Time Synchronization for Transmission Substations Using GPS and

Time Synchronization for Transmission Substations Using GPS and
CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 2, NO. 3, SEPTEMBER 2016
91
Time Synchronization for Transmission Substations
Using GPS and IEEE 1588
Peter A Crossley, Member, IEEE, Hao Guo, Student Member, IEEE, and Zhao Ma, Senior Member, CSEE
Abstract—Time synchronization systems that utilize the global
navigation satellite systems (GNSS) are widely used in the monitoring, control, and protection of transmission networks. They
ensure that phasor measurement units (PMUs) can accurately
monitor voltage phase angles, increase the accuracy of fault
locators, enhance the capabilities of disturbance recorders, and
allow differential feeder protection to use re-routable communication networks. However, concern about the reliability of GNSS
receivers used in intelligent electronic devices (IEDs) have been
reported; problems include mal-operations of differential protection, erroneous satellite timing/location messages, inappropriate
installations, and blocking of satellite signals due to illegal use
of GNSS jammers in vehicles.
Utilities now require a timing system less dependent on the
use of low cost GNSS receivers integrated into IEDs, but one that
uses Grandmaster clocks, slave and transparent clocks, and an
Ethernet communication network. The IEEE 1588-2008 synchronization protocol uses the Ethernet to disseminate a global time
reference around a substation. A future substation will probably
include duplicate 1588 grandmasters, each incorporating stable
oscillators with GNSS and terrestrial receivers, in conjunction
with a 1588 compliant Ethernet data network with slave and
transparent clocks, and redundancy boxes for interfacing with
IEDs.
Although IEEE 1588 protocol is promising for future substation automation systems, its performance and impact has to be
fully evaluated before it can be used in real substations. This
paper describes how an IEEE 1588 time synchronization testbed
is designed, constructed, and tested. Testing involves measuring
the time offset when the Ethernet is heavily loaded with other
traffic and the holdover capability of 1588 clocks. Additional
delay introduced by IEEE 1588 traffic is also measured. As there
is limited testing on GPS receivers within the power industry,
this paper also uses the testbed to evaluate the steady state and
transient behavior of GPS receivers. The results show a 1588 time
synchronization system is accurate, secure, and ideally suited for
protection and control applications, compared to a timing system
merely based on GPS receivers. The information described in this
paper should increase a utility’s confidence in applying IEEE
1588 timing in a real substation.
Index Terms—GNSS, GPS, IEDs, IEEE 1588, substations, time
synchronization, transmission networks.
Manuscript received March 11, 2016; revised June 27, 2016; accepted July
27, 2016. Date of publication September 30, 2016; date of current version
August 12, 2016.
P. A. Crossley (corresponding author, e-mail: p.crossley@manchester.ac.uk)
and H. Guo are with School of Electrical and Electronic Engineering, The
University of Manchester, (Manchester, M13 9PL) UK.
Z. Ma is with China Electric Power Research Institute, Bejing 100192,
China.
DOI: 10.17775/CSEEJPES.2016.00040
I. I NTRODUCTION
I
NTELLIGENT electronic devices (IEDs) used in substations ensure a transmission network operates in a stable
state. But when an abnormal event occurs, the IEDs must
rapidly restore the network from the abnormal state back to
a normal state. An IED measures power system quantities
(voltage, current, frequency, power, etc.) and the operational
states of switches and circuit breakers. This data is processed
by the local IED and communicated to other IEDs in the
substation, remote IEDs, network controllers, and/or system
operators where further processing and analysis is carried
out. The resulting knowledge is used by one or more IEDs
to respond to the event that first triggered the change in
the measured quantities. The event might be a short-circuit
fault or an extreme overload that should be solved locally,
or an event such as switching action, generator or feeder
tripping, a voltage or frequency excursion, which requires
remote responses. These measurements, acquired in each bay
of each substation, only describe the actual real time operating
state of the power system if they are synchronized to a
common time reference.
IEDs can be synchronized using either a dedicated timing
system based on standalone cabling and signal repeaters, or a
network timing system shared with other automation applications and normally based on Ethernet cables and switches [1].
Dedicated timing systems often use one pulse per second
(1-PPS) to provide an accurate timing reference at the start
of each second, or the IRIG-B time code for time and
date information. GPS (or GNSS) receivers convert the time
information received from GNSS into a 1-PPS signal and
the IRIG-B time code, which are then used to synchronize
all the IEDs in a substation [2]. An example of a dedicated
timing network that operates independent of the substation
communication network is shown in Fig. 1. When the 1PPS signal travels from the GPS receiver to the device, a
propagation delay is introduced. During commissioning, this
delay must be measured and compensated.
