GB2102972A - Capacitor voltage transformers - Google Patents

Abstract

A capacitor voltage transformer (CVT) comprising: a chain of capacitors (C1, C2, C3) adapted for connection across a voltage to be monitored; an electromagnetic transformer unit (3) connected across a part (C2) of the capacitor chain; and means (OP, 7) for injecting in series with the electromagnetic transformer unit output a voltage equal and opposite to the error in the electromagnetic transformer unit output during transient conditions, thereby to remove such errors from the CVT output. <IMAGE>

Description

SPECIFICATION Capacitor voltage transformers This invention relates to capacitor voltage transformers.

For monitoring very high alternating voltages, e.g. in a high voltage a.c. power transmission system, capacitor voltage transformers (CVTs) are used almost universally, in preference to electromagnetic voltage transformers, for reasons of cost.

A CvT of conventional form comprises a chain of capacitors adapted for connection across a voltage to be monitored, and an electromagnetic transformer unit connected across a part of the capacitor chain, via which unit the CvT output is derived.

Normally the capacitor chain is connected between a line at the voltage to be monitored and ground, and the transformer unit is connected across a capacitor at the grounded end of the capacitor chain.

The electromagnetic transformer unit typically comprises an iron-cored transformer, together with an extra inductance which tunes the unit to the rated frequency with the capacitance of the chain, and further devices such as spark gaps and means for suppressing ferroresonance.

Under steady state conditions the CVT output is in phase with and proportional to the line voltage. However, under rapidly changing conditions, e.g. when a fault occurs on the line, transients are generated in the electromagnetic transformer unit and the CVT output cannot follow the line voltage closely.

It is an object of the present invention to provide a capacitor voltage transformer having improved transient response.

According to the present invention there is provided a capacitor voltage transformer comprising: a chain of capacitors adapted for connection across a voltage to be monitored; an electromagnetic transformer unit connected across a part of the capacitor chain; and means for injecting in series with the electromagnetic transformer unit output a voltage substantially equal and opposite to the error in the electromagnetic transformer unit output during transient conditions.

In a preferred arrangement in accordance with the invention said means comprises an amplifier responsive to the difference between the voltage appearing across a further part of the capacitor chain, other than that across which the electromagnetic transformer unit is connected, and the output of the electromagnetic transformer unit, the amplifier output being connected in series with the electromagnetic transformer unit output.

Where the capacitor chain is grounded at one end, said further part of the capacitor chain suitably comprises a part of the chain between the part of the capacitor chain across which the transformer unit is connected and ground.

The amplifier output is suitably applied to the primary of a further electromagnetic transformer whose secondary is connected in series with the output of the CVT electromagnetic transformer unit. In such an arrangement the amplifier may be arranged to be responsive to the difference between the voltage across said further part of the capacitor chain and the voltage appearing across the series connection of the secondary of said further transformer and the CVT electromagnetic transformer unit output.

The invention will now be further explained and two embodiments of the invention described with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a CVT of conventional form; Figures 2A to 2D are waveforms illustrating the operation of the CVT of Figure 1; Figures 3 and 4 are diagrams illustrating the principle of the present invention; and Figures 5 and 6 are diagrams illustrating two practical embodiments of the present invention.

Referring to Figure 1, in a conventional arrangement employing a CVTfor monitoring the voltage on a line 1 the CVT comprises a capacitor chain C1, C2 connected between the line 1 and ground. An electromagnetic transformer unit 3 is connected across a part of the chain at its grounded end, represented by the single capacitor C2 in Figure 1, the remaining capacitance in the chain being represented by a single capacitor C1 in Figure 1. The capacitors C1 and C2 have values such that nearly all the line voltage is dropped across C1. For example, in a 440 kV, 50 Hz system C1 and C2 typically have values of 2 x 103 and 34x 103 pf respectively.

The transformer unit 3 includes an iron-cored transformer 5 having its primary connected across the capacitor C2. The output of the CVT is derived from the secondary of the transformer 5 and is typically applied to a metering circuit, or where the line 1 is a conductor of an a.c.

transmission system, to protective relay equipment for the system.

The transformer unit 3 normally further includes a variable inductance for tuning the unit to the rated frequency of the voltage on line 1 and some form of ferroresonance damping, but these are omitted in Figure 1 for simplicity.

