Engineer's Notes: protection coordination
Fast where it must be, slow where it should be
Good protection clears every fault quickly with the smallest possible outage. The device closest to the fault trips first; each device upstream waits a grading margin longer as backup. On a time-current chart that means curves stacked with clear gaps at every fault level that matters.
The transformer sits in the middle
The HV relay must protect the transformer against faults and through-fault damage, ride through magnetising inrush, and still wait for the LV ACB on LV faults. Its high-set element goes above the LV fault seen on HV and the inrush; its inverse element grades above the ACB.
Unit protection for bigger transformers
Differential (87T) and restricted earth fault (REF) only see faults inside their zone, so they trip instantly without grading. They are common on transformers above about 2.5 to 5 MVA and on critical supplies such as hospitals and data centres.
Know your network earthing
Earth fault protection depends completely on how the HV neutral is earthed. Solid, resistance, resonant and isolated systems need different relays and settings. Always get the utility earth fault level and earthing method before setting 51N.
Frequently asked questions
What grading margin is needed between protection relays?
A coordination time interval of about 0.3 to 0.4 s between numerical relays is common. It covers the downstream breaker opening time, relay overshoot and timing errors. Between a relay and a downstream LV breaker or fuse, smaller margins may be used, but utilities often state their own minimum.
Why does the HV relay need a high-set element above the LV fault current?
The high-set (instantaneous) element should trip quickly for HV faults but must not see faults on the LV side, which the LV breaker should clear. Setting it above about 1.3 times the LV terminal fault current referred to HV, and above transformer inrush, keeps it selective.
How does transformer differential protection avoid tripping on inrush?
Magnetising inrush flows only on the energised side, so it looks like an internal fault. Inrush is rich in second harmonic, so the relay blocks when the second harmonic exceeds a set percentage of the fundamental, typically 15 to 20 percent.
What is restricted earth fault protection?
Restricted earth fault (REF) compares the sum of the phase CTs with a CT in the transformer neutral. It only sees earth faults inside the protected winding, so it can be set very sensitive and fast. The high-impedance version needs a stabilising resistor and CTs with enough knee-point voltage.
Why use a short-time delay on the LV main ACB?
The short-time delay lets the downstream MCCB clear a feeder fault first, so only that feeder is lost. The ACB still clears a busbar fault after the delay, and the HV relay is graded above it as backup.
Topics: relay coordination · TCC · IDMT curves · TMS · high-set · grading margin · discrimination · inrush · transformer damage curve · ACB LSI · MCCB · 87T differential · bias characteristic · 2nd harmonic blocking · REF · stabilising resistor · metrosil · earth fault · network earthing
Related labs
