It is a device used to decrease the network voltage into values that can be handled by other equipment. Also called potential transformers (PT), are a parallel-connected type of instrument transformer, used for metering and protection in high-voltage circuits.

• Equation of VT: Vp/Vs=Np/Ns

• VT ratios are typically expressed as primary to secondary 132000/√3:120/√3

• A 110000/√3:110/√3 VT has a “VTR” of 1000

A. Electromagnetic type

The electromagnetic potential transformer is a wire-wound transformer. An electromagnetic voltage transformer is a voltage transformer that converts a primary voltage into a secondary voltage by electromagnetic induction. This transformer does not add other electrical components such as capacitors that change the primary voltage.

B. Optical type

Optical Current Transformers (OCTs) and Optical Voltage Transformers (OVTs) are an alternative to conventional transformers for protection and metering purposes with a much smaller footprint and weight.

The OVT presents a dynamic response capable of satisfactorily measuring disturbances and it can be used as a power quality monitor for distribution systems. OVT is capable to reproduce up to the 41st harmonics without significative distortion and impulsive surges up to 2.5 kHz.

When compared with conventional systems to monitor power quality, the OVT can be remote-monitored, and therefore, be installed at strategic locations on lines to be monitored kilometers away, without the need to be electrically powered

B. Capacitive Voltage Transformer

The size of an Inductive Voltage Transformer is proportional to its Nominal Voltage. In High & Extra High Voltage Systems, the voltage transformers would have:

• Large no. of Windings as

• High and Big Insulating Material

The highly insulated transformer is quite expensive as compared to the normal transformer. For reducing the cost, the Capacitive Voltage Transformer is used in the system.

Also, for OHL Capacitance to ground required a Capacitive voltage transformer to prevent resonance in the case of Inductive Voltage Transformers

A Capacitor Voltage Transformer (CVT) is an Instrument Transformer used in power systems ((over 100 kV)).

a. Step-down High & Extra high voltage signals and provide low voltage signals for Measuring devices or Protective Relays.

b. Power Line Communications “PLC” between two substations by the Capacitive Coupling Voltage Transformers “CCVT”.

Capacitive Voltage Transformer Components are:

• Two Capacitors

In practice, Capacitor C1 ((Stack of smaller capacitors connected in series)). This provides a large voltage drop across C1 and a relatively small voltage drop across C2. As the majority of the voltage drop is on C1, this reduces the required insulation level of the voltage transformer.

The capacitance in the voltage divider, in series with the inductance of the transformer and the series reactor, could be resonant with the external circuit capacitance and reactance and that may saturate the iron core of the transformer by various disturbances in the network. This phenomenon can also overheat the electromagnetic unit or lead to insulation breakdown.

• Compensating reactor: -

Compensates the phase angle shift caused by the capacitive voltage divider.

• Transformer: -

Used to isolate and further step down the voltage for the instrumentation or protective relay.

The trimming windings are used for fine-tuning the output signal to correspond with the required accuracy class requirements.

• Ferro-resonance suppression circuits: -

a. Active Ferro-resonance Suppression Circuits (AFSC) based on a series-parallel RLC filter.

b. Passive Ferro-resonance Suppression Circuits (PFSC), based on a saturable inductor with iron core in series with a damping resistance

Its inclusion may affect considerably the transient response of the transformer, causing mis-operation of protective relays and associated systems

The accuracy class of the Voltage Transformer could be combined, below is the explanation of VT Class 0.2/3P

• Metering Class: The VT core could behave as a metering core with accuracy ≤ 0.2% if the voltage is between 80 and 120% of the rated primary voltage and for any load between 25 and 100% of the rated accuracy power with an inductive power factor of 0.8.

• Protection Class: The VT core could behave as a metering core with accuracy ≤ 3% if the voltage is between 5% to 80% and between 120% to 190% of the rated primary voltage and for any load between 25 and 100% of the rated accuracy power with an inductive power factor of 0.8.

• Also, at 2 % of rated voltage, the limits of error and phase displacement with burdens between 25 % and 100 % of rated burden at a power factor of 0,8 lagging will be twice of those of protection class [either 2x3% or 2x6%].

A. 3 Phases VT connections

The electromagnetic potential transformer is a wire-wound transformer. An electromagnetic voltage transformer is a voltage transformer that converts a primary voltage into a secondary voltage by electromagnetic induction. This transformer does not add other electrical components such as capacitors that change the primary voltage.

B. Open and Broken Delta Connections

• Broken Delta

It is to connect the secondary windings of the voltage transformer together in the form of delta while leaving the ends of the delta open to connect an EF relay that operates with the directional sum of the three-phase voltages.

The source is isolated from the ground.

• Open Delta

Only two phases are connected to the voltage transformer, so that they are connected in series and in a Star way, and here the voltage value becomes 1/root (3) of the delta configuration voltage, and this is why the open delta gives only 57.7% of the power of conventional transformers