Referring now to Figure 2, under transient-free steady state conditions the CVT output (Figure 2B) is a replica on a reduced scale of the voltage on line 1 (Figure 2A). When a rapid transient occurs in the line voltage, e.g. the line voltage falls to zero, as illustrated in Figure 2A, the CVT output temporarily deviates from the desired true value due to the action of energy storage elements in the transformer unit. The CVT output may thus be considered to be two voltages Vss and VTR in series (see Figures 2C and 2D) where V55 is a true replica of the line voltage and VTR represents all deviations from the true replica.

In accordance with the present invention the error is corrected by injecting a voltage in series with the CVT transformer unit output of value VTR to produce a resultant voltage which at all times is the required replica of the line voltage.

The voltage Vm may be injected on the grounded or line side of the output of the transformer unit, as illustrated in Figures 3 and 4 respectively.

Referring now to Figure 5, in one particular embodiment of the invention the required correction voltage is produced by an operational amplifier OP having its inverting input supplied with the output voltage VCVT of the transformer unit 3 and its non-inverting input supplied with the voltage across a capacitor C3 inserted at the grounded end of the CVT capacitor chain. The value of capacitor C3 is large compared with C1 so that it appears as a very small error e.g. 0.05% in the value of C1. Hence the voltage Cc3 across capacitor C2 is a scalar proportion of the voltage across capacitor C1 and is given by the expression

where K is a scaling factor depending on the values of C1 and C3.

The factor C1 C2 results from the fact that while transient voltage VTR appears across both C1 and C2, a much larger fraction of the line voltage is dropped across C1 than C2 so that the ratio of transient voltage to line voltage across C1 and hence C3 is C1 C2 of the ratio of these voltages across C2.

With appropriate scaling the two inputs to the amplifier OP may be arranged to be C1 VC3=VSS VTR C2 VCVT=VSS+VTR and the difference of the amplifier inputs is then

As indicated above C2 typically has a value seventeen times that of C1, so that to all intents and purposes the amplifier input is the required injection voltage.

The output of the amplifier OP is applied across the primary of a transformer 7 whose secondary is connected in series with the output of the transformer unit 3.

It will be appreciated that the amplifier OP is required in order that the capacitor C3 is lightly loaded, thereby maintaining good transient response and avoiding phase errors in the voltage across C3, and in order that a correction voltage independent of the current demanded by the CVT load is provided.

The transformer 7 serves to isolate the CVT output from the capacitor chain. However in other arrangements where such isolation is not required, the amplifier output may be directly connected in series with the transformer unit output.

Referring now to Figure 6, where an isolation transformer is provided the inverting input of the amplifier OP may be derived from across the series connection of the secondary of transformer 7 and the output of the CVT transformer unit 3, instead of from the output of the unit 3. In this way the transformer 7 is incorporated in the feedback loop to compensate for all sources of errors arising in transformer 7, due for example, to copper losses, localised winding temperature rise or turns ratio errors in transformer 7.

Calibration of the CVT can be easily effected by nulling the output of the error correction arrangement under steady state conditions of the monitored voltage. A slow acting automatic nulling circuit (not shown) may, if desired, be provided. A particular advantage of this is that it reduces the stability requirements for capacitor C3.

In use of a CVT incorporating a transient error correction arrangement in accordance with the invention in an a.c. power transmission system protective arrangement, various practical difficulties may arise. However, in general, in power transmission system protection the primary situation in which response of the protective equipment to rapid transients in line voltage, and hence CVT transient error correction, is important is reduction in system voltage as a result of a fault on the system, and an arrangement in accordance with the invention will operate effectively in such a situation.

In other situations, however, it may be necessary to take steps, for example temporarily disabling the transient error correction arrangement, to permit the CVT to operate satisfactorily without transient error correction.

Such a situation can arise, for example, due to the trapping of charge on the capacitor C3, as can occur on de-energisation of the CVT by disconnection, by circuit breaker or isolator, of the section of the transmission system to which the CVT is coupled, or due to flashover in the CVT capacitor chain. Due to the high impedance presented to the capacitor C3 by amplifier OP the trapped charge may persist for several seconds.

Such trapped charge will give rise to a d.c. offset in the input to amplifer OP with resultant saturation of the error correction transformer, or more importantly, voltage transformers in the protective relay equipment connected to the CVT output. To overcome this problem the transformer 7 may be designated to ensure it saturates in such a situation thus effectively switching out the error correction arrangement and allowing the protective relay equipment to operate satisfactorily from the CVT output without error correction.

A CVT arrangement in accordance with the invention, in particular the means for injecting the voltage whereby transient errors are corrected, may be provided with a power supply separate from the voltage being monitored, but for reasons of simplicity, security and voltage isolation an attractive alternative is to power the CVT from the voltage being monitored via the CVT itself.

