A typical protection scheme is shown in Figure 11.28(a), in which the differential circulating-current arrangement is used. The specification and arrangement of the current transformers is complicated by the main transformer connections and ratio. Current-magnitude differences are corrected by adjusting the turns ratio of the current transformers to account for the voltage ratio at the transformer terminals. In a differential scheme the phase of the secondary currents in the pilot wires must also be accounted for with star-delta transformers. In Figure 11.28(a) the primary-side current transformers are connected in delta and the secondary ones in star. The corresponding currents are shown in Figure 11.28(b) and it is seen that the final (pilot) currents entering the connections between the current transformers are in phase for balanced-load conditions and hence there is no relay operation. The delta current-transformer connection on the main transformer star-winding also ensures stability with through earth-fault conditions which would not be obtained with both sets of current transformers in the star connection. The distribution of currents in a Y-D transformer is shown in Figure 11.29.
Troubles may arise because of the magnetizing current inrush when energization operates the relays, and often restraints sensitive to third-harmonic components of the current are incorporated in the relays. As the inrush current has a relatively high third-harmonic content the relay is restrained from operating.
Faults occurring inside the transformer tank due to various causes give rise to the generation of gas from the insulating oil or liquid. This may be used as a means of fault detection by the installation of a gas/oil-operated relay in the pipe between the tank and conservator. The relay normally comprises hinged floats and is known as the Buchholz relay (see Figure 11.30). With a small fault, bubbles rising to flow into
Figure 11.28 Differential protection applied to Y-Д transformers, (a) Current transformer connections, (b) Phasor diagrams of currents in current transformers
the conservator are trapped in the relay chamber, disturbing the float which closes contacts and operates an alarm. On the other hand, a serious fault causes a violent movement of oil which moves the floats, making other contacts which trip the main circuit breakers.
Figure 11.29 Currents in Y-Д transformer differential protection. OP = operate input; R = restraint input
Figure 11.30 Schematic diagram of Buchholz relay arrangement