Substation applications, such as SCADA or disturbance
recorders, requiring timing accuracy in the millisecond range,
can use a network time protocol (NTP) system operating over
an existing Ethernet communication network, as illustrated in
Fig. 2. However, most substation applications require timing
accuracies in microseconds and consequently NTP is not
suitable. For example, phasor measurement units (PMUs),
travelling wave fault locators (TWFL) and IEC61850-9-2
sample value merging units (SV-MU) require an accuracy of
c 2016 CSEE
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CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 2, NO. 3, SEPTEMBER 2016
±1 μs. Whilst current differential protection (CDP) generally
requires 20 μs and IEC61850-8-1 GOOSE messaging requires
accuracy better than 1 ms.
GPS clock
IRIG-B over coaxial cable
Protection
relays
Substation HMI
IEC 61850, FTP,
DNP3, Modbus etc
Ethernet switch
SCADA gateway
Fig. 1.
Separate timing and communication networks [1].
GPS clock as NTP master
Substation HMI
NTP
protection
relays
II. 1588 T IME S YNCHRONIZATION
A. Introduction to 1588 Time Synchronization
An increasing number of IEDs are using the Ethernet data
network for communication purposes [3] and most experts
expect Ethernet to become the communication backbone for
future transmission substations. Considering the cost, complexity and reliability, it would be ideal if high accuracy
synchronization can utilize the Ethernet. Unfortunately, an
Ethernet based NTP timing system only achieves 1–4 ms
accuracy, which is inadequate for most applications, including
IEC 61850-9-2 SVs, CDP, TWFL, and PMUs. An alternative
to NTP is the IEEE 1588 timing standard, which operates
over a data network and achieves significantly improved timing
accuracy. The IEEE 1588-2008 standard (IEEE 1588v2) [4],
also known as the Precise Time Protocol Version 2 (PTPv2),
is an IP/Ethernet based time synchronization protocol that
realizes sub micro-second timing accuracy.
A master-slave IEEE 1588 synchronization network consisting of different types of IEEE 1588v2 devices is shown in Fig.
3, where each 1588v2 device is referred to as a clock. Four
types of clock are available in IEEE 1588v2 standard [4]:
Other time
Ordinary Clock-2
reference
(Master as backup for Grandmaster)
Ordinary Clock-1
(Grandmaster)
GPS input
Ethernet switch
End-to-end transparent
Clock-1 (or non-1588 switch)
Peer-to-peer
transparent Clock-1
SCADA gateway
Fig. 2.
Boundary clock
Combined timing and communications [1].
The IEEE 1588-2008 (IEEE1588v2) synchronization protocol uses a data network to disseminate the time reference
similar to NTP, but delivers timing accuracy better than ±1 μs.
Consequently, IEEE 1588 synchronization is expected to become the preferred choice for future Ethernet based protection
and control systems. However, before it can be applied in a real
substation, various tests should be conducted to help users gain
better insight into the feasibility, performance, and limitations
of the 1588 protocol. Since testing on an Ethernet system in a
real U.K. substation is impossible, the aim of this research is to
build a laboratory based testbed to investigate and evaluate the
performance and impact of an IEEE 1588 timing system. In
addition, many GPS manufacturers claim their GPS receivers
can deliver timing accuracy better than one micro-second, but
there are few published data for those used in the power
industry. Consequently, the performance of GPS receivers is
also investigated using the testbed. The goal of this research
is to help utilities identify the limitations of timing systems
based on GPS and IEEE 1588, and then develop solutions that
enhance reliability and increase the confidence in applying the
technology.
This paper is organized as follows. Section II provides
background information on IEEE 1588, followed by test
arrangements, results, and discussions in Section III and the
conclusions in Section IV.
Peer-to-peer
transparent Clock-2
Ordinary Clock-3
(Slave)
Fig. 3.
Ordinary Clock-4
(Slave)
End-to-end transparent
Clock-2 (or non-1588 switch)
Ordinary Clock-5
(Slave)
Network of IEEE 1588v2 clocks.
1) An ordinary clock is an end device and can be a
grandmaster clock for the whole substation, or a master
clock if it supplies the time source for other clocks on
a single communication path, or a slave clock if it is
synchronised by a grandmaster or master clock.
2) A boundary clock is basically a network bridge or switch
and is the combination of slave clocks and master clocks.
It will act as a slave clock for the connected upstream
master/grandmaster clock and becomes the master clock
for its downstream slave clocks. In terms of non-1588
messages and IEEE 1588 management messages, the
boundary clock will forward them as a normal Ethernet
bridge or switch. On the contrary, IEEE 1588v2 messages related to synchronization and clock selection will
terminate in the boundary clock.
