What’s a Potential transformer?

Potential transformers are important devices within any electrical system. Whether you are a beginner or want to brush up on your comprehension of the subject, this guide will enable safe and sustainable methods of high-voltage measurement using potential transformers. We will explain everything such as their components, working principles, different types, applications, and more in simple words. So it’s wise not to skip!

Figure no 1 Potential transformers

1) What is a potential transformer?

“A Potential transformer (PT) is a special type of device used for the purpose of measuring high voltages in electrical transmission systems.”

It essentially works on reducing the voltage levels from the high level to the optimum, more manageable, and usable levels. It is all so that you can measure safely using instruments like voltmeter and other necessary applicable protective relays. This advancement is critical in controlling and monitoring power systems in industries, substations, and other electrical applications.

Electromagnetic induction is the operating principle of a pt transformer. It consists of two windings: a primary winding and a secondary winding. Primary winding has a connection with the high-voltage circuit and secondary winding connects with measuring devices. The transformation ratio associated with the primary high voltage and low voltage secondary winding is determined by the turns ratio of the windings. That is to say, in a case where the primary winding has 1,000 turns and the secondary has 10, the voltage will be in the ratio reduced by the factor of 100:1.

Figure no 2 Pt transformer

They are also designed with insulation to withstand high voltage factors and have safety features that help avoid damage to electrical wires in cases of faults internally.

Potential transformers are indispensable for both the security and high performance of electrical systems. You know the presence of high voltages makes it impossible to measure the voltages directly without a high risk of equipment damage and danger to the handlers. So, PTs are vital in assisting in accurate measurements that ensure both equipment and personnel safety at all times.

2) Construction of a potential transformer

The construction of this type of transformer is basic but careful in dimensions. The parts that comprise a potential transformer include the core, windings, and insulation.

• Core: The core consists of coated steel in layers to reduce energy loss. This core is shaped in a way that it focuses the magnetic field produced during use.

• Windings: As we discussed earlier, they consist of two coils of wire: The primary winding and secondary winding. The primary/first winding is the one fueled in the high voltage circuit, and the secondary/top winding is linked with measuring instruments like a voltmeter, or protective relays.

• Insulation: There is insulation done in between the windings and the core so that the high voltage would be efficiently applied. This allows the circuit not to touch causing short-circuits. Moreover, it doesn’t allow the transformer to short due to electrical breakdown.

• Cover: The whole structure is enclosed in a protective cover. Such a cover is usually filled with insulating oil or resin so that insulation fluid does not overheat the device. Terminals for connecting with circuits might also be incorporated in the PT.

Figure no 3 Pt transformer construction

Keep in mind that in certain designs, a fuse or circuit breaker is incorporated to provide the PT with protection in case of overload or one of the circuits faulting. There is a base or supporting structure for the PT to ensure its stability during operation.

3) What is the working principle of a potential transformer?

You will be wondering how does a potential transformer works! Okay! The working principle of a potential transformer (PT) is based on electromagnetic induction. Let’s start! Firstly in potential transformer connections, a higher voltage primary circuit is supplied with the primary winding. As current passes through this winding, a magnetic field is generated within the core of the transformer. In this case, the magnetic field induces a voltage in the secondary winding.

In the situation of an autotransformer, we calculate the resultant voltage by looking at the turns ratio of primary to secondary (the greater the windings in the primary coil, the less the voltage). To illustrate, a 10:1 ratio in windings means a voltage of 10,000V will result in only 1,000V on the secondary coil.

The secondary coil (depending on the reduced voltage) feeds into circuits containing voltmeters or relays. So in practice, large voltages can be measured without worrying too much about the risks of direct contact.

4) What are the types of potential transformers?

Potential transformers may be of different types based on their construction, operating voltage, and specific functions in electrical systems.

Types on the basis of Construction

i) Electromagnetic Potential Transformer: This type employs a core along with winding for reduction of high voltage. It is the most self-explanatory type in electrical systems.

ii) Capacitor Potential Transformer (CPT): This type employs a transformer with capacitors to divide large voltages for example transmission line voltages of electric power.

Figure no 4 Capacitor potential transformer

Types on the basis of operating voltage

i) Low-Voltage Potential Transformer: These are rated for a maximum operating voltage of less than 1000 Volts. Their common use is in small power systems, say residential as well as light to medium industrial setups.

ii) High-Voltage Potential Transformer: These are rated for greater operating voltages of more than 1000 volts. Typical uses of this equipment are in power plants and extensive electrical systems.

