METHOD FOR SYNCHRONIZING SMART ELECTRONIC UNITS IN A LOCALLY LIMITED NETWORK

Description

[0001] To synchronize the time settings of devices connected via a network, a network time protocol was created, known as the Network Time Protocol (NTP). In addition, the Precision Time Protocol (PTP) was introduced. PTP offers greater accuracy in timekeeping for devices connected via a network compared to NTP.

[0002] A locally confined network is implemented, for example, in electrical substations. Such substations serve to step down or step up the voltages present in the supply network using a transformer, the operation of which is well understood. In addition to a transformer, substations have switching units such as circuit breakers, which, upon receiving a switching signal, disconnect the substation's conductors from the rest of the supply network.

[0003] These switching signals are generated by protection and automation devices that monitor the current and voltage waveforms in the substation conductors for fault conditions. If a fault condition is detected, a switching signal is generated and sent to one or more selected switching units, causing them to switch to their interrupted position. In the interrupted position, the contacts of the selected switching units are separated from each other, thus preventing current flow through them. A conductor connected to one contact of the switching unit is then disconnected from the rest of the supply network, which is connected to the other contact of the same switching unit.

[0004] To enable the protection and automation devices to monitor the current and voltage profiles in the substation conductors for fault conditions, they must be continuously supplied with time-dependent current and voltage values. Current and voltage transformers are used to provide these values. These transformers measure the current and voltage in the conductors at a measuring point in the substation and provide a calibrated measurement signal on the secondary side. This signal is sampled at a defined sampling rate to acquire sample values. The sample values ​​are then digitized. Furthermore, a timestamp is permanently assigned to each sample value. This is done using so-called "merging units" or with the help of other intelligent electronic devices (IEDs) in the substation. To compare the measured values, the time recording of the IEDs, which are connected via a process bus, must be synchronized.This is the purpose of the aforementioned PTP.

[0005] The PTP is therefore used in digital substations for electrical power supply to synchronize intelligent electronic devices (IEDs), for example in the so-called IEC 61850 process bus.

[0006] The comparability of measured values ​​from different IEDs is crucial. A loss of or insufficient time synchronization between these devices leads to a blockage of the protective functions in the protection and automation devices, or to a faulty tripping of a switching unit, resulting in an unintentional interruption of the power supply.

[0007] The Figures 1a and 1bThe diagram illustrates the process of time recording according to PTP within a process bus communication network 1, wherein two time units 2 and 3 are provided, which send PTP synchronization messages according to the specified structure "Announce", "Sync" and "Follow_Up" via an Ethernet switch 4 to IEDs 5 such as protection devices, which are also connected to the process bus communication network 1. Figures 1a and 1b A dashed arrow represents a PTP announce message, while a solid arrow represents a sync or follow-up message. The IEDs 5 are in the slave state defined by the PTP protocol. Therefore, they are passive with regard to time value acquisition and adopt the time specified to them by a time reference unit in the so-called (grandmaster) role.

[0008] The time units 2 and 3 can be connected to a primary reference clock (PRC), such as the Global Navigation Satellite System (GNSS). However, due to their susceptibility to jamming or other means (e.g., spoofing), such satellite-based global reference clocks pose a risk and, depending on their application, are often not permitted. In such cases, an internal oscillator integrated into one of the time units 2 and 3 can be used. This oscillator then provides a relative time reference in the form of time values ​​within the process bus communication network 1, independent of external sources.

[0009] Relative time values ​​provided by the local oscillator in the time specification units 2, 3 are sufficient for synchronization in the application of a process bus communication network 1 in a substation, since no absolute time values ​​are required.

[0010] The use of two time units serves redundancy and thus availability purposes. Under fail-safe conditions, a single time unit 2 is selected as the active grandmaster using a so-called best-master clock algorithm. This unit sends PTP synchronization messages via the Ethernet switch 4. The other time unit 3 does not send PTP synchronization messages and is in slave mode. It receives PTP synchronization messages from the grandmaster time unit 2 and synchronizes its own internal oscillator with that of time unit 2, so that the internal oscillator of time unit 3 oscillates at nearly the same speed as that of time unit 2. Time unit 3 is then ready to take over the active role.

