Medium Voltage Application Guide

13763.A
13762.A
13764.A
13765.A
Voltage Transformers
SWITCHGEAR
A voltage transformer (VT) or instrument transformer is used to produce a lower secondary voltage which is directly
proportional to the primary voltage both in value and phase angle.
In medium voltage switchgear, a 3-phase voltage transformer arrangement is typically derived by using three, single
phase transformer poles. Each pole consists of a single primary and secondary winding encapsulated in epoxy resin
and encased with insulating material. In most single phase pole designs, the primary winding has integrated fusing.
In a 3-phase arrangement, the primary windings of each individual pole are externally connected in star configuration.
Each end of the secondary winding is brought out to a customer termination box. The secondary windings can be
externally connected in star or delta configuration and must always be separately fused. Star connection of the
secondary winding is preferable, as this provides voltage stability through solid earthing of the neutral point and
3-phase and neutral is available for voltage measurement.
Switchgear installations use either fixed or withdrawable voltage transformers. Withdrawable voltage transformers
are mounted on a draw-out truck arrangement.
The power rating and accuracy of a transformer arrangement will depend on its application. For metering,
protection and indication, power ratings are small, with accuracies in the range of ±0.5-3.0%. A voltage transformer
used to provide a control supply may have a power rating above 5 kVA. In this case, accuracy is not as important.
Relevant standards: IEC 61869-3, IEEE C57.13.
Voltage transformer (3-phase, fixed)
Control supply transformer (single phase, fixed)
1
L1 L2 L3
2
1
L1 L2 L3
A
A
A
F1
3.3 kV-R3
B a
B a
B a
A
110 VAC-R3
b
b
b
B
l1
l2
l3
N
3
P1
P2
F1
4.2 kV/2A
F2
4.2 kV/2A
CPT
3.3 kV/110 VAC
1000 VA
S1
S2
2
A
B
Mains supply
Mains supply
3 x single phase VT poles 110 VAC secondary (external fusing required)
Primary winding
Secondary winding
3-phase and neutral, 110 VAC phase-to-phase
(external fusing required)
IEC ratings
The main standard applying to MV voltage transformers is 61869-3.
Nominal voltage
Example: 3.3 kV/110 VAC
Denotes a primary phase-to-phase voltage rating of 3300 V and
a secondary phase-to-phase voltage rating of 110 VAC.
3300/110 VAC
Example: 3.3 kV-3/110-3 VAC
Denotes a primary phase-to-phase voltage rating of 3300 V and
phase-to-earth rating of 1905 V
(ie: 3300 x √3), and a secondary phase-to-phase voltage rating of
110 VAC and phase-to-earth rating of 63.5 VAC (ie: 110 x √3)
3300- 3/110- 3 VAC
710-12280-00A Medium Voltage Application Guide Page 127