Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R23/00—Arrangements for measuring frequencies; Arrangements for analysing frequency spectra
  • G01R23/16—Spectrum analysis; Fourier analysis
  • G01R23/20—Measurement of non-linear distortion

Definitions

• a method of instantaneously determining signal distortion rates on an AC power grid, and associated device is provided.
• the subject of the present invention is a method for instantaneous determination of a signal distortion rate on an alternating electrical network, and in particular on an alternating electrical network distributing variable frequency signals. It also relates to an electronic device capable of implementing such a method.
• the main purpose of the invention is to propose a solution for providing real-time information for the characterization of disturbances present on an electrical network, in particular an electrical network whose frequency of the voltage available on the network is variable.
• the method according to the invention is however directly transferable to electrical networks whose frequency of the available voltage is fixed.
• the field of the invention is, in general, that of the analysis of AC electrical networks, and in particular the determination of the disturbances likely to be present on such networks.
• Such an analysis may especially consist of an operation to qualify the linearity of the characteristic of a system connected to the considered power grid. If this characteristic is linear, the system responds to a sinusoid by a sinusoid, otherwise, it introduces a distortion and the output signal is no longer sinusoidal, but has acquired harmonics
• the rate of distortion also called harmonic distortion rate, abbreviated as THD, is a quantity which makes it possible to evaluate, by means of a unique number, the disturbance of a current or of a voltage at a point of a given electrical network, considering the deformation the sinusoidal magnitude of the signals of said network.
• THD is defined as the ratio of the overall rms value of harmonics (i.e., the quadratic sum of harmonics) to the rms value of the fundamental component, according to formula (1):
• THD I ) H k 2) / H 0>
• Ho represents the value of the root mean square or rms value of the fundamental component of the respective signal
• H ⁇ represents the value of the root mean square of the second harmonic K.
• the formula (1) is a formula for calculating the harmonic distortion rate adapted to a frequency approach of the signal to be processed.
• the THD thus provides a quantitative evaluation of the frequency content of a signal to be measured by indicating the energy importance of the harmonics with respect to the fundamental component of the signal.
• FFT Fast Fourier Transform
• Such an algorithm gives the amplitude and the phase of each frequency step.
• a second calculation corresponding to formula (1), makes it possible to determine the THD.
• Such calculations are suitable for stationary signals. For obvious practical reasons, they involve a number of points previously defined, very often equal to a power of two to optimize the calculation time.
• the mentioned FFT algorithms calculate the THD over a large number of signal periods. With such a principle, it is not possible to distinguish a fast variation of THD on a signal whose duration is low compared to the duration of the calculation window, because such a rapid variation would be averaged by the rest of the calculation window. measured signal.
• variable frequency electricity grids there are now industrial sectors where the use of variable frequency electricity grids is developing. This is particularly the case in the aeronautics sector.
• the jumbo jet For example, the jumbo jet
• A380 Airbus implements such an electrical network.
• the I Arthur380 alternative electricity grid is a three-phase network whose generation is provided by four variable frequency generators, abbreviated as VFG, directly coupled to the high pressure stage of each reactor.
• VFG variable frequency generators
• the excitation of the alternator is controlled so as to obtain a controlled output effective voltage of 115 volts at 200 volts, the frequency of the network being between 360 Hz and 800 Hz approximately.
• Each reactor drives a GFV that powers its own alternative main busbar.
• the increasing use of electricity in this field of application is motivated in particular by a reduction in the mass of the apparatus in question, by making it possible to simplify heavy hydraulic networks that are constraining in terms of maintenance.
• a general problem that the object of the invention seeks to solve is thus to overcome the lack of means for calculating in real time an instantaneous value of THD, while adapting to a network. generating a variable frequency signal.
• the object of the invention proposes a solution to the problems and disadvantages which have just been exposed.
• the invention proposes a method for instantaneous determination of distortion rate on variable frequency signals, and an associated device, in which a harmonic distortion rate is calculated over a shortest possible time window, corresponding to to the duration of a fundamental period of the signal considered.
• it is sought to precisely determine the value of the frequency of the signal whose THD is to be calculated, and an iteration of certain measurements carried out during a given calculation for calculating the THD on subsequent signals.
• Such a method can moreover be applied directly to alternative fixed frequency electric networks.
