Synchronization of data acquisition devices of an online surveillance system of an electrical distribution network by means of zero-crossing detection

Description

technical field

[0001] The present invention relates to the general field of monitoring the proper functioning of elements present in an electrical distribution network, in particular electrical cables, and more specifically the synchronization between at least two data acquisition devices belonging to an online monitoring system of an electrical distribution network. Technological background

[0002] One of the main problems likely to affect the operation of an electrical transmission and / or distribution network is the occurrence of partial discharges on cables, transformers, switching equipment, cable junctions, etc., which can lead to their progressive degradation and, ultimately, to destructive faults.

[0003] The detection and localization of discharges can provide crucial information to the network operator regarding the insulation condition of operating distribution cables and equipment in general.

[0004] Monitoring systems have already been proposed based on measurements at one end of a network cable, using time-domain reflectometry (TDR) and signal processing techniques. However, these systems are primarily used offline and have strict limitations regarding their effectiveness and scope.

[0005] Other known systems, called online monitoring systems, can detect and locate events that may represent anomalies, such as partial discharges, without affecting the normal operation of the network. Information regarding the progression of the phenomenon over time can prevent the occurrence of destructive faults, thereby improving network reliability indices and preventing short-circuit currents from stressing other equipment. Consequently, such online monitoring systems contribute to one of the important aspects of the smart grid: the optimal use of existing assets through the implementation of optimized preventive maintenance and intelligent asset condition monitoring.

[0006] As depicted on the figure 1, which partially schematizes an example of a meshed electrical distribution network, the principle of online monitoring consists of placing a plurality of online data acquisition devices 1 at predefined locations in the network, for example at the ends of electrical cables present in this network. In the non-limiting example of the figure 1Three of these online data acquisition devices 1 are shown positioned at three known locations, illustrated by points A, B, and C. The two devices 1 at points A and B can detect events corresponding to a partial discharge occurring at any position on a cable or network equipment, for example, between points A and B or even outside of these points. Similarly, the two devices 1 at points B and C can detect events corresponding to a partial discharge occurring at any position on a cable or network equipment, for example, between points B and C, or even outside of these points.

[0007] The known principle for locating a partial discharge with this type of online monitoring system is as follows: If a partial discharge 2 occurs between points A and B of the network, two corresponding pulse signals uA(t) and uB(t) will propagate in opposite directions within the network. The signal uA(t) is detected by the data acquisition device 1 located at point A at time toaA, and the signal uB(t) is detected by the data acquisition device 1 located at point B at time toaB. The location ZPD of the partial discharge 2 can thus be determined according to the following relationship: Z PD = t c − Δt oa 2 t c ⋅ l c with Δt oa = t oa A − t oa B t C , the flight time between points A and B; and I c the known length of cable separating points A and B.

[0008] In order to be able to determine the quantity Δt oa, and consequently, the location Z PD, it is therefore necessary to synchronize the two data acquisition devices 1 located at points A and B. In other words, the arrival times t oaA and t oaB of the signals u A (t) and u B (t) acquired by each of the two data acquisition devices 1 must be determined in a common time reference frame.

[0009] As seen on the figure 1It is already known to associate each data acquisition device 1 of the monitoring system with a receiver 10 of a satellite navigation system, for example, a GPS receiver 10. Each event detected by each of the data acquisition devices 1, for example, the preceding signal uA(t) or uB(t) generated by a partial discharge, can thus be time-stamped in a common reference system. This synchronization method is described, for example, in document WO 2021 / 138569. The difference between the arrival times of the signals uA(t) and uB(t) at the two data acquisition devices 1, expressed in a common time reference, and consequently, the location ZPD of the partial discharge, can then be determined by applying relations (1) and (2) above.

