Evolving Time Synchronization Mike MacLaren Randy Kimura AltaLink Management Ltd. AltaLink Management Ltd. Calgary, Canada Calgary, Canada

Abstract— Time synchronization is a very interesting topic for the LAN electric power industry. Our requirements and expectations of time MPLS Multiprotocol Label Switching synchronization have evolved over time with new applications NTP Network requiring improved accuracy and resiliency. New technologies and PTP techniques are required to achieve the additional accuracy and RTU resiliency. SCADA Supervisory Control and Data Acquisition SOE Sequence of Events Index Terms—IRIG-B, Precision Time Protocol, Time SNTP Simple synchronization III. ALTALINK I. INTRODUCTION As Alberta’s largest regulated In a traditional SCADA system time synchronization transmission company, AltaLink accuracy is required to meet event and alarm reporting owns more than half the provincial requirements, typically 1 millisecond. Newer applications, for transmission system with a example synchrophasors and sampled values, may require a 212,000 square kilometre service higher degree of accuracy and resiliency while other area. Our network of more than applications, for example security event reporting, may 13,000 kilometres of transmission tolerate a less accurate time synchronization methodology. lines and 300 substations delivers This paper describes the approach used by AltaLink to align electricity safely, reliably and time synchronization technologies with the accuracy efficiently to more than three requirements and the capabilities of the devices. million Albertans. We transport power from the generation This paper discusses the evolution of time synchronization facilities where it is created to the within AltaLink to meet past, present and future time communities, businesses and synchronization requirements beginning with manufacturer industries that need it every day. proprietary SCADA communication protocols. The pros and cons of the more recent approaches, IRIG-B, NTP, and DNP3, are discussed with a focus on the future feasibility of each technology. AltaLink’s pilot project deploying Precision Time Protocol (PTP) grandmaster and boundary clocks, using the To get a correct sequence of events across different places TeleCom and Power Profiles, will be described. Future in the power system a time tagging with a precise global time deployments which include transparent and slave clocks, has to be provided. Therefore, all related devices shall be Parallel Redundancy Protocol (PRP) networks, and High- synchronized with the requested accuracy. Common for events availability Seamless Redundancy (HSR) networks will be is 1 ms.[1] discussed highlighting the concerns and triggers required for TABLE I summarizes the time synchronization classes and future deployments. applications defined in IEC 61850 [1]. Time synchronization class T1 must be achieved for the power system event time II. NOMENCLATURE tagging.

CIP Critical Infrastructure Protection TABLE I. TIME SYNCHRONIZATION CLASSES DNP Distributed Network Protocol DST Daylight Saving Time Class Accuracy Application GLONASS Global Navigation Satellite System (µsec) GNSS Global Navigation Satellite System TL >10000 Low time synchronization - miscellaneous GPS Global Positioning System T0 10000 Time stamping of events with an accuracy of 10 ms IEC International Electrotechnical Commission IEEE Institute of Electrical and Electronics Engineers T1 1000 Time stamping of events with an accuracy of 1 ms IRIG Inter-Range Instrumentation Group T2 100 Time tagging of zero crossings Class Accuracy Application In order to time stamp power system events with one (µsec) millisecond accuracy the equipment clock must be time T3 25 Miscellaneous synchronized before it drifts a millisecond. For the RTU clock with 100 ppm accuracy an update period less than 10 seconds T4 4 Time tagging of synchronized samples is required. For the high accuracy oscillator an update period T5 1 High precision time tagging of samples less than 500 seconds (8 minutes) is required. TABLE III. UPDATE FREQUENCY Accuracy One Millisecond Drift Update Frequency IV. STANDALONE EQUIPMENT (ppm) (minutes) (seconds) The early systems consisted of one or more pieces of 1 16.67 1000 substation equipment operating in isolation, relying on someone manually setting the clocks. Setting the clock will 2 8.33 500 be very inconvenient as the equipment must be visited in order 5 3.33 200 to set the clock, therefore it should be assumed that the clock 10 1.67 100 will be infrequently set. It is likely that the clock will be initially set during the commissioning/installation phase and 100 0.167 10 never updated. After an extended period of time, the equipment's clock drift may affect the clock accuracy. A. Communication Path Delay An RTU that previously had significant market share has a A 's use of millisecond resolution free running clock accuracy of 100 ppm. TABLE II does not guarantee accurate equipment clocks. Whenever a summarizes the potential clock drift accumulated over an Master Station issues a time synchronization command, it hour, day and a year. After one hour the referenced RTU clock sends the current time. Upon receipt of the command the has drifted 360 milliseconds making the 1 millisecond equipment will update its clock with the time and date requirement unachievable. The accumulated error after a year contained in the command. exceeds 52 minutes. Figure 2 illustrates the communication protocol based time The RTU manufacturer offered a high accuracy crystal synchronization sequence: oscillator version with a clock accuracy of 2 ppm. Although 1. The Master Station sends a time synchronization the high accuracy crystal oscillator version is a significant command with the current time, t . improvement, the one millisecond time stamping requirement 1 remains unachievable after an hour. 2. Sometime later, t2, the equipment receives the last

