Coordinated Resynchronization of Clocks in Power Grid Systems

Abstract

Disclosed herein is a method for synchronizing clocks of a power grid device. The method, performed by a clock reader unit, includes receiving an indication of a new clock signal that a plurality of clock follower units should follow for use in synchronizing the clocks of the plurality of clock follower units and sending instructions to the plurality of clock follower units to calibrate based on the new clock signal. The method then includes obtaining confirmation that the plurality of clock follower units have completed the calibration and, in response to obtaining the confirmation, sending instructions to the clock follower units to follow the new clock signal. By coordinating the transitions of the clock follower units to follow the new clock signal, synchronization of time operations between the clock follower units can be better maintained.

Description

[Technical Field]

[0001] Technical Field The present invention relates to power grid systems, and more particularly to a method for synchronizing clocks of power grid devices. [Background technology]

[0002] background A power grid, such as an alternating current (AC) power grid, may comprise a large number of power grid devices, e.g., in power (sub)stations, transmission networks, etc. Many of these devices may need to synchronize their operation in some way.

[0003] For example, a fault detection system may register a fault if it determines that the phase angle deviates by more than a threshold amount. The deviation may be determined by comparing simultaneously obtained phase angle outputs from measurement devices located at different locations on the power grid. The phase angle outputs may be time-stamped so that similarly time-stamped outputs are compared.

[0004] In such an example, if one of the measurement devices is not operating in synchronization with another of the measurement devices, this can result in malfunctions, such as incorrect alignment of samples when comparing data from multiple devices, resulting in angle deviation errors, which can falsely detect phase angle deviations that exceed a threshold amount and unnecessarily activate a fault response.

[0005] As an alternative to such an example, other values ​​that are time-stamped by measurement, control, or other devices may be used in various parts of the power grid system, where synchronization of operation is important for their proper functioning.

[0006] For a variety of reasons, the clock signal used by a power grid device may change, requiring a resynchronization process to occur so that the power grid device can continue to operate properly.

[0007] Some power grid devices (e.g., so-called "station clocks" at electrical substations) may communicate with an external clock signal source, such as a Global Positioning System (GPS) clock signal, which may be considered a high-precision source of clock signals. Other power grid devices may track this clock signal provided by the station clock to ensure that they are operating synchronously with one another.

[0008] However, for many possible reasons, there may be a change (e.g., a jump) in the clock signal received and distributed by the station clock. This may occur, for example, as a result of a loss of communication with an external clock signal source and its subsequent restoration. Therefore, the station clock may need to provide a new clock signal to any devices that rely on the station clock for their synchronization.

[0009] Devices that track the clock signal provided by the station clock may (re)calibrate or (re)synchronize based on the new clock signal from the station clock. Depending on the particular device and how different its internal clock synchronization is from such new clock signal, different devices may take different amounts of time to (re)calibrate. Summary of the Invention [Problem to be solved by the invention]

[0010] overview As part of this disclosure, it is recognized that during resynchronization of power grid devices after a new clock signal has been acquired by a station clock, for whatever reason the new clock signal may occur, some devices may adjust to follow the new clock signal before other devices. Thus, there is a risk that the devices may be significantly out of sync with each other during the resynchronization process, particularly if the new clock signal is significantly different from the clock signal previously followed by the devices. [Means for solving the problem]

[0011] Thus, according to one aspect of the present disclosure, there is provided a method for synchronizing clocks of power grid devices that overcomes at least some of the problems discussed above.

[0012] In particular, a computer-implemented method for synchronizing clocks in power grid devices is provided, the method being performed by a clock reader unit.

[0013] A clock reader unit may be a designated device for coordinating / coordinating the resynchronization process among multiple devices, which may be called a "clock follower unit" because they follow the instructions of the clock reader unit. The clock reader unit may be implemented as software and / or hardware and may be incorporated into the station clock device (i.e., the "time master" responsible for receiving and distributing the clock signal) or one of the power grid devices that communicates with the station clock.

