Accuracy Class Of Current Transformer

Current transformers are used for measurement of electrical parameters like ampere, power, and energy. CTs are also used for measuring the current at fault condition and tripping of the upstream breaker to avoid damage to the electrical network. From the perspective of the protection of the electrical network, the current transformer accuracy class is significant, and it plays a vital role in ensuring the security of the electrical network.

There are three types of current transformers.

• Metering Class CT
• Protection Class CT
• Special Protection Class CT

Metering Class CT

The metering class CT is used for the measurement of electrical parameters. The crucial role of CT is that it should measure the current accurately without much error. The accuracy class parameter is very important for metering CT. The following specifications need to be checked for the metering class current transformer.

Important Parameters of Metering Class CT

CT Ratio

The current transformation ratio(CTR) is defined as the ratio of the input current to the output current. The current transformer of 300/5 CTR means if the primary current is 300 Amp, then the secondary current is 5 Amp.

CT Burden

The burden of the current transformer is expressed in VA. The total total burden should be taken into account when CT is used for measuring or protection purposes. To calculate the total burden of the secondary circuit of the CT, the total resistance of the secondary side of the CT should be considered. The secondary winding of CT is connected to the measuring equipment or protection circuit through pilot wires. The total resistance of the secondary circuit is the sum of CT secondary winding resistance, connecting wires resistance, and the resistance of the relay/meter.

Example

If the relay resistance is 0.1 ohm, the connecting wire resistance is 0.2 ohm, and the secondary winding resistance of CT is 0.1 ohm. The total resistance of the secondary circuit is 0.1+0.2+0.1= 0.4 ohm. If the rated secondary current of the CT is 5 Amp, then the secondary voltage is (Is*Rburden)= 0.4*5=2 volts. The burden of the current transformer is =Is*Vs= 5*2= 10 VA.

The standard accuracy classes, according to IEC, are classes 0.2, 0.5, 1, 3, and 5. The CT with accuracy classes of 0.1, 0.2, 0.5, and 1.0 are used for the measurement of electric current. 0.1 and 0.2 accuracy class CT are used in a revenue metering application. 0.2 class metering CT means the CT functions within the specified accuracy limit at 100 % and 120 % of the rated CT current, and the accuracy limit error is 0.2 %. The CT operates in the linearity zone of the magnetization curve, and it consumes a very low magnetizing current. The 0.3 class CT reads 0.993 to 1.003 at 100% rated current, and at 10% current, the CT reads in the range of 0.994 to 1.006.

The core of metering CT gets saturated when the current above its rated current flows through it. The current is limited within the device. This protects the connected metering device from overloading in case of the fault current. The salient features of metering CT are as follows.

• High accuracy in a smaller range
• Less core material is required
• This leads to lower saturation voltages

The metering CT has less core material compared to the core material of the protection class CT. The metering CT specification is written in the form of 0.3 B 1.8. The first number is the current transformer accuracy class, B stands for the metering class, and 1.8 is the maximum burden that can be connected to the CT.

0.2s and 0.5s class CT are used in revenue metering applications. The 0.2 s and 0.5s class CT have a ratio error of 0.2 % for current from 20 to 120% of the rated current.

The ratio and phase angle error for measuring the current transformer of 0.2s -0.5s class are given below.

The ratio and phase angle error for measuring current transformer of 0.1 -1.0 class are as given below.

Protection Class CT

Protection class CT is connected to the protection relay that gives a tripping command to the circuit breaker at the time of fault condition. The protection class CT has the following features.

• CT is required to perform in fault current
• Moderate accuracy over a broader range
• More core material is needed

At the time of fault, the primary current of CT increases abnormally high and the core can get magnetized above its rated capacity, and whatever fault current flowing in the circuit can’t be reflected in the secondary side of the CT. This phenomenon is known as the saturation of CT. If CT gets saturated at the time of the fault, the protection relay will not operate.

Therefore, it is very important to ensure that the protection relay must operate at the time of fault. The protection class CT is designed to take care of fault current. To ensure this, the Protection CT requires an Accuracy Limit Factor (ALF). Accuracy Limit Factor (ALF) is the multiple of rated current up to which CT will operate, complying with accuracy class requirement.

According to IEEE C57.13-2008, C200 CT has the following specifications.

C 200

Here 200 is the Secondary terminal voltage which the CT must maintain within the C Rating.

C Rating:

– Less than 3% ratio error at rated current

– Less than 10% ratio error at 20 times rated current

– Standard burden 200V/ (5A x 20) = 2Ω

Example:

5P10 class CT

If the primary current is 10 times the rated primary current of the CT, the CT will function perfectly within the accuracy limit of 5 %. A 5P20 CT has an Accuracy limit of 5% at 20 times rated current (Accuracy Limit Factor). The accuracy class of the current transformer of this CT at rated current is 1%.

Marking on CT

The accuracy class of the current transformer is written after the rated VA of the CT E.g

• 10VA5P10
• 15VA10P10
• 30VA5P20

Protection Special(PS) class CT

PS class CT is used for differential protection of the generator, motor, and transformer. The manufacturer requires the following parameters for the design of the CT.