This invention relates to a gas insulated switchgear.
In a conventional gas insulated switchgear, a system utilizing a current transformer is used for protecting the switchgear against grounding and short-circuiting. That is, the system may have a group of interrupters, for example, in the region to be protected which are opened when the difference between the current flowing into the region to be protected and the current flowing out from the region to be protected exceeds a predetermined value.
FIG. 1 is a schematic electrical connection diagram showing a conventional protective system. The illustrated system includes two bus conductors 2 and 3 between which four electrical paths each including two series connected disconnectors 4 are connected. The junction between the two disconnectors 4 of each parallel electrical path is connected to an electrical series circuit each including a circuit interrupter 1 and a disconnector 5. A pair of current transformers 6 and 7 are disposed at both sides of the circuit interrupter 1. The illustrated system is connected to load circuits through respective terminals 8. For example, the load circuit may comprise a power cable 40 extending from a terminal 8 and connected to a bus 19 via a cable head 14, a disconnector 15, a current transformer 17a, a circuit interrupter 16, a second current transformer 17b, and a second disconnector 18. The system also comprises a series circuit connected across the bus conductors 2 and 3 and including a circuit interrupter 9 and a current transformer 10 and a disconnector 11 on the one side and a current transformer 12 and a disconnector 13 on the other side.
When the region to be protected is assumed to be interrupters, current transformers 6 and 7 as well as 10 and 12 are disposed on the both sides of interrupters 1, 9 and arranged to detect the fact that a grounding or a short-circuit condition occurs on either side of the bus conductors 2 and 3, whereby the interrupters 1 associated with the line to be isolated are opened.
With the above conventional gas insulated switchgear, grounding or the like cannot be detected unless it is an electric current higher than a predetermined level due to the errors in the current transformer, and since the grounding or the like is detected by a current, erroneous operation or inoperation may occur, leading to a problem of low reliability.