Since any circuit may be transferred from one busbar

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Unformatted text preview: isolator switches, these and the associated tripping circuit must also be switched to the appropriate • 239 • 15 • zone by 'early make' and 'late break' auxiliary contacts. This is to ensure that when the isolators are closing, the auxiliary switches make before the main contacts of the isolator, and that when the isolators are opened, their main contacts part before the auxiliary switches open. The result is that the secondary circuits of the two zones concerned are briefly paralleled while the circuit is being transferred; these two zones have in any case been united through the circuit isolators during the transfer operation. Busbar P rotection 15.7.2 Location of Current Transformers Ideally, the separate discriminating zones should overlap each other and also the individual circuit protections. The overlap should occur across a circuit breaker, so that the latter lies in both zones. For this arrangement it is necessary to install current transformers on both sides of the circuit breakers, which is economically possible with many but not all types of switchgear. With both the circuit and the bus protection current transformers on the same side of the circuit breakers, the zones may be overlapped at the current transformers, but a fault between the CT location and the circuit breaker will not be completely isolated. This matter is important in all switchgear to which these conditions apply, and is particularly important in the case of outdoor switchgear where separately mounted, multi-secondary current transformers are generally used. The conditions are shown in Figure 15.10. Figure 15.10(a) shows the ideal arrangement in which both the circuit and busbar zones are overlapped leaving no region of the primary circuit unprotected. Figure 15.10(b) shows how mounting all current transformers on the circuit side of the breaker results in a small region of the primary circuit unprotected. This unprotected region is typically referred to as the ‘short zone’. The fault shown will cause operation of the busbar protection, tripping the circuit breaker, but the fault will continue to be fed from the circuit, if a source of power is present. It is necessary for the bus protection to intertrip the far end of the circuit protection, if the latter is of the unit type. With reference to Figure 15.10(b), special ‘short zone’ protection can be provided to detect that the circuit breaker has opened but that the fault current is still flowing. Under these conditions, the protection can initiate an intertrip to the remote end of the circuit. This technique may be used, particularly when the circuit includes a generator. In this case the intertrip proves that the fault is in the switchgear connections and not in the generator; the latter is therefore tripped electrically but not shut down on the mechanical side so as to be immediately ready for further service if the fault can be cleared. 15.7.2.1 CT locations for mesh-connected substations The protection of busbars in mesh connected substations gives rise to additional considerations in respect of CT location. A single mesh corner is shown in Figure Mesh corner (Note 1) (a) (b) Bus protection Line protection relay Note 1: Only 1 connection to the mesh corner permitted (a) CT arrangements for protection including mesh corner • 15 • Fault Transformer protection Mesh corner (Note 2) Circuit protection Line protection Mesh corner protection a. Current transformers mounted on both sides of breaker -no unprotected region b. Current transformers mounted on circuit side only of breaker -fault shown not cleared by circuit protection Note 2: Multiple circuits may be connected to the mesh corner (b) CT arrangements for protection additional mesh corner protection required Figure 15.10: Unprotected zone with current transformers mounted on one side of the circuit breaker only Figure 15.11: Mesh-corner protection • 240 • Network Protection & Automation Guide 15.11(a). Where only one connection to the mesh is made at a corner, CT’s located as shown will provide protection not only to the line but the corner of the mesh included between them. However, this arrangement cannot be used where more than one connection is made to a mesh corner. This is because a fault on any of the connected circuits would result in disconnection of them all, without any means of determining the faulted connection. Protection CT’s must therefore be located on each connection, as shown in Figure 15.11(b). This leaves the corner of the mesh unprotected, so additional CT’s and a relay to provide mesh-corner protection are added, as also shown in Figure 15.11(b). An equivalent circuit, as in Figure 15.12, can represent a circulating current system. G H RLG RCTG RLH RCTH R R 15.8 HIGH IMPEDANCE D I F F E R E N T I A L P R OT E C T I O N ZEG ZEH Id> This form of protection is still in common use. The considerations that have to be taken into account are detailed in the following sections. Figure 15.12: Equivalent circuit of circulating current s...
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