Current transformer and voltage transformer measuring instrument transformers are designed to operate at low current and low voltage. Safety is also a reason for not connecting test instruments to sources operating at high voltage. An instrument transformer reduces high voltages and currents to safe values.
Instrument transformers performs two functions.
- It serves as a ratio device so that low-voltage and low-current instruments can indicate accurate values for high-voltage and high current equipment.
- It serves as an isolating device, protecting the test equipment and operating personnel from high voltages
There are two kinds of instrument transformers.
- Potential transformers
- Current transformers
Potential transformers are the types of instrument transformers that establish a known ratio between the potential difference across the equipment and the potential difference across a voltmeter. For example, suppose a potential transformer establishes a ratio of 500 to 1. If a voltmeter connected across the secondary terminals reads 132 V, you know the potential difference across the primary terminals is 66,000 V or 66 kV.
Current transformers are the types of instrument transformers that establish a known ratio between the current in the equipment and the current measured by an ammeter. These transformers provide a means of measuring currents beyond the normal range of most ammeters. In addition, the transformer isolates the equipment from the operator. A current transformer increases the potential difference, but decreases the current to a measurable value. The secondary then contains more turns than the primary. The ratio of the primary to the secondary current is inversely proportional to the ratio of the primary to the secondary turns.
The primary winding in a current transformer may be nothing more than a solid copper busbar. An iron core fits around the busbar, and the secondary winding is wound around the core. Another kind of current transformer consists of a doughnut-shaped secondary coil that you slip over a feeder cable.
Safe Use of Current Transformer Safety
To prevent damage to the equipment and injury to yourself and other people, the secondary winding of the current transformer must be short-circuited before removing the transformer from or connecting it to any equipment. Most current transformers have a shorting device that shorts the secondary winding automatically.
Instrument Transformers Application
- To measure high value of electric current i.e Current Transformer(CT)
- For measure high value of electric voltages or potential difference i.e Potential Transformer(PT)
- To measure electric power with uses of both CT and PT
You can see in the above diagram how to measure the current, potential difference, and power in a circuit having a high current and high potential difference. Notice that the potential transformer is always connected in parallel of a circuit and current transformer is connected in series of the circuit.
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