One of the many challenges faced when operating a high voltage AC transmission network is how to monitor critical measurements in the system. Watts are important in calculating load flow and direction, accurate current readings (amps) are needed to ensure components are not overloaded, and voltages must be strictly maintained within the tolerances of the switchgear and transmission lines. 
This function is achieved by the use of "instrument transformers". These are highly engineered and accurate current transformers (CTs) and voltage (also called "potential") transformers (VTs/PTs).
Effectively, these devices transform extremely high currents & voltages into values that can be tolerated by low voltage sensitive measuring and protection equipment. Using the derived current and voltage measurements, power flow direction and magnitude(watts), frequency (Hz), phase angle, harmonics, and others can be calculated.
They are typically connected in sets of three, one transformer on each phase, and strategically located to gain the most benefit from the least number of devices.
The current transformer displayed outside our museum was designed to transform up to 3000 amps at 500,000 volts to 5 amps which can then be easily handled by the low voltage measuring instruments and protective relays within the substation.

Initially purchased as a spare unit, this current transformer was donated to the Edmonton Power Historical Foundation in 2016 by EPCOR Utilities when it was made obsolete by an upgrade to the Genesee Switchyard.