Protection for the check zone one extra module not

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Unformatted text preview: dule, which contains the common alarm circuits and the bias resistors. The power supplies are also fed in through this module. Bias All zones of measurement are biased by the total current flowing to or from the busbar system via the feeders. This ensures that all zones of measurement will have similar fault sensitivity under all load conditions. The bias is derived from the check zone and fixed at 20% with a characteristic generally as shown in Figure 15.15(b). Thus some ratio mismatch is tolerable. Network Protection & Automation Guide Stability with saturated current transformers The traditional method for stabilising a differential relay is to add a resistor to the differential path. Whilst this improves stability it increases the burden on the current transformer for internal faults. The technique used in the MBCZ scheme overcomes this problem. Busbar P rotection Figure 15.17 shows the correlation between the circuit breakers and the protection modules for a typical double busbar installation. In practice the modules are mounted in a multi-tier rack or cubicle. The MBCZ design detects when a CT is saturated and short-circuits the differential path for the portion of the cycle for which saturation occurs. The resultant spill current does not then flow through the measuring circuit and stability is assured. This principle allows a very low impedance differential circuit to be developed that will operate successfully with relatively small CT's. Operation for internal faults If the CT's carrying fault current are not saturated there will be ample current in the differential circuit to operate the differential relay quickly for fault currents exceeding the minimum operating level, which is adjustable between 20%-200% rated current. When the only CT(s) carrying internal fault current become saturated, it might be supposed that the CT saturation detectors may completely inhibit operation by short-circuiting the differential circuit. However, the resulting inhibit pulses remove only an insignificant portion of the differential current, so operation of the relay is therefore virtually unaffected. • 249 • • 15 • Supervision Differential Current Buswire Selection Links c =1 c = Check m = Main r = Reserve t = Transfer 1 CT Fault m Enable 1 Protection fault r t Alarm OR Supervision Biased Differential Enable Trip m =1 Bias Trip Buswire Selection Links c 1 r Biased Differential Trip t Out of service Figure 15.18: Block diagram of measuring unit Busbar P rotection Discrepancy alarm feature • 15 • As shown in Figure 15.18, each measuring module contains duplicated biased differential elements and also a pair of supervision elements, which are a part of a comprehensive supervision facility. This arrangement provides supervision of CT secondary circuits for both open circuit conditions and any impairment of the element to operate for an internal fault, without waiting for an actual system fault condition to show this up. For a zone to operate it is necessary for both the differential supervision element and the biased differential element to operate. For a circuit breaker to be tripped it requires the associated main zone to be operated and also the overall check zone, as shown in Figure 15.19. to operate the two busbar sections as a single bar. The fault current will then divide between the two measuring elements in the ratio of their impedances. If both of the two measuring elements are of low and equal impedance the effective minimum operating current of the scheme will be doubled. This is avoided by using a 'master/follower' arrangement. By making the impedance of one of the measuring elements very much higher than the other it is possible to ensure that one of the relays retains its original minimum operation current. Then to ensure that both the parallelconnected zones are tripped the trip circuits of the two zones are connected in parallel. Any measuring unit can have the role of 'master' or 'follower' as it is selectable by means of a switch on the front of the module. Transfer tripping for breaker failure Main zone Check zone S1 + ve D1 S1 S2 D2 S2 D2 Serious damage may result, and even danger to life, if a circuit breaker fails to open when called upon to do so. To reduce this risk breaker fail protection schemes were developed some years ago. D1 Trip Figure 15.19: Busbar protection trip logic Master/follower measuring units When two sections of a busbar are connected together by isolators it will result in two measuring elements being connected in parallel when the isolators are closed These schemes are generally based on the assumption that if current is still flowing through the circuit breaker a set time after the trip command has been issued, then it has failed to function. The circuit breakers in the next stage back in the system are then automatically tripped. For a bus coupler or section breaker this would involve tripping all the infeeds to the adjacent zone, a facility that is included in the busbar protection schem...
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