IEEE 1588 Precision Time Synchronization Solution for Electric Utilities

Introduction

Precision timing solutions serve to increase an electric utility’s efficiency and uptime by improving monitoring and troubleshooting capability while also reducing capital expenses by converging timing and data networks. There are several timing protocols already available today so why IEEE 1588 v2? This standard has been ratified by the IEEE in March 2008 and has been designed to overcome the inadequacies of previous solutions such as accuracy, scalability and cost. The electric power industry has recognized that with 1588 v2 there now exists a network based precision time synchronization protocol that is reliable and accurate enough (i.e. sub one us) for use in electric power applications and has adopted a power profile for IEEE 1588 v2 specifically for the electric power industry. IEEE 1588 v2 is also currently being considered for inclusion in IEC 61850 Edition 2. IEEE 1588 v2 is important for electric utilities because it meets the timing accuracy needs for the applications of today and the future while reducing the cost to install and maintain a separate dedicated timing network. With 1588, the cabling infrastructure requirement is reduced by allowing time synchronization information to be transported over the same Ethernet medium as the data communications. This convergence of timing and data information networks can be carried out right to the network edge and converted to IRIG-B for synchronization of existing devices that are not capable of 1588, allowing them to be kept in service even while updating the timing and data network infrastructure. Siemens provides the necessary pieces to implement green-field Precision Time Protocol networks based on IEEE 1588 v2 while supporting existing IRIG-B devices over the same Ethernet network.

Benefits over existing protocols

Until relatively recently the time synchronization of electronic devices in power systems has been realized via dedicated wiring used for distribution of GPS, IRIG-B or 1PPS signals. With the proliferation of modern IEDs capable of Ethernet communications new methods of time synchronization based on network protocols are available. These methods are NTP/SNTP protocols and the latest standard IEEE 1588.


The existing Network Time Protocol (NTP) or Simple Network Time Protocol (SNTP) have the advantage of being able to synchronize computers over a local area network but do not have the accuracy required for the most demanding substation applications such as IEC 61850-9-2 Process Bus or IEEE C37.118-2005 Synchrophasors. Typical SNTP implementations in normal network conditions provide the accuracy of 2-3 milliseconds; even a tuned network running SNTP can achieve accuracy only in the millisecond range which is far short of what is required for utility synchronization applications. There exists some good implementations in IEDs that are close to one millisecond, however if the application requirement is strictly one millisecond then NTP or SNTP should not be used as its behavior is indeterministic in heavily loaded networks.

The typical implementations of unmodulated (DC-shifted mode) IRIG-B timing protocol in IEDs provide accuracy in the 100 microsecond range which is accurate enough to be used for some for time stamping applications such as sequence of events recording and fault waveform capture, but not accurate enough for IEC 61850 Process bus or Synchrophasor applications which require microsecond accuracy. Additionally IRIG-B installations require dedicated coax or twisted pair cable to transport the timing signals and a single output can only drive a limited number of devices, depending on cable length and device load. These restrictions limit the scalability and increase deployment and maintenance costs for IRIG-B.

GPS is a highly accurate solution but does not scale well due to cost and complications of attaching antennas to every device.

IEEE 1588 v2 solves these problems by:

  • using an Ethernet network to propagate the timing signals, eliminating the extra cabling requirements of IRIG-B

  • using mechanisms that increase accuracy by accounting for switching time and peer to peer propagation delays that occur as the timing signals traverse the network

  • using ‘transparent clocks’ in Ethernet switches that eliminate the need for end-to-end delay measurement, reducing traffic congestion and eliminating switch jitter

The most important features introduced in IEEE 1588 v2 that correct the above problems are Peer-to-Peer Path Delay Measurement and Transparent Clock mode.

Method

Typical Accuracy in
substation with given
method

Provides date and time
of day indication

Dedicated
cabling
not required

Cost effective

Scales well with large
number of devices

IRIG-B(AM)

1ms

IRIG-B
(DC-shifted)

100us

1PPS

1us

GPS

1us

NTP

1-10ms

IEEE 1588 v1

1us

IEEE 1588 v2

1us

Table 1. Comparison of different time synchronization methods available in today’s substations. The listed accuracy is not the theoretical accuracy possible to be achieved with each method; it is instead the typical observed accuracy that also takes into account the protocol implementation in the IEDs.

