Section 6

Feeder Overcurrent
Protection
Feeder Overcurrent Protection

By far the most common type of protection for radial

distribution feeders is overcurrent protection. Typical
distribution system voltages are 44 kV, 33 kV & 25 kV.
The point of supply is normally a few kilometres from the
load.
Feeder Overcurrent Protection

The ideal way of protecting any piece of power system

equipment is to compare the current entering that piece of
equipment, with the current leaving it. Under normal healthy
conditions the two are equal. If the two currents are not equal,
then a fault must exist. This ‘Differential Protection’ principal
will be covered later when we discuss bus protection, and
transformer protection, etc. It is not economic or practical to
provide a communication channel between the ends of a feeder
to enable the currents entering and leaving the feeder to be
compared.
Feeder Overcurrent Protection

With radial feeders there is only one possible point of

supply, and the flow of fault current is in one direction
only. Overcurrent protection can therefore be used to
provide adequate protection.
Feeder Overcurrent Protection

The current entering the feeder at the circuit breaker is

measured by means of a Current Transformer located at
the base of the breaker bushing. The C.T. secondary
current is supplied to the overcurrent relays. These
overcurrent relays must then operate and initiate tripping
if a fault condition is detected on the feeder.
Feeder Overcurrent Protection
Feeder Overcurrent Protection

The overcurrent protection at the supply end of the

feeder must operate for all faults on the feeder, but
should not operate for faults beyond the remote station
‘B’. If we first consider an instantaneous overcurrent
relay, then the setting is determined by the magnitude of
the fault current at the end of the feeder.
Feeder Overcurrent Protection

Let us assume that the fault current at that point is 4800

amps. Ideally the relay will be set for 4800 primary amps,
(or amps x secondary amps) and it should not
operate for any fault beyond the bus at the remote
station.
Feeder Overcurrent Protection

However, in practice it is not possible to be so precise for

the following reasons:
a. It is not possible for the relay to differentiate
between faults which are very close to, but which
are on each side the Bus ‘B’, since the difference in
the currents would be extremely small.
b. Inaccuracies in the C.T’s and relays, and the effects
of distortion of the current waveform under
transient conditions produce errors in the response
of the protection scheme.
Feeder Overcurrent Protection

c. The magnitude of the fault current cannot be

accurately established since all of the parameters
may not be known, and the source impedance of the
power system changes as generators are put in and
out of service.
Feeder Overcurrent Protection
One solution to this problem is to set the instantaneous
overcurrent relay to ‘overreach’ the remote terminal,
(i.e. a setting less than 4800 primary amps), and
introduce a definite time delay in the tripping. This time
delay will allow the fuses or overcurrent relays at the
remote station to operate to clear faults beyond bus ‘B’
before the time delayed tripping can take place at the
supply station ‘A’. This type of time delay has the major
disadvantage that all faults will be slow clearing, even
very ‘close-in’ faults, which have the highest magnitude
of fault current.
Feeder Overcurrent Protection

This time-delayed clearing of high fault currents is usually

unacceptable, and the most common feeder protection
scheme, which overcomes the problem utilizes an inverse
time overcurrent relay in conjunction with the
instantaneous overcurrent relay. The application of this
feeder protection scheme, utilizing both instantaneous
and inverse time overcurrent relays is described next.
Feeder Overcurrent Protection

In order to ensure that the instantaneous overcurrent

relay will not unnecessarily operate for faults at the
remote station, (which should be cleared by the
overcurrent protection or fuses at that station) then it
must be set to protect only part of the feeder. A safe
maximum for most types of relay is 80% of the feeder
length.
Feeder Overcurrent Protection

The limit is determined by the characteristics of the relay

used, and the length of the feeder. If the feeder is long a
high percentage of the line can be protected; but with
short lines it may be less; and with very short lines it may
not be possible to apply instantaneous overcurrent
protection.

This type of protection is known as High-Set

Instantaneous overcurrent protection.
Feeder Overcurrent Protection

With such a relay set to detect faults on 80% of the

feeder, the remaining 20% is left unprotected. This is, of
course, not acceptable. To provide protection for the last
20% of the feeder a time-graded, or inverse definite
minimum time relay can be used.
Feeder Overcurrent Protection

This type of relay provides timed overcurrent protection,

and maintains coordination with the fuses or overcurrent
relays at the remote station. The operating time of the
relay is inversely proportional to the current.

i.e. For very high fault currents the relay will operate in
it’s minimum time; and for fault currents only slightly
above the relay pick-up current there will be a very long
operating time.
Feeder
Overcurrent
Protection

The ‘Inverse definite minimum time’ relay has a

characteristic as shown above.
Feeder Overcurrent Protection
Feeder Overcurrent Protection
If we superimpose the fuse characteristic of one of the
transformer fuses at the remote station, onto the above
overcurrent relay characteristic, we can see how the
relay settings at the supply station are coordinated with
the transformer fuse. With this scheme of protection,
utilizing High-Set overcurrent relays, Inverse Definite
Minimum Time overcurrent relays, and fuses, we will
consider the response of the protection scheme to faults
at various locations.
Feeder Overcurrent Protection
Feeder Overcurrent Protection
1. For a Fault at point X on the feeder, ONLY the High-
Set Instantaneous overcurrent relay will operate and
clear the fault with no intentional time delay.

