAMS based control schemes, including the voltage stability control scheme proposed in this paper. It can also be applied to evaluate the risk of a generalized system that contains a regular power system, communication networks, control and measurement systems. As a natural and rational extension of WAMS application, the dissertation proposes a set of new voltage stability indices based on the measurements of WAMS, which can be used to identify weak buses or circuits in the system and accurately predict their transfer capability limits. The indices have four major advantages:â‘ Compared to traditional power flow-based voltage stability indices (such as those based on the maximum power method or the Jacobian matrix singularity), the proposed indices do not require any computation associated with system-wide power flow and therefore is much faster.â‘¡Compared to majority of the voltage indices based on local measurements, the proposed indices include the effect of overall system outside the local network using a new equivalent model of the system. This assures accuracy of the indices in modeling.â‘¢The WAMS based indices could automatically include time-dependent frequency and voltage related load characteristics and start up the emergency protection and control scheme to prevent voltage collapse in time.â‘£The proposed indices require a small amount of WAMS measurements and therefore has less reliance on the reliability performance of WAMS. This feature is very useful in practical application of the indices. The simulation results of 4 IEEE standard test systems indicate that the proposed indices can accurately recognize weak buses and circuits, and predict the margin from the voltage collapse point.Based on the proposed voltage stability indices, the dissertation establishes a minimum load shedding Optimal Power Flow (OPF) model with voltage stability constraints. The power level required to transfer on the weak buses and circuits is reduced by the OPF model through shedding partial loads. This can effectively reduce the risk of voltage instability in the system. The Predictor Corrector Primary Dual Interior Point Method (PCPDIPM) is used to solve the OPF model. The effectiveness of load shedding is confirmed using the Q-V mode analysis method, which can be applied to compute eigenvalues of a reduced Jacobian matrix. The numerical results of 2 IEEE standard test systems demonstrate that the proposed OPF model is very effective since it automatically identifies the weak buses and circuits and calculates minimum load shedding.