TABLE II. CLOCK DRIFT byte of the time synchronization. Accuracy Hourly Drift Daily Drift Yearly Drift 1 2 (ppm) (milliseconds) (milliseconds) (seconds) 1 3.6 86.4 31.536 t t1 t2 2 7.2 172.8 63.072 Figure 2 Communication Protocol Time Synchronization 5 18 432 157.680 The time difference, t (the time the last byte of the time 10 36 864 315.360 2 synchronization command is received) – t1 (the time 100 360 8640 3153.600 synchronization message was built), is the communication propagation time. If the clock is set using the time provided by the Master Station, t1, than the error will be the I. SCADA PROTOCOLS communication propagation time: Communication protocols provided the ability to remotely update equipment clocks, removing the inconvenience of t t t  visiting the equipment. This system is automated, allowing for error 2 1 the clocks to be updated at a known frequency. Appropriately selecting the frequency will eliminate the effects of clock drift Some communication protocols will attempt to measure errors. the communication propagation time and adjust the clock to account for this delay. TABLE IV summarizes some communication protocol capabilities to adjust a clock for the Master Station communication path time.

TABLE IV COMMUNICATION PATH ADJUSTMENT Protocol RTU Time Sync Protocol Clock Adjustment Ack Conitel 3000 Not supported. Figure 1 Master Station Based Synchronization Not supported. Protocol Clock Adjustment Backup Master Master Station DNP3 Calculates and adjusts for communication path Station delays.

Protocol 1 Protocol 2 RTU A single master station may used to time synchronize Time Sync Time Sync devise in multiple substations (shown in Figure 3). Ack Ack Figure 5 Backup Master Station Master Figure 6 illustrates the difficulty accurately identifying the Station order of events when the substation equipment is synchronized

Protocol Time Sync from two sources. RTU Ack 5 Time Sync RTU Ack 4 2

Time Sync RTU 1 3 Ack Figure 3 Single Master Station t t t If the Master Station clock is incorrect, all the substation 2 1 equipment will be set to the incorrect time. The first piece of Figure 6 Clock Oscillation substation equipment reports an event at time t and the 1 1. The substation equipment is synchronized to the second piece of substation equipment reports another event at second Master Station's clock. time t2. Although the time is incorrect, all of the system clocks are set to the same time and the Master Station can correctly 2. An event occurs and is time stamped with time t1. determine the order in which the events occurred and the time which occurred between the two events. Accuracy errors in a 3. 10 milliseconds later the substation equipment is single Master Station system are often within tolerable limits synchronized with the first Master Station's clock. and have minimal impact on the overall functionality. 4. This results in the substation equipment clock being moved back in time 500 milliseconds. B. Dual Source Oscillation 5. 10 milliseconds later another event occurs and is time Existing systems, using “legacy”, communication stamped with time t2. protocols, are often upgraded with replacement Master Stations and an “open” communication protocol. There may The clock adjustment which occurred in step 3 has created be a period, shown in Figure 4, when the new and old Master a problem. Although the second event occurred 20 Station are in simultaneously in service. This approached milliseconds after the first event, the second event time stamp permits the operators access to the system they are familiar is 490 milliseconds before the first event. The correct order, in with while developing the skills and knowledge for the new which the events occurred or the time between the two events, system. cannot be determined.

