Current transformers (CTs) is a type of “instrument transformer”, that plays a crucial role in power systems, providing accurate current measurements for protection, metering, and control applications. Selecting the right current transformer as shown in Figure 1 is essential to ensure the reliability and efficiency of a power system. This comprehensive guide will explore the key factors to consider when choosing a current transformer.
Before diving into the selection process, it’s essential to understand the basic principles of current transformers. A current transformer is a device designed to produce an alternating current in its secondary winding proportional to the current flowing in its primary winding. The primary winding is connected in series with the current-carrying conductor, while the secondary winding is connected to the measuring or protection devices. The transformation ratio (R) of the current transformer is the ratio between the primary-rated current and the secondary-rated current. It is necessary for quality verification and assessment of possible damage.
Current Transformers Types
There are several types of current transformers, each designed for specific applications. The main types include:
- Bar-type Current Transformers
- Suitable for indoor applications.
- Consists of a bar-shaped core that encircles the conductor.
- Wound Current Transformers
- Ideal for outdoor applications.
- Have a wound core and are often oil-insulated for outdoor use.
- Window-type Current Transformers
- Allow the primary conductor to pass through a window in the core.
- Convenient for retrofitting existing systems without disconnecting the primary circuit.
- Rogowski Coil Current Transformers
- Flexible and don’t have a solid core.
- Ideal for irregular-shaped conductors
Advantages of Current Transformers
Here are some advantages of current transformers:
- Less Power Loss
CTs play a role in efficient energy management. They reduce power wastage by enabling accurate measurements, that too at lower currents.
- Accurate Scaling
Choosing the right CTs means you’ve chosen accuracy, meaning that these devices scale down the currents to a level that is suitable enough for measurement. As a result, reliable readings are observed.
- Compatibility
These devices are compatible with all kinds of instruments and relays. No wonder why they’re called as “Universal Remote Control” as well.
- Safety
CTs allow measurements without contacting directly with high currents. This minimizes the risks of shocks as well.
- Importance of the type of Power Meter to Find the Current Transformer
A Current Transformer is compatible with quite a few meters, but that doesn’t make it a great choice. However, some CTs are compatible with certain instruments while others are not. Similarly, a clamp-on CT is better because you can move it between panels. But meter choice impacts your choice of CT in one way or another. For example, you must ensure before time whether your meter is designed for amp output or mV output. Also, have you installed the meter permanently or are you willing to move it from one location to another?
It is also important to know how many amps you’re going to measure with it. One thing is obvious CTs perform well when the flow of current is between 10% and 100% of their full-scale range. So, make sure that you take all these factors into account which impact the choice of CT in one way or another.
Working Principle of Current Transformer
A current transformer is different from a normal voltage transformer. It also has two coils of wire, like the voltage transformer. One coil makes AC in the other coil. This happens when AC goes through the first coil, and makes a changing magnetic field. The second coil has a very low resistance, so it acts like a short circuit. This means that the current in the second coil only depends on the current in the first coil, and not on the resistance of the load.
What happens if the second coil is open?
The second coil of a current transformer is connected to a load and is always closed when it is working. When the current goes through the first coil, it also goes through the second coil, because they have the same number of turns, except for a small difference. This small difference is used to make the magnetic core. If the second coil is open and the current still goes through the first coil, there is no current in the second coil to cancel the magnetic field.
The magnetic field from the first coil will be very strong in the core because there is nothing to oppose it. This will make the core very hot, and lose energy. It will also make a very high voltage across the second coil.
This high voltage can damage the insulation and make the current transformer less accurate. This is why the second coil of a current transformer should never be open when the first coil has current.
Key Considerations of Current Transformer
- Accuracy Requirements:
Determine the required accuracy class for your application. Common classes include 0.1, 0.2, 0.5, and 1.0. High-accuracy classes are suitable for metering applications, while lower-accuracy classes may be sufficient for protection.
- CT Output (Z)
The burden is the impedance imposed on the secondary winding by the connected devices. It is important for technical persons like solar engineers or area technicians they know the output compatibility of their equipment. Engineers should choose a current transformer with an appropriate output rating to ensure accurate measurements. The turns ratio is important to consider in this case as it is also helpful during load calculation and selection of the current transformer. The turn ratio can be calculated easily using the below formula.
