Electrical

What is a voltage transformer?

The voltage transformer is a type of instrument transformer, these transformers would reduce or step down the system voltage to a safer and measurable value. The voltage transformer would allow the energy meter to measure the electrical service connections that have a higher voltage than what the meter is actually capable of. This transformer would create a proportional amount of voltage in the second terminal when compared to the primary. So this transformer would step down the voltage and in order to do that the primary windings must have more turns than that of the secondary windings. The voltage transformer would convert the higher voltage to a lower voltage in a constant and linear manner.

What are the functions of a voltage transformer and why do we need a voltage transformer?

  • It will be very useful for the measurement of the energy consumed
  • This device would protect the electrical systems from faults and disturbances
  • Voltage signals can be used for event logs, synchronizing, and event recorders
  • During the measurement of the voltage, we can compute the power or impedance of the system along with its direction
  • We can determine the O/V, U/V, O/F, U/F, and over fluxing protection by measuring the voltage

How is a voltage transformer constructed?

  • The size of the conductor and the core will be big
  • It has good accuracy because of its design
  • Voltage transformers are constructed in core and shell-type
  • Core type voltage transformer is used for high voltages and for lower voltages shell type is used
  • There is less leakage reactance because both the primary and the secondary winding is coaxial
  • High-quality material is used for the core and operates at low flux densities so that there would be a small magnetizing current and because of this no-load losses will be reduced

How voltage transformers are connected?

This transformer has more number of turns in its primary winding and this will be connected across to the line for which the measurement or the protection must be done. The secondary winding of this transformer has lesser turns and it will be connected to measuring equipment, the secondary winding could be connected to relays or other control devices. Mostly these transformers could be single-phase or three-phase transformers. So if the secondary winding is connected to a voltmeter or other meters, then a small current would flow through the secondary of the potential transformer due to the high impedance of the meter’s potential coil.  Due to the small current, the voltage transformer would act like a normal two-winding transformer that operates on no-load. Because of this low load on the potential transformer, the VA ratings on this transformer are really low, so on the secondary side, one end will be connected to the ground for safety. 

V-V connections

This type of connection is used where there is no need for phase to neutral voltage input. So the connection will be done in this manner that one VT primary is connected across the RY phase and the other will be connected to the YB phase, and the secondary should also have identical voltage connections.

Star/star connection

In this type of connection, there could be three separate single-phase VT’s, or there could be a single three-phase, three limb VT is used. The secondary and the primary will be connected in star and the neutral and star will be grounded and thus each of the primary phases is connected between the phase to earth of the supply circuit.

How does a voltage transformer work?

So as we discussed above the voltage transformer has a very high primary number of turns and less secondary number of turns. So this is a step-down transformer and while it steps down the voltage it increases the current. The voltage will be stepped down in a known ratio and it is called a step-down ratio. So the line will be connected to the primary winding and the voltmeter will be connected to the secondary, the voltmeter would transmit the transformed voltage value to the secondary. So in order to determine the actual voltage at the primary then we can divide the voltmeter deflection by the transformed ratio.

What are the major errors that can be seen in a potential transformer?

We can see two major types of errors in a potential transformer and they are ratio and phase angle error. The ratio error will be the error between the primary and the secondary ratio error. This error is by the variation of the actual ratio of transformation from the nominal ratio. There is a phase difference between the primary voltage and the secondary voltage. This phase angle would be there if the difference between the primary voltage and the secondary voltage is reversed. So in order to limit this error, the windings must have been designed in a way that it should have proper magnitudes of reactance and resistance. We need to make sure that the core must have minimum magnetizing and core-loss components of the exciting current.

What is the test that should be done in a voltage transformer?

Insulation resistance test

In this test, the DC voltage test will be done at the rated crest voltage and three tests will be conducted

  • High voltage to ground
  • High voltage to low voltage
  • Low voltage to ground

Dielectric withstand test

This is the AC voltage test at seventy-five percent factory test voltage, in this test the secondary will be grounded and the test would last long for one minute.

Ratio and polarity test

So in this test, the primary winding will be excited at one volt per turns ratio and the secondary is open-circuited. Then compare the H1-H2 to X1-X2. Calculate the phase ratio. So if the phase difference is zero then the polarity will be correct. After that, we should compare the ratio with the nameplate.

What are the types of voltage transformers?

Capacitive voltage transformer

In this type of voltage transformer, a capacitive potential divider will be connected between the main-line phase and the ground. This transformer can be considered as a combination between the capacitive potential divider and also a magnetic transformer. These transformers are called intermediate transformers of very small ratio. The capacitive potential divider is a stack of high voltage capacitors, and these transformers can be used in voltage metering and also as a protective device. So the capacitors would reduce the primary voltage and then the lowered voltage will be stepped down with a transformer to the secondary.

Electromagnetic voltage transformer

This type of transformer is similar to the oil-filled wire wound transformer. In this type of transformer, the primary voltage would be entirely transformed to the secondary voltage by using electromagnetism.

What are the applications of the voltage transformer?

  • It is used for electrical metering
  • It can protect the electrical instruments
  • It would protect the feeder
  • Protection of the generators
  • Electrical industry

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