C. Summer Time New Master Old Master Station Station Daylight saving time (DST) or summer time is the practice of advancing clocks during summer months by one hour so Protocol 1 Protocol 2 RTU that evening daylight lasts an hour longer, while sacrificing Time Sync Time Sync Ack normal sunrise times. Typically, regions with summer time Ack adjust clocks forward one hour close to the start of spring and Figure 4 Master Station Replacement adjust them backward in the autumn to standard time [2]. Ideally the time synchronization on the older system will be disabled, leaving a single time synchronization source form 1 the new Master Station. Older systems may not permit the 5 2 time synchronization to be disabled resulting in two time 4 3 synchronization sources for a single piece of equipment. In Figure 4 the RTU is time synchronized by the old and new t Master Stations. t2 t1 A similar situation occurs when time synchronization Figure 7 Summer Time cannot be disabled or suspended in a backup Master Station. Figure 7 illustrates the difficulty accurately identifying the order of events when the clock was adjusted backward for the autumn adjustment: 1. The equipment is synchronized to the current time. 2. A short time later an event occurs and is time- Initial deployments were based on IRIG Standard 200-98 stamped with time t1. using the IRIG-B format shown in Figure 9. This version of the IRIG-B format transmits the day of the year, a number 3. 10 milliseconds later (time = t1 + 10) Daylight Saving between 1 and 365 (or 366 on leap years). The year must be Time moves the time back one hour. manually set in the equipment otherwise the clock may have 4. The substation equipment is synchronized, resulting the correct time, month, and day with an incorrect year.

in its clock being moved back one hour. The clock is 0 10 20 30 40 50

now set to a time 59,990 milliseconds before t1 (time Seconds Minutes Hours Day = t1 + 10 – 60000). 1 2 4 8 10 20 40 1 2 4 8 10 20 40 1 2 4 8 10 20 1 2 4 8 10 20 40 80 100 200 P0 P1 P2 P3 P4 P5

5. 10 milliseconds later an event occurs and is time- 50 60 70 80 90 100

stamped with time t2. This time is 59,980 Control Functions Time of Day

20 21 22 23 24 25 26 27 28 29 210 211 212 213 214 215 216 milliseconds before t1. P6 P7 P8 P9 P10

.01 sec 8 msec 5 msec 2 msec Reference Binary '0' Binary '1' The clock adjustment which occurred in step 3 has created (Typical) Marker (Typical) (Typical) a problem. Although the second event occurred 20 Figure 9 200-98 IRIG-B Format milliseconds after the first event, the second event is time stamped 59.980 milliseconds before the first event. The IEEE 1344 (IEEE Standard for Synchrophasors for Power correct order, in which the events occurred or the time Systems) included year data in the control bits of the IRIG-B between the two events, cannot be determined. The removal of format. This variation, shown in Figure 10, is commonly the clock adjustment will eliminate the event reporting referred to as IEEE 1344 extensions. problems. 50 60 70 80

II. SATELLITE CLOCKS Year Time Offset Time Quality Accurate system time is maintained using a satellite-based 1 2 4 8 10 20 40 80 LSP LS DSP DST ± 1 2 4 8 ½H P clock. The clock synchronizes with GPS satellites and P6 P7 P8 produces an IRIG-B signal or SNTP message for time Figure 10 IEEE 1344 Extensions synchronizing substation equipment as shown in Figure 8. In 2004, IRIG Standard 200-04 was updated to include the The United Stated Department of Defense satellite based year data. Global Positioning System (GPS) time is received by a GPS clock. The precise Positioning Service (PPS) level of accuracy It would impractical to use more than one external clock is limited to authorized military users. The Standard source in a substation with multiple devices. The daisy chain Positioning Service (SPS) level of accuracy is available to the approach, shown in Figure 11, may be used when there are a general public and provides the high precision time. limited number of devices within close proximity to the clock source. Clocks may support multiple IRIG-B outputs, This satellite based system monitors 21 satellites in six permitting the implementation of a number of IRIG-B daisy orbital planes 20000 km above earth. At least 4 satellites can chains. be simultaneously monitored from every point on the globe. The system computes the GPS time by accurately measuring the propagation of signals between the satellites and GPS Clock clock. IED

