Transformer Protection .pdf
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The followings are the possible faults that may occur in transformer. The types of
protection scheme normally used for the corresponding faults are shown.
1. Phase to phase faults(rare)
Percentage differential relay as primary
Phase to ground fault (very common) protection & O/C relay as back up protection
2. High voltage surge due to lightning → Lightening arrestor
3. Faults inside the tank below oil level → Gas actuated relay (Buchholz relay)
4. Tank earth fault protection
→ Earth fault protection
5. Magnetizing inrush current
→ Differential protection with harmonic restraint
The choice of suitable protection is governed by economic considerations. This factor is based
on different ratings of transformers which vary from few KVA to several hundred MVA. Only
the simplest protection, such as fuse, can be justified for transformer of lower ratings; whereas
those of the highest ratings should have the best protection that can be designed.
Percentage differential protection:
CT connection for biased differential protection of Y-Y transformer
CT connection for biased differential protection of ∆-Y transformer
CT connection for biased differential protection of ∆-∆ transformer
Magnetizing inrush current
The phenomenon of magnetizing inrush is a transient condition which occurs primarily when a
transformer is energized. It is not a fault condition. The pick value of the inrush current may be
high as 6 to 8 times as the normal full load current. Inrush current affects the operation of the
transformer differential protection relays.
Two aspects are of significance:
1. The current flow only in one of the two windings of the transformer (primary winding) and it
is as good as an internal fault as far as protection scheme as concerned.
2. The wave shape of the inrush current differs from the usual fault current because it contains
a high percentage of second harmonic components (above63%) of the fundamental. It thus
follows that a relay designed to detect the second harmonic component in the magnetizing
inrush current can be made to utilize this as a means of discrimination between inrush
condition and the internal fault condition.
To take into the account the magnetizing inrush currents, a high speed biased differential relay
incorporating harmonic restraint features is used. This can be achieved by the use of a second
harmonic filter which is arranged to inject an additional biased current in the relay circuit
proportional in the second harmonic component.
Backup protection by over current relays:
A backup protection scheme by O/C relays is shown in the figure below:
Restricted earth fault protection
A simple O/C and earth fault protection system will not give good protection cover for a star
connected primary winding, particularly if the neutral is earthed through impedance. The degree
of protection is very much improved by the application of a unit differential earth fault system,
called restricted earth fault protection as shown in the figure below:
I rs = I as + I bs + I cs
Limitation of Buchholz relay:
1. Faults below oil level and detected.
2. Setting of mercury switch cannot be too sensitive because any vibration e.g. earth quake,
mechanical shock can cause false operation.
3. It is slow in operation ( min operating time is 0.1 sec)
Principle: The faults in the early stage in the transformer tank below oil level actuate Buchholz
relay so as to give an alarm. When a fault occurs inside the transformer tank below oil level, gas
is usually generated, slowly for a fault at an early stage and violently for heavy faults. The heat
produced by high local current causes the transformer oil to decompose and produce a gas which
can be used to detect the winding faults. Buchholz relay is the simplest form of protection which
is commonly used in all transformers provided with conservator. It consists of a chamber
connected in the upper side of the pipe run between the oil conservator and the tank, and
containing two cylindrical floats one near the top of the chamber and other opposite to the orifice
of the pipe to the transformer.
Tank earth fault protection
The connection configuration of a tank earth fault protection is shown in the figure below;
The following two basic protections are provided for every motor:
1. Thermal over load protection
2. Short circuit protection
The switchgear used for the motor protection can also be classified into the following two groups
depending on the size of the motor:
1. For small motors (up to 150 hp), fuse and thermal over current protection are used.
2. For large motors, circuit breakers and associated relays are used.
For small motors:
Short circuit protection:
Fuse will provide the short circuit protection of stator winding. The operating time current
characteristics of the fuse should be such that the fuse should not blow during the motor starting
which could be 5 to 7 times the motor full load current. The fuse should blow at currents more
than those which can be interrupted by the contactors.
Thermal relay should provide the overload protection. Thermal relay should not operate during
starting period of the motor. Starting period is generally considered to be 5-10 sec. Motor stall
current (locked rotor current) is equal to the motor starting current. The only magnitude of
current cannot be used as a means to distinguish between starting and stalling of motors. So, the
only way to distinguish these two conditions is to use the time as another parameter.
For large motors:
Overload and short circuit protection:
Over current relays and earth fault relay (either instantaneous or inverse time or both depending
on the importance of the motor) are used to protect against phase faults and the earth faults on
stator winding. If the motor is very large and expensive, it is essential to provide differential
protection for the winding. The short circuit protection characteristic is set just above the
maximum starting current of the motor.
Limitation of O/C protection:
As mentioned before, the O/C relays are set at higher current value than that may occur during
starting period. For phase to phase faults near the neutral point of a wye connected motor, the
fault current may be less than the O/C setting. Hence the relay will not operate instantaneously
and thus the fault can cause extensive damage therefore, biased differential relay is most suitable
in this case.
Phase unbalance protection
Relays that operate when the unbalance in the line currents exceeds a given value are often used
to protect motors when running with unbalanced supply voltages. These relays suffer from the
a) They operate only on the difference in magnitude of the line currents, not on the phase and
magnitude difference. They cannot therefore measure accurately the value of the negative
sequence current which is the main cause of the additional heating in the rotor winding due
b) They tend to be too sensitive under both single phasing conditions and at small values of
unbalance, thus disconnecting the motors unnecessarily.
For small motors separate phase unbalance protection is not provided as the thermal relays sense
the increased current in healthy phases due to single phasing and thereby offer adequate
For small motors:
I a − Ic
Ic − I b
Ib − I a
Ia − Ic
A 30MVA, 11.5/69 kV, Y-∆ power transformer is to be protected by a biased differential
protection. The high voltage side lags behind low voltage side phase by 300. Formulate the
complete biased differential protection for the transformer by selecting CT ratios, CT
connections. The continuous current carrying capacity of restraining coil of the relay should not
Ib − I a
Ic −I b
Full load current for a 30MVA, 11.5 kV Y/69 kV ∆ power transformers
On 11.5kV side,
Ip = 3000/ (√3×11.5) =1506A
Choose a CT ratio of 3000/5=600
Then CT secondary current, Is=1506/600=2.51A
Since 11.5 kV side is star connected, CT secondaries will be delta connected.
Hence current fed into pilot wire from 11.5 kV side CT secondary’s is √3×2.51=4.35A
On 69KV delta side, Ip=3000/ (√3×69) =251A
CT secondary’s of this side is connected in Y; hence current in CT secondary is equal to the
current in the pilot wires.
CT secondary should be 4.35A
So CT ratio =251/4.35=57.7; select a CT ratio of 60 secondary current is 5A; so, primary
current=60×5=300A. Therefore CT on 69kV ratio is 300/5.
Show the connections of CTs and the relay of the differential protection of a 33/6.6 kV star/delta
connected transformer. The CTs on the LT side have a ratio of 300/5. What should be the CT
ratio on the HT side?
Ia − Ic
Ib − I a
Ic − I b
Assume, 300A is flowing in the lines on the LT side
So, √3×6.6×300= √3×33×I, when I is the current in the HT side
I = 60A
Secondary current of the CTs on the high side is 5/√3A
CT ratio on the HT side = 60/ (5/√3)
Ic −I b
Ib − I a
Ia − Ic
CTs on the delta side are star connected. Hence the secondary phase currents are equal to
secondary line currents i.e. currents in the pilot wires. CTs on the star side are delta connected;
hence currents in the secondary phase are equal to the currents in the pilot wire divided by √3.
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