High impedance relays

In order to restrict the detection of earth faults to those within the stator winding, and those from the stator terminals to the switchgear current transformers, a sensitive relay (64) is used. Three current transformers are used in the stator live lines and one in the star to NER connection. All four current transformers are connected in parallel such that any unbalance in the currents due to an internal fault is detected by the restricted earth fault relay (64). A sensitive high impedance relay is used to achieve an instantaneous response. However, if a high impedance is connected across a current transformer it is possible that very high voltages will appear across the impedance. This is due to the action of... 

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

Figure 15.27 Three-element high-impedance circulating current relay scheme...

Relay - High impedance differential protection reiay. It operates for the fault occurring within the protection zone /pi - Fault current through ground for fault on phase 8... 

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

As with FCs, the current recorded by the respective EM must first be converted to a voltage. This is carried out using a high impedance amplifier. This voltage is then passed onto a voltage/frequency converter to allow the signal to be relayed in units of cps. Once the noise associated with these types of detectors is removed (see Section 4.2.3.3.2.1), EMs are easily capable of recording individual ion counts per... 

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. 

The measuring head connects the sample plate via high frequency capable relay matrices (multiplexers, model KRE-2450-TFCU, MTS-Systemtechnik, 3) to the measuring instruments (1, 2). The setup is usable for complex impedance spectroscopy... 

Note For small motors, e.g. 22 kW and below, the earth loop impedance inclnding the feeder cable armouring may be too high. When this is the situation a risk of electric shock exists dnring a short circnit at or near to the motor. To reduce the exposure to the risk it is necessary to nse a 51 N or a 50 N core balance current transformer and relay at the motor control centre. The choice of a 50 N is preferred subject to the contactor being properly coordinated with its upstream fuses. 

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