Accuracy Limit Factor and Instrument Security Factor of Current Transformer

Before understanding accuracy limit factor and instrument security factor of CT or current transformer we must try to understand different technical parameters of instrument transformer i.e. of current transformer and voltage transformer.

Technical Parameters of Instrument Transformer

Following are some of the normally used terms associated with instrument transformer, that is, both for current transformer and voltage transformer.

Rated Primary Current of Current Transformer

This is the value of rated primary current of current transformer on which the CT is designed to perform best. Hence rated primary current of current transformer is an optimum value of primary current at which, error of the CT are minimum and losses in the CT are also less, that means in few words, performance of the CT is best; with optimum heating of the transformer.

Rated Secondary Current of Current Transformer

Like rated primary current, this is the value of secondary current due to which errors in the CT are minimum. In other words, rated secondary current of current transformer is the value of secondary current on which the best performance of the CT is based.

Rated Burden of Current Transformer

Whatever is connected externally with the secondary of a current transformer is called burden of current transformer. In electrical power transformer the secondary is connected with load, but in case of current transformer, electrical consumer load is not connected to the secondary. In electrical power transformer we loaded the secondary of the transformer by connecting consumer’s one by one to the secondary side. But in case of current transformer or other instrument transformer, we connect, metering instrument and protection relays to the secondary, which obviously behave like load of the instrument transformer but do not have any direct relation with the load of the electrical power system. That is why, all the instruments, wires etc connected with the secondary of the instrument transformer or IT is called burden rather load. In this way, we distinguish the secondary circuit of a current transformer or voltage transformer from other purpose electrical transformer. Although literally, load and burden carry nearly same meaning in English language. Rated burden of current transformer is the value of the burden to be connected with the secondary of CT including connecting load resistance expressed in VA or ohms on which accuracy requirement is based. Similarly rated burden of resistance expressed in VA or ohms on which accuracy requirement is based.

Rated Frequency of Current Transformer

The value of the system frequency on which the instrument transformer operates.

Accuracy Class of Current Transformer

There is always some difference in expected value and actual value of output of an instrument transformer current error and phase angle error count in CT, as because primary current of current transformer has to contribute the excitation component of CT core. Accuracy class of current transformer is the highest permissible percentage composite error at rated current. The standard accuracy classes of current transformer as per IS-2705 are 0.1, 0.2, 0.5, 1, 3 and 5 for metering CT.
The accuracy class or simply class of measuring current transformer is 0.1, means the maximum permissible limit of error is 0.1%, more clearly, if we try to measure 100 A with a 0.1 class CT, the measured value may be either 100.1 or 99.9 A or anything in between these range.
The standard accuracy class for the protection current transformer, as per IS-2705 are 5 P, 10 P, 15 P.
Here in the protection current transformer, 5 P means 5%, 10 P means 10 %, and 15 P means 15 % error and ′P′ stands for protection.

Limits of Error in Current Transformers

Class 0.1 to 1.0 : The Current Error and phase displacement Error at the rated frequency shall not exceed the values given below when the secondary burden is any value from 25 % to 100 % of the rated burden.

  Limit of % current error at % of rated current Limit of phase angle error at % of rated current
Class 10 20 100 120 10 20 100 120
0.1 ± 0.25 ± 0.2 ± 0.1 ± 0.1 ± 10° ± 8° ± 5° ± 5°
0.2 ± 0.50 ± 0.35 ± 0.2 ± 0.2 ± 20° ± 15° ± 10° ± 10°
0.5 ± 1 ± 0.75 ± 0.75 ± 0.5 ± 60° ± 45° ± 30° ± 30°
1.0 ± 2 ± 1.5 ± 1.5 ± 1.0 ± 120° ± 90° ± 60° ± 60°

Application of CT Core According to Their Class

Application Accuracy Class
  As per IS As per BS
Precession Metering 0.1 or 0.2 BL
Commertial Metering 0.5 or 1.0 AM, BM, CM
Ammeters 1 or 3 CD
Protection Relays 5P10 or 5P20 STU
Special Protection PS  

Rated Short Circuit Current of a Current Transformer

In some abnormal condition like huge short circuit fault, the current transformer faces a huge current, flows through the CT primary. Although this fault current will not continue for long lime as because every fault in the system is cleared by electrical protection system within very short time. So current transformer should be designed in such a way that it can withstand this huge fault current at least for certain amount of time. It is unnecessary to design any equipment for withstanding short circuit current for long period of time since the short circuit fault is cleared by protection switch gear within fraction of second. For CT rated short circuit current of current transformer is defined as the rms value of primary current which the CT will withstand for a rated time with its secondary winding short circuited without suffering harmful effects.

Rated Voltage for Current Transformer (CT)

The RMS value of the voltage used to designate the CT for a particular highest system voltage is rated Voltage for current transformer. The voltage of electrical power system is increased if load of the system is reduced. As per standard, the system voltage can be raised up to 10% above the normal voltage during no load condition. So every electrical equipment is such designed so that it can withstand this highest voltage. As current transformer is an electrical equipment, it should also be designed to withstand highest system voltage.

Instrument Security Factor

ISF or instrument security factor of current transformer is defined as the ratio of instrument limit primary current to the rated primary current. The instrument limit primary current of metering CT is the value primary current beyond which CT core becomes saturated.

Accuracy Limit Factor

For protection current transformer, the ratio of accuracy limit primary current to the rated primary current is called accuracy limit factor of current transformer.

Knee Point Voltage of Current Transformer

Knee point voltage of current transformer is significance of saturation level of a current transformer core mainly used for protection purposes. The sinusoidal voltage of rated frequency is applied to the secondary terminals of CT, with other winding being open circuited which increased by 10%, cause the exiting current to increase by 50%.

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