Working of Potential transformer
IntroductionPotential transformer (voltage transformer) is used for protection and measurement. Actually they are protective type voltage transformer. They may be single phase or three phases. Voltage transformer is necessary for directional, voltage, distance protection. The primary of voltage transformer is connected to power circuit between phase and ground. The volt-ampere rating of potential transformer is less than power transformer. Hence potential transformer is that type transformer which protect from over voltage and also in earth fault protection. It works on law mutual induction as in simple transformer. It’s also used in calibration of measuring instrument.
Potential transformer is step down type transformer which steps down the voltage from high voltage to low voltage.
There are two type of construction
1. Electromagnetic potential transformer.
In which primary and secondary are wound on magnetic core like in usual transformer.
2. Capacitor potential transformerIn which potential transformer the primary voltage is applied to a series capacitor group. The voltage across one of the capacitors is taken to auxiliary voltage transformer. The secondary of auxiliary voltage transformer is taken for measurement or protection.
specification of potential transformer
1. Rated primary voltage
The primary voltage marked on the rating plate of the voltage transformer. The method of connection of primary winding to system and system voltage should be considered while selecting the VT of correct primary voltage rating. There are several values of standard primary voltages.
3. Rated secondary voltage100/ √3 =63.5V or 110/ √3 = 190V. it is the value of secondary voltage marked on the rating plate.
4. Residual voltage.
it is defined as vector sum of three lime to earth voltages.
5. Residual voltage transformerA three phase voltage transformer or a group of three single phases residually connected voltage transformer in which residual voltage appears across secondary terminals when three-phase voltages error is given in to primary winding.
6. Ratio errorPercentage ratio error sometimes called percentage voltage error is given in below.
7. Voltage factorThe upper limit of operating voltage (primary) is given by rated primary voltage x voltage factor, is specified for certain time.
Testing of potential transformer
1. Error measurementsThe errors are generally measured by comparison method that is comparing the voltage transformer under test with a sub-standard voltage transformer of high accuracy and known errors. Errors are measured for various primary voltages. For burdens.
Measurement of core loss and exciting current are made to chek the quality of core material and short-circuits in winding between turns.
2. Core losses
3. Insulation tests.Routine insulation tests are of two kinds, applied and induced over voltage tests. In applied tests, the primary winding is short-circuited and test voltage is applied between primary winding and earth, for specified time and of specified value.
4. High voltage tests. Power frequency with stand test and impulse withstand tests.
5. Polarity testMatch the polarity of PT.
Application potential transformer
• Voltage transformer used of residual connection
• For step down the voltage from high voltage to low voltage
• It also provide over voltage protection
• Range reduction of voltmeter
• For Provide low output voltage
• For provide supply in different type of panels