POWER TRANSFORMER PROTECTION
This Project is
designed with Peripheral Interface Controller (PIC 16F877A). Utility companies
have enormous amounts of money invested in transformers of all types, including
distribution and power transformer. Operating,
maintaining, and inspecting all power transformers are not an easy work. In
order to reduce burden on maintenance of such transformers a new idea has been
discovered.
This project is mainly used to protect the transformer from
getting worn out due to electrical disturbances. The electrical parameters like
current, voltage of the transformers are fed as base values, using a keypad to
the Peripheral Interface Controller and the output signal is provided to operate
a relay by comparing the base values with the operating electrical parameters. The
application consists of a board of electronic components inclusive of a PIC
16F877A microcontroller with programmable logic. It has been designed to work
with high
accuracy. The electrical parameters of the power transformer such as voltage
and current are fed to the Peripheral Interface Controller as base values. The
voltage and current value during the operation of the power transformer is
monitored and fed to the controller. These values are monitored using a LCD
display. By comparing these values the Peripheral Interface Controller produces
a trip signal which operates the relay and in turn the connectivity between
main supply and the power transformer is cut off, thus protecting the power
transformer from malfunctioning. In proposed method, monitoring and protecting the power
transformer from over voltage and over current are performed automatically by
using PIC microcontroller.
Components of
the project:
Rectifier,
filter and Regulating circuit (Power circuits)
Voltage
measuring circuit using Potential Transformer
Current
measuring circuit using Current Transformer
Keypad
and LCD display
Driver
circuit and a Relay
PIC
16F877A microcontroller board
The
protection system of transformer is inevitable due to the voltage fluctuation,
frequent insulation failure, earth fault, over current etc. Thus the following
automatic protection systems are incorporated.
1.
Buchholz devices:
A Buchholz relay, also called a gas
relay or a sudden pressure relay, is a safety device mounted on some oil-filled
power transformers and reactors, equipped with an external overhead oil
reservoir called a conservator. The Buchholz Relay is used as a protective
device sensitive to the effects of dielectric failure inside the equipment. It
also provides protection against all kind of slowly developed faults such as
insulation failure of winding, core heating and fall of oil level.
2.
Earth fault relays:
An earth fault usually involves a
partial breakdown of winding insulation to earth. The resulting leakage current
is considerably less than the short circuit current. The earth fault may
continue for a long time and creates damage before it ultimately develops into
a short circuit and removed from the system. Usually provides protection
against earth fault only.
3.
Over current relays:
An over current relay, also called as
overload relay have high current setting and are arranged to operate against
faults between phases. Usually provides protection against phase -to-phase
faults and overloading faults.
4.
Differential system:
Differential system, also called as
circulating-current system provides protection against short-circuits between
turns of a winding and between windings that correspond to phase-to-phase or three
phase type short-circuits. It provides protection against earth and phase
faults.
The complete
protection of transformer usually requires the combination of these systems.
Most of the transformers are usually connected to the supply system through series
fuses instead of circuit breakers. In existing method the transformer does not
have automatic protective relays for protecting the transformer.
Common
Transformer faults
As
compared with generators, in which many abnormal conditions may arise, power
transformers may suffer only from:
1. Open circuits
2. Overheating
3. Winding short-circuits
1 Open circuit Faults:
An
open circuit in one phase of a 3-phase transformer may cause undesirable
heating. In practice, relay protection is not provided against open circuits
because this condition is relatively harmless. On the occurrence of such a
fault, the transformer can be disconnected manually from the system.
2 Overheating Faults:
Overheating
of the transformer is usually caused by sustained overloads or short circuits
and very occasionally by the failure of the cooling system. The relay
protection is also not provided against this contingency and thermal
accessories are generally used to sound an alarm or control the banks of fans.
3 Winding Short-circuit Faults:
Winding
short-circuits (also called internal faults) on the transformer arise from
deterioration of winding insulation due to overheating or mechanical injury.
When an internal fault occurs, the transformer must be disconnected quickly
from the system because a prolonged arc in the transformer may cause oil fire.
Therefore, relay protection is absolutely necessary for internal faults.
Conclusion
Transformers are static devices, totally enclosed and
generally oil immersed. Therefore chances of faults occurring on them are very
rare. However the consequences of even a rare fault may be very serious unless
the transformer is quickly disconnected from the system. This necessitates to provide
adequate automatic protection for transformers against possible faults. The
major faults on transformers occur due to short circuits in the transformers or
in their connections. The basic system used for protection against these faults
is the differential relay scheme.
Protection of power transformer is a big challenge
nowadays. By the help of microcontroller based relay, protection of transformer
is performed very quickly and accurately. This system provides a better and
safer protection than the other methods which are currently in use. The
advantages of this system over the current methods in use are fast response,
better isolation and accurate detection of the fault. This system overcomes the
other drawbacks in the existing systems such as maintenance and response time.
Block Diagram
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