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IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
IEEE Power & Energy Society
Sponsored by the Power System Relaying Committee and Substations Committee
IEEE 3 Park Avenue New York, NY 10016-5997 USA
IEEE Std C37.238™2011
14 July 2011
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IEEE Std C37.238™-2011
IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications Sponsor
Power System Relaying Committee and
Substations Committee of the
IEEE Power & Energy Society Approved 16 June 2011
IEEE-SA Standards Board
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Abstract: A common profile for the use of Precision Time Protocol (PTP) of IEEE Std 1588-2008 in power system protection, control, automation, and data communication applications utilizing an Ethernet communications architecture is specified. Keywords: grandmaster clock, IEEE 1588, power substation, precise time synchronization, Precision Time Protocol (PTP), sample synchronization, slave-only clock, synchrophasors, transparent clock
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Introduction This introduction is not part of IEEE Std C37.238-2011, IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications.
This standard specifies a common profile for use of IEEE 1588 Precision Time Protocol (PTP) in power system protection, control, automation, and data communication applications utilizing an Ethernet communications architecture. Typical Ethernet-based time distribution architecture consists of a reference clock, bridges, and end devices. Bridges with boundary clock functionality may also be used at interconnection points between different PTP domains or PTP profiles. In addition to distributing global time that is traceable to a recognized standard time source, the profile has a provision for distributing local time for the cases when connectivity to recognized standard time sources is lost. The profile can be used for precise time synchronization of the devices in a substation, and between substations in a larger geographical area, if performance requirements of this standard are met. The use of different physical layer communication technologies to carry Ethernet frames, including SONET/SDH and wireless technologies, is not precluded if they can meet performance requirements of this standard. Time distribution specified in this standard is based on the following basic assumptions:
All devices that participate in time distribution support this standard.
All devices are in the same time distribution domain.
All devices have point-to-point connections to their neighbors.
Transmit and receive cable delay for each point-to-point connection is assumed to be symmetrical. Known asymmetry in cable delay can be configured and corrected.
The use of security techniques is an important consideration and, based on the application, may be desirable or mandated. Security extensions and network engineering methods for hardening the PTP-based time distribution system against malicious attacks are not covered, and are outside of scope of this standard. If security techniques are used, they should not impair the ability of devices to achieve performance, specified in this standard. Future improvements in IEEE 1588 security mechanism will be considered in a revision of this standard. Redundancy is an important consideration; some applications recommend or mandate support for different time distribution technologies, e.g., Global Positioning System (GPS) and Inter-Range Instrumentation Group B (IRIG-B). Support for multiple time distribution technologies at the same time is out of scope of this standard.
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Participants At the time this standard was submitted to the IEEE-SA Standards Board for approval, the IEEE 1588 Profile for Power System Applications Working Group had the following membership: Galina S. Antonova, Chair William Dickerson, Vice Chair Alex Apostolov Christoph Brunner Dac-Phuoc Bui Michael Dood Heiko Gerstung Didier Giarratano Richard Harada Clemens Hoga Christopher R. Huntley Hubert Kirrmann
Stanley Klein Tom Ko Steven A. Kunsman Kenneth Martin Gary Michel René Midence Bruce Muschlitz Roger Moore R. Jay Murphy Bogdan Popescu Craig Preuss
Farnoosh Rahmatian Markus Renz Veselin Skendzic Fred Steinhauser Tim Tibbals Jean-Charles Tournier Benton A. Vandiver Jon Waters Justin Wu Baihua Xue
The following members of the individual balloting committee voted on this standard. Balloters may have voted for approval, disapproval, or abstention. William J. Ackerman Mark Adamiak Satish K. Aggarwal Galina S. Antonova James Ariza Ali Al Awazi Chris Bagge Philip Beaumont P. Stephan Bedrosian Kenneth Behrendt William Bloethe Chris Brooks Gustavo Brunello Christoph Brunner Arvind K. Chaudhary Prashanth Chittapuram Stephen Conrad James Cornelison Patrick Diamond Gary Donner Michael Dood Randall Dotson Neal Dowling Lee Eccles Gary Engmann Ronald Farquharson Kenneth Fodero Heiko Gerstung Jeffrey Gilbert Roman Graf Ron Greenthaler Stephen Grier Randall Groves
Roger Hedding Gary Heuston Werner Hoelzl Gary Hoffman David Horvath Christopher R. Huntley Innocent Kamwa Piotr Karocki Bogdan Kasztenny Yuri Khersonsky Stanley Klein Joseph L. Koepfinger Jim Kulchisky Chung-Yiu Lam Greg Luri Wayne W. Manges Kenneth Martin Pierre Martin William McBride David Mcginn Gary Michel René Midence William Moncrief Georges Montillet Jerry Murphy R. Murphy Bruce Muschlitz Michael S. Newman Chris Osterloh Lorraine Padden Donald Parker Charles Petras Robert Pettigrew Ulrich Pohl
Craig Preuss Michael Roberts Robert Robinson Charles Rogers M. Sachdev Bartien Sayogo Thomas Schossig Sam Sciacca Gil Shultz Tarlochan Sidhu Mark Simon Veselin Skendzic James Smith Jerry Smith Aaron Snyder John Spare Gary Stoedter Walter Struppler John Tengdin Michael Thompson Joe Uchiyama Eric Udren Benton A. Vandiver John Vergis Jane Verner Ilia Voloh John Wang Jon Waters Stephen Webb Karl Weber Ludwig Winkel Phil Winston Oren Yuen
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When the IEEE-SA Standards Board approved this standard on 16 June 2011, it had the following membership: Richard H. Hulett , Chair John Kulick , Vice Chair Robert M. Grow, Past Chair Judith Gorman, Secretary Masayuki Ariyoshi William Bartley Ted Burse Clint Chaplin Wael Diab Jean-Philippe Faure Alexander Gelman Paul Houzé
Jim Hughes Joseph L. Koepfinger* David J. Law Thomas Lee Hung Ling Oleg Logvinov Ted Olsen
Gary Robinson Jon Walter Rosdahl Sam Sciacca Mike Seavey Curtis Siller Phil Winston Howard L. Wolfman Don Wright
*Member Emeritus
Also included are the following nonvoting IEEE-SA Standards Board liaisons: Satish K. Aggarwal, NRC Representative Richard DeBlasio, DOE Representative Michael Janezic, NIST Representative Lisa Perry IEEE Standards Program Manager, Document Development Matthew J. Ceglia IEEE Standards Program Manager, Technical Program Development
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IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications IMPORTANT NOTICE: This standard is not intended to ensure safety, security, health, or environmental protection. Implementers of the standard are responsible for determining appropriate safety, security, environmental, and health practices or regulatory requirements. This IEEE document is made available for use subject to important notices and legal disclaimers. These notices and disclaimers appear in all publications containing this document and may be found under the heading “Important Notice” or “Important Notices and Disclaimers Concerning IEEE Documents.” They can also be obtained on request from IEEE or viewed at http://standards.ieee.org/IPR/disclaimers.html .
1. Overview
1.1 Scope This standard specifies a common profile for the use of IEEE 1588 Precision Time Protocol (PTP) in power system protection, control, automation, and data communication applications utilizing an Ethernet communications architecture. The profile specifies a well-defined subset of IEEE 1588 mechanisms and settings aimed at enabling device interoperability, robust response to network failures, and deterministic control of delivered time quality. It specifies the preferred physical layer (Ethernet) higher level protocol used for PTP message exchange and the PTP protocol configuration parameters. Special attention is given to ensuring consistent and reliable time distribution within substations, between substations, and across wide geographic areas.