[0014] The method performed by the clock reader unit may include receiving an indication of a new clock signal to be followed by the multiple clock follower units for use in synchronizing the clocks of the multiple clock follower units. As used herein, "following" a clock signal may refer to synchronizing an internal clock (e.g., an internal oscillator) of a device with the clock signal in a sustained, continuous manner so that the followed clock signal governs the time operations of the device.

[0015] The clock signal provided by the time master may be derived from an external clock signal source, such as using a GNSS receiver. For example, the time master may be a station clock in an electrical substation with an IRIG Time Code B (IRIG-B) timing board installed therein. The time master may provide clock signals to devices using a pulse-per-second (PPS) signal, a precision time protocol (PTP) signal, IRIG-B, or another suitable time synchronization signal.

[0016] The indication of the clock signal change may be received from a time master (e.g., a station clock) of the power grid system in which the power grid device is included. For example, the clock signal may be provided as a PPS signal from the time master, and / or a PTP signal (e.g., using ptp4l or other PTP implementation) may contain information about an event that may affect the PPS signal. Alternatively, the indication may be received from one of the power grid devices itself, where a clock signal change or jump is first "announced" by such device. That is, in some cases, the clock signal may be interrupted or otherwise changed without a PTP signal providing information that the change will occur.

[0017] In response to an indication that a new clock signal has been received at the clock reader unit, the method may further include sending, by said clock reader unit, instructions to a plurality of clock follower units to calibrate based on the new clock signal. The instructions may be multicast or broadcast, for example, depending on how many / which clock follower units the clock reader unit "knows" are participating in the resynchronization process.

[0018] It will be appreciated that the status of a device as a clock reader unit may be predetermined / pre-assigned or may be dynamically determined such that a clock reader unit may be elected in response to the occurrence of a new clock signal. A clock reader unit may be selected, for example, based on proximity to an external clock signal source, e.g., minimum communication delay with a time master.

[0019] As used herein, “calibrating” the clock of a power grid device functioning as a clock follower unit may refer to the process by which the device's internal clock is adjusted or otherwise aligned with a provided clock signal.

[0020] For example, the clock follower unit may include an internal clock (e.g., an oscillator) that oscillates with a period P, and kP can be determined such that kP is equal to the pulse rate, index count interval, 1 second, etc., as defined by the clock signal (e.g., provided as a PPS signal). The value of k may be changed until kP satisfies such a condition, which may include a control algorithm such as a PID control algorithm.

[0021] At some point after sending instructions to the plurality of clock follower units to calibrate based on the new clock signal, the method may further include obtaining confirmation that the plurality of clock follower units have completed the calibration. The confirmation may be obtained directly or indirectly, depending on the implementation.

[0022] In some examples, obtaining confirmation that the multiple clock follower units have completed calibration may include waiting for a delay period to expire, the delay period being configured to ensure that each of the clock follower units has time to complete calibration.

[0023] The delay period may be predetermined, for example, 30 seconds, 60 seconds, or more or less, depending on the amount of time expected for all clock follower units to complete calibration of their internal clocks. As noted above, the amount of time it takes to perform such calibration may vary from device to device, and therefore the delay period may be selected to be at least as long as the longest expected time for calibrating the internal clocks of the clock follower units.

[0024] According to such an approach, a command to calibrate based on a new clock signal may be broadcast from the clock leader unit to multiple clock follower units, so that it is not necessary to know how many clock follower units depend on the time master (providing the new clock signal) for their clock signals. Furthermore, advantageously, the clock follower units do not need to have transmitters, since only receivers are required. Therefore, the amount of communication between power grid devices during the resynchronization process can be reduced.

[0025] In some examples, obtaining confirmation that each of the clock follower units has completed calibration may include receiving an indication from each of the clock follower units that they have completed calibration.

[0026] For example, when a clock follower unit receives a command to calibrate based on a new clock signal, the clock follower unit may begin calibration substantially immediately thereafter, and then, when the clock follower unit determines that its internal clock is properly synchronized with the new clock signal, i.e., calibrated based on the new clock signal, the clock follower unit may generate an indication that it has completed calibration and provide this indication to the clock reader unit.