[0010] Nevertheless, although GPS accuracy can be very high, several factors can introduce errors, such as the effects of multiple propagation of the GPS signal, satellite positioning errors, atmospheric conditions, and, above all, installation difficulties. Indeed, the correct use of GPS systems requires antennas that must be installed in open space to receive the satellite signal. However, besides the fact that these antennas are expensive, many high-voltage and medium-voltage power distribution networks are underground, and the need to install surface equipment is minimized.

[0011] Another known method, described in WO 2004 / 013642 A2, involves injecting high-frequency synchronization pulses at one end of a cable being monitored by a distribution network, using an inductive coupler. The data acquired by the monitoring systems at both ends of the cable, generated by partial discharge pulses in the cable, thus contain both partial discharge and synchronization pulses. By aligning the data sets and using the time delay between the partial discharge pulse and the synchronization pulse, it is possible to determine the location of the partial discharge. This time synchronization method uses the power cable as the transmission medium for the synchronization pulses, which mitigates the drawbacks of satellite invisibility and weather conditions associated with GPS.However, the accuracy of this method is significantly affected by the attenuation and dispersion of the synchronization pulses propagating through the cable. This results in a loss of the time-series data transmitted via the cable. Summary of the invention

[0012] The present invention aims to overcome the drawbacks of methods and systems for synchronizing at least two data acquisition devices of an online monitoring system of an electrical distribution network.

[0013] More specifically, the present invention relates to a method for synchronizing at least a first and a second data acquisition device of an online monitoring system for an electrical distribution network, each data acquisition device being located at a known point in the network and being configured to detect high-frequency events during data acquisition phases, the method comprising: a first phase of estimating the period of the electrical signal traversing the network consisting of: taking the electrical signal traversing the network at the level of the first data acquisition device and deducing a first estimate T' A of the period of the electrical signal by detecting the zero crossing times of the taken signal, time-stamped locally by a first time-stamping means associated with the first data acquisition device; taking the electrical signal traversing the network at the level of the second data acquisition device and deducing a second estimate T' B of the period of the electrical signal by detecting the zero crossing times of the taken signal, time-stamped locally by a second time-stamping means associated with the second data acquisition device;followed by a second synchronization phase comprising the following steps: sending by the first data acquisition device of an information signal at a first detection instant t ZCA1,1, locally time-stamped, of a new zero crossing of the signal taken at the level of the first data acquisition device; local time-stamping of the instant of reception of said information signal by the second data acquisition device; triggering, by the first data acquisition device, of a first phase of acquisition of high-frequency events over a plurality of successive cycles of predefined cycle duration, the first phase of acquisition being triggered at a first trigger instant t RA determined locally by the first time-stamping means and separated from the first detection instant by a duration corresponding to the first estimate T' A of the period of the electrical signal;after the elapsed time corresponding to half of the second estimate T' B of the period of the electrical signal following said instant of reception, triggering, by the second data acquisition device, of a second phase of acquisition of high frequency events over a plurality of successive cycles of predefined cycle duration, the second phase of acquisition being triggered at a second triggering instant t RB determined locally by the second time-stamping means and corresponding to a second instant of detection of a new zero crossing of the signal taken at the level of the second data acquisition device;and determination of a synchronization difference Δt oa between a first high-frequency event and a second high-frequency event acquired, on a given cycle, respectively by the first data acquisition device at a first acquisition instant time-stamped locally by the first time-stamping means and by the second data acquisition device at a second acquisition instant time-stamped locally by the second time-stamping means by calculating the difference between the second trigger instant t RB and the first trigger instant t RA. ;

[0014] In one possible embodiment, the predefined cycle time for each cycle of the first data acquisition phase corresponds to the first estimate T' A of the period of the electrical signal, and the predefined cycle time for each cycle of the second data acquisition phase corresponds to the second estimate T' B of the period of the electrical signal.

[0015] In one possible embodiment, the successive cycles in the first and second phases of data acquisition are consecutive.

[0016] Alternatively, the successive cycles in the first and second phases of data acquisition are separated two by two by a predefined spacing duration Ts corresponding to a predefined number of consecutive zero crossings.