1.2 Purpose The purpose of this standard is to facilitate adoption of IEEE Std 1588-2008 for power system applications requiring high precision time synchronization. 1 It specifies a common subset of PTP parameters and options to provide global time availability, device interoperability, and failure management. This set of PTP 1
Information on references can be found in Clause 2.
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
parameters and options allows IEEE 1588-based time synchronization to be used in mission critical power system protection, control, automation, and data communication applications.
2. Normative references The following referenced documents are indispensable for the application of this document (i.e., they must be understood and used, so each referenced document is cited in text and its relationship to this document is explained). For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies. IEEE Std 1588 ™-2008, IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems. 2, 3
3. Definitions, special terms, and word usage For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary: Glossary of Terms & Definitions should be referenced for terms not defined in this clause. 4
3.1 Definitions accuracy: The mean of the time or frequency error between the clock under test and a perfect reference clock, over an ensemble of measurements. See also: precision. boundary clock: A clock that has multiple Precision Time Protocol (PTP) ports in a domain and maintains the timescale used in the domain. It may serve as the source of time (i.e., be a master clock) and may synchronize to another clock (i.e., be a slave clock). clock: A device participating in the Precision Time Protocol (PTP) that is capable of providing a measurement of the passage of time since a defined epoch. NOTE—In the case of PTP ordinary and boundary clocks that are properly synchronized, the epoch is the epoch of the timescale in use. In the case of PTP transparent clocks (TCs), the epoch is locally defined and not necessarily aligned with the timescale. 5
device: A node with one or more Ethernet ports that is compliant to IEEE Std C37.238-2011. NOTE— Devices are network devices and ordinary clocks.
default: When applied to attribute values and options means the configuration of a Precision Time Protocol (PTP) device as it is delivered from the manufacturer. domain: A logical grouping of clocks that synchronize to each other using the protocol, but that are not necessarily synchronized to clocks in another domain.
2
IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 088544141, USA (http://standards.ieee.org). 3 The IEEE standards or products referred to in Clause 2 are trademarks owned by the Institute of Electrical and Electronics Engineers, Incorporated. 4 The IEEE Standards Dictionary: Glossary of Terms & Definitions is available at http://shop.ieee.org/. 5 Notes in text, tables, and figures of a standard are given for information only and do not contain requirements needed to implement this standard.
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
end device: An application specific device with slave-only clock functionality. epoch: The origin of a timescale. event: An abstraction of the mechanism by which signals or conditions are generated and represented. grandmaster-capable clock: A boundary or an ordinary clock that is currently capable of Precision Time Protocol (PTP) Master state. grandmaster clock: Within a domain, a clock that is the ultimate source of time for clock synchronization using the protocol. holdover: A clock previously synchronized/syntonized to another clock (normally a primary reference or a master clock) but now free-running based on its own internal oscillator, whose frequency is being adjusted using data acquired while it had been synchronized/syntonized to the other clock. It is said to be in holdover or in the holdover mode, as long as it is within its accuracy requirements. intelligent electronic device (IED): Any device incorporating one or more processors with the capability to receive or send data/control from or to an external source (e.g., electronic multifunction meters, digital relays, controllers). link: A network segment between two Precision Time Protocol (PTP) ports supporting the peer delay mechanism of this standard. The peer delay mechanism is designed to measure the propagation time over such a link. multicast communication: A communication model in which each Precision Time Protocol (PTP) message sent from any PTP port is capable of being received and processed by all PTP ports on the same PTP communication path. network device: A device used on a Precision Time Protocol (PTP) communication path to connect grandmaster clock to one or more ordinary clocks directly or through one or more network devices. NOTE—Bridges are network devices. 6
one-step clock: A clock that provides time information using a single event message. ordinary clock: A clock that has a single Precision Time Protocol (PTP) port in a domain and maintains the timescale used in the domain. It may serve as a source of time (i.e., be a master clock) or may synchronize to another clock (i.e., be a slave clock). NOTE—An ordinary clock can be grandmaster-capable or slave-only clock.
parent clock: The master clock to which a clock is synchronized. peer-to-peer transparent clock (P2P TC): A transparent clock that, in addition to providing Precision Time Protocol (PTP) event transit time information, also provides corrections for the propagation delay of the link connected to the port receiving the PTP event message. In the presence of P2P TCs, delay measurements between slave clocks and the master clock are performed using the P2P delay measurement mechanism. portNumber: An index identifying a specific Precision Time Protocol (PTP) port on a PTP node.
6
A bridge is often referred to as a layer2 switch or simply switch.
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
precision: A measure of the deviation from the mean of the time or frequency error between the clock under test and a perfect reference clock. See also: accuracy; traceability. Precision Time Protocol (PTP): The protocol defined by IEEE Std 1588-2008. As an adjective, it indicates that the modified noun is specified in or interpreted in the context of IEEE Std 1588-2008. primary reference: A source of time and or frequency that is traceable to international standards. See also: traceability. profile: The set of allowed Precision Time Protocol (PTP) features applicable to a device. Precision Time Protocol (PTP) communication: Information used in the operation of the protocol, transmitted in a PTP message over a PTP communication path. Precision Time Protocol (PTP) communication path: The signaling path portion of a particular network enabling direct communication among ordinary and boundary clocks. Precision Time Protocol (PTP) message: One of the message types defined in IEEE Std 1588-2008. Precision Time Protocol (PTP) port: A logical access point of a clock for PTP communications to the communications network. preferred grandmaster clock: A grandmaster-capable device that is intended to be the best or backup grandmaster in the network. Typically a preferred grandmaster has an independent connection to a recognized standard time source. There should be at least two preferred grandmaster clocks in a system. recognized standard time source: A recognized standard time source is a source external to Precision Time Protocol (PTP) that provides time and/or frequency as appropriate that is traceable to the international standards laboratories maintaining clocks that form the basis for the International Atomic Time (TAI) and Universal Coordinated Time (UTC) timescales. Examples of these are Global Positioning System (GPS), NTP, and National Institute of Standards and Technology (NIST) timeservers. residence time: The time difference between ingress and egress time stamps for an IEEE C37.238 Sync message. slave-only clock: An ordinary clock that is not currently capable of Precision Time Protocol (PTP) Master state. stability: A measure of how the mean of the time or frequency error between the clock under test and a perfect reference clock varies with respect to variables such as time, temperature, etc. See: accuracy. synchronized clocks: Two clocks are synchronized to a specified uncertainty if they have the same epoch, and their measurements of the time of a single event at an arbitrary time differ by no more than that uncertainty. syntonized clocks: Two clocks are syntonized if the duration of the second is the same on both, which means the time as measured by each advances at the same rate. They may or may not share the same epoch. timeout: A mechanism for terminating requested activity that, at least from the requester’s perspective, does not complete within the specified time. timescale: A linear measure of time from an epoch. TimeInaccuracy: A device’s estimate of current worst-case time error (its magnitude) between time that the device provides and traceable time.
