Power transformers and switchgear are the heavy equipment that changes the voltage of electricity and protects the power grid when something goes wrong. A transformer is a large machine that raises or lowers voltage. Switchgear is the set of switches, circuit breakers and protection relays around it. A circuit breaker is a switch that opens by itself to cut off the power when there is a fault. A protection relay is the device that senses the fault and tells the breaker to open.
Why change the voltage at all? A wire loses some power as heat, and the loss grows with the current. For the same amount of power, a higher voltage needs a smaller current. So near a power plant, step-up transformers raise the voltage, and the lines can carry power over hundreds of kilometres with little loss. But homes cannot use such high voltage. At a substation, step-down transformers lower it again, so that local lines can carry the power to homes and shops. A transformer works only with alternating current, which is why the grid uses AC. Inside, it sits in a tank of oil, which cools it and insulates it.
Take, for example, a substation near Surat. A step-down transformer there receives power at 132,000 volts and sends it out at 11,000 volts to the local lines. Say a branch falls on one of those lines. The current jumps suddenly. A protection relay senses it within a fraction of a second and tells the circuit breaker to open. Only that one line goes dark, not the whole area. Without the breaker, the fault current would keep flowing and could damage the transformer.
The energy management system in the control room can also open and close breakers from a distance, to move power around. Sensors on the machine feed asset performance management software, which tracks the health of every transformer a utility owns. One common example is transformer monitoring. Electric railways need the same kind of equipment to power their trains. See railway track and electrification.
The buyers are utilities. In India, they are mostly state-owned transmission and distribution companies, and they buy through government tenders. A large transformer takes many months to make, and it is built to last for decades. A failed one is expensive and slow to replace. That is why utilities watch the health of their transformers with predictive maintenance.