A slightly more onerous condition may arise when two

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Unformatted text preview: closing of primary isolators. It is generally desirable to attain an effective primary operating current that is just greater than the maximum load current, to prevent the busbar protection from operating spuriously from load current should a secondary circuit wiring fault develop. This consideration is particularly important where the check feature is either not used or is fed from common main CT's. 15.9.3 Check Feature Busbar P rotection For some low impedance schemes, only one set of main CT's is required. This seems to contradict the general principle of all busbar protection systems with a check feature that complete duplication of all equipment is required, but it is claimed that the spirit of the checking principle is met by making operation of the protection dependent on two different criteria such as directional and differential measurements. • 15 • The usual solution is to route all the CT secondary circuits back to the protection panel or cubicle to auxiliary CT's. It is then the secondary circuits of the auxiliary CT’s that are switched as necessary. So auxiliary CT's may be included for this function even when the ratio matching is not in question. In static protection equipment it is undesirable to use isolator auxiliary contacts directly for the switching without some form of insulation barrier. Position transducers that follow the opening and closing of the isolators may provide the latter. Alternatively, a simpler arrangement may be provided on multiple busbar systems where the isolators switch the auxiliary current transformer secondary circuits via auxiliary relays within the protection. These relays form a replica of the busbar and perform the necessary logic. It is therefore necessary to route all the current transformer secondary circuits to the relay to enable them to be connected into this busbar replica. Some installations have only one set of current transformers available per circuit. Where the facility of a check zone is still required, this can still be achieved with the low impedance biased protection by connecting the auxiliary current transformers at the input of the main and check zones in series, as shown in Figure 15.16. In the MBCZ scheme, described in Section 15.9.6, the provision of auxiliary CT's as standard for ratio matching also provides a ready means for introducing the check feature duplication at the auxiliary CT's and onwards to the relays. This may be an attractive compromise when only one set of main CT's is available. Main zone Check zone 15.9.4 Supervision of CT Secondary Circuits In low impedance schemes the integrity of the CT secondary circuits can also be monitored. A current operated auxiliary relay, or element of the main protection equipment, may be applied to detect any unbalanced secondary currents and give an alarm after a time delay. For optimum discrimination, the current setting of this supervision relay must be less than that of the main differential protection. In modern busbar protection schemes, the supervision of the secondary circuits typically forms only a part of a comprehensive supervision facility. 15.9.5 Arrangement of CT connections It is a common modern requirement of low impedance schemes that none of the main CT secondary circuits should be switched, in the previously conventional manner, to match the switching of primary circuit isolators. Main zone Check zone Figure 15.16: Alternative CT connections 15.9.6 Static Low Impedance Biased Differential Protection - Type MBCZ The Type MBCZ scheme conforms in general to the principles outlined earlier and comprises a system of standard modules that can be assembled to suit a particular busbar installation. Additional modules can be added at any time as the busbar is extended. A separate module is used for each circuit breaker and also one for each zone of protection. In addition to these there is a common alarm module and a number of power supply units. Ratio correction facilities are provided within each differential module to accommodate a wide range of CT mismatch. • 248 • Network Protection & Automation Guide Zone 1 Zone 2 Zone 3a Bus coupler 1 Feeder 1 Z1 Z3a Zone 3b Feeder 2 Bus section Feeder 3 Z2 Z3b Check Feeder 4 zone Bus coupler 2 Intermodule plug-in buswire connections Figure 15.17: Type MBCZ busbar protection showing correlation between circuit breakers and protection modules The modules are interconnected via a multicore cable that is plugged into the back of the modules. There are five main groups of buswires, allocated for: i. protection for main busbar ii. protection for reserve busbar iii. protection for the transfer busbar. When the reserve busbar is also used as a transfer bar then this group of buswires is used iv. auxiliary connections used by the protection to combine modules for some of the more complex busbar configurations v. protection for the check zone One extra module, not shown in this diagram, is plugged into the multicore bus. This is the alarm mo...
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