Voltage transformers are extensively used in electric power networks in order to provide standard values for metering and relays. Similarly capacitive voltage transformer exists for providing secondary voltages. The standard value is usually 110 V or 110Ö3. The VA ratings and class of accuracy of these devices is about SO - 100 VA and 0.S respectively. Some designs use an amplifier in addition to the voltage transformer or capacitive divider in order to provide improved accuracy.
Draw back of prior art
The first problem with the amplifier aided electromagnetic voltage transformer is that it requires an additional VT of the same nominal ratio. For example, a 11 kV /110 V VT requires a second 11 kV/110 V VT along with an amplifier in order to improve its accuracy. Similarly the capacitive VT has a poor bandwidth due to its inherent design. Purely capacitive dividers with an amplifier have the drawback in that the amplifier needs to supply the entire burden power.
New proposal to overcome the drawback
The new proposal, that is, this invention comprises a combination of a capacitive tap, an electromagnetic VT and an amplifier to avoid the drawbacks mentioned above. The advantages are that it has in a single unit a capacitive divider, and electromagnetic VT and an amplifier. It results in very high accuracy and greater bandwidth than existing designs.
The combined capacitive and electromagnetic voltage transformer, according to this invention, comprises a voltage transformer, an isolation transformer, an amplifier connected on the secondary side of the isolation transformer characterised by a capacitive divider connected on the primary side of the isolation transformer, between line and earth of the voltage transformer.

The invention will now be described in former detail by reference to the accompanying drawings which illustrate, by way of example, and not by way of limitation one of possible embodiments of the combined capacitive and electromagnetic voltage transformer, Fig. 1 illustrating the embodiment with a unity gain amplifier Fig. 2 illustrating the embodiment with a high gain amplifier Fig. 3 illustrating the construction of the voltage transformer of the said embodiment
Fig. 1 and Fig. 2 shows the methods providing a combined capacitive divider C1,C2 form the capacitive divider, VT is the voltage transformer, A is the amplifier and TX is the isolation transformer. Since A is an ideal amplifier, it draws no current from the C1, C2 combination. Any errors in VT corrected by an increased current from A making the error of VT small. Fig. 1 uses a unity gain amplifier (voltage follower) and Fig. 2 uses a high gain amplifier.
1. Unity gain amplifier
The proposed scheme of voltage transformer compensation using unity gain amplifier is shown in Fig. 1. Here VT is the voltage transformer whose secondary potential is to be compensated for. Cl, C2 along with isolation transformer TX forms the voltage divider whose division exactly matches with the nominal ratio of the VT. Two terminals of similar polarity say, S1x and SIT of TX and VT, and one terminals of the burden are connected together. SOT is earthed, while S2X is connected the input of voltage follower (VF-unity gain amplifier), whose output is connected to the other end of the burden.
In the absence of voltage follower, the burden would normally be directly across the terminal S1x - Sue and the voltage across it would possess considerable

magnitude and phase error. Which again vary with the value and nature of the burden. In Fig. 1 the voltage follower forces the voltage across the burden to assume a value equal to the open circuited voltage across C* independent of the value of burden, as long as the error voltage is compatible with the permissible output swing of the voltage follower. The secondary of TX is essentially open-circuited as the terminals S2x is connected to the non-inverting input of the operational amplifier.
Since A is an ideal amplifier, it draws no current from the C1, C2 combination. Any errors in VT corrected by an increased current from A making the error of VT small.
2. High gain amplifier
High gain amplifier scheme for voltage transformer compensation is shown in Fig. 2. VT is the voltage transformer, the error of which is to be compensated for. C1, C2 combination with TX forms the auxiliary voltage transformer of the same nominal ratios as VT. Two terminal of similar polarity, say S1 for the two transformers and one terminal of the burden are connected together, the other end of the burden being grounded. In the absence of the amplifier, the burden would be connected directly across the secondary terminal of VT, one end of which would be earthed and TX would be supplying essentially the same output current and consequently having the same terminal voltage V2T. The terminal S2X of TX is connected to the input of the inverting amplifier of gain A, whose output is connected to S2X of VT. The voltage across the input terminal of the amplifier is equal to the difference in the voltage across the burden and the secondary voltage TX, whose secondary is essentially on open-circuited, being connected to an amplifier of high input impedance.

The amplifier provides an output voltage which acts in series with the secondary terminal voltage of VT to develop the required voltage across burden with sufficiently large gain, the output voltage of the amplifier gets adjusted in magnitude and phase position till burden potential approaches secondary voltage of TX, thus reducing the voltage error and phase displacement as viewed from the burden. In brief since A is an ideal amplifier, it draws no current from the Cl, C2 combination. Any errors in VT corrected by an increased current from A making the error of VT small.