2. For a fault at point Y on the feeder, it is beyond the

‘reach’ of the High-set instantaneous relay, therefore
that relay will not operate. The inverse timed
overcurrent relay will operate after a time delay
determined by the magnitude of the fault current and
the relay characteristic.
Feeder Overcurrent Protection

3. For a fault at point Z , it is again beyond the ‘reach’ of

the Highset Instantaneous relay. The Inverse Timed
overcurrent relay will start timing, but the fuse on the
feeder Fl will operate first and clear the fault. The
inverse timed overcurrent relay at station ‘A’ will then
reset.
Feeder Overcurrent Protection
Feeder Overcurrent Protection
Now let us look at a typical utility feeder which supplies
customer transformers at many different points along it’s
length. The same High-Set Instantaneous Overcurrent
and Inverse Timed Overcurrent relays are used, and the
H.S. relay must be set such that it does not operate for
faults beyond the first tap. The High-Set relay will
therefore be set to operate for faults up to 80% of the
distance to the first tap.
Feeder Overcurrent Protection
The criteria used for setting the Inverse-Timed
Overcurrent relay are:
1. The relay must not operate for the maximum load
current that will be carried by the feeder.
2. The relay setting must be sensitive enough for the
relay to operate and clear faults at the very end of
the feeder.
3. The relay operating characteristic must be set to
coordinate with other protection devices, such as
fuses, ‘downstream’ from the supply station.
Feeder Overcurrent Protection

This type of protection scheme will provide adequate

protection for feeders. However, there are some
disadvantages with this arrangement, particularly on long
overhead feeders. The main disadvantage is that most
faults will be slow in clearing because the inverse time
overcurrent relay must operate. This slow fault clearing is
usually disturbing to customers on the affected feeder.
Feeder Overcurrent Protection

As mentioned earlier, there is a very high incidence of

faults caused by lightning on overhead feeders,
particularly at the lower distribution voltages.
Consequently, the great majority of faults on such
feeders are transient in nature, and can be cleared by
opening the breaker, with no permanent damage
resulting.
Feeder Overcurrent Protection
Protection schemes for this type of feeder can be
enhanced by adding a Low-Set Instantaneous
Overcurrent relay, and providing Auto-Reclosing of the
circuit breaker after fault clearance. The low set
instantaneous overcurrent relay is set to operate for the
minimum fault current at the very end of the feeder. This
means that it will ‘Overreach’, and operate for faults in
the transformers tapped on the feeder. All faults will
therefore be first detected by the Low-Set relay.
Feeder Overcurrent Protection

This relay then trips the breaker, and also initiates Auto-
Reclose. For about 90% of the faults this auto-reclose will
be successful, and the interruption to the customers is for
only about 0.5 seconds. If, however, the fault is
permanent, such as a broken pole or a tree on the line,
then the auto-reclose will be unsuccessful. After the
circuit breaker has autoreclosed the tripping from the
Low-Set overcurrent relay is disabled for 10 seconds. This
means that proper protection coordination will then take
place: i.e.
Feeder Overcurrent Protection
1. If the fault is in a transformer, then the fuse will
blow to isolate only the faulted transformer, and
leave the remainder of the feeder in service.
2. If the fault is on the feeder, beyond the first tap,
then the inverse timed overcurrent relay will
operate after a time delay, and the feeder will trip
a second time and “Lock Out”.
3. If the fault is close to the supply station, then the
High-Set overcurrent relay will operate and trip
the feeder a second time, with no intentional time
delay, and ‘Lock Out’.
Feeder
Overcurrent
Protection
H.S. INV. T. L.S.
R

W
H.S. INV. T. L.S.
B 30A

H.S. INV. T. L.S.

