GB2105928A - A.c. voltage sensor - Google Patents

Abstract

An a.c. voltage sensor is used to monitor the output level of an a.c. voltage generator and to produce a control signal which is operative to hold the a.c. output voltage at a predetermined level. A nominal d.c. voltage is derived from the output of the a.c. generator by means of a bridge rectifier 5 and compared with a reference level. The nominal d.c. signal contains an a.c. ripple component, and the invention enables the magnitude of this ripple to be greatly reduced without the need to provide a filter circuit which would adversely extend the time constant of the control circuit. This is achieved by providing resistance (R2, R3, R4) in each input path to the bridge rectifier and a resistor (R1) across the bridge rectifier output, the values of the resistances (R2, R3, R4) w.r.t. that of the resistor (R1) being chosen such that the output ripple has twice the frequency it would have in the absence of the resistances. <IMAGE>

Description

SPECIFICATION A.c. voltage sensor This invention relates to an a.c. voltage sensor which is operative to produce a d.c. signal having a level which is representative of the magnitude of an a.c. signal applied to it. It is often necessary to control the output level of an a.c. generator, and this can be achieved by monitoring the actual output level which it produces, and modifying the operating characteristics of the generator to bring the monitored output level into agreement with a required value. This can be accomplished by deriving a control signal having a mean level which represents the level of the generator output signal and comparing the control signal with a reference value representing the required output level.Ideally, the control signal which is then applied to the generator is a steady d.c. level, but, in practice, this is not generally possible as a ripple is inevitably present which is related to the frequency and the number of phases of the a.c. signal. Although in principle low pass filters can be used to reduce the ripple to a satisfactory level, the long time constants associated with such a filter can seriously impair the operation of the control loop.

The present invention seeks to provide an improved a.c. voltage sensor in which this difficulty is reduced.

According to this invention, an a.c. voltage sensor includes means for applying a polyphase a.c. signal to a rectifier which has a number of rectifying arms related to the number of phases of the a.c. signal and wherein a resistance is provided in the input path of each rectifying arm; an output resistor connected in shunt with the output of said rectifier, and wherein the value of said resistance in relation to said output resistor value is arranged to produce an output ripple signal which has twice the frequency which it would have in the absence of said resistances.

Although the principle is applicable in general to high order polyphase a.c. signals, the most common polyphase a.c. signal has only three phases. In such a case, a three arm rectifier is used to produce an output signal having a ripple which has twice the frequencythatwould be produced by a conventional a.c. voltage sensor. The effect of increasing the frequency in this way is to very greatly reduce the magnitude of the ripple signal present at the output of the rectifier, and consequently relatively little subsequent filtering is needed, thereby enabling the time constant of the a.c. voltage sensor to be minimised. This enhances the stability of a feedback loop which incorporates the a.c. sensor, whilst permitting itto follow relatively rapid variations in the level of the applied a.c. voltage.

If the frequency of the applied a.c. voltage is f, the following table shows the frequency f, of the output signal in accordance with the invention as compared with the ripple frequency f2 of a conventional rectifier in which the resistances in the input paths are not present.

No. of phases No. of rectifier f2 fi arms 1 2 2f 2f 3(or6) 3(or6) 6f 12f 2 (or 4) 4 4f 8f 5 5 10f 20f 7 7 14f 28f From the table it will be apparent that the invention is applicable only to polyphase signals, i.e. it does not apply to an a.c. signal having only one phase. However, a modification of the invention enables a single phase a.c. signal to be converted to a polyphase signal (usually a three phase signal) which can then be applied to the rectifier.

A proper six phase system would produce the same ripple as a three phase system and the benefits of the invention would be applicable. However, the term six-phase is often used to mean a twin three phase signal with 30 (not 60 displacement), but the invention is not applicable to this case.

The invention is further described by way of example with reference to the accompanying drawings, in which Figure 1 shows an a.c. generator which is controlled by means of a known a.c. voltage sensor, Figure 2 shows an improved a.c. voltage sensor in accordance with this invention, Figures 3, 4 and 5 are explanatory diagrams illustrating the mode of operation of the circuit shown in Figure 2, and Figure 6 shows a modification of an a.c. voltage sensor.