Master Station IED Figure 11 IRIG-B Daisy Chain Protocol IRIG-B RTU Clock Time Sync Fiber-optic transceivers can be used to extend the IRIG-B Ack network over longer distances. The layout shown in Figure 12 Figure 8 Satellite Clock has been used to extend the IRIG-B network between control Inter-range instrumentation group time codes, commonly buildings. known as IRIG time codes, are standard formats for transferring timing information. Atomic frequency standards R Fiber RS-232/485 Transceiver Clock and GPS receivers designed for precision timing are often T equipped with an IRIG output. The standards were created by R Fiber RS-232/485 T Transceiver IED c i t p the Working Group of the U.S. o - r e b i F military's Inter-Range Instrumentation Group (IRIG), the R Fiber RS-232/485 IED standards body of the Range Commanders Council. Work on T Transceiver these standards started in October 1956, and the original Figure 12 Fiber-optic IRIG-B standards were accepted in 1960 [3]. Some RTUs are capable of replicating the IRIG-B signal and redistributing the signal to devices. This approach is typically used for RS-232 interfaces. An unused pair of wires The test was conducted using two RTUs and the result was within the RS-232 cable is used for IRIG-B. This approach, unexpected. The characteristics of RTU 1 are: leveraging the existing RS-232 cable, eliminates the requirements to wire a dedicated pair of wires for IRIG-B.  Embedded device, with the hardware and firmware supplied by a single supplier.

IRIG-B  All patches are contained within a firmware release. Clock

IRIG-B  Uses a hard real time operating system. RTU IED The characteristics of RTU 2 are: IRIG-B IED  The hardware, application software, and operating Figure 13 IRIG-B Sub-master software may be sourced from different suppliers  The application software and operating system may A. Accuracy be independently patched. The adoption of substation automation resulted in a DNP3 communication interface between the RTU and substation  A soft real time operating system is used. devices. Elimination of the IRIG-B network and time DNP3 time synchronization was rejected due to the synchronization using the communication protocol was accuracy dependency on the RTU make, model, or firmware. identified as a potential reduction in engineering complexity and project delivery cost. This change required confirmation The addition of an IRIG-B time synchronization source is that the communication protocol based time synchronization similar the addition or upgrade of a second Master Station. could achieve accuracies comparable to the IRIG-B The traditional approach is disabling the time synchronization implementation. service between the Master Station and RTU. When the IRIG- B signal is lost, the RTU clock is free running and will drift The introduction of a communication path error was over time. If the IRIG-B signal is lost for significant period of described in Figure 2. Communication protocol capability to time the RTU clock will eventually become inaccurate. measure and adjust for the communication path error is summarized in TABLE IV. The setup, shown in Figure 14, The same technique can be implemented when the Master was used to compare the two time synchronization methods. Station time synchronization functionality cannot be disabled. The RTU returns a positive response to Master Station without

IRIG-B modifying the internal clock. The RTU is time synchronized Clock I/O using the IRIG-B signal.

IRIG-B Protocol RTU I/O Time Sync Master Ack Station Figure 14 Communication Protocol Test Setup Protocol IRIG-B RTU Clock The first I/O module is directly time synchronized by the Time Sync Ack clock using IRIG-B. The second I/O module was time synchronized via an RTU using DNP3 on a serial Figure 15 Ignore Master Station Time communication channel. The first I/O module represents the The presence of multiple time sources provides an traditional approach while the second I/O module represents opportunity to create a redundant time synchronization the communication protocol method under review. The time scheme for the RTU. Loss of the IRIG-B signal is detected stamp generated by the first I/O module is the baseline. The and the RTU asserts an indication. This indication can be used time stamp of the second I/O module was compared to the to troubleshoot the system and invoke time synchronization first with the differences summarized in TABLE V. failover where the RTU will automatically start processing the TABLE V TIME SYNCHRONIZATION ACCURACY Master Station time synchronization messages. RTU 1 RTU 2 Master 0 37 Station -1 42 Protocol IRIG-B RTU Clock -1 42 Time Sync Ack -1 35 Figure 16 Master Station Time Failover -1 44 The failover logic is straight forward. The RTU will accept -1 44 and process the Master Station time synchronization messages 0 45 whenever:  The IRIG-B signal between the RTU and GPS clock is lost.  IRIG-B processing is disabled in the RTU. these event logs the time synchronization accuracy requirement was downgraded from one millisecond to several seconds. Changing the accuracy permitted the use of Network Time Protocol (NTP), a technology already supported by the IRIG-B Signal EMS Time Sync networking equipment. IRIG-B Enabled The Center for Security Critical Security Control 6.2 recommends: Figure 17 IRIG-B Failover Logic GPS clocks may have the capability to detect the loss of Include at least two synchronized time sources from which signal, for example a break in cabling between the antenna all servers and network equipment retrieve time and the clock, destruction of the antenna or an obstacle information on a regular basis so that the timestamps in blocking the signal to the antenna. The GPS clock can be logs are consistent [5]. configured to assert a relay contact when the loss of the GPS When possible, backup NTP servers are configured. signal is detected. A generic I/O module is used to interface Improved redundancy is achieved by selecting NTP servers in the relay contact, allowing the RTU to monitor the status of different geographical areas. the antenna and GPS clock. The time synchronization failover logic has been updated IV. PRECISION TIME PROTOCOL to include the status between the antenna and GPS clock. The Precision Time Protocol (PTP) was developed to RTU will accept and process the Master Station time simultaneously synchronize several devices on a local area synchronization messages whenever: network (LAN) with sub-microsecond accuracy. The protocol  The IRIG-B signal between the RTU and GPS clock was developed to fill a void left by previous technologies. is lost. Network Time Protocol (NTP) was not accurate enough to meet a lot of industries technical requirements and multiple  RTU IRIG-B processing is disabled. GPS receivers in the system was not a cost effective solution.  The GPS signal between the antenna and GPS clock PTP was originally defined in IEEE 1588-2002 (1588v1) is lost. in order to target LAN applications. In 2008, IEEE 1588-2008 (1588v2) was released to provide greater accuracy, precision and robustness targeting networks. Unfortunately IEEE 1588v2 is not backwards compatible with IRIG-B Signal IRIG-B Enabled EMS Time Sync IEEE 1588v1. Clock Relay Contact PTP utilizes an algorithm to self-organize all the clocks in a system into a master-slave hierarchy as shown in Figure 19. Figure 18 IRIG-B and GPS Failover Logic

