What a distance relay is (21)
A distance relay protects a line, not a point. It does not watch current: it measures V/I and obtains an impedance, which is proportional to the distance to the fault. That is why it is drawn on an R-X plane and not on a time-current curve, and why it does not share a screen with a 50/51.
| Zone | Typical setting | Time | What for |
|---|---|---|---|
| Z1 | 80% of the line | Instantaneous | Trips right away. It stops short on purpose: it must never see past the remote bus. |
| Z2 | 120% | ~0.3 s | Covers the 20% that Z1 does not see, and reaches into the next line. It waits, to give the relay over there its turn. |
| Z3 | 180% | ~1 s | Remote backup: if the next line's relay fails to clear, this one does. |
Your protections and the grid's: three different questions
Inside the plant, selectivity is achieved with time. Fuse, inverter, string box, feeder, incomer: every series pair has to be separated by the CTI, on the order of 0.2 to 0.3 seconds. The one below clears first and the one above waits. That is the entire content of a coordination table.
Looking outward, the question changes: a fault in your medium voltage has to be opened by your incomer before the transmission operator's relay at the point of connection decides the problem is theirs. If your step is slower than their step, they take out the whole line, and your fault becomes an outage for everyone on it.
And from the outside comes the one that gets forgotten most: a fault in the grid sinks the voltage at your point of connection, and your interface protections — 27 undervoltage, 59 overvoltage — will see that sag. If you trip, you disconnected the plant over somebody else's problem, and the grid code obliges you to ride it out. That is the ride-through envelope, and it is the reason the 27/59 settings are not chosen by looking at the plant alone.
What the transmission operator asks of you: infeed
Your plant injects current at the point of connection. For a fault further down the line, that injection changes what the grid's distance relay measures: the contribution entering between the relay and the fault makes it read more impedance than is really there, and its zone shortens. That is the under-reach effect, and it means their zone 2 stops covering what it covered before you existed. The interconnection study asks for exactly that number: how far each zone moved with the plant connected and without it.
What arc flash is, and why the calculation sometimes refuses to run
If an arc strikes inside a switchgear cubicle while somebody is working there, the question is not how much current flows: it is how much energy the person receives, in cal/cm². That defines the personal protective equipment and the approach boundary. The calculation consumes the bolted-fault current, the clearing time — that is, your own protection settings —, the conductor gap, the electrode configuration and the enclosure size. That is why arc flash runs after coordination and never before: if you change the coordination, the incident energy changes.
There are two legitimate reasons a bus may be left declared as not calculated:
- It is outside the range of the standard. The IEEE 1584-2018 model is empirical, fitted with tests between 208 V and 15 kV. A 33 kV bus falls outside, and the correct move is to refuse rather than extrapolate: a regression evaluated outside its range, printed in a report, is an invented number. For higher voltages, use the Lee method or the transmission operator's practice.
- A datum is missing that nobody can assume for you. The electrode configuration — VCB, VCBB, HCB, VOA, HOA — and the equipment class change the result substantially. Without that explicit choice, the result is not ratified and should not be printed.
The functions: the ANSI vocabulary
The functions are named by ANSI/IEEE C37.2 with a number, and that number is the vocabulary you use to talk to the transmission operator. The function lives in the relay; what interrupts is the breaker. A flawless 87T is worthless if the breaker does not open, and that is why 50BF and 74TCS exist.
