Depending on the method of transformer earthing and fault location, some transformer earth faults result in only a small increase in phase current, which transformer differential protection may not detect. Conversely, the amount of current in the neutral may be sufficient to detect most or all earth faults, again depending on the earthing method. By connecting an REF relay to CTs installed in correct locations on the transformer, one can use REF protection to complement differential protection in detecting transformer earth faults. Obtaining maximum benefit from REF protection requires that one consider many factors, including whether to select high-impedance REF or low impedance REF relays. In making this selection, one should understand the theory behind each option.
Historically, only high-impedance REF protection was available, because of equipment and technology limitations. Today, numerical protection relays include low-impedance REF elements for transformer protection. Both types of protection have advantages and disadvantages; the relays do not perform equally well in all applications. One key advantage of low-impedance REF protection included in a numerical relay is the ability to use CTs with different ratios and specifications without the need for interposing CTs. One key advantage of high-impedance REF is proven immunity (relay security) to CT saturation for external faults. Key to either type of protection is the ability to provide maximum winding coverage against earth faults. There is also speculation, as yet unsubstantiated, that a high-impedance REF element provides superior sensitivity and coverage against earth faults.
To apply REF protection on star-connected transformers, connect the three-phase CTs in star, and connect this combination to a CT in the neutral leg of the transformer, NER or NEC, as shown in Fig.1. These CT connections provide a path for the zero-phase sequence currents to circulate in the CTs during external faults, but they force the current through the relay for internal faults. Therefore, the REF relay provides protection for all earth faults that fall in the area between the phase and neutral CTs. Any fault outside this area should be covered by alternative protection functions.
Fig. 2 shows an external earth fault on the star (source) side of a transformer, and Fig. 3 shows an in-zone fault on the delta side of a transformer. Currents are in per unit. In Fig. 2, the zero-sequence infeed for an upstream red phase-to-earth fault circulates between the neutral CT and the red-phase CT, and no operating current can flow through the relay element. In Fig. 3, in the case of the in-zone fault on the delta winding, it is clear that all the fault current flows through the neutral CT (IFp = Inp) and nothing flows in the phase CT. Therefore, the secondary CT current has to flow through the relay element for this element to operate. At this stage, we simplify the scenario by not taking into account the magnetization of the other CTs.
A hybrid REF protection function may also be applied to a delta winding without an NEC. The hybrid REF excludes the neutral CT from the circuit and the three phase CTs are all connected in parallel with the relay element. This is called a balanced earth-fault connection. The zone of protection is still only the delta winding of the transformer. The hybrid REF function can also be applied to an unearthed star winding.