Abstract:
The present application relates to a method for providing a corrected measuring signal indicating a high voltage on a high-voltage node (HV), including: injecting a periodic injection signal into a voltage divider coupled between the high-voltage node (HV) and a reference potential; obtaining a sensing signal at a sensing node (S) of the voltage divider, wherein the sensing signal depends on the periodic injection signal; from the sensing signal, separating a first sensing signal portion resulting from the high voltage and a second sensing signal portion resulting from the periodic injection signal; and depending on the second sensing signal portion, correcting the first sensing signal portion corresponding to the high-voltage signal in order to obtain the corrected measuring signal.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to high-voltage measuring units, in particular to measuring units for measuring high AC or DC voltages using a voltage divider. Furthermore, the present invention relates to methods for increasing the accuracy of high-voltage measuring units. 
       TECHNICAL BACKGROUND 
       [0002]    Voltage measuring units to be applied in high-voltage power systems commonly apply resistive or capacitive voltage dividers. When applying a voltage divider for measuring the high voltage, a precisely known attenuation factor needs to be preset. Such a voltage divider is usually configured with a series connection of two dipoles, wherein the upper dipole is connected to the voltage line carrying the voltage to be measured and a lower dipole is connected to a ground potential. In order to achieve a desired attenuation of around 70 dB to 100 dB, the upper dipole has a significantly higher impedance than the lower dipole. The impedance of the upper dipole is high so that the connection to the voltage line can be considered as galvanically insulated, which is essential for application in HV power systems as high voltages to be measured in HV power systems may have voltages of several tens of kV. For instance, impedances of the upper dipole of about 300 M| may be applied in order to limit the current flowing through the voltage divider to a few tens of ∝A. 
         [0003]    Manufacturing tolerances of the impedances of the dipoles as well as aging effects may lead to varying attenuation of the voltage divider. Furthermore, temperature variations may also affect the impedances of the dipole so that measurement errors may occur. 
         [0004]    US 20130335730A1 discloses a temperature compensation for an optical current or voltage sensor. The signal to be measured is obtained from a resitive voltage devider. A reference signal generator provides a periodic square wave reference signal which is added to the signal obtained from the voltage divider by means of a summing integrator. 
         [0005]    JP061 30089 A 1 discloses an electro-optical voltage sensor with a temperature compensation by means of a reference voltage. The signal to be measured is obtained from a capacitive voltage divider. 
         [0006]    It is therefore an object of the present invention to provide a means for compensating for the above effects of aging, temperature variations and manufacturing tolerances in order to facilitate a more accurate measurement of the high voltage on the high-voltage line. 
       SUMMARY OF THE INVENTION 
       [0007]    The above object is achieved by the method for providing a corrected measuring signal indicating a high voltage on a high-voltage node according to claim  1  as well as by the measuring unit and the high-voltage measurement system according to the further independent claims. 
         [0008]    Further embodiments are indicated in the dependent subclaims. 
         [0009]    According to a first aspect, a method for providing a corrected measuring signal indicating a high voltage on a high-voltage node is provided, comprising the steps of:
       injecting a periodic injection signal into a voltage divider coupled between the high-voltage node and a reference potential;   obtaining a sensing signal at a sensing node of the voltage divider wherein the sensing signal depends on the periodic injection signal;   from the sensing signal, separating a first sensing signal portion resulting from the high voltage and a second sensing signal portion resulting from the injection signal; and   depending on the second sensing signal portion, correcting the first sensing signal portion corresponding to the high-voltage signal in order to obtain the corrected measuring signal
           wherein correcting the first sensing signal portion includes providing a correction signal, wherein the correction signal is applied on the first sensing signal portion,   wherein the correction signal is obtained depending on the second sensing signal portion,   wherein the periodic injection signal is provided by an injection signal source,   
           wherein the correction signal is obtained depending on a difference signal between the injection signal exposed to an attenuation, which corresponds to an attenuation of the voltage divider, and the second sensing signal portion.       
