A potential or voltage transformer is a step - down transformer employed to transform high - voltage values into fractional values. Measuring instruments such as ammeters, voltmeters, and wattmeters are designed for low - voltage operation. Connecting these measuring devices directly to high - voltage lines for measurement can cause them to burn out or be damaged. Hence, a potential transformer is used for measurement purposes.
The primary windings of the potential transformer are directly linked to the measurand line, and its secondary terminals are connected to the measuring meter. The potential transformer converts the high voltage of the measurand line into a fractional value suitable for the measuring instrument.
The construction of a potential transformer is nearly identical to that of a power transformer, yet there are some minor differences:
- The construction of a potential transformer takes into account factors like cost, efficiency, and regulation. In contrast, a potential transformer is designed with a focus on performance parameters. Specifically, the ratio of voltage to the number of turns remains constant, and the phase difference between the input and output signals is minimized.
- Power transformers may encounter temperature - rise issues due to overloading. Since the output of a potential transformer is relatively small, the problem of overheating does not occur in it.
Parts of a Potential Transformer
The following are the essential components of a potential transformer.
Core
The core of a potential transformer can be of either the core - type or shell - type. In a core - type transformer, the windings surround the core. Conversely, in a shell - type transformer, the core surrounds the windings. Shell - type transformers are designed for low - voltage operations, while core - type transformers are utilized for high - voltage applications.
Windings
The primary and secondary windings of a potential transformer are arranged coaxially. This configuration is adopted to minimize the leakage reactance.
Note on Leakage Reactance: Not all of the flux generated by the primary winding of a transformer is coupled to the secondary winding. A small fraction of the flux is associated with only one of the windings, and this is called the leakage flux. The leakage flux induces self - reactance in the winding it links with. Reactance, in general, refers to the opposition offered by a circuit element to the change in voltage and current. This self - reactance is known as leakage reactance.
In a low - voltage transformer, insulation is placed adjacent to the core to mitigate insulation - related issues. A single coil serves as the primary winding in a low - potential transformer. However, in a large - potential transformer, the single coil is divided into smaller parts to reduce the insulation requirements between layers.
Insulation
Cotton tape and cambric materials are commonly used as insulation between the windings of a potential transformer. In low - voltage transformers, compound insulation is not typically employed. High - voltage transformers use oil as an insulation medium. Transformers with a rating exceeding 45kVA use porcelain as an insulator.
Bushing
A bushing is an insulated device that enables the connection of the transformer to the external circuit. The bushings of a transformer are usually made of porcelain. Transformers that use oil as an insulating medium employ oil - filled bushings.
A two - bushing transformer is used in systems where the line it is connected to is not at ground potential. Transformers connected to the ground neutral require only one high - voltage bushing.
Connection of Potential Transformer
The primary winding of a potential transformer is connected to the high - voltage transmission line whose voltage is to be measured. The secondary winding of the transformer is connected to the measuring meter, which determines the magnitude of the voltage.
