What is Potential Transformer?Construction of PT,Classification Of Potential Transformer,Ratio of PT.,Uses of PT,Difference between PT and CT,Diff. b/w power Transformer and PT.

Potential Transformer

               We can define Potential Transformer as an instrument transformer used for the transformation of voltage from a high value to the lower value,In fact potential transformer step down the voltage to a safe limit value which we can easily measured by the ordinary low voltage instrument like a voltmeter,watt meter and watt-hour meter etc.

                Potential transformer is a step down transformer which lower the voltage circuit.Potential transformer are connected across or parallel to the line which is being measured to record phase angle error or ratio errors.

Construction of Single Phase PT

     Core

                Core of Potential transformer may be core type or shell type .In core type transformer the windings surrounding the core and in the shell type transformer the core surrounded the winding.The core type transformers are used for high voltage applications where as shell type transformers are used for low voltage applications.

Windings

                The Primary and secondary windings are placed co axially for reducing the leakage reactance of the potential transformer.In low voltage transformer the insulation is placed next to the core for reducing the problems of insulation.In case of low potential transformers single coil is used as the primary winding.In case of large potential transformers single coil is subdivided into small parts for reducing the insulation between the layer.

Insulation

                          We can use cotton tape and the cambric material as an insulation between the winding of potential transformer.In case of low voltage transformer we can not use the compound insulation.We can use oil as an insulation medium for high voltage transformer.For the transformers having rating higher than 45KVA we use porcelain as an insulator.

Bushing

                          Bushing is an insulated device which the transformer is connected to the external circuit .Porcelain is used for making the bushing of the transformer.Transformers which uses the oil filled bushing.

                The transformer which connects to the ground neutral uses only one high voltage bushing where as two bushings transformer is used in the system when the line through which it is connected is not at ground potential .

Classification of Potential Transformer

            Potential transformers are classified into two types.

1. Out door Potential Transformer

                          These are single phase or three phase potential transformers available for different range of operating voltages that are used for out door relaying and metering application up to 33kv these of electromagnetic type single and three phase voltage transformers.Above 33kv single phase out-door potential transformers are of two type 

• Electromagnetic or wound type conventional Potential transformer.
• Capacitive voltage transformers(CVTS).

     2.In door Potential Transformers

                            Indoor Potential Transformers are available as single or three phase moulded magnetic type.The mounting mechanism can be fixed or draw out type.In this type of potential transformer all parts of primary winding are insulated from each at its rated insulation capacity.These type of transformers are designed to operate relays measuring instruments and other control devices in door services with high accuracy.Potential transformers are used as metering voltage transformers and protection voltage transformers.

Ratio of Potential Transformers

                          The PT is typically described by its voltage ratio from primary to secondary.A 1000:100 PT will provide an output voltage of 100volts when 1000volts are at its primary winding.Standard secondary voltage ratings are compatible with standard measuring instruments.

Uses of Potential Transformer

                         Potential transformers or voltages transformers are used to transform high distribution level voltages to a lower voltage usually 120 volts that can be safely measured by standard watt-hour meters.However all 2400volts and above applications for metering and most relaying  applications require potential transformers.Potential transformers are used in substations for metering purpose to reduce the cost of metering equipment for high voltages.

Difference between PT and CT

• the primary winding of the CT is connected in series with the load so that it carries the full line current but there is only a small voltage across it.Whereas the primary winding of the PT has the full supply voltage applied across it.
• The secondary of the CT is almost short circuit,however the secondary of the PT is practically a open circuit.
• In CT the excitation current Io and flux density  vary over a wide range however in PT they vary over limited range only.

Difference between Power transformer and Potential Transformer

                      Power transformer is used for transferring power from low voltage circuit to high voltage circuit and vice verse.They can be step up or step down transformer.Where as potential transformer is a step down transformer which step down the voltage to a safe limit value which we can measured easily by the ordinary low voltage instruments.