Discharge circuit for rapidly eliminating charge trapped in a capacitor voltage divider used for monitoring high voltage AC

The voltage divider comprises a capacitor column (11) for dropping nearly all of the high voltage and a base capacitance (10) connected in series therewith so that a small voltage appears there across. Said discharge circuit comprises a diode rectifier bridge (15, 16, 17, 18) connected by its AC terminals in parallel with the base capacitance, and two identical windings (20, 21, 22, 23) connected in parallel respectively with two of the diodes (15, 16) of the bridge, both of said winding-shunted diodes being connected to the same one of the AC terminals of the diode bridge.

The present invention relates monitoring high voltage AC, particularly as 
used in high tension power transmission grids. 
BACKGROUND OF THE INVENTION 
Monitoring high voltage AC, such as that found in electricity power 
transmission grids, is done by means of a measurement signal derived from 
a voltage divider which, for reasons of cost, is generally a capacitive 
divider constituted by a capacitive column that drops nearly all of the 
high voltage connected in series with a base capacitance that can only 
withstand a low voltage and which delivers the measurement signal. The 
measurement signal is a faithful image of the waveform of the high voltage 
AC provided that it is fed into an instrument having an input impedance 
that is several tens of times greater than the impedance of the base 
capacitance, and except for certain special circumstances when charge is 
trapped in the base capacitance and causes the measurement waveform 
voltage to change abruptly. Charge becomes trapped due to the difference 
between the discharge time of the capacitive column and that of the base 
capacitance. When there is an interruption in the high voltage AC, the 
same amount of charge remains trapped in the capacitive column as in the 
base capacitance, but the discharge rates of the two halves of the 
capactive voltage divider are very different and the amount of charge 
remaining in each half rapidly becomes very different. Unless the AC is 
re-established very quickly, or after a long enough period for both halves 
to have discharged fully, there will be more charge on the base 
capacitance than on the capacitive column, thereby wrongly dividing the AC 
at the moment it is re-established. This is because the base capacitance 
discharges slowly through a high impedance, giving a slowly decaying DC 
measurement signal with a discharge time constant of several tens of 
seconds. 
Unless special precautions are taken, this error voltage saturates the 
monitoring equipment which receives the measurement signal and makes it 
inoperative. One known way of protecting the monitoring equipment is to 
insert a selective filter centered on the frequency of the high voltage AC 
in between the monitoring equipment and the capacitive divider. This is 
done each time there is a danger of trapped charge appearing, i.e. for an 
appreciable period of time on some occasions that the high voltage is 
re-established. Unfortunately, such a filter also modifies the measurment 
signal and considerably reduces the accuracy of measurements made for the 
hundred or so seconds it is inserted. 
Preferred embodiments of the present invention reduce the drawbacks due to 
trapped charge by eliminating it very quickly without causing too much 
disburbance to the measurement signal. 
SUMMARY OF THE INVENTION 
The present invention provides a discharge circuit for rapidly eliminating 
charge trapped in a capacitor voltage divider used for monitoring high 
voltage AC, said voltage divider comprising a capacitor column for 
dropping nearly all of the high voltage and a base capacitance connected 
in series therewith so that a small voltage appears there across, wherein 
said discharge circuit comprises a diode rectifier bridge connected by its 
AC terminals in parallel with the base capacitance, and two identical 
windings connected in parallel respectively with two of the diodes of the 
bridge, both of said winding-shunted diodes being connected to the same 
one of the AC terminals of the diode bridge. 
Advantageously, the circuit further comprises: a controlled switch 
connected in series between the diode bridge and the terminals of the base 
capacitance; an AC detector circuit connected across the terminals of the 
base capacitance; and a timing circuit responsive to the AC detector 
circuit to close the controlled switch a few seconds after the AC detector 
circuit detects that the AC has been interrupted, and to open the 
controlled switch a few tens of milliseconds after the AC detector circuit 
detects that AC has been re-established.

