When travelling by the local train in Mumbai, many times the lights and fans get switched off for a few seconds.Ever wondered Why does it happen? Here is the explanation to your doubts.
To understand this, you will have to first understand the concept of neutral zone & substations.
As you might be knowing, the overhead wires running above the trains better known as OHE(Overhead Electrical wires) are supplied with single phase ,25 kV AC supply. This supply is not readily available but is stepped down to this level at the Traction Substations from the 132 or 33 kv received from the grid by transformers.
Now each substation serves a particular area of about 50–100 KMS. Now care must be taken that there shouldn’t be 2 sources of supply in an area at the same time because it will cause difficulties as parallel operation of transformers is dependent on several factors like same KVA ratings, same phase sequence, same percentage impedance and what not. So between the end of the jurisdiction & the start of the area covered by another substation, a small region is created where there is no supply in the OHE. As there is no supply, it is called as neutral zone & spams only for a few 100 meters. While crossing this section, all the pantographs must be in “down” condition meaning that the train travels on its inertia & is expected to cross over to the area of the other substation before stopping completely.
There are boards which indicate when to lower the pantograph at distances of 500 & 250 metres before the neutral zone.
Now naturally, since the pantographs have been lowered in the neutral zone, the power supply to the train is not available for that small amount of time. Hence the lights and fans get switched off for a few seconds.
Hope this helps.