[0017] In one possible embodiment, the first high-frequency event detected by the first data acquisition device and the second high-frequency event detected by the second data acquisition device in a given cycle correspond to two signals generated by the same partial discharge at a point in the network located between the first and second data acquisition devices, and the method further includes a step of calculating the location ZPD of the partial discharge according to the relation Z PD = TOF ′ − Δt oa 2 TOF ′ ⋅ l c where I c is a cable length between the first and second data acquisition devices. In particular, TOF' denotes the time of flight between point A and point B. In particular, especially in the above relation, Δt oa denotes the difference between the time of arrival of the partial discharge at point A time-stamped locally by the first time-stamping means and the time of arrival of the partial discharge at point B time-stamped locally by the second time-stamping means, corrected with the difference between the second triggering time t RB and the first triggering time t RA.

[0018] The present invention also relates to an online monitoring system for an electrical distribution network comprising at least a first and a second data acquisition device, each data acquisition device being located at a known point in the network and being configured to detect high-frequency events during data acquisition phases, the online monitoring system being characterized in that it comprises: a first local timestamping means and a first zero-crossing detection module, associated with the first data acquisition device, and configured to sample the electrical signal traversing the network at the level of the first data acquisition device and detect the zero-crossing times of the sampled signal, locally timestamped by the first timestamping means; a second local timestamping means and a second zero-crossing detection module, associated with the second data acquisition device, and configured to sample the electrical signal traversing the network at the level of the second data acquisition device and detect the zero-crossing times of the sampled signal, locally timestamped by the second timestamping means;and synchronization means configured to: during a first estimation phase, deduce a first estimate T'A of the period of the electrical signal from zero-crossing times successively time-stamped locally by the first time-stamping means, and a second estimate T'B of the period of the electrical signal from zero-crossing times successively time-stamped locally by the second time-stamping means, and to perform a second synchronization phase comprising the following steps: sending by the first data acquisition device of an information signal at a first detection time tZCA1,1, time-stamped locally, of a new zero-crossing of the signal taken at the level of the first data acquisition device; local time-stamping of the time of reception of said information signal by the second data acquisition device;triggering, by the first data acquisition device, of a first phase of high-frequency event acquisition over a plurality of successive cycles of predefined cycle duration, the first acquisition phase being triggered at a first trigger instant t RA determined locally by the first time-stamping means and separated from the first detection instant by a duration corresponding to the first estimate T' A of the period of the electrical signal;after the elapsed time corresponding to half of the second estimate T' B of the period of the electrical signal following said instant of reception, triggering, by the second data acquisition device, of a second phase of acquisition of high frequency events over a plurality of successive cycles of predefined cycle duration, the second phase of acquisition being triggered at a second triggering instant t RB determined locally by the second time-stamping means and corresponding to a second instant of detection of a new zero crossing of the signal taken at the level of the second data acquisition device;and determination of a synchronization difference Δt oa between a first high-frequency event and a second high-frequency event acquired, on a given cycle, respectively by the first data acquisition device at a first acquisition instant time-stamped locally by the first time-stamping means and by the second data acquisition device at a second acquisition instant time-stamped locally by the second time-stamping means by calculating the difference between the second trigger instant t RB and the first trigger instant t RA. ;

[0019] In one possible embodiment, the first timestamping means and the second timestamping means are N-bit counters, where N is an integer greater than or equal to 16.

[0020] In one possible embodiment, the first time-stamping means is integrated into the first data acquisition device, and / or the second time-stamping means is integrated into the second data acquisition device.

