Understanding the Accuracy Class of Current Transformers (Protection Class)

Current transformers and voltage transformers are extensively used in power system to scale down the operating current and voltage. The scaling down of the current and voltage is required chiefly for measuring, metering and protection purposes. The relays and other measuring instrument operates and tolerates much lower values of current and voltage for their safe operation.

In terms of usage, CTs are broadly classified as Protection CT or Metering CT. This article shall focus on Protection Class Current Transformers. It is imperative here to mention the instrument transformers (CTs & VTs) may have multiple cores, in which case separate cores may be used for metering and protection purposes, all in one instrument transformer. Multi Core CTs and VTs may have different designated class for each core.

Before dwelling into explaining accuracy class of Current Transformers let us first outline the following terms:

Accuracy Limit Factor

At the time of fault, heavy current flows through the Current Transformers therefore it is important that the CT must maintain appropriate amount of accuracy during high saturation conditions. Accuracy limit factor signifies the current limit up till which the performance of the CT shall remain sufficiently accurate. It is expressed in terms of multiple of the rated current.

Burden

The burden of the Current transformer is the impedance presented to the secondary of the CTs. The burden is essentially due to the CT internal impedance, connecting wire impedance and any device such as relay or meter connected to the secondary. The burden is usually expressed in Volt Amperes (VA) or ohms.

Composite Error (Ratio Error & Phase displacement)

Due to the saturation of the core, the secondary current is not the exact multiple of the primary current, this is known as ratio error. In addition the phase displacement between the primary and reversed secondary current must be ideally zero however there is some displacement which is known as phase displacement error. Composite error is defined by IEC 61869 as the R.M.S value of the difference between primary and secondary current. The composite error includes ratio error, phase displacement and error due to harmonics.

Accuracy Class of Current Transformer

IEC 61869-2 defines the accuracy of the current transformer for protection purposes. The preferred accuracy classes are 5P (5 percent maximum error) and 10P (10 percent maximum error). IEC associates accuracy limit factor as well. The most commonly used values for ALF are 10 and 20. For Example We have a current transformer whose name plate reads as 30 VA 5P20, which means 30 VA the maximum burden that can be fed with CT and 5% error at 20 times of the rated current.

Class PX Current Transformer

PX class transformer are normally used for unit protection scheme such as for bus bar protection. Where transient performance are critical to safe operation of the protection scheme. Knee Point Voltage, Exciting Current at knee-point and Secondary Impedance are important characteristics that define PX class CT.

Knee Point voltage is a point in excitation curve at which 10% increment of voltage would require an increase of 50% in excitation current.

IEEE defines two fundamental relaying accuracy designations, one headed by a “C” and the other by a “T” designation. T class accuracy CTs are seldom used today. C class designation is usually followed by a number which represents the secondary terminal voltage the CT (C200, C400 etc.) will maintain operating with in error limits. The C designation limits the ratio error to 10% at 20 times the rated current. So secondary terminal voltage has direct relation to the burden of the CT. However, in IEEE, the secondary burden has a 60-degree impedance angle, whereas in IEC the secondary burden is purely resistive. Therefore, an IEEE current transformer with a limiting error of 10 percent with the IEEE burden will have a limiting error of 5 percent with the IEC resistive burden. Therefore, for IEC equivalency, the accuracy is a 5P class rather than 10P. A table is presented below as an example showing IEEE and IEC equivalent.

200	B-2.0	2.0	C200	50VA-5P20
400	B-4.0	4.0	C400	100VA-5P20

Expressing Burden in Ohms (An-Example)

Considering the example of CT rated as 100/5 Amps 30 VA 5P20

Volt Amperes = 30 VA

Secondary Rated Current = 5 A

Therefore

Secondary Rated Voltage = 30/5 = 6 V

Burden in Ohms = 6/5 = 1.2 Ohms

About the Author

Muhammad Adeel Khan has over 7 years experience in the field of power systems working with leading companies in the electrical industry. . He received his Bachelors degree in Electrical Engineering from NED University in 2011. He also holds Masters degree in Power Systems from NED University.