Multi ratio current transformers (CT) allow users to pick current transformation ratio needed for specific application from a given CT. Various winding taps are provided allowing user to pick and choose the desired ratio. Many models allow up to ten different ratios and these CTs are primarily used in relaying applications. While multi ratio CTs are very convenient, using lower ratio also reduces the accuracy rating of CT. In relaying applications where accuracy is important this needs to be factored into. Multi ratio current transformer may have a name plate diagram like the one shown below. How do you pick the correct tap for your application from this diagram?
In the figure above, X1, X2, X3, X4, X5 are the tap connections available to user. Full rating of CT is 600:5 when connected across taps X1-X5. If user needs a different current rating, then he has to pick another tap. To do this the numbers listed on the drawing has to be understood. These numbers are the number of turns between various taps. For example, in figure 1 above there are 20 turns between X1 and X2, 10 turns between X2 and X3, 80 turns between X1 and X4 etc.
Example: Assume we need to use above 600:5 CT for a 150:5 application. First divide 150/5 which gives 30. From the figure above find out which combination gives total of 30 turns (answer X1-X3). Connect CT secondary leads between X1 and X3 and leave the other taps open. Now we have a 150:5 CT!
Accuracy and Multi ratio CT
Accuracy class stamped on the CT is based on full rating of the CT. In figure 1, CT accuracy rating of C100 applies only when CT is connected between X1 and X5 (full winding). If any other lower tap is used, accuracy is proportionally reduced.
Formula for calculating effective accuracy class when applied on a lower tap is as follows:
For the example above 600:5, C100 CT is used at a lower tap of 150:5. Number of turns used is 30 (X1-X3) and total turns is 120 (X1-X5). New accuracy class can be calculated as below:
Sometimes multi ratio CT tap information is provided not as shown in figure 1 but as in figure 2 in the form of actual winding ratios available- much easier for user.
In this case the same accuracy class formula can be modified as below:
Consider the same example as of 600:5 C100 CT used at a lower tap of 150:5.