[0021] In one possible embodiment, the first zero-crossing detection module is integrated into the first data acquisition device, and / or the second zero-crossing detection module is integrated into the second data acquisition device. Brief description of the figures

[0022] The following description, with reference to the accompanying drawings, given by way of non-limiting examples, will clearly explain what the invention consists of and how it can be implemented. Regarding the accompanying figures: there figure 1The diagram, already described above, partially and schematically illustrates an example of an electrical distribution network with an online monitoring system comprising data acquisition devices synchronized in a known manner based on a GPS-type navigation system; figure 2 partially and schematically illustrates an example of an electrical distribution network with synchronized data acquisition devices according to a possible embodiment of the invention; figure 3 schematically illustrates a data acquisition device conforming to a possible embodiment of the invention; the figure 4 illustrates possible steps for a synchronization process according to the invention; the figure 5 schematically illustrates certain steps in the process of the figure 4 ; there figure 6 schematically illustrates other steps in the process of the figure 4 . Description of method(s) of implementation

[0023] In the figures, identical or equivalent elements will bear the same reference symbols. The various diagrams are not to scale.

[0024] In the following, the synchronization between at least two data acquisition devices of a monitoring system according to the invention will be described in the non-limiting case where the online monitoring system is configured to identify and locate partial discharges in the network. The synchronization principle can nevertheless be extended to any online monitoring system with multiple data acquisition devices that need to be synchronized.

[0025] There figure 2 partially illustrates an electrical distribution network similar to the network of the figure 1, including an event monitoring system. The electrical distribution network is, for example, a high-voltage or medium-voltage network, composed of a plurality of electrical cables, connection or linking accessories, switchgear, and / or transformers. The system includes a plurality of acquisition devices 1 placed at various known points in the network, such as points A, B, and C shown on the figure 2 The points A, B, C where the data acquisition devices are located are preferably situated at the ends of cables or cable sections. In the non-limiting case of an underground network, the data acquisition devices are preferably placed in easily accessible locations, for example at transformers.

[0026] Since the devices 1 are here dedicated, without limitation, to the detection and localization of partial discharges, each device 1 typically comprises, as schematically illustrated on the figure 3 , detection means 11 capable of detecting impulsive events caused in the cables by partial discharges, such as the high-frequency pulses uA(t) and uB(t) generated by the impulsive discharge 2 of the figure 2 The means 11 are, for example, a non-invasive sensor, preferably an inductive sensor 11, located around the cable at the device's location point. In place of the GPS receiver 10 of the figure 1 The online monitoring system also includes: a first local timestamping means 13 and a first zero-crossing detection module 12, associated with the first data acquisition device 1, and configured to sample the electrical signal traversing the network at the level of the first data acquisition device 1 and detect the zero-crossing times of the sampled signal, locally timestamped by the first timestamping means 13; a second local timestamping means 13 and a second zero-crossing detection module 12, associated with the second data acquisition device 1, and configured to sample the electrical signal traversing the network at the level of the second data acquisition device 1 and detect the zero-crossing times of the sampled signal, locally timestamped by the second timestamping means 13.

[0027] By "associated" is meant that each local timestamping means 13 and / or each zero-crossing detection module 12 is either electrically and functionally connected to each device 1, or integrated into each device 1, as illustrated in the figure 3 .

[0028] Each data acquisition device 1 may advantageously include a mobile (4G or higher) or Ethernet communication module 14, enabling it, in particular, to receive control signals emitted by a remote server (not shown) included in the online monitoring system, or to transmit information, such as acquired data, to this server or to any other device 1 of the monitoring system. Each device 1 is also capable of emitting information signals to any other device 1 included in the monitoring system, or of receiving such information signals. These information signals may be transmitted via the detection means 11, used in active mode, the information signal then being injected into the network, in particular into the cable at the location point of device 1, and retrieved via the detection means of another device 1.Alternatively, information signals can be transmitted / received via the 14 mobile communication modules (or via Ethernet included in each device 1.

[0029] Each local timestamping means 13 is preferably a precise local clock, or a counter of 16 bits or more. Such a local timestamping means 13 makes it possible to timestamp locally everything that happens at the level of each device 1, including zero-crossing detections, the transmission or reception of any information signal, and each event detected by the high-frequency sensor 11 at the location point of the data acquisition device 1, during a data acquisition phase.