[0027] According to one example, the indication received from the clock follower unit to indicate a completed calibration may include an identifier of the clock follower unit sending the indication. The clock reader unit may maintain a list or register of the clock follower units responsible for their resynchronization process. Thus, verifying that multiple clock follower units have completed calibration may include determining that all of the registered clock follower units have provided an indication that they have completed calibration.

[0028] It will be appreciated that in lieu of such a register, the clock reader unit may instead determine the number N of clock follower units that depend on a particular time master (with which the clock reader unit is associated) for the clock signal. The clock reader unit may then obtain confirmation that multiple clock follower units have completed calibration by determining that N clock follower units have sent an indication that they have completed calibration.

[0029] In some examples, if the indication that a clock follower unit has completed calibration includes an identifier of said clock follower unit, data may be collected regarding the time required for the clock follower unit to calibrate its internal clock based on the new clock signal. This data may be collected for all clock follower units and may be used, for example, to inform decisions regarding which devices to elect as potential future clock leader units, the length of time of the delay period, etc.

[0030] According to the methods described herein, in response to receiving confirmation that a plurality of clock follower units have completed calibration, the clock reader unit may send instructions to the clock follower units to follow the new clock signal.

[0031] Thus, once it has been determined that all clock follower units are ready to transition to follow the new clock signal, a coordinated command is sent (e.g., broadcast, multicast, or otherwise) to all clock follower units. Thus, even if some clock follower units have long been calibrated to the new clock signal (relative to the slowest calibrating clock follower units) by the time the command to start following the new clock signal is sent, they will not transition to following the new clock signal until all clock follower units are ready to do so. Thus, this approach can ensure that no substantial differences in synchronization between power grid devices occur during the resynchronization process because this process is coordinated by the clock leader units.

[0032] Thus, the number of potential failures or errors due to lack of synchronization between power grid devices may be reduced, and therefore the "uptime" and reliability of power grid systems including such devices may be advantageously improved.

[0033] During calibration, the clock follower unit may not be tracking the clock signal at all. Alternatively, the clock follower unit may enter what is called a "holdover" mode, in which case time operations (e.g., timestamps) of the clock follower unit may be performed using the clock follower unit's internal clock while the internal clock is not tracking the clock signal.

[0034] The clock follower unit may comprise two internal clocks, such that, from the perspective of the device functioning as the clock follower unit, a method for time synchronizing a power grid device may include receiving an instruction to calibrate based on a new clock signal and performing a time operation using the first internal clock.

[0035] The first internal clock may be in the aforementioned "holdover" mode so that it does not follow any clock signal, but may be suitably configured to maintain a fairly consistent internal clock signal for a short period of time so that the first internal clock remains substantially synchronized with the old clock signal that it was following before the clock follower unit received the command to calibrate based on the new clock signal.

[0036] In some examples, in response to receiving the instruction for the new clock signal, the clock reader unit may instruct each of the clock follower units to stop following the old clock signal. Alternatively, this instruction may be implicit in the instruction to calibrate based on the new clock signal. That is, the clock follower units may be following the old clock signal with their first internal clock and, in response to receiving the instruction to calibrate based on the new clock signal, stop following the old clock signal.

[0037] In some examples, instead of entering a "holdover" mode, the first internal clock may follow the old clock signal. That is, the time master may be configured to provide two clock signals: an old clock signal and a new clock signal. When a new clock signal is detected, the time master may provide this new clock signal in addition to the old clock signal, the old clock signal being the clock signal provided to the device before the arrival of the new clock signal.

[0038] The clock follower unit may, for example, calibrate the second internal clock based on the new clock signal while the time operations of the clock follower unit are performed using the first internal clock.

[0039] As described above, calibrating the second internal clock may include adjusting the second internal clock (e.g., an oscillation or a multiplier thereof) or otherwise aligning the second internal clock with a new clock signal provided from a time master.

[0040] Once the second internal clock has been calibrated to the new clock signal, the clock follower unit may consider the calibration complete and may send an indication of this to the clock reader unit and / or may wait until it receives an instruction to follow the new clock signal.

[0041] Then, in response to receiving a command to follow the new clock signal, the clock follower unit may perform a timed operation using the second internal clock. For example, the clock follower unit may transition from performing a timed operation (e.g., a timestamp) using the first internal clock to performing a timed operation using the second internal clock.