 
         [0018]    One idea of the above method for providing a corrected measuring signal is to correct a sensing signal tapped on a sensing node of a voltage divider by means of a periodic injection signal. The voltage divider may have a first and second dipole providing an impedance ratio for a high attenuation of the high voltage with respect to the sensing node. The injection signal is injected into the voltage divider as a periodic AC injection signal, which causes a current to flow through the voltage divider to the low impedance high-voltage node. The injection signal having a specific periodicity results in a sensing signal on the sensing node. The sensing signal is an overlaid signal of the signal resulting from the voltage division of the high voltage which corresponds to the first sensing signal portion, and the injecting of the injection signal which corresponds to the second sensing signal portion. So the sensing signal substantially depends on the impedances of the first and the second dipole of the voltage divider. 
         [0019]    By means of the specific frequency/-ies of the injection signal, the second sensing signal portion of the divider voltage at the sensing node related to the injection signal can be extracted or respectively separated from the sensing signal at the sensing node. Hence, the injected voltage signal strongly depends on the impedance ratio of the dipoles. As for high-voltage measurement the voltage divider has a first dipole with much higher impedance than the second dipole, the sensing signal strongly depends on a variation of the impedance of the first dipole. In case of a variation of the impedance of the first dipole, the voltage level of the second sensing signal portion caused by the injection signal will vary accordingly. 
         [0020]    As the second sensing signal portion at the sensing node will also vary according to the variation of the impedance of the first dipole, a correction value can be determined which can be applied on the first sensing signal portion to correct for the variability of the high impedance of the first dipole of the voltage divider. 
         [0021]    The above method has the advantage that impedance variations of the first dipole caused by manufacturing tolerances, temperature changes and aging effects can be compensated for in order to provide a corrected measuring signal which is substantially independent of variations of the high impedance of the first dipole. 
         [0022]    Furthermore, the correction allows the use of general-purpose elements for implementing the voltage divider, thereby saving costs for high-accuracy material and manufacturing as measurement errors can be reliably corrected. 
         [0023]    Furthermore, no manual calibration is required with the application of the above measurement method. As the correction can be permanently or periodically carried out on the sensing signal at the sensing node without substantially affecting the first sensing signal portion, no maintenance is required during the lifetime of the measurement system in order to ensure the expected accuracy. 
         [0024]    Furthermore, correcting the first sensing signal portion may include providing a correction signal, wherein the correction signal is applied on the first sensing signal portion of the sensing signal. 
         [0025]    It may be provided that the periodic injection signal is injected into a compensation voltage divider of a compensation unit, which compensation voltage divider is coupled between the injection signal source and the reference potential. 
         [0026]    According to an embodiment, a compensation signal provided by the compensation voltage divider of the compensation unit is fed to an input of a subtraction unit or of a differential amplifier, and the second sensing signal portion provided by the correction unit is fed to another input of the subtraction unit or of the differential amplifier. 
         [0027]    According to an embodiment, the injection signal may be provided with a square waveform, wherein the difference signal has a square waveform the amplitude of which is associated to the correction signal. 
         [0028]    Moreover, the correction signal may correspond to a digital correction value to be applied on the first sensing signal portion, particularly by multiplication. 
         [0029]    The injection signal may be provided with a frequency which is different from the frequency of the high voltage, wherein separating the second sensing signal portion from the sensing signal is carried out by applying a digital or analog second filtering on the sensing signal. 
         [0030]    Furthermore, separating the first sensing signal portion from the sensing signal may be carried out by applying a digital or analog first filtering on the sensing signal. 
         [0031]    It may be provided that correcting the first sensing signal portion includes associating the correction signal to the second sensing signal portion 
         [0032]    According to a further aspect, a measuring unit for providing a corrected measuring signal indicating a high voltage on a high-voltage node is provided, comprising:
       an injection unit for injecting a periodic injection signal into a voltage divider coupled between the high-voltage node and a reference potential; and   a correction unit which is configured
           to receive a sensing signal from a sensing node of the voltage divider depending on the injection signal;   to separate a first sensing signal portion resulting from the high voltage and a second sensing signal portion resulting from the injection signal; and   to correct the first sensing signal portion corresponding to the high-voltage signal in order to obtain the corrected measuring signal depending on the second sensing signal portion   a compensation unit configured to provide a compensation signal which is related to the periodic injection signal and which is for eliminating variations of the second sensing signal portion due to temperature variations and   wherein the injection unit comprises an injection source for supplying the periodic injection signal.   