[0011] Figure 1bThis illustrates a situation in which time unit 2 has failed or is disconnected from the network. The absence of time unit 2 is detected by time unit 3 due to the lack of PTP announce messages from time unit 2. Time unit 3 then assumes the grandmaster role.

[0012] The IEDs detect the Grandmaster's switch from time unit 2 to time unit 3 based on the PTP messages. This does not disrupt the IEDs' time synchronization, as the internal oscillator of time unit 3 was synchronized with the oscillator of time unit 2 before the switchover. Time synchronization among the IEDs continues seamlessly. There is no time jump during or after the Grandmaster switchover.

[0013] When the malfunction of time unit 2 has resolved or the disconnected time unit 2 is reconnected to the process bus communication network 1, the BMCA, in accordance with the state of the art, immediately decides that time unit 2 assumes the grandmaster role. As grandmaster time unit 2, it resumes sending PTP synchronization messages.

[0014] During the absence of Time Unit 2, the local oscillator of Time Unit 3 runs at its own speed and drifts out of sync with the local oscillator of Time Unit 2. The time and frequency difference between the local oscillators of Time Units 2 and 3 can be considerable at the time of takeover by the restored or reconnected Time Unit 2. After the takeover, the IEDs detect a time jump and resynchronize their internal oscillator with the new local oscillator of the new Grandmaster Time Unit 2. This resynchronization process can take up to 20 seconds.

[0015] The IEDs block their protective functions during this resynchronization to prevent a possible false tripping of the circuit breakers. This represents a significant disadvantage.

[0016] Figure 2The above is illustrated with the help of a two-dimensional diagram, on whose abscissa time and on whose ordinate 6 the time offset of the time unit 3 to the time unit 2 according to Figure 1 The graph shows the values ​​in arbitrary units. The distance of 20 between the dashed lines represents the permissible time offset between time units 2 and 3 for the protective applications of IEDs. The solid curve thus represents this time offset as a function of time. If the curve lies between the dashed lines, the time offset is acceptable.

[0017] In the time range referenced at 7, time unit 2 operates without errors and is connected to the process bus communication network 1. In this range 7, it functions as grandmaster time unit 2. At time 8, time unit 3 assumes the grandmaster role because PTP announce messages from time unit 2 cease, and this is detected by time unit 3. In time range 9, time unit 3 then operates as grandmaster time unit 3. At time 10, the grandmaster role reverts to time unit 2. Because the oscillators of time units 2 and 3 were no longer synchronized in time range 9, the time values ​​of time unit 3 have diverged from those of time units 2. In time range 11, this deviation exceeds the permitted limit.The IEDs detect the aforementioned time jump and resynchronize their internal oscillator until, at time 12, the IEDs are synchronized with the recurring Grandmaster time unit 2. The current method has the disadvantage that the IEDs' protection algorithms are blocked in the dashed time range 11.

[0018] The following documents represent the relevant state of the art.

[0019] YASUYUKI KOZAKAI ET AL: "Keeping clock accuracy on a master clock failure in substation network", PRECISION CLOCK SYNCHRONIZATION FOR MEASUREMENT CONTROL AND COMMUNICATION (ISPCS), 2010 INTERNATIONAL IEEE SYMPOSIUM, IEEE, PISCATAWAY, NJ, USA, September 27, 2010 (2010-09-27), pages 25-29, ISBN: 978-1-4244-5978-0. This paper proposes preventing the slave from switching to a new grandmaster clock for a waiting period when the operator installs a new grandmaster clock. The waiting period depends on the duration of the slave's control loop convergence.

[0020] NGRAM DAVID ME ET AL: "Quantitative Assessment of Fault Tolerant Precision Timing for Electricity Substations", IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, IEEE, USA, Vol. 62, No. 10, October 1, 2013 (2013-10-01), pages 2694-2703, ISSN: 0018-9456, DOI: 10.1109 / TIM.2013.2263673

[0021] DE 11 2018 007928 T5 (MITSUBISHI ELECTRIC CORP [JP]) May 27, 2021 (2021-05-27).

