Current Transformer vs Voltage Transformer (Potential Transformer)
A transformer is a device that transfers electrical energy from one circuit to another using electromagnetic induction through inductively coupled conductors, also called the transformer’s coils. Based on the number of turnings in the secondary coil, an electromotive force and a related current are induced in the secondary coil. It is used to control the current and hence the voltage in the secondary circuit.
Depending on the significant output in the secondary coil (current / voltage), the transformer is called a voltage (potential) transformer or current transformer. Voltage transformers and current transformers are mainly used in instrumentation hence collectively known as instrument transformers. Its other uses are power system protection and control.
More about Voltage (Potential) Transformer
A transformer is a device that is used to increase or decrease the voltage of a system, keeping the net power loss minimal. A transformer used to increase the voltage is known as a step-up transformer, and a transformer used to decrease the voltage is known as the step down transformers. The output voltage of a potential transformer is proportional to the number of turnings in the secondary coil, which is a step down transformer.
Suppose in the primary and the secondary coils, number of turnings are NP & NS, and the voltages are VP & VS. Then the voltage in the secondary can be obtained by VS/VP = NS/NP.
Potential transformers are used in instrumentation, to obtain a precise voltage output with a manageable potential difference across the load. Usually the secondary voltage of a potential transformer is rated for 69 V or 120 V for a given rated primary voltage, designed to be compatible with the input ratings of protective relays.
More about Current Transformer
A current transformer is a transformer designed to provide the secondary current proportional to the current flowing in its primary coil. Current transformers are commonly used in measuring instruments and protective relays used in the electrical power networks, where they permit measuring large currents safely, which are often accompanied with high voltages. The current transformer can safely isolate the measurement and control circuitry in the instrument from high voltages that are usually present in the power transmission circuits.
Current transformers usually consist of a single primary turning and a well-insulated toroidal secondary with multiple turnings. Current in the secondary can be obtained by Is/IP = NS/NP. The current transformers are usually denoted by the current ratio of the primary to secondary. Precaution must be taken not to disconnect the secondary circuit while the current is flowing through the primary, because a large voltage is induced in the secondary coil.
What is the difference between Current Transformer and Voltage Transformer (Potential Transformer) ?
• Potential transformers lower the voltage with an increase of current in the secondary, while current transformers lower the current with an increase in the voltage.
• The potential transformers are used as high voltage voltmeters and ordinary voltmeters. The current transformers are used in place of the ordinary ammeters, to measure the high values of currents in the high voltage power applications.
• In potential transformers, primary may have several windings, but in the current transformer primary usually has one turning.
• In three phase power transmission, for measurement in the same line, three current transformers must be used, while only a single potential transformer is sufficient.