Voltage Transformer and current Transformer are known as Instrument Transformer. They are used in the substation to transform high magnitude voltage and current to low magnitude voltage and current suitable for metering and protection purposes.
While the main purpose of the instrument transformer is metering and protection it also isolate the high voltage side from the low voltage side comprised of measurement and protection devices and circuits. Before proceeding further one should have some basic knowledge about working of the transformer. Here in this article we discuss about the voltage transformer (also called as potential transformer) and in the next article we will discuss about current transformer. The voltage transformers are broadly of two types. These are inductive VT and Capacitive VT (CVT). Let us first consider the inductive (electromagnetic) Voltave Transformer.
We Know the two fundamental laws of the inductive transformer.
For an ideal transformer,
While the main purpose of the instrument transformer is metering and protection it also isolate the high voltage side from the low voltage side comprised of measurement and protection devices and circuits. Before proceeding further one should have some basic knowledge about working of the transformer. Here in this article we discuss about the voltage transformer (also called as potential transformer) and in the next article we will discuss about current transformer. The voltage transformers are broadly of two types. These are inductive VT and Capacitive VT (CVT). Let us first consider the inductive (electromagnetic) Voltave Transformer.
We Know the two fundamental laws of the inductive transformer.
For an ideal transformer,
V_{p }/ N_{p} = V_{s }/ N_{s}
I_{p }N_{p} = I_{s } N_{s}
Subscripts 'p' and 's' are used for primary and secondary sides of the transformer. N is the number of turns of the respective side of the transformer.
The voltage and current transformers are used for measuring or protection purpose. Hence in the ideal case we desire to get the value of secondary voltage which is proportionate to the primary voltage. The voltage transformer (potential transformer) is designed to closely follow the formula V_{p }/ N_{p} = V_{s }/ N_{s} for a specified range of operation.
The equivalent circuit of an actual voltage transformer is shown in FigB. R_{p }and L_{p} are primary side resistance and leakage reactance of transformer. R_{s }and L_{s} are for the secondary of transformer. R_{c }and L_{m} are the core loss component and magnetising reactance component respectively. Error is mainly introduced in the measurement of voltage and phase angle due to these parameters of transformer.
In comparison to current transformer the voltage transformers operate at a relatively higher point of the operating curve. In the design process care is taken to limit the exitation current otherwise the increased exciting current will result in excessive voltage drop in the series impedances, so the error is increased.
The inductive voltage transformer is constructed similar to power transformer. The secondary(low voltage) side winding has few turns wound over the magnetic core and the primary (high voltage) side winding is comprised of several turns wound over the primary winding. The cross sectional area of the secondary side conductor is considerably more than the primary side conductor. The secondary side voltage adopted is usually 100 volt or 110 volt.
A sketch of voltage transformer is shown in FigC. The porcelain insulator provide required creepage distance for HV terminal from ground. The tank made from galvanized steel filled with oil contains the magnetic core wound with primary and secondary windings of VT. In a voltage transformer the core size is comparatively more so that a low flux is maintained at operating point.
Burden and Error
The instrument transformers are classified according to the allowed percentage error and burden. The load on the secondary side of the voltage transformer is called as burden(For instrument transformers burden terminology is used instead of load on the secondary side).The rated burden is specified in voltampere or VA. The Total burden of all the instruments connected to the secondary of the voltage transformer (VT) should be less than the rated burden. For example the VT secondary may be connected to a voltmeter, a watt meter, Integrating meter, a synchroscope and some relays. The sum of the burdens of all these equipments should be less than the rated burden of the VT. More over if the conductor lead used for connecting to these instruments is very long, then the burden due to this long lead should also be added to the burdens of all the equipmets connected to the secondary of the Voltage Transformer. The burden of the VT can also be specified by impedance value in Ohm.
The voltage transformers has a specified rated transformation ratio. If k_{n} is the rated transformation ratio then voltage error in percentage is given as,
Voltage Error = ( k_{n} * V_{s } V_{p} ) *100 / V_{p }
V_{p } and V_{p} are the actual primary and secondary voltage.
And K_{n} is the ratio of rated primary voltage to rated secondary voltage.
The Accuracy class of voltage transformer (VT & CVT) is defined by the IEC. The table below display the limits specified for the accuracy classes.
Voltage Transformer Accuracy Classes (As IEC 600442)
Accuracy Class

Voltage Error (%)

Phase Error (Minutes)

Application

0.1

+0.1

+5

Precise Measurement

0.2

+0.2

+10

Measurement

0.5

+0.5

+20

Measurement

1.0

+1.0

+40

Measurement

3.0

+3.0



Measurement

3P

+3.0

+120

Protection

6P

+6.0

+240

Protection

The protection VTs are less accurate than the metering VTs. For revenue metering purposes the VT with accuracy class 0.2 may be preferred. For indicating meters less accuracy class like 1.0 may be chosen.
For both metering and protection VTs, the above accuracy of VT should be valid for the burden between 25% to 100%. of rated burden.
Capacitor Voltage Transformer (CVT)
The above described inductive voltage transformer is usually economical for system voltage rating upto 132 kV. For higher system voltage at Extra High Voltage (EHV) and Ultra High Voltage(UHV), Capacitor Voltage Transformers (CVT) are used. At system voltage above 38 kV the inductive VT is not cost effective. The CVT is basically comprised of a capacitor voltage divider (see figure below) and an inductive Voltage transformer (as described above). The tapped voltage from the last unit of capacitor voltage divider is fed as input to the inductive VT. By using the capacitor voltage divider the system voltage is reduced to a voltage level suitable as input to the transformer.
As the circuit is capacitive a reactor L is connected in the primary so that the sum of the reactance L and the leakage reactance of the transformer compensate the capacitive effect at power frequency.
The phenomon of ferroresonance considerably influences the design of CVT. Under the conditions of various network disturbances or fault conditions the divider capacitor and inductor in the CVT form a series tuned resonating circuit. In resonance the magnetic circuit may saturate and overheat the transformer. It is necessary to damp out ferroresonance in CVT. So the CVTs are equipped with ferroresonance damping circuit as shown in the figure above.