Study On The Fault Analysis Method And Relay Protection Of The Power Grid With Distributed Generators

Recently, in order to meet the increasing power demand, as well as cope with the great pressure caused by the gradually exhaustion of fossil energy and environmental protection, the distributed generators (DGs), represented by the wind power generator and photovoltaic system, has been developed rapidly in China. Moreover, the development of DGs has been one of the key development strategies in the Chineses field of energy.The power generation modes and grid connected strategies of DGs are diversified, and the transient and steady-state characteristics of the fault currents provided by DGs are much different from those of the fault currents provided by conventional synchronous generator. Moreover, the output power of DGs is greatly affected by the natural environment and climate, which has the features of remarkable randomness and intermittency. The complicated fault current characteristics and versatile operating conditions of DGs make the performance of the relay protection applied in the power grid with penetration of DGs deteriorate greatly, accordingly, the safe operation of the power grid cannot be ensured. Meanwhile, these also make the short circuit current analysis theory which is based on the conventional synchronous generator cannot meet the fault analysis requirements of the power grid with penetration of DGs, which bring the study of relay protection severe challenges. Generally speaking, the complicated fault current characteristics and versatile operating conditions of DGs greatly deteriorate the performance of conventioanl relay protection and badly impact the safe and stable operation of the power gird. These technical problems have greatly resticted the further development and application of DGs and need to be solved. In this condition, as a part of the project "The Construction Mode and Principle Study of Smart Distribution Network Relay Protection Coping with the Large-scale Penetration of Distributed Generators"(Grant No.51177058) which is supported by the Natural Science Foundation of China, the dissertation implements an in depth study on the fault current characteristics of DGs, the fault analysis method and improved relay protection of the power grid with penetration of DGs.The basic task of relay protection is to identify and isolate the fault element according to the variation characteristics of the electrical quantities (or non-electrical quantities) after a fault occurs. Hence, the analysis of the power grid fault characteristics is the basis of the study of relay protection. According to the fault characteristics presented by different kinds of DGs, the DGs are divided into two types:generators of asynchronous type which are directly interfaced to the power grid (like doubly-fed induction generator) and inverter-based generators which are interfaced to the power grid with inverters (like photovoltaic system). Accoridngly, the fault current characteristics of these two types of DGs are studied specially with theoretical analysis and simulation study.For the doubly-fed induction generator (DFIG, the generator of asynchronous type), on condition that a severe fault which causes the grid voltage drop greatly occurs, in order to ensure the safety of DFIG, the crowbar protection is activated to short circuited the rotor windings with the crowbar resistance and divert the current from rotor-side converter (RSC). On the other hand, in cases of non-severe faults which occur far away from DFIG (meanwhile, the stator voltage of DFIG is large enough), the crowbar protection will not be activated and the rotor windings are still excited by the AC/DC/AC converter. The fault current characteristics of DFIG, such as transient components and damping time constant, are much different under the two conditions. Moreover, they are both different from those of conventional synchronous generator. Therefore, the fault current characteristics of DFIG under these two conditions are studied separately in the dissertation.In order to study the fault current characteristics of DFIG on condition that a severe fault occurs and the crowbar protection is activated, a fault current analysis method based on the simplified calculation model of stator flux linkage is proposed. Firstly, the dynamic model of DFIG in stator stationary reference frame is developed to meet the fault current study requirements under symmetric and asymmetric fault conditions. Then the dynamic response of stator flux linkage is studied and a simplified calculation model of stator flux linkage is established with consideration of accuracy as well as simplicity. Based on it, the fault current characteristics DFIG under conditions of symmetric and asymmetric faults are analyzed and the analytical expressions are obtained. Besides, the equivalent calculation models of fault current are built.During non-severe fault conditions, the dynamic response of RSC has a large influence on the fault current characteristics of DFIG, and a fault current analysis method considering the dynamic response of RSC is proposed. Firstly, the steady-state control strategy and the improved control strategy for low-voltage ride through (LVRT) are analyzed. Then, the dynamic response of RSC is studied when the inner rotor current control loop is designed to be first order or second order system. Meanwhile, the simplified calculation models of rotor fault current are established. Based on it, the fault current of DFIG during non-severe fault condition is studied and the analytical expressions are obtained. Finally, simulation examples in PSCAD/EMTDC verify the theoretical analysis results.Most types of DGs interface to the power grid with inverters, which can be classified as inverter interfaced distributed generators (IIDGs). The fault current characteristics of IIDGs are significantly affected by the corresponding improved control strategy for LVRT. Hence, the IIDG fault current analysis method based on the LVRT control strategy is proposed. Firstly, in order to meet the grid code, a control strategy for LVRT of IIDG is proposed. Based on it, with theatrical analysis and simulation study, the dynamic responses of inverter controller and fault current characteristics of IIDG are analyzed under conditions of symmetric fault and asymmetric faults. Moreover, the calculation model of the fault current of IIDG is established.The special fault current characteristics of DGs make the conventional fault analysis method can be no more applied for the power grid with penetration of DGs. In order to meet the fault analysis method of the power grid with penetration of DGs, as well as establish theoretical basis for the study of relay protection, it is necessary to propose a fault analysis method which can be used for the power grid with penetration of DGs. Hence, the conventional fault analysis method based on the synchronous generator is studied. Then take the power grid with penetration of IIDGs for example, the problems of the conventional fault analysis method are pointed out. Based on it, a novel fault analysis method for the power grid with penetration of IIDGs is proposed. Finally, the simulation study shows the effectiveness and validity of the proposed fault analysis method.The complicated fault current characteristics and versatile operating conditions of DFIGs make the performance of the relay protection applied on the tie line of DFIG-based wind farm deteriorate greatly and cannot meet the safe operation requirements of the power grid. In order to address this issue, a novel distance protection scheme is proposed. Firstly, the proposed distance protection is based on the differential equation of the transmission line; hence its performance will not be affected by the activation of crowbar protection and the damping components in the fault current of DFIG. Secondly, the second-order Butterworth low-pass filter is used to effectively limite the influence of the high frequency components in the measured voltages and currents on the performance of the proposed distance protection. Then, the stability and accuracy of the fault distance calculation results are ensured, with the reconstruction of faulty phase voltage at the fault point and the iterative calculation of the fault distance. A variety of simulation tests show that the perfoemance of the proposed distance protection is much better than that of the conventional distance protection, because the proposed distance protection can distinguish the internal faults and external faults, and no matter what kind of fault occurs in the prorection scope, the proposed distance protection can trip correctly and quickly.In the end, systematical summary about the research works is given and the further work needed to be further implemented is pointed out.