Voltage transformers (VT), also called potential transformers (PT), are a parallel-connected type of
instrument transformer Instrument transformers are high accuracy class electrical devices used to isolate or transform voltage or current levels. The most common usage of instrument transformers is to operate instruments or metering from high voltage or high current circu ...
. 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.
Ratio
The PT is typically described by its voltage ratio from primary to secondary. A 600:120 PT will provide an output voltage of 120 volts when 600 volts are impressed across its primary winding. Standard secondary voltage ratings are 120 volts and 70 volts, compatible with standard measuring instruments.
Burden and accuracy
Burden and accuracy are usually stated as a combined parameter due to being dependent on each other.
Metering style PTs are designed with smaller cores and VA capacities than power transformers. This causes metering PTs to saturate at lower secondary voltage outputs saving sensitive connected metering devices from damaging large voltage spikes found in grid disturbances. A small PT (see nameplate in photo) with a rating of 0.3W, 0.6X would indicate with up to W load (12.5 watts
) of secondary burden the secondary current will be within a 0.3 percent error parallelogram on an accuracy diagram incorporating both phase angle and ratio errors. The same technique applies for the X load (25 watts) rating except inside a 0.6% accuracy parallelogram.
Markings
Some transformer winding primary (usually high-voltage) connecting wires are of many types. They may be labeled as H
1, H
2 (sometimes H
0 if it is internally designed to be grounded) and X
1, X
2 and sometimes an X
3 tap may be present. Sometimes a second isolated winding (Y
1, Y
2, Y
3) (and third (Z
1, Z
2, Z
3) may also be available on the same voltage transformer. The primary may be connected phase to ground or phase to phase. The secondary is usually grounded on one terminal to avoid capacitive induction from damaging low-voltage equipment and for human safety.
Types of Voltage Transformers
There are three primary types of potential transformers (PT): electromagnetic, capacitor, and optical. The electromagnetic potential transformer is a wire-wound transformer. The capacitor voltage transformer (CVT) uses a capacitance potential divider and is used at higher voltages due to a lower cost than an electromagnetic PT. An optical voltage transformer exploits the
Faraday effect
The Faraday effect or Faraday rotation, sometimes referred to as the magneto-optic Faraday effect (MOFE), is a physical magneto-optical phenomenon. The Faraday effect causes a polarization rotation which is proportional to the projection of the ...
, rotating polarized light, in optical materials.
![cvt](https://upload.wikimedia.org/wikipedia/commons/6/63/Cvt.png)
A capacitor voltage transformer (CVT), also known as capacitor-coupled voltage transformer (CCVT), is a
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
used in
power systems
An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of a power system is the electrical grid that provides power to homes and industries within an extended area. The ...
to step down
extra high voltage
High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, ''high voltage'' refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant spec ...
signals and provide a
low voltage In electrical engineering, low voltage is a relative term, the definition varying by context. Different definitions are used in electric power transmission and distribution, compared with electronics design. electrical safety codes define "low vo ...
signal, for metering or operating a
protective relay
In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detecti ...
.
In its most basic form, the device consists of three parts: two
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
s across which the transmission line signal is split, 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 C
1 is often constructed as a stack of smaller capacitors connected in series. This provides a large voltage drop across C
1 and a relatively small voltage drop across C
2. As the majority of the voltage drop is on C
1, this reduces the required insulation level of the voltage transformer. This makes CVTs more economical than the wound voltage transformers under high voltage (over 100kV), as the latter one requires more winding and materials.
The CVT is also useful in communication systems. CVTs in combination with wave traps are used for filtering high-frequency communication signals from power frequency.
[Stanley H. Horowitz, Arun G. Phadke ''Power system relaying third edition'', John Wiley and Sons, 2008 pages 64–65] This forms a
carrier communication network throughout the transmission network, to communicate between substations. The CVT is installed at a point after
Lightning Arrester
A lightning arrester (alternative spelling lightning arrestor) (also called lightning isolator) is a device, essentially an air gap between an electric wire and ground, used on electric power transmission and telecommunication systems to protect ...
and before
Wave trap
In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass filter. A notch filter is a b ...
.
References
{{Electric transformers
Electric transformers