PROCEDURE FOR SYNCHRONIZING INTELLIGENT ELECTRONIC UNITS IN A LOCALLY LIMITED NETWORK

Abstract

The invention relates to a method for synchronizing intelligent electronic devices (IEDs) (5) in a locally confined network (1), wherein the locally confined network (1) has at least two time units (2, 3), wherein: - for a functioning or reconnected time unit (2), it is checked whether there is another time unit (3) in the network that assumes an active grandmaster role (1), - the first time unit (2) is forced into its slave state if another time unit in the grandmaster role has been detected in the locally confined network (1), wherein the first time unit (2) synchronizes with the time unit (3) in the grandmaster role, - after synchronization, the enforcement of the slave state for the first time unit (2) is lifted, - the first time unit (2) switches to the role of the grandmaster time unit.if it is better suited for the grandmaster role, or remains in its slave state if another time unit (3) in the network (1) is better suited for the grandmaster role.

Description

[0001] To synchronize the time settings of devices connected via a network, a network time protocol was developed, called Network Time Protocol (NTP). Furthermore, the Precision Time Protocol (PTP) was introduced. Compared to NTP, PTP provides greater accuracy in recording the time of devices connected via a network.

[0002] A locally limited network is implemented, for example, in electrical power substations. Such substations serve to reduce or increase the voltages prevailing in the supply network using a transformer whose operating principle is well known. In addition to a transformer, substations have switching units such as circuit breakers, which, upon receiving a switching signal, isolate the substation's conductor feeders 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's conductors for the presence of fault conditions. If a fault condition exists, a switching signal is generated and sent to one or more selected switching units, causing the switching units to be switched to their breaker position. In the breaker position, the contacts of the selected switching units are separated from each other, preventing current flow through the switching units. A conductor strand 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 switching unit.

[0004] In order for the protection and automation devices to monitor the current and voltage curves in the substation conductors for the presence of fault conditions, they must be continuously supplied with time-dependent current and voltage values. Current and voltage transformers are provided to provide these current and voltage 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, which is sampled at a specified sampling rate to obtain sample values. The sample values ​​are then digitized. Furthermore, a time stamp is permanently assigned to the sample values. This is done using so-called "merging units" or with the help of other intelligent electronic devices (IEDs) in the substation. To be able to compare the measured values, the time recording of the IEDs, which are connected to each other via a process bus, must be synchronized.This is what the PTP is for.

[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. Lost or insufficient temporal synchronization between these devices leads to blocking of the protection functions in the protection and automation devices or to the erroneous tripping of a switching unit, resulting in an unintentional interruption of the power supply.

[0007] The Figures 1a and 1billustrate the process of time recording according to PTP within a process bus communication network 1, where two time indication 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 that are also connected to the process bus communication network 1. In the Figures 1a and 1b A dashed arrow corresponds to a PTP Announce message, while a solid arrow represents a sync or follow-up message. The IEDs 5 are in the slave state specified by the PTP protocol. They therefore behave passively with regard to time acquisition and adopt the time specified by a time-indicating unit in the so-called grandmaster role.

[0008] The time indication units 2, 3 can be connected to a primary reference clock source – the so-called Primary Reference Clock (PRC) – e.g., the Global Navigation Satellite System (GNSS). However, due to their susceptibility to interference by jamming or other means (e.g., spoofing), such satellite-based global reference clock sources pose a risk and, depending on their application, are often not permitted. In such a case, an internal oscillator can be used, which is built into one of the time indication units 2, 3. These then specify a relative time reference in the form of time values ​​in the process bus communication network 1 independently of the outside.

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

[0010] The use of two time units serves redundancy and availability purposes. Under non-failure conditions, a single time unit 2 is elected as the active grandmaster using a so-called best master clock algorithm. This unit sends PTP synchronization messages via Ethernet switch 4. The other time unit 3 does not send PTP synchronization messages and is in the slave state. It receives PTP synchronization messages from the grandmaster time unit 2 and synchronizes its own internal oscillator to the oscillator of time unit 2, so that the internal oscillator of time unit 3 oscillates at almost the same speed as the internal oscillator of time unit 2. However, time unit 3 is ready to transition to an active role.

