Installation, Maintenance, Testing &
Commissioning of Substations
A Study of Power System Protection by Overcurrent Method
situation where overload current causes overheating of the
protected equipment.
Power System protection:
is a branch of electrical power engineering that deals with the protection of
electrical power systems from faults through the isolation of faulted parts from
the rest of the electrical network. The objective of a protection scheme is to
keep the power system stable by isolating only the components that are under
fault, while leaving as much of the network as possible still in operation.
The overcurrent protection is a bigger concept so that the overload
protection can be considered as a subset of overcurrent protection.
The overcurrent relay can be used as overload (thermal) protection when
protecting the resistive loads, etc., however, for motor loads, the overcurrent
relay cannot serve as overload protection. Overload relays usually have a
longer time setting than the overcurrent relays.
1. Types of protection Schemes Protection schemes can be divided into two
major groupings.
3. Types / Characteristics of Overcurrent Relays
1.1 Unit Type Protection:
2.3 Sensitive Earth Fault Protection:
Unit type schemes protect a specific area of the system, i.e., a transformer,
transmission line, generator or bus bar. The unit protection scheme is
based on Kirchhoff's Current Law, sum of all the currents entering and
leaving a junction must be equal to zero. Any deviation from this must
indicate a fault current. In these schemes, the effects of any disturbance or
operating condition outside the area of interest are totally ignored and the
protection is stable above the maximum possible fault current that could
flow through the protected area.
1.2 Non-Unit type protection:
The non-unit schemes, while also intended to protect specific areas but
have no fixed boundaries. As well as protecting their own designated areas,
the protective zones can overlap into other areas. While this can be very
beneficial for backup purposes i.e an area to be isolated if a fault is
detected by non-unit schemes.
The sensitive earth fault relay is a protective device that works by measuring the
residual current across the three phases and neutral (if available) in a system.
This is done using a Core Balance Current Transformer (CBCT). In the ideal
condition, the residual current will be zero as all the currents flowing through
the three wires and neutral (if available) and their magnetic fields cancel each
other out.
Definite time relay operates instantaneously when the current reaches a
predetermined value. Operates in a definite time when current exceeds its Pickup value. Its operation criterion is only current magnitude (without time delay).
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2.1 Phase Overcurrent Protection:
It is intended to protect equipment when an insulation fault occurs, for
instance a direct contact between a live conductor and earth. In such a
situation, great fault currents will flow back to the transformer through its
neutral point when it is connected to earth.
Prepared By: Babar Shahzad
Instantaneous Overcurrent relay
In the event of a fault, the residual current over the three phases will not be
equal to zero as the current from the faulted phase flows through the earth.
Normally used on systems where the earth fault current is limited to a very low
value (low or high impedance earthed systems) or if sensitive protection is
required.
2. Overcurrent Protection
2.2 Earth Overcurrent Protection:
4.
2.
The most simple of these schemes measures current and incorporates an
inverse time characteristic into the protection operation to allow
protection nearer to the fault to operate first.
This is the simplest of the ways to protect a line and therefore widely used.
In the event of fault the current would increase between phases to a value
several times greater than maximum load current. It has a limitation that
it can be applied only to simple and non-costly equipments.
3.
Instantaneous Overcurrent Relay
Definite Time Overcurrent Relay
Inverse Time Overcurrent Relay (IDMT Relay)
1.
2.4 Primary requirement of Overcurrent protection
The protection should not operate for starting currents, permissible
overcurrent, and current surges. To achieve this, the time setting is
provided. The protection should be co-ordinate with neighboring
overcurrent protection.
Operating time is constant.
There is no intentional time delay.
It operates in 0.04s (40ms) or less (20ms)
4.1 Definite Time Overcurrent Relay
In this type, two conditions must be satisfied for operation (tripping), current must
exceed the setting value and the fault must be continuous at least a time equal to
time setting of the relay.
2.5 Difference between Overcurrent and Overload protection
Overcurrent protection protects against excessive currents or
currents beyond the acceptable current ratings, which are
resulting from short circuits, ground faults and overload
conditions. While, the overload protection protects against the
Date: 21-03-2022 Rev:02
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A Study of Power System Protection by Overcurrent Method
Installation, Maintenance, Testing &
Commissioning of Substations
adjusting the 'time dial setting / time multiplier'. The lowest time dial setting (fastest
operating time) is generally 0.05 and the slowest is 10.
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Relays may contain more than one stage of protection, each stage includes
each own current and time setting.
