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A current transformer is defined as "as an instrument transformer in which the secondary current is substantially proportional to the primary current (under normal conditions of operation) and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections." This highlights the accuracy requirement of the current transformer but also important is the isolating function, which means no matter what the system voltage the secondary circuit need be insulated only for a low voltage.
The current transformer works on the principle of variable flux. In the "ideal" current transformer, secondary current would be exactly equal (when multiplied by the turns ratio) and opposite to the primary current. But, as in the voltage transformer, some of the primary current or the primary ampere-turns is utilized for magnetizing the core, thus leaving less than the actual primary ampere turns to be "transformed" into the secondary ampere-turns. This naturally introduces an error in the transformation. The error is classified into two-the current or ratio error and the phase error.
Zonal CT s are designed to minimise the errors using the best quality electrical steels for the core of the transformer. Both toroidal (round) and rectangular CT s are manufactured.
Rated primary current: The value of current which is to be transformed to a lower value. In CT parlance, the "load" of the CT refers to the primary current.
Rated secondary current: The current in the secondary circuit and on which the performance of the CT is based. Typical values of secondary current are 1 A or 5 A. In the case of transformer differential protection, secondary currents of 1/ root 3 A and 5/ root 3 A are also specified.
Rated burden: The apparent power of the secondary circuit in Volt-amperes expressed at the rated secondary current and at a specific power factor (0.8 for almost all standards)
Accuracy class: 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. 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.
Composite error: The rms value of the difference between the instantaneous primary current and the instantaneous secondary current multiplied by the turns ratio, under steady state conditions.
Accuracy limit factor: The value of primary current upto 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.
Short time rating: The value of primary current (in kA) that the CT should be able to withstand both thermally and dynamically without damage to the windings, with the secondary circuit being short-circuited. The time specified is usually 1 or 3 seconds.
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Principle of operation
The standards define a voltage transformer as one in which "the secondary voltage is substantially proportional to the primary voltage and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections."
This, in essence, means that the voltage transformer has to be as close as possible to the "ideal" transformer. In an "ideal" transformer, the secondary voltage vector is exactly opposite and equal to the primary voltage vector, when multiplied by the turns ratio.
In a "practical" transformer, errors are introduced because some current is drawn for the magnetization of the core and because of drops in the primary and secondary windings due to leakage reactance and winding resistance. One can thus talk of a voltage error,which is the amount by which the voltage is less than the applied primary voltage ,and the phase error, which is the phase angle by which the reversed secondary voltage vector is displaced from the primary voltage vector.
Zonal designs its VT's so that the resistance and reactance drops are minimized. It also uses the best grades of cold rolled grain oriented electrical steels which enables operation at optimum levels of magnetic induction, thus reducing both the size and the cost of the VT.
Typical terms used for specifying a voltage transformer (VT)
- Rated primary voltage: This is the rated voltage of the system whose voltage is required to be stepped down for measurement and protective purposes.
- Rated secondary voltage: This is the voltage at which the meters and protective devices connected to the secondary circuit of the voltage transformer operate.
- Rated burden: This is the load in terms of volt-amperes (VA) posed by the devices in the secondary circuit on the VT. This includes the burden imposed by the connecting leads. The VT is required to be accurate at both the rated burden and 25% of the rated burden.
- Accuracy class required: The transformation errors that are permissible, including voltage (ratio) error and phase angle error. Phase error is specified in minutes. Typical accuracy classes are Class 0.5, Class 1 and Class 3. Both metering and protection classes of accuracy are specified. In a metering VT, the VT is required to be within the specified errors from 80% to 120% of the rated voltage. In a protection VT, the VT is required to be accurate from 5% upto the rated voltage factor times the rated voltage.
- Rated voltage factor: Depending on the system in which the VT is to be used, the rated voltage factors to be specified are different. The table below is adopted from Indian and International standards.
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