• the invention therefore essentially relates to a method of instantaneous determination of a distortion rate on an alternative electric network, characterized in that it comprises the various steps of:
• the method according to the invention may have one or more additional characteristics among the following:
• the alternating electric network is a variable frequency alternating electric network, the input signal being a variable frequency signal;
• the step of selecting, with the first processing means, a single period of the input signal comprises the various operations consisting of:
• the sample signal by reporting on the input signal the alternating start and end times of the identified processed signal.
• the step of identifying the period of the filtered signal to be processed and alternating start and end times of the filtered signal to be processed comprises the additional preliminary operation of transmitting, from the selection means to the third means, processing, information relating to the end of alternation time of a filtered signal portion directly preceding the sample signal.
• the filtering device comprises a first filter presenting a first cutoff frequency and a second filter having a second cutoff frequency, the input signal being transmitted to the first filter and the second filter, producing respectively a first filtered signal and a second filtered signal, respectively transmitted to a first detection means passing zeros and a second zero crossing detecting means of the third processing means, generating zero crossing information and each transmitting to a single logical comparator said zero crossing information, said logical comparator interpreting said information; obtained to determine the fundamental frequency to be considered in the operation of identifying the start and end times of alternation.
• the first cutoff frequency is equal to the maximum frequency observable on the variable frequency network, and in that the second cutoff frequency is less than or equal to twice the value of the minimum frequency observable on the variable frequency network.
• the first cutoff frequency is equal to 800 Hz and the second cutoff frequency is equal to 600 Hz.
• the step of selecting, with the first processing means, a single alternation of the input signal comprises the various operations consisting of:
• the method comprises the prior step of transmitting, from the selection means to the fourth processing means, information relating to the end of alternation time of a signal portion directly preceding the sample signal.
• the calculation step, with the second processing means, of the rate of Harmonic distortion of the sample signal includes the operations of:
• the step of calculating, with the second processing means, the harmonic distortion ratio of the sample signal comprises the operations of:
• the input signal is a digital signal.
• the input signal is an analog signal.
• the present invention also relates to a device for instantaneous determination of a distortion rate on signals of an alternating electric network, able to implement at least one embodiment of the method according to the invention, said device receiving as input an input signal, characterized in that the device comprises in particular:
• first processing means for selecting a single period of the input signal to obtain a sample signal
• second processing means receiving as input the sample signal, for calculating the harmonic distortion rate of the received sample signal
• the device according to the invention may have one or more additional characteristics among the following:
• the alternative electric network is a variable frequency network
• the first processing means comprises:
• a filtering device filtering the fundamental frequency of the input signal, to obtain a filtered signal to be processed; a third processing means identifying the period of the filtered signal at processing and the instants of beginning and end of alternation of the filtered signal to be processed by detecting zero crossings of the fundamental present in the filtered signal to be processed;
• a selection means determining the sample signal by transferring to the input signal the start and end times of alternation of the identified processed signal.
• the filtering device comprises a first filter having a first cutoff frequency and a second filter having a second cutoff frequency, the first filter and the second filter receiving the input signal and producing respectively a first filtered signal and a second signal; filtered, and the third processing means comprises: a first zero passage detection means and a second zero crossing detection means, respectively receiving the first filtered signal and the second filtered signal, and producing passage information; zero respectively of the first filtered signal and the second filtered signal; a single logic comparator receiving the information relating to the zero crossing and interpreting said obtained information to determine the frequency of the fundamental to be considered.
• FIG. 4 a detailed exemplary embodiment of the exemplary embodiment of FIG. 2; in Figure 5, a first schematic exemplary embodiment of a second processing means involved in the device according to the invention; - In Figure 6, a second schematic exemplary embodiment of the second processing means involved in the device according to the invention.
• FIG. 1 shows very schematically an example 100 of the device according to the invention.
• An input signal Sin is provided by the variable frequency electrical network.
• the input signal Sin is received by a first processing means 1, which selects, in a first step of the method according to the invention, a single period of the input signal Sin.
• the power grid in question provides a variable frequency alternating current
• the term "input signal period” is used to denote the period of the input signal characterizing, during a given time window, said input signal, the latter being actually periodic over the time window considered.
• the single period selected by the first processing means is then transmitted, via a sample signal Sper, to a second processing means 2 of the device according to the invention.
• the second processing means 2 performs, according to a second step of the method according to the invention, a calculation of THD for each signal period transmitted to it via the sample signal Sper.
