Sethuraman Srinivasan
Differential Protection
Differential Protection is a unit type protection for a specified equipment or specified zone. It
is since any fault within an electrical equipment would cause the current entering it to be
different from leaving the equipment. Thus, differential protection compares the two currents
magnitude or angle or both and issue a trip if there is a difference. This method of detecting
fault is easy applied when both the end of the equipment protected are physically located
near each other. So, differential protection can easily apply to equipment’s like transformer,
generator, busbar or maybe a motor. When it comes to transmission line protection then ends
of the line is far apart to apply a differential protection scheme to be applied.
I. Principle of simple differential protection
The advantage of adding a differential
protection for internal fault detection are as
follows:
a) Differential protection provides faster
detection of faults that appears on the
internal zone of protection
b) The location of the fault is determined by
the size of the equipment protected
c) The high-speed clearing of in-zone
faults can significantly lower the arc
flash incident energy levels.
The principle of operation of differential
protection is that vector difference of
current passes through the operating coil.
For example, from figure 1 if the CT ratio
of the protected equipment is 1:1 on both
the end, the operating current Id which the
difference between the current entering at
one end and the current leaving the other
end can be expressed as,
Id = I1 + I2 (or) Id = I1 + (-I2)
Where,
Id
is the differential operating current
I1
is the current entering at one end
I2
is the current leaving at another end
For differential protection following factors
to be considered transformation ratio of the
CT, Vector group and CT connection for
avoiding mal-operation.
II. During External fault
Figure 1
In normal operating conditions the two
currents at two ends of the protected
equipment are equal and balance. So, no
current flows through the operating coil and
hence relay remains inoperative. But when
any internal faults like phase to phase or
phase to ground fault or inter turn fault the
difference in current starts to flow through
the operating coil and it trips the circuit
breaker.
Figure 2
Simple differential protection scheme
should always remain stable for any fault
which is outside protected zone. From the
Figure 2 for an external fault such as
through fault current at the terminals or
maybe on the load connected the current
entering and leaving the protected zone is
same. Assuming CT ratio is same on both
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Sethuraman Srinivasan
end as 1500/ 1A. There is not fault current
flowing in split path or the operating coil.
III. During Internal Fault
Figure 3
Now if any fault occurs within the protected
zone internally within the protected
equipment maybe a phase to phase fault or
phase to ground fault the resultant fault will
starts to follow through the operating coil
and if the current exceeds the pickup value
of the relay it will initiate trip to circuit
breaker to protect the equipment.
Therefore, from the above understanding if
the fault or spill current is more than the
pickup value of the simple differential relay
it will initiate the trip. The minimum
internal fault current that the operating coil
of the simple differential relay is given by,
Id, min = CT ratio × Differential pickup value.
IV. Relay Stability on internal and external
fault conditions
So far for understanding the differential
principle we have assumed CT’s are ideal.
But in practical scenario’s CT’s are
subjected to magnitude and phase angle
errors. Both errors depend on CT’s Burden.
These burdens depend on the lead lengths
and the impedance of the relay coil.
Generally, these CT errors occurs during
any external faults or maybe during sudden
increase of primary current of the CT. So,
during the external fault or during a through
fault current goes on increasing various
imperfections of CT’s get magnified. This
cause the spill current on the operating coil
to build up. The imperfections of the CT’s
on both ends will not same practically. Due
to this there will be difference between the
secondary current on CT’s exceeds the
pickup value settings and relay initiate trips.
So, without any internal fault due to CT
errors simple differential relay may maloperate. In such condition relay lost its
stability due to the external through fault
occurred. The ability of the simple
differential relay to restrain (block) for the
external through fault current is defined as
Through fault limit. Which means beyond
this limit relay loses it stability.
When it comes to internal fault, the
minimum internal fault required to sense by
the operating coil of the simple differential
relay is decide by the pickup value of the
relay in the operation coil path. So, for
defining the minimum internal fault relay
needs to operate and the maximum fault
current beyond which the simple
differential relay mal-operates, the stability
ratio is defined.
𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑡ℎ𝑟𝑜𝑢𝑔ℎ 𝑓𝑎𝑢𝑙𝑡
𝑠𝑡𝑎𝑏𝑙𝑒 𝑙𝑖𝑚𝑖𝑡
𝑠𝑡𝑎𝑏𝑖𝑙𝑡𝑖𝑦 𝑟𝑎𝑡𝑖𝑜 =
𝑀𝑖𝑛𝑖𝑚𝑢𝑚 𝑝𝑖𝑐𝑘𝑢𝑝 𝑣𝑎𝑙𝑢𝑒
𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑓𝑜𝑟 𝑖𝑛𝑡𝑒𝑟𝑛𝑎𝑙 𝑓𝑎𝑢𝑙𝑡
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