               
 
         [0040]    It may be provided that the compensation unit comprises a compensation voltage divider, and wherein the injection unit injects the periodic injection signal into the voltage divider coupled between the injection unit and the reference potential. 
         [0041]    The compensation voltage divider my have the same impedance ratio as the voltage divider. 
         [0042]    The compensation signal provided by the compensation voltage divider of the compensation unit may be supplied to an input of a subtraction unit or to an input of a differential amplifier, and the second sensing signal portion provided by the correction unit may be supplied to another input of the subtraction unit or of the differential amplifier. 
         [0043]    It may be provided that the output of the subtraction unit or the output of the differential amplifier is associated with a correction value and may be fed to a multiplier and may there be multiplied with the first sensing signal portion to correct variations of the portion of the sensing signal. 
         [0044]    Moreover, the injection unit may include a current mirror or a transformer for injecting a reference signal into the voltage divider as an injection signal. 
         [0045]    The correction unit may include a first filter unit and/or a second filter unit configured to separate the first sensing signal portion and the second sensing signal portion from the sensing signal. 
         [0046]    It may be provided that the injection unit comprises an injection source for supplying the injection signal, wherein a compensation unit is configured to provide a compensation signal referring to the injection signal and to eliminate variations of the second sensing signal portion due to temperature variations by applying the compensation signal. 
         [0047]    According to a further aspect, a measurement system for providing a corrected measuring signal indicating a high voltage on a high-voltage node is provided, comprising:
       a voltage divider having a first dipole and a second dipole serially connected between the high-voltage node and a reference potential and providing an intermediate sensing node; and   the above measuring unit.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0050]    Embodiments are described in more detail in conjunction with the accompanying drawings, in which: 
           [0051]      FIG. 1  schematically shows the high-voltage measurement system; 
           [0052]      FIGS. 2 a -2 c    indicate a variance for positioning the signal injection element; 
           [0053]      FIGS. 3 a -3 b    show options for injecting the injection signal as an injection current by means of a current source; 
           [0054]      FIG. 4  schematically shows a more detailed implementation with a temperature compensation of the injection signal generator; and 
           [0055]      FIG. 5  schematically shows a further embodiment of the measurement system. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0056]      FIG. 1  shows a schematic diagram of a high-voltage measurement system  1  having a voltage divider  2  which is coupled between a high-voltage node HV, the voltage of which is to be measured, and a reference potential, such as a ground potential GND. The high voltage node HV may carry a DC or AC high voltage. The high-voltage measurement system  1  serves for measuring a high voltage at the high-voltage node HV and for providing sufficient galvanic insulation of the high-voltage node HV through the high-voltage measurement system  1 . 
         [0057]    The voltage divider  2  has a first dipole  21  and a second dipole  22  which are connected in series between the high-voltage node HV and the ground potential GND, wherein a sensing node S located between the first dipole  21  and the second dipole  22  is used to tap a sensing signal. To provide a high attenuation of 60 dB to 100 dB for the high voltage at the high-voltage node HV, the first dipole  21  is provided with a high impedance while the second dipole  22  has a substantially lower impedance. To achieve galvanic insulation with respect to the high-voltage node HV, the impedance of the first dipole  21  may be selected to be very high, such as higher than 100 M|, for instance 300 M|. The impedance of the second dipole  22 , which is connected between the sensing node S and the ground potential GND, may be as low as several tens of kQ, such as 30 kQ. 
         [0058]    In the above example of 300 M| and 30 kQ as impedances, an attenuation of about 70 dB can be achieved. Applying high impedances for the first dipole  21  allows for substantial galvanic insulation of the sensing node S from the high-voltage node HV. 
         [0059]    The sensing node S is coupled with a measuring unit  3  to measure a sensing voltage at the sensing node S as an indication of the voltage level of the high-voltage node HV. 
