Current Transformer (CT) Cable Losses and Accuracy
Current Transformer (CT) Cable Losses and Accuracy
In this blog we discuss the limitations of Current Transformers with respect to maintaining the intended accuracy of a measurement system.
The main limitation we discuss only applies to 1 Amp, or 5 Amp output CT's or coils centres around the term burden, which can also be considered the "load" on the cable or wire used to connect the CT to the meter. The meter itself, the cable and the terminations all add to the burden and therefore must be considered.
5 Amp or 1 Amp output CT's
For 5 or 1 Amp output CT's, there is a simple and sometimes overlooked rule for selecting and sizing CT's and CT cable. The rule is as follows.
The burden shall not exceed the maximum specifications of the CT in order to maintain accuracy in the measurement system.
The simple formula is
CT Burden ≥ VA(Burden) of Meter + VA(Burden) CT Cable + VA(Burden of Terminations)
If we assume the termination burden is negligible then we should take one example and refer to the following table.
Example
We plan to use a 5 Amp CT or Meter with 0.5mm² diameter core wire and we wish to run 2M length to the meter. Our meter (in this case Cube 400 by Northern Design) has a burden of 0.1VA, and the cable has a burden of 3.67VA. Therefore our totoal burden is 0.1VA + 3.67VA = 3.77VA.
This means we need to select a CT with at least 3.8 or 4 VA
In practice there maybe be some nominal losses asscoiated with the terminations, so if you are extending cables throuhg addtional terminations then you need to consider the additional burden or losses introduced.
What if there is not CT available with the required burden?
In this situation you have at least three possible options;
- Use larger guage wire
- Reduce cable length with redesign
- User large CT rating and put multiple turns through the CT. Each turn will double the measured current and maintain the required accuracy of the secondary CT rating.
The second and third option might not be possible in retrofit apllications. if you need any further advice or clarifications on what CT's are available, feel free to contact us so we can confirm with any of our numerous CT suppliers.
0.333V Output CT's
0.333V output types usually found with USA based equipment such as Solar Edge, or also for inverters such as Fronius specify the maximum wire length acceptable for connection to the system. In many cases the wire is proved or had wired into the Current Sensor.
