GROUND FAULT PROTECTION OF POWER TRANSFORMERS
Faults in Δ-connected transformer windings
A residual relay, device 51N, as shown in Figure 8 and Figure 9, will detect ground faults within the Δ winding of the transformer and in the phase conductors between the CTs and the winding, when an external source of zero sequence current is available. Instantaneous overcurrent relays may be used, but sensitive settings will probably result in incorrect operations from dissimilar CT saturation and magnetizing inrush.
This can be avoided by using a short-time overcurrent relay with a sensitive setting. The scheme is particularly valuable in plants or systems where the transformers are remote from the circuit breakers. By using CTs at the circuit breaker, sensitive detection is obtained for cable, bus, Δ winding, and bushing faults.
A single window or doughnut CT supplying an instantaneous relay (as commonly used in motor protection) is secure, but is limited to cases of low and medium voltages where all three conductors can be fitted through the CT window.
Figure 8—Complete ground fault protection of a Δ-Y transformer using a residual overcurrent and differentially connected ground relay
Figure 9—Complete ground fault protection of a Δ-Y transformer using a residual overcurrent and directional relay
Faults in grounded Y-connected transformer windings
To successfully detect faults in grounded Y-connected transformer windings, the relay system should discriminate between faults internal and external to the protected zone. The ground differential relay, device
87G in Figure 8, typically an overcurrent relay, or the directional ground relay, device 67G, connected as in Figure 9, is satisfactory. Both relay schemes will operate correctly for any internal ground faults with the circuit breaker in the circuit to the grounded Y winding open or closed.
They will operate correctly with an external zero sequence current source, and they will not operate for external ground faults. The auxiliary CT is necessary if the phase and neutral CTs are of different ratio. Both schemes are particularly applicable where the ground fault current is limited and phase differential relays may not respond.
The device 67G operating coil or element current is zero for an external fault with CT ratios matched. Therefore, it is wise to select the auxiliary CT ratio to give positive nontrip bias to device 67G for an external ground fault (auxiliary CT secondary current slightly greater than the transformer neutral CT secondary current). Unequal CT action can produce residual error current during external phase faults. No transformer neutral current is produced and sensitive relays could misoperate.
On a grounded neutral system, it is possible to isolate the transformer case from ground except for a single point. A CT and overcurrent relay at this grounding point would detect any internal ground fault or bushing flashover.
Although effective, several problems are encountered. The system should be tested periodically to determine that no accidental grounds have been added.
Incorrect operations can result from accidental grounds from power tools and transformer auxiliary equipment. Careful coordination between auxiliary equipment circuit breaker or fuse curves, arrester characteristics, and a time overcurrent trip relay can minimize this danger.
Transformer differential relays may not be sensitive enough to operate on ground faults where the transformer bank or system is grounded through an impedance. In these cases, it may be necessary to apply a sensitive time overcurrent relay in the transformer impedance-grounded neutral or a time overvoltage relay connected across the neutral impedance.
These relays should be coordinated with any feeder and line protection relays that they may overlap. It is possible to provide high-speed protection and to avoid the need for coordination by using sensitive product-type relays, which are connected to trip only for ground in the protected zone.
An overcurrent relay connected to a neutral CT is torque-controlled by the blocking contacts of a plunger-type instantaneous relay in the neutral of the main breaker CTs. Since the transformer differential relays may not operate for such ground faults within the differential zone, these ground fault relays must trip the source-side circuit breakers.
Ground relays also used for sensitive ground fault protection
The primary advantage of ground relays over phase relays is their sensitivity. In systems where the ground fault current is purposely limited, their use may be vital. Ground relays can normally be applied with sensitivities of 10% or less of full load current.
This compares very favorably with differential relays, whose pickup current may be from 20–60% of full load current under the most advantageous conditions. It is common practice in the United Kingdom and other countries influenced by the U.K. to protect all power transformers with the restricted earth relay.
The term restricted earth is an expression referring to a sensitive ground relay system that is designed to detect ground faults within a well-defined protective zone (similar to that in Figure 8)