Voltage Dip Evaluation In Power Grid Of Fault Conditions And Its Application

Evaluation of power grid voltage dips in detail is the premise and basis of analyzing the occurrence of voltage dip in power system and making corresponding improvement and governance program. What’s more, it’s an important foundation on which to further analyze its effect on the power supply reliability. The voltage dip problem caused by a various types of short-circuits and ground faults with arbitrary complex grid topologies will be the research object. This thesis makes deep study on these following aspects:calculation of the fault voltage of the focus bus in the system; the influence of which the line protection and reclosing operation characteristics make on the duration of the voltage dip; the distributed generator optimization options to reduce the frequency of voltage dips in the target network and the power supply reliability considering the voltage dip.This thesis has proposed a bus voltage dip assessment algorithm with the duration considering the line protection and reclosing action characteristics, to improve the insufficient that we usually concern about the voltage dip amplitude and frequency in the traditional voltage dip evaluation algorithm but ignore another important parameter-the voltage dip duration. The fault voltage expression of any buses in the network which is concerned can be given when the short circuit or ground fault which happen on the power transmission lines, whether in the radial network or in a complex grid topology. By calculating the mutual impedance between the fault point and other buses and the self-impedance of the fault point we can extended the original bus impedance matrix and calculate the value of the fault resistance, by which we can explain the fault voltage of any buses as the analytical expression contained with the fault point location parametric and the fault resistance. Combining the protection scope and movement time limit of the power transmission lines protections (over current protection, distance protection, differential protection, etc.) and the fault voltage calculation method given for the single-end tripping of the protection action line, and also the failure frequency and the distribution of the failure probability in the system, to determine the duration of voltage dip and the voltage dip assessment result in each node. Because of considering the two main parameters-the amplitude and duration of voltage dips, the result of the proposed assessment algorithm will be more accurate, comprehensive and with higher practicability. The proposed method has been verified the correctness in the standard IEEE-57bus test system.This thesis has proposed a distributed generator optimal access configuration algorithm based on the genetic algorithm, when the target power grid has the requirement of the distributed generator. The algorithm regards reducing the frequency of the voltage dips as the goal, bases on the genetic algorithm to optimize the distributed generator access, in order to minimize the total frequency of the voltage dip for all the buses or the voltage dip frequency for some certain sensitive load connection buses. The algorithm is based on the number of distributed generators required to access, parameters and the actual situation of the original system, to determine the reasonable access vector encoding. It creates the corresponding objective function and penalty function according to the optimization objectives and constraints, then uses the global optimization features of genetic algorithm to strike the distributed power optimal access solution of the voltage dip problems. The simulation results show that both the non-optimized or optimized access of distributed generators can reduce the frequency of voltage dip problems for some buses in some extent and also the frequency of total voltage dips for the power system. The optimized distributed generators access system can’t guarantee that the frequency of voltage dip in each bus reaches the best, but from the overall aspect, we can achieve the target that the biggest reduction of the frequency for the total system voltage dips after the optimization. The simulation results verify the effectiveness of the optimized access algorithm.The traditional power grid reliability assessment system mainly reflects the ability that the electricity sector can provide continuous and uninterrupted power to the electricity customers. But it doesn’t consider the supply voltage fluctuation impact on the power users, which is caused by the voltage dip that the duration is only a few cycles or seconds. For this problem, this thesis presents a load outage judgment in the case of voltage dips, and power supply reliability analysis algorithm based on the assumption that the voltage dip economic loss is equivalent to the outage time. This thesis put the voltage dip impact on the users into the scope of the system reliability assessment. Firstly, it uses the previously mentioned voltage dip evaluation with the duration of voltage dip to give the key parameters of the amplitude, the duration and the frequency of voltage dip. Secondly, it classifies the load equipments in the users to determine the capacity and number of each load and the weighting factors, capacity factors, number factors and the importance factors of the load equipments according to the extent of voltage sensitivity. It Combines the original equipment voltage-sensitive curve to synthesis the voltage integrated sensitive curve for the users and buses, and then to establish the load outage probability judgement function for users and buses, and inputting the voltage dip assessment to determine the load outage situation. Finally, according to the load outage economic loss and the power users’outage time economic loss, we can calculate the equivalent outage duration and then get the power grid reliability indexes considering the voltage dip. The power supply reliability algorithm which regards the economic loss of voltage dip as the outage time is an important supplement to the existing power supply reliability algorithms. This can lay the foundation of providing high quality power supply to voltage sensitive user. The simulation results in the IEEE-14buses system show the feasibility of the method.