CONVENIOS REGIONALES

Voltage transformers have two main functions:

adapting the HVA current value of the primary to the characteristics of the measurement or protection devices by a low proportional secondary voltage isolating the power circuits from the measurement and/or protection circuit.

3.4.1.1. Composition and type

They consist of a primary winding, a magnetic circuit and one or more secondary windings, all coated in an insulating resin. There are two types, according to their connections:

phase/phase: primary connected between two phases

phase/earth: primary connected between one phase and earth.

3.4.1.2. General characteristics

They are defined by the IEC 60044-2 standard and have the following characteristics (see table):

Voltage transformer characteristics table

Characteristics Rated values

isolating voltage (kV) 3.6 - 7.2 - 12 17.5 24 36 power frequency withstand voltage (kV)

1 min 10 20 28 38 50 70

lightning surge withstand (kV - peak) 40 60 75 95 125 170 frequency (Hz) 50 - 60

primary voltage Upn (kV)

(divided by e if single phase) 3 -3.3 -6 -6.6 -10 -11 -13.8 -15 - 16.5 - 20 -22 secondary voltage Usn (V) 100 - 110 or 100 / 3 - 110 / 3

rated power (VA) 10 - 15 - 25 - 30 - 50 - 75 -100 - 150 - 200 - 300 - 400 - 500

Table 12: Operating characteristics of a VT

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Rated voltage

In practice, it is the rated voltage of the installation (e.g.: Ur = 24 kV).

Rated isolation level

maximum 1min power frequency withstand voltage (Ud in kV) maximum impulse withstand voltage (Up in kV peak)

E.g.: with 24 kV the 1 min withstand voltage is Ud = 50 kV and the impulse withstand voltage is Up = 125 kV peak.

Rated frequency

Rated primary voltage Upn

Depending on their design the voltage transformers are connected:

either between phase and earth and in this case Upn = Ur / e either between phases and in this case Upn = Ur

Rated secondary voltage Usn

In practice, in Europe 100 V or 110 V is used for phase/phase voltage transformers. For single phase phase/earth transformers the secondary voltage must be divided by 3 (100 /

3 or 110 / 3)

Rated transformation ratio

It is the ratio between the rated primary and rated secondary currents: Kn = Upn / Usn This ratio is constant and the secondary voltage is independent of the burden.

Rated power

It is the apparent power (in VA) which the VT can supply at the secondary for the rated secondary voltage for which the accuracy is guaranteed (secondary connected to the rated burden). See the standardised values in the characteristics table.

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Accuracy class

It defines the guaranteed error limits on the transformation ratio and on the phase error in specified power and voltage conditions.

Voltage error (ε %)

It is the error which the transformer introduces in the voltage measurement when the transformation ratio is different to the rated value.

Phase difference or phase error (ψ in minutes)

Phase difference between the primary and secondary voltages, expressed in minutes of angle.

Rated voltage factor KT

It is the factor, which is a multiple of the rated primary voltage and which determines the maximum voltage for which the transformer must meet the specified heating and accuracy requirements. This maximum operating voltage depends on the network's neutral point arrangement and on the primary winding earthing conditions (see "Voltage factors table").

The voltage transformer must be able to withstand it until the fault is eliminated.

Voltage

factor Rated time Primary winding

connection method Network neutral point arrangement continuous between phases any

1.2 continuous

between the neutral point of the star-connected transformer and earth

any

1.2 continuous

1.5 30 s between phase and earth directly earthed 1.2 continuous

1.9 30 s between phase and earth earthed by limiting resistor with automatic elimination of the earth fault 1.2 continuous

1.9 8 h between phase and earth isolated neutral without automatic elimination of the earth fault 1.2 continuous

1.9 8 h between phase and earth earthed by limiting resistor with automatic elimination of the earth fault

Table 13: Rated voltage factor KT

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Heating power

It is the apparent power which the transformer must supply in continuous operating conditions at its rated secondary voltage without exceeding the temperature limits in the standards.

Operation of a VT

The operation of a VT is more simple that that of a CT because the secondary voltage is almost independent of the burden since it is connected to a high impedance (operates almost as an open circuit).

Also, the secondary must not be short-circuited. In these conditions an excessively high current would damage the transformer.

Connecting a VT

Figure 28: Simplified diagram and connection of a VT

It can be between phases or between phase and earth (see diagram) and the connections are made on the terminals with France markings, as for the CTs.

VT configurations

There are several measurement configurations possible (see figure)

3 transformers in star configuration: requires 1 isolated HVA terminal per transformer

2-transformer configuration, called a V configuration: requires 2 isolated HVA terminals per transformer.

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Figure 29: Example of a VT configuration Summary

Voltage transformers (VT) meet the IEC 60044-2 standard. Their function is to supply a voltage at the secondary which is proportional to that of the HVA circuit in which they are installed. The primary, installed in parallel on the HVA network between phases or

between phase and earth is subject to the same overvoltages as the network.

The secondary delivers an almost constant voltage whatever the burden. The secondary must never be short-circuited.

In an isolated neutral point arrangement, all the phase-neutral VTs must be suitably loaded to avoid the risks of ferroresonance.