Relaying and breaking: the nearest relay clears first

Drag the fault current and compare the two relays

The two curves are the relays' time-current characteristics: the horizontal axis is the current as a multiple of the pickup setting, I/Is (log scale), and the vertical axis is the operating time t. The lower gold curve is the downstream relay nearest the fault, and the upper blue curve is the upstream backup relay. Drag across the figure to raise the fault current and both dots come down together, so both relays get faster, but the gold dot always stays below, so the near relay operates first. The time gap between the two dots is the coordination interval, and thanks to it the closest breaker clears only the faulted section.

Faster for bigger current — inverse time

An overcurrent relay has an inverse-time characteristic: the larger the current, the shorter its operating time. The curves in this figure use the IEC standard inverse form t = TMS · k / ((I/Is)^α − 1) with k = 0.14 and α = 0.02, so for an overload just above the pickup setting Is the relay waits 1 to 3 seconds, while for a short circuit of 20 times Is it operates within 0.2 to 0.5 seconds. The aim is to ride through light overloads for a while and cut only dangerous faults quickly; tripping at once on every small disturbance would cause needless outages.

Selectivity · the near one first

On a line with several relays in series, each relay further upstream is set with its curve raised so that it operates later. In this figure the backup relay has a TMS of 0.2, twice that of the near relay (0.1), so at any current it takes exactly twice as long. So even when both see the same fault current, the downstream relay trips first, and only if its breaker fails does the upstream relay trip as backup after the coordination interval. The gap shrinks as the current grows, though: 0.33 s at the default 8 times, but only 0.23 s at 20 times. Coordination is checked by whether a margin of about 0.3 s remains at the largest fault current, so where the fault current can reach 20 times, this setting would need a higher backup TMS. Cutting out only the faulted section and keeping the rest alive in this way is selectivity, and it is the basis of the distribution reliability seen in C5.

The relay judges, the breaker acts

The relay is the brain that measures current and voltage and decides whether there is a fault; the circuit breaker is the hand that receives its signal and actually opens the circuit. When the breaker's contacts part, an arc forms in the gap, and an interrupting medium such as vacuum, SF6 gas or oil cools the arc so that the current is cut as it passes through 0. The breaker itself opens within a few cycles, tens of milliseconds, after receiving the trip signal, and primary protection on transmission lines clears a fault within about 0.1 s including the relay's decision. Overcurrent relays that deliberately add delay, as in this figure, are used mainly on distribution feeders and for backup, and even that delay cannot be stretched freely, because the longer a fault lasts, the more the large fault current damages equipment and the more likely generators are to lose synchronism and shake the whole system.

Back to the first screen

As the fault current rose, both dots came down and both operating times shortened, yet the gold near relay always stayed below the blue backup relay, which means it always came first. The coordination interval between them shrank as well, which is why the setting is chosen by checking that this margin remains at the largest fault current. That time gap is what lets the closest breaker clear only the faulted section and keep the rest alive. The fault current found in D3 became the input that drives the relays here, and as a result the “outage confined to the faulted section” seen in C5 actually happens; protection, in the end, is opening the right breaker at the right moment.

So far we have followed the path of generating, sending and protecting power, all on the assumption of large synchronous generators driven by thermal, hydro and nuclear plants. The final section E is where that assumption changes, and the next unit (PW-E1) takes up grid connection of solar and wind. Inverters push renewable output, which comes out as DC or at variable frequency, into the grid matched to its frequency and voltage, and the intermittency of sun and wind together with the lack of rotating inertia poses new challenges for stability.