Overcurrent Protection

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What is Overcurrent Protection?

Overcurrent protection is widely applied at all voltage levels
to protect lines,transformer, generators, and motors from current overloads. They comes in the form of removable protection modules(relays).


What is Overcurrent Protection Relay?

Think of it as an expensive fuse with some added functions.

Basically, Overcurrent Proctection Relay consists of 2 types :

  • Instantaneous
  • Time-Delay

Instantaneous Overcurrent Relays

  • Operate without any intentional time delay.
  • Used for faults close to the source when the fault current is very high.
  • Can detect and respond to a fault in a few cycles.

Time-Delay Overcurrent Relay

  • Operate with an adjustable time delay.
  • The actual time delay depends on the current through the relay coil.
  • Higher currents will cause a faster operation of the relay.
  • Minimum current (at which the relay will respond ,called the pick-up value) is also

How It Works

Instantaneous overcurrent protection is considered the simplest protection scheme.
It is widely used because of its quick reaction time.

How It Works

Transient sag/swell is not an uncommon phenomenon in power systems, especially in
high voltage transmission systems. The instantaneous overcurrent relay will usually treat
tolerable transients as faults and trip the circuit breaker. It is therefore necessary to add
some time delay to allow the transients to decay. This is when a time-delay relay is used.
Coincidently, a typical electromechanical relay has some degree of time delay.
A reasonable time delay is necessary so that relays can be coordinated with each other.

The current is made to pass through the protective relay and was constantly monitored. The overcurrent protection would activate should the input current reaches the pick up value set by the relay.

The relay pick up value is commonly set to a value anywhere between 125-135%
of the maximum load current and 90% of the minimum fault current.
These values help to minimize unnecessary responses from the relay.

The calculation of the pick-up value is shown here

The picture below describes a radial distribution linethat has three major segments.

Notice that each segment has one overcurrent protection relay connected to it.

The protection system should be designed to satisfy the following requirements:

  1. Under normal conditions the breakers are not tripped
  2. Under fault conditions only the breaker closest to the fault will trip
  3. If the closest breaker fails to operate, then the next breaker closest to the fault
    should trip.

For example, if a fault occurs at the third segment, then breaker 3 should trip. Breaker 1
and breaker 2 should remain closed so that power can still be delivered to loads on the
first two segments. But if breaker 3 fails to trip, breaker 2 should trip after a time delay
while breaker 1 remains closed.

There are two settings that must be applied to all time-delay overcurrent relays:

  1. the pickup value and
  2. the time delay.

Time-delay overcurrent relays are designed to
produce fast operation at high current and slow operation at low current; hence, an
inverse time characteristic. Relays from different manufactures may have different
inverse time characteristics.