REG216, CompactREC216 Numerical Generator ProtectionNumerical Control Unit

Basic requirements A restricted E/F scheme must be able to detect E/F’s in the protection zone remain stable during both phase and earth through-faults. The scheme is designed to remain stable in the case of a solidly grounded star-point for an external E/F current in the case of an impedance grounded star-point for the highest external phase and earth fault current. When designing a scheme, it is assumed that one c.t. is fully saturated and none of the others are.

Design 

The E/F current is determined by a) the generator and step-up transformer reactances when the HV circuit-breaker is open (see Fig. 4.1) b) in addition to a) by the HV power system when the HV cir cuit-breaker is closed (see Fig. 4.2). As a result of the current distribution for a through-fault, the star point c.t. conducts the highest current in the case of a solidly grounded transformer as shown in Fig. 4.2. Apart from the burden, the high fault level results in a high c.t. flux and the probability of it saturating is then also high. The influence of through faults on the circulating current circuit is limited, especially if the connections between the c.t. cores can be kept short. It is for this reason, that phase faults are ne glected when designing a scheme for a solidly grounded system. Phase faults have to be taken into consideration, however, where a system is impedance grounded. The value of the stabilising resistor is chosen such that the volt age drop caused by the highest external E/F and possibly phase fault current across the secondary winding and leads of the saturated c.t. cannot reach the pick-up setting of the protection (see Fig. 4.3).

The knee-point voltage of the c.t’s is specified such that the c.t’s can supply sufficient current during an internal fault to enable the protection to trip. The knee-point voltage Uk of the c.t’s must therefore be appreciably higher than the voltage drop Ua

Symbols used:

IE I2 I2N I1N IN R2 RL Ua , Ui Uk Û I RS IF Equations: primary star-point current (AC component) for a through-fault secondary current of the non-saturated c.t’s c.t. secondary rated current c.t. primary rated current protection rated current secondary resistance of the saturated c.t. at 75°C lead resistance according to Fig. 4.3 voltage drop across the circulating current circuit for external, respectively internal faults. knee-point voltages of the c.t’s peak value of voltage current setting stabilising resistor highest primary fault current (AC component) for an internal E/F.

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