Electric Power Systems and GPS
Alison Silverstein,
NASPI Project Manager
alisonsilverstein@mac.com

Civil GPS Service Interface Committee
September 12, 20161

Overview and punchlines• Power system owners and operators use GPS
for navigation, position and timing.
• If we lose GPS today, it will complicate (highercost, longer duration, lower efficiency), but
not kill, grid operations.
• GPS and alternate time sources (and the ways
we deliver and use them) need to become
more reliable for time-synchronizedapplications to become mission-critical in thefuture.
2

Time & power measurement resolutioninstrument sampling
rate 512/cycle
GPS time stamp:differential absoluteclockaccuracy10-9nanosecond

0.1o

phasormeasurements
reported 120/sec

1 cycle

1o

10-6

10-3

microsecond

millisecond

Micro-PMUs
sample at 512
samples/cycle

Grid standards
require event
time datasynchronized at
2 ms accuracy

SCADA
measurements

1time scale in sec

Phasormeasurement units
sample 30 to 120
samples per second

SCADA samples
every 4 to 6
seconds

60 Hertz (Hz) = 60 cycles/second = frequency of alternating current sine waves on theNorth American power system1 cycle = 1/60th second = 16.67 milliseconds = duration of one sine wave cycle at 60 Hz
Lightning strikes (frequent cause of line faults) last a few microseconds
Voltage sags last from a few milliseconds to several cycles
Relays operate in 1 to 1.5 cycles and breakers interrupt current in 1.5 to 3 cycles, so total
fault clearing time could be up to a few hundred ms
Household clocks blink after momentary line interruptions up to two seconds long3

Key grid timing use technologies
Most grid time-using devices use GPS antennas as the timing source,and distribute time within the substation primarily using copper
cabling (IRIG-B), with growing use of Ethernet (IEC 1588).
• Relays – system protection workhorse. Monitors local grid conditions
down to the microsecond, and actuates control operations (includingbreaker operation) for line trips and other system protection
measures. Time-synchronized with GPS (mostly) or SONET over fiber.
• Synchrophasor technology – Phasor Measurement Units (PMUs) dohigh-speed grid monitoring, time-synchronized to UTC withmicrosecond accuracy. Used mostly at transmission level. Now
sampling at 30 to 120 samples/sec; timing must be accurate within 1
μs. When timing delivery mechanisms become more reliable,
synchrophasor technology can become a mission-critical tool.
• Micro-PMUs being developed for distribution system monitoring,analysis & control.
4

Power system GPS (or PNT) uses
• Position & navigation– Fleet direction and tracking– Mobile crew management– Customer information systems– Physical asset tracking & management– LIDAR for vegetation management– LIDAR and 3D modeling for substation and lineconstruction• Timing– Absolute time – system protection, including fault locationand fault clearing– Synchronized time – see slide 8• Also heavy telecom and IT network dependence, which relyon timing delivery services
5Why time resolution matters on the grid -same event, four time slices

Pacific DC Intertie Trip April 2

4:00 to 8:00 am

4:58 to 5:30 am
Pacific DC Intertie Trip April 28, 2015

5:06:38 to 5:07:33 am

5:06:45 to 5:06:54 am

micro-PMU data from Lawrence Berkeley
National Laboratory, viewed through theBerkeley Tree Database (BTrDB) plotter atdifferent time resolutions

6

FarmWhy
Oscillations
time resolution matters on the grid –same event, two time slices

ds3-14Hz
as high as

wind farms?
d toe wind farms
problem atwith thes to curtail(35 minutes)

Source: OG&E, “Synchrophasors at OG+E,”
Austin White, IEEE PES GM 2016

(2 seconds)
7

Inter-area oscillations in Western Interconnection
Aligning synchronized wide-area measurements from multiple locations
reveals the power system’s dynamic state; you can’t see and analyze
these oscillations from slower SCADA data or from unsynchronized localPMU measurements. Timing errors cause false conclusions about gridconditions.
Western Interconnection Frequencies - August 4 2000 Oscillation

60.06
Grand Coulee
Malin
Devers60.05

Frequency (Hz)

60.04
60.03
60.02
60.01
60
59.99
59.98
-50

5

10Time (sec)152025Power system uses of time-dependent data
On-line, real-time uses
(current and emerging uses)
Absolute time
• Fault detection & location (100 ns)*
• Fault clearing• Lightning correlation (1 ms)• Synchronize a generator to the grid
Synchronized time
• Frequency management• Voltage management*• Wide-area situational awareness• Automatic event detection &notification*• Oscillation detection• Islanding control• Black-start system restoration• Integrate distributed resources, including
rooftop PV and EVs*
• Dynamic line management*• Remedial action schemes (<50 ms)*