3) An end-to-end transparent clock is a network bridge or
switch capable of measuring the time required for a
1588 message to pass through the end-to-end transparent
CROSSLEY et al.: TIME SYNCHRONIZATION FOR TRANSMISSION SUBSTATIONS USING GPS AND IEEE 1588
clock. In comparison, this clock passes all non-1588
packets as a normal bridge/switch. For all 1588 “timing”
packets, the end-to-end transparent clock measures the
residence time and accumulates the value in a special
field (correctionField) of the IEEE 1588 message. The
final destination of the message (i.e., a slave clock) can
then compensate the residence delay time, as shown in
Fig. 4.
Message at egress
PTP message
Network
payload
protocol Preamble
correctionField headers
Message at ingress
Event message
Network
payload
protocol Preamble
correctionField headers
93
hierarchy can be established and ports in the master state will
start to send out 1588 messages. After that, the intermediate
IEEE 1588v2 transparent clocks (network bridges/switches
supporting 1588v2 features) will measure the delay of 1588
messages between the port in the master state and the port in
the slave state. This will be used by the port in the slave state
to adjust the clock’s local time. Note: ports in the passive
state will neither transmit nor receive 1588 messages. Once
the master-slave hierarchy is established, the slave clock(s)
will estimate the time offset between itself and the Master
Clock using data packets containing time information. With
reference to Fig. 6; the relationship between timestamps t1
and t4 is:
+
t1 + time offset + propagation delay = t4
time offset = t4 − t1 − propagation delay
+
Ingress timestamp
Ingress
Egress timestamp
+
Residence time bridge
4) A peer-to-peer transparent clock is a network bridge
or switch with the ability to measure the residence
time of an IEEE 1588 message and the delay of the
link on which the receiving port locates. Similarly, the
peer-to-peer transparent clock will forward all non-1588
messages as a normal bridge/switch. For specific 1588
packets, the peer-to-peer transparent clock will measure
the residence time and link delay and then update the
value of the “correctionField” so that the receiver of the
messages (i.e., a slave clock) can compensate for both
the switching delay and the path delay. The residence
time and link delay measurement model of a peer-topeer transparent clock is shown in Fig. 5.
GPS timing source
Master
- Master clock is synchronized
with clock source
(2)
Egress
Fig. 4. End-to-end transparent clock measuring 1588 message residence
time [4].
path + residence time
(1)
correctionField
Transparent clock
Slave
- Peer to peer delay mechanism
measures path propagation time
- Transparent clock measures
residence and path delay
Industrial Ethernet
Fig. 5. Peer-to-peer transparent clock measuring 1588 message residence
time and path delay [5].
B. Working Principle of 1588v2
The 1588v2 synchronization process requires the selection
of the grandmaster or master used to synchronize the slaves
and subsequently the establishment of master-slave hierarchy.
The best master clock algorithm defined in [4] can determine
which clock in the network is the best clock so that it can
be selected as the grandmaster clock. The best master clock
algorithm can then determine the state of each port (i.e.,
master, slave or passive) on a clock. Once the clock selection
and state determination are accomplished, the master-slave
Hence, it is necessary to measure the propagation delay before calculating the time offset. When using 1588v2 in power
industry, the IEEE 1588 Power Profile [6] specifies the use
of Peer Delay Request-Response to measure the propagation
delay. More specifically, this measures the propagation delay
between the Master port and associated Slave port(s) using the
messages “Sync, Pdelay Req and Pdelay Resp” as shown in
Fig. 6. If a 1-step clock is used, there is no Follow Up and
Pdelay Resp Follow Up message. If a 2-step clock is used, a
Follow Up and a Pdelay Resp Follow Up are required. A 1step clock measures the actual time t1 when the Sync message
is sent out and encapsulates the precise timestamp in the
Sync message. Whilst the 2-step clock only packs a coarse
timestamp in the Sync message and the actual time is carried
in the associated Follow Up message later.
The synchronization process using the Peer Delay RequestResponse mechanism is described below:
1) Master clock generates the Sync message and transmits
it; the time t1 when the Sync message is sent out is
carried in Sync message (or in the Follow Up message).
2) When a Sync message arrives at a peer-to-peer transparent clock, timestamp t2 is recorded by the clock
as the ingress timestamp. When the Sync message is
forwarded, timestamp t3 is generated as the egress timestamp. The time difference between t3 and t2 is the residence time that is accumulated in the correctionField of
Sync (or Follow Up). The peer-to-peer transparent clock
also accumulates the link delay in the correctionField
of Sync. If there are multiple peer-to-peer transparent
clocks, the residence time and link delay measurement
and accumulation process is repeated in each clock.
3) Slave clock receives the Sync message (and Follow Up
message), records the receiving time t4 , and extracts the
timestamp t1 and correctionField data.
Once the slave clock receives the sending and receiving
timestamps with propagation delay of Sync, it can calculate
the time offset between the master clock and the slave clock.
