Studies On Power System Islanding Based On Wide Area Measurements

Since the U.S.-Canadian blackout on 14 August, 2003, a number of blackouts have occurred worldwidely, the whole world has been shocked by the blackouts. As a result, more and more experts are making great efforts on the research about the reasons of blackouts and the measures how to deal with an unstable power system. The reason of almost all the serious disasters in power system is that there are no countermeasures set before the occurrence of instabilities, and so the outages are transferred to the other part of the system. For a power system, considering the preventive measures to save it from possible instability is extremely important. The long time experiences for power system operation have shown that no matter how strict the requirements for power system stability, and how perfect the measures are, there would be some unpredictable casual faults working together to cause instability. However, the expense for too strict security requirements is very high. A weaker system with good countermeasures for possible instability will perform much better than a strong one without countermeasures. So the subject of countermeasures to an unstable power system has become a hot field.In China, when an interconnected grid is subjected to large disturbances and becomes out-of-step, splitting the out of step area from the main grid has been used widely as the last resort for preventing widespread blackout. But most of the out-of-step devices are based on local information, their criteria only reflect one or several types of out-of-step phenomena, and it is very difficult for different devices to cooperate, too. In addition, most of the devices are designed based on equivalent two-machine system model, however, under complex conditions of three or more groups of generators operate asynchronously, both the out of step criteria and the pre-decided islanding spots will not be suitable.Two pivotal problems about active islanding are when and where to split the system. Islanding cutset searching is a NP-hard problem, and heuristic approaches are employed to solve it. The approaches mentioned in literature need much calculation time, and can not be used online. Some references use a setting time after faults or a number of oscillation periods to determine islanding time, but the system condition is not been considered. Thus, it is very urgent to do research on proper islanding spots, suitable islanding time, and valid measures to keep the islands stable.Power system is modeled by a vertex-weighted graph, the system buses are put into a series of subsystems. Then out of step among generators is converted to the one among subsystems. Based on WAMS information, out of step area identifying and islanding cutset searching can be made online, and islanding time can be determined adaptively. The main contributions of the dissertation include the following.1 Based on power system dispatching area and generator coherency, an approach, which divides an interconnected power system into a series of subsystems, is proposed. After setting some faults on the current system, the generator power angles' curves are got by simulation. Employing the curves, the generators are grouped into a number of coherency groups. If the generators in a dispatching area are not coherent, they can be put into several coherent groups. Considering the dispatching area and generator coherent grouping, the generators of a system are put into as many as possible groups, that is, the system is divided into the same number of subareas. The generator buses are the core of the subareas. Finally, the nongenerator buses of the system are put into the corresponding subareas employing breadth first searching method. Non-generator buses are the outer layer buses of the subareas. The approach has been used to a practical power system.2 Based on power system areaing, an islanding cutset searching approach is proposed. It is assumed that all the dynamic data on generators can be obtained from WAMS. The subareas where there are out-of-step generators are called out-of-step subareas. Combine the interconnected out-of-step subareas to an out-of-step area (OOSA). An OOSA may include one or more out-of-step subareas, the other part of the system is called remaining system (RS). The subareas in OOSA, which have direct electrical connections with RA, are called OOSA boundary. By searching for the tie lines that connect OOSA boundary and RS, the cutset can be obtained The problem of cutset searching is classified as a NP-hard one. After system areaing, there is no need to scan all the inner branches of OOSA, this reduces lots of calculation burden, which makes the calculation complexity of cuset searching proportional to the number of the outer layer buses of the OOSA. Simulation results for the IEEE 30-bus system and a practical 1318-bus Shandong power system demonstrate the effectiveness of the proposed approach.3 An adaptive scheme to decide "when to split" a system disturbed by power oscillation is proposed. Generator's operation frequency limits are surveyed, and generatorωand dω/dt are used to estimate the level of affection made by the oscillation. It is proposed that islanding measures must be taken before the affection can be eliminated by islanding and load/generation shedding. Based on WAMS information, it can be judged that some generators may go out of step from other generators, and the subareas they are belonged to are called "possible out of step subareas". Combine the connected "possible out of step subareas" to an area, named "possible out of step area (POOSA)". According to generator frequency limits, a scheme, which decides islanding time adaptively, is proposed. If the current maximalωof POOSA generators or the predicted values ofωin 0.1s gets setting threshold values, the POOSA should be split from the main power system immediately. After out of step occurs, if currents of generators or the main power stations' voltages, the maximalωof OOSA generators or the predicted value ofωin 0.1s gets setting thresholds, the OOSA should be split immediately. Otherwise, the system may operate asynchronously for a setting time. The simulation results show that splitting the system at the time determined by the scheme, and taking some load shedding measures when they are needed, the stability of the islands can be ensured.4 If an area which will be split from the main grid has large generation deficit, an integrated measure including islanding load shedding, underfrequency and undervoltage load shedding is given. The frequency characteristics of the generators in islands with generation lack are analyzed. The conclusion is that the frequency increases, at the beginning of the forming of some islands with generation deficit, so the measures based on under-frequency are invalid at this case. Furthermore, the more the generation deficit is, the higher the island frequency may get. Sometimes, the frequency may be so high that the OPC protection of the generator unit will operate, and the most serious situation is that the generators would trip cascadingly or simultaneously. Because the generator frequency in the islands with generation deficit increase, a new measure is proposed, that is, shedding a proportion of loads at the same time of system splitting, which can improve the voltage profile, increase the generators' output power, and limit the maximal frequency at the beginning of the island's forming. After a while, if the frequency reaches a setting point of under-frequency devices, some loads would be shed again. If the voltage profile is low, the under-voltage load shedding devices will work. Simulation results show that the measures proposed in this paper can keep the frequency of the islands stable.