[0022] The object of the invention is to provide a method of the type mentioned above in which the blocking of the protection algorithms of the IEDs can be reduced in time or even completely avoided.

[0023] This problem is solved according to the invention by the features of claim 1.

[0024] Variants of this invention are the subject of the dependent patent claims.

[0025] Within the scope of the invention, it was recognized that the disadvantage of the prior art described above is caused by the fact that a grandmaster-capable PTP time unit assumes the grandmaster role as soon as it has determined that it is the best PTP time unit in the network according to the BMCA (Best Master Clock Algorithm). A grandmaster-capable time unit according to PTP is a time unit that, based on the BMCA, can assume the grandmaster role and simultaneously become the sole synchronization source, the grandmaster, in a network. The problem identified by the inventors of unsynchronized local oscillators when switching back from the grandmaster role is not taken into account in the BMCA and the PTP port state machine. This leads to the described time jump and the blocking of the protection functions (protection algorithms).

[0026] The present invention proposes that a restored or reconnected, locally synchronized time unit capable of grandmaster functionality does not immediately assume the grandmaster role, but instead undergoes the following steps: First, it checks whether there is currently an active PTP grandmaster in its own PTP domain, which is defined by its domain number. This check is performed by waiting to receive a PTP announcement message. If the time unit capable of grandmaster functionality does not receive a PTP announcement message from another time unit within a previously defined timeframe, it is assumed that no active grandmaster exists in the domain. If no other grandmaster is detected on the network, the time unit executes the BMCA (Building Management Computing Action). It then switches to the grandmaster role.

[0027] However, if a Grandmaster time unit is detected on the network, the time unit switches to its slave state and begins to synchronize its own internal oscillator with that of the current Grandmaster.

[0028] Once the time unit has synchronized its own internal oscillator with the required accuracy to the grandmaster, the enforcement of its slave state is lifted. The BMCA is executed, and depending on the result of this check, the time unit will either switch to the grandmaster role or remain in the slave state.

[0029] The crucial point is that the restored or reconnected grandmaster-capable time unit first synchronizes its own internal oscillator with that of the current PTP grandmaster time unit, if present, before it assumes the grandmaster role. This prevents a time jump.

[0030] A local area network (LAN), as defined in the invention, is a spatially limited network. For example, a local area network is defined in the IEC standard IEC 61850. Advantageously, the local area network features structured cabling. In a preferred embodiment, the network is a process bus communication network. The spatially limited network includes intelligent electrical devices (IEDs) that communicate, for example, via wired communication lines or wirelessly. . 5G wireless networks are interconnected. However, the use of Ethernet technology is preferred within the scope of the invention. In principle, other locally limited networks are also possible within the scope of the invention.

[0031] In the context of this invention, the abbreviation IED refers to an intelligent electronic device. An IED is, for example, a protection or automation device, a relay, or a field control device, used, for instance, in the field of protection and control technology in substations. An IED is often also referred to as a processor-based controller.

[0032] Advantageously, the invention utilizes the PTP, whereby the slave state is enforced by setting the priority of said time unit. If the priority of a grandmaster-capable time unit is set to, for example, 254 or 255, it will remain in its slave state during a check by the BMCA.

[0033] Advantageously, the test to determine which time unit is better suited for the active Grandmaster role is carried out according to the Best Master Clock algorithm of the IEEE 1588 protocol.

[0034] Further advantages arise if each time input unit has its own oscillator. Such an oscillator, for example, has a quartz crystal whose oscillations are converted into a time standard. Oscillators are well known to those skilled in the art, so their precise operation need not be discussed in detail here. However, the oscillator used in a time input unit can be synchronized with another oscillator within the scope of the invention.

[0035] According to another practical option, the locally limited network is a wireless network. Advantages arise particularly if the wireless network is a 5G network.

