Method and system for synchronizing substation devices The present invention relates to a method and system for syn ^¬ chronizing substation devices and more particularly, to a method and system for fail safe time synchronization of merging units installed in an electrical substation. An electrical substation is an important part of electrical power generation, transmission and distribution system.

Transformation of voltage from low to high or high to low are mainly performed at the substation. The substation has transformers along with switching, protection and control devices. Another important task performed at the substation is to monitor and control the flow of power of the electrical power generation, transmission and distribution system along with the protection of machines and devices installed in the sub ^¬ station. Few decades back, the monitor, control and protec- tion actions performed at the substation needed lot of manual effort for data collection, analysis, maintenance and so on. However in present time, by using power system automation techniques, the manual effort for controlling the substation has been significantly reduced.

The power system automation is an act of automatically controlling a power system by using instrumentation and power devices. In case of substation automation i.e. automatically controlling a substation, a remote user monitors and controls the functionality of the substation with the help of data re ^¬ ceived from various sensors and/or intelligent electronic de ^¬ vices (IED) installed within the substation. Tasks performed under substation automation can be broadly categorized in three categories namely, data acquisition, monitor and con- trol. In data acquisition, data like analog current and/or voltage values are measured and collected from various sen ^¬ sors and/or IEDs installed at various sections of the substa ^¬ tion. The acquired data can be used by the device collecting it, can be sent to another IED or a processing terminal or a database for further analysis. The analysis of the data can be done either by using some predefined algorithms i.e. auto ^¬ matically or manually.

In monitoring task, the data acquired from various devices is analyzed by a computer process or by a supervisor like an en ^¬ gineer or an operator, for checking the status and conditions of various devices, machines and processes associated with the substation. The control task refers to sending command messages to various devices installed within the substation. The command messages are based on the analysis of the ac ^¬ quired data and are created automatically or by the engineer or operator who is monitoring the acquired data. Other tasks performed under the substation automation are sending and receiving data and the command messages. For a reliable opera ^¬ tion of the substation, it is necessary that all the tasks performed under the substation automation i.e. data acquisi ^¬ tion, monitoring and control are mutually synchronized along with the communication of the data and the command messages.

The devices installed within the substation for substation automation includes measuring devices like electronic current transformers, electronic voltage transformers, or other in- strumentation transformers, data collection and routing devices like merging units, data transmitting mediums like ca ^¬ bles, and processing units like laptops, computers, etc.

A merging unit is used for receiving and combining different measure types and then to produce a homogeneously defined digital data stream created from the received measured data. The merging units received analog data from various measuring devices like current transformers and voltage transformers. The merging unit output digital data stream is defined ac- cording to IEC61850 standard and is send to various protec ^¬ tion devices, power quality devices and other devices that are supporting the substation automation system. In the substation automation system multiple merging units are installed and send data to multiple IEDs like protection devices. For a reliable execution of the station automation system, a precise timing synchronization between all the merging units is a key necessity, particularly when one IED is receiving digital data stream from more than one merging units. If timing synchronization is not accurate or if the timing synchronization fails, sampling values i.e. the digital data provided by the merging units, will be no longer useful for IEDs specially the protection devices since the sampling instants could have become mutually asynchronous. The processing of sampling values that are mutually asynchro ^¬ nous the protection devices could lead to wrong measured val ^¬ ues and false triggering of the safety functions of the pro- tection devices.

There are various methods of time synchronization are known in the state of the art. Most common method is external syn ^¬ chronization with the help of second impulses, GPS or similar mechanisms known in the prior art. But the external synchro ^¬ nization methods require additional wiring which increase the cost of the system. In addition to this, mostly the external synchronizations are designed in redundant manner hence con ^¬ sidered less fail safe.

