Contents

CORRECTION FACTOR TO BE USED FOR DETERMINING

THE ACTUAL POWER/ENERGY AFTER CONSIDERATION OF

RATIO AND PHASE ANGLE ERRORS OF C.TS. AND P.TS.

USED IN METERING.


It would be seen from the vector diagram that the meter reading shall be corresponding to Vs.Is.Cos a which does not correspond to the power/energy which shall be VICos f. A correction needs to be applied to the meter reading depending upon the errors of C.Ts. and P.Ts.

Phase angle seen by the meter is  a which obviously  is

a = f - q ± d

In the vector diagram d is shown as positive that is P.T. secondary voltage leads the primary voltage reversed. The phase angle of meter pressure coil is ignored and is assumed to be zero.

V  - System Voltage 

I   - System Current 

f - Load P.F

Is -  C.T Secondary Current

Vs - P.T Secondary Voltage

q - Phase angle error C.T

d - Phase angle error P.T


a - Phase angle seen by meter 

Thus, the correction corresponding to phase angle errors of C.Ts

and P.Ts. shall be

 

K = Cos f  

Cos (f - q ± )                                                                                       (1)

Note - d may be positive or negative.

 

About ratio errors of C.Ts and P.Ts. let the C.T. ratio error be e%

and P.T. ratio error be p%.

 

 

Actual Ratio of C.T. = RCTA  =  Primary Current

Actual Secondary Current

 

 

 

 

Actual Ratio of P.T. = RPTA  =  Primary Voltage

Actual Secondary Voltage

 

Considering the rated primary current (Ip) as primary current

(100%) for C.T. and rated primary voltage Vp (100%) as primary

voltage for P.T, (Errors e and p are assumed to be at rated primary

current and rated primary voltage with rated burden connected

across the secondary.

 

Actual Ratio of C.T.                    =                  Ip          

Is  -  e x Is                                                                              100                                             (2)                              Actual Ratio of P.T                     =               Vp           

   Vs  -  p x  Vs

               100                        (3)

 

 

 

where Is and Vs are rated secondary current for C.T. and rated

secondary voltage for P.T. respectively.

 

Dividing numerator and denominator of equation 2 and 3 by Is

and Vs respectively

 

           Actual Ratio of C.T.            =    Ip / Is                    

                                                            1-  e x Is / Is

                                                                    100                                            (4)

 

  

 

            Actual Ratio of P.T.           =        Vp / Vs          

                                                               1-  p x Vs / Vs

                                                                          100                                      (5)

 

 

 

 

or rewriting equation 4 and 5 it will be seen that

 

 

   

             Actual Ratio of C.T.            =    Nominal Ratio

                                                                 1-    e  

                                                                      100                                        (6)

 

           

            Actual Ratio of P.T.              =     Nominal Ratio

                                                                    1-     p   

                                                                          100                                     (7)

 

 

 

 

            or simply for C.Ts A.R.         =        N.R.      

                                                                  1-     e                                                                           100                                         (8)