- Using ROGOWSKI Coils
The advantages of Rogowski coil CTs include lightweight flexibility, a broad range of current sensing, no saturation point, and a large window size. But if the meter accepts specific readings such as 333mV, 1A or 5A, it may not work with the Rogowski coil. But the use of an integrator will still make it work somehow. This way, you can solve compatibility issues with ease.
- Primary Current Rating
This is the most important factor to consider. Related solar engineers should select a current transformer with a primary current rating that matches the expected maximum current in the system. Oversized or undersized transformers can lead to inaccurate readings.
- Saturation Characteristics
Saturation characteristics of the current transformer should be known to solar supervisors or technicians. Saturation can occur if the current exceeds a certain level, leading to inaccurate measurements. Choose a CT with a saturation point well above the maximum expected current. It can be calculated using below formula:
Where:
VSEC = Secondary Voltage in open-circuit conditions
Z = Burden impedance/ CT Output
- Load Size
Just like any other physical dimension, load size is an important parameter to consider. The amperage range or current input range tells about the size of the load they can measure. If there are any fluctuations in the load, it is better to choose a CT with a broad range. But if the reading goes out of the sensor’s range, the load will not be measured by the meter accurately.
- Accuracy Rating
Choosing the equipment with the highest accuracy is really important. So, before you choose a revenue grade sensor, check which industry’s accuracy standards they’re following. The most common accuracy standard in this regard is IEC 60044-1 0.5 class. But if you have to collect data for overall consumption only, a sensor with only 1% accuracy is good enough.
- Form Factor
In retrofit applications, using a split-core or Rogowski coil current transformer is more convenient due to easy installation around conductors without wire disconnection, despite higher initial costs. In new construction, solid-core CTs are cost-effective as they don’t disrupt operations, but the lower upfront price may not outweigh uncalculated installation costs with shutdowns and disconnections.
- Environmental Conditions
Environmental conditions should be analyzed by the engineers or technicians where the CT will be installed. Outdoor installations may require weather-resistant or oil-insulated transformers. For corrosive environments, choose transformers with appropriate coatings.
- CT Expected Life
There are different types of current transformers (CTs) like split core, clamp-on, and Rogowski coil CTs. Some of them, like the solid core CT, require you to disconnect the wires to install them through a window opening. This can be inconvenient, especially if you need to move the meter frequently. No matter which type of CT you use, it’s important to turn off the power in the circuit you’re working on and follow all safety guidelines to stay safe.
- Accuracy under Fault Conditions
Evaluation of the accuracy of the current transformer under fault conditions should be done under the supervision of solar engineers. Some transformers may exhibit accuracy issues during short circuits or other fault conditions. This is important because it will reflect your solar plant accuracy in the future so it’s advisable not to ignore the accuracy feature.
- Conductor Size Considerations
Conductor size is a critical factor in current transformer (CT) selection, as it must fit around the conductor while avoiding unnecessary oversizing to save cost and space. Flexible Rogowski coils are a practical solution for crowded electrical panels, providing a compromise between window size and flexibility.
- Compliance with Standards
Its responsibility of engineers to ensure that the current transformer complies with relevant industry standards. Common standards that are being followed almost in every country include IEC 60044 for international applications and IEEE C57.13.
Selecting Current Transformer for Solar Plant
Let’s suppose you want to know the ratio of your CT as which one to install in your system. If we have 2MVA transformer with 11/0.433 KV rating and you want to select the CT that aligns with the transformer rating, then you can use formula to first find the secondary current:
Now, all the transformers have some 10-20% margin over the full-load ratings. If we use 20%, then the secondary current would come out as:
Now you have to select the CT from the above two ratios as these are the standard sizes available for specified ratings. Selecting 5A CT secondary will need you to add relays and other circuit components of higher current ratings. Engineers or technicians thinking of installing new CT should must consider such parameters to avoid interruption in power supply.
And for integration with energy storage current transformers can be used
Conclusion
Choosing the right current transformer is a critical step in ensuring the reliability and accuracy of current measurements in a power system. By carefully considering factors such as accuracy requirements, burden, saturation characteristics, and environmental conditions, engineers can select a current transformer that meets the specific needs of their application. Additionally, staying informed about industry standards and technological advancements is crucial for making informed decisions in the rapidly evolving field of power systems. With the right current transformer in place, power systems can operate efficiently and safely, contributing to the overall reliability of electrical infrastructure.
