In transmission and distribution systems it is impossible to measure such high voltages and currents using normal meters. Measurement of high currents and voltages using shunts and multipliers respectively is not preferred as they have many disadvantages like high power consumptions, insulation problems, no electrical isolation, etc.

In order to measure these high voltages and currents, there is a need for instrument transformers in addition to normal meters. The instruments transformers are of two types potential and current transformer.

## Current Transformer :

To measure high alternating currents, an ammeter of a small range can be used in conjunction with a current transformer (CT). A current transformer is basically a voltage step-up transformer and thus the current is stepped down depending upon the transformation ratio, n = n_{2}/n_{1}. Hence, the ammeter in the secondary circuit measures the current of I_{L}/n.

#### Where,- I
_{L} = load current

So, by multiplying the ammeter reading by n, we get the total load current flowing through CT's primary winding.

_{L}= load current## Potential Transformer :

To measure high alternating voltage, a voltmeter of a small range can be used in conjunction with a potential transformer (PT). A potential transformer is basically a voltage step-down transformer whose primary is connected across the supply and secondary across a voltmeter of small range. Hence the voltage indicated by the voltmeter depends upon the transformation ratio n_{2}/n_{1}.

#### Where,- n
_{1} = number of primary winding turns - n
_{2} = number of secondary winding turns

So, the voltmeter reading is divided by the transformation ratio to obtain the magnitude of the supply voltage.

_{1}= number of primary winding turns_{2}= number of secondary winding turns## Difference Between Current and Potential Transformer :

Current Transformer | Potential Transformer |
---|---|

The primary of the CT is connected in series with the line carrying the current to be measured. | The primary winding of the PT is connected across the line carrying the voltages to be measured. |

CT acts as a series transformer with its secondary almost shorted. | PT acts as a parallel transformer with its secondary almost open-circuited. |

The line current and exciting current of a CT vary over a wide limit. | The line voltage and exciting current of PT vary only over a small limit. |

The primary current of a CT is independent of the load on the secondary winding. | The primary current of a PT depends on the load on the secondary winding. |

The primary winding of a CT consist of very few turns. | The primary winding of a PT consist of comparatively more turns. |

The secondary winding of CT consist of more number turns. | The secondary winding of PT consist of fewer turns. |

The primary winding of CT carries a full line current. | The primary winding of a PT carries full line voltage. |

The current range of secondary winding of a CT will be 1-5A. | The voltage range of the secondary winding of a PT will be 110V. |

The primary impedance of a CT is very low. | The primary impedance of a PT is high. |

The transformation ratio of CT is High. | The transformation ratio of PT is low. |

Depending upon construction there are wound type and bar-type current transformers. | There are two types of PT's Electromagnetic type and Capacitor voltage type. |

There are two types of errors in CTs. They are ratio and phase angle error. | There are two types of errors in PTs. They are ratio and phase angle errors. |

Current transformers can be used for measuring current and power, for relay operation, Monitoring the power grid. | Potential transformers can be used for measuring voltage and power, electrical protection systems, for synchronizing generators. |