Notes on the class accuracy of current transformers
Current transformers are divided into classes according to their accuracy. Depending on the product category, Beckhoff current transformers are available in the standard accuracy classes 0.5; 1 and 3. The class designation corresponds to an error curve with regard to current amplitude and angle error. Beckhoff current transformers conform to the IEC 61869 standard.
The accuracy classes of current transformers are related to the rated current. If current transformers are operated with a low current in relation to the rated current, the measuring accuracy decreases. The following tables show the error limits taking into account the rated current values:
Percentage current error (±) when the operating current is a percentage of the rated current | ||||||
|---|---|---|---|---|---|---|
I/IN [%] | 5 % | 20 % | 50 % | 100 % | 120 % | |
Accuracy class | 0.5 | 1.5 % | 0.75 % | - | 0.5 % | 0.5 % |
1 | 3.0 % | 1.5 % | - | 1.0 % | 1.0 % | |
3 | - | - | 3 % | - | 3 % | |
Phase shift/misalignment (±) with percentage of operating current to rated current | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Minutes [ ´ ] | Degrees | |||||||||
I/IN [%] | 5 % | 20 % | 50 % | 100 % | 120 % | 5 % | 20 % | 50 % | 100 % | 120 % | |
Accuracy class | 0.5 | 90‘ | 45‘ | - | 30‘ | 30‘ | 1.5° | 0.75° | - | 0.5° | 0.5° |
1 | 180‘ | 90‘ | - | 60‘ | 60‘ | 3° | 1.5° | - | 1° | 1° | |
3 | - | - | - | - | - | - | - | - | - | - | |
The following graph shows the error limits taking into account the rated current values for the accuracy classes.
Fig.16: Characteristic curves accuracy classes/primary current | Transformer load In order to use the optimum accuracy of the transformer, you must ensure that the cabling between the transformer and the measuring terminal provides a suitable transformer load in your application. This is achieved by using 0.25 to 1.0 times the rated power of the transformer. |