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AC Metrology
11 questions By Tony R. Kuphaldt
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Question 1 of 11
In power distribution systems, it is very important to be able to measure line voltage. You cannot control what you cannot measure, and it is important to control power line voltage so as to not exceed the insulators’ ratings.
But how do you safely measure the voltage of a 750 kV power line? Obviously, no voltmeter small enough to be located on a control panel could safely handle 750,000 volts applied to it, as a voltage that high is capable of arcing several feet through the air (not to mention the safety hazards of having wires behind the meter panel connecting straight to the power line!).
In industry, specialized transformers are used to safely measure the high voltages on power lines. Describe what is special about these “potential transformers,” and how they are implemented to measure dangerous voltages.
Reveal answerA “potential transformer,” or “PT,” is a step-down transformer with a very precise winding turns ratio, so that the secondary voltage is a precise and known fraction of the primary voltage.
Follow-up question: in addition to stepping the line voltage down to relatively safe levels, potential transformers also provide one more important safety feature for voltage measurement. Describe what this extra feature is, and why it is important. Hint: all transformers except for autotransformers provide this feature!
Notes:Ask students to draw a rough schematic diagram of how a potential transformer would be placed in a complete voltage-measurement circuit, with power lines, panel-mounted voltmeter mechanism, safety fuses, etc.
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Question 2 of 11
A common instrument used for measuring high AC currents in power systems is a current transformer, abbreviated “CT”. Current transformers usually take the form of a “donut,” through which the current-carrying conductor passes:
The purpose of a current transformer is to create a secondary current that is a precise fraction of the primary current, for easier measurement of current in the power conductor.
Given this function, would current transformers be considered a “step-up” or “step-down” transformer? Also, draw how the secondary windings of a current transformer are arranged around its toroidal core.
Reveal answerFrom the perspective of voltage, which is usually how the terms “step-up” and “step-down” are referenced, a current transformer is a “step-up” transformer. Its secondary windings are wound perpendicular to the magnetic flux path, as typical in all transformers.
Notes:The question of whether the current transformer is a “step-down” or “step-up” has an important safety implication for students to realize. Ask your students to describe what conditions might prove the most dangerous when working around current transformers, given their “step-up” nature with reference to voltage.
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Question 3 of 11
How much current will be output by a current transformer if the load current is 350 amps and the CT ratio is 600:5?
Reveal answerSecondary current = 2.917 amps
Notes:This question is an exercise in mathematical ratios.
