Technical Parameters of Instrument Transformer


Technical Parameters of Instrument Transformer

Based on the function performed by the current transformer, it can be classified is follows:

  1. Measuring current transformers. These current transformers are used along with the measuring devices for the measurement of current, energy, and power.
  2. Protective current transformers. These current transformers are used along with the protection equipments such as trip coils, relays, etc.

Rated Burden(VA) of Instrument Transformer

Many of our customers regularly request quotes for standard and custom 5A ratio type current transformers but only ask for the window size and the input current with no knowledge of the “burden”. The burden is specified as VA or resistance (or impedance). We can not guarantee the accuracy without the burden.

The burden is the termination impedance of the measuring instrument. The measuring instrument can be an analog or digital energy meter, a data logger or a recorder. All instruments that use a current transformer to measure line current must terminate the CT with a resistance (impedance in same case which means there is some inductance involved).

A CT supplier should be given the burden when ordering. Technically, the total burden for a CT is the internal winding resistance, the connecting lead resistance and the measuring instrument input resistance.
Some CTs are installed a long distance from the measuring instrument and the connecting leads can have a resistance as much as 0.002 ohms per foot (both ways because the current makes a complete round trip). This resistance will cause an error if not taken into account.

Burden of instrument transformer
Burden of instrument transformer

    Accuracy class:

  • In the case of protection CT s, the CT s should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as well as composite error at the accuracy limit factor of the CT.
  • In the case of metering CT s, accuracy class is typically, 0.2, 0.5, 1 or 3.
  • This means that the errors have to be within the limits specified in the standards for that particular accuracy class.
  • The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated burden at the specified power factor.

 Accuracy limit factor:

  • The value of primary current up to which the CT complies with composite error requirements. This is typically 5, 10 or 15, which means that the composite error of the CT has to be within specified limits at 5, 10 or 15 times the rated primary current.

 Instrument security factor (ISF or Fs):

  • The ratio of rated instrument limits primary current to the rated primary current. The times that the primary current must be higher than the rated value, for the composite error of a measuring current transformer to be equal to or greater than 10%, the secondary burden being equal to the rated burden. The lower this number is, the more protected the connected instrument are against.
  • This typically takes a value of less than 5 or less than 10 though it could be much higher if the ratio is very low. If the factor of security of the CT is 5, it means that the composite error of the metering CT at 5 times the rated primary current is equal to or greater than 10%. This means that heavy currents on the primary are not passed on to the secondary circuit and instruments are therefore protected. In the case of double ratio CT’s, FS is applicable for the lowest ratio only.

Core balance CT (CBCT):

  • The CBCT, also known as a zero sequence CT, is used for earth leakage and earth fault protection. The concept is similar to the RVT. In the CBCT, the three core cable or three single cores of a three phase system pass through the inner diameter of the CT. When the system is fault free, no current flows in the secondary of the CBCT. When there is an earth fault, the residual current (zero phase sequence current) of the system flows through the secondary of the CBCT and this operates the relay. In order to design the CBCT, the inner diameter of the CT, the relay type, the relay setting and the primary operating current need to be furnished.
  • Class PS or X or PX CT:

    • In balance systems of protection, CT s with a high degree of similarity in their characteristics is required. These requirements are met by Class PS (X) CT s. Their performance is defined in terms of a knee-point voltage, the magnetizing current at the knee point voltage or 1/2 or 1/4 the knee-point voltage, and the resistance of the CT secondary winding corrected to 75C. Accuracy is defined in terms of the turn’s ratio in instrument transformer.


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