Protective relays

Electrical. The plant electrical system is sometimes more important than the steam system. The electrical system consists of the utihty company s entry substation, any ia-plant generating equipment, primary distribution feeders, secondary substations and transformers, final distribution cables, and various items of switch-gear, protective relays, motor starters, motors, lighting control panels, and capacitors to adjust power factor. 

Thiese can be drawn tor temperature versus time or current versus time as desired, depending upon the calibration of the device, such as a motor protection relay. Below we provide a brief procedure to draw these curves. 

In all these conditions protection against 2, 3, 4 and 5 is generally applicable to large LT motors, say 100 h.p. and above, and all HT motors. This protection is normally provided by a single-device motor protection relay, discussed in Section 12.5. 

These call for a closer proteciion, w hich is possible through a single point niolor protection relay (MPR). Since a single MPR provides protections against unfavourable operating as well as fault conditions, we discuss this relay separately in Section 12.5. 

Therefore a motor protection relay, which may be a normal time ora long lime, will be able to provide through one unit a comprehensive motor protection, generally requiring no more protection, except for a surge protection, which may be needed for an HT motor. 

Figure 12.29(b) PLC or micro-processor-based motor protection relay (Courtesy Alstom)... 

For meticulous protection, therefore, it is advisable to use a motor protection relay for large LT and all HT motors. The following motor details and working conditions are essential to know before making i proper selection of a protective scheme ... 

Having discussed the effect of the above parameters on the motor s performance, we will now illustrate by way of an example a general case to broadly suggest a procedure that can be followed to select the protective scheme for a motor. For more detailed selection of the motor protection relay, reference may be made to the relay manufacturer. 

Static thermal relay (discrete ICs or microprocessor based) For medium and large motors, 300 h.p. and above, this type of protective relay should be preferred to achieve optimum utilization of the motor s capacity. Consider the available setting ranges in the vicinity of... 

The latest practice for up to 6.6 k V motors is to use an HT load break, fault-making isolator in conjunction with the appropriate type and size of HRC fuses, a vacuum contactor and a motor protection relay. The contactors... 

Figure 12.46 A simple power diagram for a 6.6 kV motor control with an isolator, contactor and a motor protection relay...

Thermistor Motor Protection Relay Catalogue-SP 5QI25/54S2, Larsen and Toubro. 

The philosophy to assume the impedance of the source of supply (generator or a transformer) as the impedance of the faulty circuit may be far from reality and may give a very high fault current. In actual operation, the fault intensity may be far less, as every device and component connected in the circuit will tend to add to the effective impedance of the faulty circuit and limit the magnitude of the fault current. Figure 13.15 also subscribes to this theory. But it is customary to design the systems for the worst fault conditions which, in all likelihood, may not arise, and decide the protective scheme and the current settings of the protective relays for the minimum possible fault current. 

This is not material in voltage transformers, as neither the voltage measuring instruments nor the protective relays will carry any inrush current during a switching operation or a fault. No short-time rating is thus assigned to such transformers. 

They are used for botli measuring and protection purposes. As a measuring VT, they are used to feed a voltmeter, kW, kWh or a kVAr meter, a power factor, frequency meter or a synchroscope. As a protection VT they are used to feed a protective circuit, incorporating voltage sensitig protection relays. To save on cost and mounting... 

For high set protective schemes, where to operate the protective relays, the primary fault currents are likely to be extremely high, as in the above case. Here it is advisable to consider a higher primary current than the rated for the protection CTs and thus indirectly reduce the ALF and the product of VA X ALF. In some cases, by doing so, even one set of CTs may meet the protective scheme requirement. 

Relay - High impedance three element differential protection relay IVp - Windings ol a power equipment or section ol a power system to be protected... 

Relay - High impedance single element ground fault differential protection relay... 

Differential proteetion (Relay Code 87) To detect a stator phase-lo-phase fault by a three-pole differential protection relay, current setting 10 0%. For scheme diagrams, refer to Section 15.6.6(1). 

Normally a two-stage protection is provided for V/Hz protection. If V/Hz exceeds 1.18 per unit, a protection relay trips the generator within two seconds (typical). For V/Hz l.l to 1.18 per unit, a protection relay trips the generator within 45 seconds (typical). 

The motor protection relay, as discussed in Section 12.5, is generally provided with a delay feature to bypass these transients and delay the tripping by two or three cycles. 

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