In power utilities like TSSPDCL, it is a regular practice in 33/11KV indoor and outdoor substations that various electrical equipments like HV breaker, LV breaker, feeder breaker, battery charger, etc., are supplied with AC/DC supply through fuses. Substation operators are instructed by in-charge officers to regularly monitor the availability of power supply to these equipments through indicating bulbs, healthy trip circuits, trip supervision circuits, toggle switches, etc. But, most of the operators hardly check these power supplies to equipment. Sometimes indicating bulbs and meters do not function.
Protection equipment like circuit breakers will not function on non-availability of DC supply, which will not trip on faults and cause damage to equipment under protection. This will results in tripping of backup protection equipment i.e., pulling out LV breaker or pulling out breaker at EHT substations, causing unnecessary interruption to more number of consumers. If this failure of fuse is recognized by the operator in advance by some proper indication and annunciation system these kind of interruptions can be avoided and also burden of MRT staff coming from distance location for replacing the fuse and CBD staff arranging back feed to the feeders will be eliminated.
To reduce this type of interruption due to none functioning of circuit breakers in case of failure of its DC fuses to some extent, a digital circuit is designed to monitor the most essential DC supply to different protection equipment.
This circuit (as below) is built around integrated circuits like timer IC555, operational amplifier used as comparator IC741, IC4510 BCD counter used for data selection, IC74LS151 multiplexer and optocoupler MCT2E.
In brief, this circuit samples the DC supply available at DC fuse from all equipments and makes available at multiplexer through buffer circuit i.e., opto-coupler, for selection of the same and compare with reference voltages. A timer circuit and BCD counter circuit is used for selecting the data from breakers one by one for comparison. A multiplexer circuit sends the data to comparator one by one. This comparator circuit compares the data available at its terminal with reference data, which drives the buzzer to give alarm if the data is abnormal. Also LEDs are incorporated in the circuit to indicate the DC supply in the circuit breakers, etc. Similarly, a 16 channel multiplexer IC can monitor DC supply of 16 equipments at a time. This circuit costs around Rs. 2,000/-.
Annunciation /alarm system in this circuit will alert the operator within few seconds/minutes, as per settings, after failure of fuse. This helps the operator to take necessary action of informing the concerned officers and replacing the fuse immediately. In turn this helps to avoid the unnecessary interruptions during faults.
– By Er. Ramakrishna K K
Asst. Divisional Engineer (Elec)/Lines/Nehru Nagar- SCADA