Method

fulfils IEC 61850 Station
Bus requirements

fulfils IEC 61850 Process
Bus requirements

fulfils IEEE C37.118 Synchrophasor
Data requirements

IRIG-B (AM)

IRIG-B
(DC-shifted)

(see note 1)

1PPS

(see note 2)

GPS

NTP

(see note 3)

IEEE 1588 v1

IEEE 1588 v2

Table 2. Compliance of different time synchronization methods to substation application requirements

Note 1:
It is generally assumed that the unmodulated IRIG-B is good enough for synchrophasor applications. The IEEE C37.118 standard specifies that the accuracy of a synchrophasor measurement system shall not exceed 1% of a ‘total vector error’, where 1% of ‘total vector error’ corresponds to a phase angle error of 0.57 degrees, if no other errors are present. The error of 0.57 degree corresponds to approximately 26us at 60 Hz and 32us at 50 Hz. Therefore a PMU which introduces 100us error on IRIG-B input results in a 2.2 degrees phase angle error at 60Hz.

Note 2:
1PPS has sufficient accuracy for IEC 61850 station bus but it is not suitable for this application as it does not provide any indication of the date or time of day. Typically 1PPS is in substation automation application in conjunction with other time synchronization methods such as IRIG-B or SNTP.

Note 3:
Despite the fact that NTP/SNTP is specified in the first edition of IEC 61850 and is currently being deployed in station bus applications the user shall remember that it does not fulfill the 1us accuracy requirement.

Comparison of different timing methods shows that IEEE 1588 v2 has the accuracy required for the most demanding utility applications such as synchrophasors can scale well and is not as costly as GPS.

IEEE 1588 v2 Peer-to-Peer Path Delay Measurement

In this delay measurement mechanism introduced with version 2, the propagation delay time is measured only between the switch and its upstream peer. This is an alternate method to measuring the total end-to-end path delay from the slave clock to the master clock that eliminates the two problems likely to occur. In a large network the end-to-end method will traverse many switches, each with varying and unpredictable latency times that lead to timing inaccuracy and jitter, compounded by the possibility of asymmetrical data paths. Secondly, all the end-to-end path delay request messages must be answered by the master clock that in a large network can cause a traffic and processing bottleneck at the master. In the peer-to-peer delay mechanism, path symmetry is guaranteed and there will never be processing or traffic overloading due to the one to one relationship.

IEEE 1588 v2 Transparent Clock Mode

Store and forward Ethernet switches have latency times that can vary depending on the data that the switch is currently processing. For instance if the switch is transmitting a 1500 byte packet the latency will be much greater than if the transmit queue was empty or transmitting a 500 byte packet. In transparent clock mode, the switch timestamps the time packet as it enters the switch, measures the switch residence time, and corrects the time packet either as it leaves the switch or with a follow-up message with the correction field in it. With transparent clock mode accumulation of switch latency errors or jitter is eliminated.

IEEE 1588 v2 Applications and Solutions


Figure 2. The RS416 and RSG2288 are synchronized with IEEE 1588 v2 for IEC 61850 Station Bus and Process Bus applications. 1588 v2 signals can also be synchronized with IRIG-B for distribution to legacy devices.

Applications for Protection and Control IEDs

Precision timing is required for IEDs performing fault recording (i.e. waveform capture), sequence of events recording and any other application that requires data to be accurately time-stamped. With the events and timing recorded in the proper chronological order the pre-fault, fault and post-fault events can be can be accurately analyzed. Most of today’s protection and control IEDs have the internal clock resolution of 1ms, therefore the typical requirement for time synchronization in IEC 61850 Substation Automation Systems is to guarantee 1ms time-stamping accuracy for Station Bus. With IEEE 1588 v2 this requirement can be achieved in a cost effective way compared to legacy IRIG-B solutions. RUGGEDCOM devices are capable of receiving 1588 v2 through their Ethernet ports and distributing 1588 v2 or synchronized IRIG-B signal for legacy devices.

