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How to calculate Transformer Line Loss Compensation (TLC) for two identical Potential Transformers (PTs or Voltage Transformers) in parallel

Goals and Symptoms

When two transformers are connected in parallel, TLC parameters have to be adjusted accordingly since ION meters can only compensate for 1 PT. In this case, the system has to be simplified into 1 PT in terms of the variables that are used for TLC, i.e. %excitation current, Iron loss etc….

Causes and Fixes

Since the PTs are identical and connected in parallel, voltage on two meters will be equal. However, same assumption can not be made for the current since it will be dependent on the load. In the no-load situation, basic principles dictate that the current should be split into 2 equally, but load will not allow this assumption. Therefore, the following parameters can be calculated as:
Iron Loss: will be 2 times what one transformer would cause

%Excitation Current: will be 2 times what one transformer would require

%Impedance: Zt = (Z1*Z2)/(Z1 + Z2) where Z stands for impedance and t, 1 and 2 mean total, 1st and 2nd transformers respectively.

Copper Loss: As the current will not be split half, copper loss has to be calculated based on the current split. The current flow on each transformer has to be determined experimentally (or based on calculation with a fixed load). Since the Copper Loss is a function of I2, the ratio of Copper Loss for 2 PTs to 1 PT case can be found using the following formula:

Copper Loss for 2 PTs = [ (I1/I)2 + (I2/I)2 ] x Copper Loss for 1 PT where I, I1 and I2 indicate the total current flow and current flow on the 1st and 2nd transformers respectively.


  • Date Created: November 14, 2008
    Last Revised: February 23, 2009
    Public
    Original Author: KG
    All content © 1992-2009 Schneider Electric
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