A restricted earth fault (REF) protection element provides sensitive detection of ground faults near the neutral of a transformer wye-connected winding. The sensitivity gains from the REF element are especially more valuable if the equipment is low-impedance grounded. Security of the REF element may be jeopardized by external faults, such as a phase-to-phase-to-ground or an evolving fault with CT saturation.
Figure 1 illustrates how REF protects a wye-connected transformer winding. If a ground on a winding is internal to the transformer, then 3I0 and IN are in phase due to the corresponding CT polarities. If these two residual currents are in phase, the protection operates.
Note that there is typically also a nondirectional REF element (Figure 2) to cope with situations such as when the breaker supplying the phase currents is open (i.e., 3I0 equals zero) since this condition prevents the directional element from operating.
DEPENDABILITY IMPROVEMENTS
Improvement to existing logic (see Figure 5a) is shown in the green boxes for Figure 3 and Figure 4. An angle greater than 105 degrees with respect to the difference between 3I0 and IN indicates that the fault is external to the REF element zone of protection. The angular value has been increased to better handle scenarios with capacitive fault current contribution, such as from a medium-voltage cable network with intermittently bonded ribbon (IBR) cables.
Relay Word Bits
- REFnFP ≡ Ground fault declared internal
- REFnRP ≡ Ground fault declared external
- NDREFn ≡ Nondirectional REF Element n enabled
- RFnTCE ≡ REF Directional Element n enabled (sufficient levels of 3I0 and IN)
- REFBLKn ≡ REF Element n phase fault or external ground fault detected
- REFRn ≡ Earth Fault Outside REF Element n zone
- REFFn ≡ Directional REF element n declares an internal fault (via directional or nondirectional logic)
REF50Gn is a pickup setting that determines the residual current sensitivity. The minimum acceptable value of REF50Gn must be greater than any natural 3I0 unbalance resulting from load conditions, steady-state CT, and relay errors. To enable REF element n, IN must be greater than this pickup, and 3I0 must be greater than 80 percent of this pickup.
SECURITY IMPROVEMENTS
The FLTPn path in Figure 3 blocks REF element operation for faults that do not have ground involvement to avoid security issues from multi-phase faults. This is accomplished by checking for a sufficient level of negative-sequence current (3I2) and significantly less 3I0. All multi-phase faults produce 3I2; three-phase faults develop 3I2, but negligible 3I0, during the transient measurement period when fault initiation occurs since each faulted phase current develops differently.
External multi-phase faults that involve ground can reduce the security of the REF element. One such scenario is an external phase-to-phase fault that evolves to the ground. Once ground is involved and there is sufficient neutral current, CT saturation can occur prior to ground involvement, and the REF element can lose security as a result.
Security for external phase-to-phase-to-ground faults is improved by using REFBLKn as was seen in Figure 3. CTs take time to saturate; therefore, REFnRP should initially assert for external faults involving ground. Once REFnRP asserts for an eighth of a cycle, REFBLKn asserts, which blocks and secures the REF element for 1 second to address CT saturation.
CONCLUSION
Figure 5a is the corresponding logic in the SEL-487E instruction manual that combines the logic from Figure 3 and Figure 4. Figure 5b zooms in on the blocking logic for CT saturation. If the magnitude of 3I2 is greater than the pickup setting REF50G1 and is ten times or greater than the zero-sequence current in the wye-connected winding, the REF element is blocked from operating for the detection of an internal fault. IOPNRF1 and negated IRFFR1P identify the condition in Figure 5c in which current flows in the neutral but no current flows in the line (i.e. top left and gate).
Relay Word Bits
- REF 1 ≡ REF Element 1 enabled
- IQFR1 ≡ Negative-sequence reference quantity (derived from protected winding)
- REFBLK1 ≡ CT saturation detected, or fault is external
- REFR1 ≡ External fault detected by REF Element 1
- IOPNRF1 ≡ REF Element 1 neutral current
- IRFFR1 ≡ REF Element 1 winding zero-sequence current
- NDREF1 ≡ Nondirectional REF Element 1 enabled
- TCREF1 ≡ Torque Control for REF Element 1
- REFTQ1 ≡ REF Element 1 Torque Quantity
- THRES1P ≡ REF Element 1 internal fault detected if REFTQ1 is greater than the forward threshold (+105o)
- THRES1N ≡ REF Element 1 external detected if REFTQ1is less than the reverse threshold (-105o)
- REFF1 ≡ REF Element 1 declares an internal fault (directional or nondirectional)
Settings
REF50G1 ≡ Residual Current Pickup for REF Element 1
REFERENCE
Schweitzer Engineering Laboratories. SEL-487E-3,-4 Transformer Protection Relay Instruction Manual, Date Code 20240509.
Steve Turner is a Consultant at Sargent & Lundy. He was previously in charge of system protection for the Fossil Generation Department at Arizona Public Service Company for five years. Turner formerly held positions at Beckwith Electric Company, GEC Alstom, SEL, and Duke Energy, where he developed the first patent for double-ended fault location on overhead high-voltage transmission lines and was in charge of maintenance standards in the transmission department for protective relaying. He has BSEE and MSEE degrees from Virginia Tech University. Turner is an IEEE Senior Member and a member of the IEEE PSRC and has presented at numerous conferences.