Current transformers (CTs) and voltage transformers (VTs), also known as potential transformers (PTs), are both types of instrument transformers used in electrical power systems. While they serve different purposes, they share the common function of transforming electrical quantities to levels suitable for measurement and protection purposes. Here are the key differences between current transformers and voltage transformers:

1. Function:

   • Current Transformer (CT): A CT is used to step down high currents in power systems to levels suitable for measurement and protection purposes. It produces a secondary current proportional to the primary current flowing through the conductor passing through its core.
   • Voltage Transformer (VT) or Potential Transformer (PT): A VT is used to step down high voltages in power systems to levels suitable for measurement and protection purposes. It produces a secondary voltage proportional to the primary voltage across its terminals.

2. Connection:

   • Current Transformer (CT): CTs are typically connected in series with the primary conductor carrying the current to be measured or protected. The secondary winding of the CT is connected to measuring and protective devices such as ammeters, wattmeters, relays, or meters.
   • Voltage Transformer (VT): VTs are connected in parallel with the circuit or device whose voltage is to be measured or monitored. The secondary winding of the VT is connected to measuring and protective devices such as voltmeters, wattmeters, relays, or meters.

3. Construction:

   • Current Transformer (CT): CTs consist of a primary winding (usually one turn or a few turns of a heavy conductor) and a secondary winding (many turns of fine wire) wound around a magnetic core. The primary winding is connected in series with the circuit carrying the current to be measured.
   • Voltage Transformer (VT): VTs typically consist of a high-voltage primary winding and a low-voltage secondary winding wound around a magnetic core. The primary winding is connected in parallel with the circuit whose voltage is to be measured.

4. Ratio:

   • Current Transformer (CT): The turns ratio of a CT is determined by the ratio of the number of turns in the secondary winding to the number of turns in the primary winding. For example, a CT with a turns ratio of 1000:5 will produce a secondary current of 5 amps when the primary current is 1000 amps.
   • Voltage Transformer (VT): The turns ratio of a VT is determined by the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. For example, a VT with a turns ratio of 10:1 will produce a secondary voltage of 1/10th of the primary voltage.

5. Applications:

   • Current Transformer (CT): CTs are used for metering, monitoring, and protection of electrical systems, such as measuring current for billing purposes, monitoring load currents, and providing inputs to protective relays for detecting faults.
   • Voltage Transformer (VT): VTs are used for metering, monitoring, and protection of electrical systems, such as measuring voltage for billing purposes, monitoring voltage levels, and providing inputs to protective relays for detecting faults.

In summary, while both current transformers and voltage transformers are instrument transformers used in electrical power systems, they differ in their function, connection, construction, ratio, and applications. CTs step down high currents to measurable levels, while VTs step down high voltages to measurable levels. They are both essential components for accurate measurement and protection in power systems.