MORE DETAILED DESCRIPTION 
In FIG. 1, a capacitive voltage divider is represented by two capacitors 10 
and 11 connected in series. The capacitor 10 represents the base 
capacitance, while the capacitor 11 represents the capacitive column. The 
ends of the voltage divider are connected to the terminals of a source of 
electricity 12 which delivers the high voltage AC to be monitored. The 
terminals 13 and 14 of the base capacitance 10 provide a measurement 
signal which is supposed to reproduce the waveform of the high voltage AC 
to be monitored, but at low voltage. A discharge circuit in accordance 
with the present invention is connected across the terminals 13 and 14 of 
the base capacitance 10. The discharge circuit comprises a controlled 
switch 19 connected in series with the AC terminals of a recifier bridge 
having four diodes 15, 16, 17 and 18. Two identical windings are connected 
in parallel with respective diodes of one of the pairs of diodes connected 
to the same one of the AC terminals of the diode bridge. In the example 
shown the diodes in question are the diodes 15 and 16, and the windings 
are represented by ideal inductances 20, 21 connected in series with 
respective ideal resistances 22, 23. The controlled switch is controlled 
by a timing circuit 31 responsive to an AC detector circuit 30 which is 
connected acros the terminals 13 and 14 of the base capacitance 10. 
The controlled switch 19 is optional, and could be replaced by a short 
circuit. Its function is to connect the discharge circuit only when charge 
is likely to be trapped in the voltage divider, and to disconnect it once 
the charge has been eliminated. 
As mentioned above, trapped charge is only a problem when the high voltage 
AC is re-established after being down for a threshold period of a few 
seconds. It is thus sufficient for the controlled switch 19 to be closed 
only after said threshold period has been exceeded, and to open the switch 
again a short period after AC is re-established, said short period being 
about one hundred milliseconds, and long enough for the discharge circuit 
to eliminate any trapped charge. The AC detector 30 is a conventional 
circuit providing a binary output signal whose level corresponds to the 
presence or absence of AC across the terminals 13, 14 at an amplitude 
greater than some given threshold. The timing circuit 31 provides two 
different time delays: a first delay which prevents the switch 19 from 
being closed until several seconds, say five, have elapsed after a break 
in the high voltage AC; and a second delay which prevents the switch 19 
from being opened again until about one hundred milliseconds after the 
return of high voltage AC has been detected. The circuits that provide 
these time delays are not described in detail since such is common 
technical practice. The controlled switch 19 is preferably a simple 
conventional electromechanical relay because of its low resistance when 
closed and its effective isolation when open. There is no need for the 
high switching speed that a semi-conductor device could supply, for 
example. 
Apart from the conduction direction of the diodes, the circuit comprises 
two similar branches connected in parallel across the terminals 13, 14 of 
the base capacitance 10 by the controlled switch 19. One of the branches 
is constituted by the components 16, 18, 20 and 22 and passes trapped 
charge that would tend to make the terminal 13 of the base capacitance 10 
positive relative to the terminal 14, while the other branch comprises the 
components 15, 17, 21 and 23 and passes trapped charge that would tend to 
make the terminal 13 of the base capacitance 10 negative relative to the 
terminal 14. 
Trapped charge that would tend to make the terminal 13 of the base 
capacitance 10 positive relative to the terminal 14 passes through the 
inductance 20, the resistance 22 and the diode 18. The diode 16 loops the 
inductance 20 so that it can dissipate energy in the resistance 22 without 
transferring any to the base capacitor 10. It also prevents the potential 
at the cathode of the diode 18 from dropping below that of the terminal 14 
thereby preventing the diode 18 from conducting when the terminal 13 of 
the base capacitance 10 goes negative relative to the terminal 14. 
Similarly, trapped charge that would tend to make the terminal 13 of the 
base capacitance 10 negative relative to the terminal 14 passes through 
the diode 17, the resistance 23 and the inductance 21. The diode 15 loops 
the inductance 21 so that it can dissipate energy in the resistance 23 
without transferring any to the base capacitor 10. It also prevents the 
potential at the anode of the diode 17 from rising above that of the 
terminal 14 thereby preventing the diode 17 from conducting when the 
terminal 13 of the base capacitance 10 goes positive relative to the 
terminal 14. 
