Measure knee point on a transformer

There was a query in all this about how to measure knee point on a transformer. Knee point relates to the ability of the secondary winding of a current transformer to produce enough voltage to drive the required current around the connected burden and what excitation current is "lost" inside the transformer to do that.

So for a current transformer, leave the primary winding open circuit. Apply a small increasing voltage to the secondary and measure the current that flows into the secondary - that is the excitation current. When you find a 10% increase in voltage causes a 50% increase in current, you have found the excitation point (slightly different definitions between IEC and IEEE Standards but very close)

It doesn't really make much sense to find the knee point of a Power Transformer or a Voltage Transformer. Both of them must transform the primary voltage to the secondary voltage without saturation for the rated current - i.e. knee point must be at least the rated secondary output voltage for the rated output current. Since we do not want the power transformer or voltage transformer to operate with more than rated secondary current we will usually apply protection to prevent that.

IF - a big IF - you had to measure the knee point of either a power transformer or voltage transformer, the process would be the same. But note that injecting voltage onto the secondary means injecting at least the rated secondary voltage onto the secondary terminals (which means a test set cable of producing that voltage safely) which naturally will result in the rated primary voltage on the primary terminals. Be careful!!

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