The primary winding insulation strength test of the voltage transformer is generally applied with power frequency voltage from the secondary winding.
For transformers with multiple voltage ratios, the winding with the highest rated primary voltage is selected according to the test voltage value given on the nameplate.
If the test voltage is not specified on the nameplate, it can be negotiated with the user to select a test voltage (i.e. 2kV~3kV) in Table 2-2 that is greater than the first value but less than or equal to the second value.
If there is no negotiation, the test voltage of transformers with a rated primary voltage of 35kV and below is selected as 1.5 times the rated voltage, and others are selected as 1.3 times the rated voltage.
The power frequency test voltage (effective value) between the secondary winding and the grounding terminal is 2kV, and the test time for insulation withstand voltage is 1min.
[JJG 313-2010] 5.3.4 Demagnetization
If the manufacturer specifies the demagnetization method, demagnetization should be performed according to the marking on the nameplate or the provisions of the technical documents.
If the manufacturer does not specify, open circuit demagnetization or closed circuit demagnetization can be used according to custom.
When implementing the open circuit demagnetization method, select a winding with fewer turns in the primary (or secondary) winding and pass it with 10%~15% of the rated primary (or secondary) current.
When the other windings are open, the current is steadily and slowly reduced to zero.
During the demagnetization process, the peak voltage meter connected to the two ends of the winding with the most turns should be monitored.
When the indicated value reaches 2.6kV, demagnetization should be carried out at this current value.
When implementing the closed circuit demagnetization method, a resistor equivalent to 10~20 times the rated load (considering sufficient capacity) is connected to the secondary winding, and the power frequency current is passed through the primary winding, increasing from zero to 1.2 times the rated current, and then evenly and slowly reduced to zero.
If the core of the current transformer is wound with two or more secondary windings, one of the secondary windings is connected to the demagnetization resistor during demagnetization, and the other secondary windings are open.
[Article Interpretation] When the current of the current transformer suddenly drops, such as when the power supply is suddenly cut off under high current conditions, the secondary winding is suddenly open, etc., residual magnetism may be generated in the core of the transformer.
The transformer core has residual magnetism, which reduces the core magnetic permeability and affects the transformer performance.
Transformers that have been used for a long time should be demagnetized during calibration.
Demagnetization is to pass an alternating excitation current through the primary or secondary winding to give the core an alternating magnetic field.
Starting from 0, the alternating magnetic field (excitation current) is gradually increased to make the core reach a saturated state, and then the excitation current is slowly reduced to zero to eliminate the residual magnetism in the transformer core and achieve the demagnetization effect.
Note that the inter-turn insulation of the winding has been assessed during the demagnetization process, so the JJG313-2010 regulation cancels the inter-turn insulation test item.
[JJG313-2010] 5.3.5 Winding polarity check
The winding polarity of the current transformer for measurement is specified as subtractive polarity. When the primary current flows into the polarity end of the primary winding, the secondary current flows out from the polarity end of the secondary winding.
It is recommended to use an error measuring device equipped with a polarity indicator to check the polarity of the winding according to normal wiring.
When checking polarity using an error measuring device without a polarity indicator, it should be done when the working current is not more than 5%.
If the measured error exceeds the measuring range of the calibrator, the polarity is abnormal.
This regulation also allows the use of other methods (such as DC method or AC method) to check the polarity of the current transformer winding.
[JJG314-2010] 5.3.4 Winding polarity check
The winding polarity of the voltage transformer for measurement is specified as minus polarity.
It is recommended to use an error measuring device equipped with a polarity indicator to check the polarity of the winding according to normal wiring.
When checking polarity using an error measuring device without a polarity indicator, it should be done when the working voltage is not more than 5%.
If the measured error exceeds the measuring range of the calibrator, the polarity is abnormal.
This regulation also allows the use of other methods (such as DC method or AC method) to check the polarity of the voltage transformer winding.