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
traceable: Synchronized to a recognized standard time source. See also: primary reference; recognized standard time source; traceability. traceability: A property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties. NOTE—Adapted from the International Vocabulary of Basic and General Terms in Metrology [B12]. 7
transparent clock (TC): A device that measures the time taken for a Precision Time Protocol (PTP) event message to transit the device and provides this information to clocks receiving this PTP event message. See also: end-to-end transparent clock; peer-to-peer transparent clock. two-step clock: A clock that provides time information using the combination of an event message and a subsequent general message. See also: one-step clock.
3.2 Special terms IEEE C37.238 message: A PTP message compliant to IEEE Std C37.238-2011.
3.3 Word usage In this document, the word shall is used to indicate a mandatory requirement. The word should is used to indicate a recommendation. The word may is used to indicate a permissible action. The word can is used for statements of possibility and capability.
4. Abbreviations API
Application Programming Interface
BC
boundary clock (according to IEEE Std 1588-2008)
BMC
best master clock (according to IEEE Std 1588-2008)
CTQ
Continuous Time Quality {according to IEEE PC37.118.1™ [B4]}
DNP3
Distributed Network Protocol version 3 (according to IEEE Std 1815™-2010 [B10])
EUI
extended unique identifier
GPS
Global Positioning System
ID
identification
IED
Intelligent Electronic Device (according to IEC 61850-7-2, Ed. 2.0 [B2])
IERS
International Earth Rotation and Reference Systems Service
7
The numbers in brackets correspond to those of the bibliography in Annex F.
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
IRIG-B
Inter-Range Instrumentation Group B
LAN
Local Area Network
LN
Logical Node (according to IEC 61850-7-2, Ed. 2.0 [B2])
MAC
media access control (according to IEEE Std 802.3™-2005 [B9])
MIB
Management Information Base (according to RFC 1157)
OUI
organizational unique identifier (allocated by the IEEE)
NOTE—The OUI is typically used in specifications or the implementation of devices for the purpose of identification. It identifies the organization that owns the OUI-dependent subidentifier and may not necessarily be the organization that defines the specification or provides the hardware. 8
P2P
peer-to-peer (according to IEEE Std 1588-2008)
PMU
Phasor Measurement Unit (according to IEEE Std C37.118™-2005 [B11])
PTP
Precision Time Protocol (according to IEEE Std 1588-2008)
SCADA
Supervisory Control and Data Acquisition
SNMP
Simple Network Management Protocol (according to RFC 1157)
SV
Sampled Value (according to IEC 61850-9-2, Ed. 2.0 [B3])
TAI
International Atomic Time
TC
transparent clock (according to IEEE Std 1588-2008)
TLV
type, length, value (according to IEEE Std 802.1AB™-2009 [B7])
TQ
Time Quality (according to IEEE Std C37.118-2005 [B11])
UTC
Coordinated Universal Time
VLAN
Virtual Local Area Network (according to IEEE Std 802.1Q™-2005 [B8])
5. Standard profile for power system applications
5.1 Identification PTP Profile IEEE Standard Profile for Use of IEEE 1588 Precision Time Protocol in Power System Applications Version: 1.0
8
The IEEE OUI listing ca n be obtained at http:// standards.ieee.org/develop/regauth/oui/public.html.
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
Profile identifier: 1C-12-9D-00-00-00 This profile is specified by the IEEE 1588 Profile for Power System Applications Working Group of the IEEE Power System Relaying Committee and IEEE Power System Substation Committee. 9
5.2 PTP attribute values All nodes shall support the values for attributes as specified in Table 1.
Table 1 —PTP attribute values Attribute Value portDS.logAnnounceInterval 0 portDS.logSyncInterval 0 portDS.announceReceiptTimeout 2 for all preferred grandmaster clocks, 3 for all other grandmastercapable devices 1.0 s portDS.logMinPdelayReqInterval 0 defaultDS.priority1 128 for grandmaster-capable devices 255 for slave-only devices defaultDS.priority2 128 for grandmaster-capable devices 255 for slave-only devices defaultDS.slaveOnly FALSE for grandmaster-capable devices TRUE for slave-only devices defaultDS.domainNumber Default initialization value is 0
transparentClockdefaultDS.primaryDomain
Default initialization value is 0
Range
None None None
None None None
None
None
As specified by 7.1 and Table 2 of IEEE Std 15882008 As specified by 7.1 and Table 2 of IEEE Std 15882008
5.3 Path delay mechanism Clause 11 of IEEE Std 1588-2008 specifies two path delay measurement mechanisms: delay requestresponse mechanism and peer delay mechanism. The peer delay request mechanism specified in 11.4 of IEEE Std 1588-2008 shall be the only path delay measurement mechanism in operation when this profile is deployed. Two-step devices should populate the requestReceiptTimestamp field of the IEEE C37.238 Pdelay_Resp message as specified in 11.4.3, option (c) 8 (i) of IEEE Std 1588-2008. Path delay measurement and correction for the last connection to a slave-only intelligent electronic device (IED) is optional, if it is not functioning as a transparent clock (TC). Such IEDs shall use Timestamp and CorrectionField values to adjust their clock.
9
A copy of IEEE Std C37.238-2011 may be obtained from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854-4141, USA ( http://standards.ieee.org).
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
5.4 Best master clock algorithm 5.4.1 Default best master clock algorithm The master-slave hierarchy shall be determined using the default best master clock (BMC) algorithm, specified in 9.3.2, 9.3.3, and 9.3.4 of IEEE Std 1588-2008. clockAccuracy shall use the clock’s accuracy, not its precision; refer to IEEE Standards Interpretations for IEEE Std 1588™-2008 [B6]. All devices except slave-only clocks shall support the full functionality of the default best master clock algorithm.
5.4.2 Default settings Default device settings shall be determined by the most likely device function. Unless a device’s primary function is a grandmaster clock, its default configuration shall be slave-only clock. Slave-only clocks
shall not transmit IEEE C37.238 Announce messages, in order to avoid advertising themselves as potential grandmasters. shall have priority1 and ClockClass values set to 255, as specified in 7.6.2.2 and 7.6.2.4 of IEEE Std 1588-2008.
5.5 Management mechanism All grandmaster-capable devices shall report TimeInaccuracy and traceability to a recognized standard time source. Grandmaster-capable devices shall support a SNMP MIB. SNMP support for all other devices is optional. When supported, the IEEE C37.238 MIB shall be used. (The IEEE C37.238 MIB can be accessed at http://standards.ieee.org/downloads/C37/C37.238-2011/ C37.238-2011_MIB-D5-8.mib.) All devices that do not support the IEEE C37.238 MIB except transparent clocks shall specify whether and how the following information is made available:
TimeInaccuracy
Traceability to a standard recognized time source
Offset from the grandmaster
If the offset from a grandmaster exceeds a configurable limit.
NOTE—Refer to C.3 and C.4 for reporting mechanisms for IEC 61850 and IEEE C37.118 applications.