[0042] A plurality of clock follower units may be configured to perform this method, so that when a clock reader unit transmits (e.g., broadcast, multicast, or otherwise) an instruction to follow a new clock signal, the transition to following the new clock signal (e.g., performing a timed operation using an internal clock calibrated to the new clock signal) may occur substantially simultaneously for all clock follower units of the plurality.

[0043] Thus, as described above, it can be ensured that the clock follower units do not become desynchronized with each other even when they need to resynchronize to a new clock signal provided by the time master.

[0044] It will be appreciated that the new clock signal will not always be substantially different from the old signal, at least to the extent that uncoordinated or non-simultaneous transitions to the new clock signal may risk errors or failures.

[0045] Thus, the clock reader unit may instruct the multiple clock follower units to abort the resynchronization process in response to determining that the new clock signal has a time difference with respect to the old clock signal that is less than a threshold amount. The threshold may be, for example, 1 microsecond (μs), 4 μs, 16 μs, or less or more, depending on the implementation. This determination may, in some examples, be made on a device-by-device basis for each of the clock follower units.

[0046] By aborting the resynchronization process, the clock follower units may instead be allowed to drift from the old clock signal to the new clock signal, the difference between which may be expected to be small enough (i.e., less than a threshold amount) so that the difference in drift rate between the two clock follower units is unlikely to cause problems due to lack of mutual synchronization.

[0047] According to some example implementations, instructing the clock follower units to follow the new clock signal may include determining a transition time for the clock follower units to transition to follow the new clock signal, and communicating the transition time to the multiple clock follower units so that the clock follower units simultaneously follow the new clock signal at the transition time.

[0048] Thus, for example, time differences caused by the length of time it takes for an instruction to follow the new clock signal to reach different clock follower units may be taken into account. That is, the transition time may be selected far enough into the future to ensure that all clock follower units have received an instruction to follow the new clock signal. It will be appreciated that the transition time may be selected so that it is not so far into the future that there is a risk that the internal clocks in "holdover" mode will become substantially out of sync with one another.

[0049] The transition time may be communicated with respect to a PPS clock signal (e.g., a message equivalent to "transition at the next pulse of the PPS signal") or with respect to a PTP clock signal, which may be in universal coordinated time (UTC) format (e.g., a message equivalent to "transition at 12:00:00 UTC").

[0050] It will be appreciated that communicating the transition time in the command to follow the new clock signal may be preferable in implementations having many (eg, more than 20) clock follower units.

[0051] The above-described methods may be performed by a data processing device, such as a processor, which may be included in one or more power grid devices, each having a clock. The methods may be embodied as a set of instructions for execution on a computer or similar data processing device, or may be stored as a computer program or computer-readable medium, such that a computer executing the instructions may perform the methods.

[0052] Furthermore, the same principles described between devices can also be applied within a device, between different modules of the device.

[0053] In any event, many advantages, some of which are described above, may be realized through the coordination of devices functioning as clock follower units with devices functioning as clock reader units during the resynchronization process. These advantages, as well as others, may be further understood through the description of specific illustrated embodiments, all of which are within the scope of this disclosure.

[0054] BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments will now be described, by way of example only, with reference to the following figures: [Brief explanation of the drawings]

[0055] [Figure 1] FIG. 1 is a schematic diagram of a power grid system according to one embodiment of the present disclosure. [Figure 2A] 1 is a schematic diagram of an alternative configuration of multiple power grid devices according to an embodiment of the present disclosure. [Figure 2B] 1 is a schematic diagram of an alternative configuration of multiple power grid devices according to an embodiment of the present disclosure. [Figure 3] FIG. 1 illustrates a method for synchronizing clocks of power grid devices for execution by a clock reader unit. [Figure 4] FIG. 1 illustrates a method for time synchronizing power grid devices for execution by a clock follower unit. [Figure 5A] FIG. 1 is a schematic diagram of time synchronization of power grid devices. [Figure 5B] FIG. 1 is a schematic diagram of time synchronization of power grid devices. [Figure 5C] FIG. 1 is a schematic diagram of time synchronization of power grid devices. [Figure 5D] FIG. 1 is a schematic diagram of time synchronization of power grid devices. [Figure 5E] FIG. 1 is a schematic diagram of time synchronization of power grid devices. [Figure 5F] FIG. 1 is a schematic diagram of time synchronization of power grid devices. [Figure 5G] FIG. 1 is a schematic diagram of time synchronization of power grid devices. [Figure 5H] FIG. 1 is a schematic diagram of time synchronization of power grid devices. DETAILED DESCRIPTION OF THE INVENTION