         [0060]    The measuring unit  3  includes an injection unit  31  which serves to inject an injection signal into the current path of the voltage divider  2 . The injection unit  31  can be applied in the current path of the voltage divider  2  at different positions so that an injection current I re f flows through the current path. For instance, the injection unit  31  may be applied at a position between the second dipole  22  and the ground potential GND as shown in  FIG. 2 a   , between the first dipole  21  and the sensing node S as shown in  FIG. 2 b   , or between the sensing node S and the second dipole  22  as shown in  FIG. 2 c   . In general the injection unit  31  is not directly coupled to the high voltage node HV but so that at least the first dipole  21  is between the high voltage node HV and the injection unit  31 . 
         [0061]    The injection unit  31  can include an injection signal source  311  which outputs an injection voltage signal V re f. The injection signal source  311  may be connected in series with the voltage divider  2 . The injection signal source  311  may have a low impedance, and provide the injection signal which can be a periodic voltage signal Vref of predetermined frequency/-ies and waveform(s). In case the high voltage at the high-voltage node HV is an AC voltage, the frequency or periodicity, respectively, of the injection signal is to be selected to differ therefrom. 
         [0062]      FIGS. 3 a  and 3 b    show alternative configurations for injecting the injection signal.  FIG. 3 a    shows an injection by means of a transformer  315 , the primary side of which is coupled in series to the voltage divider  2  and the secondary side of which is coupled to a current source  312 . The current source  312  applies the injection signal to be injected into the voltage divider  2 . 
         [0063]    As shown in  FIG. 3 b   , a current mirror including two transistors  314  (interconnected at their gates) is applied for injecting a current injection signal provided by the injection current source  313  into a first side (first transistor  314 ) of the current mirror so that the injection current is introduced into the voltage divider  2  by its second side (second transistor  314 ). 
         [0064]    Back to  FIG. 1 , the measuring unit  3  further includes a correction unit  32  which is connected to the sensing node S. The correction unit  32  comprises a first gain stage  321  and a second gain stage  322 , both coupled with their inputs to the sensing node S. The gain stages  321 ,  322  have different gains to appropriately amplify the sensing signal to obtain an amplified signal in which the signal variations due to the impedance drift of the first dipole  21  can be detected. The first gain stage  321  serves for receiving the sensing signal at the sensing node S, amplifies it and outputs the amplified sensing signal to a first AD converter  323  which converts the voltage of the applied sensing signal. Following the first AD converter  323 , a first digital filter unit  324  is applied for separating a first sensing signal portion from the sensing signal at the sensing node S. In case the high voltage is an AC voltage, the first digital filter unit  324  may selectively extract the frequency portion of the frequency of the high voltage at the high-voltage node HV and harmonics thereof. The first digital filter unit  324  may include a DFT unit to act as a digital filter. The first digital filter unit  324  extracts the signal portion related to the frequency of the high voltage at the high-voltage node HV and its harmonics, which represents the HV sensor signal. 
         [0065]    Alternatively, instead of the first AD converter  323  and the first digital filter unit  324  a first analog filter can be applied upstream of the first gain stage  321  to selectively let the relevant voltage portions pass through. 
         [0066]    The output of the first digital filter unit  324  is applied to a multiplier  326  for digitally multiplying the digital value of the HV sensor signal. 
         [0067]    The second gain stage  322  is connected with its output to a second AD converter  325  which forwards the digitalized and amplified sensor signal to a second digital filter unit  327 . The second digital filter unit  327  may correspond to a high pass , band pass or low pass filter and is configured to extract (a digital representation of) a second sensing signal portion from the overlaid voltage of the sensing signal which has the same frequency as the injection signal but excludes the first sensing signal portion. Hence, the first and second digital filter units  324 ,  327  serve for selectively extracting a respective sensing signal portion from the sensing signal, i. e. the HV sensor signal and the injected voltage signal, which refer to the high voltage and the injection signal, respectively. 
         [0068]    In case the high voltage is a DC voltage, the first filter unit  324  can be omitted or formed as a low pass filter having a base frequency lower than the (lowest) frequency of the injection signal. 
