Relaying and breaking: the nearest relay clears first

Drag the fault current and watch the two relays

The two curves are time-current characteristics. The lower (gold) is the downstream relay near the fault, the upper (blue) is the upstream backup. Drag the handle to raise the fault current. Both trip faster, but the near relay always first — that time gap is the grading margin. So the nearest breaker clears only the faulted section.

Faster for bigger current — inverse time

An overcurrent relay has an inverse-time characteristic where the operating time falls as the current rises. In the form t = TMS · k / ((I/Is)^α − 1), it acts slowly for a small overload just above the setting Is, and almost instantly for a large short of many times Is. The aim is to ride through a light overload but clear a dangerous fault fast. Tripping at once on small upsets would cause frequent needless outages.

Selectivity · the near one first

On a line with several relays in series, each relay’s time-current curve is stacked apart by a grading margin (typically about 0.3 second). Then, even seeing the same fault current, the downstream relay nearest the fault acts fastest and trips first, while the upstream relay acts that much later. If the near breaker clears properly the upstream stays put, but should that breaker fail the upstream trips as backup after the margin. Removing only the faulted section while the rest stays alive is selectivity, the foundation of the distribution reliability seen in C5.

The relay judges, the breaker acts

The relay is the brain that measures current and voltage to judge a fault; the breaker is the hand that, on its signal, actually opens the circuit. When the breaker parts its contacts an arc forms in the gap, and a quenching medium — vacuum, SF6 gas or oil — cools the arc fast to interrupt at a current zero. From fault to clearing must finish within a few cycles, a few tens of milliseconds. Any later and the large fault current burns equipment, and generators lose synchronism so the whole system shakes.

Back to the first screen

Raising the fault current shortened both curves’ operating times, yet the gold near relay was always before the blue backup, and the grading margin between them held. That time gap is exactly the secret of selectivity, where the nearest breaker clears only the faulted section and saves the rest. The fault current found in D3 became the input that moves the relay here, and as a result the “only the faulted section goes dark” seen in C5 actually happens. Protection is, in the end, opening the right breaker at the right instant.

We have seen the path of making, sending and protecting power — mostly assuming the huge synchronous generators of thermal, hydro and nuclear plants. The last section E is where that assumption changes. The next unit (PW-E1) looks at grid connection of solar and wind. An inverter pushes the renewable output, which comes as DC or variable frequency, into the grid’s frequency and voltage, and the intermittence of sun and wind and the lack of rotating inertia pose new challenges to stability.