GB2037440A

GB2037440A - A capacitive A.C. voltage divider

Info

Publication number: GB2037440A
Application number: GB7936876A
Authority: GB
Original assignee: Hazemeijer BV
Current assignee: Hazemeijer BV
Priority date: 1978-10-26
Filing date: 1979-10-24
Publication date: 1980-07-09
Also published as: DE2943403C2; CH643363A5; NL7810695A; FR2439997B1; NL184588B; DE2943403A1; SE439201B; IT7969082A0; GB2037440B; NL184588C; FR2439997A1; SE7908901L; IT1119471B

Abstract

A capacitive voltage divider, for instance for medium or high voltage, comprising a series circuit of at least two capacitors C1, C2, one of which serves as a measuring capacitor C2, which series circuit may be connected between a primary voltage and a reference voltage whereby a measuring signal is derived from the measuring capacitor C2 as a secondary voltage, which measuring signal may consist of an A.C. voltage component and an unwanted D.C. voltage component and wherein a leakage inpedance R1 has been connected across the measuring capacitor for discharging the D.C. voltage component, including control means for the reception of the measuring signal and the derivation of the true unwanted D.C. voltage component from the measuring signal, characterized by compensating means for receiving the measuring signal and for compensating the D.C. voltage component in the measuring signal with the said derived true D.C. voltage component. As shown, filter F1 blocks the D.C. component so that comparator D provides only the D.C. component to a logic circuit B1 which closes S1 or S2 (depending on polarity) C3 is thus charged from a source Vref1 or Vref2. When C3 is charged to the mains voltage as C2, comparator D gives zero output, opening S1 or S2 and the measuring signal output is free from D.C. <IMAGE>

Description

SPECIFICATION A capacitive A. C. voltage divider.

The invention relates to a capacitive voltage divider for instance for medium or high voltage, comprising a series circuit of at least two capacitors one of which serves as a measuring capacitor, which series circuit may be connected between a primary voltage and a reference voltage whereby a measuring signal is derived from the measuring capacitor as a secondary voltage, which measuring signal may consist of an A. C. voltage component and a D. C. voltage component and wherein a leakage impedance has been connected across the measuring capacitor for discharging the D. C. voltage component, including control means for the reception of the measuring signal and the derivation of the true D.

C, voltage component from the measuring signal.

A similar voltage divider is known from "Bulletin Scientifique A. I. M.", June 1973, pages 154-155.

When the measuring signal derived from the measuring capacitor serves to monitor the primary voltage connected to the voltage divider, such a D.

C. voltage across the measuring capacitor may thwart or even preclude this monitoring function.

If such a D. C. voltage is present distance relays cannot intervene timely or will intervene wrongly.

Also the localizing of an earth fault or phase short circuit will be difficult or even impossible.

A D. C. voltage across the measuring capacitor may for instance occur when a primary A. C.

voltage line is disconnected at the moment at which the wave form of the alternating voltage does not pass through zero. The voltage then remaining on the line will lead to a residual charge also called the D. C. voltage component which will result in a D. C. voltage across the measuring capacitor via the capacitor connected to the primary voltage. If in the presence of such a D. C.

voltage across the measuring capacitor the voltage would return again after a few periods and a short circuit would occur immediately afterwards while the D. C. voltage is not yet diverted a zero passage of the measuring signal will consequently occur only after a few periods to which zero passage a distance relay will respond.

However, this implies that the relay will respond too late. Also overvoltages on the primary A. C.

voltage lines may lead to residual charges and D.

C. voltages across the measuring capacitor.

Hence for a correct operation of distance relays an undistorted reproduction of the changes in the primary voltage will be essential. In the known voltage divider a leakage impedance is therefore connected in parallel to the measuring capacitor whereby the D. C voltage will disappear fast enough.

The application of a leakage impedance will indeed prevent the occurrence of an inadmissable delay in the operation of protective means among others under the above mentioned conditions, but said application will also have the detrimental effect that there will occur a phase shift of the measuring signal with respect to the primary A. C.

voltage. As a result thereof there will occur a delayed response of the protective means, e.g. in case of a short circuit in the primary alternating voltage circuit. Apart from a phase shift the measuring signal will also present transient phenomena due the leakage impedance, which phenomena, as will be evident, will likely disturb an accurate operation of the protective system for the primary A; C. voltage.

