2. DIFFERENTIALPROTECTIONSCHEME
This scheme is employed for the protection of
transformers against internal short circuits.
It provides the best overall protection for internal faults.
It compares the current entering the transformer with the
current leaving the element.
If they are equal there is no fault inside the zone of
protection.
If they are not equal it means that a fault occurs
between the two ends.
4. Ip - primary rated current of transformer Is
- secondary current of transformer
CT of ratio Ip/1A at primary side and
CT of ratio Is/1A at secondary side of the transformer.
Biased Differential Protection Scheme
5.
6. Construction:
The secondaries of these both CTs are connected together in
such a manner that secondary currents of both CTs will oppose
each other.
In other words, the secondaries of both CTs should be
connected to same current coil of differential relay in such a
opposite manner that there will be no resultant current in that
coil in normal working condition of the transformer.
7. Operation:
But if any major fault occurs inside the transformer due to which
the normal ratio of the transformer disturbed then the
secondary current of both transformer will not remain the same
and one resultant current will flow through the current coil of the
differential relay, which will actuate the relay and trip both the
primary and secondary circuit breakers.
To correct phase shift of current because of star – delta connection
of transformer winding in case of three phase transformer,
the current transformer secondaries should be connected in delta
and star.
8. Limitations of differential protection of transformer:
1) Due to the magnetization characteristics of the CTs used, the ratio
errors change with respect to the circulating currents.
2) The pilot wires used may vary in length due to which the unbalance in
the secondary circuit parameter (resistance) is created that results in
improper scheme.
3) During heavy short circuit conditions the high currents create
saturation of the flux in core of CTs that lead to abnormal relaying or
unexpected behavior of the relaying circuit.
9. 4) Tap changing may lead to change in settings & improper operation.
5) Inrush of magnetizing current may lead to inadvertent operation &
hence the settings are done for higher values of fault current (higher
imbalance) due to which accuracy of sensing & operation is decreased.
11. BUCHHOLZ RELAY
It is a gas actuated relay.
When a fault develops slowly, it produces heat,
thereby decomposing solid or liquid insulating material
in the transformer.
The decomposition of the insulating material produces
inflammable gases.
The Buchholz relay gives an alarm when a specified
amount of gas is formed. The analysis of the gas
collected in the relay chamber indicates the type of the
incipient fault.
12. There is a chamber to accommodate Buchholz relay, in
between the transformer tank and the conservator.
The Buchholz relay is a slow acting device, the
minimum operating time is 0.1 s and the average time
is 0.2 s.
13.
14. The Buchholz relay also has another set of contacts
operated by a float. These contacts stay open when
the transformer tank is filled with oil. However, in case
of leakage of oil or decomposition of oil, the float
sinks causing the contacts to close.
Loss of oil will no doubt cause the transformer
temperature to rise but does not warrant immediate
tripping. Hence, normally these contacts are wired to
an alarm which alerts the operator.
Working Animation
15. RESTRICTEDEARTHFAULTPROTECTION
A percentage differential relay has a certain minimum value of
pick up for internal faults. Faults with current below this value
are not detected by the relay.
Winding-to-core faults, which are single phase to ground
type, involving high resistance, fall in this category.
Therefore for such type of faults RESTRICTED EARTH FAULT
PROTECTION is used.
16.
17. An external fault in the star side will result in current flowing in the
line CT of the affected phase and at the same time a balancing current
flows in the NCT, hence the resultant current in the relay is therefore
zero. So this REF relay will not be actuated for external earth fault.
But during internal fault the NCT only carries the unbalance
fault current and operation of Restricted Earth Fault Relay takes place.
This scheme of REF protection is very sensitive for internal earth fault
of electrical power transformer.
18. The protection scheme is comparatively cheaper than differential
protection scheme.
Restricted earth fault protection is provided in power transformer for
sensing internal earth fault of the transformer. In this scheme the CT
secondary of each phase of power transformer are connected together
as shown.
Then common terminals are connected to the secondary of a Neutral
Current Transformer or NCT. The CT connected to the neutral of
transformer is called Neutral Current Transformer .
19. OVERCURRENTPROTECTION
In an over current relay, there would be essentially
a current coil. When normal current flows through this coil,
the magnetic effect generated by the coil is not sufficient to
move the moving element of the relay, as in this condition the
restraining force is greater than deflecting force.
20. But when the current through the coil increased, the
magnetic effect increases, and after certain level of
current, the deflecting force generated by the magnetic
effect of the coil, crosses the restraining force, as a
result, the moving element starts moving to change
the contact position in the relay.
22. Fig. shows two numbers of phase fault over-current relays and one
ground-fault over-current relay for providing over-current protection
to the star-delta transformer.
The neutral current under load conditions is quite small. The neutral
current is essentially because of load unbalance.
26. The basic working principle of thermal relay is that, when a
bimetallic strip is heated up by a heating coil carrying over
current of the system, it bends and makes normally open
contacts to close and close the Trip Circuit of C.B.