The free running accuracy of the GPS clock should be Grandmaster Clock checked prior to adding the loss of GPS signal to the time synchronization failover logic. The drift of the free running GPS clock is often minimal and may fall within acceptable limits. Substation Clock III. CIP EVENT LOGGING CIP-007-5 security event monitoring requirement 4.1 requires the responsible entity to: Device Clock Log events at the BES Cyber System level (per BES Cyber System capability) or at the Cyber Asset level (per Cyber Figure 19 PTP Hierarchy Asset capability) for identification of, and after-the-fact Grandmaster Clock determines the time base for the system. investigation of, Cyber Security incidents that includes, as a minimum, each of the following types of events: Substation Clock is a slave to the Grandmaster Clock and master to the Device clocks. 1. Detected successful login attempts; Device Clock is a slave to the Substation Clock. 2. Detected failed access attempts and failed login attempts; The grandmaster clock determines the time base for the system and is typically synchronized using GPS/GLONASS 3. Detected malicious code.[4] antennas. Each slave synchronizes to its master by Some of the equipment does not support IRIG-B and an exchanging Sync, Delay_Req, Follow_Up, Delay_Resp alternative technology is required to time stamp event logs. messages. The time of these message exchanges are recorded The substation LAN equipment ( switches, serial port and the slave clock computes the time delay offset from its servers, radios, etc.) support Syslog for event reporting. For master and adjusts its local clock. The synchronization and time offset computation process can be seen in Figure 20 C. System Architecture PTP Synchronization Offset. The telecom profile (G.8265.1) allows the user to specify the IP addresses of multiple grandmaster clocks. The local substation clock will actively evaluate each of these grandmaster clocks to determine which the best available master is at a given time. A “PTP Subnetwork” was provisioned within the existing MPLS network where three (3) grandmaster clocks and the local PTP substation clock are assigned IP addresses within the subnet. The local PTP substation clock has two independently programmable Ethernet ports. The second port is assigned an IP address on the local substation subnet to distribute the PTP power profile (C.37.238-2011). All IED’s capable of accepting the PTP power profile are synced over their existing LAN connections. A second PTP clock is also synced using the PTP power profile and it then distributes legacy IRIG-B to devices not capable PTP time synchronization. The full system architecture is shown in Figure 23.

Grandmaster Clock Grandmaster Clock Grandmaster Clock Figure 20 PTP Synchronization Offset There are several optional features and attributes defined in the IEEE 1588 standard which can be selected and defined to make up a specific PTP profile. Several profiles have already been defined for specific applications and the two used MPLS Router in this technical paper are G.8265.1 Telecom Profile and Substation Clock C.37.238-2011 Power Profile. IED

A. G.8265.1 Telecom Profile Ethernet Switch IED

This profile was selected for use between the remotely Ethernet Switch Ethernet Switch Substation Clock located grandmaster clock and the local substation PTP clock. The profile was already available in our existing grandmaster IED IED IED clocks and was best suited for packet based telecom networks. Figure 21 describes the communication path in which the Figure 23 System Architecture G.8265.1 Telecom Profile is implemented across. D. Accuracy Verification

Grandmaster Clock A lab test was performed to verify the accuracy of our new PTP time synchronization scheme against the traditional antenna based IRIG-B scheme. An event was simultaneously MPLS Router triggered on two separate SCADA I/O modules. One I/O module’s clock was being synchronized using an antenna Substation Clock based GPS clock and IRIG-B while the other I/O modules clock was being synchronized by the IRIG-B outputs of the Figure 21 TeleCom Profile Communication Path PTP clock. The lab test setup is shown in Figure 24.