Finally, once the time offset is calculated, the local time of
the slave clock is adjusted to follow the master clock time.
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CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 2, NO. 3, SEPTEMBER 2016
ts1
Pdelay_Req
ts2
ts3
Pdelay_Req
Pdelay_Resp
ts4
ts6
ts7
Pdelay_Resp
ts8
Pdelay_Resp_Follow_Up
t1
Port in Master
state
ts5
Pdelay_Resp_Follow_Up
Sync
t2 Peer-to-peer
transparent
t3
clock
Follow_Up
Port in Slave
state
Sync
t4
ts9
Initially,
slave time = master time + time offset
Clock output:
1-PPS or IRIG-B
Follow_Up
Pdelay_Req
ts10
ts11
Pdelay_Resp
ts13
ts12
Pdelay_Req
Pdelay_Resp
Pdelay_Resp_Follow_Up
Timestamps known by Slave Clock
t4, also t1 if 1-step Master in use
t1, t4
ts14
ts15
ts16
Pdelay_Resp_Follow_Up
correctionField of Sync / Follow_Up =
fractional ns = CF0
correctionField of Sync / Follow_Up =
CF0 + linkDelay 1 = CF1
correctionField of Sync / Follow_Up =
CF1 + residenceTime = CF2
correctionField of Sync / Follow_Up =
CF2 + linkDelay 2
Fig. 6.
Peer delay request-response mechanism.
C. Previous Application and Research of IEEE 1588
In China, IEEE 1588 has been employed in real substations
since December 2009 and details of network design and
engineering are provided in [7]. However, no results on the
performance of IEEE 1588 timing have been given.
IEEE 1588 devices including clocks and Ethernet switches
from various manufacturers with power profile were tested
in the IEEE 1588 power profile plugfest hosted by the IEEE
Power & Energy Society (PES) Power System Relaying Committee (PSRC) in early 2010 . Results indicated the achievable
timing accuracy was in the range between a few hundred
nano-seconds and a few micro-seconds when there was no
other traffic in the network. It was also discovered the best
master clock algorithm would select a non-qualified clock as
the grandmaster clock during the transient state. The report
also suggested a multi-vendor testbed would be useful in
identifying implementation issues.
Authors in [8] integrated IEEE 1588 timing in a real
substation automation system in Italy in 2011. In this system,
the Ethernet switches did not support 1588 and the delay
request-response mechanism had to be used instead of the
Peer Delay Request-Response mechanism that is specified by
the 1588 power profile. Experimental results showed the time
of the slave clock drifted even when the background traffic
only occupied 1% of the bandwidth. It was advised that IEEE
1588v2 Ethernet switches would be necessary to achieve sub
micro-second timing accuracy.
A hardware testbed integrating IEEE 1588 and IEC 61850
was set up during the UCAIug Network Interoperability
Demonstrations at CIGRE 2012 [9]. It was reported that
good interoperability between different vendors was achieved
(detailed results were not published); however, utilities may
not obtain sufficient confidence on IEEE 1588 timing because
the testing period was too tight.
In Australia, a full IEEE 1588 hardware testbed with Peer
Delay Request-Response mechanism was built in 2012 [10].
Test results indicated that timing accuracy better than 500
ns could be obtained when three transparent clocks were
used between grandmaster and slaves. However, all tests were
conducted with no background traffic and the testing period
was only 1800 s, which was relatively short.
Therefore, this paper expands the research work by integrating assessment of long-term stability of GPS receivers and
IEEE 1588 slaves and investigation of impact of IEEE 1588
traffic on network latency.
III. P ERFORMANCE A NALYSIS FOR GPS & 1588 T IMING
A. Timing Stability of GPS
The timing system for a transmission substation needs
to maintain a high level of accuracy over a long period.
However, direct use of GPS is now considered unreliable
because of problems related to climatic conditions and signal
CROSSLEY et al.: TIME SYNCHRONIZATION FOR TRANSMISSION SUBSTATIONS USING GPS AND IEEE 1588
interference. Hence, it is necessary to investigate the longterm time differences when using different GPS receivers. This
is critically important since automation systems use phasor
measurement information from widely separated substations
and this relies on time references derived from different GPS
receivers.
The laboratory system used for measuring the time offset
between different GPS receivers is shown in Fig. 7. One of
the GPS receivers is selected as the reference and its 1-PPS
is used as the reference input to the measurement server. The
other GPS receivers, A and B, also feed their own 1-PPS to
the measurement server. The server then measures the time
difference between the rising edge of the 1-PPS from reference
GPS receiver and receiver A and B under test. Note that the
mask angle for GPS signal reception is only configurable on
GPS receiver A and the device manual suggests increased
mask angle reduces the field of view and lowers the timing
error caused by multi-path satellite signals situated low in the
sky.
