On-Line Static Voltage Stability Assessment In The Electric Power Market Environment

With the rapid and steady development of Chinese economy, the power system has also gradually grown into the era of the large-scale joint grid, ultra-high voltage, high content unit and long distance transfer. In addition, the power system is undergoing the reform to transform the generation and the power grid from the uniform-dispatch to the deregulated power industry. The interconnection of large-scale power systems and the introduction of electrical power market create opportunities and challenges to the operation of power systems. Power systems become more complex and their operation margins are reduced. In the past 10 years, many serious incidents of voltage collapse happened around the world, caused huge economic losses and social disorder, and aroused sustained concern wildly in the academic and engineering areas. Since many voltage instability are happened within a short time frame, the operators were not able to take effective actions to prevent it from happening. Moreover, it is also difficult for them to have speedy recovery after a wide area voltage collapse. In order to ensure that power systems operate with security and stability margin in the new environment, it is necessary to develop new rapid and effective approaches for voltage stability assessment. While many techniques exist, most techniques are computationally demanding and cannot be used in an on-line application. Therefore, it is necessary to develop new algorithm for on-line voltage stability assessment through advanced technology of measurement, communication and computation according to the practical operation of power systems. This dissertation considers the impacts of uneven regional load growth and economic dispatch in the power market to develop an on-line voltage stability assessment methods based on continuation power flow (CPFLOW) and voltage stability indices. Day-ahead and hour-ahead regional load forecasting and economic dispatch are used in the proposed algorithm to calculate load margin and indices. Based on PMU and power transmission paths, this dissertation also introduces the concept of voltage stability index. The framework of on-line voltage stability assessment for large scale power systems are designed to use synchronized information provided by the Wide Area Measurement System (WAMS). The main contributions of the dissertation are as follows.An algorithm for on-line voltage stability assessment is proposed. Voltage stability has been called load stability. Load characteristics is one of the most important factors which affect the voltage stability in power systems. It affects the developing of voltage instability and voltage collapse to a great extent. The algorithm applies the continuation power flow; however, the difference is that the uneven regional load growth is considered rather than the traditional uniform load growth in determine the load directions. Due to the load characteristics, economic development and weather conditions are different in regions, the regional load growths are different. Instead of minimum cost, many factors such as environment, system security, and energy price should be considered in the generation re-dispatch under deregulated market. In order to reflect the uneven regional load growth and more realistic re-dispatch scheme, this dissertation adopts the day-ahead regional load forecasting and the economic dispatch determine load directions and generation directions to estimate the voltage stability margin in power systems. The proposed algorithm can assess the system voltage stability more effectively and accurately.An algorithm of voltage stability index with the consideration of uneven regional load growth is proposed. The Hour-ahead regional load forecasting is applied, it gives the regional load in the next hour according to many factors (such as weather, society activities), and those forecasting data reflect the physical change, not only the change of load types but also the usage. The algorithm considers the regional loading in the calculation of voltage stability indices. The hour-ahead regional load forecasting is applied to determine the regional loading at a certain moment, and then the system loading can be distributed to online generator units through economic dispatch. At last the power flow is conducted to generate indices. These voltage stability indices can estimate the distance of the current operating point to the voltage marginally stable point during the system operation. Because of load forecasting and economic dispatch are already essential functions in the operation of power systems, and have pretty high precision, therefore the algorithm can reflect the actual operation and provide effective voltage stability assessment. It is also computationally efficient for on-line applications.A phasor measurement unit (PMU) based voltage stability assessment index, VSI, is presented. Based upon the optimal placement of the PMU, voltage stability assessment index can be determined by means of the local voltage measurements provided by PMU and power transmission paths. With the advancement in phasor measurement and communication technologies, the Wide Area Measurement System (WAMS) becomes possible and is the trend for the monitoring and control of modern huge interconnected power systems. WAMS can catch the real-time system information at the same time stamp from any location in the whole system; construct the data source for on-line voltage stability assessment in large scale power systems. With the information of voltage phasors in the same time stamp, the difference between voltages at the sending end node and the receiving end node can be calculated easily in a power transmission path, the ratio of the projection of the difference voltage on the voltage at the sending node to the half of the sending node voltage is defined as the transmission path voltage stability index (TPVSI). The TPVSI of the weakest power transmission path can be used to assess the system voltage stability. Compared to other indicators, this index uses synchronized voltage phasors gathered by PMUs to improve the accuracy and avoid complex matrix operation in which is more suitable for on-line voltage stability assessment.