         [0069]    The first and/or the second filter unit  324 ,  327  may include a DFT (Discrete Fourier Transformation) logic coupled with a selector to selectively extract one or a sum of amplitude values at one or more specific frequency portions of the sensing signal at the sensing node S. The output of the second filter unit  327  is forwarded to a control logic  328  which has included a predetermined function or predetermined look-up table or the like and associates to the output value of the second filter unit  327  a correction value C which is then fed to the multiplier  326 . The value of the first sensing signal portion, which is related to the high voltage to be measured at the high-voltage node HV, namely the one or the sum of amplitude values of the one or more specific frequency portions of the sensing signal, is thus multiplied with the correction value C to correct for variations of the portion of the sensing signal which is related to variations of the impedance of the first dipole  21 . Instead of multiplying the correction value C can be applied on the first sensing signal portion by adding or any other appropriate operation. Accordingly, the association of the correction value C to the output value of the second filter unit  327  is implemented such that any variations of the sensing signal due to a variation of the impedance of the first dipole  21  are compensated. The control logic  328  causes the injection signal source  311  to output an injection voltage signal V re f which is assumed to be temperature-independent. 
         [0070]    Another embodiment of the present invention is shown in  FIG. 4 . The embodiment of  FIG. 4  shows an injection unit  31  with an injection signal source  311  for applying an injection voltage signal V re f. Furthermore, a compensation unit  4  to compensate the temperature is provided. 
         [0071]    To compensate for temperature influences on the injection voltage signal V re f (which causes the second sensing signal portion at the sensing node S), the output of the second filter unit  327  is supplied to one input of the subtraction unit  41 . On another input of the subtraction unit  41 , a compensation signal V re f C  is applied. The compensation signal V re fc is generated by means of a compensation voltage divider  42  having the same impedance ratio as the voltage divider  2 . The impedances of the compensation voltage divider  42  are substantially lower than the impedance of the second dipole  22 , so that the attenuation of the voltage divider  2  is not affected by the compensation voltage divider  42  in series with the second dipole  22 . Particularly, the total impedance of the compensation voltage divider  42  may be less than ⅛, preferably less than 1/10, most preferably less than 5% of the impedance of the second dipole  22 . In other embodiments, the compensation voltage divider  42  may not be in series with the voltage divider  2 , so that the total impedance of the compensation voltage divider  42  may be independent from the impedance of the second dipole  22 . 
         [0072]    The injection signal source  311  (injection voltage source) is coupled via a buffer  43  to the compensation voltage divider  42 , the intermediate node N of which is coupled to a third gain stage  44  which may be designed to be identical to the second gain stage  322 . The output of the third gain stage  44  is coupled to a third A D converter  45 , the output of which is coupled to a third digital filter unit  46 . The third AD converter  45  and the third digital filter unit  46  may be designed identical to the second A D converter  325  and the second digital filter unit  327 , respectively. The third digital filter unit  46  may have a selector for selectively extracting the signal portion at the frequency of the compensation signal V re fc. The output of the third digital filter unit  46  provides the compensation signal V re fc. 
         [0073]    The temperature compensation unit  4  allows the generation of the compensation signal V re f C  which has experienced the same attenuation as the injection signal injected into the voltage divider  2 . Therefore, any temperature-related variations of the injection unit  31  can be eliminated by means of the subtraction unit  41 . The output of the subtraction unit  41  is applied to the control logic  328  where the signal portion at the frequency of the compensation signal V re f C  is associated to the correction value C as described above. 
         [0074]    The frequency of the injection voltage signal V re f needs to be selected to differ from the frequency of the AC high voltage. The injection voltage signal V re f may be selected to be sinusoidal, but can have any other waveform as long as at least one harmonic of the injection signal has a known amplitude and all harmonics of the injection signal are different from the frequency of the high voltage to be measured. The sensing voltage with the overlaid injection signal can be isolated from the signal, either in the digital domain as described in the embodiments of  FIG. 1  and  FIG. 4 , or in an analog domain through using analog filters, or by a combination of both. One main idea is to compare the response of the voltage divider  2  at the frequency of the injection voltage signal V re f with a reference injection signal which might be temperature-compensated (see  FIG. 4 ) or not (See  FIG. 1 ). In the embodiment of  FIG. 4 , the reference injection signal is generated to be temperature-compensated. 
         [0075]    The embodiment of  FIG. 5  differs from the embodiment of  FIG. 4  with respect to the application of a switched injection signal V re f S witc h  and using an analog filter to separate the switched injection signal V re f S witc h  from the sensing signal to be measured. 
