The substations have current and voltage transformers designed to isolate electrically the high voltage primary circuit from the low voltage secondary circuit and, thus, provide a safe means of supply for indicating instruments, meters and relays.
(a) Current Transformer (CT)
Current transformers are used in power installations for supplying the current circuits of indicating instruments(ammeter, wattmeter, etc.), meters (energy meter, etc.) and protective relays. These transformers are designed to provide a standard secondary current output of 1 or 5 A, when rated current flows through the primary. A fundamental feature of CT is its transformation ratio, expressed since a ratio of the rated primary to rated secondary current. Current transformers have two inherent errors: the current ratio and phase displacement. These two errors serve as a basis on which current transformers are classified for accuracy.
(b) Voltage Transformer or Potential Transformer (PT)
These instrument transformers are used for supplying the voltage circuit of indicating instruments, integrating meters, other measuring apparatus and protective relays or trip coils. These might be of single phase or three phase design and of the dry or oil immersed kinds. A voltage transformer or PT is rated in terms of the maximum burden (VA output) it will deliver without exceeding particular limits of error. Instead a power transformer is rated through the secondary output it will deliver without exceeding specified temperature increase. All voltage transformers are designed for a standard secondary voltage of 110 V or 110 / √3 V.