The high-end differential measurement method does not change the grounding method of the equipment, which is conducive to the safety of measurement. It should be used first.
To calibrate the three-phase five-column voltage transformer, a three-phase high-voltage power supply should be applied, a standard voltage transformer should be connected between the measured phase and the ground, and the secondary winding of the measured phase should be connected to the voltage load box, and the error should be measured by the comparison method.
If the secondary circuit load of the three-phase five-column voltage transformer is connected in a V shape, two voltage load boxes can be used to connect the secondary circuit phases into a V load during calibration, and an ungrounded standard voltage transformer should be connected between the measured phases.
Calibration should be carried out according to the ungrounded voltage transformer, and the wiring method and calibration results should be explained in the calibration certificate.
[Article Interpretation] The circuit used to check the error of the voltage transformer should be selected according to the standard device and error measurement device that can be used.
At present, the test transformer is mainly used to step up the voltage of the 6kV, 10kV35kV, and 66kV voltage transformers on site, and the precision electromagnetic voltage transformer is used as the standard.
110kV, 220kV, 330kV, 500kV voltage transformers mainly use series resonant boost power supply and capacitive voltage ratio standard.
With the maturity of standard voltage transformer manufacturing technology, 110kV~1000kV electromagnetic standard voltage transformers are also used for on-site verification. These transformers mainly use Gas insulation.
Therefore, in addition to using capacitive voltage ratio standard, electromagnetic standard voltage transformers can also be used for on-site verification of voltage transformers.
Because the load capacity of power transformers is large and the accuracy is not high, there is no significant difference between the results of high potential end difference measurement and low potential end difference measurement.
From the circuit principle analysis, when the low potential end differential is measured, the low potential end of the standard voltage transformer is not grounded, and the capacitance and conductance current flowing into the secondary winding from the primary side will flow through the secondary winding in the opposite direction and enter the ground from the high potential end; while the normal working state of the standard voltage transformer should be that the stray current enters the ground from the low potential end, which will have a certain impact on the error of the standard transformer.
The main reason for recommending high potential end differential measurement is to consider the safety of measurement.
High voltage tests have requirements for equipment grounding, including standard voltage transformers.
High potential end differential measurement just meets the grounding requirements of high voltage equipment, which is conducive to the safety of measurement, so the regulations recommend priority.
The distribution network also often uses three-phase three-column or three-phase five-column power transformer verification circuits.
The three-phase three-column voltage transformer is only used for Yy wiring, that is, three-phase three-wire wiring; the three-phase five-column voltage transformer can be used for both Yy wiring and YNyn wiring.
JJG 1021-2007 requires that three-phase voltage transformers be calibrated using three-phase power supply.
This is because three-phase operation and single-phase operation do not meet the superposition property.
Generally, the error of three-phase operation cannot be deduced from the error of single-phase operation.
Therefore, when the three-phase five-column voltage transformer is operated as a grounded voltage transformer (YNyn), the standard transformer should be replaced with a grounded voltage transformer.
When the three-phase five-column voltage transformer is operated as an ungrounded voltage transformer (Yy), the standard transformer should be replaced with an ungrounded voltage transformer.
It must be noted that both wirings use YN wiring on the primary side.
This is a unified requirement of the calibration procedure for the measurement circuit.
This ensures that the error measurement has good enough reproducibility.
If it is a three-phase three-column voltage transformer, the primary circuit cannot be connected to YN because there is no zero-sequence magnetic flux circulation circuit, otherwise the zero-sequence magnetic flux will pass through the air and produce serious eddy current losses.
Therefore, the three-phase three-column voltage transformer can only be calibrated as an ungrounded voltage transformer.
The three-phase three-column voltage transformer does not lead out the neutral point, so the neutral point 0 of the primary circuit is not grounded.
Except that the three-phase three-wire voltage is applied to the primary side during measurement, the other measurement steps are exactly the same as when the three-phase five-column voltage transformer is used as an ungrounded voltage transformer for error measurement.