B. C.37.238-2011 Power Profile Grandmaster Clock This profile was selected for use between the substation master PTP clock and its slave devices. This profile has been optimized for use in the power systems industry Figure 22 MPLS Router describes the communication path in which the C.37.238-2011 Power Profile is implemented across. Substation Clock Clock

PTP IRIG-B Substation Clock I/O I/O Ethernet Switch Ethernet Switch Figure 24 PTP Test Setup

IED IED The timestamps from five (5) different triggered events were then recorded from the event recorders of SCADA I/O Figure 22 Power Profile Communication Path #1 and #2 and compared for accuracy. The results of the test and the goal is to achieve T4 or T5 accuracy using the LAN can be found in TABLE VI. interface. IEC 61850-9-3 specifies a precision time protocol (PTP) profile of IEC 61588:2009 | IEEE Std 1588-2008 TABLE VI. LAB TEST RESULTS applicable to power utility automation which allows Event PTP Synced I/O IRIG-B Synced I/O compliance with the highest synchronization classes specified 1 13:53:52.370000 13:53:52.3700 in IEC 61850-5 (TABLE I) [7]. 2 13:54:13.085000 13:54:13.0850 Additional studies are required to determine if an antenna is required for the substation clock and the transparent clock 3 13:54:14.245500 13:54:14.2455 function is required in the Ethernet switch. 4 13:54:16.381500 13:54:16.3815 5 13:54:20.199000 13:54:20.1990 VI. CONCULSIONS Precision Time Protocol can be used to achieve T1 time The results were as expected as the timestamps from both synchronization accuracy. An antenna is not required in the devices matched for each event that was generated. The event substation because the telecom profile enables the substation recorders had a limitation of recording events only to the clock to accurately synchronize to multiple grandmaster nearest 500us which is more than enough to meet the industry clocks. Transparent clock functionality is not required for a requirement of 1ms accuracy. simple network topology. The current IRIG-B system uses a substation clock with a local antenna. The study confirmed an antenna was not REFERENCES required for the substation clock, i.e. the Grandmaster Clocks [1] Communication networks and systems for power utility automation – synchronize the substation clocks to sufficiently maintain T1 Part 5: Communication requirements for functions and device models, time synchronization accuracy. IEC 61850-5, Edition 2.0, 2013-01 [2] Michael Downing, Spring Forward: The Annual Madness of Daylight Most AltaLink substations use a simple network design, Saving Time, Washington DC, Shoemaker & Hoard, 2005 typically a single Ethernet Switch. The Ethernet switches [3] ”IRIG timecode” Wikipedia: The Free Encyclopedia. Wikimedia currently approved for AltaLink substation do not support Foundation, Inc. PTP. The lab testing confirmed PTP Transparent clock [4] Cyber Security – System Security Management, North American Electric Reliability Corporation, CIP-007-5 support is not required for the single switch network. [5] The CIS Critical Security Controls for Effective Cyber Defense, The Center for Internet Security, Version 6.1, August 31, 2016 V. MORE TO COME [6] Communication networks and systems for power utility automation – Part 90-4: Network engineering guidelines, Edition 1.0, 2013-08 The method of depends on the [7] Communication networks and systems for power utility automation – accuracy that a given application requires. Timing accuracy Part 9-3: Precision time protocol profile for power utility automation, for sampled measurement values, and synchrophasors, is Edition 1.0, 2016-05 much higher than for simple time-stamped events [6]. Time [8] Precision Time Protocol, Wikipedia: The Free Encyclopedia. synchronization class T4 or T5 (refer to TABLE I) must be Wikimedia Foundation, Inc. achieved. [9] Recent Advances in IEEE 1588 Technology and Its Applications, John C. Eidson. Agilent Laboratories, Measurement Research Lab. Synchrophasors and sampled measured values use routable protocols. The use of a separate IRIG-B LAN is undesirable