PERFECT TIMINGHOW IEEE STANDARD PC37.238 IMPACTS
SUBSTATION AUTOMATION
CRAIG PREUSS, P.E.ENGINEERING MANAGER – UTILITY AUTOMATIONBLACK & VEATCH CORPORATIONSUBSTATIONS C0 SUBCOMMITTEE CHAIRWORKING GROUP C7 MEMBER
• Why is Timing Important to a Smart Grid?
• Trusted Time
• What Time is It Today?
• What Time is It Tomorrow?
• Origins of Perfect Time
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• August 14, 2003 blackout revealed gaps in substation time synchronization
• Utilities implement synchronization different ways, including not at all
• In 2009, NERC guidelines[1] state that time synchronization accuracy should be accurate to 1 ms and utilize the most accurate method suitable for the application
• In 2009, NIST gets involved with Priority Action Plan 13 in the Smart Grid Interoperability Roadmap Release 1.0
• “Time synchronization is the key to many Smart Grid applications”
• NIST is focused on PMUs and IEC 61850 sampled measured values
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WHY IS TIMING IMPORTANT TO A SMART GRID?
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[1] NERC Security Guideline for the Electricity Sector: Time Stamping of Operational Data Logs” version 0.995, available at http://www.nerc.com/docs/cip/sgwg/Timestamping_Guideline_009-11-11_Clean.pdf
1. Fault and disturbance recording devices (DFRs and DME) are governed by NERC PRC-018-1 to be within 2 ms of UTC
2. SOE (typically 1 ms)
3. PMU IEDs (typically 1 μs)
4. Test equipment for synchronized end-to-end testing (typically 1 ms)
5. Reporting to RTU, EMS, and SCADA systems (typically 1 ms)
6. Sampled measured values in IEC 61850-9-2 require 1 μs
7. Traveling wave fault detection (towers located 800 ft apart represent 1.7 μs, so 100 ns is practical)
8. Lightning correlation (1 ms)
9. Accurate correlation of substation events with communication network events (1 s ??)
10. Special protection schemes (50 ms or less depending on scheme requirements)
11. Smart meters and revenue meters require from 1 ms to 1 μs
12. Control of fast acting switches and actuators as low as 1 μs
13. “Use of Time Synchronized Measurements in Protective Relay Applications” PSRC H14
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ARE THERE MORE THAN TWO APPLICATIONS?
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TIMING DEPENDENT APPLICATIONS
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perationsForensics
Phasor Measurements
Sampled Measured Values
Travelling Wave Fault Loc.
Lightning Strike Correlation
SCADA/EMS/SAS/DMS/DA
Comm. Network Correlation
Special Protection Schemes
Event/Disturbance Recorders
Metering
0.5s 1ms 1 sAccuracy
SOE
Test Equipment
Fast Acting Control
Future PMU Protection Apps
• IEEE Std C37.1-2007
• Clocks be set an order of magnitude greater than the actual timing requirement
• A requirement for 1 ms requires the clock to be accurate to 0.1 ms
• PSRC H3 is working on PC37.237
• Recommended practice for time tagging of power system protection events in protective relays
• Similar work starting in SUBS
• IEC 61850 Time Performance Ranges
There is a need for sub-microsecond accuracy in the Smart Grid!
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WHAT DO STANDARDS REQUIRE?
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Protection Protects equipment Includes event/disturbance
recorders
By-product:Time stamped data
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WHAT TIME IS IT?
Tele-communication
Private SDH/SONET networks for secure voice and datacommunication (SCADA)
Input:Frequency
Metering & Measurement
Metering at substations / customer
Transducers for voltage, current, phase…
Result:Time stamped data
Control Systems
SCADA/EMS/DMS DA SA Generation
Result:Time stamped data
• Absolute time matters when …
• Applications function across the Smart Grid (beyond the substation)
• It changes the measurement quality
• The measurement affects grid security (wide area control and protection)
• NERC PRC-018-1 compliance counts
• Is our timing system trusted?
• Trusted time is accurate, secure, and deterministic
• GPS is the standard time source
• GPS is not under your control, and issubject to external influences
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ATTRIBUTES OF TRUSTED TIME
PRC-018-1 Disturbance Monitoring Equipment Installation and Data Reporting
IS GPS A TRUSTED TIME SOURCE?
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Satellite Outages
GPS JammersPersonal Privacy Devices
RF, Climatic & Solar Interference
• Does the GPS system meet the N-1 contingency requirements
• Substations are designed to N-1 criteria
• Protection is designed to N-1 criteria
• Telecommunication networks assumeGPS can fail (Cesium)
• Should we depend solely on GPSfor time sensitive applications?
