(7) Secondary load.
The errors of current transformers and voltage transformers are related to the actual secondary load.
When designing transformers, manufacturers will adjust the ratio error under rated load to a negative value and the ratio error under 1/4 rated load to a positive value in order to save materials.
Therefore, when users select transformer parameters, they should make the rated secondary load greater than the actual secondary load, and 1/4 of the rated load less than the actual secondary load to ensure that the error is qualified during use.
JJG 1021-2007 does not advocate taking 1/4 rated load as the lower limit load.
The secondary circuit load of the old transformer is an electrical instrument.
The load of each instrument is generally 5VA~7.5VA, and the total load reaches several tens of VA.
Therefore, the rated secondary capacity of the transformer is generally designed to be 30VA~100VA. At present, a large number of electronic instruments and digital instruments have replaced electrical instruments.
The load of these instruments is very small, most of which are less than 1VA.
This has resulted in the phenomenon that the actual secondary load of the transformer is less than the traditional 1/4 rated load.
At present, the manufacturing standards of power transformers GB 1207-2006 "Electromagnetic Voltage Transformer", GB 1208-2006 "Current Transformer" and GB/T 4703-2001 "Capacitor Voltage Transformer" still take the lower limit load as 1/4 rated load, which will cause the transformer to exceed the tolerance during operation.
The reasons for the additional error are: when designing the transformer, in order to save materials and costs, the manufacturer generally changes the ratio difference of the transformer under the rated load and the ratio difference under 1/4 rated load by 15 whole units.
According to the linear influence calculation, the change of 1/4 rated load affects the ratio difference by 5 whole units.
From the error influence of the capacitor voltage transformer, it can be concluded that it is much more difficult for the capacitor voltage transformer to reach level 0.2 than the electromagnetic transformer, and the tolerance rate during on-site inspection is also higher than that of the electromagnetic transformer.
However, capacitive voltage transformers also have advantages.
They will not produce ferromagnetic resonance with the line and have good insulation performance.
The cost of capacitive voltage transformers used in high-voltage power grids is lower than that of electromagnetic voltage transformers, so capacitive voltage transformers still have enough room for survival.
In particular, at present, a large number of electronic devices are used in secondary loads, and the actual load is only about 1/10 of the original.
As long as the error distribution method of the capacitive voltage transformer is properly handled, it can be relatively easy to reach level 0.2 during use.
The so-called error distribution means that the rated secondary load of the capacitive voltage transformer should be reduced, and the error index saved after reducing the load is reserved as error margin and allocated to the operation variation.
For example, if the current secondary rated load of the capacitive voltage transformer is reduced from 300VA to 50VA, the error can be adjusted to level 0.1 under laboratory conditions.
After installation on site, even if there is an additional error, it can meet the accuracy requirement of level 0.2.
(8) Demagnetization.
In GB20840.3-2013 "Mutual Instrument Part 3: Supplementary Technical Requirements for Electromagnetic Voltage Transformers", GB1208-2006 and JJG313-2010, the error of the current transformer is based on the error after demagnetization.
The main reason is that the state of the current transformer core after demagnetization is reproducible. However, the error of the current transformer in the residual magnetization state is not a stable value and cannot be used to define the basic error of the transformer.
JJG1021-2007 also adopts the same processing method, stipulating that the basic error of the current transformer is defined in the demagnetized state.