1-PPS
GPS Antenna
Measurement
server
1-PPS
GPS Antenna
1-PPS
Reference GPS
receiver
95
an increased mask angle can improve the timing accuracy by
reducing the occurrence of spikes in the timing signal.
A reason for the time offset spikes is the location of a
high wall to the south of the GPS antennas. Results indicate
satellites are lost at the southern edge and the signal quality
might not be sufficient for ideal synchronization; see Fig. 9.
Fig. 9.
Log of GPS satellites using reference receiver.
According to Fig. 10, the accuracy of the 1-PPS from
receiver B is worse than receiver A; occasionally the time
offset of GPS receiver B exceeds the threshold value ±1 μs,
which might lead to mal-operation if an application requires
an accurate time reference from multiple GPS receivers. The
minimum and maximum time offset of receiver B measured
during the tests are −677 ns to 1372 ns.
Time Offset for GPS Receiver B
GPS Antenna
GPS receiver B
Fig. 7.
Measurement of time offset between receivers.
The time offset between GPS receiver A and reference GPS
receiver is shown in Fig. 8, and Fig. 10 shows the time offset
between receiver B and the reference. The time offset for
receiver A is always significantly less than ±1 μs and the time
offset spikes are reduced when the mask angle is increased.
More specifically, the minimum offset value is −379 ns and
the maximum value is 456 ns when the mask angle is less
than 8◦ . When the mask angle is increased to above 15◦ , the
timing offset varies between −346 ns to 318 ns. This verifies
Time Offset (ns)
Time Offset for GPS Receiver A
900
800
700
600
500
400
300
200
100
0
−100
−200
−300
−400
−500
Receiver A with 5 degree mask angle
Receiver A with 8 degree mask angle
Receiver A with 15 degree mask angle
Receiver A with 20 degree mask angle
0
5,000
10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000
Time (s)
Fig. 8.
Time offset between receiver A and reference.
Time Offset (ns)
GPS receiver A
1500
1400
1200
1000
800
600
400
200
0
−200
−400
−600
−800
−1000
Receiver B with unconfigureable mask angle
0
10,000
20,000
30,000
40,000
50,000
60,000
Time (s)
Fig. 10.
Time offset between receiver B and reference.
B. Timing Stability of IEEE 1588 Synchronization System
Unlike direct use of GPS, 1588 timing relies on a wired
data network to accomplish time synchronization within a
substation. This means it is more controllable as the data
network can be carefully designed, engineered and monitored
so that it could always operate correctly.
The lab setup to measure the time offset between the
reference GPS receiver (1588 grandmaster) and the 1588
slaves is indicated in Fig. 11. The reference GPS receiver can
transmit 1588 timing packets as well as output 1-PPS to the
measurement server. The 1588 slaves are synchronized and
then feed 1-PPS to the measurement Server for time offset
measurement. The Grandmaster and Slave clocks use the 1588
Power Profile and all Ethernet switches utilize the Peer Delay
Request-Response Mechanism as required by the 1588 Power
Profile. The background traffic in the data network occupies
85% of the bandwidth and is multicast traffic with priority
4. The bandwidth of the network is 100 Mb/s. Note: 1588
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Power Profile requires the worst-case time offset to be less
than ±1 μs with a network load up to 80% of total bandwidth
and the priority of 1588 traffic is 4.
GPS Antenna
1-PPS
1588 switch
(peer-to-peer
transparent
clock)
Grandmaster
clock
(reference
GPS receiver)
Fig. 11.
slaves.
Measurement
server
1-PPS
1-PPS
1588 switch
(peer-to-peer
transparent
clock)
1588 switch
(peer-to-peer
transparent
clock)
Slave Clock-1
Slave Clock-2
1588 network
Measurement of time offset between 1588 grandmaster and 1588
The measured time offsets for both 1588 Slave clocks are
shown in Fig. 12. The offsets are always less than ±150 ns
and the fluctuation is significantly less than achieved with individual GPS receivers. Note: during the measurement period,
a single 1588 packet is randomly lost due to deficiencies in
the Ethernet switch. However, the 1588 slaves always maintain
synchronization.
C. Resynchronization of GPS Timing
In a real substation, a local clock may lose global/national/
regional synchronization due to a power supply failure or the
loss of an external synchronization signal. For example, U.K.
National Grid reported a number of differential protection maloperations resulting from corruption of the 1-PPS output of
GPS receivers upon GPS restoration. To understand the issue,
the test facility shown in Fig. 7 is used to monitor the behavior
of two GPS Receivers after the loss of GPS synchronization
and then its subsequent restoration. To emulate the loss of the
GPS signal, the GPS Antennas connected to receivers A and
B are temporarily disconnected. The measurement results are
illustrated in Fig. 13 and 14. When receiver A loses the GPS
signal, the time offset between itself and the reference GPS
receiver drifts at 2 ns/s. The mask angle of receiver A is 20◦
and the worst-case initial time offset is 150 ns. Therefore,
receiver A can maintain micro-second accuracy for 425 s.