• How can we backup the time source?
• How can we survive temporary outages?
N-1 ContingencyWithstand the loss of any one item of plant/component without loss of load or adverse voltage outcome
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IS GPS SUFFICIENT?
BACKING UP THE TIME SOURCE
HoldoverUse a stable cesium oscillator to control drift over extended outages. This does not address clock failure.
IRIG-B over TDMTransporting IRIG-B over the TDM communication network(VoIP does not transport IRIG-B well)
NTP over EthernetTransporting NTP over the Ethernet/enterprise network(Not accurate enough)
IEEE 1588-2008A standards based time and frequency dissemination over Ethernet with a high rate of adoption (driven by mobile and industrial automations sectors)
IRIG-B
1588-v2
NTP
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SMART TIMING FOR A SMART GRID?
Transmission& Distribution
Master Clock
GPS
Multiple GPS(project based)
NTP
Control Center(s)
IRIG-BASCII(NTP)
DNP3
Generation
Multiple GPS(per Unit)
Time Synchronization
IRIG-B
• IRIG-B de facto time synchronization standard in substations
• Achieves microsecond precision
• Supported by many satellite clocks and IEDs
• Requires
• Satellite clock and antenna
• Dedicated timing wire
• Engineering calculations
• Careful design
Existing time distribution does not seem too smart!
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WHAT TIME IS IT?
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• US Department of Defense Global Positioning Satellite (GPS) system
• Accuracy better than 10 nanoseconds
• Signals received by antenna
• Clock calculates distance to at least four satellites
• Clock calculates propagation delay
• Inaccuracies introduce between 20 and 500 ns delay
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SATELLITE CLOCK AND ANTENNA
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• A separate timing wire is required
• Coax (with proper taps and terminations)
• Twisted shielded pair(TSP)
• Timing wire is installed next to serial and network connections to relays.
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DEDICATED TIMING WIRE
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Source: IEEE 1588 for Time Synchronization of Devices in the Electric Power IndustryFred Steinhauser, Christian Riesch, Manfred Rudigier ISPCS 2010, Portsmouth
• Satellite clock outputs
• Need to calculate load so clock is not overloaded
• Need to ensure voltage drop is not below minimum input on IEDs
• Need to ensure voltage is not above maximum input on IEDs
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ENGINEERING CALCULATIONS
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• Timing wire distances should be limited to between 50 and 100 feet
• Proper termination to avoid ringing and reflections
• Status of time synchronization
• May be available in some IEDs
• Not well documented
• Rarely implemented
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CAREFUL DESIGN
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GENERATION 1 SUBSTATION ARCHITECTURE
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Interlocking Logic
Control Center
HMI/Mimic
IED
Rel
ay
Rel
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Hardwired ParallelCopper Cabling(Relay Room)
RTU
Hardwired ParallelCopper Cabling(HV Yard)
IED
IRIG
-B
IRIG
-B
IRIG
-B
Switchgear CT/PT (VT)Switchgear CT/PT (VT)
GENERATION 2 SUBSTATION ARCHITECTURE
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Control Center
IED
IED
IED
Gateway
Hardwired ParallelCopper Cabling(HV Yard)
IED
Switchgear CT/PT (VT)Switchgear CT/PT (VT)
HMIStation Controller
Vendor protocols such as LON, MVB,DPS, Profibus, FIP, DNP 3.0, Modbus, etc.
IRIG
-B
IRIG
-B
IRIG
-B
IRIG
-B
IRIG
-B
Communication Bus Substation Clock
IRIG
-B
• Not scalable to the smart grid
• Substantially more devices than a substation
• Highly distributed environment
• Lots of antennas because devices are too far apart
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STATUS QUO
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DISTRIBUTING TIME TOMORROW
1588 Slave/Grandmaster
IEEE 1588
IEEE 1588
IEEE 1588
1588
1588
Optical PHY
TelecomSwitch
Relaying& IED’s RTU
IEEE 1588 Power ProfileLegacy (IRIG-B, PPS..)
1588Grandmaster
IEEE 1588 Slave / GPS Backup Function
2.048Mhz / 2.048mbps
Station Bus LAN
1588Grandmaster
Atomic Clock
AtomicClock
1588 Grandmaster
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THE SMART SUBSTATION
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Rel
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IED
Control Center
Bay
Con
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Bay
Con
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Gateway
CommunicationBus
IEEE C37.238 Timing (IEEE 1588 Power Profile)
Inte
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Nex
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CT/
PT (V
T)
IEEE C37.238 Timing
HMIStation Controller
Substation ClockIEC 61850 / Station Bus
Merging UnitCT/PT (VT)
Switchgear
IEC 61850 / Process Bus
LEGACY IEDS IN A SMART SUBSTATION
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1588 slave clocks can
generate IRIG-B or
SNTP for
downstream devices.
