Over Voltage and Under Voltage Control Using Relay

Topics: Relay, Diode, Rectifier Pages: 11 (3006 words) Published: February 24, 2013

a. Power supply
b. Under voltage/over voltage detector
c. NOR Gate (OR and NOT gate)
d. Bi-stable / Latch
e. Relay driver
f. Buzzer Driver


The protection system is one o the important aspect on which major manufacturers is concentrating. The companies like L&T, SIMENS etc. The protection system not only provides durability to the equipment but also avoid hazards. The protection system is one of the major fields in the electrical engineering. There are different types of protection systems out of that very common and important one is the voltage protection system. Normally all the electrical equipments are specified with a specific voltage range for its safe operation. When ever those equipments are applied with a voltage more then the upper specified limit the system may draw excess current which may cause to burn the equipment. The equipment is designed to withstand a specific voltage when ever the voltage goes beyond the limit value then automatically the insulations used in that equipment get punctured and cause a damage to the equipment. Similarly whenever the equipment is applied with a voltage lower then the lower specified voltage it draws excess current to maintain the efficiency. The out put may not be proper and the equipment may damage. The over voltage and under voltage relay in combination protect the device from fluctuation of voltage. This type of relay provides the equipment complete protection against undesired voltage condition. The over voltage and under voltage relay is one of the important protective relays which is used in the practical application at industries to protect motors transformers and other electronic and electrical equipment and power system.

The over voltage and under voltage relay is designed using potential transformers (PT). There is a PT used to sample the line voltage and converts into DC value. The DC value obtained at the rectifier and filter out put is analogous to the AC line voltage amplitude. The filter is designed with a time constant more then five time period (5T). The sampling voltage obtained at the rectifier filter out put of the PT changes after 5T only. In this manner the sampling voltage is made resistant to the effects of spics. The sampled voltage is feed to two different comparators having reference voltages analogous to the under voltage and over voltage setting. Whenever the sample voltage goes beyond the reference voltage set for over voltage condition then comparator output goes high and a fault condition is detected. Similarly whenever the sample voltage goes bellow the reference voltage set for under voltage condition in the other comparator the output goes high and a fault condition is detected. The outputs of both the comparators are feed to a logic gate to indicate the fault condition when ever any of the comparator issues a fault decision. The out put of the logic gate is feed to the bistable / latch and then relay driver and buzzer driver to protect the equipment from the fault condition and indicate the fault occurrence by blowing the buzzer.

a. Power supply

Circuit connection: - In this we are using Transformer (0-12) v, 1Amp, IC 7812, diodes IN 4007, LED & resistors. Here 230V, 50 Hz ac signal is given as input to the primary of the transformer and the secondary of the transformer is given to the bridge rectification diode. The o/p of the diode is given as i/p to the IC regulator (7812) through capacitor (1000mf/35v). The o/p of the IC regulator is given to the LED through resistors.

Circuit Explanations: - When ac signal is given to the primary of the transformer, due to the magnetic effect of the coil magnetic flux is induced in the coil(primary) and transfer to the secondary coil of the...
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