         [0076]    The switched injection signal V re f S witc h  is provided by a switched signal voltage source  51  which generates the switched injection signal V re f S witc h  as a square waveform signal with a main frequency (switching frequency) lower than the frequency of the AC voltage to be measured at the high-voltage node HV. The switched injection signal V re f S witc h  is coupled into the voltage divider  2  as described in the embodiments above. 
         [0077]    To obtain the corrected sensing signal, a demodulation unit  61  is provided which allows for separating the sensing signal from the switched injection signal V re f S witc h  by means of demodulation. The switched injection signal V re f S witc h  is applied to the demodulation unit  61  to eliminate the effect of the switched injection signal V re f S witc h  on the sensing signal. The demodulation unit  61  may be operated in an analog or a digital domain. In a digital domain, as it is shown in  FIG. 5 , an AD converter  62  is coupled upstream of the demodulation unit  61 . 
         [0078]    Furthermore, the sensing signal may be filtered by an analog filter  63  which may have low pass or band pass filter characteristics to let the (switching) frequency of the signal portion caused by the switched injection signal V re f S witc h  pass through and to block the signal portion caused by the high voltage to be measured. The output of the analog filter  63  is supplied to a differential amplifier  64  on a non-inverting input thereof. 
         [0079]    This embodiment also provides the compensation voltage divider  42  as described above. An attenuated switched injection signal V re f S witch_att is generated which has experienced attenuation in the compensation voltage divider  42  which has the same attenuation of the voltage divider  2  as described above. The attenuated switched injection signal V re fswitch_att is applied at the inverting input of the differential amplifier  64 . The differential amplifier  64  outputs a difference signal between the attenuated switched injection signal V re f S witch_att and the filtered sensing signal. The difference signal has a square waveform whose amplitude is related to the drift due to variations of the impedances in the voltage divider  2 . The difference signal can be applied to a logic unit  7  which associates a correction value C to the amplitude of the difference signal. The correction value C can be applied on the demodulated sensing signal by means of a multiplier  66  as described in the embodiments above, or alternatively, by any other operation unit, such as an adder. 
         [0080]    Without drift, the attenuated switched injection signal V_fswitch_att corresponds to the mismatch between the compensation voltage divider  42  and the voltage divider  2 . The compensation voltage divider  42  can be considered stable in time and temperature with respect to the drift of the voltage divider  2 . The gain of the differential amplifier  64  may be chosen high, such as G=10 4  or more, and shall be implemented in a manner to render the tolerance and drift of the gain G several times smaller than the tolerance and drift of the impedances of the voltage divider  2 . 
         [0081]    The differential amplifier  64  receives the filtered sensing signal and the attenuated switched injection signal V re f S witch_att with almost the same amplitude. However, the difference corresponds to the mismatch of the compensation voltage divider  42  and the voltage divider  2 . This allows for the gain to be very high in order to make it possible to read a very low level of signal changes induced by a variation of the voltage divider  2 . 
       REFERENCE LIST 
       [0000]    
       
           1  High-voltage measurement system 
           2  voltage divider 
           21  first dipole 
           22  second dipole 
           3  measuring unit 
           31  injection unit 
           311  injection voltage/signal source 
           312  current source 
           313  injection current source 
           314  transistor 
           315  transformer 
           32  correction unit 
           321  first gain stage 
           322  second gain stage 
           323  first AD converter 
           324  first digital filter unit 
           325  second A D converter 
           326  multiplier 
           327  second digital filter unit 
           328  control logic 
           4  temperature compensation unit 
           41  subtraction unit 
           42  compensation voltage divider 
           43  buffer 
           44  third gain stage 
           45  third A D converter 
           46  third digital filter unit 
           51  switched signal voltage source 
           61  demodulation unit 
           62  AD converter 
           63  analog filter 
           64  differential amplifier 
           66  multiplier 
           7  logic unit 
         C correction value 
         GND ground potential 
         HV high-voltage node 
         N intermediate node 
         S sensing node 
         Vref injection voltage signal 
         Vrefswitch switched injection signal 
         Vrefswitch_att attenuated switched injection signal