Thus, both the primary and secondary voltage is introduced in the system. Voltage Ratio Error — The voltage ratio error is expressed in regarding measured voltage, and it is given by the formula as shown below. Where K n is the nominal ratio, i. Phase Angle Error — The phase angle error is the error between the secondary terminal voltage which is exactly in phase opposition with the primary terminal voltage. The increases in the number of instruments in the relay connected to the secondary of the potential transformer will increase the errors in the potential transformers.
The burden is the total external volt-amp load on the secondary at rated secondary voltage. The rated burden of a PT is a VA burden which must not be exceeded if the transformer is to operate with its rated accuracy. The rated burden is indicated on the nameplate. The limiting or maximum burden is the greatest VA load at which the potential transformer will operate continuously without overheating its windings beyond the permissible limits.
This burden is several times greater than the rated burden. The main flux is taken as a reference. The V p is the voltage applied to the primary terminal of the potential transformer. The voltage drops due to resistance and reactance of primary winding due to primary current is given by I p X p and I p R p.
When the voltage drop subtracts from the primary voltage of the potential transformer, the primarily induced emf will appear across the terminals. This primary emf of the transformer will transform into secondary winding by mutual induction and converted into secondary induced emf E s. The conductors used here are large and the core is made from iron. Mostly the core type potential transformer is used, though both shell type and core type can be designed easily.
This reduces the cost of insulation and the risk of damaging. One more important factor while construction of the potential transformer is the phase shift between the primary and the secondary winding. It is carefully observed to have a lower voltage by varying the load. The potential transformer always connects in parallel with the mains transmission and distribution circuits. The primary of the voltage transformer directly connects to the mainline. The voltage of the mains is then calculated.
Measuring instruments like a voltmeter or wattmeter connected to the secondary output of the transformer. Therefore, the high line voltage is measured with a lot of ease with such low voltages measuring devices. Since the transformer connection is mechanically coupled and electrically isolated, therefore, the circuit is completely safe and also the measuring is done properly.
Also, the primary of the voltage transformer is rated for volts to thousands of volts. The secondary is rated for max volts. The principle of the potential transformer is based on mutual induction.
The connection of the power circuit to the voltage transformer is between phase and ground. The primary and secondary winding of the transformer is magnetically coupled through the minimum reluctance path of the core but electrically isolated. Now, the input of the transformer is given with the high voltage which produces a flux. This flux traverse through the core and induces the voltage at the secondary winding of the transformer.
Therefore, through the principle of electromagnetic induction, the high voltage is converted to a lower voltage. This happens without any physical connection between the two. Also, as the potential transformer has a larger value of impedance, therefore, the smaller value of current flows through the secondary winding.
The other function of this transformer is similar to that of the ordinary transformer. The equivalent circuit of the voltage transformer is the same as the ordinary one. The rest of the calculations is the same as for the ordinary transformer. These transformers are categorized based on their function as well as on their construction. Depending on the construction the transformer can be classified as the.
Both of these are available in both single-phase and three-phase and have good accuracy. The use of such transformers is to control and operate measuring devices like relays, etc,. Let us discuss these in detail. This type of potential transformer is similar to the wire wound oil-filled conventional transformers.
The tap tank is connected to the line terminal. To fill oil, a plug is there at the tank. It is minted on the insulator support. In this type of PT, the primary winding connects between the two phases. Or between a phase and the ground. Therefore, the one end of the primary is connected to the mains of the measuring power circuit.
And the other to the ground common ground terminal. The secondary of the transformer is given to the load, i. Once the value is calculated at the secondary then with the use of turns ratio, we can calculate the voltage of the higher primary side.
The types of potential transformers are categorized into two types, namely, the conventional wound types electromagnetic types and the capacitor voltage potential transformers. In comparison to the latter, the former capacitor voltage potential transformers are wound type and expensive due to the additional insulation requirement.
The capacitive potential transformer encompasses a magnetic potential transformer and a capacitive type potential divider circuit.
Let Vp be the primary terminal voltage of the potential transformer. The values of I pXp and I pRp depicts the primary winding voltage drop due to resistance and reactance. The primary induced EMF across terminals can be calculated by subtracting the voltage drop to the primary voltage. The mutual induction generated due to the transformation of primary EMF to the secondary winding captures the value of secondary induced emf. The effect of secondary winding resistance and reactance results in the secondary output voltage across the secondary terminal.
It is expressed as,. At the initial stage, the internal impedance across the primary winding develops the voltage drop. In the later stages, it is transformed into secondary winding through turns ratio. There again causes a voltage drop due to secondary winding impedance.
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