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This is an article on technical subject pertaining to 33/11KV substations.  In APCPDCL most of the substations are designed for two power transformers, two 33KV incoming sources (one regular and one alternate supply) and will be feeding six to eight 11KV feeders.  Generally, substations will be fed from one source.  In this condition all the feeders, station transformer, potential transformer, auxiliary equipment are fed from one source.  But when the substation is fed from two different 33 KV sources at a time due to some technical reasons, then one power transformer will be fed from one source and other power transformer will be fed from other source.  This is done by opening the bus coupler on 33KV bus and 11KV bus.  On separation of 11KV bus, the station transformer will be on one side of the bus only.  The station transformer will be feeding auxiliaries, substation lighting, fans etc and substation lighting and fans will indicate presence of incoming supply most of the cases.  When the 33KV source feeding the station transformer via power transformer fails, the operator knows it by observing the condition (off status) of fans and lights in the control room and takes action required.


But when other 33 KV source feeding other power transformer and other 11KV feeders fails (supply interrupted from 132/33KV substation), the substation operator will not know as there is no indication/alarms.  The operator will not know until somebody calls him for non availability of supply in their premises.  This will lead to increased interruption time and inconvenience to thousands of consumers.  To avoid this situation and to improve customer satisfaction and in turn to improve the revenues of the company, two types of relays have been designed.

Types of Relays
a) Current transformers based sampling and detection
b) Potential transformers based sampling and detection

Current transformers based sampling and detection
In this system the current flowing in the metering circuit of the current transformer is sampled and compared with preset value and conditions are detected.

Block diagram
The circuit is built around basic operation amplifier IC, used as comparator.  This IC compares the current in the connected network with preset value and gives alarm if found abnormal.  This device operates on 230 volts AC as well as 12 volts DC supply.

A CT coil is used as a basic transducer which converts current flowing in the connected network to a lower value, and  it is converted  into proportional voltage.  This voltage is further rectified to DC voltage and passed on to comparator IC358 inverting input  Pin-2  for comparing with preset DC value at non inverting input Pin-3.  After comparing,  the IC output will be  high or low as per DC voltage at Pin-2.  If the voltage at Pin-2 is less than voltage at Pin-3, the output Pin-1 will be high and vice versa.  This output will drive the relay with driver transistor BC517.  This relay will in turn connect the 220 Volts  DC to alarm device which give indication to operator for taking action.

Potential transformers based sampling and detection
Block diagram:

This circuit is very simple compared to previously discussed model.  Here the voltage of potential transformer available in the substation, if connected on other side of the station transformer (as discussed above) is utilized as sample voltage.  The 110 Volts AC voltage from potential transformer is stepped down, rectified with rectifier diodes and smoothed with capacitor and applied to a DC relay.  The relay gets energized if the supply is available and vice versa.  The NO or NC contacts of relay are utilized for connecting 220 Volts DC supply to alarm device.  The alarm device will alert operator for taking necessary action.

These devices are specially designed, to alert the operator when the SECOND incoming source fails, which is not existing in the present system.

These two models are successfully functioning at 33/11KV Hussainsagar substation which are designed by me.

By Er. K K Ramakrishna
ADE/Lines/Nehrunagar, Secunderabad