http://www.svri.nl/en/selecting-a-voltage-transformer-vt/

21 January 2019

nl | en Posts

Selecting a Voltage Transformer (VT)

If the ratio, burden, class and optionally voltage factor are selected, the voltage transformer (VT) or potential transformer (PT) has been specified basically. Of course additional requirements like type, frequency, environmental conditions should also be specified.

 

Voltage transformator configurations

For 3-phase metering, 1-pole voltage transformers are configured in star (Y), and 2-pole voltage transformers in open-delta (V). 

VT Configurations

In an isolated neutral system, or in a resonant earthed system, without earth-fault tripping, select a voltage factor of 1.9 Un for 8 hours for 1-pole voltage transformers. Then the voltage transformers can operate for 8 hours during an earth fault in the power system. If you don’t specify it, the manufacturer will probably assume 1.2 Un continuous.

 

Transformation ratio Up/Us

The transformation ratio is the ratio between the primary and secondary voltage.

The secondary voltage Us is mostly:
– 100 V or 110 V according to IEC for open-delta configuration.
– 100:√3 V or 110:√3 V according to IEC for star configuration.
– 120 V according to IEEE for open-delta and star configuration.

For the primary voltage Up or transformation ratio Kn you select:
– for Up the nominal power system voltage according to IEC. Kn follows out of Up and Us.
– for Kn a simple number according to IEEE. Up follows out of Kn and Us.

 

Accuracy

The accuracy of a voltage transformer depends among others of:
– burden
– class/saturation
– load
– frequency

For the burden according to IEC select a minimum value that equals 1x and a maximum value that equals 4x the actual load, from the range:
10 – 15 – 25 – 30 – 50 – 75 – 100 – 150 – 200 – 300 – 400 – 500 VA
The values underlined are preferred values. The range is applicable for pf = 0.8 lagging.

For the burden according to IEEE you can select:

M 35 VA, pf = 0.2
W 12.5 VA, pf = 0.1
X 25 VA, pf = 0.7
Y 75 VA, pf = 0.85
Z 200 VA, pf = 0.85
ZZ 400 VA, pf = 0.85

 

For the class select:

Application Class IEC Class IEEE
Very accurate metering 0.1 – 0.2 0.15
Tariff (kWh) metering 0.2 – 0.5 0.3
Instrument meters and control 1 0.6
Protection (P) 1 – 3P – 6P 1.2 – 1.2R

 

 

Examples and tips

  • If the rated generator voltage is 11 kV (50 Hz), then the specification of a voltage transformer for protection could be: 11000:√3 /110:√3 V, 10 VA, class 1
  • For kWh metering it could be: 11000:√3 /110:√3 V, 10 VA, class 0.1
  • The accuracy class is only applicable if the total load is 25 – 100% of the voltage transformer burden.
  • Voltage transformers for metering are provided with a defined accuracy for voltages of 80-120%. For 3P and 6P transformers it is 5-120%.
  • If the rated generator voltage is 13.8 kV, the IEEE specification of a voltage transformer for protection could be: 14400/120 V,  1.2 X (, 60 Hz). The transformation ratio is 120:1.

 

See also

 

References

  • IEC 61869-1 – Instrument transformers – Part 1: General requirements
  • IEC 61869-3 – Instrument transformers – Part 3: Additional requirements for inductive voltage transformers
  • IEC 60044-2 en IEC 186 are withdrawn voltage transformer standards
  • IEEE Std C57.13-2008 – IEEE Standard Requirements for Instrument Transformers