Current Transformer (CT) – Construction and Working Principle

The burden of a Potential Transformer

The burden is the overall external volt-amp power on the secondary section at the rated voltage. The rated burden of a potential transformer is a VA burden which should not be increased if the device tends to perform with its rated precision. The rated burden is determined on the nameplate.
The maximum or limiting burden is the highest VA power at which the practical transformer will constantly perform without overheating its parts beyond the permissible range. This burden is several times higher than the rated burden.
If the standard ratio of the potential transformer is varied from the practical ratio of the device due to the resistance and reactance drops, then this error will occur.

Causes of Errors

Because of the internal impedance, the voltage reduces in the primary section, and it is transmitted proportionally to its twists ratio and the secondary section. Similarly, the same thing occurs on the secondary side.

Reduction of Errors

The errors of potential transformers can be decreased or prevented by increasing the precision in the designing, magnitudes of resistance and reactance of primary and secondary sides, and minimum magnetization of the core.

Phasor Diagram of a Potential Transformer

The phasor diagram of a potential transformer is presented in the figure below.


Potential Transformer Phasor Diagram
Where,

• 
  Es – Secondary induced emf
  
• 
  Is – Secondary current
  
• 
  Xs – Secondary winding reactance
  
• 
  Rs – Secondary winding resistance
  
• 
  Vs – Secondary terminal voltage
  
• 
  Vp – Primary terminal voltage
  
• 
  Ip – Primary current
  
• 
  Ep – Primarily induced emf
  
• 
  Xp – Primary winding reactance
  
• 
  Rp – Primary winding resistance
  
• 
  Kt – Turn ratio
  
• 
  Im – Magnetizing component of Io
  
• 
  Io – Excitation current
  
• 
  Iw – Core loss component of Io
  
• 
  Β – Phase angle error
  
• 
  Φm – Main flux
  

The main flux is considered as the reference. The primary value is the vector sum of the excitation current Io, and the current identical to the reversal secondary value Is improved by the ratio of 1/kt in the instrument transformer. The Vp is the voltage provided to the primary side of the potential transformer.

The voltage reduces based on the resistance and reactance of the primary section due to the primary current, which can be obtained by IpRp and IpXp. When the voltage reduction subtracts from the primary value of the potential transformer, the primarily produced EMF (Electromotive Force) will appear through the terminals.
This basic EMF of the device will change into the secondary side by the mutual induction effect and transformed into the secondary produced EMF (Es). This EMF will reduce by the secondary side reactance and resistance, and the resultant output will appear through the secondary terminal voltage, and it is expressed by Vs.
The produced primary voltage EMF is the subtraction of reactance and resistance drops (IpXp, IpRp) from the primary voltage (Vp). The voltage reduces because of the resistance and reactance of the primary winding.
The EMF produced in the primary is transmitted into the secondary side by the mutual induction and creates the EMF in the secondary side (Es). The output voltage through the secondary side because of the EMF reduction by the reactance and resistance is Vs. The output value on the secondary side is gained by subtraction of the resistance and reactance drops (IsRs, IsXs) from the produced EMF in the secondary winding (Es).