Multi-Cascade Instantaneous Current Protection For Short Length Feeders Without Communication

In some distribution networks, there are some exceptional cases along the line which have a high demand for power supply security, when any fault must be removed immediately. To this end, the traditional method is to use the the instantaneous current protection,but sometimes the range of the instantaneous current protection can not cover the important load because of the lack of the feeder which leads to the small difference of fault current. To solve the problem,we can replace the instantaneous current protection with longitudinal differential protection, but the need to build information channels and related facilities will lead to a big cost.In order to solve the problem above,a new relay protection named multi-cascade instantaneous current protection based on the instantaneous current protection has been presented. In this scheme, the basic components of the current of different types of short circuit are segmented by series reactors which realized the function of all-line fast tripping. At the same time, there is no need to build information channels and related facilities.The following work is done in this paper:(1)The feasibility of three schemes to realize multi-cascade instantaneous current protection is analyzed in this paper, and came to a conclusion that the plan named fault location based on high frequency component of fault current extracted by capacitors and the plan named fault location using high frequency components extracted by parallel resonant circuit have little feasibility, meanwhile the plan named fault location by using series reactors to segment fault current has a high feasibility. Furthermore, the basic principle and realization method of the plan were analyzed.(2)The inductance value of the series reactors used in the multi-cascade instantaneous current protection was deduced and reduced by the fault type discriminant function of the microcomputer protection. At the same time, a simulation model was established in the PSCAD to verify the correctness of the theoretical value above, the general representation of the simulation model was also explained. Finally, considering the effects of load current and system impedance and fault angle on the Krel2, the paper got the most unfavorable situation in the actual operating conditons by simulation experiments, in which case the inductance of the series reactor was adjusted accordingly.(3)The effect on ASAI of the distribution network by multi-cascade instantaneous current protection was evaluated. The voltage loss caused by the installation of series reactors were analyzed quantitatively, meanwhile the power loss caused by series reactors was estamated.Finally, combined with the previous work, this paper made a summary of the setting process of the multi-cascade instantaneous current protection.(4)The theory of the multi-cascade instantaneous current protection was verified by taking a typical city distribution network and a typical rural distribution network as examples. As can be seen from the examples, the multi-cascade instantaneous current protection is more suitable for distribution networks which have short power supply radius.