The Optimization Design Of Protection For Medium-voltage Ring Network In Urban Mass Transit

The AC medium-voltage ring network is the"aorta"of power-supply system in urban mass transit, which is one of the key ensures for the transit running. Perfect relay protection is very important for the stable and reliable operation of medium-voltage ring network.The general protection scheme for medium-voltage ring network is made up of two parts: the main protection is fiber longitudinal differential protection, and the back-up protections are definite time over-current protection and zero-sequence current protection. The fiber longitudinal differential protection is sensitive to failure current, and acts rapidly and reliably. But it's protection scope is limited, just to the line(cable). It can't protect the failure which happens in switch cabinet or busbar. However, as the back-up protection, definite time over-current protection or zero-sequence current protection has to face the contradictions between speed and selectivity, which needs a long time to cut off the failure or makes several substations trip at the same time, as a result will extend the failure's influence section. It is obvious that, the functional limitation of general protection affects network's configuration, specially in"large series ring".In this paper, a method that using the programmable function of micro-computer protection device,is proposed to improve general protection. The"near boundary back-up over current trip accelerated"will be introduced in detail as an example. It's core idea is transferring unlock signals among devices and combining programmable function of device to accelerate the back-up protection, then, consummate the scope of protection and avoid override-trip. However, when several failures happen at the same time in the system or several protection devices conk out simultaneously, the scheme has a limitation.Current selection trip protection is raised as the optimization scheme in this paper, which relies on communication and fast computation of the protection devices. It can rapidly find out the sorts and place of failue, then, control the corresponding switch to cut off the failure section. It acts fast, and can reduce the influence of the failure by precise tripping off breakers. The drawbacks of general scheme are overcome.