[0011] Figure 1billustrates a situation in which Time Unit 2 has failed or is disconnected from the network. Time Unit 3 detects the absence of Time Unit 2 due to the lack of PTP Announce messages from Time Unit 2. Time Unit 3 switches to the grandmaster role.

[0012] The IEDs detect the grandmaster's transition 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 transition. Time synchronization among the IEDs continues smoothly. There is no time jump during or after the grandmaster transition.

[0013] When the malfunction of Time Unit 2 is resolved or the disconnected Time Unit 2 is reconnected to the process bus communication network 1, the BMCA, according to the state of the art, immediately decides that Time Unit 2 assumes the grandmaster role. As Grandmaster Time Unit 2, it begins sending PTP synchronization messages again.

[0014] During the absence of Time Unit 2, the local oscillator of Time Unit 3 runs at its own speed and moves away in time from the local oscillator of Time Unit 2. The time and frequency difference between the local oscillators of Time Units 2 and 3 can be significant at the time of takeover by the restored or connected 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 protection functions during this resynchronization to prevent possible nuisance tripping of the circuit breakers. This represents a significant disadvantage.

[0016] Figure 2illustrates the above-described with the help of a two-dimensional diagram, on whose abscissa the time and on whose ordinate 6 the time offset of the time indication unit 3 to the time indication unit 2 according to Figure 1 is shown, each in arbitrary units. The distance 20 between the dashed lines represents the permissible time offset between time units 2 and 3 for the IED's protection applications. The solid curve thus represents the said time offset as a function of time. If the curve lies between the dashed lines, it is an acceptable time offset.

[0017] In the time range referenced by 7, time unit 2 is operating error-free and is connected to the process bus communication network 1. In this range 7, it operates as grandmaster time unit 2. At time 8, the grandmaster role is taken over by time unit 3 because PTP announce messages from time unit 2 are missing 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 is switched back 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 deviated from those of time units 2. In time range 11, this deviation is greater than permitted.The IEDs detect the time jump and resynchronize their internal oscillator until, at time 12, the IEDs are synchronized with the recurring grandmaster time unit 2. The previous method has the disadvantage that the IEDs' protection algorithms are blocked in the dashed time range 11.

[0018] The object of the invention is to provide a method of the type mentioned above in which blocking of the protection algorithms of the IEDs can be reduced in time or even completely avoided.

[0019] This object is achieved according to the invention by the features of patent claim 1.

[0020] Variants of this invention are the subject of the dependent patent claims.

[0021] Within the scope of the invention, it was recognized that the disadvantage of the prior art described above is caused by a grandmaster-capable PTP time indication unit assuming the grandmaster role as soon as it has determined that it is the best PTP time indication unit in the network according to the BMCA (Best Master Clock Algorithm). A grandmaster-capable PTP time indication unit is a time indication unit that, thanks to 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 non-mutually synchronized local oscillators when switching back to 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 to the blocking of the protection functions (protection algorithms).

[0022] The present invention proposes that a restored or reconnected grandmaster-capable locally synchronized time indication unit does not immediately assume the grandmaster role, but first performs the following steps: First, a check is made to determine whether there is currently an active PTP grandmaster in its own PTP domain, which is determined by its domain number. This check is performed by waiting for receipt of a PTP Announce message. If the grandmaster-capable time indication unit does not receive a PTP Announce message from another time indication unit within a previously specified time frame, it is assumed that no active grandmaster exists in the domain. If no other grandmaster is detected in the network, the time indication unit executes the BMCA. It switches to the grandmaster role.

[0023] However, if a grandmaster timing unit is detected on the network, the timing unit enters its slave state and begins to synchronize its own internal oscillator with that of the current grandmaster.

[0024] After the timing unit has synchronized its own internal oscillator with the grandmaster with the required accuracy, its slave state is unenforced. The BMCA is executed, and depending on the result of this check, the timing unit in question will either switch to the grandmaster role or remain in the slave state.

[0025] The key point, therefore, is that the restored or reconnected grandmaster-capable timing unit first synchronizes its own internal oscillator with that of the current PTP grandmaster timing unit, if one exists, before the latter assumes the grandmaster role. This avoids a time jump.

