Calculation Method of Current Transformers in PT Switchgear
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In a PT (Potential Transformer) switchgear, potential transformers are generally the main components. However, current transformers are sometimes
also installed to perform functions such as measurement and protection. The calculation of current transformers mainly involves the following aspects:
#### Calculation of Transformation Ratio
- **Based on the Primary Rated Current**: Determine the primary rated current \(I_{1N}\) of the current transformer according to the maximum load
current during the normal operation of the line or equipment. For example, if the maximum load current of a line is expected to be 200A, considering
a certain margin, a current transformer with a primary rated current of 250A or 300A can be selected.
- **Determine the Secondary Rated Current**: The secondary rated current \(I_{2N}\) is generally a standard value, commonly 5A or 1A. If the input
current required by the measurement and protection devices is 5A, then select a current transformer with a secondary rated current of 5A.
- **Calculate the Transformation Ratio**: The transformation ratio \(K\) of the current transformer is equal to the ratio of the primary rated current
to the secondary rated current, that is \(K = \frac{I_{1N}}{I_{2N}}\). For instance, if the primary rated current is 300A and the secondary rated current is
5A, then the transformation ratio \(K=\frac{300}{5}=60\).
#### Calculation of Capacity
- **Measure the Load Capacity**: The total capacity \(S_{L}\) of the loads such as measuring instruments and relays connected to the secondary side
of the current transformer needs to be calculated. The power consumption of measuring instruments can generally be obtained from their technical
parameters. For example, if the power consumption of an ammeter is 1VA, and there are multiple instruments, then add up their power consumptions.
- **Consider the Line Loss**: There will also be a certain loss in the line connecting the secondary side of the current transformer and the loads such
as measuring instruments. The line resistance \(R_{L}\) can be calculated based on the line length, wire cross - sectional area, and material. The line
loss power \(P_{L}=I_{2}^{2}R_{L}\), where \(I_{2}\) is the secondary current.
- **Determine the Capacity of the Current Transformer**: The rated capacity \(S_{N}\) of the current transformer should be greater than the sum of
the total secondary load capacity and the line loss power, that is \(S_{N}\geq S_{L}+P_{L}\). Generally, the capacities of current transformers come in
different specifications such as 5VA, 10VA, and 15VA. Select a current transformer with an appropriate capacity according to the calculation results to
ensure measurement accuracy.
#### Calculation for Accuracy Verification
- **Calculate the Error**: Calculate the current error and phase error of the current transformer during operation according to its characteristic
curve and the actual load situation. For example, given parameters such as the excitation current \(I_{e}\) and the secondary load impedance \(Z_{L}\)
of the current transformer, the current error can be calculated using the formula \(\Delta I=\frac{I_{2}-I_{2}'}{I_{2}'}\times100\%\), where \(I_{2}\) is the
actual secondary current and \(I_{2}'\) is the ideal secondary current (the current without considering the error).
- **Verify the Accuracy**: Compare the calculated error with the required accuracy class of the current transformer. For example, the allowable current
error of a 0.5 - class current transformer is ±0.5%. If the calculated current error is within this range, it indicates that the current transformer can meet
the accuracy requirements under the current load condition; otherwise, a suitable current transformer needs to be re - selected or the load needs
to be adjusted.