Ethernet switches an
obvious place.
Transition from IRIG-B and SNTP to 1588 will be gradual. A simple migration path is essential for success.
Serial and Ethernet
IEDs need bridge
between 1588 and
legacy time sync
protocols.
• Ethernet network protocol
• Re-use Ethernet network asset (should support 1588)
• Hardware assist and delay measurements provide high precision time synchronization (nanosecond)
• No need for separate cabling (IRIG-B,PPS)
• Fault tolerance using best master clock algorithm
• Low cost to implement in IEDs
• Reduce reliance on GPS
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BENEFITS OF SMART TIMING
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• Late 1980s – computer networks need time
• Answer: NTP as RFC 1059
• Recognized today as pretty good time
• 2002 – members of the automation, robotics, test and measurement, time keeping industries, NIST, and the military recognize the need for more accurate time
• Answer – IEEE 1588 (then version 1)
• Meant to provide sub-microsecond synchronization of real-time clocks in networked distributed measurement and control systems
• Early adoption in motion control, process control, robotics, packaging, printing presses, gas turbine control, telecommunications, and military applications
• Since 2007, the International IEEE Symposium on Precision Clock Synchronization for Measurement, Control and Communication (ISPCS) has been held every year
• IEEE 1588 does work, delivers high precision, has many committed vendors, and interoperability exists
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THE ORIGINS OF PERFECT TIME
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• 2005 – 1588 working group realizes that additional work is still needed
• Known issues, formal mechanism for extensions, conformance testing, redundancy, and security
• Working group starts updating
• 2007 – IEEE PSRC subcommittee H task force 1 (HTF1) sees a need for a network time protocol that supports existing and developing requirements
• Identifies phasor measurement units or synchrophasors
• Identifies IEC 61850
• Five performance classes
• Range from 1 ms to 1 μs
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THE ORIGINS OF PERFECT TIME
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• 2008 – IEEE 1588 Version 2 is ratified and introduces some key concepts
• Ordinary clocks, boundary clocks, transparent clocks, and slave clocks that address delays introduced by the communication network
• Peer to peer path delay measurement
• Higher sync message rates
• UDP protocol mapping
• Security
• Profiles are allowed by industries looking for specific capabilities that would foster compatibility between devices
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THE ORIGINS OF PERFECT TIME
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• 2008/2009
• 2008 – IEEE PSRC HTF1 agrees a power profile to IEEE 1588 is required and working group forms (H7) to develop standard
• Later in 2009, IEEE Substations Committee requests a joint working group so scope can be expanded to whole substation
• End of 2009, first PSRC/SUBS plug fest
• 2010/2011
• Second PSRC/SUBS plug fest
• Joint working group C7 and H7 are still working on standard
• Annexes, mappings to 61850 and C37.118, management, etc
• Balloting continues in 2011TH
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THE ORIGINS OF PERFECT TIME
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IEEE 1588-2008• -2008 is also referred to as version 2
• defined for all applications and environments… barrier to interoperability
profiles define protocol elements to suit the intended application
Profiles are not interoperable (by design)
Default Profile
Defined in Annex J. of 1588 specification
LAN/Industrial Automation Application (v1)
Power Profile
Defined by IEEE PSRC (C37.238)
Substation LAN Applications
Telecom Profile
Defined by ITU-T (G.8265.1)
Telecom WAN Applications
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• Functions over LAN and WAN
• Grandmaster sends time messages to slaves
• Slaves eliminate round-trip delay and synchronize
• Accuracy is improved
• High transaction rate
• Hardware time-stamping
• PTP aware switches/routers
• Best Master Clock Algorithm is self healing system
• Can meet Telecom & Utility accuracy needs
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IEEE 1588-2008 OVERVIEW
Grandmaster
EmbeddedSlave
ExternalSlave
1588 Packet Flow1588
1588
1588
IEEE 1588-2008 MESSAGE OVERVIEW
The Grandmaster (Server) sends the following messages:
• Signaling (2 types)
• Acknowledge TLV (ACK)
• Negative Acknowledge TLV (NACK)
• Announce message
• Sync message
• Follow_Up message
• Delay_Resp(onse)
The Slave (Client) sends the following messages:
• Signaling (3 types)
• Request announce
• Request sync
• Request delay_resp(onse)
• Delay_Req(uest)
Message Headers entering the PHY are the “on-time” marker
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IEEE 1588 ROUTING OPTIONS
• Grandmaster sends PTP packets directly to PTP slaves
• Switches/Routers forward PTP packets directly to slaves
• Unicast Sync Interval; Telecom Profile:
• User defined Sync interval up to 128Hz
• Many subscribers supported
• Grandmaster broadcasts PTP packets to a Multicast IP address.
