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Step-down substation Big Picture Challenge

Students are asked to order a transformer for one of the substations. They learn about how heat loss can be avoided when electricity is transported, and how transformers work.

Students then choose the appropriate formulas to work out the power specification and the transformer efficiency, then they are guided through working out the number of turns needed and which cooling system is required.

• Understand why the voltage of electricity is stepped up before it is transported.
• Understand what a transformer is used for and how it works.
• Know that current x voltage = power
• Know that useful output ÷ useful input x 100% = efficiency %

Answers for Big picture challenge: Step-down substation

Power specification

Correct formula:
Power (W)  =  Current (A) x Voltage (V)

Primary coil:
110 A x 400,000 V  =  44,000,000 W

Secondary coil:
2156 A x 20,000 V  =  43,120,000 W

Transformer efficiency

Correct formula:
Useful output (power in secondary coil, MW) x 100 / Useful input (power in primary coil, MW)

(43.12 MW / 44 MW) x 100  =  98%

Number of turns

Correct formula:
(Voltage (primary, V) / Voltage (secondary, V))  =  (Number of turns (primary)) / (Number of turns (secondary))

Cooling system

Oil filled, oil circulated by pumps and forced air cooling via fans