[0036] According to a preferred embodiment of the invention, the locally limited network is a process bus communication network of a substation. Ethernet technology is preferably used in the process bus communication network.

[0037] Within the scope of the invention, the number of time units is not limited to two. Thus, for example, five or more time units can communicate with each other in the network.

[0038] It is advantageous to have at least one time-defining unit integrated into an IED, or in other words, built in. It is therefore a component of the IED and located within its housing.

[0039] Further advantages arise if at least one IED is a protection or automation device of an electrical power supply network.

[0040] Another variant of the method according to the invention consists in the fact that the grandmaster-capable time unit, once available or reconnected to the network, does not immediately resume the grandmaster role after synchronizing with the current grandmaster. Instead, it remains in slave mode and continues to synchronize with the current grandmaster time unit. Only when the absence of a grandmaster time unit is detected does the time unit switch to the grandmaster role due to the BMCA (Borderless Control Alert). This reduces the number of grandmaster switching operations in the network, which is advantageous with regard to the stability of the PTP (Point-to-Point) synchronization.

[0041] The present invention further relates to a time-display unit for a locally limited network through which intelligent electronic devices (IEDs) are interconnected. According to the invention, at least this time-display unit is configured to carry out one of the methods mentioned above.

[0042] For the purposes of this invention, a time reference unit is understood to be any unit capable of generating a time standard. This unit may be a separate device or a component or part of another device, for example, a protective or automation device.

[0043] The invention is explained in more detail below with reference to exemplary embodiments, wherein the same reference numerals refer to identically functioning components and wherein Figure 1a shows an embodiment of a process bus communication network with two functioning time units, Figure 1b shows an embodiment of a process bus communication network with one failed or disconnected time unit, Figure 2 shows a two-dimensional diagram illustrating a method for assuming the grandmaster role according to the prior art, Figure 3 shows a two-dimensional diagram illustrating a method for assuming the grandmaster role according to the invention, and Figure 4 schematically illustrates an embodiment of the method according to the invention using a flowchart.

[0044] On the Figures 1a, 1b and Figure 2 This has already been addressed in connection with the assessment of the state of the art.

[0045] Figure 3 Figure 1 illustrates an embodiment of the method according to the invention. It schematically shows the course of the waveform in a local network according to the inventive diagram. Figure 1a specified time reference to which the IEDs of network 1 synchronize, using a two-dimensional diagram, on whose abscissa the time and on whose ordinate 6 the time offset of the time reference unit 3 to the time reference unit 2 according to Figure 1 is represented in arbitrary units.

[0046] The distance of 20 between the dashed lines again represents the permissible time offset between time units 2 and 3, as permitted by the protective applications of the IEDs. The solid curve thus depicts the time offset between the time units as a function of time. If the curve lies between the dashed lines, the time offset is acceptable.

[0047] In the time range referenced at 13, time unit 2 operates without errors and is connected to the process bus communication network 1. In this range 13, it functions as grandmaster time unit 2. At time 14, time unit 3 takes over the grandmaster role. . During time interval 15, the time display unit 3 then operates as Grandmaster time display unit 3. At time 16, the time display unit 2 has regained its full functionality. In another embodiment, it was disconnected from the network and reconnected to network 1 at time 16.

[0048] Unlike the state of the art, it does not immediately resume the grandmaster role, but instead synchronizes itself, or in other words, its oscillator, with that of grandmaster time unit 3. This synchronization is achieved at time 17. Using the BMCA, it is determined that time unit 2 is better suited for the grandmaster role. The grandmaster role switches at time 17. In time 18, time unit 2 again fills the grandmaster role.

[0049] Within the scope of the invention, the time delay of the IEDs never exceeds the permissible limits. Blocking of the IEDs' protection algorithms is avoided within the scope of the invention.

[0050] To avoid immediately assuming the grandmaster role within the scope of the invention, i.e., to force the slave state of the respective time unit, this embodiment of the invention, which is based on PTP, uses so-called priorities. It should be noted, however, that forcing the slave state can also be achieved by other means without departing from the scope of the invention.