Types on the basis of function

i) Measuring Potential Transformer: They are measuring devices that tend to measure potential accurately within moderate range reliability. Voltmeters or other devices used for similar measuring specialists will receive or take these readings.

ii) Protective Potential Transformer: In the electric system fault or abnormality resets existing conditions, They are placed in the protective systems for relay.

5) Measuring voltage using a potential transformer?

It can be observed that the method of voltage measurement with potential transformer PT is relatively safe as well accurate. Because the transformer has the role of reducing the measure voltage in this case.

Firstly, the PT is connected directly with the high-voltage line through its primary winding. The voltage reached at primary winding is reduced in ratio that depends on the winding of both coils (primary and secinday winding) of transformer. This reduced voltage is then delivered to measuring devices such as a voltmeter or an energy meter.

According to statistics, potential transformers in most cases are used on systems operating at voltages between 1 kV and over 765 kV. You can take example as, in a 132 kV power station, a PT whose ratio is 132,000:110 can step down voltage to 110 volts for measurement purposes.

Point to be noticed is that the error is usually between 0.5% and 1%. Additionally, it isolates measuring devices and operators from direct exposure to high voltage.

 

6) Phasor Diagram of a potential transformer

“The phasor diagram of a potential transformer demonstrates the relation of the current and voltage in primary and secondary windings.”

A potential transformer has the following inputs and outputs:

• Primary Voltage (VP): This is the voltage which is applied to the primary winding of the transformer.
• Secondary Voltage (VS): It is the voltage that is at the secondary winding but it is lesser than the primary voltage.
• Primary Current (IP): This indicates the current in the primary winding of the transformer.
• Secondary Current (IS): This shows the current in the secondary winding of the transformer.
• Flux (Φm): The main magnetic field in the core.

Figure no 5 Phasor diagram

• A few words of other variables in the diagram:

• RP and RS: Resistances of the primary and secondary windings respectively.
• XP and XS: Reactances of the primary and secondary windings respectively.
• Io: Excitation current, consists of components
• Im: magnetizing current.
• Iw: current of core loss.
• Kn: Turn ratio of the transformer
• EP and ES: Induced EMFs in the primary and secondary respectively.
• β: Phase angle error between primary and secondary voltages

Phase voltage Vp keeps the line of the coordinate system with the primary current whose power factor is φm. The biggest flux (Φm) is denoted by the current in the very center of the diagram. The primary voltage (VP) drops as a function of losses due to a reduction in the resistance (RP) and reactance (XP). Likewise, secondary volts (VS) are secondary EMF minus the losses incurred in RS and XS.

The specific Energy loss current (Io) consists of two components specifically Im and Iw. The current (IP) originates from Io, while the current from the secondary (IS) is modified by the term Kn or the windings ratio. In a nutshell, all the transformation of energy from primary to secondary is due to mutual induction between windings. That’s all!

7) What are the advantages and disadvantages of a potential transformer?

• Potential Transformer Benefits:

+ Safety: Due to reduced high voltage into lower small amount, there is no harm to measuring devices and personnel.

+ Precision: Resulting from accurate voltage measurement which is important for the supervision and control of electrical systems.

+ Isolation: It electrically prevents the high voltage side from the measuring devices thus the chances of destruction are minimized.

+ Durability: Able to withstand high voltages. You know, it operates satisfactorily for a persistent desired duration.

+ Cost Effective: It is a reasonable approach to high voltage measurement, particularly in industrial and power systems.

• Disadvantages of a Potential Transformer:

- Limited load: Its only purpose is not being used for high-load applications, rather it is used with a few small measuring devices.

- Phase angle: Phase is the angle, which can marginally vary. The fact is that during application it may introduce inaccuracies, especially in delicate applications.

- Voltage drop: The loss in windings may provide slight differences in the output voltage.

- Requires maintenance: After a certain time period, the insulations and windings may get old and worn out thus exhibiting a certain degree of timeliness.

- Size with high voltage: At very high voltages, there will be nuclear-sized potential transformers which will cost extraordinarily.

8) Conclusion

To summarize, potential transformers are very critical apparatuses that allow for the measurement of high voltages safely and accurately. Their construction allows for great efficiency, protection, and reliability. Thereby, making them indispensable in power systems and industries. The transformers do have some limitations and weaknesses, however, are far fewer than the benefits. For potential transformers that are sturdy and work effectively, consult Delixi Electric, who are reputable and experienced in the production of electrical equipment.