6 as operation due to mechanical shock or mistakes

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Unformatted text preview: al shock or mistakes made by personnel. Faults in the low voltage auxiliary wiring must also be prevented from causing operation by passing current to earth through the switchgear frame. A useful check is provided by a relay energised by the system neutral current, or residual current. If the neutral check cannot be provided, the frame-earth relays should have a short time delay. When a check system is used, instantaneous relays can be used, with a setting of 30% of the minimum earth fault current and an operating time at five times setting of 15 milliseconds or less. Figure 15.7 shows a frame-leakage scheme for a metalclad switchgear installation similar to that shown in Figure 15.4 and incorporating a neutral current check obtained from a suitable zero sequence current source, such as that shown in Figure 15.2. + In Out 64A-1 GH CSS-G 64B-1 _ Trip relays 64CH-1 K L1 CSS-H L2 Insulation barriers M Zone J L5 Busbar P rotection M Zone G j1 g 64CH-2 H L h1 j2 15 • I > 64B-2 74-1 K 74-2 I > Zone G relay + In Zone H relay I Out L3 L4 > CSS-G CSS-H j1 L1 L6 74 Alarm cancellation relay CSS Control selector switch protection in/protection out L3 Busbar protection in service lamp L4 Busbar protection out of service lamp L5 Tripping supply healthy lamp L6 Alarm and indication supply healthy lamp M1 M2 L3 L4 _ g1 K • 64A-2 Tripping relays L2 Figure 15.7: Typical tripping and alarm circuits for a frame-leakage scheme h1 N j2 GH D.C. Zone bus wires Busbar isolator auxiliary switches Figure 15.6: Frame-earth scheme for double busbar substation 15.6.4 Frame-Earth Protection - Check System On all but the smallest equipments, a check system should be provided to guard against such contingencies The protection relays used for the discriminating and check functions are of the attracted armature type, with two normally open self reset contacts. The tripping circuits cannot be complete unless both the discriminating and check relays operate; this is because the discriminating and check relay contacts are connected in series. The tripping relays are of the attracted armature type. • 238 • Network Protection & Automation Guide It is usual to supervise the satisfactory operation of the protection scheme with audible and visual alarms and indications for the following: a. busbar faults b. busbar protection in service c. busbar protection out of service The scheme may consist of a single relay connected to the bus wires connecting all the current transformers in parallel, one set per circuit, associated with a particular zone, as shown in Figure 15.8(a). This will give earth fault protection for the busbar. This arrangement has often been thought to be adequate. If the current transformers are connected as a balanced group for each phase together with a three-element relay, as shown in Figure 15.8(b), additional protection for phase faults can be obtained. e. alarm supply healthy To enable the protection equipment of each zone to be taken out of service independently during maintenance periods, isolating switches - one switch per zone - are provided in the trip supply circuits and an alarm cancellation relay is used. 1 5 . 7 D I F F E R E N T I A L P R OT E C T I O N P R I N C I P L E S The Merz-Price principle is applicable to a multi-terminal zone such as a busbar. The principle is a direct application of Kirchhoff's first law. Usually, the circulating current arrangement is used, in which the current transformers and interconnections form an analogue of the busbar and circuit connections. A relay connected across the CT bus wires represents a fault path in the primary system in the analogue and hence is not energised until a fault occurs on the busbar; it then receives an input that, in principle at least, represents the fault current. The phase and earth fault settings are identical, and this scheme is recommended for its ease of application and good performance. 15.7.1 Differential Protection for Sectionalised and Duplicate Busbars Each section of a divided bus is provided with a separate circulating current system. The zones so formed are over-lapped across the section switches, so that a fault on the latter will trip the two adjacent zones. This is illustrated in Figure 15.9. Tripping two zones for a section switch fault can be avoided by using the time-delayed technique of Section 15.6.2. However instantaneous operation is the preferred choice. Zone B Zone A G H J Busbar P rotection d. tripping supply healthy BS K Id> B BC Differential f relay • Zone C G H Typical feeder circuits Figure 15.9: Zones of protection for double bus station A B C N Differential relay Id I> Id> b) Phase and earth fault circulating current scheme using three-element relay Figure 15.8: Circulating current scheme Network Protection & Automation Guide For double bus installation, the two busbars will be treated as separate zones. The auxiliary busbar zone will overlap the appropriate main busbar zone at the bus coupler. Since any circuit may be transferred from one busbar to the other by...
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This note was uploaded on 02/18/2013 for the course EE 45 taught by Professor Kjald during the Spring '13 term at Aachen University of Applied Sciences.

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