Difference between Metering CT and Protection CT

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In electrical power systems, an instrument transformer is used in measurement and protection applications.

A current transformer is nothing but a specially designed voltage step-up and current step-down electrical transformer which is used to reduce the high currents in power lines to a safe value for metering and protection applications. In practice, a current transformer is referred to as a CT.

Depending on the application, there are two main types of current transformers (CTs) namely, metering current transformer and protection current transformer. Both types of current transformers are different from each other in many aspects.

The primary difference between metering CT and protection CT is that a metering CT is used for measuring the flow of current in the power line, while the protection CT is used for detecting fault currents in the power lines.

Read this article to understand all the important differences between metering CT and protection CT.

What is a Metering CT?

Metering CT stands for metering current transformer. It is a type of current transformer or CT which is specially designed for measuring high electric currents flowing in the power distribution and transmission lines.

The output terminals of the metering CTs are connected to the measuring instruments like ammeters. These types of CTs are required to have a good accuracy in current measurement approximately up to 120% of the rated current.

Also, the metering CTs generally use a core made up of nickel-iron alloy with a low excitation current and low flux density.

The metering CTs are rated as 100/1 A, 200/2 A, etc. and have accuracy classes as 0.1, 0.2, 0.5, 1, 3, and 5.

What is a Protection CT?

A protection CT (current transformer) is a type of current transformer which is designed to sense faults in the system and operate the protective relay in response. Therefore, the primary purpose of protection CTs are to protect system from faults and abnormal conditions.

These CTs are so designed that they can handle both normal operating currents and fault currents. The protection CTs continuously monitor the flow of current in the power lines and operate if any abnormal current flows in the system.

At the output side, a protective relay is connected across its secondary winding which initiate the operation of a circuit breaker if required.

The core of protection CTs is design in a way that they can operate for a wide range of currents under fault conditions and does not get saturated. In practice, the core of protection CTs is made up of gain-oriented silicon steel having a high saturation flux density.

The accuracy classes for protection CTs are defined as PS, 5P20, 5P10, etc.

Consider a 5P10 CT. It has 5% error, 20 times of the rated current, and P is the protection class. Thus, this protection CT allows a flow of fault current of 20 times of the rated current with a 5% error in the secondary current.

This is all about the basics of metering CT and protective CT used in electrical power systems for measurement of currents and protection applications respectively. We will now highlight the key differences between metering CT and protective CT.

Difference between Metering and Protection CT

The significant differences between metering and protection CTs are highlighted in the following table -

Conclusion

Metering CTs are the current transformers designed for monitoring and measuring the current flowing in the power lines and these CTs are connected to a measuring instrument like ammeter. In contrast, protection CTs are the current transformers designed for detecting faults in the power system and they are connected to a protective relay.

FAQs Related to Metering and Protection CTs

Here is a list of some commonly asked questions related to metering CTs and protective CTs -

1. Why is CT metering not used for protection?

We cannot use the metering CT for protection of power system because the core of metering CT has low saturation flux density. Thus, its core gets saturated when the circuit current becomes 1.2 times of the rated current. We know that fault current is generally many times higher than the rated current. Hence, we cannot use the metering CT for protection.

2. What are the different types of CT protection?

The following are the main types of protections CTs -

• PVC tape wound CT
• Class PS/X CT
• Resin Cast CT

3. How do I choose a CT for protection?

To select an appropriate CT to protect a system, we must consider several important factors which are listed below -

• System voltage and current levels under normal operating conditions.
• System voltage and current levels under fault conditions.
• Accuracy class of CT depending on the accuracy required in fault detection and relay used on the output side.
• Saturation level of the CT core.
• Response time of the CT, etc.

After knowing all these factors, we select a protection CT that fulfil all these parameters.

4. What is 5P10 in CT?

In this case of CT, 5P10 represents the following parameters -

• 5P represents the accuracy class of the CT. Where, 5 is the ratio error in the output current and P stands for protection.
• 10 represents the accuracy limit factor of the CT.

So, we can state that when 10 times current flows in the primary winding of the CT, then there can be a 5% error in the secondary current.

5. What CT class is used for metering?

The CT classes used for metering application are Class 0.1, Class 0.2, Class 0.2S, Class 0.5, Class 0.5S, Class 1, Class 3, and Class 5.