• the device according to the invention is able to perform a calculation of THD, and to provide, under the form of an output signal Sout, a numerical value, or, in the case of a completely analog processing, an analog form of the result of the calculation.
• the signal processing which has just been described is implemented either directly on analog signals, or on digital samples, obtained in a conventional manner by passing an analog signal in an analog / digital converter. digital.
• FIG. 2 shows a first exemplary embodiment of the first processing means 1.
• the first processing means 1 consists of a filtering device 11, a third processing means 12, a selection device 15, and an iteration circuit 16.
• the filtering device 11 receives the input signal Sin at its input, its function is to filter the input signal Sin so as to keep only the frequencies close to the frequency of the fundamental component, also called fundamental, of the input signal. Sin, and thus provide a filtered signal to be processed Sf.
• the filtered signal to be processed Sf is then transmitted to the third processing means 12, the function of which is to carry out a zero crossing detection operation of the fundamental, in order to identify the period of the filtered signal Sf and the start times of alternation and end of alternation of the filtered signal Sf.
• alternation designates the portion of the signal comprised between a first instant, called the beginning of alternation, and a second instant, called the end of alternation, the amplitudes of the signal at the instants of the beginning of alternation and end of alternation being equal, but not necessarily zero, the signal considered having evolved on a single whole period between the instants of beginning of alternation and end of alternation.
• cycle designates a particular alternation, for which the amplitudes noted at times - or dates - of beginning and end of alternation are zero.
• Information relating to the start and end times of alternation is transmitted in the form of a signal Sp at a first input of a selection device, which moreover receives, at a second input, the signal d Sin input.
• the selection device 15 then has the function of only selecting, by transferring the instants of beginning of alternation and end of alternation on the input signal Sin, a single period of the input signal Sin, the period thus selected corresponding to the sample signal Sper which is then transmitted to the second processing means 2.
• a first information signal Sm corresponding to end of processing information and including in particular the end of alternation period of the signal Sin which comes to be processed, are transmitted from the selection means 15 to an iteration circuit 16.
• the latter uses this information to transmit, via a second information signal Sn, an input data.
• the third processing means 12 the latter being able to extract from the second information signal Sn alternating start information of the next period of the signal Sin to be processed. The latter will be the next directly following Sin signal, the entire Sin signal can be processed.
• FIG. 3 shows a second exemplary embodiment of the first processing means 1.
• the first processing means 1 consists of a fourth processing means 13, a fifth processing means 14, the selection device 15, and the iteration circuit 16.
• the fourth processing means 13 receives as input the input signal Sin, from which it extracts a portion corresponding to a previously defined time window of the signal Sin and forming a truncated signal; fourth processing means 13 is to perform a frequency analysis of the truncated signal, in order to extract the frequency content and identify the fundamental frequency of the truncated signal.
• the information produced by the fourth processing means 13 is transmitted in the form of a third information signal Sg to the fifth processing means 14, the function of which is to determine, from the transmitted frequency information, the start times and the end of alternation of the input signal Sin, corresponding to the cycle start and cycle end dates of the fundamental of the truncated signal.
• the information relating to the instants of start and end of alternation are transmitted in the form of the information signal Sp at the first input of the selection device 15, whose operation is the same as that described in FIG. first example.
• the signals supplied and received by the iteration circuit 16 are identical to those of the first example described. However, in the second example, the second information signal Sn is transmitted to the fourth processing means 13, which is thus aware of the new date from which the search of the fundamental of the input signal must be sought.
• FIG. 4 shows a detailed exemplary embodiment corresponding to the more general device represented in FIG. 2.
• This detailed example is particularly adapted to the case where the fundamental frequency of the input signal Sin is likely to vary over a frequency range included between a minimum frequency and a maximum frequency greater than double the minimum frequency. This is the case, for example, for variable frequency electrical networks whose frequency can vary over a range from 360 Hz to 800 Hz.
• the filtering device 11 by means of a first filter 111 and a second filter 112, the first filter and the second filter being for example low-pass filters having different cut-off frequencies, respectively equal to 800 Hz and 600 Hz in the example.
• the cutoff frequencies chosen are different from those mentioned.
• a first cut-off frequency of between 750 Hz and 850 Hz is selected, and a second cut-off frequency of between 550 Hz and 650 Hz.
• the first filter 111 and the second filter 112 each receive the input signal Sin at the input. , which is previously duplicated, and respectively produce a first filtered signal Sf1 and a second filtered signal Sf2.
