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Transformer Current Ratio Calculator

Given primary voltage = 69 kV ; secondary voltage = 12.47 kV
secondary current = 1,296.447 A ; Calculate the expected primary current




Application of Current Ratio Formula in Estimating Short Circuit Fault Current

Given the normal operational current (ampere) data of 320 MVA , 3 Phase Transformer, 525 / 230 kV as shown in table below.

Estimate the snapshot of temporary transient primary short circuit current, when there is SLG Fault in transmission line primary voltage source due to disturbance using the 10 kV instantaneous voltage drop recorded by PI historian during SLG fault.

Voltages(kV) Amperes
525 Primary Voltage 803.29 Secondary Current
230 Secondary Voltage 351.92 Primary Current
10 Primary SLG Fault 803.29 Secondary Current
230 Secondary Voltage 18,475.67 Short Circuit Primary Current




Given the normal operational current (ampere) data of 600 MVA , 3 Phase Transformer, 525 / 230 kV as shown in table below.

Estimate the secondary short circuit current, when the secondary voltage source is shorted and the instantaneous voltage recorded by PI historian during short circuit is 25 kV.

Voltages(kV) Amperes
525 Primary Voltage 1506.18 Secondary Current
230 Secondary Voltage 659.85 Primary Current
525 Primary Voltage 13,856.85 Short Circuit Secondary Current
25 Secondary SLG Fault 659.85 Primary Current

Rationale Behind Transformer Current Ratio

In a no loss , ideal transformer , 100% efficient power transfer is usually assumed.

Power transformer efficiency based on efficiency test report is usually from 95% - 98%. So it is reasonable enough to assume a 100% efficiency.

What does 100% power transfer efficiency means? It means the
power generated in primary winding = power generated in secondary winding

Formula recall of power ( P = I V ).

Power Primary winding = Ip Vp

Power Secondary winding = Is Vs

IpVp = Is Vs

By algebraic manipulation.

Vp / Vs = Is / Ip

By applying this knowledge and using PI historian snapshot value, we can estimate the magnitude of short circuit current and estimate the magnetic forces to the primary or secondary winding of a power transformer depending on where the SLG fault happened. Even better if you have Pi historian data, you can create a streaming data of SLG Fault by using PI expression equation.

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