Voltage Transformers
Voltage transformers (VT), also called potential transformers (PT), are a
parallel-connected type of instrument transformer. They are designed to
present a negligible load to the supply being measured and have an
accurate voltage ratio and phase relationship to enable accurate secondary
connected metering.
Types of Voltage Transformers:
Electromagnetic Voltage Transformer (EMVT):
Single-phase Voltage Transformer: Used to measure the voltage in a single-
phase circuit.
Three-phase Voltage Transformer: Used in three-phase power systems to
measure phase-to-phase and phase-to-ground voltages.
Capacitive Voltage Transformer (CVT):
Uses a series of capacitors to step down high voltage to a lower level. It is
commonly used in high-voltage applications, such as in substations for
voltage measurement and protection.
Optical Voltage Transformer (OVT):
Uses optical fibers and the Pockels effect to measure voltage. It offers
advantages like immunity to electromagnetic interference and is used in
specific high-voltage applications.
Inductive Voltage Transformer (IVT):
Operates on the principle of electromagnetic induction and is similar to
traditional power transformers but designed for accurate voltage
measurement rather than power transfer.
Applications of Voltage Transformers:
Metering: Accurate measurement of voltage for billing and monitoring.
Protection: Providing inputs to protective relays to detect faults and
abnormal conditions.
Control: Feeding control systems with voltage information for maintaining
system stability.
Voltage transformers are essential components in power systems,
ensuring safety and accuracy in the management of electrical energy.
Accuracy
The accuracy of a voltage transformer indicates how closely the output
voltage (the stepped-down voltage) represents the actual high voltage in
the system. Accuracy is affected by factors such as:
Ratio Error: The difference between the actual transformation ratio
and the nominal transformation ratio.
Phase Angle Error: The phase difference between the primary voltage
and the secondary voltage, which ideally should be zero.
Class
The class of a voltage transformer defines the permissible error limits
within which the transformer operates under specified conditions. The
class designation is a standardized way to express the accuracy of the
transformer, typically given as a percentage.
Common Accuracy Classes:
Class 0.1: Very high accuracy, with an error margin of ±0.1%.
Class 0.2: High accuracy, with an error margin of ±0.2%.
Class 0.5: Moderate accuracy, with an error margin of ±0.5%.
Class 1: An error margin of ±1%.
Class 3: An error margin of ±3%.
Applications of Different Classes:
Class 0.1 and 0.2: Used in precision metering where high accuracy is
critical, such as in revenue metering and laboratory equipment.
Class 0.5 and 1: Commonly used in general metering and control
applications in power systems.
Class 3: Used where accuracy is less critical, often in situations
where only approximate voltage measurements are required.
Key Standards for Voltage Transformers
1. IEC 61869-3 (International Electrotechnical Commission):
o Title: Instrument Transformers – Part 3: Additional
requirements for inductive voltage transformers.
o Scope: This standard specifies the requirements for inductive
voltage transformers, including accuracy classes, testing
procedures, and performance characteristics.
o Application: Widely used in Europe and other parts of the world.
2. ANSI/IEEE C57.13 (American National Standards Institute/Institute
of Electrical and Electronics Engineers):
o Title: Standard Requirements for Instrument Transformers.
o Scope: This standard covers the requirements for both current
and voltage transformers, including accuracy, ratings, and
testing.
o Application: Commonly used in North America.
3. BS EN 61869-3 (British Standard/European Norm):
o Title: Instrument Transformers – Part 3: Additional
requirements for inductive voltage transformers.
o Scope: Similar to the IEC standard, this British and European
standard specifies requirements for inductive VTs.
o Application: Used in the United Kingdom and European Union
countries.
4. IS 3156 (Indian Standards):
o Title: Specification for Voltage Transformers.
o Scope: Specifies the requirements, accuracy classes, and
testing methods for voltage transformers used in India.
o Application: Applicable in India for various voltage levels and
applications.
5. CSA C60044-2 (Canadian Standards Association):
o Title: Instrument Transformers – Part 2: Additional
requirements for inductive voltage transformers.
o Scope: Aligns closely with IEC 61869 but with specific
adjustments for use in Canada.
o Application: Used in Canada for voltage transformer standards.
6. DIN VDE 0414-2 (German Standards):
o Title: Instrument Transformers; Voltage Transformers.
o Scope: Specifies the technical requirements for voltage
transformers used in Germany.
o Application: Applicable in Germany and often referenced in
other German-speaking countries.
Key Aspects Covered by These Standards:
Accuracy Classes: Defines the permissible error limits for different
classes of VTs.
Thermal Performance: Specifies the maximum temperature rise
under normal operating conditions.
Insulation Requirements: Sets the minimum insulation levels to
ensure safety and reliability.
Dielectric Tests: Includes procedures for testing the insulation's
ability to withstand high voltages.
Short-Circuit Withstand Capability: Defines the ability of the VT to
withstand short-circuit conditions without damage.
Marking and Labeling: Specifies the information that must be marked
on the VT, including the ratio, accuracy class, and rated voltage.
A capacitor voltage transformer (CVT), is a transformer used
in power systems to step down extra high voltage signals and provide
a low voltage signal to the actual VT (voltage transformer) used for
operating metering/protective relays due to a lower cost than an
electromagnetic PT.
In its most basic form, the device consists of three parts: a two
capacitor voltage divider across which the transmission line, an
inductive element to tune the device to the line frequency, and a
voltage transformer to isolate and further step down the voltage for
metering devices or protective relay.
The tuning of the divider to the line frequency makes the overall
division ratio less sensitive to changes in the burden of the
connected metering or protection devices.The device has at least
four terminals: a terminal for connection to the high voltage signal, a
ground terminal, and two secondary terminals which connect to the
instrumentation or protective relay.
Capacitor C1 is often constructed as a stack of smaller capacitors
connected in series. This provides a large voltage drop across C1 and
a relatively small voltage drop across C2. As the majority of the
voltage drop is on C1, this reduces the required insulation level of the
voltage transformer. This makes CVTs more economical than the
wound voltage transformers under high voltage (over 100 kV), as the
latter one requires more winding and materials.
In communication systems, CVTs in combination with wave traps are
used for filtering high-frequency communication signals from power
frequency .This forms a carrier communication network throughout
the transmission network, to communicate between substations.
Applications:
Normal VT:
o Low to Medium Voltage Systems: Commonly used in low to
medium voltage systems (up to 220 kV) for metering and
protection purposes.
o Accuracy: High accuracy for precise voltage measurement.
CVT:
o High Voltage Systems: Typically used in high-voltage
transmission systems (above 220 kV), such as in substations,
for voltage measurement and relay protection.
o Power Line Carrier Communication (PLCC): CVTs are often
used for coupling communication signals onto high-voltage
power lines, which is a critical application in long-distance
power transmission.
A Transformers Turns Ratio
emf = turns x rate of change
Where:
ƒ – is the flux frequency in Hertz, = ω/2π
Ν – is the number of coil windings.
Φ – is the amount of flux in webers
Power in a Transformer
Transformer Efficiency