For Operation of Definite Time Overcurrent Relay operating time is
constant
2. Its operation is independent of the magnitude of current above the pickup value.
3. It has pick-up and time delay settings, desired time delay can be set.
4. Easy to coordinate.
5. Constant tripping time independent of in-feed variation and fault location
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Operates when current exceeds its pick-up value.
Operating time depends on the magnitude of current.
It gives inverse time current characteristics at lower values of fault current and
definite time characteristics at higher values
An inverse characteristic is obtained if the value of time dial or multiplier is
below 10, for values between 10 and 20 characteristics tend towards definite
time characteristics.
Widely used for the protection of distribution lines.
1.
4.1.1 Definite time overcurrent relay is used as:
Back up protection of distance relay of transmission line with time delay.
2. Back up protection to differential relay of power transformer with time
delay.
3. Main protection to outgoing feeders and bus couplers with adjustable
time delay setting.
4.2.1 Type of Inverse Curves;
There are several inverse curves, faster or slower with different time
multipliers (Also called Time dial TD or time multiplier k). The most popular
curves are,
• Normal Inverse or Standard inverse (Mostly used in Pakistan)
• Very Inverse
• Extremely inverse.
1.
With higher time multiplier setting, protection operation is slower while with
lower time multiplier protection operation is faster
4.2 Inverse Time Overcurrent Relays (IDMT Relay)
IEC Normal inverse curve with different time multipliers (k) or (TD)
In this type / characteristic of relays, operating time is inversely changed with
current. So, high current will operate overcurrent relay faster than lower
ones. There are mainly standard inverse, very inverse and extremely inverse
types and several other manufacturers customized.
Inverse Time relays are also referred to as Inverse Definite Minimum Time
(IDMT) relay.
The operating time of an overcurrent relay can be moved up (made slower) by
Prepared By: Babar Shahzad
Date: 21-03-2022 Rev:02
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Installation, Maintenance, Testing &
Commissioning of Substations
A Study of Power System Protection by Overcurrent Method
4. Time Graded Over Current Protection
This may also be referred simply as over-current protection of electrical
power transmission line. Let' discuss different schemes of time graded
over current protection.
4.1 Protection by Definite Time Relay
The relay nearest to the end of the line has minimum time setting while
time setting of other relays successively increased, towards the source.
For example, suppose there is a source at point A
4.2 Advantages of Definite Time Line Protection
The main advantage of this scheme is simplicity. The second major
advantage is, during fault, only nearest CB towards the source from fault
point will operate to isolate the specific position of the line.
4.3 Disadvantage of Definite Time Line Protection
If the number of sections in the line is quite large, the time setting of
relay nearest to the source, would be very long. So, during any fault
nearer to the source will take much time to be isolated. This may cause
severe destructive effect on the system.
5. Practical Exercise:
Each participant to test Over current / Earth fault protection to measure,
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Pickup level
Definite operate time
Inverse operate time
Draw definite & Inverse characteristics (at least two different time
multipliers) by applying different fault currents via secondary
injection instrument.
An excel calculator is provided to each participant to verify tipping times at
different inverse time curves
4.4 Over Current Line Protection by Inverse Relay
The drawback as we discussed in definite time over current protection of
transmission line, can easily be overcome by using inverse time relays. In
inverse relay the time of operation is inversely proportional to fault
current.
At point D the circuit breaker CB-3 relay is installed with definite time of
relay operation 0.5 sec. Successively, at point “C” another circuit breaker
CB-2 relays is installed with definite time of relay operation 1 sec and
similarly relays at CB-1 & CB with increased times
Now, assume a fault occurs at point F. Due to this fault, the faulty current
flow through all the current transformers or CTs connected in the line.
But as the time of operation of relay at point D is minimum the CB-3
associated with this relay will trip first to isolate the faulty zone from rest
part of the line. In case due to any reason, CB-3 fails to trip, then next
higher timed relay will operate the associated CB to trip. In this case, CB-2
will trip. If CB-2 also fails to trip, then next circuit breaker i.e. CB-1 will trip
to isolate major portion of the line.
In the above figure, overall time setting of relay at point D is minimum
and successively this time setting is increased for the relays associated
with the points towards the point A. In case of any fault at point F will
obviously trip CB-3 at point D. In failure of opening CB-3, CB-2 will be
operated as overall time setting is higher in relay at point C.
Although, the time setting of relay nearest to the source is maximum but
still it will trip in shorter period, if major fault occurs near the source, as
the time of operation of relay is inversely proportional to faulty current.
End of Training;
Hope after going through all above, each participant has a greater level of
understanding regarding overcurrent protection.
Prepared By: Babar Shahzad
Date: 21-03-2022 Rev:02
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