• Switches/Routers…
• With IGMP snooping, forwards multicast packets to subscribers
• Without IGMP snooping all multicast traffic broadcast to all ports
• Multicast Sync Interval; Default Profile:
• 0.5 Hz, 1Hz & 2 Hz (1 packet/ 2 seconds up to 2 packets/second)
Unicast Multicast
Unicast (1:1)
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UNICAST STARTUP SEQUENCE
IEEE 1588 Processor
Network protocol stack& OS Processing
Physical layer
Sync detector & timestamp generator
IEEE 1588 Processor
Network protocol stack& OS Processing
Physical layer
Sync detector & timestamp generator
Master/Server Slave/Client
Network
Master Clock Slave Clock
Server clock sends:
2. Signaling (Announce granted)
4. Signaling (Sync granted)
6. Signaling (delay_resp granted)
Client clock sends:
1. Signaling (Request Announce)
3. Signaling (Request Sync)
5. Signaling (Request delay_resp)
10
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50
60
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80
90
100
120
110
130
140
150
Time
Time
The process is repeated before the lease expires(typically halfway through the lease period).
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• In lieu of signaling, the Grandmaster is self-declared, based on user priority, clockClass… and the lowest clockid
• Ordinary Clock is the IEEE 1588term for Grandmaster and/or Slave
• Consider the example where OC1 loses the GPS reference:
• OC1 clockClass changes (6 to 7)
• OC2 assumes Grandmaster mode
• OC1 enters passive state
MULTICAST STARTUP SEQUENCE
Ordinary Clock 3
Ordinary Clock 1
Ordinary Clock 2Network
Announce
Priority 2GPS Reference
Grandmaster
Priority 2GPS Reference
GrandmasterAnnounce
Syn
c
Del
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resp
Passive Mode
Sync
Delay_resp
clockClass Definition
6 Clock synchronized to a Primary Reference time source
7 Clock previously in clockClass 6 but is in holdover within holdover specs
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TIME TRANSFER TECHNIQUEMaster Clock Slave Clock
The process is repeated up to 128 times per second.(Announce rate is lower than Sync rate)
Switch/R
outer Layer
Time
Time
t2
t3
Data AtSlave Clock
Leap second offset
t2 (& t1 for 1-step)
t1,t2
t1, t2, t3
t1, t2, t3, t4
Round Trip DelayRTD = (t2 - t1) + (t4 - t3)
Offset:(slave clock error and one-way path delay)
OffsetSYNC = t2 – t1 OffsetDELAY_REQ = t4 – t3
We assume path symmetry, thereforeOne-Way Path Delay = RTD ÷ 2
Slave Clock Error = (t2 - t1) - (RTD ÷ 2)
The protocol transfers TAI (Atomic Time).UTC time is TAI + leap second offset from the announce message
t1
t4
TAI instant 1 January 1961 00:00:01.422818 exactly was identified as UTCinstant 1 January 1961 00:00:00.000000
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TIME TRANSFER EXAMPLEMaster Clock Slave Clock
Switch/R
outer Layer
Time
Time
t2
t3
Data AtSlave Clock
t1 = 100 secondst2 = 152 seconds
(150+2)
t3 = 157 seconds(152+5)
t4 = 109 seconds(100+2+2+5)
Assume at an instant in time:Master clock value = 100 secondsSlave clock value = 150 seconds
(the slave clock error = 50 seconds)One way path delay = 2 seconds
Sync message is sent at t = 100 secondsFor illustration, Delay_Req is sent 5 seconds after the Sync message is received:
Round Trip DelayRTD = (t2 - t1) + (t4 - t3)RTD = (152 - 100) + (109 - 157)RTD = 4 seconds
Slave clock error eliminated.Slave Clock Error = (t2 - t1) - (RTD ÷ 2)
= (152 - 100) - (4 ÷ 2)= 50 seconds
Round trip error eliminated
If the slave clock is adjusted by -50 seconds, the Master & Slave will be synchronized
t1
t4
2s
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ANNOUNCE MESSAGEMaster Clock Slave Clock
Switch/R
outer Layer
Time
Time
t1
t2
t3
t4
The announce message carries no Sync information.It does transport the leap second offset
Leap Second Information
Grandmaster clockClassGrandmaster Accuracy
Flags
Grandmaster Clock Type
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SYNC MESSAGEMaster Clock Slave Clock
Switch/R
