GB2282893A - External simulator of current transformers - Google Patents

Abstract

According to the present invention a auxiliary CT Tm is connected to the secondary loop of the main CT Tn. Tm's iron core has the same magnetisation characteristics as Tn's. Tm and Tn have the common impedance when the rated secondary current are same and different impedance when the rated secondary current are different. A high gain amplifier PA is connected as differential amplifier. Its input is the difference between Tn's secondary voltage and Tm's secondary voltage. This circuit is so designed that the current output of the high gain amplifier proportionally to Tn's primary current including harmonics ad unidirectional component. So it is realizable to use this invention to get true primary current externally. It is suitable for new CT's and for CT's already installed in power systems. <IMAGE>

Description

EXTERNAL SIMULATOR OF CURRENT TRANSFORMERS This invention relates to improvement in current transformation by use of an external simulator of the current transformers.

The current transformer (CT) performance and its errors during short-circuit conditions are very important for the proper functioning of protective devices in power systems. The classic CT are defined and produced to ensure correct transformation of alternating current component of the short-circuit current within the limits specified by accuracy classes P, (5P or lOP). During the transients in the network, the unidirectional component of the primary current with time constants from several tens of milliseconds (ms) up to several seconds occur in short-circuit currents. The transformer core saturation owing to the unidirectional component of the primary current causes both considerable distortions of the secondary current waveform and a phase shift. The transient, which can take several minutes can lead to considerable errors in respective protective device. These are the main reasons for the need to reduce flux in the core and for an external simulator to obtain true primary current including unidirectional component. The simulator can be applied to new protective CT's and CT's already installed in the systems.

According to the present invention an auxiliary CT Tm is connected to the secondary loop of the main CT Tn as shown in Fig. 1. and in Fig. 2. Tmts iron core has the same magnetisation characteristics as Tn's. Tm and Tn have the common impedance when the rated secondary current are same in Fig. 1 and different impedance when the rated secondary current are different in Fig.2. A high gain amplifier PA is connected as differential amplifier. Its input is the difference between Tn's secondary voltage ( ion - 12m)Z2 in Fig 1. and i2nZ2n in Fig. 2. and Tm's secondary voltage which is ( I2n - 12m)Z2 in Fig.1. and I2mZ2m in Fig.2.. This circuit is so designed that the current output of the high gain amplifier proportionally to the main CT's primary current including harmonics and unidirectional component. The amplifiers Al and A2 which connected with resistor Rl, R2 and R3, R4 separately are used to scale Tn's secondary voltage and Tm's. A specific embodiment of the invention will be described by way of example with reference to the accompanying drawing on Fig 2 in which: Tn is, with primary winding Nin and secondary winding N2n, the main CT which is to be simulated and Z2n is its dummy burden which do not need to large. Tm with primary winding Nim and secondary winding N2m, is the auxiliary CT which is used to simulate Tn and Z2m is its dummy burden. Tm's and Tn's iron cores have the same magnetisation characteristics If using lom to represent the magnetising current of Tm and Ion to Tn, the magnetic potential equations for Tn and Tm are as follows: IlnNIn = I2nN2n + lonNln (1) I1mN1m = I2mN2m + I0mN1m (2) Because Tn and Tm are designed to have the similar magnetisation characteristics, this relationship can be written as: lonNin = kl (3) I0mN1m The connection ofthe amplifier is such that the output I1m tend to make AU to be zero, so: I2nZ2n R2/R1# I2mZ2m R4/R3 (4) It is not difficult to adjust Z2n and Z2m to be: Z2n=k2Z2m (5) Substituting equ.5 into equ.4 yields: 12n=12m/k2 (6) Adjusting Z2m, N1m, N2m, Ri, R2, R3 and R4 to make: N2n/N2m = k1k2 (7) Substituting equ.3 and equ.7 into equ.1 and equ.2 yields: Iln=k3Iim (8) Where b. . k2, ks are constants. From equ.8 it is quit clear that using this simulator the Tm's primary current lirn can be made proportional to the current I1n in the external burden Zb. The invention calls for an amplifier that will give the current balance as stated in equ.8 & the value of the burden does not enter this equation provided the amplifier which is capable of developing the voltage necessary to drive lim through Zb & Tm.

Claims (3)

1. Claim 1. An external simulator of current transformers comprises an auxiliary CT Tm in which the core has the same magnetisation characteristics as the CT Tn to be simulated, a high gain electronic amplifier PA is connected as differential amplifier. Its input is the difference between Tn's secondary voltage and Tm's secondary voltage. This circuit is so designed that the current output of the high gain amplifier PA in the external burden Zb proportional to Tn's primary current including harmonics and unidirectional component.
2. Claim 2. An external simulator of current transformers as claimed in claim 1 wherein Tm and Tn have the common impedance when the rated secondary current are same and different impedance when the rated secondary current are different.
3. Claim 3. An external simulator of current transformers as claimed in claim 1 and claim 2 wherein the amplifiers Al and A2 which connected with resistor RI , R2 and R3, R4 separately are used to scale the secondary voltages of Tn's and Tm's.