[0051] Priorities are parameters defined in the IEEE 1588 standard, which are evaluated by the BMCA. Accordingly, each time unit 2 can, in principle, be assigned priority 1 and priority 2. After the failure of time unit 2, its priority 1 is limited to 255 and 254. The priority value for time unit 3 remains fully configurable by the user. Priority 2 therefore remains at the value previously preset by the network user.

[0052] Under normal, undisturbed conditions, the user-configured priority parameter is used. At time 16, the priority 1 value of time unit 2 is set to the highest possible value of 255, which corresponds to the lowest priority for assuming the grandmaster role. Therefore, during the execution of the BMCA, time unit 2 reliably remains in its slave state.

[0053] At time point 17, the internal oscillator of time unit 2 is synchronized with the required accuracy to the current PTP grandmaster, namely the oscillator of time unit 3. At time point 17, the priority 1 value of time unit 2 is reset to its previously user-configured value. The enforcement of the slave state is therefore lifted. Time unit 2 resumes its grandmaster role by executing the BMCA.

[0054] Figure 4The inventive method is illustrated by a flowchart. For reasons of space, the following is shown in the Figure 4 Instead of a time unit, the Grandmaster role refers to a master clock.

[0055] When a time unit – for example, time unit 2 – becomes functional again or reconnected to the network, the first step is to determine whether there is a time unit in the network that holds the grandmaster role, i.e., is a master clock. If no master clock is present, time unit 2 reverts to its normal operation. A BMCA (Building Management Computing Agent) will then determine that time unit 2 should assume the grandmaster role. This change is then implemented.

[0056] However, if a master clock is detected on the network, the priority 1 of time unit 2 is set to 255. This forces time unit 2 into slave mode. A loop then searches for the master clock, and time unit 2 synchronizes with it, in this case, time unit 3. . This loop continues as long as the time offset between the master clock and time unit 2 is greater than the predefined limit and the master clock is still present. Afterwards, the priority 1 value is reset to its preconfigured value.

Claims

1. Method for synchronizing intelligent electronic units, IEDs, (5) in a locally restricted network (1), wherein the locally restricted network (1) has at least two timing units (2, 3), in which - for a first timing unit (2) which is functional again or is connected to the network again, a check is carried out in order to determine whether the network contains a further timing unit (3) which assumes an active grandmaster role (1), - the first timing unit (2) is forced into its slave state if another timing unit in the grandmaster role was detected in the locally restricted network (1), wherein the first time unit (2) synchronizes with the timing unit (3) in the grandmaster role, - after synchronization, the enforcement of the slave state is cancelled for the first timing unit (2), - the first timing unit (2) changes to the role of the grandmaster timing unit if it is better suited to the grandmaster role, or remains in its slave state if another timing unit (3) in the network (1) is better suited to the grandmaster role.

2. Method according to Claim 1, characterized in that the slave state is forced using the PTP by setting the priority of said timing unit (2).

3. Method according to Claim 1 or 2, characterized in that the check as to which timing unit (2, 3) is better suited to the active grandmaster role is carried out according to the best master clock algorithm of the IEEE 1588 protocol, version 2008.

4. Method according to Claim 3, characterized in that each timing unit (2, 3) in the network (1) has an oscillator.

5. Method according to one of the preceding claims, characterized in that the locally restricted network (1) is a domain.

6. Method according to one of the preceding claims, characterized in that the locally restricted network is a process bus communication network (1) of a substation.

7. Method according to one of the preceding claims, characterized in that at least one IED (5) is a protection or automation device.

8. Method according to one of the preceding claims, characterized in that at least one timing unit is installed in an IED.

9. Method according to one of the preceding claims, characterized in that a timing unit with grandmaster capability that is available again or is connected to the network again remains in the slave state and synchronizes continuously with the current grandmaster timing unit and changes to the grandmaster role only when the grandmaster timing unit is removed.

10. Timing unit (2, 3) for a locally restricted network (1), via which intelligent electronic units IEDs (5) are connected to one another, characterized in that it is configured to carry out a method according to one of the preceding claims.