Current Transformer, core-balance

Protection of a domestic or an industrial single phase system Ground fault on an LT system Ground fault protection in hazardous areas. Ground leakage in an HT system Core-balanced current transformers (CBCTs). Ground fault (G/F) protection schemes... 

An alternative and less expensive scheme uses a core-balance current transformer at the sending end of each cable. Such a scheme is shown in Figure 12.14. 

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. 

A core balance current transformer functions more reliably and is more sensitive than a set of three current transformers connected in parallel. A three-transformer system is prone to responding to the initial inrnsh cnrrent of the motor. To avoid this the current setting needs to be higher than would be preferred. 

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. 

Neither is the technique of earth leakage detection restricted to low voltage systems. The technique is employed on high voltage systems although the core balance method is not the only one used. For example, another way to detect earth fault current is to monitor the amount of current that flows in the earthing conductor at the point of supply, using a current transformer. If the amount of current exceeds a particular value, a circuit breaker will operate to cut off the supply. 

The current transformer (CT) overcomes the power loss and circuit isolation problems of the current shunt, bnt like the VT, it introduces ratio and phase-displacement errors. The constraction of a current transformer (Fig. 26) is different from that of a power transformer, although the basic theory of all transformers is the same, that is, (a) the voltage induced in a transfonner winding is proportional to N mf (where N is the number of turns in the winding, is the flux in the core, and / is the frequency of operation) and (b) the ampere-tums of the windings balance. 

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