Accuracy Limit Factor and Instrument Security Factor of Current Transformer

The accuracy limit factor of a protection current transformer is the ratio of accuracy-limit primary current to rated primary current. The instrument security factor of a metering current transformer (CT) is the ratio of instrument-limit primary current to rated primary current. Both ratios sit among the other ratings of an instrument transformer, including current and voltage transformer ratings.

Technical Parameters of Instrument Transformer

These are the terms used with an instrument transformer, for both a current transformer and a voltage transformer.

Rated Primary Current of Current Transformer

The rated primary current of a current transformer is the value at which the CT is specified to run. At this current, errors and losses are small. Heating stays within design limits and the CT meets its accuracy figures.

Rated Secondary Current of Current Transformer

Like rated primary current, this is the secondary current at which CT errors are smallest. Rated secondary current of a current transformer is the secondary current on which the CT accuracy and heating figures are based.

Rated Burden of Current Transformer

Whatever is connected externally with the secondary of a current transformer is called burden of current transformer. In an electrical power transformer the secondary is connected with load, but in a current transformer the consumer load is not connected to the secondary. In an electrical power transformer we load the secondary by connecting consumers one by one to the secondary side. On a current transformer or other instrument transformer we connect metering instruments and protection relays to the secondary. Those devices behave like a load on the instrument transformer but have no direct relation with the load of the electrical power system. That is why all instruments, wires and similar items connected to the secondary of the instrument transformer or IT are called burden rather than load. In this way we distinguish the secondary circuit of a current transformer or voltage transformer from other purpose electrical transformer circuits. In ordinary English, load and burden are close in meaning. Rated burden of current transformer is the burden connected to the CT secondary, including connecting-lead resistance, expressed in VA or ohms, on which the accuracy requirement is based. Rated burden of a voltage transformer is the resistance expressed in VA or ohms on which the accuracy requirement is based.

Rated Frequency of Current Transformer

The system frequency at which the instrument transformer is specified to operate.

Accuracy Class of Current Transformer

There is always some difference between the expected and actual output of an instrument transformer. Current error and phase-angle error appear in a CT because part of the primary current supplies the excitation of the 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 of a measuring current transformer, such as 0.1, means the maximum allowable ratio error at rated current is 0.1%. For example, if measuring 100 A with a 0.1 class CT, the reading could be between 99.9 A and 100.1 A.
The standard accuracy classes for the protection current transformer, as per IS-2705, are 5 P, 10 P, 15 P.
For those protection classes, 5 P means 5%, 10 P means 10 %, and 15 P means 15 % composite 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.

Application of CT Core According to Their Class

Rated Short Circuit Current of a Current Transformer

In a heavy short-circuit fault, a large current flows through the CT primary. That fault current does not last long, because the electrical protection system clears the fault quickly. The current transformer must withstand that fault current for a stated time. It is unnecessary to design equipment to carry short-circuit current for a long period, since the fault is cleared by protection switch gear in a fraction of a second. For a CT, rated short-circuit current is 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 the rated voltage for a current transformer. The voltage of an electrical power system can rise if load falls. This page treats a 10% rise above normal voltage at no load as the design allowance. Electrical equipment, including a current transformer, is therefore specified to withstand that highest system voltage.

Instrument Security Factor

ISF or instrument security factor of a current transformer is the ratio of instrument-limit primary current to rated primary current. The instrument-limit primary current of a metering CT is the primary current beyond which the CT core saturates.

Accuracy Limit Factor

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

Knee Point Voltage of Current Transformer

Knee point voltage of current transformer marks the saturation level of a current transformer core used mainly for protection. A sinusoidal voltage of rated frequency is applied to the secondary terminals of the CT, with the other winding open-circuited. The knee point is the voltage at which a 10% increase causes the exciting current to increase by 50%.