When receiver A regains GPS signal, an 8 μs timing spike
in the time offset occurs and its duration is several seconds.
This could cause a problem for an IED if precise timing is
required [2].
Time Offset for 1588 Slaves
Time Offset for GPS Receiver A
200
1588 Slave C
1588 Slave D
100
Time Offset (ns)
Time Offset (ns)
150
50
0
−50
−100
−150
0
5,000
10,000 15,000 20,000 25,000 30,000 35,000 40,000
Time (s)
Time offset between 1588 slaves and grandmaster.
The results presented in this paper demonstrate IEEE 1588
delivers long term timing accuracy within ±150 ns when 1588
Power Profile is enabled on 1588 Grandmaster and Slave
clocks and all Ethernet switches support 1588 Peer Delay
Request-Response mechanism, regardless of the network load
(85% bandwidth in this test) and the loss of 1588 packets. The
variation in the timing accuracy is −79 ns to 107 ns for slave
C and −109 ns to 72 ns for slave D.
In comparison, conventional GPS timing system delivers
inferior long term timing accuracy as factors such as signal
interference, antenna positions, and system installation techniques degrade the performance of the system. The situation
often becomes worse when a large number of distributed GPS
antennas and receivers are used and each IED obtains the
time signal from its own GPS Receiver. In addition, the clock
adjustment algorithm used in different GPS receivers is not
the same, and in some cases the time offset between GPS
receivers can fail to satisfy the ±1 μs requirement. However,
when 1588 timing is used, only one GPS receiver operates as
the 1588 grandmaster for the whole substation at any point in
time, and all 1588 Slaves follow this grandmaster and deliver
±1 μs accuracy more easily.
Loss of GPS
Restoration of GPS
100
200
300
400
500
600
700
800
900
Time (s)
Fig. 13.
Receiver A time offset: loss and recovery of GPS.
When receiver B loses the GPS signal, the time offset
between itself and the reference receiver drifts at 1.33 ns/s.
When GPS synchronization is restored, Receiver B recovers
synchronization very quickly, and no significant time offset
spike is observed. Hence, the transient performance of receiver
B is acceptable for IEDs that require precise timing.
Time Offset for GPS Receiver B
600
Time Offset (ns)
Fig. 12.
2,000
1,000
0
−1,000
−2,000
−3,000
−4,000
−5,000
−6,000
−7,000
−8,000
−9,000
−10,000
0
Loss of GPS
400
200
0
Restoration of GPS
−200
0
100
200
300
400
500
Time (s)
Fig. 14.
Receiver B time offset: loss and recovery of GPS.
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D. Resynchronization of 1588 Timing
The test facility shown in Fig. 11 is used to investigate how
1588 slave clocks behave when 1588 timing packets are lost
and then recovered. The loss is emulated by disconnecting the
1588 slaves from their adjacent Ethernet switches. The impacts
on slave C and D are shown in Fig. 15 and 16 respectively.
When slave C is disconnected from the data network, it drifts
at 0.8 ns/s, resulting in a 1 μs accuracy holdover time of
about 1116 s. Once the connection is restored and it starts to
receive the 1588 packets, it immediately synchronizes to the
grandmaster. In comparison, when slave D is disconnected,
it drifts at −3.6 ns/s and the holdover time is about 248 s.
Again, it rapidly synchronizes to the grandmaster when the
communication is restored.
Time Offset for 1588 Slave C
150
Recovery of 1588 Packets
Time Offset (ns)
100
Two extremely important components of the network traffic
are the IEC 61850 9-2 sample values (SV) and the IEC 61850
GOOSE messages. These maintain the correct operation of the
primary plant used in a substation and ensure the power system
operates in a stable state. As defined in the IEC 61850 standards and IEEE 1588 Power Profile, 1588, SV and GOOSE
traffic all have the same priority value of 4. This means a
particular packet from any of these sources would be randomly
placed in the output queue as shown in Fig. 17, which means
the residence time within a switch is un-deterministic. If a
1588 Ethernet switch is used, un-deterministic residence time
values will not affect the 1588 timing accuracy since the
delays are measured and compensated and this is previously
proved by the tests described in Section B. However, a data
packet such as a SV message is very sensitive to the network
latency [11]. Hence, it is necessary to investigate how the 1588
traffic affects the data traffic from other applications that share
the same Ethernet network.
50
Loss of 1588 Packets
0
−50
−100
0
100
200
300
400
500
1588
with priority = 4
8
7
6
5
4
3
2
1
61850 9-2 SV
with priority = 4
Mixed traffic
Time (s)
Fig. 15.