| ANSI | What it does |
|---|---|
| 50 | Instantaneous phase overcurrent. Above the threshold, it trips without waiting. For the bolted fault close to the relay. |
| 51 | Time-delayed phase overcurrent, IEC 60255 inverse curve: more current, less time. It is the one graded with the CTI. |
| 50N / 51N | The same, but on the residual current: the sum of the three phases differing from zero indicates a ground fault. A much lower threshold, because a ground fault can be a few amperes and still kill. |
| 49 | Thermal image. It does not watch the peak, it watches accumulated heat. For transformers and cables, which withstand overload for a while and not a while longer. |
| 67 / 67N | Overcurrent with direction. Indispensable when current can come from both sides: a medium-voltage ring, or a plant that injects. Without 67, in a ring the wrong breaker opens and the healthy half is lost too. |
| ANSI | What it does |
|---|---|
| 87T | Transformer: compares what goes in with what comes out. If it does not balance, the fault is inside. Instantaneous and coordinated with nobody, because its zone is its own current transformers. |
| 87B | The same over a bus. |
| 87L | The same over a line, with a communications channel between the two ends. |
| 64 REF | Restricted earth fault: catches ground faults near the transformer neutral, exactly where the 87T loses sensitivity. |
| 21 | Distance: measures impedance, which is distance. For long lines, where the 51 no longer discriminates between here and there. |
| ANSI | What it does |
|---|---|
| 27 / 59 | Undervoltage and overvoltage. |
| 81U / 81O | Underfrequency and overfrequency. |
| 81R | Rate of change of frequency (df/dt): detects that the plant was left in an island. |
| Anti-islanding | The combined function: do not keep feeding a section the grid has already let go. |
| ANSI | What it does |
|---|---|
| 24 | Overexcitation (V/Hz): high voltage or low frequency saturates the core. |
| 63 | Buchholz: a mechanical gas and pressure relay. An internal arc generates gas before it becomes measurable current. It is the one that saves the transformer. |
| 25 | Synchronism check before closing: that the two ends are in phase. |
| 79 | Automatic reclosing. Most overhead-line faults are fleeting. |
| 50BF | Breaker failure: a trip was ordered and current keeps flowing, so trip the ones upstream. |
| 60FL | Voltage transformer supervision. If the voltage signal is lost, a 21 or a 67 starts lying: block them, do not let them opine. |
| 60CTS | The same over the current transformer. |
| 74TCS | Trip circuit supervision: verifies the wire to the trip coil is healthy. A trip that does not arrive is having no protection, and without this function nobody finds out until the day of the fault. |
The apparatus: what actually interrupts
| Level | Apparatus |
|---|---|
| DC | String fuse · load-break switch · surge arrester · insulation monitor |
| Low voltage | Moulded-case breaker · air circuit breaker · fuse · surge arrester · residual-current device |
| Medium voltage | Circuit breaker · load-break switch · disconnector · earthing switch · fuse · surge arrester · recloser · ring main unit · neutral earthing resistor · zig-zag earthing transformer |
| High voltage | Circuit breaker · disconnector · earthing switch · surge arrester |
| Measurement | Protection relay · current transformer · voltage transformer · billing meter · power-quality meter |
The last two in medium voltage interrupt nothing: the neutral resistor and the zig-zag define how the system is earthed, which is what decides how much ground-fault current exists — and therefore whether the 51N can see it. It is a project decision taken before a single relay is chosen.
And the third concept, the one you count when you talk about coverage: the protectable elements.
- DC string · combiner box · inverter AC output · low-voltage bus.
- Medium-voltage block transformer · medium-voltage feeder · medium-voltage collector bus.
- Step-up transformer · high-voltage bus · high-voltage line to the point of connection.
The typical scheme, level by level
| Where | What it carries |
|---|---|
| String | Fuse, only if there are three or more strings in parallel (IEC 62548) |
| DC side | Insulation monitor and arc-fault detection |
| Inverter AC output | Moulded-case or air circuit breaker |
| MV block transformer | 50/51 + 50N/51N · 63 · 87T if it is large |
| MV feeder | 50/51 + 50N/51N · 67 if it is a ring |
| MV collector bus | 87B · 50BF |
| Step-up transformer | 87T · 64REF · 63 · 49 · 24 |
| HV bus and line | 21 · 87L · 79 · 25 · 50BF · 60FL |
| Point of connection | 27/59 · 81U/81O/81R · anti-islanding |
The last row is the only one negotiated with the transmission operator: those are the settings that go into the protection coordination settlement. Everything above it belongs to the project, and its engineering defines it.