It may be advantageous in such a case to provide a

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Unformatted text preview: ent phase and earth fault scheme, mainly to reduce the number of current transformers paralleled into one group. Extra-high-voltage substations usually present no such problem. Using the voltage-calibrated relay, the current consumption can be very small. Busbar P rotection A simplification can be achieved by providing one relay per circuit, all connected to the CT paralleling buswires. Zone R c1 c2 H D Zone M1 Zone M2 b1 a1 F E G • c1 c Zone M2 Zone R Bus wires Check zone Bus wires B C A B C N 95 CHX-2 Zone relay same as check Zone M2 relay same as check Zone M1 relay same as check M1 First main busbar M2 Second main busbar R Reserve busbar Stabilising Resistor + _ Id> Id Supervision Relay Metrosil o (non-linear resistor) Figure 15.13: A.C. circuits for high impedance circulating current scheme for duplicate busbars Network Protection & Automation Guide • 243 • I High Impedance Circulating Current Relay 15 • + In Out 87M1-1 CSS-M1 M1 M2 R 87M2-1 CSS-M2 96 D2 a1 87R-1 96 E c1 CSS-R 96 F1 96 F2 b1 87CH-1 96 D1 96 G c2 96 H1 96 H2 D.C. Buswires 80 T 87CH-2 87M1-2 87M2-2 30 M2 87R-2 95M1-1 95 M1X 95M2-1 95R-1 95 RX 95CH-1 30M1-1 30M2-1 74-1 30R-1 30 M1 30 R 95 M2X 95 CHX 74 74-2 95M1X-1 95M2X-1 Busbar P rotection 95RX-1 95CHX-1 In Out L1 L2 CSS-M1 L1 L2 CSS-M2 • L1 L2 15 • CSS-R 30 74 80 87 95 80 I Zone indicating relay Alarm cancellation relay D.C. volts supervision relay High impedance circulating current relay Bus wires supervision relay 95X CSS L1 L2 Zone bus wires shorting relay Control selector switch Indicating lamp protection in service Indicating lamp protection out of service Figure 15.14: D.C. circuits for high impedance circulating current scheme This enables the trip circuits to be confined to the least area and reduces the risk of accidental operation. 15.8.3 Check Feature Schemes for earth faults only can be checked by a frameearth system, applied to the switchboard as a whole, no subdivision being necessary. For phase fault schemes, the check will usually be a similar type of scheme applied to the switchboard as a single overall zone. A set of current transformers separate from those used in the discriminating zones should be provided. No CT switching is required and no current transformers are • 244 • Network Protection & Automation Guide 15.8.4 Supervision of CT Secondary Circuits Any interruption of a CT secondary circuit up to the paralleling interconnections will cause an unbalance in the system, equivalent to the load being carried by the relevant primary circuit. Even though this degree of spurious output is below the effective setting the condition cannot be ignored, since it is likely to lead to instability under any through fault condition. Supervision can be carried out to detect such conditions by connecting a sensitive alarm relay across the bus wires of each zone. For a phase and earth fault scheme, an internal three-phase rectifier can be used to effect a summation of the bus wire voltages on to a single alarm element; see Figures 15.13 and 15.14. The alarm relay is set so that operation does not occur with the protection system healthy under normal load. Subject to this proviso, the alarm relay is made as sensitive as possible; the desired effective setting is 125 primary amperes or 10% of the lowest circuit rating, whichever is the greater. Since a relay of this order of sensitivity is likely to operate during through faults, a time delay, typically of three seconds, is applied to avoid unnecessary alarm signals. 15.8.5 Arrangement of CT Connections It is shown in Equation 15.4 how the setting voltage for a given stability level is directly related to the resistance of the CT secondary leads. This should therefore be kept to a practical minimum. Taking into account the practical physical laying of auxiliary cables, the CT bus wires are best arranged in the form of a ring around the switchgear site. In a double bus installation, the CT leads should be taken directly to the isolator selection switches. The usual routing of cables on a double bus site is as follows: a. current transformers to marshalling kiosk b. marshalling kiosk to bus selection isolator auxiliary switches c. interconnections between marshalling kiosks to form a closed ring The relay for each zone is connected to one point of the ring bus wire. For convenience of cabling, the main zone relays will be connected through a multicore cable between the relay panel and the bus section-switch marshalling cubicle. The reserve bar zone and the check zone relays will be connected together by a cable running to the bus coupler circuit breaker marshalling Network Protection & Automation Guide cubicle. It is possible that special circumstances involving onerous conditions may over-ride this convenience and make connection to some other part of the ring desirable. Connecting leads will usually be not less than 7/0.67mm (2.5mm2), but for large sites or in other difficult circumstances it may be necessary to use cables of...
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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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