97
Switch
Time offset slave C: Loss-recovery 1588 packets.
87 6 5 4 3 2 1
1
2
3
4
5
6
7
Time Offset for 1588 Slave D
100
Loss of 1588 Packets
Time Offset (ns)
0
2 6 2 5 4 1 3 1 2 1
61850 GOOSE
with priority = 4
−100
Fig. 17.
Interaction of traffic with same priority.
−200
−300
Recovery of 1588 Packets
−400
0
100
200
300
400
500
600
700
Time (s)
Fig. 16.
Time offset slave D: Loss-recovery 1588 packets.
Based on these observations, re-synchronization of 1588
timing ensures 1588 slave clocks follow the grandmaster in
a fast and secure manner. In comparison, restoration of the
GPS signal in a conventional GPS receiver might introduce a
time offset spike that corrupts the output of the receiver and
could cause mal-operation of an IED.
The test setup illustrated in Fig. 18 is used to investigate the
impact of 1588 traffic on the latency of SV packets through
an Ethernet switch. The grandmaster injects 1588 messages
into the switch with a rate up to 128 packets/s and the
latency of SV packets can be measured under various 1588
traffic load conditions. More specifically, a MU produces the
SV stream which will be copied by an Ethernet tap before
entering the 1588 switch. The copied SV packet is captured
Wiring Legend
Original SV stream
Copied SV stream
1588 traffic
E. Impact of 1588 Traffic
The IEEE1588 protocol uses data packets within an Ethernet
network to disseminate the time reference from the grandmaster. Sharing an Ethernet network between 1588 traffic
and traffic related to other IED applications can result in
an inevitable interaction. This might affect a critical IED
especially if the traffic is not carefully managed and the
network is not appropriately engineered.
Grandmaster clock
(1588 traffic generator)
MU
Capture
card
Ethernet tap
1588 switch
T2
T1
Fig. 18.
Measurement of latency of SV packets.
98
CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 2, NO. 3, SEPTEMBER 2016
and timestamped by an Ethernet capture card at T1 . After the
original SV packet is sent out from the switch, it is captured
and timestamped by the capture card at T2 . Hence, the latency
of a SV packet is (T2 −T1 ). The measurement error introduced
by the Ethernet tap and the capture card is < ±60 ns, which
is negligible for the network latency measurement.
The measurement results are shown in Fig. 19; the average
latency for a SV packet is about 18 μs. This is because the
size of the SV packet from the MU is 141 bytes or 1128 bits
and the transmission delay within the switch is 1128/(100 ×
106 ) = 11.28 μs @ 100 Mb/s. According to IEC 61850-904 [12], there is a minimum switching delay of 8 μs within an
Ethernet switch and thus the total latency for a SV packet is
about 19.28 μs, which is similar to the measurement results.
It is also observed latency can increase to about 24 μs when
there is no 1588 traffic. The reason is that an Ethernet switch
has to periodically send a bridge protocol data unit (BPDU)
according to the Ethernet standard [13]. A BPDU contains 64
bytes or 544 bits, which causes additional latency of 544/(100
× 106 ) = 5.44 μs @ 100 Mb/s if it is transmitted before a
SV packet.
Density
SV Latency Within an IEEE 1588 Ethernet Switch under Different 1588 Traffic
0.020
0.018
0.016
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0
SV latency without 1588 traffic
SV latency with 96 Sync/second
SV latency with 128 Sync/second
1818.5 1919.5 2020.5 21 21.5 2222.5 23 23.5 2424.5 2525.5 2626.5 2727.5 28 28.5 2929.5 30
Latency (μs)
Fig. 19.
Impact of 1588 on SV latency.
When 1588 traffic is present in the network, the maximum
latency for a SV packet increases from 24 μs to 29 μs.
Considering a 1588 Sync message contains 72 bytes or 576
bits, the transmission latency is thus 576/(100 × 106 ) =
5.76 μs. 29 μs indicates a BPDU and a 1588 message are
transmitted before a SV packet. Consequently, if 1588 traffic
shares the data network with critical automation applications,
it will introduce additional delay of about 5.76 μs at a data
rate of 100 Mb/s or 0.576 μs at 1000 Mb/s. This has to be
considered when planning and designing the data network.
IV. C ONCLUSION
Centralized time synchronization based on the IEEE1588
protocol has numerous advantages, including: the ability to
use high quality antennas and receivers; simplified installation requirements; automatic compensation of the propagation
delay from the time reference; removal of the need for
manual measurement of signal delay; operation over general
substation Ethernet network; removal of the need to construct a
dedicated time distribution network; reduced requirements on
number of output modules on GPS receivers; future scalability;
immunity to GPS synchronization degradation and loss; and
ease of maintenance and replacement. However, if high timing
accuracy (< 1 μs) is required from IEEE 1588, two extra
requirements are required, which significantly increase the
capital cost: all Ethernet switches must be 1588 compliant and
additional 1588 slave clocks are required to translate 1588 and
provide 1-PPS and IRIG-B signals for non-1588 IEDs.