[0056] Detailed Description The present disclosure is described below by means of several illustrative examples, which it will be understood are provided for purposes of illustration and description only and are not intended to limit the scope of the disclosure.

[0057] Furthermore, while the examples may be presented in the form of individual embodiments, it will be recognized that the present disclosure also encompasses combinations of the embodiments described herein.

[0058] 1 schematically illustrates a power grid system 100 that includes a power grid 102 and multiple power grid devices 104a, 104b, 104c (collectively referred to as power grid devices 104 or simply "devices 104") that collectively implement the power grid 102. The power grid 102 may include, for example, a generation network for generating electricity and a transmission network for transmitting the generated electricity to electrical loads.

[0059] The power grid devices 104 may be located in any portion of the power grid 102. For example, the devices 104 may form part of a substation or a transmission substation. The devices 104 may be configured to perform time operations such as time-stamping measurements (e.g., measurements of current, voltage, phase angle, etc.), contributing to converter control, or other such time-sensitive operations.

[0060] The devices 104 may each include an internal clock 106. The clock 106 may include an oscillator, such as a piezoelectric crystal or ceramic resonator or similar hardware and / or software clock. The clock 106 within the device 104 may be used to perform timed operations, such that the rate or timing at which the timed operations are performed by the device 104 is governed by the clock 106.

[0061] Some power grid devices 104 may act as time masters, meaning that they are treated as the authority among devices 104 as to when their clocks 106 should follow. Devices 104 acting as time masters may communicate with an external clock signal source, such as a GNSS signal.

[0062] Power grid devices, for example 104a and 104b, which may not be in direct communication with an external clock signal, may have the clock signal provided to them from another device, for example 104c, acting as a time master.

[0063] Thus, the clocks 106 of all devices 104 may all be synchronized with each other, thereby allowing time operations performed by said devices 104 to be consistent in time.

[0064] However, from time to time, the clock signal provided by time master device 104c may change, such as jumping forward or backward, which may be caused by a loss of communication with the external clock signal source or some other reason. In such cases, a new clock signal needs to be distributed from time master device 104c to the other devices 104a and 104b so that they can resynchronize their respective clocks 106 based on the new clock signal.

[0065] During such a resynchronization process, some devices, e.g., device 104a, may synchronize their clocks 106 with the new clock signal faster than other devices, e.g., device 104b, and therefore risk performing time operations using clocks 106 that are not synchronized with each other. This can lead to errors or failures or their false detection.

[0066] Thus, this specification describes a scheme for assigning devices 104 as being either "clock readers" (effectively time shift managers for a group of devices) or "clock followers" (effectively time consumers that follow the instructions of a clock reader unit (or "clock reader")).

[0067] 2A and 2B illustrate several alternative configurations of multiple power grid devices 202 , 204 , 206 in an exemplary power grid subsystem of a power substation 200 .

[0068] Substation 200 may include a station clock 202 that acts as a time master for substation 200. Station clock 202 may receive a clock signal C1 from an external clock signal source (not shown), such as a GNSS satellite. Station clock 202 may then distribute this clock signal C1 among multiple devices 206A, 206B, 206C...206N, collectively referred to as "devices 206."

[0069] Device 206 may be a computing device having a CPU and I / O boards and may be configured to perform control or monitoring operations for one or more converters in substation 200, for example.

[0070] 1, devices 206 may each include an internal clock (not shown) that tracks clock signal C1 provided to them by station clock 202. That is, the internal clocks of devices 206 may periodically or on-going synchronize their internal clocks according to clock signal C1 provided by station clock 202, which may be provided as a pulse-per-second (PPS) signal, a precision time protocol (PTP) signal, or another type of clock signal.