[0030] Each zero-crossing detection module 12 comprises a low-frequency sensor 15 capable of picking up the sinusoidal electrical signal traveling through the network at the fundamental frequency of 50 Hz or 60 Hz, depending on the country, at the location point (A, B, or C) of the device 1 under consideration, and a zero-crossing detection circuit 16 that receives the signal picked up by the low-frequency sensor 15. The low-frequency sensor 15 is preferably a non-invasive sensor, for example, a non-contact ELF magnetometer or an inductive ELF sensor (ELF being the initials for "Extremely Low Frequency"). Low-frequency capacitive sensors may nevertheless be considered. The circuit 16 can implement any known zero-crossing detection algorithm.The zero-crossing detector circuit 16 can be any known comparator circuit capable of detecting the voltage of the sampled signal when it passes from the positive level to the negative level and from the negative level to the positive level.

[0031] The online monitoring system also includes time synchronization means configured to implement the steps of a synchronization process according to the invention, which will now be explained.

[0032] The synchronization method used by the data acquisition devices 1, according to the present invention, is illustrated in the figure 4 For the sake of simplicity, the explanation is given with regard to the two devices 1 located at points A and B, but can easily be extended to all data acquisition devices 1 present in the electrical network:

[0033] The synchronization process 100 begins with a phase of estimating the period of the electrical signal transiting through the network, performed at each data acquisition device 1, and in particular at each zero-crossing detection module 12. More specifically, the estimation phase 110 consists of: on the one hand, to take the electrical signal traveling through the network at the level of the first data acquisition device 1, located at point A, and to deduce a first estimate T' A of the period of the electrical signal by detecting the times of zero crossings of the taken signal, time-stamped locally by a first time-stamping means 13 associated with the first data acquisition device 1; on the other hand, to take the electrical signal traveling through the network at the level of the second data acquisition device 1, located at point B, and to deduce a second estimate T' B of the period of the electrical signal by detecting the times of zero crossings of the taken signal, time-stamped locally by a second time-stamping means 13 associated with the second data acquisition device.

[0034] As schematically illustrated on the figure 5And given that the sampled signal corresponds to the sinusoidal electrical signal at the fundamental frequency of the electrical grid (i.e., 50 Hz or 60 Hz depending on the country), this signal crosses zero twice during each signal period. In other words, each zero-crossing detection module will be able to detect a pair of zero crossings for each period of the periodic signal sampled at device 1 located at point A, and respectively, for the periodic signal sampled at device 1 located at point B. Each device will also be able to locally timestamp each zero-crossing detection via its timestamping method 13. If we note: ZC A 1 1 ZC A 1 2 , ZC A 2 1 ZC A 2 2 , … , ZC A M 1 ZC A M 2 , respectively ZC B 1 1 ZC B 1 2 , ZC B 2 1 ZC B 2 2 , … , ZC B M 1 ZC B M 2 , M successive pairs of zero crossings detected by device 1 located at point A, respectively at point B, and t ZC A 1 1 t ZC A 1 2 , t ZC A 2 1 t ZC A 2 2 , … , t ZC A M 1 t ZC A M 2 , respectively t ZC B 1 1 t ZC B 1 2 , t ZC B 2 1 t ZC B 2 2 , … , t ZC B M 1 t ZC B M 2 , the pairs of associated instants obtained by local time stamping via the time stamping means 13 associated with the data acquisition device 1 located at point A, respectively at point B, the estimate T'A, respectively T'B, of the period of the periodic signal is calculated according to the relation: T A ′ = 2 × ∑ i = 1 M t ZC A i 2 − t ZC A i 1 respectively T B ′ = 2 × ∑ i = 1 M t ZC B i 2 − t ZC B i 1

[0035] The larger the integer M, the more we are protected from fluctuations that could affect the periodic signal due to overload or the presence of equipment in the network. The integer M can be, for example, equal to 100, or even 200 or 300.