Referring to Figure 1, there is shown therein a three phase a.c. generator 1 which can take the form of a rotating a.c. machine or a static inverter. Its actual form is not of significance, but it is of the kind in which the magnitude of its a.c. output is controllable by means of a control signal applied over lead 2 to the generator. In the case of a rotating a.c.

machine, the winding 3 represents the field coil.

With a.c. generators, it is common to use a servo control or voltage regulator feedback loop to control the output level and to hold it at a required value.

The output of the a.c. generator 1 is fed via a transformer 4to a three-arm rectifier 5, which produces a rectified a.c. signal having a significant ripplefrequency which is six times that of the frequency output of the generator 1. The rectified signal is applied to a low pass filter 6, which attenuates the ripple and which is represented by a resistance 12 and capacitance 13. The filtered output is compared at a com parator 7 with a reference level 8 which represents the required signal level. The operation of the feed back loop is such as to bring the output of the low pass filter 6 into agreement with the reference level applied to terminal 8.A resistor R, is connected in shunt with the output of the rectifier 5, and it acts as a dominant load which prevents the rectifier 5 and the filter 6 operating as a pear detector and it also allows similar responses to co@@@@ going and nega tive going transient signals . It is necessary to include the filter 6 as too short a time constant results in malfunctioning of the feedback loop, e.g. the feed back amplifier forming part of the comparator 7 can saturate on peaks of the ripple. Conversely, if the filter 6 has too long a time constant, the frequency response of the loop detriorates causing excessive recovery times following abrupt variations in the output level of the generator 1, which can be caused by variations in the load (not shown) connected to terminals 9, 10 and 11.

The present invention enables the amplitude of the ripple signal provided atthe output of the rectifier 5 to be very greatly reduced, and this very significantly enables the performance of the feedback loop to be improved. The a.c. sensor in accordance with the invention is illustrated in Figure 2. For the sake of clarity, the generator 1 has been omitted, and it will be seen that as compared with Figure 1, three additional resistances R2, R3 and R4 have been inserted between the transformer 4 and the rectifier 5. Each of these additional resistances had an ohmic value which is nominally 58% of R1, but these are theoretical values and in practice it may be necessary to modify them slightly to allow for voltage drops occurring across rectifier diodes, waveform distortion and unbalance between the different output lines of the generator 1.

In the arrangement shown in Figure 1, the theoretical value of the ripple level is 14% and is given by the expression (generator output peak value - peak value of ripple) (x100)/mean d.c. value. The ripple frequency is 6f, where f is the output frequency of the generator.

In Figure 2, the ripple present on resistance R1 is now only 3.4% at a frequency of 12f. In practice, this means that the ripple attenuation filter 6 may have its time constant reduced to about 12% of the value which would be required in Figure 1 to give a similar performance. A consequence of the resistors R2, R3 and R4 is that the output level of the rectifier is only half that shown in Figure 1, but this can readily be compensated by changing the ratio of the transformer 4 or by adjusting the gain of the amplifier 7.

Although the resistances R2, R3 and R4 are shown as separate resistors inserted in the paths between the transformer 4 and the rectifier 5, it is possible to incorporate equivalent values into the windings of the transformer 4.

The theory behind the operation of the circuit shown in Figure 2 is explained with reference to Fig ures 3, 4 and 5 in which an a.c. signal has three phases VA, Vs, and Vc. As is indicated in Figure 3, the transformer 4 and rectifier 5 arrangement of Figure 1 produce a control signal VDC having a ripplefrequency of 6f with signal peaks occurring at angles of 0 , 60 , 120 with respect to the incoming phase neutral voltage waveforms. This represents the con ventional response of the rectifier 5.

Figure 4 shows that if the transformer voltage source assumed for Figure 3 were replaced by a cur rent source, the ripple peaks of the resulting signal V,c' would be displaced to occur at angles of 30 , 90 , 250 because the peaks now occurwhen the sum of currents is maximum. The resistance R2, R3 and R4 are chosen to provide a value of source impedance which will combine the ripple waveforms shown in Figures 3and 4such that the 6fwaveforms cancel leaving only a remanent ripple having a frequency of 1 2f. This 1 2f waveform VDC" is shown in Figure 5, and it clearly possesses a very much lower amplitude.