The above drawbacks are avoided by the capacitive A. C. voltage divider according to the invention characterized by the presence of compensating means for receiving the measuring signal and for compensating the D. C. voltage component in the measuring signal with the said derived true D. C. voltage component.

In the capacitive A. C. voltage divider according to the invention of D. C. voltage component in the measuring signal, i.e. the charge on the measuring capacitor will indeed be discharged via a leakage impedance applied to that effect, but in anticipation thereof the D. C. voltage component in the measuring signal caused by the said charge will be compensated immediately and remain compensated until the disappearance of the charge. The term "immediately" as used above means within a short time in comparison with the time of one cycle which at 50 cycles amounts to 20 ms.

In a preferred embodiment of the present invention the compensating means consist of a compensating capacitor and a leakage impedance connected in parallel thereto, the common connection terminal of which is connected to the said reference voltage; switching means for supplying a given further reference voltage to the other common connection terminal of the compensating capacitor and the leakage impedance connected in parallel thereto, which switching means may be controlled by a signal derived from the true D. C. voltage component, the value of said further reference voltage and the time for switching on of said switching means being chosen such that the voltage at the compensating capacitor may quickly compensate the D.C. voltage component in the measuring signal; a differential amplifier to a input of which there is supplied the measuring signal and to an other input of which there is supplied the voltage at the compensating capacitor while the output signal compensated with respect to the D. C.

voltage component is supplied to the control means. Considered in absolute value the voltage at the compensating capacitor may be made equal to the true D. C. voltage component at the measuring capacitor. The differential amplifier may then have an ampiification factor of 1 for both signals.

It should be remarked that Dutch Patent Applications 7711655 and 7802969 to applicant also propose a capacitive A. C. voltage divider having an additional capacitor. By means thereof there will be generated two separate signals i.e. a narrow band fundamental wave signal hence corresponding to the true A. C. voltage component and a broad band signal including also the D. C.

voltage component. Both may serve as a control signal whereas the latter may serve as a measuring signal for monitoring or protective purposes. The control signals will cause the control means to eliminate D. C. voltage components optionally present in the broad band signal. The method disclosed in the afore said patent applications for obtaining two more or less independent channels by means of a high voltage capacitor may meet difficulties in certain situations as regards the cross talk or the practical feasibility. Although a high degree of independence between the two both mentioned signals may be attained by choosing the capacities appropriately there remains need of a capacitive A. C. voltage divider capable of realising a complete independency.This independency may be obtained by means of a double voltage divider, this also being a subject of the above mentioned patent applications. In that case however practical objections may arise also especially when the measuring capacitor has a considerable capacity in which case problems arise due to the application of generally preferred semi conductive switches, because the switching currents become too large. This is for instance the case when updating an old CVT-measuring means in which a large additional capacitor has to be inserted in the series arrangement in order to create an additional measuring point. In practise it will then be more attractive to apply one fixed leakage impedance.

Now a feature of the capacitive A. C. voltage divider according to the present invention is the provision of both a simplified practical feasibility and a complete independence of both the above mentioned signals. For the voltage divider according to the present invention the control velocity is at least equal to that of the best embodiment according to the two afore said patent applications while the band width remains amply sufficient for the envisaged purpose.

In case of the voltage divider according to the present invention the undesired charge on the measuring capacitor is not eliminated within a short time by short circuiting this capacitor by means ofa switch but is contrary thereto the D. C.

voltage component on the capacitor compensated with for instance as high a voltage of the same or opposite potential by combining both voltages n a differential amplifier.

The invention will now be described in greater detail with reference to the drawings in which by way of example some embodiments are shown.

Fig. 1 shows a first embodiment of a compacitive A. C. voltage divider according to the present invention in combination with the associated control means; Fig. 2 shows an embodiment modified with respect to Fig. 1 of the capacitive A. C. voltage divider according to the present invention; and Fig. 3 shows a preferred embodiment in which a second differential amplifier has been added.

Like in the known arrangement as well as in the arrangements according to the afore said patent applications to applicant, the capacitive A. C.

voltage divider comprising the series circuit of the capacitors C1 and C2 and the leakage impedance R1 connected across thereto is arranged between the primary voltage Vp and a reference voltage, for instance the earth. The leakage impedance R1 will have a value chosen so large that the phase fault in the signal at the junction of C1 and C2 may be neglected or may be simply compensated for the A. C voltage frequency.