[0036] Once each device has been able to locally estimate the period T'A or T'B of the sampled periodic signal, process 100 continues with a second synchronization phase comprising the following steps (see jointly the figures 4 And 6The data acquisition device 1, located at point A, sends, during step 120, an information signal s(t) at a first detection instant tZCA1,1, locally time-stamped, of a new zero crossing of the signal sampled at this data acquisition device 1. As seen previously, this signal s(t) can be sent via any transmission channel (via the network itself by injecting the signal into the cable, or via cellular communication, or via Ethernet). The signal s(t) can be of any type.

[0037] During step 130, the information signal s(t) is received by the data acquisition device 1 located at point B, after a duration corresponding to the time of flight τ between the two points A and B. Note that this time of flight T is generally on the order of microseconds for a maximum distance of approximately 15 km separating the two points A and B. During this step 130, the moment of reception of this information signal s(t) is locally time-stamped by the local time-stamping means d 13 associated with this device 1.

[0038] During a step 140, the localized triggering data acquisition device 1 at point A triggers a first phase of high-frequency event acquisition by its sensor 11, over a plurality of successive cycles of predefined cycle duration Tm, the first phase of acquisition being triggered at a first trigger instant tRA determined locally by the first local time-stamping means 13 and separated from the first detection instant tZCA1,1 by a duration corresponding to the first estimate T'A of the period of the electrical signal.

[0039] Furthermore, after the elapsed time corresponding to T' B / 2 following the instant of reception of the signal s(t), the data acquisition device 1 located at point B triggers, during a step 150, a second phase of acquisition of high frequency events by its own sensor 11, over a plurality of successive cycles of predefined cycle duration, the second phase of acquisition being triggered at a second trigger instant t RB determined locally by the second time-stamping means 13 and corresponding to a second instant of detection of a new zero crossing of the signal taken at the level of the second data acquisition device 1.

[0040] In a particularly advantageous embodiment, the predefined cycle time for each cycle of the first data acquisition phase corresponds to the first estimate T'A of the period of the electrical signal, and the predefined cycle time for each cycle of the second data acquisition phase corresponds to the second estimate T'B of the period of the electrical signal. Consequently, the cycle times for the two phases are identical.

[0041] As shown by figure 6 , the triggering times T RA and T RB, although measured locally by each time-stamping means 13, are assumed to coincide in a common time reference frame.

[0042] It follows that it is possible to determine, during a step 160, determination, a synchronization difference Δt oa between a first high-frequency event and a second high-frequency event acquired, on a given cycle, respectively by the data acquisition device 1 located at point A, at a first acquisition instant time-stamped locally by its own time-stamping means 13, and by the second data acquisition device 1 located at point B, at a second acquisition instant time-stamped locally by its own time-stamping means 13, by simply calculating the difference between the second trigger instant t RB and the first trigger instant t RA. This calculation can be performed locally (for example at the level of the first device 1 located at point A) or centrally at the remote server.

[0043] In one possible embodiment, the successive cycles in the first and second phases of data acquisition are separated two by two by a predefined spacing duration Ts corresponding to a predefined number of consecutive zero crossings.

[0044] Alternatively, the successive cycles in the first and second phases of data acquisition are consecutive, which means that the spacing duration Ts is zero.

[0045] In all cases, a cycle (n+1) for device 1 located at point A, respectively at point B, begins at a time t R A n + 1 , respectively t R B n + 1 , which can be expressed in terms of the previous cycle n, according to the relation: t R A n + 1 = t oa A n + T m + T s , respectively t R B n + 1 = t oa B n + T m + T s so that, for each cycle n, the relationship is preserved Δt oa n = t oa B n − t oa A n = t R B n + 1 − t R A n + 1

[0046] Any high-frequency event that can be detected by the detection means 11 of device 1 located at point A or of device 1 located at point B during data acquisition phases will therefore be able to be time-stamped first locally, via the local time-stamping means 13, then in a common reference base thanks to the knowledge of the synchronization difference Δt oa between the two data acquisition devices 1.

