What are instrument transformers?
Instrument transformers are used in connection with the measurement of voltage, current, power and power in alternating current circuits. There are mainly two reasons for the use of instrument transformers in measurement:
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To extend (multiply) the range of the measurement instrument
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To isolate the measurement instrument from a high-voltage line.
In power systems, levels of currents and voltages handled are very high, and therefore it will affect the measuring instrument, so the direct measurement is not possible. In such a case, instrument transformers can be effectively used to step down the voltage and current within range of the existing measuring instruments of moderate size.
There are two types of instrument transformers:
- Current transformers (CT)
- Potential Transformers (PT)
Current transformers (CT):
The primary winding of a current transformer is connected in series with the line carrying the main current. The secondary winding of the CT, where the current is many times stepped down, is directly connected across an ammeter, for measurement of current; or across the current coil of a wattmeter, for measurement of power; or across the current coil of a watt-hour meter for measurement of energy; or across a relay coil.
Potential Transformers (PT):
Potential transformers or PTs are used in such cases to operate voltmeters, potential coils of wattmeters, relays and other devices to be operated with high-voltage lines. The primary winding of the PT is connected across the high-voltage line whose voltage is to be measured and the measuring instruments are connected across the secondary of the PT. For all these purposes, it is essential that the secondary voltage be a definite fraction of the primary voltage, and in some applications, they need to be in the same phase as well.
Difference between CT and PT:
| Current Transformers (CT) | Potential Transformer (PT) |
|---|---|
| Reduce the main power line current to be measured by normal range instruments i.e. current is stepped down from ptimary to secondary | Reduce the main power line voltage to be measured by normal range of instrument. i.e., voltage is stepped down from primary to secondary |
| Primary winding of CT is connected in series with the main power line to sense current | Primary winding of PT is connected across the main power line to sense current |
| Primary winding has less number of turns as compared to the secondary winding | Primary winding has more number of turns as compared to the secondary winding |
| CT secondary side should never be open circuited while energized, to restrict accidental over-voltage | PT secondary can be safely open circuited even if the PT energized, since secondary voltage is always restricted by the turn ratio |
| In many cases, such as in bar type, in single primary winding type and in clamp-on type CT, the primary winding is nothing but main power line conductor itself | In all the PTs, separate primary as well as secondary windings are necessary. Primary winding terminals are connected across the main power line |
| While using CT for measurement of power, secondary winding is connected in series with the coil of wattmeter | While using PT for measurement of power, secondary winding is connected in parallel with the coil of wattmeter |