8.2.1 8.2.1.2.1 8.2.1.2.2 8.2.1.2.3 8.2.1.3.1.1 8.2.1.3.1.2 8.2.1.3.1.3 8.2.1.4.1 8.2.1.4.2 8.2.1.4.3 8.2.1.4.4 GM identity to be transmitted in IEEE_C37_238 TLV. networkTimeInaccuracy to be transmitted in IEEE_C37_238 TLV. This value is set at grandmastercapable clocks. Engineered networkTimeInaccuracy in nanoseconds. This value is set at the end device to represent the worst networkTimeInaccuracy from this device to all preferred grandmasters. Maximum TimeInaccuracy that the device contributes to the networkTimeInaccuracy. Offset from Master Limit to generate OfstExceedsLimit event. 8.2.2 8.2.2.2 8.2.2.3 TimeInaccuracy contribution of the device in nanoseconds.a 8.2.3 8.2.3.2 8.2.3.2 8.2.3.3 8.2.3.4 8.2.3.5 8.2.3.6 8.2.3.7 8.2.3.7 8.2.3.7 8.2.3.8 8.2.3.9 GM identity received in IEEE_C37_238 TLV grandmasterTimeInaccuracy received in IEEE_C37_238 TLV, nanoseconds networkTimeInaccuracy received in IEEE_C37_238 TLV, nanoseconds
8.2.1.2.1 GM identity received in IEEE_C37_238 TLV. Indicates Network Protocol in use. Port VLAN ID Port Priority grandmasterTimeInaccuracy received in IEEE_C37_238 TLV. 82 NetTimeInacc networkTimeInaccuracy received in IEEE_C37_238 TLV. 83 LocTimeInacc TimeInaccuracy contribution of the local clock in nanoseconds. 84 8.3.3 ieeeC37238TCPortDS 85 PortNumber 8.3.3.2.1 86 LMinPdlyRInt 8.3.3.3.1 87 Faulty 8.3.3.3.2 88 MeanPDly 8.3.3.3.3 89 DlyAsymm Path delay asymmetry. Boundary and ordinary clock objects (optional for transparent clocks) 90 ieeeC37238Events 91 ChangeOfMaster Indicates that new grandmaster has been selected. 92 MasterStepChange Indicates that a step change occurred in current grandmaster time. 93 FaultyState Indicates that a clock has entered faulty state. 94 PortStateChange Indicates that port state has changed. 95 OfstExceedsLimit Indicates that for a clock in a slave state Offset from Master exceeds configurable limit. 96 OtherProfileDetect Indicates that another PTP profile has been detected. 97 LeapSecAnnounced Indicates that a leap second has been announced. 98 PTPServiceStarted Indicates that PTP service has started. 99 PTPServiceStopped Indicates that PTP service has stopped.
R/W
R/W R R/W R/W R/W R R R
R R/W R R R/W
a
LocTimeInacc of the current grandmaster clock shall be identical to the value that it writes in the GMTimeInacc field of the IEEE_C37_238 TLV.
5.6 Transport mechanism The PTP over IEEE 802.3 transport mapping, specified in Annex F of IEEE Std 1588-2008, shall be the only transport mechanism in all IEEE C37.238 devices used in a substation system. All IEEE C37.238 messages shall be mapped into IEEE 802.1Q-tagged Ethernet frames with configurable priority and configurable VLAN ID. The default priority shall be 4. The default VID shall be 0. The full range of priority and VID values shall be supported per IEEE Std 802.1Q-2005 [B8]. Bridging devices shall follow the rules defined in IEEE Std 802.1Q-2005 [B8]. By default, all devices shall accept IEEE C37.238 messages that have had their IEEE 802.1Q tags removed, and tagged IEEE C37.238 messages with their configured VID value. NOTE—IEEE 802.1Q tags are often removed at bridge edge ports.
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5.7 Clock types 5.7.1 One-step and two-step clocks All devices’ ingress ports shall support both one-step and two-step modes. All devices’ egress ports may support either one-step or two-step modes. It is recommended that one-step operation would be the only mode of operation supported as it results in a less network traffic, faster possible IEEE C37.238 Sync messages rates, and potentially simpler implementations. However, allowing two-step operation allows for potentially technically more feasible silicon solutions.
5.7.2 Slave-only clocks IEDs may be designed as slave-only clocks, as specified in 9.2.2 of IEEE Std 1588-2008. NOTE—It is recommended that slave-only clocks use the IEEE C37.238 message bytes as described in Annex D. D.
5.8 Communication model The multicast communication model specified in 7.3 of IEEE Std 1588-2008 shall be used. The multicast MAC addresses shall conform to Annex F of IEEE Std 1588-2008: 01 – 80 – C2 – 00 – 00 – 0E for IEEE C37.238 Pdelay messages and 01 – 1B – 19 – 00 – 00 – 00 for all other IEEE C37.238 messages.
5.9 Timescale 5.9.1 PTP timescale The timescale of a substation system shall be the PTP timescale, as specified in 7.2 of IEEE Std 1588-2008.
5.9.2 Grandmaster clockClass Any clock specifically designed to be the grandmaster clock shall be designed to operate as a clockClass 6 clock, as specified in 7.6.2.4 of IEEE Std 1588-2008.
5.9.3 Grandmaster clockClass degradation Any clock specifically designed to be the grandmaster clock shall be designed to degrade to a clockClass 7 or 187 clock when appropriate, as specified in 7.6.2.4 of IEEE Std 1588-2008.
5.9.4 Grandmaster clockAccuracy degradation Any clock specifically designed to be the grandmaster clock shall be designed to degrade its clockAccuracy when appropriate (e.g., when in holdover), as specified by 7.6.2.5 of IEEE Std 1588-2008.
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5.9.5 Grandmaster timescale All ordinary and boundary clocks in a substation system shall be designed to support the PTP timescale in the event they become the grandmaster.
5.9.6 Grandmaster ID Grandmaster clocks shall have a configurable 1-byte ID. Grandmaster ID shall be communicated using profile specific IEEE_C37_238 TLV, defined in 5.12.2. 5.12.2.
5.9.7 Transient behavior of grandmaster-capable grandmaster-capable clocks Grandmaster-capable clock vendors shall document devices’ resynchronization behavior by specifying the offset from primary reference, a threshold, at which the clock will step its time. When the offset is less than the threshold the clock shall slew its time with a constant slew rate that shall also be specified.
5.10 Clauses 16 and 17, Annexes K and L of IEEE Std 1588-2008 From the options specified in Clause 16, Clause 17, Annex K, and Annex L of IEEE Std 1588-2008, only ALTERNATE_TIME_OFFSET_INDICATOR TLV is required
5.11 clockIdentity As specified in 7.5.2.2 of IEEE Std 1588-2008, clockIdentity values shall be EUI-64 constructed based on EUI-48.
5.12 TLVs 5.12.1 IEEE 1588 TLVs The following TLV defined in Clause 14 of IEEE Std 1588-2008 shall be supported by all devices:
ALTERNATE_TIME_O ALTERNATE_TIME_OFFSET_INDIC FFSET_INDICATOR ATOR TLV (specified by 16.3 of IEEE Std 1588-2008) for determining Local time.
ORGANIZATION_EXTE ORGANIZATION_EXTENSION NSION (specified by 14.3 of IEEE Std 1588-2008), as defined in 5.12.2.
These TLVs shall be appended to IEEE C37.238 Announce messages. The order of the mandatory TLVs shall be as follows:
ORGANIZATION_EXTENSION (IEEE_C37_238 TLV)
ALTERNATE_TIME_OFFSET_INDICATOR TLV.
Additional TLVs, if present, shall follow the two mandatory TLVs.
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The use of additional TLVs is beyond the scope of this standard.
