Current Transformers for Energy MetersGoSwitchgear2023-08-28T12:14:59+04:00
Energy meters are vital equipment that fulfills a range of requirements. They measure the amount of energy that homes, farms, and industries consume, and owners will have to pay bills according to this amount. Therefore, energy meters must be as accurate as possible to satisfy all parties. Generally, the energy meters you are accustomed to within your homes are self-contained, meaning all the electric energy passed directly through it, and it measures the parameters.
However, industries and factories use current transformers for metering due to the large amounts of electric energy they consume. These current transformers, or CTs, are crucial to how the energy meters operate. We will delve into great detail about these current transformers, how to choose the right one, and what factors you need to consider.
Without further delay, let’s dive into it.
What are current transformers (CTs)?
Current transformers are used in transformer-rated energy meters to measure the total power consumed and the current measure flow. Transformer-rated energy meters are different from self-contained energy meters used within homes as the electric energy involved is very low. When the current involved is more than 200 Amperes, as with most industries and factories, transformer-rated meters with current transformers are required.
The purpose of the current transformer is to take the primary current of the electric energy that needs to be measured and transform it into a secondary current that is much smaller and easier to manage. This current can then be passed safely through the meter to measure the electric energy. Current transformers are also used with measuring instruments like voltmeters, ammeters, and kilowatt-hour meters. CTs are available in various options with different parameters and features. You will need to choose the right one for you, and we will look at how you can do so next.
What factors to consider when choosing a current transformer?
CT Input and Output
Current transformers are available in numerous output options, which include 5A, 333mV, and 80mA options. Additionally, the input into these current transformers will vary depending on the facility and the measured energy load. Therefore, when choosing a current transformer, you will need to consider the CT input and output. There are two critical parameters to consider: CT Ratio and CT Burden.
The current transformer ratio (CTR) is the ratio between the input and output current. This ratio will dictate how the input current is transformed into the voltage current. For example, a 300/5 CTR means that the CT will convert a 300A input current into a 5A output current. Similarly, a 200/5 CTR means that the CT will transform a 200A input current into a 5A output voltage. For a 300/5 CT, the turns in the transformer are in a 60/1 ratio. In the 200/5, the turns are in a 40/1 ratio. There are various CTR values, and you can choose the current transformer based on your desired output value, which is what your metering equipment will require.
CT Burden is the burden your current transformer will have to undertake. You can measure this value by considering the secondary circuit’s resistance within the current transformer. The total resistance is the sum of the secondary circuit resistance, connecting wires resistance, and the meter’s resistance. After all, the current transformers will have to handle many loads, likely fluctuating throughout the day. In addition to having a higher CT burden, the transformer should also have a broader current sensing range if the load fluctuates significantly. If the load is out of the sensor range, it may not be able to measure the electric energy accurately.
The accuracy rating of the current transformer is another crucial factor you will need to consider. This rating dictates how accurate the CT is, which tells you what metering equipment it is suitable for. Generally, we all want the highest possible accuracy when measuring, but low accuracy ratings are ideal for simple measurements. Current transformers are available in various accuracy ratings and have their applications. For example, accuracy classes for 0.1 and 0.2 are suitable for measuring current in revenue metering applications. If you want to see the exact information regarding accuracy classes, here is a table:
Transformer Size And Type
The size and dimensions of your transformer are essential to consider, as they will need to fit the dimensions of your setup and metering equipment. After all, the current transformer will need to fit around the conductor, where the electric energy will need to be measured. There are three main types of current transformers:
- Wound current transformers
- Split-core current transformers
- Bar-type current transformers.
Standard wound current transformers are a great option as they are made by various manufacturers, making them affordable while having a small compact design. Additionally, it is available with high accuracy ratings. However, it can be challenging to install. This is where you can consider a split-core current transformer that can be easily installed and removed around the conductor.
However, this CT is expensive and inaccurate compared to its standard counterparts. Alternatively, bar-type current transformers use the main circuit’s bus bar as the primary circuit. This setup’s advantage is that it is fully insulated from the system with such a high operating voltage.
Different countries have different regulations when it comes to electrical equipment. This is mainly for safety reasons but is also to ensure that that operation is at a high enough standard and accuracy. Similarly, there are regulatory requirements for current transformers. This means you may be restricted in your choices and have to choose a CT that complies with your country’s regulations.
There can be a wide variety of regulations you will have to look at the courtesy of marks on the CTs. For example, the CE mark shows that the current transformer conforms to European health, safety, and environmental standards. Similarly, there may be more specific regulations, such as in Canada’s case. Current transformers need to be Measurement Canada approved in Canada. This means they must be 100mA, 80mA, or 5A output devices with a solid core and an accuracy rating of at least 0.6%.
Therefore, check the regulation requirements of your country and ensure the CT you are getting fulfils these requirements.
The bottom line
Energy meters are crucial technology for measuring the electric energy consumed. Houses and industries have to pay bills according to these measurements, which means they should be as accurate as possible. Standard energy meters are self-contained, but energy meters for high currents used in industries require current transformers. These CTs take the primary current from the measured source and convert it into a safer current that the meter can measure. There are a variety of factors and parameters that you need to consider when buying a current transformer. The input that will be fed into the current transformer and the desired output is crucial to consider. You will need to choose a CT with the right CTR and burden values to ensure you get an output that the meter can handle. It will also ensure that the CT can handle load fluctuations. Similarly, the accuracy rating is crucial as it will tell you how accurate the currents and measurements will be. The transformer’s type and size are also necessary as they will have to fit in specific dimensions and prioritize different features, depending on your requirements. All these factors must fit your country’s regulatory requirements, so ensure you conduct proper research.
We hope this article proves insightful and enables you to choose the suitable current transformer for your energy meters. Thank you for reading!