[0047] In the non-limiting case where the data acquisition devices 1 are dedicated to detecting high-frequency events corresponding to signals uA(t) and uB(t) generated by the same partial discharge 2, the method further includes a step (not shown) of calculating the location ZPD of the partial discharge according to the relation Z PD = TOF ′ − Δt oa 2 TOF ′ . I c in which IC is a cable length between the first and second data acquisition devices (1). In particular, TOF' denotes the time of flight between point A and point B. In particular, especially in the above relation, Δt oa denotes the difference between the time of arrival of the partial discharge at point A, locally time-stamped by the first time-stamping means, and the time of arrival of the partial discharge at point B, locally time-stamped by the second time-stamping means, corrected by the difference between the second triggering time t RB and the first triggering time t RA. In particular, this is clear from the passage in paragraph

[0007] and the passage in paragraph

[00042] .

[0048] This calculation step can, for example, be performed at the remote central server level.

[0049] Steps 110 to 160 are preferably repeated periodically (e.g., once or several times a day) to compensate for drifts that may affect the network, such as temperature changes, overloads, and / or dispersion in the meters 13).

Claims

1. A method for synchronization between at least a first data acquisition device (1) and a second data acquisition device (1) of an online monitoring system for monitoring an electrical distribution network, each data acquisition device (1) being located at a known point in the network and being configured to detect high-frequency events during data acquisition phases, the method comprising: a first phase (110) of estimating the period of the electrical signal travelling through the network, including: - sampling the electrical signal travelling through the network at the first data acquisition device (1) and deducing therefrom a first estimate T'A of the period of the electrical signal by detecting times of zero crossings of the sampled signal, which are locally timestamped by a first timestamping means (13) associated with the first data acquisition device (1); - sampling the electrical signal travelling through the network at the second data acquisition device (1) and deducing therefrom a second estimate T'B of the period of the electrical signal by detecting times of zero crossings of the sampled signal, which are locally timestamped by a second timestamping means (13) associated with the second data acquisition device (1); followed by a second synchronization phase, including the following steps: - the first data acquisition device (1) sending (120) an information signal at a first time of detection tZCA1,1, which is locally timestamped, of a new zero crossing of the signal sampled at the first data acquisition device; - the second data acquisition device (1) locally timestamping (130) the time of reception of said information signal; - the first data acquisition device (1) triggering (140) a first phase of acquiring high-frequency events over a plurality of successive cycles having a predefined cycle duration, the first acquisition phase being triggered at a first triggering time tRA determined locally by the first timestamping means (13) and separated from the first time of detection by a duration corresponding to the first estimate T'A of the period of the electrical signal; - after a duration corresponding to half the second estimate T'B of the period of the electrical signal following said time of reception has elapsed, the second data acquisition device (1) triggering (150) a second phase of acquiring high-frequency events over a plurality of successive cycles having a predefined cycle duration, the second acquisition phase being triggered at a second triggering time tRB determined locally by the second timestamping means (13) and corresponding to a second time of detection of a new zero crossing of the signal sampled at the second data acquisition device (1); and - determining (160) a synchronization difference Δtoa between a first high-frequency event and a second high-frequency event acquired, over a given cycle, respectively by the first data acquisition device (1) at a first acquisition time locally timestamped by the first timestamping means (13) and by the second data acquisition device (1) at a second acquisition time locally timestamped by the second timestamping means (13) by calculating the difference between the second triggering time tRB and the first triggering time tRA.

2. The method according to claim 1, wherein the predefined cycle duration for each cycle of the first data acquisition phase corresponds to the first estimate T'A of the period of the electrical signal, and the predefined cycle duration for each cycle of the second data acquisition phase corresponds to the second estimate T'B of the period of the electrical signal.