5.12.2 Profile specific TLVs The following profile specific TLV shall be supported:
IEEE_C37_238 TLV
This TLV shall be used to communicate the following:
Grandmaster ID
GrandmasterTimeInaccuracy
NetworkTimeInaccuracy
The grandmaster ID is user configurable. The implementation of this TLV shall conform to the terms of 14.3 of IEEE Std 1588-2008. The IEEE_C37_238 TLV field values shall be as follows:
tlvType (Enumeration16); ORGANIZATION_EXTE ORGANIZATION_EXTENSION NSION value = 0003 hex
lengthField (UInteger16) value = 000C hex. The number of octets in the dataField
organizationId (Octet[3]); (Octet[3]); The OUI value assigned by IEEE RAC = 1C129D hex
organizationSubType (Enumeration24) (Enumeration24) value = 000001hex
dataField (12 octets) as follows:
grandmasterID (2 octets) value 0003-00FE hex, most significant byte is reserved and shall be 0. Other values are invalid and indicate that grandmasterID is not configured.
grandmasterTimeInaccuracy (Uinteger32) value 00000000-FFFFFFFF hex, in nanoseconds, where FFFFFFFF indicates the maximum value (approximately 4.29 s) has been exceeded.
networkTimeInaccuracy10 (Uinteger32) value 00000000-FFFFFFFF hex, in nanoseconds, where FFFFFFFF indicates the maximum value (approximately 4.29 s) has been exceeded. reserved (2 octets) value 000000000000 hex shall be transmitted.
The IEEE C37.238 Announce messages shall not be sent until the grandmaster clock has been assigned a unique grandmaster ID and data for the ALTERNATE_TIME_OFFSET_INDICATOR TLV has been configured. IEEE C37.238 Announce messages without two mandatory TLVs attached shall be ignored by the BMC algorithm.
10
networkTimeInaccuracy is provided for experimental purposes, and its use is undefined.
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5.13 TimeInaccuracy IEEE C37.238 TimeInaccuracy provides a mechanism by using profile specific IEEE_C37_238 TLV, defined in 5.12.2, for the applications to determine whether the worst-case error estimate in the delivered time is acceptable. All clocks shall provide TimeInaccuracy using profile specific IEEE_C37_238 TLV, defined in 5.12.2. TimeInaccuracy includes the following:
grandmasterTimeInaccuracy
networkTimeInaccuracy
GrandmasterTimeInaccuracy is provided by grandmaster clocks. This value shall be a more precise value of the clockAccuracy of the clockQuality field specified in 7.6.2.5 and Table 6 of the IEEE Std 1588-2008. NetworkTimeInaccuracy is configured at the grandmaster and shall be set to 0 by default. This value may be set to TimeInaccuracy accumulated in the worst network path. LocalTimeInaccuracy is maximum TimeInaccuracy that the device contributes to the networkTimeInaccuracy. Device manufactures shall specify LocalTimeInaccuracy. This value shall not exceed the value specified in Annex B. TimeInaccuracy shall be calculated during network engineering stage based on device specifications and network design. TCs shall not forward any IEEE C37.238 messages, while their LocalTimeInaccuracy exceeds its specified maximum LocalTimeInaccuracy. NOTE—TCs could use the following parameters to define their TimeInaccuracy:
Error introduced by the Timestamping hardware.
Error of the PDelay measurements on the receiving port.
Syntonization error, i.e., uncertainty combined with the residence time of previous Sync messages.
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Annex A (informative)
Operating modes
A.1 Overview This profile for use of IEEE 1588 PTP in power system applications describes a robust, Ethernet-based method for precise clock synchronization. Although optimized for stringent timing applications, it is versatile enough to allow simple implementations while retaining good balance between time precision and the complexity of implementation. Substation applications using PTP time distribution can be broadly divided into following categories: 1)
General purpose, visually observed time indicators (< ± 100 ms)
2)
Time stamping of SCADA data and operational data logs (< ± 1 ms)
3)
IRIG-B replacement applications with limited distance to the PTP grandmaster (< ± 1 μs)
The first three categories operate as slave-only clocks and are not allowed to transmit IEEE C37.238 Announce messages to participate in the BMC election process. They are located at the network edge (last device) and cannot pass traffic to others, meaning they are not required to support TC functionality. Devices in the first three categories are expected to process IEEE C37.238 Sync, Follow_Up, and Announce messages, but do not need to support Pdelay exchange or transmit IEEE C37.238 Pdelay messages. They process all IEEE C37.238 Sync message fields (including correctionField), support onestep and two-step operation (with and without Follow_Up), and correctly process the leap second events, as specified in Annex D.
A.2 Description Category 1 devices typically target general purpose processors and do not require PTP-specific hardware. Although some evaluation of the software stack delays may be required, this category does not generally need specialized software techniques. The software stack needs to be capable of receiving and analyzing layer 2 messages. Category 2 devices may target general purpose processors and do not necessarily require PTP-specific hardware. Due to additional precision requirements, this implementation may need to use interrupts or careful/customized software implementation. While it may be possible to a use general-purpose software stack, some optimization is likely to be required. Category 3 devices are intended for direct replacement of IRIG-B; for example, for pulse per second IRIG-B distribution networks in which the distance between the last PTP source (PTP grandmaster, boundary clock, or transparent clock) and the category 3 slave does not exceeding 100 m (propagation delay < 0.5 μs). It is intended to replicate current applications that do not explicitly compensate IRIG-B cable length. Category 3 devices may target general purpose microprocessors, but depending on operating system, hardware, and other factors, may require PTP-specific hardware support (hardware-based timestamping). Category 3 devices are expected to emulate the Category 4 slave behavior with the
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exception of not measuring the length of the last cable/fiber segment (not supporting IEEE C37.238 Pdelay message exchange). Category 4 devices support full implementation of this profile. They typically require hardware assisted timestamping and participate in the IEEE C37.238 Pdelay message exchange. Category 4 devices may be placed anywhere in the network.
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Annex B (normative)
Steady-state performance requirements This annex specifies steady-state performance requirements for the devices and networks providing time distribution service to the end devices. These devices include grandmaster clocks, transparent clocks, and ordinary clocks. Performance requirements for the end devices are application dependent and are out of scope of this annex. Network devices shall be able to distribute time from a grandmaster clock to the end devices over 16 network hops. The worst-case time error between the time delivered to the end device and recognized standard time source shall be ± 1 μs. This shall apply for network loads up to 80% wire-speed (line rate) on each link. Random-length Ethernet frames shall be used: 80% with priority 4 and 20% with lower priority. TimeInaccuracy provided by grandmaster clocks compliant to IEEE Std C37.238-2011 shall be better than 0.2 μs. NOTE—Grandmaster clocks with TimeInaccuracy that exceeds 0.2 μs may be used for shorter PTP communication paths (less than 16 network hops), if the TimeInaccuracy accumulated in the communication path is better than 1 μs.
TimeInaccuracy provided by TCs compliant to IEEE Std C37.238-2011 shall be better than 50 ns. Holdover drift of grandmaster-capable devices shall be within 2 μs for up to 5 s at a constant temperature. The time it takes a device to report its status and start IEEE C37.238 associated stack and processes on start-up shall be specified by the manufacturer.
