Current transformers relays

FIG. 29-8 Typical high-voltage ac motor starter illiistrating several protective schemes fuses, overload relays, ground-fault relays, and differential relays with the associated current transformer that act as fault-current sensors. In practice, the differential protection current transformers are located at the motor, hut the relays are part of the starter. 

Relays have inputs from several current transformers (CTs) and the zone of protection is bounded by these CTs. While the CTs provide the ability to detect a fault inside the zone, circuit breakers (CBs) provide the ability to isolate the fault by disconnecting all of the power equipment within the zone. Thus, a zone bonndai y is usually defined by a CT and a CB. When the CT is part of the CB it becomes a natural zone boundaiy. When the CT is not an integral part of the CB, special attention must be paid to the fault detection and fault interruption logic. The CT still defines the zone of protection, but communication channels must be used to implement the tripping function. Figure 1 shows the zones of protection in a typical system. 

Protective devices, usually relays or fuses, are installed at supply points in a power system to accurately detect and quantify a disruptive disturbance in the system. The variable most freqnently used for detection is the supply line current, and in most sitnations this is detected throngh the nse of current transformers. Occasionally direct acting devices are used e.g. fuses for voltages up to about... 

The star-point connection to earth is provided with a current transformer and a sensitive relay. The relay (51 G) is of a definite time delay or inverse time delay type so that it can be graded as back-up protection to earth fault relays at downstream feeders and consumers. 

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... 

Where a bus-section circuit breaker is used to divide the busbars (during abnormal operating conditions) each set of busbars is protected as a separate zone. Each zone consists of the incomers, the outgoing circuits and the bus-section circuit breaker(s). An accurate current transformer is connected in each line of each circuit. All the current leaving the zone must be balanced by current from the incomer circuits. A fault in the zone will be detected by the (87) relay. Rapid operation is required... 

It is therefore necessary to provide a sensitive method for detecting earth fault currents. The most common method is to provide a core balance current transformer at the circuit breaker or contactor. This current transformer has a current or turns ratio, which is independent of the ratios used by the transformers connected in the three-phase conductors. This is because a particular level of current is to be detected rather than a fraction or multiple of the stator load current. The switchgear manufactnrer will normally recommend the ratio of the core balance transformer and the matching relay. The relay will be either instantaneous 50 N or an inverse time 51 N type depending upon whether the motor is controlled by a circuit breaker or a contactor. 

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. 

It should be noted that this function is assigned only to a relay which detects the flow of current from the frame of a machine or enclosing case or structure of a piece of apparatus to ground, or detects a ground on a normally ungrounded winding or circuit. It is not applied to a device connected in the secondary circuit or secondary neutral of a current transformer, or in the secondary neutral of current transformer, connected in the power circuit of a normally grounded system. 

Under Voltage Relay Current Transformer—reduces high voltage to instrumentation ... 

Electrical isolators include fiber-optic and photo-electric couplers, transformer-modulated isolators, current transformers, amplifiers, circuit breakers, and relays. Isolators used in NPPs are designed to prevent the maximum credible fault in the transverse mode on the non-Class IE side of the isolator from degrading the performance of the Class IE side of the isolator below an acceptable level. 

There must also be a range of instruments including insulation testers, multimeters, potential indicators, loop impedance testers, RCD testers, current transformers, wattmeters and perhaps relay testing equipment. Expensive instruments, such as oscilloscopes, which may be only occasionally required and which may require expert handling are probably not worth buying, but can be hired when needed or can be provided by a specialist called in to deal with a problem that cannot be tackled in-house. 

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. 

Consider a system being fed through a transformer of 1500 kVA, 11/0.433 kV, having a rated LV current of 2000 A. The protection CT ratio on the LV side for the high set relay may be considered as 4000/5 A (depending upon the setting of the relay) rather than a conventional 2000/5 A, thus reducing the ALF of the previous example from 20 to 10. Now only one set of 15 P10 CTs will suffice, to feed the total protective scheme and have a VA x ALF of not more than 150. 

These are protection CTs lor special applications such as biased differential protection, restricted ground fault protection and distance protection schemes, where it is not possible to easily identify the elass of accuracy, the accuracy limit factor and the rated burden of the CTs and where a full primary fault current is required to be transformed to the secondary without saturation, to accurately monitor the level of fault and/or unbalance. The type of application tind the relay being used determine the knee point voltage. The knee point voltage and the excitation current of the CTs now form the basic design parameters for such CTs. They are classified as class PS CTs and can be identified by the following characteristics ... 

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