This invention relates to a self-powered electronically error compensated current transformer.
The known electronic error compensation current transformer (hereinafter also called E(T for the sake of brevity) derives power for its operation from an extraneous source. However, this invention proposes to render the ECT self-powered.
The various other features of this invention and its advantages will be clear from tin-following description.
The self-powered electronically error compensated current transformer, according to this invention, comprises the known electronic error compensation current transformer (ECI ). characterised by an auxiliary core carrying a primary winding and two identical secondary windings:, the primary winding of the auxiliary core being connected in series with the mam primary of the ECT, both primaries being energised by the same current that is required to ho measured, the secondary windings on the auxiliary core feeding a rectifier-filter-regulator circuit furnishing a d.c. voltage at its output fir being fed to the amplifier of the ECT.
This invention will now be described with reference to-the accompanying' drawings which illustrate.
In Figs.l (a) and 1 (b) a single core and two core ECT respectively
and
in Fig.2(a) and 2 (b), by way of example and not. by way of limitation, embodiments of the single core and two core versions of this invention.
High accuracy current transformers (C.T.) can be built using either large quantities of copper and special soft magnetic core or by resorting to electronic error compensation. While the former results in a bulky and expensive unit, the latter needs auxiliary power. This invention proposes a novel construction that exploits the advantages of electronic, error compensation, retaining the simplicity of a passive Current Transformer,
A Current Transformer has its ratio error and phase error. One has to resort to the liberal use of expensive high quality Nickel-Iron cores to achieve appreciable reduction m

the ratio and phase errors. Such high accuracy Current Transformers would invariably be
bulky and expensive. Employing electronic feedback compensation technique for error
reduction results in small-size, cost-effective units. Such electronically error-compensated
Current Transformers find application in the sensing of currents in thyristor-drive circuits
and in the accurate monitoring of power and energy in electrical'utility systems. These
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current transformers use the 'zero-flux principle' and are of two types when m.ade for ax.' application.
(i) In the single-core arrangement shown in Fig. 1(a), an additional third winding, namely, the detector winding of ND turns, is also wound on the Current Transformer core, apart from the usual primary and secondary windings of N, and N2 turns. An operational (high-gain) amplifier G gets as input, the induced voltage in the detector winding which is proportional to the time-derivative o\' the flux in the Current Transformer core. -The opamp's output feeds the . secondary winding of the Current Transformer and the burden. The polarities are chosen to ensure overall negative feedback, so that zero voltage is induced ' in the detector (ND) winding. Zero induced voltage in the detector winding is possible if and only if the alternating flux in the core is reduced to zero. Zero-flux conditions are established in the core, with the amplifier supplying the voir ampere required by the burden and the resistance of the secondary winding of the Current Transformer. In other .words, the opamp will send a current I-, through the secondary winding such that exact ampere-turn balance (N,l, NJ2) is almost achieved, resulting in a near-ideal Current Transformer with very low errors. The core just serves as an ampere-turn balance.
(ii) In the two-core scheme illustrated in Fig. 1(b), both the cores A and B carry the usual primary and secondary windings. Additionally, core A is wound with the detector winding of ND turns and core B with a compensating winding of N, turns. Here too, the opamp ensures that the induced voltage in the detect*it-winding and thereby, the ax. flux in core A are reduced to zero. Core B supplies most of the burden VA by normal transformer action and the amplifier puts in a small amount of VA corresponding to the error power.

Two sources of a.c. power are used as input in the electronically error-compensated systems reported so far. One supply, coming from the measuring circuit, provides the primary current of the Current Transformer. An additional a.c. voltage source is used to derive a regulated d.c. power supply for the amplifier via a trans former-rectifier-filler arrangement. If the need for the second a.c: supply is avoided, retaining the electronic error-compensation facility, we can expect a considerable simplificatipn in the design of the overall current-measuring device, which will then become a self-powered one. We propose here such a Current Transformer, incorporating an additional core with suitable windings.
The incorporation of the above self-powering, device into the actual ECT is illustrated in Fig. 2(a) for the single-core scheme and in Fig. 2(b) for the two-core arrangement. An auxiliary core C, used in addition to the main ECT core(s), carries one primary winding and two identical secondary windings IS1-IS2, 1S3-IS4. The primary winding of core C is connected in series with the main ECT primary and is energised by the same current that is being measured. The secondary windings on core C feed individual, conventional bridge-' rectifier circuits with filter-regulator combination as shown in Fig. 2(a) and 2 (b). The outputs of the regulators are connected so as to realise a ± Vcc supply for the amplifier