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Annex C (informative)
Time performance parameters and their use for IEC 61850, IEEE Std C37.118, and IRIG-B applications This annex describes the following:
IEEE C37.238 time performance parameters
Time performance parameters mapping into IEEE C37.238 MIB
Use of time performance parameters for IEC 61850, IEEE C37.118, and IRIG-B applications
IEC 61850 Logical Node for bridge function and IEEE Std C37.238
C.1 Time performance parameters Power substation applications require accurate time synchronization and time quality information for phasor estimations relative to UTC time, sample synchronization, event time stamping, and other functions. Time quality information generally includes the following:
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Slave clocks may need to track the performance of their parent in order to determine whether the parent’s signal is a valid signal, as required by A.4 of IEEE Std 1588-2008. Remote substations utilizing PTP may be subject to grandmaster clock failure when time or frequency can become incorrect, plus deliberate effects such as GPS spoofing that substitutes the real GPS signals with other signals. Mechanisms to provide this performance monitoring may be considered in a future revision of the IEEE Std C37.238.
C.2 Use of IEEE C37.238 MIB for time performance parameters TimeInaccuracy and traceability to a standard recognized time source are available in the IEEE C37.238 MIB as specified in Table 1 and Annex E.
localTimeInaccuracy is available as ieeeC37238currentDSLocTimeInacc, object 3 in the current dataset ieeeC37238currentDS.
grandmasterTimeInaccuracy is available as ieeeC37238parentDSGMTimeInacc, object 13 in the current dataset ieeeC37238parentDS
networkTimeInaccuracy is available as ieeeC37238parentDSNetTimeInacc, object 14 in the current dataset ieeeC37238parentDS
engineeredTimeInaccuracy is available as ieeeC37238defaultDSEngTimeInacc, object 13 in the current dataset ieeeC37238defaultDS
timeTraceable flag is available as ieeeC37238DefaultDSTimeTraceable, object 5 in the time properties dataset ieeeC37238DefaultDS.
The currently selected grandmaster identity is communicated via IEEE_C37_238 TLV and is available in the IEEE C37.238 MIB, object 12 in the parent dataset ieeeC37238parentDS and object 8 in the TC default dataset ieeeC37238TCDefaultDS. Grandmaster identity to be used when a clock becomes a grandmaster is available as IEEE C37.238 MIB object 11 in the default dataset ieeeC37238DefaultDS.
C.3 Use of IEEE Std C37.238 for IEC 61850 applications IEC 61850 specifies TimeStamp type for time stamping all data provided by IEDs and SmpSynch parameter for time quality information for the sampled value service (IEC 61850-7-2 [B2], IEC 61850-9-2 [B3]).
C.3.1 IED data timestamping The IEC 61850 TimeStamp represents a UTC time with the epoch of midnight (00:00:00) of 1970-01-01 (IEC 61850-7-2 [B2]). The PTP TimeStamp represents a time with the epoch of 31 December 1969 23:59:51.999918 UTC. 11 The IED should therefore set the following IEC 61850 fields as shown: If the required TimeStamps’ timescale = UTC LeapSecondsKnown = True
11
Historic note: Although these statements indicate a difference of 8 s and 82 Ps between the two epochs, such difference does not exist; the difference between these epochs is actually exactly 8 s because since 1972 the seconds rollover of UTC and PTP have been aligned.
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SecondsSinceEpoch = PTP Time (seconds) – currentUtcOffset
12
FractionOfSecond 13 = (Integer) PTP Time (nanoseconds) × 2 24 × 10 –9 If the required TimeStamps’ timescale = PTP LeapSecondsKnown = False SecondsSinceEpoch = PTP Time (seconds) FractionOfSecond 5 = (Integer) PTP Time (nanoseconds) × 2 24 × 10 –9 The IEEE C37.238 TimeInaccuracy provides information for setting IEC 61850 TimeAccuracy value. Per IEC 61850-7-2 [B2], the IEC 61850 TimeAccuracy is the number of significant bits n in the TimeStamp’s FractionOfSecond field (the latter being the number of 2 – 24 s (approximately 60 ns) in the current UTC second). Mapping between the IEEE C37.238 TimeInaccuracy and the IEC 61850 TimeAccuracy is shown in Table C.1, refer to IEC 61850-5 [B1].
C.3.2 Sampled Value (SV) service IEC 61850 SmpSynch contains time quality information for the SV service (IEC 61850-9-2 [B3]). The IEEE C37.238 timeTraceable flag corresponds to IEC 61850 reporting in SmpSynch that SV are synchronized by a global area clock. The IEEE C37.238 grandmaster ID should be used in IEC 61850 SmpSynch to report what local area clock the SV are synchronized to.
C.3.3 IEC 61850 Logical Node for bridge function and IEEE Std C37.238 A future edition of IEC 61850 may include a part on Logical Node (LN) for bridge function. The intention is to create a data model with a dedicated LN for the bridge function, which could include IEEE Std C37.238. Such an IEC 61850 LN cannot be integrated yet, but may be considered in a future revision of IEEE Std C37.238.
12
During a leap-second event, if leap-second indication is communicated, this value should be changed before the updated currentUtcOffset is received (using flagField[9:8]). 13 For the most accurate results, 1/2 the quantization step of 2 –24 seconds must be added either at this IED (doing the conversion from IEEE C37.238 to IEC 61850) or somewhere else (e.g., at the devices using the IEC 61850 messages), but only once.
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
Table C.1—Mapping between IEEE C37.238 TimeInaccuracy and IEC 61850 TimeAccuracy IEEE C37.238 TimeInaccuracy, plus uncertainty (in ns) in process generating the IED data
IEC 61850-7-2 TimeAccuracy code n n
> 500 000 001
0
250 000 001 to 500 000 000
1
125 000 001 to 250 000 000
2
62 500 001 to 125 000 000
3
31 250 001 to 62 500 000
4
15 625 001 to 31 250 000
5
7 812 501 to 15 625 000
6
3 906 251 to 7 812 500
7
1 953 126 to 3 906 250
8
976 563 to 1 953 125
9
488 282 to 976 562
10
244 141 to 488 281
11
122 071 to 244 140
12
61 036 to 122 070
13
30 518 to 61 035
14
15 259 to 30 517
15
7 630 to 15 258
16
3 815 to 7 629
17
1 908 to 3 814
18
954 to 1 907
19
477 to 953
20
239 to 476
21
120 to 238
22
60 to 119
23
0 to 59
24
Resulting time –n accuracy (2 )
Time performance class defined in IEC 61850-5
Approx. 7.8 ms
10 ms (performance class T0)
Approx. 0.9 ms
1 ms (performance class T1)
Approx. 61 Ps
100 μs (performance class T2)
Approx. 15 Ps
25 μs (performance class T3)
Approx. 3.8 Ps
4 μs (performance class T4)
Approx. 0.9 Ps
1 μs (performance class T5)
C.4 Use of IEEE Std C37.238 for IEEE C37.118 applications The IEEE C37.118 TimeStamp represents a UTC time with the epoch of midnight (00:00:00) of 1970 01 01. The PTP TimeStamp represents a time with the epoch of 31 December 1969 23:59:51.999918 UTC. 14 The IED should therefore set the following IEEE C37.118 fields as shown:
14
Historic note: Although these statements indicate a difference of 8 s and 82 Ps between the two epochs, such difference does not exist; the difference between these epochs is actually exactly 8 s because since 1972 the seconds rollover of UTC and PTP have been aligned.
