GB2271188A

GB2271188A - External compensator of current transformers

Info

Publication number: GB2271188A
Application number: GB9316005A
Authority: GB
Inventors: Qifeng Xu
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-09-28
Filing date: 1993-08-02
Publication date: 1994-04-06
Anticipated expiration: 2013-08-02
Also published as: GB2271188B; GB9316005D0; CN2150619Y

Abstract

Nominal ratio and phase angle error between the fundamental component of current in the primary and the secondary of a Current Transformer Tn is due to its magnetising current, which is a non-linear function of primary current and burden Z2n. Based on the principle of physical similarity a specially designed model T1m (a small CT) which has the same magnetisation characteristics as the CT to be compensated is wired in the secondary circuit of this CT. The excitation current is obtained from the model and sent to the burden Z2n of the main CT so that its errors are compensated. It is suitable for new CT's and for CT's already installed in power systems. The model T1m is associated with an active zero magnetic flux sensor including a second transformer T2m and a high gain amplifier supplying a compensating current to burden a load Z2n of the main current transformer. <IMAGE>

Description

EXTERNAL COMPENSATOR FOR CURRENT TRANSFORMERS This invention relates to an external compensator for current transformers.

Nominal ratio and phase angle errors between the fundamental component of current in the primary and the secondary of a current transformer (CT) is due to the CT's magnetising current, which is a non-linear function of primary current and burden. In power system it is very difficult to improve the precision of high-tension current transformer to 0.5 degree or better owing to the non-linear property of magnetic core material, the bulk of the insulation between the primary and secondary windings and the limitations of the traditional compensation methods. In the present invention a small model CT, designed on the principle of physical similarity to the main CT, is used to generate a current equal to the magnetising current of the main CT .The excitation current thus obtained from the model is sent to the burden so that ratio and phase angle errors are compensated. Compensation can be applied to new CT's and CT's already installed in systems.

According to the present invention a double cored CT is added to the secondary loop of the main CT. The first core of the auxiliary CT has the same magnetisation characteristics as the main CT's and is with a model burden used to simulate the state of magnetisation ofthe main CT. The second core is used with a high gain amplifier as an active zero magnetic flux sensor of the magnetic state of the first core. The sensor is so designed that the current output of the high gain amplifier equals the magnetising current of the main CT and is circulated through the burden of the main CT.A specific embodiment of the invention will be described by way of example with reference to the accompanying drawing on Page 5 in which: Tn is, with windings Ngn and N2n, the main CT which is to be compensated and Z2n is its burden.Core T1", and T2,., with windings Nlzn, N2wn, N3m and N4", is the double stage CT in which Tln is the first stage that has the same magnetisation characteristics as Tn and T2m is the second stage high my core. Z2n} is the burden of the first stage CT which is adjustable to match Z2n . T22}} with windings N3m and N4m and the high gain amplifier A compose an active zero flux CT whose output is the magnetising current I0n of T2m .Because the amplifier has thigh input impedance, N4m is effectively open-circuited and proportional to the magnetic potential of T2wn. The amplifier will produce a compensating current I'am. I'omN3m gives a demagnetising ampere turn which makes magnetic flux in T2", tend to zero, that is I0n# O. I2n is the secondary current of the main CT. I2aw is the secondary current of the double stage CT.If using Io", to represent the magnetisino current of Tlm, the magnetic potential equation for Tlm is as follows: I2nN1m-I2mN2m=I0mN1m (1) The magnetic potential equation for Tn is: I1nN1n-I2nN2n=I'0nN1n (2) where I'On is the magnetising current of Tn. When Tn and Tin are designed to have the similar magnetisation characteristics, this relationship can be written as: I'0n/I0m=k1 (3) Where ki is a constant. The magnetic potential equation for T2m is: I0mN1m-I'0mN3m=I0nN1m (4) By ion O, I'0m#I0mN1m/N3m=I0mk2 (5) Where k2=N1m/N3m is also a constant. Substituting equ.3 into equ.5 yields I'0n/I'0m=k1/k2 (6) Adjusting N1m , N3m and N2n and designing Tin to make kl /k2 =1, then: I'0m = ItOm (7) Sending I'On to the burden of the main CT the errors is compensated completely.

The invention is suitable not only for improving the precision of high-tension CT but also for instrument CT.

Claims

Claim 1. An external compensator for current transformers wired in the secondary circuit and comprising a double cored transformer where one core models the main CT loaded with a model of the main burden and the second core constitute a zero flux magnetic potential detector CT driven by a high gain amplifier which also returns to the main burden the magnetising current of the model CT scaled to represent the magnetising current of the main CT.
Claim 2. An external compensator for current transformers as claimed in claim 1 wherein the double cored transformer composed of the first iron core Tim, the secondary iron core T2wn and windings Nlm, N2m, N3m and N4EZ. N1m and N2", are wound on Ti,11 and T2m. Tim with windings Nim and N2m has the same magnetisation characteristics as Tn the CT to be compensated while N3m and N4", is wound only on T2m.
Claim 3. An external compensator for current transformers as claimed in claim 1 and claim 2 wherein an active zero flux CT is composed of a high mu metal core T2nz with windings N3m and N4m and a high gain amplifier A.