When assuming that the phase correction may be omitted, the signal at the junction of the capacitors C1 and C2 in the present embodiment first passes the amplifier V1, in this case a differential amplifier, whereupon the signal directly arrives as the measuring signal B at the input 2 of the comparison circuit D. The output signal of the amplifier V, is moreover supplied to a A. C. voltage blocking filter F1 whereupon the signal arrives at the first input of the comparison circuit D as a narrow band fundamental wave signal. In this comparison circuit D the two signals are compared with each other in order to determine a D. C. voltage component in the broad band measuring signal B.

In this case the compensating circuit comprises the already mentioned differential amplifier V1, the compensating capacitor C3 including the leakage impedance R2 arranged parallel thereto as well as a number of additional resistors R3 and R4 and switches S1 and S2.

A junction of the compensating capacitor C3 and the leakage impedance R2 is connected to a reference voltage that may also be earth in this instance. The other junction is connected to the second input of the differential amplifier. To this other junction there are also connected two resistors R3 and R4, each of which are connected perse to a different connecting conductor by means of switches S1 and S2 respectively, which on their turn are connected to additional reference voltages Vref 1 and Vref 2 respectively. These switches S1 and S2 may be operated by the output signal of the comparison circuit D, that is to say by a signal dependent on the D. C. voltage component determined by the comparison circuit D. This D. C. voltage component is first supplied to a logical decisive circuit B1. This logical decision circuit has been provided with a number of outputs. Each one of these outputs receives a signal corresponding to a predetermined magnitude and polarity of the D. C. voltage component and one of these output signals may switch on the switch S1 or S2, respectively, whereupto the reference voltage Vref 1 or Vref 2is supplied to the junction of the compensating capacitor C3 and the leakage impedance R3 connected to the differential amplifierV1. On the compensating capacitor C3 as large a D. C voltage is thereby built up as present on the capacitor C2.

The build up of this compensating voltage is dependent on the value of the resistances R3 and R4. These values will be chosen in account therewith and dependent on the desired compensating velocity. The same applies to the value of the reference voltages Fret 1 and Ref 2' When D. C. voltages on the capacitors C2 and C3 have become alike the differential amplifier V1 will not contain anymore a D. C.

voltage component in its output signal i.e. the desired meansuring signal. Then both the signals supplied to the inputs 1 and 2 of the comparison circuit D will yet only consist of the true A. C.

voltage signal. Hence the comparison circuit D will not detect anymore a D. C. voltage component so that the switches S1 and/or S2 are again opened.

It has already been indicated above that there has been arranged a leakage impedance R2 across the compensating capacitor C3. The leakage impedance R1 across the measuring capacitor C2 is of such a high value that the cut off frequency lies sufficiently far below the frequency of the alternating voltage to be measured in order to keep the introduced phase and amplitude fa,ults at a low level. This implies however a large time constant. Hence C2 will discharge slowly through R1 whereby the D. C. voltage component at the junction C1, C2 will thus decrease gradually.

The capacitor C3 may discharge by way of the leakage impedance R2, the latter leakage impedance preferably having such a value that the time constant R2. C2 will be as large as the time constant Ri .C2. After charging C3 to the correct compensating voltage there will be required no corrections in principle anyone more for C2 as well as C3 will discharge by way of the associated leakage impedances R1 and R2, respectively, along similar curves. In case of dissimilar time constants there will occur corrections after some time in order to maintain a complete compensation.

In the embodiment of Fig. 2 there is applied only one switch S1 supplying a reference voltage to the capacitor C3 through the resistor R3. In this case the reference voltage consists of the difference between the broad band measuring signal, i.e. the signal with the true A. C. voltage component and the D. C. voltage component and the signal supplied by the D. C. voltage blocking filter F1, i.e. the true A. C. voltage component. This means that Vref and the D. C. voltage component derived by the comparison circuit D will in general be alike. Upon detection of a D. C. voltage component in the broad band signal the switch S1 will be closed whereby C3 will be quickly charged to the difference voltage, i.e. the output voltage of the comparison circuit D. The D.C. voltage component in the broad band measuring signal is then compensated and S1 will again be opened.

The capacitors C2 and C3 will discharge again with the same velocity.