The rate of charge flow through the circuit depends on its transient 
behavior. Ignoring the forward voltage drop of the diodes, the transient 
behavior is that determined by the inductance L of the ideal inductors 20 
and 21, and the resistance R of the ideal resistances 22 and 23 connected 
across the terminals of a charged capacitor, except that the discharge 
current is prevented from reversing after it has fallen to zero. To reduce 
the time taken for the trapped charge to escape, the first current zero 
should be made to occur as soon as possible, i.e. an oscillating discharge 
should be aimed for. This occurs when the well known relationship: 
EQU R.sup.2 &lt;4L/C (1) 
is satisfied, where C is the base capacitance. The duration of the 
discharge flow is then: 
##EQU1## 
The effect of the circuit on the measurement signal depends on the current 
drawn from the capacitance 10 by the circuit if it remains connected by 
the switch 19 under normal operating conditions. To evaluate this effect, 
the current passing through the inductances 20 and 21 can be calculated 
for the case where the circuit has AC applied thereto. The inductance 20 
can be used for a worked example, supposing that the terminal 13 of the 
base capacitance 10 is maintained at an alternating voltage V sin wt 
relative to the terminal 14. 
During each first half-period (0 to .pi./w) of each cycle, the circuit 
branch constituted by the components 16, 18, 20 and 22 will be in forced 
excitation with the diode 18 biased ON and the diode 16 biased OFF. During 
the remainder of the time, i.e. during each second half-period (.pi./w, 
2.pi./w) of each cylce it will be freely damped with the diode 18 OFF and 
the diode 16 ON. 
The current i(t) passing through the inductance 20 during the first 
half-periods of forced excitation for the said branch 16, 18, 20 and 22 is 
given by the differential equation: 
##EQU2## 
whose general solution is: 
##EQU3## 
and i.sub.10 is the value of the current at instants t=0+ 2K.pi. 
The current i(t) passing through the inductance 20 during the second 
half-periods when the branch 16, 18, 20 and 22 is freely damped is given 
by the differential equation: 
##EQU4## 
whose general solution is: 
##EQU5## 
where i.sub.11 is the current at t=.pi./w+2K.pi. 
In normal operation the current through the inductance 20 at the end of 
each first half-period is the same as that at the beginning of each second 
half-period, whence: 
##EQU6## 
and consequently the current during forced excitation (i.e. the first 
half-periods) can be completely determined as follows: 
##EQU7## 
This is the current draw from the source V sin wt by the discharge circuit 
during the first half-periods while the diode 17 in the other branch is 
OFF. 
The current drawn from the source V sin wt by the discharge circuit during 
the second half-period is due to the other branch, and can be deduced from 
that calculated for the first branch by taking account of the direction 
current flow and the .pi./w phase shift. 
##EQU8## 
The total current drawn by the discharge circuit is the sum of these two 
currents and may be expressed: 
##EQU9## 
where t=t.sub.1 modulo .pi./w and n=(t-t.sub.1).times.w/.pi. 
This current may be decomposed into two parts, a first part: 
##EQU10## 
which is the current drawn from the source by an inductance L in series 
with a resistance R; and a second part: 
##EQU11## 
which is a pulsating current. 
It will be observed that the current component i.sub.B can be considered to 
be a correcting component present during the first half of the positive 
half-periods of the source V sin wt, tending to counteract the phase shift 
effect of the current component i.sub.A, provided that i.sub.B is 
approximately: 
##EQU12## 
at the beginning of the positive half-periods of the source V sin wt, i.e. 
where: 
##EQU13## 
and provided it becomes a negligeable quantity after one half-period. 
##EQU14## 
Under these conditions, the current i.sub.A can be ignored when 
investigating the effect of the discharge circuit on the measurement 
signal during normal operation. 
The second condition is included in the first which can be expressed by 
replacing sin .phi. by its value: 
##EQU15## 
This condition is easy to satisfy in practice. To do this, it is sufficient 
to put R/Lw&gt;0.2. The condition then becomes: 
EQU 1.12&lt;.epsilon. 
For R/Lw=1 the condition becomes: 
EQU 0.03&lt;.epsilon. 
and for R/Lw=5 the condition becomes: 
EQU 2.9.times.10.sup.-8 &lt;.epsilon. 
Assuming that equation (2) is satisfied, the effect of the discharge 
circuit on the measurement signal during normal operation is less than the 
effect of connecting an inductance L in series with a resistance R 
directly across the terminals of the base capacitance 10. 
The value of the base capacitance 10 is much larger than that of the column 
11 (to divide the voltage). Putting the value of the base capacitance 10 
as C, the relative amplitude of connecting said LR circuit across the 
capacitance C would be: 
##EQU16## 
where v is the voltage across the terminals of the base capacitance 
without the LR circuit and v' the same voltage with the LR circuit 
present. 
