VOLTAGE TRANSFORMER

Definitions A device used to step down voltage from high value (primary voltage) to a low value (secondary voltage).

Low value voltage is easy to measure, no need to develop high voltage measuring equipment. The secondary voltage is substantially proportional to the primary voltage and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections.

VOLTAGE TRANSFORMER

How Transformer Works When an electric current passes through a long, hollow coil of wire then there will be a strong magnetic field inside the coil and a weaker field outside it.

The lines of the magnetic field pattern run through the coil, spread out from the end and go round the outside and in at the other end.

VOLTAGE TRANSFORMER

How Transformer Works

VOLTAGE TRANSFORMER

How Transformer Works With a hollow coil the lines form complete rings. If there is an iron core in the coil it becomes magnetised, and seems to make the field become much stronger while the current is on.

VOLTAGE TRANSFORMER

How Transformer Works

The iron core of a transformer is normally a complete ring with two coils wound on it. One is connected to a source of electrical power and is called the 'primary coil'; the other supplies the power to a load and is called the 'secondary coil'.

The magnetisation due to the current in the primary coil runs all the way round the ring. The primary and secondary coils can be wound anywhere on the ring, because the iron carries the changes in magnetisation from one coil to the other. There is no electrical connection between the two coils. However they are connected by the magnetic field in the iron core.  

VOLTAGE TRANSFORMER

How Transformer Works

When there is a steady current in the primary there is no effect in the secondary, but there is an effect in the secondary if the current in the primary is changing. A changing current in the primary induces an e.m.f. in the secondary. If the secondary is connected to a circuit then there is a current flow.

VOLTAGE TRANSFORMER

How Transformer Works

VOLTAGE TRANSFORMER

How Transformer Works

The iron core is itself a crude secondary (like a coil of one turn) and changes of primary current induce little circular voltages in the core. Iron is a conductor and if the iron core were solid, the induced voltages would drive wasteful secondary currents in it (called 'eddy currents'). So the core is made of very thin sheets clamped together, with the face of each sheet coated to make it a poor conductor. The edges of the sheets can be seen by looking at the edges of a transformer core.

VOLTAGE TRANSFORMER

VOLTAGE TRANSFORMER

How Transformer Works

Function 1. Protection

Voltage transformer connected to protection equipment (protection relay). The protection relay can be set to identify whether the the input voltage (voltage transformer’s secondary voltage) indicate that fault occur in the system.

  2.   Measurement

Voltage transformer connected to measurement equipment (voltmeter, energy meter).

VOLTAGE TRANSFORMER

Construction Type

Single Phase

VOLTAGE TRANSFORMER

Construction Type

Single Phase

VOLTAGE TRANSFORMER

VOLTAGE TRANSFORMER

Construction Type

Single Phase Two Winding

Construction Type

Three Phase

VOLTAGE TRANSFORMER

Construction Type

Three Phase

VOLTAGE TRANSFORMER

Construction Type

Three Phase

VOLTAGE TRANSFORMER

Construction Type

VOLTAGE TRANSFORMER

Connection to Measurement or Protection Devices  Measurement or protective devices are load to the voltage transformer. The circuit connection shall be in parallel.

Current transformer not connected to the load when installed to the switchgear shall be left open circuit.

Open circuit is a condition when the voltage transformer is not connected to its load.

Shorting the secondary terminal to ground during operation will damaged the voltage transformer (or burst).

VOLTAGE TRANSFORMER

Connection of Voltage Transformer 

v

v

v

Single Line Diagram For 2 Winding Transformer

Single Line Diagram 3 Winding Transformer

v

VOLTAGE TRANSFORMER

Typical Voltage Transformer Connection

VOLTAGE TRANSFORMER

Typical Voltage Transformer Connection

VOLTAGE TRANSFORMER

Typical Voltage Transformer Connection

VOLTAGE TRANSFORMER

Typical Current Transformer Description

132kV/110V/110V A.C - Voltage Ratio3 3 3

CL.: 0.2 - Accuracy Class

100VA/Phase - Rated Burden

V.F.: 1.9, 30s - Voltage Factor

VOLTAGE TRANSFORMER

Term & Definitions  Rated Primary VoltageThe value of primary voltage which specified by manufacturer (shall be based on customer/purchaser requirements). E.g. 132kV, 275kV, 33kV etc.

Rated Secondary VoltageThe value of secondary voltage which specified by manufacturer or customer. E.g. 110V A.C.

Rated BurdenThe burden assigned by manufacturer at which the voltage transformer performs with specified accuracy. Burden is a load connected across the secondary of voltage transformer expressed in VA.

VOLTAGE TRANSFORMER

Term & Definitions  Accuracy ClassThe class assigned to the voltage transformer with the specified limits of ratio error and phase angle error. Accuracy Class: 5, 0.5, 0.2 For metering.Accuracy Class :3P, 5P For protection

Rated Voltage Factor (V.F)The upper limit of operating voltage (primary voltage) where voltage transformer should be able to withstand during operation.

VOLTAGE TRANSFORMER

Term & Definitions

Rated Voltage FactorDepending on the system in which the VT is to be used, the rated voltage factors to be specified are different. The table below is adopted from International standards.

VOLTAGE TRANSFORMER

Term & Definitions

Turns RatioThe ratio between the number of actual turns of wire in each coil is the key in determining the type of transformer and what the output voltage will be. The ratio between output voltage and input voltage is the same as the ratio of the number of turns between the two windings.

The relationship between the number of turns in the secondary and the number of turns in the primary is commonly called the Turn Ratio or Voltage Ratio.

VOLTAGE TRANSFORMER

Testing  Polarity CheckTo prove that a V.Ts has been manufactured and installed with the correct polarity.

Insulation ResistanceDielectric insulation tests such as power frequency withstand voltage test on primary and secondary windings for one minute and insulation resistance test.

VOLTAGE TRANSFORMER

Testing

Accuracy Test & Ratio TestInjection from primary winding and measurement at the secondary winding (at the terminal).

Gas Pressure Indication (GIS V.T)Gas pressure low 1st stage and 2nd stage indication to control room by reducing the gas pressure inside the V.T compartment.

VOLTAGE TRANSFORMER

V.T Loading Calculation 1) Identify equipment connected to the voltage transformer.

2) Identify the cable loop length from voltage transformer terminal’s to the connected equipment.

3) Calculate all the connected load.

VOLTAGE TRANSFORMER

Typical Installation Inside The Gas Insulated Switchgear 

VOLTAGE TRANSFORMER

VOLTAGE TRANSFORMER

VOLTAGE TRANSFORMER