Difference between current transformers and Potential transformers

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 are 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:

PropertyCurrent transformerPotential transformer/
Voltage transformer
PurposeTransforms high current to low, measurable value. Read moreSteps down high voltage to low, measurable value. Read more
WindingsPrimary: Typically Single turn.
Secondary: A large number of turns.
Primary: A large number of turns.
Secondary: Fewer turns.
Winding thicknessPrimary: Heavy conductor capable of carrying high currents.
Secondary: Thin conductor capable of carrying 5A-20A.
Primary: Thin conductor.
Secondary: Thick conductor.
Flux DensityLowHigh
Primary
Connection
The primary winding is connected in series to the current carrying conductorThe 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 ratingThe secondary rated current can be either 1A or 5AThe secondary rated voltage can be 100/√3 – 120/√3 or 100/3 – 120/3 V
Primary ImpedanceHighLow
Symbols Symbol of CT- Electrical Classroom Symbol of Potential transformer or Voltage transformer
Types1. 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

Current transformer connections

CT connection diagram

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 where as they are connected in parallel to the PT-secondary.

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