• the first filtered signal SfI and the second filtered signal Sf2 are respectively transmitted to a first module 121 and to a second module 121 'of the third processing means 12, the first module and the second module being means of detection of zero crossing which are in this example, identical.
• the zero crossing detection information generated by the modules 121 and 121 ' is then transmitted to a logic comparator 122 of the third processing means 12 which, based on said zero crossing detection information, determines the frequency of the fundamental to be considered. . This determination is made as follows:
• the frequency of the signals detected in the first filtered signal Sf1 will be between 720 Hz and 800 Hz, corresponding to the first harmonic, and the frequency of the signals detected in the first signal.
• second filtered signal Sf2 will be between 360 Hz and 400 Hz;
• the frequency of the signals detected in the first filtered signal Sf1 will be between 400 Hz and 600 Hz, and the frequency of the signals detected in the second filtered signal Sf2 will also be between 400 Hz and 800 Hz;
• the frequency of the signals detected in the first filtered signal SfI will be between 600 Hz and 800 Hz, and no signal will be detected in the second filtered signal Sf2; input signal being here completely cut by the second filter 112.
• the logic comparator 122 detects the presence of different signals from the first filter 111 and the second filter 112, it considers only the second filtered signal Sf2 from the second filter 112 as containing the fundamental of the input signal; when the logic comparator 122 detects the presence of identical signals from the first filter 111 and the second filter 112, it considers indifferently the first filtered signal Sf1 or the second filtered signal Sf2 as containing the fundamental of the input signal; when the logic comparator 122 detects the presence of a signal from the first filter 111 and the absence of a signal from the second filter 112, it considers only the first filtered signal Sf1 from the first filter 111 as containing the fundamental of the signal d 'Entrance.
• the logic comparator 122 thus makes it possible to transmit, after processing by the modules 121 or 121 ', the good filtered signal, that is to say the signal containing the fundamental of the input signal Sin, by means of selection 15.
• the calculation of the THD performed by the second processing means 2 can then be performed according to different methods.
• the second processing means 2 carries out a complete frequency analysis of the received sample signal Sper by establishing the frequency spectrum, that is by extracting the amplitude values from it. of each of the harmonics and the fundamental of the signal considered. Once these values are determined, we apply the formula (1) previously given.
• a second method a temporal approach of the signal to be processed is made. Such a method appears to be more efficient in terms of computing time, and therefore more suitable for real-time applications, and can be used on digital signals as on analog signals. In such an approach, it is the effective values of the different signals that must be considered.
• the second processing means 2 which receives as input the sample signal Sper, comprises a first processing module 21 which, from the sample signal Sper, develops a signal Sfond, relative to the frequency of the fundamental, which includes only the frequency content relative to the fundamental frequency of the signal.
• the first processing module makes, for example, a filtering operation, digital or analog depending on the nature of the signal Sper, in particular with a bandpass filter centered on the fundamental.
• a second processing module 22 of the second processing means 2 uses the signal Sfond and the sample signal Sper to perform a calculation of THD according to the following formula (2):
• THD I 00 * V (( V S per ⁇ / S background) 2 -1) where V S p er represents the rms value of the sample signal Sper and Vsfond represents the rms value of the Sfond signal.
• the second processing means 2 which receives the sample signal Sper as input, comprises a third processing module 23 which, from the sample signal Sper, separates the signal Sfond relating to the fundamental frequency of a signal Sharm complementary, corresponding to the Sper sample signal to which the Sfond signal was removed.
• the first processing module makes, for example, a filtering operation, digital or analog depending on the nature of the signal Sper, including a bandpass filter centered on the frequency of the fundamental.
• a fourth processing module 24 of the second processing means 2 then uses the Sfond and Sharm signals to perform a THD calculation according to the following formula (3):
• THD I 00 * (V Sh armA / S background) where Vsharm represents the rms value of the Sharm signal, and where V S f 0n d represents the rms value of the Sfond signal.
• the formulas (2) and (3) here constitute formulas for calculating the harmonic distortion rate adapted to a temporal approach of the signal to be processed.

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• Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• Mathematical Physics (AREA)
• General Physics & Mathematics (AREA)
• Measurement Of Current Or Voltage (AREA)
• Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
• Measuring Frequencies, Analyzing Spectra (AREA)
• Monitoring And Testing Of Transmission In General (AREA)
• Supply And Distribution Of Alternating Current (AREA)

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