outer Layer
Time
Time
t1
t2
t3
t4
Flags (same as announce)
t1
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DELAY_REQ(UEST) MESSAGEMaster Clock Slave Clock
Switch/R
outer Layer
Time
Time
t1
t2
t3
t4
The delay_req(uest) message optionally carries timing information in the Timestamp field
Flags (same as announce)
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DELAY_RESP(ONSE) MESSAGEMaster Clock Slave Clock
Switch/R
outer Layer
Time
Time
t1
t2
t3
t4
Flags (same as announce)
t4
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TIME TRANSFER (UNICAST)
Time
Time
t2
t3
t1
t4
Grandmaster Slave Clock
Slave Initiated ProcessLease establishm
ent
Repeats @
Lease duration interval
Repeats @ Sync Interval rate
Repeats @ Announce Interval rate
Grandmaster/Server Slave/Client
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TIME TRANSFER (MULTICAST)
Grandmaster Initiated Process
No signaling occurs
Syn
cinterval is pre-determ
ined, lease is infinite
Time
Time
t2
t3
t1
t4
Master Clock Slave Clock
Repeats @ Sync Interval rate
Repeats @ Announce Interval rate
Grandmaster
Grandmaster/Server Slave/Client
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IEEE 1588-2008 TRAFFIC IMPACT
• Signaling (request) 96 bytes (54)
• Signaling (ACK/NACK) 98 bytes (56)
• Announce message 106 bytes (64)
• Sync message 86 bytes (44)
• Follow_Up message 86 bytes (44)
• Delay_Resp(onse) 96 bytes (54)
• Delay_Req(uest) 86 bytes (44)
• Using the following typical values:
• Announce interval 1 per second
• Sync interval 64 per second
• Lease duration 300 seconds
• Delay_Req(uest) 64 per second
• Delay_Resp(onse) 64 per second
• Peak traffic transmitted in one second:
(96x3)+(98x3)+106+64x(86+96+86)
= 17840 bytes
= 0.017% of Fast Ethernet (100mbps)
= 0.00166% of GigE
• () 1588 only message size in bytes
Message Packet Sizes In-band Traffic Rate
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IMPROVING ACCURACY1-Step Clock 2-Step Clock
Master Clock Slave Clock
Switch/R
outer Layer
Time
Time
t1
t3
t4
Estimated value oft1 received at Slave
Real value oft1 received at Slave
Master Clock Slave Clock
Switch/R
outer Layer
Time
Time
t1
t3
t4
Real value oft1 received at Slave
Packet encryption may prevent the real-time stamp from being inserted into the Sync
message
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IMPROVING ACCURACY
• Switch, not a clock
• Measures 1588 packet delay inside the switch (“residence time”)
• Modifies (adds) residence time to the correction field
• Limited to non-encrypted networks
• Correction field must be accurate
• Switch with built-in clock
• Internal clock synchronized via 1588 to the upstream master
• Slave on 1 port, master on others
• Interrupts the Grandmaster sync flow
• Regenerates 1588 messages
• Essentially a client one side being used to discipline a GM on the other
Transparent Clock Boundary Clock
Transparent Clock
Residence Time = Egress Time - Arrival Time
ArrivalTime
EgressTime
PTP Packet PTP Packet
Boundary Clock
Slave
GMC
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• PTP has demonstrated sub microsecond time synchronization
• The work by PSRC H7 and SUBS C7 will create a power profile for PTP
• Accuracy for all smart grid applications will be possible
• Redundancy will be possible
• Switch vendors have embraced work
• IED vendors need to get involved
• Migration path possible using hybrid solution that combines PC37.238 and IRIG-B
• Timing solutions sold today may not have migration path
• Timing’s impact on bandwidth is minimalPE
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CONCLUSIONS
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• IEEE PSRC working group website
• http://www.pes-psrc.org/h/H7.html
• IEEE 1588 PTP website
• http://ieee1588.nist.gov/
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FOR MORE INFORMATION
49
PERFECT QUESTIONS
CRAIG PREUSS, P.E.ENGINEERING MANAGER – UTILITY AUTOMATIONBLACK & VEATCH CORPORATIONSUBSTATIONS C0 SUBCOMMITTEE CHAIRWORKING GROUP C7 MEMBER
