POWER TRANSFORMER PROTECTION USING MICROCONTROLLER BASED RELAY

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