[0026] A locally limited network, referred to in English as a "local area network" and abbreviated as "LAN," is understood within the meaning of the invention to be a spatially limited network. The locally limited network is, for example, a network defined in the IEC standard IEC 61850. Advantageously, the locally limited network has structured cabling. According to a preferred variant, the network is a process bus communication network. The spatially limited network comprises intelligent electrical devices (IEDs) that are interconnected, for example, via wired communication lines or radio, e.g., 5G radio networks. However, the use of Ethernet technology is preferred within the scope of the invention. In principle, however, other locally limited networks are also possible within the scope of the invention.

[0027] For the purposes 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 bay control device used, for example, in the field of protection and control technology in substations. An IED is often also referred to as a processor-based controller.

[0028] Advantageously, PTP is used within the scope of the invention, 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.

[0029] Advantageously, the test to determine which time unit is more suitable for the active grandmaster role is carried out according to the Best Maser Clock algorithm of the IEE 1588 protocol.

[0030] Further advantages arise when 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 known to those skilled in the art, so their exact functioning need not be discussed further here. However, the oscillator used in a time input unit can be synchronized with another oscillator within the scope of the invention.

[0031] According to another practical variant, the locally limited network is a radio network. This is particularly advantageous if the radio network is a 5G radio network.

[0032] 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.

[0033] Within the scope of the invention, the number of time-indicating units is not limited to two. Thus, for example, five or more time-indicating units can communicate with each other in the network.

[0034] Conveniently, at least one time display unit is integrated into an IED, or in other words, built into it. It is therefore a component of the IED and located in its housing.

[0035] Further advantages arise if at least one IED is a protection or automation device of an electrical power system.

[0036] A further variant of the method according to the invention consists in the fact that the grandmaster-capable time unit that is once again available or reconnected to the network does not immediately reassume the grandmaster role after synchronizing with the current grandmaster. Instead, it remains in the slave state and continuously synchronizes with the current grandmaster time unit. Only when the absence of a grandmaster time unit is detected does said time unit switch to the grandmaster role due to the BMCA. This reduces the number of grandmaster switchovers in the network. This is advantageous with regard to the stability of PTP synchronization.

[0037] The present invention further relates to a time indication unit for a locally limited network via which intelligent electronic devices (IEDs) are interconnected. According to the invention, at least this time indication unit is configured to carry out one of the above-mentioned methods.

[0038] For the purposes of the invention, a time indication unit is any unit capable of generating a time standard. This unit can be a separate device or a component or part of another device, for example, a protection or automation device.

[0039] The invention is explained in more detail below using exemplary embodiments, wherein the same reference numerals refer to components with the same effect and wherein Figure 1a shows an embodiment of a process bus communication network with two functional time display units, Figure 1b shows an embodiment of a process bus communication network with a failed time display unit or one that is disconnected from the network, Figure 2 shows a two-dimensional diagram to illustrate a method for assuming the grandmaster role according to the prior art, Figure 3 shows a two-dimensional diagram to illustrate 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 flow chart.

[0040] On the Figures 1a, 1b and Figure 2 has already been discussed in connection with the assessment of the state of the art.

[0041] Figure 3 illustrates an embodiment of the method according to the invention. It shows schematically the course of the data in a local network according to Figure 1specified time to which the IEDs of network 1 synchronise, using a two-dimensional diagram, on the abscissa of which the time and on the ordinate 6 the time offset of the time indication unit 3 to the time indication unit 2 according to Figure 1 is shown, each in arbitrary units.

[0042] The distance 20 between the dashed lines represents the time offset allowed by the IED's protection applications between time units 2 and 3. The solid curve thus represents the time offset between the time units as a function of time. If the curve lies between the dashed lines, it is an acceptable time offset.

[0043] In the time range referenced by 13, time unit 2 operates error-free and is connected to the process bus communication network 1. It operates in this range 13 as grandmaster time unit 2. At time 14, time unit 3 assumes the grandmaster role. In time range 15, time unit 3 then operates as grandmaster time unit 3. At time 16, time unit 2 has regained full functionality. In another embodiment, it was disconnected from the network and reconnected to network 1 at time 16.