Assuming that the condition: 
EQU LCw.sup.2 &gt;&gt;1 (3) 
is satisfied, then the relative error is little different from: 
##EQU17## 
It is small because of the condition (3). 
The relative phase error on the measurement signal due to an LR circuit 
would be: 
##EQU18## 
Assuming LCw.sup.2 &gt;&gt;1, then 
##EQU19## 
By way of a worked example, consider monitoring a high voltage line at 127 
KV and 50 Hz using a 4 nF capacitive column and a 10 .mu.F base 
capacitance as a voltage divider and a measuring instrument with an input 
impedance of 2 M ohms connected across the base capacitance. A winding 
with an inductance of 10 H and a resistance of 1890 ohms is chosen. 
Conditions (1) and (2) become: 
EQU .epsilon.=15% LCw.sup.2 =9.87 
The initial assumption that the discharge circuit could be modelled during 
normal operation on an LR circuit of high impedance relative to the base 
capacitance is therefore justified. The resulting errors in the 
measurement signal are about 9% in amplitude and 3.degree. in phase. 
Trapped charge escapes in 86 ms instead of the 20 or seconds that would be 
required without the discharge circuit, where 20 seconds is the time 
constant of 10 .mu.F and 2 M ohms. 
If better performance is required, i.e. shorter discharge time without 
increased relative phase and amplitude error in the measurement signal, 
auxiliary coils may be connected in parallel with all or part of the 
windings, each being controlled by controlled switches which open one 
after the other when high voltage is re-established. FIG. 2 shows a 
circuit that includes an auxilliary coil connected in parallel with each 
of the windings. Much of the circuit is the same as that of FIG. 1, and 
components that are unchanged from one to the other have the same 
reference numerals. Each auxiliary coil is shown as an inductance 24 or 25 
connected in series with a respective resistance 26 or 27. The coils are 
connected in parallel with respective damping diodes 28 and 29 and in 
series with respective controlled switches 33 and 34, under the control of 
a timing circuit 32 which is slightly different from that of FIG. 1. 
The timing circuit 32 is excited by the AC detector circuit 30. It provides 
three different time delays. The first simultaneously closes all three 
switches at a suitable period after a break is detected in the AC, say 
after 5 seconds. The second time delay applies only to opening the 
auxilliary switches 33 and 34 some tens of milliseconds after AC is 
re-established, while the third time delay applies to opening the switch 
19 about one hundred milliseconds after AC is re-established. 
The damping diodes 28 and 29 serve to loop the current passing through the 
auxiliary coils when the auxiliary controlled switches 33 and 34 are 
opened. 
Placing auxiliary coils in parallel on the windings and disconnecting them 
progressively on high voltage AC being re-established serves to reconcile 
the requirement for low inductance for the charge to escape through 
quickly, and high inductance for minimum effect on the measurement signal. 
Since, whatever happens, the measurement signal is going to be badly 
effected while the trapped charge escapes, a high percentage error is 
acceptable during the escape. However, the circuit shown in FIG. 1 can not 
just be used with low inductance windings, since it then creates a new set 
of distortions to the measurement signal when the controlled switch 19 is 
opened, with new trapped charge causing the measurement signal to jump by 
a percentage error that is as great as the percentage error due to the 
discharge circuit itself. To avoid further jumps in the measurement 
signal, the inductance of the winding should be increased in steps, each 
of which traps less and less charge to be absorbed in the discharge 
circuit until the disruption to the measurement signal is small enough for 
the discharge circuit to be completely disconnected with a tolerable jump 
in the measurement signal. 
Without going beyond the scope of the invention, it is possible to make 
various modifications to the circuit or to replace various means by 
equivalent means. For example, more auxiliary coils can be used with a set 
of controlled switches having staggered opening times, to provide a more 
progressive increase in the effective inductance of the discharge circuit 
on high voltage AC being re-established. 
A capacitor may be connected between the AC terminals of the rectifier 
diode bridge. Its capacitance should be calculated so that it resonates 
with the windings 20, 22, 21 and 23 via the diode bridge at the frequency 
of the high voltage AC. This leads to a very high impedance being 
connected across the terminals of the base capacitance 10 by the 
controlled switch 19, and in particular, the resulting phase shift is very 
small.