Medium Voltage Application Guide

SWITCHGEAR
Many manufacturers use a continuous overload rating of 1.2 times the primary voltage rating without exceeding
thermal capabilities which can lead to winding and insulation failure.
Output power
This is the apparent output power rating of a transformer when nominal voltage is applied to the primary.
For a 3-phase voltage transformer
arrangement, S = √3 x U x I
Where:
S = apparent output power (VA)
U = secondary line voltage rating (V)
For a single phase voltage transformer
arrangement, S = U x I
I = secondary line current rating (A)
Standard values are: 10, 15, 25, 30, 50, 75, 100, 120 VA
Control supply power transformers are usually single phase transformers, with an output power rating of 500 VA to
5000 VA. For further details, refer to Control power supply transformer sizing on page 128.
Accuracy class
Designates the maximum error of the transformed voltage and phase angles at rated primary voltage.
IEC 61869-3 specifies standard accuracy classes for voltage transformers.
Accuracy classes for metering applications
Class
Metering class 0.2
Metering class 0.5 and 1.0
Metering class 3.0
Application
High accuracy applications (eg revenue metering)
General use
Rarely used
Accuracy classes for protection applications
Class Voltage error ±%
(between 0.5~1.5 x U p for earthed systems;
between 0.5~1.9 x U p for unearthed systems)
Phase shift in minutes (60 minutes = 1 degree)
(between 0.5~1.5 x U p for earthed systems; between
0.5~1.9 x U p for unearthed systems)
3P 3 120
6P 6 24
Control supply power transformer sizing
Sizing a control supply power transformer (CPT) requires analysis of the expected secondary load.
Information required:
total inrush VA of the load
total sealed VA of the load
acceptable voltage drop level on the secondary of the transformer at inrush stage.
In most cases, the inrush period is 20 to 100 milliseconds. The power factor of the inrush current is assumed to be
0.4 (for other values, use the adjustment factor in the table below).
To select a CPT:
1. Determine the acceptable voltage drop on the secondary of the transformer at inrush stage. In the appropriate
voltage drop column, select an Inrush VA value larger than the calculated total inrush VA of the secondary load.
Note: the total inrush VA of the secondary load must include any resistive loading.
2. Find the corresponding transformer power rating, in the Nominal VA rating column.
3. Check that the Nominal VA rating value is larger than the calculated total sealed VA of the transformer
secondary load.
4. If the inrush VA power factor is different from 0.4, multiply the Nominal VA rating by the Power Factor
Adjustment factor.
Page 128 Medium Voltage Application Guide 710-12280-00A