N
30A
Feeder Overcurrent Protection
A typical feeder overcurrent a.c. schematic diagram,
showing all three phases, is shown above. The diagram
includes High-Set Instantaneous, Inverse Time, and
Low-Set Instantaneous relays. Very often the high-set
instantaneous and inverse time overcurrent relays are
built into a single relay case. Until a few years ago, all of
these relays were electro-mechanical, and often in
separate relay cases. i.e. The h.s. instantaneous -
attracted armature, and the inverse time - induction
disc. More recently electronic relays were used, and the
settings are applied by changing the position of ‘DIP’
switches. These electronic overcurrent relays were much
more compact, and were functionally identical to the
electro-mechanical overcurrent relays.
Feeder Overcurrent Protection
Today, almost all overcurrent relays being installed are
microprocessor- based, and have many functions in the
one relay. As well as the protection functions described,
these relays have many more features available, such as
event recording, waveform capture, fault location and
frequency trend load-shedding. These features of modern
microprocessor-based relays will be discussed later.
Feeder
Overcurrent
Protection
Feeder Overcurrent Protection
The d.c. tripping circuit for such an overcurrent
protection scheme is shown above: A typical 27.6 kV
feeder arrangement is shown on the next page. The fault
levels at various points on the feeder are indicated, and
the overcurrent protection settings are shown.

The protection coordination curves for the various relays

and fuses are also shown.
Feeder Overcurrent Protection
Feeder Overcurrent Protection
Criteria for Setting the Inverse-Timed
Overcurrent Relay
1. The relay must not operate for the maximum load
current that will be carried by the feeder.
i.e. cold load pick-up and back-to-back feeder
loads
2. The relay setting must be sensitive enough for the
relay to operate and clear faults at the very end of
the feeder.
3. The relay operating characteristic must be set to
coordinate with other protection devices, such as
fuses, ‘downstream’ from the supply station
Criteria for Setting the High-set
Instantaneous Overcurrent Relay

1. The relay must be set to operate for faults up to,

but not beyond, the first tap from the feeder.
2. In practice, the relay is set to operate for faults up
to 80% of the distance to the first tap.
3. This provides high-speed clearance for the high
level faults close to the supply station.
Criteria for Setting the Low-set
Instantaneous Overcurrent Relay

1. The relay must operate for all faults on the feeder,

right up to the feeder end.

This provides high-speed initial clearance for all faults

on the feeder.

For 10 seconds after the feeder breaker autorecloses,

the tripping from the low-set relay is blocked.
Directional Overcurrent Protection
If there is generation connected to a distribution feeder,
the system is no longer RADIAL.

Fault current can then flow in either direction — into the

feeder from the power system or out of the feeder from
the generator

A directional relay or element must be used to supervise

the overcurrent relay elements to allow the overcurrent
protection to trip only if the fault current flows into the
feeder from the power system.
Directional Overcurrent Protection

Overcurrent protection is used extensively on radial

distribution systems, where the fault current can only
flow in one direction. If there is generation connected to
a distribution feeder, then fault current can flow in
either direction, and the system is no longer radial. If the
generation is large (typically above about 5 MW) in
comparison to the normal load on the feeder, then the
feeder overcurrent protection at the supply station
requires directional supervision.
Directional Overcurrent Protection

A directional relay or element is used to supervise the

overcurrent relay elements to allow the overcurrent
protection to trip only if the fault current flows into the
feeder from the power system. The directional relay
prevents tripping if fault current from the generator
flows out from the feeder to a fault elsewhere on the
power system.
Testing of Feeder Overcurrent Protection

The individual C.T’s of the feeder overcurrent protection

scheme are tested as described earlier. With overcurrent
protection the C.T. polarity is not of critical importance.
However, the relative polarity of all three phases must be
the same. The individual relay elements are tested by
injecting a variable test current into the C.T. secondary
circuit, via links or switches on the front of the relay
panel.
Testing of Feeder Overcurrent Protection
The ‘pick-up’ current of the instantaneous relays is
verified, and for the inverse time relays 3 or 4 values of
current are injected, and the relay operating time is
verified in comparison to the relay characteristic curve.

With the feeder ‘on-load’, the current in the C.T.

secondary circuit should be measured, and compared to
the indicating ammeter readings, and with the secondary
current from the C.T’s on the opposite side of the circuit
breaker.
Microprocessor-Based Feeder Protection Relays

Most feeder protection relays being installed today are

microprocessor-based, and include many functions within
the one relay.

As well as the basic instantaneous and inverse- timed

overcurrent functions, these relays also include many
other protection functions and additional features.
Microprocessor-Based Feeder Protection Relays

• Directional Supervision
• Undervoltage and Overvoltage
• Bus underirequency & Rate-of-change
• Synchronism Check
• Negative Sequence Voltage
• Auto-reclose
• Event Recording
• Oscillography, or Waveform Capture
• Fault Location
COURSE RECAP