The document summarizes an experiment on a percentage differential relay. The experiment had three aims: to study the operating characteristics of the relay at 15% and 30% bias settings, to use the relay for single-phase transformer protection, and to study the harmonic restraint feature. Observations showed that increasing the bias setting required more current to trigger the relay. Testing the relay on a single-phase transformer demonstrated it would only operate for internal faults, verifying its protection zone. Introducing a DC branch also demonstrated the relay's harmonic restraint by increasing its tripping current.

1. Experiment 04: Percentage Differential Relay
Aim:
 To study and determine the operating characteristics of a percentage differential
relay for 15% and 30% bias setting
 To use the transformer differential relay forsingle phase transformer protection
 To study the harmonic restrained feature of transformer differential protection
Basic Theory:
 A differential relay is one that operates when the vectordifference of two or
more similar electricalquantities exceeds a pre-determined value. Thus the two
quantities must have a phase difference(approx. 180 degrees) for the operation
of the relay.
 The Merz-Pricedifferential protectionscheme assumes that twocurrent
transformers used are identical. This however is not true, especially under
transient conditions. Due to different amounts of residual magnetism one of the
CTs may saturate earlier during a short circuit current and thus the twoCTs
transform their primary currents differentially even fora through fault
condition.
 To accommodatethis feature, the “biased differential protectionscheme” or
“percentage differential scheme” is used: wherein the relay is biased as a
modification to Merz Pricescheme.
 As in figure 1, an operating and a restraining coil is connected.The torque
developed by the operating coil is proportional to the ampere turns:
T  (i1 – i2) n0
Where (i1 – i2) is the differential current through the coil, n0: no. of turns in the
operating coil
 T  (i1 + i2) n0/2
Torque due to restraining coil is proportional to the equivalent current in
restraining coiltimes the no. of turns in restraining coil.
 Thus:
2(i1 – i2)/(i1 + i2) = nr/ no
The ratio of the differential operating current to the average restraining current
is a fixed percentage in the “percentage differential relay”.
Figure1

2. Apparatus used:
1. Ammeters – 15A – 2
2. Ammeter – 5 A – 1
3. Autotransformers -2
4. Rheostats – 100 ohm, 5A-2
5. Current transformers -2
6. Percentage differential relay -1
Observations:
 Study of operating characteristics of a percentage differential relay:
a) Bias = 15%
i1 (A) i1 – i2 (A) i2 (A)
1.9 0.32 1.65
2.09 0.39 2.5
3.35 0.48 3.0
4.05 0.72 4.72
b) Bias = 30%
i1 (A) i1 – i2 (A) i2 (A)
1.6 0.62 2.25
2.75 0.75 2.05
3.65 1.0 2.7
4.25 1.18 3.15
c) Bias = 45%
i1 (A) i1 – i2 (A) i2 (A)
2.3 0.9 1.5
3.1 1.18 2.0
4.0 1.5 2.55
Following are the plots of the ratio of differential operating current to the
average restraining current forthe three cases:

3.  Using the differential relay scheme fora single-phase transformer protection as
in figure 2, it is observed that the relay does not operate fora fault outside the
protection zone, howevera short circuitfault inside the protection zone triggers
the relay.
Figure2
 To verify the harmonic restrained feature of transformer differential protection,
a circuit as in figure 3 is created. It is observed that with the DC branch
connected the relay trips at around 0.88A; withoutit, it trips at 0.2 A; thus
verifyingthe restraint to current harmonics demonstrated by the scheme.
Figure3
Conclusion:
 The plot of differential current vs. average restraining current is a straight line in
a percentage differential relay
 More the relay bias setting, more will the initial amount of current required to
trigger the relay
 The percentage differential relay protection scheme displays harmonic restraint.