When quickly charging the capacitor C3 to the associated reference voltage by closing one of the switches S1 and 52 there will occur transient phenomena. These transient phenomena caused by charging C3 may however may reach also the true A. C. voltage signal by way of the differential amplifier V2, the latter signal being supplied to the input 1 of the comparison circuit D by way of the D. C. blocking filter F1. This drawback may be avoided by introducing a second differential amplifier V2 and by diverting th summation point exclusively to the branch of the broad band signal supplied to the input 2 of the comparison circuit D.

In this case the differential amplifier V1 serves as a buffer amplifier. The mentioned phenomena generated by quickly charging C3 will then only be present in the patter signal and not in the signal supplied to the other input of the comparison circuit D by way of the blocking filter F 1.

Consequently this signal will thus always be a more accurate narrow band fundamental wave signal than in the former embodiments in which upon filtering in F1 there may yet be a residue of transients in A when the band width of the D. C.

voltage blocking filter F1 is not particularly small.

In the embodiment of Fig. 1 as well as in the other embodiments the differential amplifier V1 may have an amplification factor of 1.

it goes without saying that in the charging circuit for capacitor C2 represented by the switches S1 and 82, respectively and R3 and R4, respectively, many variations are possible that have already been discussed in the two afore said .patent applications to applicant.

The important advantages of the arrangement according to the invention are that the performances i.e. the operation velocity will be at least the same as those of an arrangement including two parallel capacitive A. C. voltage dividers according to for instance Fig. 7 of Dutch Patent Application 771 1655. Moreover a voltage divider according to the invention may simply be added to an existing voltage divider without essential modifications of the voltage divider as such, such an essential modification positively being required for instance in case of the voltage divider according to Dutch Patent Application 7711655.

The invention being thus described, it will be obvious that the same is not restricted to the embodiment represented and discussed above but that modifications and variations thereof are possible without departure from the spirit and the scope of the invention.

Claims

1. A capacitive voltage divider, for instance for medium or high voltage, comprising a series circuit of at least two capacitors one of which serves as a measuring capacitor, which series circuit may be connected between a primary voltage and a reference voltage whereby a measuring signal is derived from the measuring capacitor as a secondary voltage, which measuring signal may consist of an A. C. voltage component and a D. C. voltage component and wherein a leakage impedance has been connected across the measuring capacitor for discharging the D. C. voltage component, including control means for the reception of the measuring signal and the derivation of the true D. C. voltage component from the measuring signal characterized by compensating means for receiving the measuring signal and for compensating the D.C. voltage component in the measuring signal with the said derived true D. C. voltage component.

2. The capacitive voltage divider of claim 1 characterized in that said compensating means consist of a compensating capacitor and a leakage impedance connected in parallel thereto, the common connection terminal of which has been connected to the said reference voltage; switching means for supplying a given further reference voltage to the other common connection terminal of the compensating capacitor and the leakage impedance connected in parallel thereto, which switching means may be controlled by a signal derived from the true D. C. voltage component the value of said further reference voltage and the time for switching on of said switching means being chosen such that the voltage at the compensating capacitor may quickly compensate the D.C. voltage component in the measuring signal; a differential amplifier to an input of which there is supplied the measuring signal and to another input of which there is supplied the voltage at the compensating capacitor, while the output signal compensated with respect to the D.

C. voltage component is supplied to the control means.

3. The capacitive voltage divider of claim 2 characterized in that the compensating capacitor and associated leakage impedance have the same RC-time as the measuring capacitor and the leakage impedance connected in parallel thereto.

4. The capacitive voltage divider of claim 2 or 3 characterized in that the switching means consist of a plurality of parallel switches each one thereof being connected to a different predetermined reference voltage.

5. The capacitive voltage divider of claim 2 or 3 characterized in that said further reference voltage equals the true D. C. voltage component.

6. The capacitive voltage divider of anyone of the preceding claims characterized in that a buffer amplifier has been connected between the measuring capacitor and the differential amplifier.

7. The capacitive voltage divider of anyone of the preceding claims characterized in that the control means consist of a comparison circuit to one input of which there is supplied through a D.

C. blocking filter the true A. C. voltage signal derived from the measuring signal and to another input of which there is supplied the complete measuring signal while the switching means for the compensating capacitor are controlled by the output signal by means of logical decisive means.

8. The capacitive voltage divider of claims 6 and 7 characterized in that the input of the D. C.

blocking filter is directly connected to the output of the buffer amplifier.

9. A capacitive voltage divider substantially as hereinbefore described with reference to any one of the accompanying drawings.