In general all the 33/11KV Indoor substations with ABB, Siemens, Schneider Electric, etc breakers, are provided with heaters in each breaker compartment to avoid absorption of moisture by current transformers, cable end- terminations, epoxy bus insulators etc., during winter and rainy seasons so that good IR values are maintained between phase to phase and phase to neutral to avoid short circuits.
The Operators at substations are switching on the heaters only on specific instructions. Though Operator switches on the heater, he/she is not sure whether the heater is functioning, as these heater circuits are not provided with any indicator bulbs and or alarms indicating functioning of heaters.
During rainy and winter seasons if heaters fail to function, current transformers absorbs moisture formed inside the breaker compartments and create dead short circuit on 33KV and 11KV cable feeders.
To avoid this type of very serious breakdowns, a simple circuit is designed to caution the operator at times of heater failure and is as follows.
This circuit is built around popular operational amplifier IC 358 as comparator and IC 555, IC 4017 as timer. In this circuit the current flowing through heaters is sampled by CT coil and converted to a proportional DC voltage. This DC voltage is compared with a preset value of DC voltage at operational amplifier wired as a comparator. If the sampled voltage i.e., current flowing through heaters is below the preset value, the comparator output will be high and this high level voltage at comparator output will turn on alarm device via driver circuit and an indicator bulb will also “ON” stating heater is faulty.
A timer circuit with IC 555 and a counter IC 4017 is added to this circuit to disconnect the supply to heaters for few minutes per hour to avoid over heating of heaters and its connecting cables. This will improve the life of heaters and connecting wires.
This device is designed, assembled, tested and installed at 33/11KV Chinna Thokatta SS and 33/11 KV Addagutta SS on trial basis and are functioning satisfactorily.
“This circuit is specially designed to avoid dead faults on 33KV feeders and 11KV feeders – which not only avoids damage to the substation equipment but also reduces abnormal interruption to consumers. Also, the device cautions the operator in case of failure of heater inside the breaker compartment.”
By Er. K K Ramakrishna
ADE/Lines/Nehrunagar – Secunderabad