[0071] 3 may be described in conjunction with the description of FIGS. 2A and 2B for ease of understanding. The station clock 202, or a designated or elected (to be) clock reader unit 204, may monitor 302 for changes in the clock signal C1. In some examples, one of the devices 206 may detect an unstable or variable clock signal C1 and may signal such a change to the responsible monitor, i.e., clock reader unit 204.

[0072] In some examples, a change in clock signal C1 may be signaled in an incoming PTP signal or may be initially noticed by one of devices 206. A change in clock signal C1 may have many causes and may include, for example, jumping forward or backward in time to correct alignment between station clock 202 and an external clock signal source.

[0073] At step 304 of method 300, a new clock signal (e.g., C2, not shown) may be detected (step 304:Y), which may trigger the election of a clock reader unit 204 if it has not already been pre-selected or if a dedicated unit such as the extended station clock 203 shown in FIG. 2A is provided.

[0074] 2B, one of the devices 206, namely device 206D, may be elected as the clock reader unit 204. This election may then be temporary or permanent and may be based on one or more of proximity to the station clock 202 (e.g., in terms of communication latency), central location among a group of devices 206 (e.g., for equal latency in communicating therewith), or some other factor.

[0075] Devices 206 that are not elected as clock reader units 204 may be referred to as clock follower units 206. That is, in Figure 2A, all of the devices 206 are clock follower units 206 because the clock reader units 204 are separate components incorporated into the extended station clock 203. However, in Figure 2B, device 206D is elected as clock reader unit 204, such that the remaining devices 206A, 206B, 206C...206N are clock follower units.

[0076] Continuing in method 300, after a new clock signal is detected (Y in step 304), the clock reader unit 204 may send 306 instructions to the multiple clock follower units 206 to calibrate based on the new clock signal.

[0077] The clock reader unit 204 may then determine whether confirmation has been obtained regarding whether the multiple clock follower units 206 have completed calibration, see step 308 of the illustrated method 300.

[0078] Obtaining confirmation that the multiple clock follower units have completed calibration 308 includes starting a timer T E until the time when the clock signal is received from all N clock follower units 206 that depend on the station clock 202, i.e., all clock follower units 206 that the clock reader unit 204 is responsible for during the resynchronization process. CF_1→NThe method may include waiting 310 until the received signal is received.

[0079] Once confirmation is obtained (step 308:Y), the clock reader unit 204 may send 312 instructions to the multiple clock follower units 206 to follow the new clock signal.

[0080] The clock follower units 206 may transition to follow the new clock signal immediately upon receiving the command from the clock reader unit, or the command from the clock reader unit may further include information regarding the transition time at which the transition should occur. In the former case, the device 206D may be selected so that the latency in communication between the device 206D and the other devices 206 is substantially uniform for all devices 206, and thus this device 206D may be elected as the clock reader unit 204 so that the clock follower units 206 receive commands from them substantially simultaneously.

[0081] Figures 4 and 5A-5H illustrate an exemplary implementation of the aforementioned method 300 from the perspective of a clock follower unit 506. That is, Figure 4 illustrates a method 400 for execution by the clock follower unit 506 for synchronizing clocks 508A, 508B.

[0082] In the illustrated arrangement 500 of FIG. 5A, the arrangement 500 comprises a clock follower unit 506 having a first internal clock 508A and a second internal clock 508B which may actually be separately operable parts of the same clock.

[0083] The clock follower unit 506 may be in data communication with the station clock 502 (similar in function to the station clock 202 described above) and the clock reader unit 504 (similar in function to the clock reader unit 204 described above). The clock follower unit 506 may be connected to the station clock 502 and the clock reader unit 504 via any suitable wired or wireless data connection and using any suitable data communication protocol or language.

[0084] Clock follower unit 506 may be part of a device responsible for time-stamping record 510, which may be, for example, a measurement of the phase angle of an AC voltage signal output or input to a converter or converter controller, or some other measurement or determination related to power grid operation. The timestamp applied to record 510 may be in UTC format or another suitable time format for use in comparing with other records having matching timestamps (e.g., derived from similar devices), for example, as part of a control or fault detection system.

