Current transformers (CT) and Potential Transformers (PT) are instrument transformer types used to step down transmission or distribution line voltages and current to the level that can be measured using measurement devices or controlled using control devices. The principle of operation of a certain type of CT and PT is similar to power transformers. Here are the basic difference between CT and PT. Potential transformers are also known as Voltage transformers in some countries.
Difference between Current transformer and Potential transformer / Voltage transformer
Below are the differences between current transformers and potential transformers:
| Property | Current transformer | Potential transformer/ Voltage transformer |
|---|---|---|
| Purpose | Transforms high current to low, measurable value. Read more | Steps down high voltage to low, measurable value. Read more |
| Windings | Primary: Typically Single turn. Secondary: A large number of turns. | Primary: A large number of turns. Secondary: Fewer turns. |
| Winding thickness | Primary: Heavy conductor capable of carrying high currents. Secondary: Thin conductor capable of carrying 5A-20A. | Primary: Thin conductor. Secondary: Thick conductor. |
| Flux Density | Low | High |
| Primary Connection | Connected in series to the ammeter or other instruments. | The primary of a potential transformer is connected across the conductor and earth. |
| Secondary Connection | Connected in series to the ammeter or other instruments. | Connected across the voltmeter or other instruments. |
| Secondary rating | The secondary rated current can be either 1A or 5A | The secondary rated voltage can be 100/√3 – 120/√3 or 100/3 – 120/3 V |
| Primary Impedance | High | Low |
| Symbols |  |  |
| Types | 1. Window CT or toroidal CT 2. Bar CT 3. Wound CT | 1. Electromagnetic induction type 2. Capacitive-coupled type 3. Optical type |
| Safety Consideration | The secondary terminals of a current transformer must never be open-circuited. | The secondary terminals of a voltage transformer must never be short-circuited. |
Connection: Current transformer vs Potential transformer
Connection difference between CT and PT
The connection difference between CT and PT can be understood from the below diagram. It can be noted that the current transformer is connected in series to the load whereas the potential transformer or voltage transformer is connected across the load. Also, the measurement devices is connected in series to the CT-secondary whereas they are connected in parallel to the PT-secondary.
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