Testing of Current Transformers - Silsbee's & Arnold's Method

Testing of Current Transformers:

1)Mutual Inductance Method of Testing:

This is one of the oldest ways of current Transformers testing.This is an absolute method using the null technique.The connections are shown in the below figure.

            Rp and Rs are low resistance, non-inducts voltage non-conductive shunts, Rs is variable while Rp is fixed.Rs has a slide wire for fine adjustment of resistance.The voltage drop across resistance Rp is matched against voltage drop across Rs.A vibration galvanometer is put in the circuit to indicate the balance conditions.Assuming, for the moment that there is no phase difference between Ip and Is the vibration galvanometer will indicate zero deflection. 

                        if Ip Rp =Is Rs or Ip /Is =Rs/Rp  

           Therefore, Rs and Rp should be so chosen that the ratio Rs / Rp is nearly equal to the nominal ratio of the current transformer.Resistance Rs is adjusted to render the two voltage drops equal.In order to obtain zero deflection the magnitude and also the phase of the voltage drops should be same. 

       Thus a mutual inductance M is put to compensate for phase difference between Ip and Is as without any phase compensating device it will be impossible to obtain balance with resistance alone.The figure below represents the phasor diagram for mutual inductance method under balance conditions.

       

           

           It should be noted that the secondary load circuit includes the resistance Rs impedance of primary winding of mutual inductance and also the impedance marked burden.This must be taken into account while stating the burden at which the errors have been measured.

2)Silsbee's method of Testing:

         Silsbee's method is a comparison method used for Current Transformers testing.There are two types of Silsbee's methods; deflectional and null.Only the deflectional method is described here. The arrangement for this method is shown schematically in below figure.Here the ratio and phase angle of the test transformer X are determined, in terms of that of a standard transformer S having the same nominal ratio.

       The two transformers are connected with their primaries in series. An adjustable burden is put in the secondary circuit of the transformer under test.An ammeter is included in the secondary circuit of the standard transformer so that the current may be set to the desired value.W1 is a wattmeter whose current coil is connected to carry the secondary current of the standard transformer. 

        The current coil of wattmeter W2 carries a current ∆I which is the difference between the secondary currents of the standard and test transformers. The voltage circuits of the wattmeters (i.e., their pressure coils) are supplied in parallel from a phase shifting transformer at a constant voltage V.The phasor diagram is shown in the below figure.

1.The phase of the voltage is so adjusted that wattmeter W1 reads zero. Under these conditions, voltage V is in quadrature with current Iss.The position of voltage phasor for this case is shown as Vq.

        Reading of wattmeter, W1, 

                                  W1q = Vq Iss cos 90°= 0

       Reading. of wattmeter, W2,

                  W2q = Vq x component of current ∆I in phase with 

                 Vq = VqIq = Vq Isx sin(θx - θs)

 where        θx = phase angle of current transformer under test, 

                  θs = phase angle of standard current transformers. 

2. The phase of voltage V is shifted through 90° so that it occupies a position Vp and is in phase with Iss 

               Reading of wattmeter W1, 

                                        W1p = Vp Iss Cos θ = VpIss

            Reading of wattmeter W2, 

                                    W2p = Vp x component of current ∆l in phase with Vp

                                           = Vp x ∆Ip = Vp[Iss —Isx Cos(θx — θs)] 

If the voltage is kept same for both sets of readings, then 

                                      V = Vp - Vq. 

We have, 

                          W2q = VIsx Sin(θx -θs), W1p =VIss 

                                 = VIss — Isx Cos(θx - θs) 

                                 = VIss —VIsx Cos(θx - θs) 

                                 ≈ W1p —VIsx cos(θx - θs) ≈ W1p - VIsx 

as (θx - θs) is very small and, therefore, cos (θx - θs) =1. 

For above, VIsx =W1p — W2p. 

Actual ratio of current transformer under test, Rx = Ip / Isx. 

Actual ratio of the standard transformer, Rs = Ip /Iss. 

           

           Hence if the ratio and phase angle errors of the standard transformer are known, we can compute the errors of the test transformer.W2 must be a sensitive instrument.Its current coil may be designed for small values. It is normally designed to carry about 0.25 A for testing Current Transformers having a secondary current of 5A. 

3)Arnold's method of Testing Current Transformer:

          This is a comparison current transformer testing involving null techniques.Arnold's method is used for getting very accurate results.The errors of the transformer under test X are compared with those of standard transformer S.In order to isolate the measuring circuit from the secondaries of CTs, a 5/5 current transformer T is used.This current transformer has negligible errors.

Conclusion:

          Now here we have learnt Current Transformers Testing - Silsbee's & Arnold's Method. You can download this article as pdf, ppt.

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