Another known method of achieving the time synchronization between the merging units is use of IEEE 1588 standard of synchronization through Ethernet. In this method, all the de ^¬ vices are connected and synchronized through an Ethernet net- work, as shown in FIG 1 and the same network is used for sending and receiving the sampled values. FIG 1 illustrates a process bus 100 which is a part of the substation automation, not shown in FIG 1, as known in the state of the art. The process bus 100 includes three merging units 102, 104, 106 and one protection device 108. The three merging units 102, 104, 106 are connected to the protection device 108 through three Ethernet switches 110, 112, 114 respectively. The time synchronization in the process bus 100 is achieved by a mas- ter clock 116. Other known redundancy mechanisms in Ethernet like parallel redundancy protocol (PRP) , high-availability seamless redundancy (HSR) etc. can also be used in the proc ^¬ ess bus 100 for time synchronization. It will improve the safety of the system in case of network component failure.

A drawback of using IEEE 1588 standard of synchronization through Ethernet as shown in FIG 1 is, if the master clock 116 fails and a new master clock will be negotiated using an algorithm like best master clock algorithm or any other algorithm known in the state of the art. During this switch-over time i.e. the replacement of the master clock 116 with the new master clock, no valid time master will be available. Hence all the components run will fall out of synchronization with respective component specific inaccuracies which will lead to a failure of time synchronization of the merging units 102, 104 and 106.

Another disadvantage of using IEEE 1588 standard through Ethernet is, if a part of the network like a connection fails and isolates the merging units from the master Clock, a new master clock which works independent of the original master clock is formed in the sub-network. For example, in FIG 1, if the network connection between the switch 112 and the switch 114 fails then the merging unit 106 will be isolated from the master clock 116 and a local clock, associated with the switch 114, if capable enough then will start working as new master clock which will be independent from the original mas ^¬ ter clock 116. The function of the local clock as a new mas- ter clock which is independent of the original master clock

116 will not be a suitable condition for maintaining the time synchronization between the merging units 102, 104 and 106.

Another disadvantage of the method known in the state of the art is, if a merging unit 106 falls out of synchronization with the master clock 116 but retains the connection to the safety device 108 then the merging unit 106 will continue to forward sampled values to the safety device 108, based on a local frequency associated with the merging unit 106 and the switch 114, as shown in FIG 1. The local frequency associated with the merging unit 106 and the switch 114 will not related in any way to the time synchronization of the safety device 108. Hence this condition could lead to hyper-function of the safety device 108.

Failure of the synchronization between the merging units is the failure of all functions that use the sampling values of the merging units. There are few strategies known in the state of the art that are used to mitigate this problem. A common strategy is using emergency functions that can manage with the sampling values of a merging unit or reducing the sensitivity of the safety function. All the measures of the merging units will lead to an incomplete or unselective emer ^¬ gency protection in the event when the synchronization between the merging units fails. But implementation of the emergency functions incurred additional cost for engineering and commissioning the emergency functions. Another strategy that commonly used when the mutual synchronization fails is grouping together all important signals of a merging unit for a function. But to implement this strategy, the required merging units should be configured in a flexible manner and should have variable number of channels which makes the merg- ing units expensive.

From the foregoing it is clearly evident that there is no less expensive method and system for maintaining time syn ^¬ chronization between the merging units of process bus appli- cations under all possible adverse conditions including the conditions mentioned above is available. Hence there is a strong need of a method and system for maintaining time synchronization between substation devices like merging units. It is therefore an object of the present invention to provide a method and system for synchronizing merging units of a substation automation system. The object is achieved by providing a method for synchroniz ^¬ ing one or more sample values in a process bus according to claim 1, and a system for synchronizing one or more sampled values in a process bus according to claim 7. Further embodi- ments of the present invention are addressed in the dependent claims .

In a first aspect of the present invention, a method for syn ^¬ chronizing one or more sample values in a process bus is dis- closed. A first step of the method disclosed in the first as ^¬ pect of the present invention is, receiving the one or more sampled values with one or more sync telegrams at one or more substation devices from one or more merging units. At next step, one or more time shifts for the one or more sampled values are identified from the one or more sync telegrams and at least one local sampling time signal at the one or more substation devices. At a final step of the method disclosed in the first aspect of the present invention, the one or more sampled values are shifted in accordance with the one or more time shifts identified at the one or more substation devices.

