Study Of A New Current Protection Strategy Based On Sufficient Criterion In Distribution Network With DGs

With the shortage of traditional resource and environmental problems, more and more people have paid attention to the renewable resources. By distributed generation technologies, the clean resources have been connected to the distributed network, which is one of the major methods they offer power to customers. However, beside the alleviation of power, the integration of DGs has great influence on the traditional distributed network, bringing out a large amount of problems of protection strategies. The contradiction between the applicability and the dispersion of the lacked information in the network, as well as the contradiction between the changeable characteristics of DGs and the setting problem of current protection, makes the protection schemes inapplicable and unsatisfactory. In order to simplify the setting problem and ensure the selectivity, the paper proposes novel direction modules for the detection of the forward and reverse faults, based on the theory of sufficient criterion. The strategy needs some changes for the setting of the normal current protection and with the corporation between the direction detector and the changed current protection, it can make a balance between the cost and the reliability of the protection.Based on a quantitative analysis of the levels of the short-circuit currents of different types of DGs, the paper proposes sufficient criterion-based direction modules to avoid the bad effects of DGs connected to network on the selectivity and sensitivity. According to the large difference of the short-circuit capacity between DGs and the distributed system, the current-magnitude direction module is proposed based on the feature of the short-circuit current. The direction module can effectively solve the selectivity and sensitivity problems in traditional protection stategy. Taking the capacity of the DG self and the whole limited capacity of DGs into consideration, a new direction module, called direction module on coefficient of DGs'short-circuit current capacity, is proposed, where km is introduced for the calculated value for coefficient of DGs' short-circuit current capacity. This model can operate well even in high DG penetration cases, ensuring the sufficient selectivity and high sensitivity in most cases. The two direction modules only need the magnitudes of short-circuit currents and simple bool information communication and can operate in T-contruction network and assist the transition resistances well.The amended three-sect current protection based on the direction modules is introduced and with the direction modules, the sufficient criterion-based current protection strategy is composed. With the cooperation of the amended three-sect current protection and the direction modules, the synthetical cerition will gain a higer sensitivity and remains sufficient selectivity. For a few cases that the strategy can not locate the faults, DG isolation technology is applied.There is a simulation on MATLAB/SIMULINK platform, where one 500MVA of 35KV distribution network model is constructed to test and verify the strategy. The final simulations validate the strategy's reliability and the sufficient selectivity.