IEEE C37.118 and IEEE PC37.118.1/2 Synchrophasor applications require information on time quality and traceability to a recognized standard time source, e.g., UTC (IEEE Std C37.118-2005 [B11], IEEE PC37.118.1 [B4], IEEE PC37.118.2™ [B5]). IEEE Std C37.238 provides time quality and traceability via TimeInaccuracy and timeTraceable field as described in C.1. IEEE PC37.118.1 requires providing a time traceable and quality indication with each measurement. Since it only covers measurement requirements and not any external reporting, the format and representation is not specified. For IEEE PC37.118.2, IEEE C37.238 timeTraceable flag maps directly to bit[13] of the STAT in the Synchrophasor Data frame. This bit is cleared if the time provided is traceable to UTC and set if it is not locked to a traceable source. IEEE PC37.118.2 also provides Time Quality in the data frame, both for the overall frame in the FRACSEC word (MSG_TQ) and for each phasor measurement unit (PMU) segment in the STAT word (PMU_TQ). These indications should be derived from the IEEE C37.238 TimeInaccuracy value. The IEEE C37.238 TimeInaccuracy represents the worst-case time error provided to the end device, in this case the PMU. The PMU should process the timing signal internally as it is applied to the measurement and data. This internal processing may add some uncertainty to the timing precision. The TimeInaccuracy value provided under IEEE C37.238 should be increased sufficiently to cover these internal PMU inaccuracies before assignment of the measurement time quality value required for IEEE PC37.118.1/2. The time quality fields provided by IEEE PC37.118.2 are a 3-bit (PMU_TQ) and a 4-bit (MSG_TQ) code that covers subsets of the range provided by IEEE C37.238. Each code represents an interval of error. The TimeInaccuracy should be mapped to the code representing the interval into which it falls. Consult the draft standard for details (IEEE PC37.118.2 [B5]). For example, if the IEEE C37.238 TimeInaccuracy value is 60 500 ns (the value is an unsigned integer in ns), and the estimated uncertainty of processing in the PMU is 500 ns, then the worst-case error is 61 000 ns. The MSG_TQ is 0 if the signal is locked to a traceable source. This should be determined from the timeTraceable bit in IEEE C37.238. If this bit is cleared (indicating time is not traceable), then this uncertainty will map into code 6 (binary 0110), which covers the interval between 0.00001 s (10 000 ns) and 0.0001 s (100 000 ns). The PMU_TQ is active at all times. The PMU_TQ for this total error would be code 4 (binary 100), which covers the interval from 0.00001 s (10 000 ns) and 0.0001 s (100 000 ns). The message time quality flag in provided by IEEE PC37.118.2 additionally provides the mechanism required to differentiate records with identical time stamps (an artifact from the use of UTC time); the IEEE C37.238 flagField[9:8] bits provide the information needed for this.
C.5 Use of IEEE Std C37.238 for IRIG-B time-distribution applications Some applications using IRIG-B for time distribution require information on time quality and traceability to a recognized standard time source, e.g., UTC (IEEE Std C37.118-2005 [B11]). IEEE Std C37.118-2005 specifies the use of IRIG-B control bits P71-P74 to show if the source (e.g., a grandmaster clock) is locked to a traceable source, or (if not locked) the estimated worst-case time error.
15
During a leap-second event, if leap-second indication is communicated, this value should be changed before the updated currentUtcOffset is received (using flagField[9:8]). 16 Other quantization steps are allowed by IEEE Std C37.118 [B11] (for other applications).
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
Refer to IEEE PC37.118.1 [B4] for the mapping between IEEE C37.238 TimeInaccuracy (when the clock is not locked) and the IRIG-B control bits P71-P74 (IEEE Std C37.118-2005 [B11], IEEE PC37.118.2 [B5]). For example, if the IEEE C37.238 TimeInaccuracy value is 60 500 ns (the value is an integer in ns) and the clock is not locked to a traceable source, then this uncertainty will map into TQ code 6 (binary 0110), which covers the interval between 0.00001 s (10 000 ns) and 0.0001 s (100 000 ns). If the clock is locked to a traceable source, then the TQ will be 0 regardless of the TimeInaccuracy value. The IEEE PC37.118.1 proposed revision of IEEE Std C37.118 specifies the use of IRIG-B control bits P76P78 for a new field (CTQ for Continuous Time Quality) to show the estimated worst-case time error at all times (i.e., whether the source is locked or not). Refer to IEEE PC37.118.1 for the mapping between IEEE C37.238 TimeInaccuracy (when the clock is not locked) and the IRIG-B control bits P76-P78 (IEEE Std C37.118-2005 [B11], IEEE PC37.118.2 [B5]). For example, if the IEEE C37.238 TimeInaccuracy value is 60 500 ns (the value is an integer in ns), this uncertainty will map into CTQ code 4 (binary 100), which covers the interval between 0.00001 s (10 000 ns) and 0.0001 s (100 000 ns).
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
Annex D (informative)
Use of IEEE C37.238 messages for slave-only clocks (IRIG-B replacement) This annex describes how slave-only clocks can use the IEEE C37.238 message bytes. This suggestion does not provide automatic compensation for the cable delay to the destination IED with a slave-only clock as this is normally negligible, being only approximately 5 ns/m and, if desired, could be compensated in a manner similar to the compensation for the cable delay to GPS antennae (a configurable delay-correction field). The non-negligible delays introduced by the network bridges are compensated. NOTE—For this annex, the position of a byte in a frame is designated by the integer B n, with n starting at 0 for the byte after the frame’s EtherType field.
D.1 Use of received IEEE C37.238 Sync and Follow_Up messages to obtain a PTP time Detect IEEE C37.238 Sync messages, these are Ethernet frames with the following five fields:
A Destination MAC Address of 01-1B-19-00-00-00
An EtherType field of 88F7 16
B0 = 0016 (identifier for IEEE C37.238 Sync messages)
B2, B3 = 4410 (indicates the message has 44 bytes)
B20 through B27 (clockIdentity) = B53 through B60 (grandmasterIdentity) of IEEE C37.238 Announce message
If B6[1] = 0, the PTP Timestamp (the time when the one-step IEEE C37.238 Sync message entered the cable, as the number of seconds since the start of 1970, TAI) comprises the sum of the following three fields. In the IEEE C37.238 Sync message:
The 48-bit unsigned-integer B34 through B39 (48-bit seconds of the originTimestamp)
The 32-bit unsigned-integer B40 through B43 (32-bit nanoseconds field of the originTimestamp)
The 64-bit signed-integer B8 through B15 (correctionField in 2 –16 nanoseconds)
If B6[1] = 1, the PTP Timestamp (the time when the two-step IEEE C37.238 Sync message entered the cable, as the number of seconds since the start of 1970, TAI) comprises the sum of the following four fields. In the IEEE C37.238 Sync message:
64-bit correctionField B8 through B15 (first correctionField in 2 –16 nanoseconds)
In the IEEE C37.238 Follow_Up message (same bytes except B 0 = 0816):
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
The 48-bit unsigned-integer B34 through B39 (48-bit seconds field of the originTimestamp)
The 32-bit unsigned-integer B40 through B43 (32-bit nanoseconds field of the originTimestamp)
The 64-bit signed-integer B8 through B15 (second correctionField in 2 –16 ns)
D.2 Use of received IEEE C37.238 Announce messages to determine UTC time and local time NOTE—Each IEEE C37.238 Announce message comprises an IEEE 1588 Announce message, extended using an IEEE 1588 ORGANIZATION_EXTENSION TLV (for grandmaster ID and TimeInaccuracy), followed by an IEEE 1588 ALTERNATE_TIME_OFFSET_INDICATOR TLV (for local time).