Further in accordance with the first aspect of the present invention, time data from a master clock is received at the one or more substation devices. The time data received from the master clock is the at least one sampling time signal. In a preferred embodiment of the present invention, the at least one sampling time signal can be a time signal originated at the one or more substation devices. Furthermore, in accordance with the first aspect of the pre ^¬ sent invention, the one or more sync telegrams are generated by one or more virtual clocks of the one or more merging units before the one or more sampled values and the one or more sync telegrams are received at the one or more substa- tion devices.

In addition to this, the method disclosed in accordance with the first aspect of the present invention, the one or more sync telegrams are compared with the at least one local sam ^¬ pling time signal for identifying the one or more time shifts in the sampled values received at the one or more substation devices from the one or more merging units.

In accordance to a second aspect of the present invention, a system for synchronizing one or more sampled values in a process bus is disclosed. The system includes one or more merging units for generating the one or more sampled values with one or more sync telegrams and one or more substation devices for receiving the one or more sampled values and the one or more sync telegrams from the one or more merging units. The one or more substation devices of the system com ^¬ prises one or more clock recovery modules for identifying one or more time shifts between the one or more sync telegrams of the one or more sampled values and at least one local sam ^¬ pling time signal. The one or more substation devices also comprises one or more re-samplers for shifting the one or more sampled values in accordance to the one or more time shifts identified by the one or more clock recovery module.

Further, in accordance with the second aspect of the present invention, the system comprises at least one master clock for generating time data for the one or more substation devices wherein the time data generated by the master clock can be the at least one local sampling signal.

Furthermore, in accordance with the second aspect of the pre ^¬ sent invention, the system further comprises at least one switch for connecting the one or more merging units and the at least one master clock with the one or more substation devices.

Also in accordance with the second aspect of the present in- vention, the one or more merging units comprises one or more virtual clocks for generating the one or more sync telegrams in accordance with the one or more sampled values. In addition to this, the one or more substation devices dis ^¬ closed in the second aspect of the present invention, also comprises at least one local clock (310) connected with the one or more clock recovery modules (306) and the one or more re-samplers. In an embodiment of the present invention the at least one local clock generates the at least one local sam ^¬ pling time signal.

Furthermore in accordance with the second aspect of the pre- sent invention, the one or more substation devices further comprises at least one local sampling clock which is in con ^¬ tact with the one or more re-samplers. In another embodiment of the present invention, the at least one local sampling clock generates the at least one local sampling signal.

Accordingly, the present invention provides a method for synchronizing one or more sample values in a process bus by us ^¬ ing a system for synchronizing one or more sample values in a process bus.

The present invention is further described hereinafter with reference to illustrated embodiments shown in the accompany ^¬ ing drawings, in which: FIG 1 illustrates a process bus in accordance with the state of the art;

FIG 2 illustrates a modified process bus in accordance with an embodiment of the present invention; and

FIG 3 illustrates a block diagram of the modified

process bus illustrated in FIG 2 in accordance with an embodiment of the present invention. Various embodiments are described with reference to the draw ^¬ ings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident that such embodiments may be practiced without these specific details. FIG 2 illustrates a modified process bus 200 in accordance with an embodiment of the present invention. The process bus 200 includes three merging units 202, 204, 206, along with the master clock 116 and three Ethernet switches 110, 112, 114. The three merging units 202, 204 and 206 are connected to the safety device 108 through the three switches 110, 112, 114 respectively, as illustrated in FIG 2. Three virtual clocks 208, 210, 212 are implemented in the three merging units 202, 204, 206 respectively. In other words, the merging unit 202 includes a virtual clock 208, the merging unit 204 includes a virtual clock 210 and the merging unit 206 in ^¬ cludes a virtual clock 212, as shown in FIG 2. The merging units 202, 204, 206 are capable of sending sync telegrams along with sample values to the safety device 108. The sync telegrams are a kind of time stamps and the merging units 202, 204, 206 generates the sync telegrams by using the vir ^¬ tual clocks 208, 210, 212. The virtual clocks 208, 210, 212 are in a precise time synchronization with the master clock 116 of FIG 2. The safety device 108 can relate the sample values and the sync telegrams received from the merging units 202, 204, 206 with time data received from the master clock 116. In a preferred embodiment of the present invention, the safety device 108 is capable of synchronizing the sample val ^¬ ues by using the sync telegrams received from the merging units 202, 204, 206 along with the time data received from the master clock 116 with a safety device system time.