[0044] In contrast to the prior art, it does not immediately reassume the grandmaster role, but rather synchronizes itself, or in other words, its oscillator, with that of grandmaster timing unit 3. This synchronization is achieved at time 17. Using the BMCA, it is determined that timing unit 2 is better suited for the grandmaster role. The grandmaster role is switched at time 17. In time range 18, timing unit 2 again assumes the grandmaster role.

[0045] Within the scope of the invention, the permissible limits regarding the IED time offset are never exceeded. Blocking of the IEDs' protection algorithms is avoided within the scope of the invention.

[0046] To avoid the immediate assumption of the grandmaster role within the scope of the invention—in other words, to force the respective time-indicating unit into slave mode—this embodiment of the invention, which is based on PTP, uses so-called priorities. However, it should be noted that forcing the slave mode can also be accomplished by other means without departing from the scope of the invention.

[0047] Priorities are parameters defined in the IEEE 1588 standard that are evaluated by the BMCA. Accordingly, each time unit 2 can generally 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 of time unit 3 remains fully user-configurable. Priority 2 therefore remains at the value previously preset by the network user.

[0048] Under normal, fault-free 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, when the BMCA is executed, Time Unit 2 reliably remains in its slave state.

[0049] At time 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 17, the priority 1 value of time unit 2 is reset to its previously user-set value. The slave state is therefore no longer enforced. Time unit 2 reassumes the grandmaster role by executing the BMCA.

[0050] Figure 4illustrates the method according to the invention by means of a flow chart. For reasons of space, Figure 4 Instead of a time unit, the Grandmaster role refers to a master clock.

[0051] If a time unit—for example, time unit 2—becomes functional again or reconnects to the network, the first step is to determine whether the network has a time unit in the grandmaster role, i.e., a master clock. If no master clock is present, time unit 2 returns to normal operation. A BMCA will determine that time unit 2 should assume the grandmaster role. This is then implemented.

[0052] However, if it is detected that a master clock is present in the network, the priority 1 of time unit 2 is set to 255. This forces time unit 2 into slave state. A loop is then run to search for the master clock, and time unit 2 synchronizes with the master clock, in this case, time unit 3. This loop is run as long as the time offset between the master clock and time unit 2 is greater than the specified limit and the master clock is still present. The priority 1 value is then reset to its preconfigured value.

Claims

1. A method for synchronizing intelligent electronic units (IEDs) (5) in a locally limited network (1), wherein the locally limited network (1) has at least two time units (2, 3), in which - for a time unit (2) that is functional again or reconnected to the network, it is checked whether there is another time unit (3) in the network that assumes an active grandmaster role (1), - the first time unit (2) is forced into its slave state if another time unit in the grandmaster role has been detected in the locally limited network (1), wherein the first time unit (2) synchronizes with the time unit (3) in the grandmaster role, - after synchronization has taken place, the enforcement of the slave state for the first time unit (2) is canceled, - the first time unit (2) switches to the role of the grandmaster time unit,if it is better suited to the grandmaster role, or remains in its slave state if another time unit (3) in the network (1) is better suited to the grandmaster role.

2. Method according to claim 1, characterized in that using PTP, the slave state is enforced by setting the priority of said time indication unit (2).

3. Method according to claim 1 or 2, characterized in that the check as to which time unit (2, 3) is better suited for the active grandmaster role is carried out according to the Best Master Clock algorithm of the IEE 1588 protocol.

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

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

6. Method according to one of the preceding claims, characterized in thatthe locally limited 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 time indication unit is installed in an IED.

9. Method according to one of the preceding claims, characterized in that a grandmaster-capable time unit that is once again available or once again connected to the network remains in the slave state and continuously synchronizes with the current grandmaster time unit and only switches to the grandmaster role when the grandmaster time unit is no longer available.

10. Time indication unit (2, 3) for a locally limited network (1) via which intelligent electronic units IEDs (5) are interconnected characterized in thatwhich is designed to carry out a method according to one of the preceding claims.