6.3.4.2 Comparison method circuit using standard current transformer
The P end of the primary winding of the current transformer under test is connected to the L, end of the standard current transformer, and the S, end of the secondary winding is connected to the K end of the standard current transformer.
The secondary winding terminals of other current transformers that share the primary winding are short-circuited and grounded with wires.
[Article interpretation] One of the differences between power current transformers and instrument current transformers is that they have multiple core windings.
One core winding is a current transformer, and they share the primary conductor.
During the test, a primary current needs to be applied to the primary conductor to generate magnetic flux in all the cores at the same time, so that the primary circuit has a relatively large impedance.
In order to minimize the impedance of the primary circuit, the secondary winding of the transformer that is not being tested at the time should be short-circuited and grounded during the test.
This is also to prevent high voltage from being induced in the secondary winding of the current transformer, affecting the safety of the test equipment and personnel.
Current transformer error measurement circuit
The circuit is used to measure the error of the high-voltage current transformer.
As long as the high-voltage leakage current influence of the high-voltage current transformer is within the allowable range, the low-voltage method measurement result can be used as the verification result.
The current transformer in the combined transformer is verified by the circuit.
As long as the influence of the current and voltage transformers on each other is within the allowable range, the measurement results of the separate tests can be used as the verification result.
Generally, it is not necessary to apply voltage and current verification at the same time, because the current transformer is subjected to different power loads during operation, and the voltage transformer is also subjected to different grid voltages.
It is not only a lot of work to measure the errors under these current and voltage combinations, but also meaningless.
If the actual error correction of the combined transformer is to be considered, then other error influences need to be corrected, such as residual magnetism, environmental interference, grid harmonics, secondary loads, etc.
This will make the energy metering very complicated.
An important step in verifying the current transformer is to generate a large current in the primary current loop.
In addition to the large current transformer, the correct connection of the large current wire cannot be ignored.
According to the electrical contact theory, when the current passes through the contact surface of two metal plates, many contact points with a certain area will be generated under the action of mechanical pressure.
When the current path is bent, contraction resistance will be generated.
The sum of the areas of many micro-contact points is equivalent to the compressive contact surface generated by the deformation of the material, which is calculated in material mechanics by pressure and HB value representing the hardness of the material.
Within a certain range, the greater the pressure, the larger the micro-contact area and the smaller the contact resistance value.
When connecting the conductive ear plate, sufficient pressure must be applied to keep the product of the contact voltage drop and the current (the electrical power borne by the contact point) within a safe range.
Theoretical calculations show that the contact voltage drop of the copper joint should not exceed 0.1V, and the contact voltage drop of the brass joint should not exceed 0.25V, otherwise the contact point will overheat, oxidize or even weld.
A single large-capacity booster transformer is very heavy and is now only used in laboratories.
When conducting large current verification on site, multiple booster transformers should be used in parallel and in series, and the capacity of a single unit should be around 6kVA.
The primary current busbars should be close to each other and can be partially twisted if necessary to reduce the loop inductance voltage drop.
At the same time, the non-measured secondary winding of the tested current transformer should be short-circuited to ground to minimize the impedance voltage of the primary circuit of the tested current transformer.
Since the reactive power required by the line is very large, the most reasonable method is to use a resonant power supply.
Considering the versatility of the equipment, the series resonant line is more suitable than the parallel resonant line, because the parallel resonance requires the use of a step-up transformer, and its primary and secondary windings have resistance losses, and its capacity is fixed.
To output different voltages, multiple taps must be prepared, which also adds inconvenience to manufacturing.
The series resonant line uses a bus-type step-up transformer, which does not require a secondary winding or a tap, and is easy to provide sufficient voltage and current through multiple parallel and series connections.