However, bearing in mind that accurate monitoring of power system voltage on occurrence of a power system fault is a primary requirement, it will be understood that such a self-powered arrangement must be capable of maintaining power supply to the CVT at least for as long as significant transients may persist after a fault. This period is typically about 80 milliseconds so that the necessary maintenance of power supply can easily be achieved by employing storage capacitors in the power supply.

With a self-powered arrangement a further difficulty arises on re-energisation of the power supply system, since clearly the power supply for the CVT is unlikely to be fully restored until after any transients resulting from re-energisation of the power system have died away, and the CVT output will consequently be adversely affected.

An effective way to overcome this problem is to intentionally delay recommencement of operation of the error correction arrangement, e.g. by delaying switch-on of the power supply for the error correction arrangement of the CVT, until a short time after re-energisation of the power system. The CVT will then monitor the power system voltage after re-energisation, until the end of the delay, as effectively as a conventional CVT without error correction. The length of the delay is such, e.g. 3 seconds, as to allow any trapped charge on the capacitor C3 to discharge before the error correction arrangement is brought back into operation.

It will be appreciated that since the purpose of a CVT in accordance with the invention is to ensure that no transient current flows in the CVT load, the power required to correct transient error is zero, neglecting losses. In this connection it is pointed out that an arrangement according to the invention exhibits a significant advantage over an arrangement wherein a load is connected directly across capacitor C3 via an amplifier. In such an arrangement the power which can be delivered to the load without affecting the transient response of the capacitor C3 is very small. Clearly no such difficulty arises in an arrangement according to the present invention.

However, in a practical arrangement in accordance with the invention the losses which occur in the amplifier OP and transformer 7 may be appreciable since the amplifier output, being in series with the transformer unit output, must deliver a current equal to that delivered to the transformer unit 3. In a CVT in accordance with the invention it may therefore be necessary to reduce this loss for example; by de-rating the CVT output; by splitting the CVT loads into two classes and only driving the loads requiring good transient response with an error corrected output; or reducing power dissipation during quiescent conditions.

One factor which alleviates this difficulty in the context of a power transmission system protection-arrangement is that during transmission system fault conditions when error correction is most important, the steady state signal is reduced with consequent reduction in power dissipation and hence in the required CVT power supply rating for efficient operation.

Claims (9)

Claims

1. A capacitor voltage transformer comprising: a chain of capacitors adapted for connection across a voltage to be monitored; an electromagnetic transformer unit connected across a part of the capacitor chain; and means for injecting in series with the electromagnetic transformer unit output a voltage substantially equal and opposite to the error in the electromagnetic transformer unit output during transient conditions.

2. A capacitor voltage transformer according to Claim 1 wherein said means comprises an amplifier responsive to the difference between the voltage appearing across a further part of the capacitor chain, other than that across which the electromagnetic transformer unit is connected, and the output of the electromagnetic transformer unit, the amplifier output being connected in series with the electromagnetic transformer unit output.

3. A capacitor voltage transformer according to Claim 2 wherein said further part of the capacitor chain comprises a part of the capacitor chain between the part of the capacitor chain across which the transformer unit is connected and ground.

4. A capacitor voltage transformer according to Claim 2 or Claim 3 wherein the amplifier output is applied to the primary winding of further electromagnetic transformer whose secondary is connected in series with the output of the CVT electromagnetic transformer unit.

5. A capacitor voltage transformer according to Claim 4 wherein the amplifier is responsive to the difference between the voltage across said further part of the capacitor chain and the voltage appearing across the series connection of the secondary of said further transformer and the CVT electromagnetic transformer unit output.

6. A capacitor voltage transformer according to any one of Claims 2 to 5 wherein said means is arranged to be temporarily disabled in the event of trapping of charge on said further part of the capacitor chain.

7. A capacitor voltage transformer according to Claim 6 when dependent on Claim 4 or Claim 5 wherein said temporary disablement is effecting by arranging for said further transformer to become saturated in the event of trapping of charge on said further part of the capacitor chain.

8. A capacitor voltage transformer according to any one of the preceding claims wherein said means is arranged to be powered from the voltage being monitored, and recommencement of operation of said means on restoration of the voltage being monitored following failure of said monitored voltage is temporarily delayed.

9. A capacitor voltage transformer substantially as hereinbefore described with reference to Figure 3 or Figure 4 or Figure 5 or Figure 6.