3. The method according to any one of the preceding claims, wherein the successive cycles in the first and second data acquisition phase are consecutive.

4. The method according to any one of claims 1 to 2, wherein the successive cycles in the first and second data acquisition phase are separated in pairs by a predefined spacing duration TS corresponding to a predefined number of consecutive zero crossings.

5. The method according to any one of the preceding claims, wherein the first high-frequency event detected by the first data acquisition device (1) and the second high-frequency event detected by the second data acquisition device (1) over a given cycle correspond to two signals generated by the same partial discharge (2) at a point of the network located between the first and second data acquisition devices (1), and in that the method furthermore comprises a step of calculating the location ZPD of the partial discharge using the relationship Z PD = TOF ′ − Δt oa 2 TOF ′ . I c in which IC is a length of cable between the first and second data acquisition devices (1).

6. An online monitoring system for monitoring an electrical distribution network having at least a first data acquisition device (1) and a second data acquisition device (1), each data acquisition device (1) being located at a known point in the network and being configured to detect high-frequency events during data acquisition phases, the online monitoring system comprising: - a first local timestamping means (13) and a first zero crossing detection module (12), which are associated with the first data acquisition device (1), and configured to sample the electrical signal travelling through the network at the first data acquisition device (1) and detect the times of zero crossings of the sampled signal, which are locally timestamped by the first timestamping means (13); - a second local timestamping means (13) and a second zero crossing detection module (12), which are associated with the second data acquisition device (1), and configured to sample the electrical signal travelling through the network at the second data acquisition device (1) and detect the times of zero crossings of the sampled signal, which are locally timestamped by the second timestamping means (13); and - synchronization means configured: in a first estimation phase, to deduce a first estimate T'A of the period of the electrical signal from times of zero crossings that are successively locally timestamped by the first timestamping means (13), and a second estimate T'B of the period of the electrical signal from times of zero crossings that are successively locally timestamped by the second timestamping means (13), and to perform a second synchronization phase, comprising the following steps: - the first data acquisition device sending (120) an information signal at a first time of detection tZCA1,1, which is locally timestamped, of a new zero crossing of the signal sampled at the first data acquisition device (1); - the second data acquisition device (1) locally timestamping (13) the time of reception of said information signal; - the first data acquisition device triggering (140) a first phase of acquiring high-frequency events over a plurality of successive cycles having a predefined cycle duration, the first acquisition phase being triggered at a first triggering time tRA determined locally by the first timestamping means (13) and separated from the first time of detection by a duration corresponding to the first estimate T'A of the period of the electrical signal; - after a duration corresponding to half the second estimate T'B of the period of the electrical signal following said time of reception has elapsed, the second data acquisition device triggering (150) a second phase of acquiring high-frequency events over a plurality of successive cycles having a predefined cycle duration, the second acquisition phase being triggered at a second triggering time tRB determined locally by the second timestamping means (13) and corresponding to a second time of detection of a new zero crossing of the signal sampled at the second data acquisition device (1); and - determining (160) a synchronization difference Δtoa between a first high-frequency event and a second high-frequency event acquired, over a given cycle, respectively by the first data acquisition device (1) at a first acquisition time locally timestamped by the first timestamping means (13) and by the second data acquisition device (1) at a second acquisition time locally timestamped by the second timestamping means (13) by calculating the difference between the second triggering time tRB and the first triggering time tRA.

7. The system according to claim 6, wherein the first timestamping means and the second timestamping means are N-bit counters, N being an integer greater than or equal to 16.

8. The system according to any one of claims 6 and 7, wherein the first timestamping means (13) is integrated into the first data acquisition device, and / or the second timestamping means is integrated into the second data acquisition device (1).

9. The system according to any one of claims 6 to 8, wherein the first zero crossing detection module (12) is integrated into the first data acquisition device (1), and / or the second zero crossing detection module (12) is integrated into the second data acquisition device (1).