Detect IEEE C37.238 Announce messages, these are Ethernet frames with the following six fields:
A Destination MAC Address of 01 – 1B – 19 – 00 – 00 – 00
An EtherType field of 88F7 16
B0 = 0B16 (identifier for IEEE C37.238 Announce messages)
B64 = 000316 (identifier for IEEE 1588 profile-specific TLV)
B68 through B70 = 1C129D16 (OUI for IEEE C37.238 TLV. This guarantees rejection of other IEEE 1588 profiles)
B90 through B91 = 000916 (identifier for IEEE 1588 ALTERNATE_TIME OFFSET_INDICATOR TLV)
D.2.1 Determining UTC time The frame’s bytes B44B45 have the number of seconds to be subtracted (from the PTP Timestamp) to get UTC time. If the frame’s byte B 7[2] is set, the above should be considered invalid (this bit is currentUtcOffsetValid).
D.2.2 Determining future leap-second adjustments to UTC time If the frame’s byte B 7[0] is set, the last minute of the current UTC day will have 61 s. If the frame’s byte B 7[1] is set, the last minute of the current UTC day will have 59 s.
D.2.3 Determining local time To determine local time, the slave clock should add the signed 32-bit (seconds) field currentUtcOffset comprising B95 through B98 to the PTP time.
D.2.4 Determining daylight-time savings events A pending daylight-time savings event is indicated by a non-zero signed 32-bit jumpSeconds field comprising B99 through B102. This indicates that when the PTP seconds field becomes equal to the unsigned 48-bit timeOfNextJump field comprising B 103 through B108, the currentOffset is incremented by jumpSeconds value.
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
NOTE—Due to network delays, the updated IEEE C37.238 Announce messages’ currentOffset field will not be received for perhaps a few seconds, and therefore should be ignored for a few seconds after an event.
D.2.5 Determining the name of local time Byte B109 is the number of following bytes that encode the name of the local time (e.g., “PST”) using UTF-8, which is the Basic Latin block specified by ISO/IEC 10646 [B13] (to conform with IEEE Std 15882008). If the number of characters is odd, the following byte (padding) is set to 00 16. Examples of UTF-8 codes are given in Table D.1. 17
Table D.1—Examples of UTF-8 codes
D.3 Use of received IEEE C37.238 Announce messages (with TLVs) to determine the time’s quality Slave-only clocks require a given time quality for their application functions as well as traceability to a recognized standard time source. They also may need to report these parameters in an application specific format.
D.3.1 Time traceability To determine traceability of received time, the slave-only clock should
17
Check that the frame’s B 7[3] bit is set (denotes timescale is PTP).
Check that the frame’s B 7[4] bit is set (denotes that time is traceable to a recognized standard time source, e.g., GPS signal is present).
Refer to http://www.cs.tut.fi/~jkorpela/ucs.html8 for more details.
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
D.3.2 Time inaccuracy The TimeInaccuracy provides an estimate of the worst-case error, in nanoseconds, of the time being provided; this is the sum of the source clock’s uncertainty and the uncertainties of all the other (e.g., transparent) clocks transporting the messages. To determine the received time’s inaccuracy, the slave clock should add the following three fields: B76 through B79 = grandmasterTimeInaccuracy (in nanoseconds) B80 through B83 = networkTimeInaccuracy (in nanoseconds) locally configured engineeredTimeInaccuracy (in nanoseconds) The last two values are calculated by the system designer by summing the individual localTimeInaccuracy for each TC in the cascade from the grandmaster clock. This value is set either in networkTimeInaccuracy in grandmaster clocks or engineeredTimeInaccuracy in end devices.
D.3.3 Determining the grandmaster clock sourcing the time The grandmasterIdentity field B 53 through B60 identifies the grandmaster clock source, also referred to as clockIdentity. This ID is needed to ensure that only Sync messages from an IEEE C37.238 clock are used for determining the time
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
Annex E (normative)
Management Information Base IEEEC37-238-MIB DEFINITIONS ::= BEGIN --===================================================================== -- MIB for support of IEEE C37.238 Standard Profile for Use of -- IEEE 1588 Precision Time Protocol in Power System Applications --===================================================================== IMPORTS MODULE-IDENTITY, OBJECT-TYPE, NOTIFICATION-TYPE, Unsigned32, Integer32 FROM SNMPv2-SMI -- [RFC2578] TEXTUAL-CONVENTION, TruthValue FROM SNMPv2-TC -- [RFC2579] MODULE-COMPLIANCE, OBJECT-GROUP, NOTIFICATION-GROUP -- [RFC2580] FROM SNMPv2-CONF ifGeneralInformationGroup FROM IF-MIB; -- [RFC2863] ieeeC37238TSMib MODULE-IDENTITY LAST-UPDATED "201105050000Z" -- May 5, 2011 ORGANIZATION "IEEE PSRC / SUB Working Group H7/SubC7" CONTACT-INFO "WG-URL: http://www.pes-psrc.org/h/ WG-EMail: Contact: Galina S. Antonova Postal: 600-3731 North Fraser Way Burnaby, BC V5J 5J2 Canada E-mail: [email protected]" DESCRIPTION "The Management Information Base module for IEEE C37.238 time synchronization protocol." REVISION "201105050000Z" -- May 5, 2011 DESCRIPTION "Published as part of IEEE PC37.238 Draft D5.8 Copyright (C) IEEE (2011)." -- IEEE C37.238 MIB info. ::= { iso(1) org(3) ieee(111) standards-association-c-seriesstandards (3) std-c37 (37) part238 (238) 9999 } ieeeC37238Notifications OBJECT IDENTIFIER ::= { ieeeC37238TSMib 0 } ieeeC37238Objects OBJECT IDENTIFIER ::= { ieeeC37238TSMib 1 } ieeeC37238Conformance OBJECT IDENTIFIER ::= { ieeeC37238TSMib 2 } --===================================================================== -- Textual Conventions --=====================================================================
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IEEE Std C37.238-2011 IEEE Standard Profile for Use of IEEE 1588™ Precision Time Protocol in Power System Applications
UNITS "2**-16 ns * 2**64" MAX-ACCESS read-write STATUS current DESCRIPTION "Offset from Master Limit to generate OfstExceedsLimit event." ::= { ieeeC37238DefaultDS 15 } --===================================================================== -- The Current data set represents this system’s topological location -- relative to the known Grandmaster system. --===================================================================== ieeeC37238CurrentDS OBJECT IDENTIFIER ::= { ieeeC37238Objects 2 } ieeeC37238CurrentDSStepsRemoved OBJECT-TYPE SYNTAX Integer32(-32768..32767) MAX-ACCESS read-only STATUS current DESCRIPTION "The number of communication paths traversed between the local clock and the grandmaster clock. The initialization value shall be 0." REFERENCE "IEEE Std 1588-2008 8.2.2.2" DEFVAL { 0 } ::= { ieeeC37238CurrentDS 1 } ieeeC37238CurrentDSOfstFrMaster OBJECT-TYPE SYNTAX IEEEC37238TimeInterval UNITS "2**-16 ns * 2**64" MAX-ACCESS read-only STATUS current DESCRIPTION "Implementation-specific representation of the current value of the time difference between a master and a slave as computed by the slave; i.e., =