In comparison with timing approaches directly using 1-PPS
and/or IRIG-B from GPS receivers, 1588 timing over a fully
compliant 1588 network has the following advantages: better
long term accuracy; no time offset spike when synchronization
is restored; correct operation under heavy network load conditions and during the loss of 1588 packets; negligible bandwidth consumption and negligible impact on sample values or
GOOSE messages. In conclusion, time synchronization system
based on IEEE 1588 is an ideal method for accurate timing
of IEDs in a transmission substation.
R EFERENCES
[1] D. Ingram and B. Smellie. (2014, Oct.). Solving electrical substation
timing problems: A white paper on the use of the precision time protocol
for substation protection and control systems. [Online]. Available: http://
www.chronos.co.uk/files/pdfs/tek/SOLVING ELECTRICAL
SUBSTATION TIMING PROBLEMS.pdf
[2] W. An, N. Tart, D. Barron, M. Bingham, and A. Hackett, “A transmission
utility’s experience to date with feeder unit protection systems,” in 2012
International Conference on Developments in Power Systems Protection
(DPSP), 23–26 Apr. 2012, pp. 1–6.
[3] C. Strauss. Practical Electrical Network Automation and Communication Systems. Oxford, UK: Newnes, 2003.
[4] IEEE Standard for a Precision Clock Synchronization Protocol for
Networked Measurement and Control Systems, IEEE Standard 1588TM 2008, 2008.
[5] Siemens AG. IEEE 1588 precision time synchronization solution for
electric utilities. [Online]. Available: http://w3.siemens.com/mcms/
industrial-communication/en/rugged-communication/technologyhighlights/ieee-1588-precision-time-synchronization-solution-forelectric-utilities/Pages/ieee-1588-precision-time-synchronizationsolution-for-electric-utilities.
[6] IEEE Stanadard Profile for Use of IEEE 1588 Precision Time Protocol
in Power System Applications, IEEE Standard C37.238-2011, 2011.
[7] R. Moore, R. Midence, and M. Goraj, “Practical experience with IEEE
1588 high precision time synchronization in electrical substation based
on IEC 61850 process bus,” in 2010 IEEE General Meeting, July 2010,
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[8] C. M. De Dominicis, P. Ferrari, S. Rinaldi, and M. Quarantelli, “On
the use of IEEE 1588 in existing IEC 61850-based SASs: Current
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[9] M. Goraj, H. Kirrmann, R. Mackiewcz, and C. Hoga. (2012, Dec.).
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(Industry Reports). Available: https://www.pacw.org/no-cache/issue/
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[10] D. M. E. Ingram, P. Schaub, and D. A. Campbell, “Use of precision time
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[11] Communication Networks and Systems for Power Utility Automation –
Part 5: Communication Requirements for Functions and Device Models,
IEC 61850-5: 2013 (E), 2013.
[12] Communication Networks and Systems for Power Utility Automation
– Part 90-4: Network Engineering Guidelines, IEC 61850-90-4: 2013,
2013.
[13] IEEE Standard for Ethernet – Section 1, IEEE Standard 802.3-2015,
2016.
CROSSLEY et al.: TIME SYNCHRONIZATION FOR TRANSMISSION SUBSTATIONS USING GPS AND IEEE 1588
Peter A. Crossley (M’95) received the B.Sc. degree
from UMIST, Manchester, U.K., in 1977 and the
Ph.D. degree from the University of Cambridge,
U.K., in 1983. He is a Professor of electrical power
systems engineering at the University of Manchester,
U.K. and Director of the EPSRC Centre for doctoral
training in power networks. He has published over
250 technical papers on protection and is an active
member of CIGRE, IEEE and IET.
Hao Guo (S’12) received the B.Eng. degree from the
University of Manchester, Manchester, U.K. in 2012.
He is currently a Ph.D. student at The University of
Manchester. His research interests include time synchronization, Ethernet redundancy and engineering
within power substations.
99
Zhao Ma rreceived the B.Eng. degree from Xian
Jiaotong University in 1982 and the Ph.D. degree
from the Staffordshire University, U.K., in 1996. He
is National Distinguished Expert of “1000 Elite Program” and Chief Expert – Smart Power Distribution
of China Electric Power Research Institute (CEPRI).
He is CEng. FIET, current CIGRE China and the
Asia Pacific Region SC6 Chairman. His main work
areas include: smart distribution network planning
and asset management; intelligent T&D equipment,
in particular for design and development; technical
consulting; and MVDC and Energy Internet.
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