Voltage Transformer
Trainee-Electrical Engineer at Oman Electricity Transmission Company
2 articles Following
VT(Voltage transformer) or Potential transformer:
A voltage transformer is a device that is used to transfer higher voltage from the power
grid to lower voltage that can be fed to protection, measurement, and control devices.
The primary voltage of the transformer is proportional to the secondary voltage i.e.
when the primary voltage increases the secondary will also increase. The primary
winding of VT will be connected in parallel to the power grid and the secondary will be
connected to the devices.
What are some of the important things that you have to understand when we talk about
VT :
• VT ratio:
It is the ratio between the primary and secondary voltage. The Popular rating of
secondary voltage that will be connected to the devices is 110v. Furthermore, the turn
ratio of VT can be express as “primary voltage / 110v” E.g. 11KV/110V.
• Burden:
It is the total load that will be connected to the secondary. The burden of VT can be
expressed as the total impedance in ohms of the circuit or the maximum load in VA
(Accuracy Power) that can be applied to the VT secondary. Moreover, VT is usually
connected to a very high impedance and used as an almost open circuit i.e. the burden
is high in VT. The standardized values for the burden are: 10 - 15 - 25 - 30 - 50 - 75 - 100
- 150 - 200 - 300 - 400 - 500 VA.
• Rated voltage factor (Kt):
In order to determine the maximum voltage for the transformer to attain the required
level of heat and accuracy, we multiply the rated voltage factor by the rated primary
voltage i.e. [Max voltage= Kt*Vp ]. Furthermore, the rated voltage factor is determined
by the maximum operating voltage, which depends on the network earthing system
and the way the VT’s primary winding is connected.
• Rated primary voltage (Vp) or (Up):
The voltage of the system that the VT will be connected to. This will depend on the
design of connections which have two types:
1. Phase to phase which will be written as [ 30KV/110V]
2. Phase to earth which will be written as [ 30kv/seq(3) / 110v/seq(3) ]
• Rated secondary voltage (Vs) or (Us):
The voltage will be 110v or 100v that will be connected to the devices in the secondary.
• Voltage ratio error:
This is the error that the transformer introduces in the voltage measurement and we
can find it by: voltage ratio error % = ((Kn*Vs)-Vp / Vp )*100.
• Accuracy class:
This defines the guaranteed transformation ratio and phase error limits in specified
power and voltage conditions. for VT used for measurement the standardized IEC
accuracy classes are 0.1- 0.2- 0.5- 1- 3 and for the majority of cases, VT will have 0.5 or
1 and rarely uses 3.
E.g. : if VT have cl 1 this mean that the voltage error will +0.1 or -0.1 and phase
displacement will not exceed 40 minuets
For VT used for protection, the standardized IEC accuracy classes are 3P or 6P (below a
table is given that shows the phase and voltage errors in relation to the accuracy class
for protective VT).
• Rated thermal limiting output:
It’s the apparent power that the transformer can permanently supply at its rated
secondary voltage without exceeding the heating limits.
• When you want to select VT you have to consider:
1. Rated voltage factor
2. Rated primary voltage (system operating voltage)
3. Rated Secondary voltage ( can be 110v or 100v)
4. B.I.L (Basic Level of Insulation)
5. Rated load on Secondary side (Burden)
6. Accuracy Class Rating
• Famous types of VT
1. Medium voltage transformer ( used for voltage up to 36 kV)
2. Inductive voltage transformers ( used in HV line for voltage up to 520 kV)
3.CVT i.e. Capacitive voltage transformers ( used in HV line for voltage 820 kV )
Remember:
1. Connected in parallel
2. The VT secondary should always be an open circuit (high impedance)
3. N(prim) > N(sec)
4. Burden is high