[0085] 5A, the method 400 may begin with the clock follower unit 506 using a first internal clock 508A to timestamp a record 510. The first internal clock 508A may follow a first clock signal C1 (hereinafter referred to as the "old" clock signal) provided by the station clock 502 while being used for the timestamp.

[0086] At some point, as described above, a new clock signal C2 may be detected arriving at and provided by the station clock 502. In Figure 5B, the station 502 is shown notifying the clock reader unit 504 of this change. However, in an alternative arrangement 500A shown in Figure 5C, the clock follower unit 506 itself (e.g., using the first internal clock 508A) may detect the changed clock signal C2, which may appear as an unstable time, and notify the clock reader unit 504.

[0087] In response to detecting the new clock signal C2, the clock reader unit 504 may send an instruction to the clock follower unit 506 to calibrate based on the new clock signal C2. The clock follower unit 506 may monitor for such an instruction, as shown in step 404 of the method illustrated in FIG.

[0088] In response to receiving the instruction to calibrate (step 404:Y), clock follower unit 506 may stop following the old clock signal C1 and begin calibrating based on the new clock signal C2, as shown in Figure 5D. In some examples, clock reader unit 504 may explicitly instruct clock follower unit 506 to stop following the old clock signal C1, while in other examples this may instead be implicit.

[0089] The first internal clock 508A may then be disconnected from the clock signal provided by the station clock 502 and may enter what may be referred to as a "holdover" mode, whereby the first internal clock 508A continues to oscillate as before, but no longer maintains synchronization based on the input clock signal from the station clock 502.

[0090] During such a holdover mode, the first internal clock 508A may be substantially aligned with the old clock signal C1 and may remain approximately synchronized therewith due to the inherent inertia of the first internal clock 508A. Thus, for at least some period of time, different clock follower units may be substantially synchronized with each other despite being in holdover mode. However, it will be appreciated that the longer the first internal clocks 508A of the clock follower units are not tracking a clock signal, the more likely they are to become out of synchronization with each other.

[0091] 5E, the first internal clock 508A of the clock follower unit 506 may not stop following the old clock signal C1. Instead, the station clock 502 may be configured to provide two clock signals C1 and C2 such that the new clock signal C2 can be provided in parallel with the old clock signal C1, at least until the clock follower unit 506 is calibrated to the new clock signal C2.

[0092] While the second internal clock 508B is calibrating to the new clock signal C2, time operations may be performed using the first internal clock 508A, i.e., the first internal clock 508A may be used to provide timestamps for the records 510 during calibration.

[0093] Then, once the second internal clock 508B has completed calibration, meaning it is properly synchronized to the station clock 502 and is ready to follow the new clock signal C2, the clock follower unit 506 will issue an indication I that it has completed calibration, as shown in FIG. 5F. CF may be transmitted to the clock reader unit 504.

[0094] In an alternative arrangement 500C shown in FIG. 5G, the clock follower unit 506 receives an indication I that it has completed calibration. CFInstead, the clock reader unit 504 may transmit a signal T E You may wait for

[0095] Once the clock reader unit 504 receives confirmation that the clock follower unit 506 has completed calibration, the clock reader unit 504 may send an instruction to the clock follower unit to follow the new clock signal C2.

[0096] The clock follower unit 506 may monitor for an instruction to follow the new clock signal C2, as shown in step 412 of the method 400 shown in Figure 4. As shown in step 414, while the clock follower unit 506 has not yet received an instruction to follow the new clock signal (step 412:N), the clock follower unit may perform time operations using the first internal clock 508A, for example, while in holdover mode as described above.

[0097] Next, when the clock follower unit 506 receives an instruction to follow the new clock signal C2 (step 412:Y), the clock follower unit 506 transitions to performing a time operation to follow the new clock signal C2, i.e., time stamping the record 510, as shown in FIG. 5H; see step 416 of the method 400 of FIG. 4.

[0098] As shown in FIG. 5H, the clock follower unit 506 may transition from using a first internal clock 508A in holdover mode to time-stamping the records 510 using a second internal clock 508B that follows a new clock signal C2 provided by the station clock 502.