The system and method for synchronization illustrated in FIG 2 is an exemplary embodiment of the present invention. In another embodiment of the present invention, one or more merg- ing units with one or more virtual clocks send one or more sync telegrams along with one or more sample values to one or more IEDs and the one or more IEDs synchronize the one or more sample values received from the one or more merging units by using the one or more sync telegrams with time data received from one or more master clocks.

FIG 3 illustrates a block diagram 300 of components of the modified process bus 200 of FIG 2 in accordance with an em ^¬ bodiment of the present invention. The block diagram 300 of the modified process bus 200 includes the merging units 202, 204 with the virtual clocks 208, 210 and the master clock 116. The merging units 202, 204 and the master clock 116 are connected with a field device 304 through a switch 302. The switch 302 can be a switch similar to the Ethernet switches 110, 112, 114 shown in FIG 2. The field device 304 can be the safety device 108 shown in FIG 2. The field device 304 com ^¬ prises a clock recovery module 306, a re-sampler 308, a local clock 310 and a main board 312. The main board 312 performs one or more prime functions of the field device 304 and also includes a local sampling clock 314 as illustrated in FIG 3. A basic function of the clock recovery module 306, the re- sampler 308, the local clock 310 and the local sampling clock 314 of the field device 304 is synchronizing the sample val ^¬ ues received from the merging units 202, 204 along with the time data received from the master clock 116.

The merging units 202, 204 send the sampling values along with respective time stamps from the virtual clocks 208, 210 as described in FIG 2 through IEC61850 protocol. The clock recovery module 306 receives the sampled values with the time stamps from the merging units 202, 204. The clock recovery module 306 also receives a local sampling time desired at the field device 304 from the local clock 310. The clock recovery module 306 evaluates a time shift required for the synchroni ^¬ zation of the sample values of the merging units 202, 204 from the local sampling time and the time stamps i.e. the sync telegrams received from the merging units 202, 204. The sampling values received from the merging units 202, 204 are shifted in time domain according to the time shift identified by the clock recovery module 306 by the re-sampler 308 shown in FIG 3. The re-sampler 308 time shifts the sampling values based on the time shift identified by the clock recovery mod ^¬ ule 306 by using time data received from the local sampling clock 314 of the main board 312 of the field device 304. In a preferred embodiment of the present invention, the sample values received from the merging units 202, 204 are synchro ^¬ nized with time data received from the master clock 116 by the clock recovery module 306 and the re-sampler 308 of FIG 3. It is clearly evident from the system and method for synchro ^¬ nizing the sample values of the merging units described in FIG 2 and FIG 3 that if a new master clock replaces the mas ^¬ ter clock 116 then the sample values are synchronized by the clock recovery module 306 and the re-sampler 308 according to the new master clock. The new master clock can be a new system designed for replacing the master clock 116 and/or either or both of the virtual clocks 208, 210 functions as master clock for the merging units 202, 204 respectively. Also, for the merging units 202, 204 of the modified process bus 200, explained in FIG 2 and FIG 3 no major modifications are required so the cost of the system remains less in com ^¬ parison to merging units required for synchronization known in the state of the art.

It is evident from the foregoing description and related ad ^¬ vantages of the present invention that the invention provides a low cost system and method for synchronizing merging units of a substation automation system.

While the present invention has been described in detail with reference to certain embodiments, it should be appreciated that the present invention is not limited to those embodi ^¬ ments. In view of the present disclosure, many modifications and variations would present themselves, to those of skill in the art without departing from the scope of various embodi ^¬ ments of the present invention, as described herein. The scope of the present invention is, therefore, indicated by the following claims rather than by the foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims ar to be considered within their scope.