Off-line uses
•

•
•
•

•

Power system modeling– Generators– Loads– System model
Event reconstruction and analysis(1 ms)
Equipment mis-operations
identification and diagnosis
Baselining (statistical eventcharacterization) to develop operator
decision support tools*
Disaggregate distributed generationfrom loads behind the meter** = emerging uses
9

The power system today and tomorrow

One-way power flows, limited resource diversity, centralized management moving
toward two-way power flows serving diverse loads and resources with high coordination
and IT and communications requirements
Source: IEEE via
https://smartgrids.wikispaces.com/Smart+Grid+Future+Capabilities

10Some of the ways timing goes badfrom the grid user’s perspective•••

From space– Ionospheric problems – sunspots, geomagnetic disturbances– Events – leap seconds, satellite constellation changes
On-site– GPS receiver – poor quality, software bugs, no firmware updates, badlocation, local jamming or spoofing or other radio interference, lostwire to the PMU, no correction for PNT broadcast problems– PMU – poor interoperability with GPS receiver, slow firmware patches,
lost wire to GPS receiver, sloppy program for time-handling, nodetection of timing problems, no back-up time source– In substation timing delivery (rare) – problems with cabling or
ethernet distribution of time signal to slave clocks
Phasor Data Concentrator and applications – inadequate detection oftiming anomalies or gaps and computational errors resulting from thoseproblems. Also sometimes inadequate timing standards and protocols…
11

What happens to synchrophasor measurements
if GPS goes bad?
• If there is an error or spoof of the time signal to a phasor
measurement unit (PMU), that error will cause false calculations ofphase angle and mis-alignment of measured grid conditions relative
to other PMUs
• In the case of the leap second:– Where the GPS clocks skipped the second or were early/late, PMUmeasurements were too early or too late, causing PDCs to ignore thePMU measurements– Where there were duplicate time stamps, there were “duplicate” PMU
measurements– Phase angle error depends on accurate time information; bad timestamps mean erroneous phase angle calculation*** These are all PMU or PMU-clock problems, not GPS problems – but
the user doesn’t recognize that…
12Calculating phase angle with a time error
PMU performance during the June 2016 leap second– Above -- for 17 seconds, it appears that the phase has a 36 degree error (at 59.9Hz)– Note that there are no reports for the second immediately following the leap second, andthere are two sets of reports for the second between 17 and 18 seconds after.
withLeap
Second- 28th June’15– EncounterAnd different PMU
modelshandledthe leap
second differently.
Randomchanges
observed methods
in trendof angular– Dueto inconsistenttime-determination(below),some PMUs in India reported
difference in some PMUs after 23:29 at National
wildlyfluctuating phase angles.
Source: “2015 NIST Investigation of PMU
Control Center.

Response to Leap Second”, Allen Goldstein,
DJ Anand & Ya-Shian Li-Baboud, NIST, March2016

Source: POSOCO, P.K. Agarwal,“Encounter with the Leap Second,”
NASPI Work Group, March 23, 2016
23 March 2016

Encounter with Leap Second

613

Power system punchlines• Timing errors from the time source can cause
incorrect synchrophasor data– Such errors can create false analytical conclusions and in
the future could drive undesirable and possibly dangerous
automated grid operations with synchrophasor-basedcontrols• The power sector needs to protect future grid
operations with better timing tools and practices toimprove robustness and resilience– We need to assume that PNT could be unreliable at both
source and receiving points– We need to start implementing measures to assure
accurate, reliable time stamps against multiple failure
modes
14

Some timing remedies and options
At the PNT level:• Improved signal robustness checks• Multi-frequency – L1 C/A, L2C, L5 (but multi-frequency receivers areexpensive)• Multi-system – GPS, GLONASS, GALILEO, eLoran, good internal oscillators• Multiple receivers• Jamming, spoofing and interference detection and/or prevention
GPS-independent networks:• Telecom network is capable of time transfer -- avoids dependence onsatellites and transmitter sites and requirement for large receiver network
installation and maintenance• Network-distributed time can receive accurate time from multiple sources(GPS, NTP, CDMA, PTP), some IRIG-B• Distributed clock networks, some IEEE 1588
• Holdover clocks in key devices for short-term back-up

15

What we’re doing about it –NASPI Time Synchronization Task ForceScope – Time synchronization awareness and problemsolving for electric sector, with synchrophasor focus• Document problems of current PNT solutions• Identify specific, near-term solutions and mitigations that canaddress multiple failure causes (redundant timing sources,
better installation and maintenance practices, detection ofbad or anomalous time signals, specs for good-quality
equipment, etc.)• Develop and share how-to information for these solutions• Recommendations for longer-term research needs (timing
problem detection, equipment interoperability, standards
updates, etc.) within grid sector and beyond
16Questions?
Alison Silverstein
NASPI Project Manager
alisonsilverstein@mac.com
www.naspi.org

Source: BBC17