What is a voltage transformer

What is a transformer

Transformers are devices which are used to transfer electric power, this device consists of two or more windings which link with a common magnetic field. A transformer has played an important role to create an electrified world. The transmission of alternating current won’t be possible without the transformer.

What is the difference between current and voltage transformer

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12 types of transformers

What is a voltage transformer

Voltage transformers are also known as potential transformers, they are used in high voltage circuits. These transformers will allow protective relaying equipment to be operated at lower voltages. The voltage transformer is an instrument transformer that transforms voltage from one level to another level, a high voltage measurement is not possible because the measuring device won’t be able to handle high voltages. The voltage transformer is a high precision transformer in which the ratio of primary voltage to secondary voltage is a known constant, which changes very little with a load. Secondary voltage is almost in phase with the primary voltage, the nominal secondary voltage is 115V irrespective of what the primary voltage maybe and this will allow the standard instruments and relays to be used on the secondary side. The voltage transformer is used to monitor or measure the voltage on transmission lines and to isolate the metering equipment from these lines.

What is an ideal voltage transformer

An ideal voltage transformer is a transformer in which under no-load conditions where the load current is zero and the voltage drop is only caused by the magnetizing current and is thus negligible.

How does a voltage transformer work

The working of a voltage transformer is based on electromagnetic induction. These transformers can convert voltage from high to low and it would be good for transmission of power and to the meters which need to handle the voltage. Voltage transformers are used in three and single-phase systems. Voltage transformers are attached to a point in a system where it can be measured.

How is a voltage transformer constructed

Voltage transformer construction is similar to the conventional transformers, but the insulation between the primary and secondary windings must be great to withstand the full line voltage on the HV side. Regarding this one terminal of the secondary winding is always connected to the ground so the fatal shock danger can be eliminated when touching one of the secondary leads. It would look like the primary is isolated from the secondary but the distributed capacitance between the two windings makes an invisible connection and it could produce a high voltage between the secondary and the ground, so by grounding one of the secondary terminals the highest voltage between the secondary and the ground is limited to 115V. The nominal voltage of a potential transformer is usually 500 VA and because of that, the volume of insulation is greater than the volume of copper. Voltage transformers are installed in HV lines to measure the line to neutral voltage and this will eliminate the need for two HV bushings because one side of the primary is connected to ground.

Why voltage transformer is required

The power system has two basic needs, one is the metering of energy consumed and the other one is the protection of the electrical system from faults and disturbance. Faults can be of many types some faults can be detected by current measurement but current won’t provide any discretion about nature and location of the fault. So in order to know that voltage is also measured along with current during faults and by that we can compute the power or impedance of system along with its direction and over-fluxing protections are also configured from voltage transformer. Voltage signals are also used for synchronizing, disturbance recorders and event logs.

How voltage transformer is connected

Voltage transformers have a primary and one or more secondary windings, metering and protection devices are connected to the secondaries of the voltage transformer. In voltage operation or shunt mode, the primary winding is connected in parallel with the power system to transform the phase voltage to usually 63.5 volts which is suitable for the meter or relay.

There are three types of voltage transformer connections they are

V-V connection

This type is used for measurement and for those protections which do not require phase to neutral voltage input. The primary of voltage transformers is connected in voltage with identical voltage connection for the secondary.

Star to star connection

In this connection either three separate single-phase VT’s or a single three-phase, three limb VT is used. Both the primaries and secondaries are connected in star with both star neutrals solidly grounded. So the primary limb is connected between phase to the earth of the supply and a similar phase to earth voltage is replicated on the secondary.

Star/open delta connection

Primary windings are connected in star with star neutral solidly grounded and the secondaries are connected in series to form an open delta connection. This type of connection is called residual connection and requires either 3 single-phase VTs or a single 3 phase 5 limb VT. This residual connection is used for polarizing directional earth fault relays or for earth fault detection in a non-effectively grounded or isolated neutral system.

What is the voltage factor in a voltage transformer

The voltage factor determines the maximum output voltage for voltage transformers expressed in per unit of rated voltage, which in turn dependent on the system and voltage transformer earthing conditions. Voltage transformers used in the non-effectively earthed system have high voltage factors since, in the event of an earthed fault in one of the phases, the healthy phase voltage may rise to phase value.

What are the types of voltage transformer

Voltage transformers are of two types they are electromagnetic voltage transformer and protective voltage transformer. The electromagnetic voltage transformer is similar to the small power transformer and differs only in details of design that control ratio accuracy over the specified range of output, cooling, and insulation. In the case of high system voltages, the cost of the conventional potential transformer would be high. CVT is more economical than EVT when the high voltage capacitors can also serve for carrier current coupling. The capacitor will provide end to end communications between substations for distance relays telemetry, supervisory and voice communication.

How to do the protection of voltage transformer

Protection of voltage transformer from accidental overloads and short circuits across its second terminal is achieved by incorporating fuses or MCB in secondary circuits located near to the transformer as possible. Short circuit in the secondary winding gives only a few amperes in the primary winding and it is not sufficient to rupture a high voltage fuse at the primary side.