Power System Protection Lab.
Manual
Handout no. 7
Name Syed Aqeel Haider
Reg. No 2019-EE-517
Marks/Grade
EXPERIMENT # 7
Differential Protection for Power Transformer
Objective:
At the end of this lab session students will be able to
Understand the working of Differential Relay.
Model Differential relay in Matlab Simulink.
Differential protection:
A differential protection relay is a protective device used in electrical power systems
to detect and isolate faults that occur within the protected zone. It operates based on the
principle of comparison between the current entering and leaving the protected zone, and any
difference in these currents indicates the presence of a fault. The differential protection relay
is typically installed at the ends of the protected zone, and it monitors the current flowing into
the zone and the current flowing out of the zone. The relay compares these currents and trips
the circuit breaker if the difference between the currents exceeds a certain threshold, which
indicates the presence of a fault.
.
Laboratory Task:
Implement a Differential-Relay that protects the Power-Transformer against all the faults
within the protected zone. Protected zone is the region between two current-transformers on
either sides of Power-Transformer. Relay should not operate for the faults outside the protected
zone.
Procedure:
Step 1:
Draw a power system having a three phase source, three phase circuit breakers, three-
phase VI measurement units, three phase transformer (two winding) and three phase RLC series
load. Connect the blocks as shown in figure and set the parameters as specified.
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Power System Protection Lab. Manual
In above figure, two inputs ‘L.V Side’ and ‘H.V Side’ of ‘Differential Relay’ are currents from
low voltage side and high voltage side of transformer respectively.
Three-Phase
Step 2:
In subsystem “Differential Relay”, connect the blocks as shown in the diagram and set the
parameters of blocks as specified.
Step 3:
Now connect three three-phase fault blocks as shown in figure and set their parameters as
specified.
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Power System Protection Lab. Manual
Simulation
Run the simulation and observe the waveforms of current on both scopes.
In above figure Fault 1 and Fault 3 are outside the protected zone of Differential relay so
relay does not trip. But Fault 2 occurs inside the protected zone so relay picks the fault and opens
the circuit breakers to clear it.
Subsystem
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Power System Protection Lab. Manual
Now implement same relay in which comparison should be done between sinusoidal
currents instead of their RMS values. In the differential relay block make changes as shown
below.
In above logic of differential relay RMS blocks have been removed and a delay of 20ms has been
introduced by using counter. These counters are in their free running state and their counting rate
is 1000 counts per second. To insert this delay ‘relational operators’ prior to the counter blocks
have been changed from ‘>’ to ‘<’.
Run the simulation after these changes. You will observe that relay trips the circuit breaker
initially. This is due to the phase difference between two currents. The both primary and
secondary currents are 180O out of phase due to the connection of second CT ( Three-Phase V-I
Measurement2).
Reverse its connection as shown below to make the both currents in phase.
Waveform of single phase to Ground Fault
Current Waveform of LV side
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Power System Protection Lab. Manual
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Power System Protection Lab. Manual
Current Waveform of HV Side
Two Phase to Ground
Current Waveform of LV side
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Power System Protection Lab. Manual
Current Waveform of HV side
Line to Line Fault
Current Waveform of LV side
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Power System Protection Lab. Manual
Current Waveform of HV side
Conclusion:
The differential protection relay is an important protective device in electrical
power systems. It operates by monitoring the current entering and leaving a protected zone
and tripping the circuit breaker if the difference between the currents exceeds a certain
threshold, indicating the presence of a fault. The differential protection relay is commonly
used to protect transformers, generators, motors, power cables, and busbars. It provides fast
and reliable protection, which helps to minimize damage to the power system and reduce
downtime. The use of differential protection relays is essential for maintaining the reliability
and safety of electrical power systems.
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