IEC 61850 Process Bus

The IEC 61850-9-2 “Process Bus” standard reduces conventional copper wiring and total installed cost by allowing sampled values of current and voltage as well as control commands to switchgear in the switchyard to be distributed over an Ethernet network (a.k.a. a “Process Bus”) for connection to protection and control IEDs. Accurate time synchronization with the precision requirement of sub one microsecond is essential for a properly functioning Process Bus. Sampled Measured Values of voltage and currents need to be synchronized between the Merging Units (MUs) and the receiving IEDs that perform the critical protection and control functions. Merging Units are the interface electronic devices that enable digital communication over the Ethernet network using Sampled Measured Values between the process level and the bay level. Merging Units continuously measure multiple analog CT/VT values from primary equipment and digitize them according to IEC 61850-9-2 standard. Data shifted at the receiving IEDs by just microseconds will result in protection algorithm malfunction.

Once converted to digital format and timestamped, the Sampled Values are then sent via unsolicited multicast onto the high speed fiber optic Ethernet network, a.k.a. IEC 61850-9-2 “Process Bus”. The RUGGEDCOM RSG2288 Ethernet switch with IEEE 1588 v2 and IRIG-B support makes implementation of the IEC 61850-9-2 “Process Bus” more economical, more practical and easier to deploy by providing a reliable and precise time synchronization over the substation Ethernet network.

Synchronous Phasor Measurement (Synchrophasors)

Synchrophasors are used in the utility industry to analyze the state of the power system and implement controls to maintain stability on the grid. Phasor Measurement Units, (PMUs), take measurements at given locations on the power system and calculate phasors that are synchronized to absolute time. The IEEE C37.118 Synchrophasor for Power System standard has provided a definition for synchrophasors and how they should be time-stamped and communicated. To achieve the total accuracy of less than 1% as defined in the standard it is required that the timing accuracy is in the order of microseconds and on a switched Ethernet network the only solution is to use IEEE 1588 v2.

Deploying IEEE 1588 v2 on an IEC 61850 Network

Siemens is the leading IEEE 1588 Time Synchronization Solution provider for electric utility companies. From Master Clock, Transparent Clock and IRIG-B conversion the RUGGEDCOM RSG2288 and RS416 devices are specifically designed for harsh environments to meet the most demanding needs of Electrical Utilities. Deploying IEEE 1588 v2 in substations means anticipating the future as this time synchronization method is expected to be officially specified in the standard documents of IEC 61850.

Synchronizing IEEE 1588 v2 with IRIG-B

In many situations it may be desirable to move to IEEE 1588 v2 in stages to examine the performance and benefits of the new protocol while still employing the existing protection and control IEDs that are using IRIG-B. RUGGEDCOM devices provide the solution by allowing a time conversion mechanism of the 1588 v2 protocol at the Ethernet ports to synchronize the IRIG-B signals on output. The IRIG-B signals are available over dedicated cabling with PWM or PPS outputs on the RSG2288 Ethernet switch or integrated into each of the 16 serial port connections on the RS416 serial server for legacy devices. Figure 3 shows how the IEEE 1588 time synchronization signal is distributed over Ethernet from the IEEE 1588 Master Clock. The non-1588 IEDs are synchronized by their local RSG2288 switch with Slave and Transparent Clock functionality via the IRIG-B output.


Figure 3. Conversion into IRIG-B of IEEE 1588 time signal distributed over Ethernet by local IEEE 1588 Slave / Transparent Clocks.