The conditions for 6f cancellation are satisfied if two-diode conduction changes to three-diode conduction at angles of 15 75O etc.

Now VA= k sin#, and let k = 1, i.e. VA = sin# etc.

R1 = 1 (ohm) andR2= R3 = R4= R (ohm) At phase angle 15 , during two-line conduction from Vc to Vs, Vx = Vc(R+1)+V5.R}1(2R+1) Vy=iVcR+VB(R+ 1)}/(2R+ 1) To satisfy changeover at 15 , VA = Vx Substituting, V, (2R + 1)= Vc (R + 1) + VB.R or(2R + 1) sin 0= (R + 1) sin (0 + 120) + Rsin (0- 120) or (2R + 1) sin 15 = (R + 1) sin 45 - R sin 75 This givesR = 0.58 of the value of R1 Two line conduction occurs overthe period 150 to +15", i.e. the d.c. output voltage overthis period is given by:: = = Vx - Vy ={VcVB}/(2R + 1) = A/3 cos V3cos01(2R+ 1) i.e. at + 15 , VDC" = V3COS 15/(2R + 1) at 0 , VDc" = V3 1/(2R + 1) i.e. peak-peak ripple = (1 - cos 15) V3/(2R + 1) and mean d.c. output = sin 15 (12/#) V3(2R + 1) Therefore, % ripple = (1 - cos 15) (7r/12)/sin 15 x 100% = 3.4% By way of comparison the ripple for Figure 1 is given by the expression (1 - cos 30) (ir/6)sin 30 x 100% i.e. % ripple = 14.0% The mean d.c. output for Figure 2 is given by sin 15 (12/7r) \/3/(2R + 1) By way of comparison the corresponding value for Figure 1 is given by sin 30 (6/#) A/311 i.e. output is reduced to 2 sin 15/sin 30 (2R + 1), for Figure 2 and this is 50% of the value for Figure 1.

Figure 6 shows a modification in which the three phases which are applied to the rectifier 51 are derived from a single phase input by a phase splitting technique. This can be advantageous in certain servo loop applications, and enablesthe benefits of the present invention to be achieved even for a single phase a.c. signal. The single phase signal is applied to terminals 52 and 53 and is converted to a three phase signal by a phase splitting network comprising resistors 54, 55, 56, and capacitor 60. In determining the values of resistors which correspond to resistors R2, Rand R4 shown in Figure 2, it is necessary to take into account the resistances of the phase splitting network. Typical resistance values are shown for two of the source resistances 57 and 58, but that resistance which is theoretically present at point 59 is included in the value of resistances 54 and 55 shown for the phase splitter.

The circuit shown in Figure 6 can be extended to produce higher phase values if required. Although the calculations given previously are applicable to only a three phase system, the principle is applicable to polyphase systems in general and can be advantageous even when the input waveforms are not sinusoidal.

Claims (6)

1. An a.c. voltage sensor including means for applying a polyphase a.c. signal to a rectifier which has a number of rectifying arms related to the number of phases of the a.c. signal and wherein a resistance is provided in the input path of each rectifying arm; an output resistor connected in shunt with the output of said rectifier, and wherein the value of said resistance in relation to said output resistor value is arranged to produce an output ripple signal which has twice the frequency which it would have in the absence of said resistances.

2. An a.c. voltage sensor including means for applying a three phase a.c. signal to a rectifier which has three rectifying arms and wherein resistances are provided in the three input paths to the rectifying arms; an output resistor connected in shunt with the output of said rectifier, and wherein the values of said resistances in relation to said output resistor value are arranged to produce an output ripple value which has tweive times the frequency of the applied three-phase a.c. signal.

3. A sensor as claimed in claim 1 or 2 and which includes means for comparing a signal derived from said rectifier with a reference signal level representing a predetermined level of the applied a.c. signal.

4. A sensor as claimed in claim 3 and wherein a low pass filter is provided between said rectifier and said comparison means.

5. A sensor as claimed in any of the preceding claims and wherein means are provided for receiving a single phase a.c. signal, and for converting it to a polyphase signal which is applied to said rectifier.

6. An a.c. voltage sensor substantially as illustrated in and described with reference to Figure 2 or 6 of the accompanying drawings.