[0099] Method 400 may then repeat if further changes to clock signal C2 are detected, although it will be understood that in such repeated executions of the method, first internal clock 508A and second internal clock 508B may exchange their functions as described above.

[0100] The above description of method 400 performed by clock follower unit 506 may be performed by each clock follower unit in the plurality of clock follower units, which may all obtain their clock signals from station clock 502 and communicate with clock reader unit 504.

[0101] Because the clock reader unit 504 waits for confirmation that all clock follower units have completed calibration before instructing the clock follower units to start following the new clock signal C2, it can ensure that the timestamps of the records performed by each device functioning as a clock follower unit can remain substantially synchronized with each other even during the resynchronization process, thereby advantageously reducing the risk of a failure or its false detection.

[0102] While the present disclosure is susceptible to various modifications and alternative forms, specific embodiments have been shown and described by way of example in connection with the drawings in order to clearly explain various advantageous aspects of the disclosure. It should be understood, however, that the detailed description herein and the drawings accompanying this specification are not intended to limit the disclosure to the particular forms disclosed. Rather, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the following claims.

Claims

1. 1. A computer-implemented method for synchronizing clocks in power grid devices, the method being performed by a clock reader unit, the method comprising: receiving an indication of a new clock signal to be followed by a plurality of clock follower units for use in synchronizing the clocks of the plurality of clock follower units; sending instructions to the plurality of clock follower units to calibrate based on the new clock signal; obtaining confirmation that the plurality of clock follower units have completed calibration; in response to receiving the confirmation, sending instructions to the plurality of clock follower units to follow the new clock signal; A method comprising:

2. 2. The method of claim 1, wherein obtaining confirmation that the plurality of clock follower units have completed calibration includes waiting for a delay period to expire, the delay period being configured to ensure that each of the clock follower units has time to complete calibration.

3. The method of claim 1 , wherein obtaining confirmation that each of the clock follower units has completed calibration comprises receiving an indication from each of the clock follower units that it has completed calibration.

4. The method of claim 3 , wherein an indication received from a clock follower unit to indicate a completed calibration includes an identifier of the clock follower unit sending the indication.

5. instructing the plurality of clock follower units to stop following the old clock signal in response to receiving the indication of a new clock signal. The method of claim 1 further comprising:

6. In response to determining that the new clock signal has a time difference with respect to the old clock signal less than a threshold amount, sending a command to one or more of the plurality of clock follower units to abort a resynchronization process. The method of claim 5 further comprising:

7. Instructing the clock follower unit to follow the new clock signal comprises: determining a transition time for the clock follower unit to transition to follow the new clock signal; communicating the transition time to the plurality of clock follower units so that the clock follower units follow the new clock signal simultaneously at the transition time; The method of claim 1 , comprising:

8. The indication of the new clock signal may be: from one of the clock follower units, or From an external clock source The method of claim 1 wherein the signal is received.

9. The clock reader unit Pre-assigned clock reader status, and / or Proximity to external clock sources 10. The method of claim 1, wherein the power grid device is selected from the plurality of power grid devices based on:

10. 1. A method for time synchronizing power grid devices, comprising: receiving a command to calibrate based on a new clock signal; performing a time operation using a first internal clock; calibrating a second internal clock based on the new clock signal; receiving a command to follow the new clock signal; performing a timed operation using the second internal clock in response to receiving the command to follow the new clock signal; and A method comprising:

11. tracking an old clock signal together with the first internal clock; in response to receiving a command to calibrate based on a new clock signal, ceasing to track the old clock signal; The method of claim 10 further comprising:

12. A data processing device comprising means for carrying out the method according to any one of claims 1 to 11.

13. A computer program comprising instructions which, when executed by a computer, cause the computer to carry out a method according to any one of claims 1 to 11.

14. A computer readable medium comprising instructions which, when executed by a computer, cause the computer to perform the method of any of claims 1 to 11.

15. 1. A power grid system comprising: a plurality of power grid devices in data communication, each power grid device having at least one clock; 13. A power grid system, wherein at least one of the plurality of power grid devices comprises the data processing device of claim 12.

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