In case of a green-field application where IEDs with IRIG-B input have been specified it is still beneficial to use IEEE 1588 to distribute the time synchronization signal from the substation clock and convert it to IRIG-B inside the relay cabinets, just 1 or 2 meters away from the IEDs. This solution reduces installation costs and complexity as hundreds of meters of copper signaling cable used for distributing IRIG-B signal can be eliminated. A good example of such application is large high voltage AIS substations where protection and control cabinets are located in containers or kiosks installed in the switchyard in close proximity to the primary equipment. Another example can be MV relay cabinets placed on an extensive area in large power plants or industrial installations. In the above applications there can be significant distances between all IED cabinets and the clock signal source. In cases where the communications network is based on Ethernet then IEEE 1588 can be used for sending time synchronization to all IEEE 1588 enabled Ethernet switches in each relay cabinet. The switches in the relay cabinets act as IEEE 1588 slave and transparent clocks and can have IRIG-B output permitting the IEDs mounted in the same rack to be synchronized with IRIG-B. As a result the installation permits the usage of IEDs with high accuracy IRIG-B input while at the same time reduces the total cost of installation by eliminating large copper cable runs with dedicated time synchronization wiring. Furthermore such installation is future-proof as the Ethernet switches support IEEE 1588. In case of a possible future upgrade of IEDs to IEEE 1588 no modifications would be required to the switches in all protection and control cabinets as these are already IEEE 1588 enabled. An example of the solution described above has been illustrated in the figures below. Figure 4 shows multiple IED cabinets located 50 meters from each other, the control room where the clock source with GPS antenna is installed and located 300 meters from the nearest IED cabinet. On this figure it is shown that IRIG-B signal is distributed via dedicated coaxial or twisted pair copper wiring to all relay cabinets. In figure 5 we can see that only Ethernet wiring is connecting the control room and all the relay cabinets. Significant amount of copper cabling can be saved as IRIG-B signal is only distributed locally inside each cabinet, where the distances do not exceed 2 meters. Figure 6. shows in an all Ethernet environment clock signal distribution is done over the network eliminating all the dedicated IRIG-B cabling.


Figure 4. Conventional approach with dedicated wiring for distribution of high precision time signal. The communication network is based on Ethernet and Serial communication.


Figure 5. Distribution of IEEE 1588 time signal over the network and local conversion into IRIG-B in every IED cabinet. IRIG-B is distributed via dedicated cabling or through serial connection.


Figure 6. Distribution of IEEE 1588 time signal over the network in an all Ethernet environment completely eliminates any dedicated IRIG-B cabling.

Precision Timing Source: Master / Grandmaster Clock Mode

The RUGGEDCOM RSG2288 has the flexibility to accept many time reference inputs including GPS in order to become the timing network Grandmaster clock. With these modes of operation, the RSG2288 can receive timing signals directly through GPS and with 1588 v2 over the network as a backup. The RSG2288 uses the Best Master Clock, (BMC), algorithm to determine the most accurate clock source and select this as its timing input source. If there are existing GPS clocks in place with IRIG-B output, the RSG2288 can accept this IRIG-B signal as the timing source and convert it to IEEE 1588 v2 for distribution across the network.


Transparent Clock Mode

The RSG2288 in transparent clock mode provides accurate propagation of timing signals without accumulation of timing jitter errors. The transparent clock measures peer to peer path delay and internal switch residence time.


Ordinary Clock Mode

RUGGEDCOM devices can be configured to fall back to their internal clock that has been synchronized to temporarily become the network timing master in case timing signals from GPS or other clock sources become lost, allowing the timing network to be kept up and running. The ordinary clock mode also permits the synchronization of IRIG-B with the 1588 timing network.

Figure 7. In master clock mode RSG2288 can be synchronized with a variety of clock sources. Transparent clock eliminates timing errors due to switching latency and propagation delay.

Summary

The Electric Utilities realize the accuracy and cost saving benefits of using a Precision Timing Solution based on IEEE 1588 v2 and will adopt the standard into IEC 61850 Edition 2.

Key benefits of RUGGEDCOM's Precision Timing Solution:

  • eliminates the extra cabling requirements of IRIG-B by using common Ethernet cabling

  • achieves guaranteed millisecond accuracy for substation sequence of event timing

  • sub-microsecond accuracy for critical applications like IEC 61850-9-2 Process Bus

  • eases the deployment of precision timing networks in modern ‘All-Ethernet’ substations

  • facilitates the migration path from legacy solutions and paves the way towards IEC 61850


Key technical challenges solved in IEEE 1588 v2:

  • increases accuracy by using sophisticated methods to account for internal switch processing delays and link propagation delays

  •  eliminates the traffic congestion caused by the need for end-to-end delay measurement

  •  provides the accuracy similar to GPS without the need for antennas at every node