Time Synchronization in Electrical Power Transmission and Distribution Systems

Allen Goldstein NIST Synchrometrology Lab

Abstract

Synchronization of measurements in electrical power systems with Coordinated Universal Time (UTC) is expected to become mission critical worldwide over the next few years. This paper describes some of the applications for time-synchronized measurements, the requirements for accuracy and reliability, the ways timing information is received and distributed in electrical substations, and the risks which must be mitigated before the power system becomes reliant on time synchronization. It explains the need for alternative time synchronization sources redundant with Global Positioning System (GPS).

2 Time Synchronization in Electrical Power Transmission and Distribution Systems Time Synchronized Measurements

. not a new concept or application in electrical power systems . multiple spatially distributed . as technology advances, the measurements which are time frame of synchronized synchronized to occur within information has been steadily a specified window of time reduced from minutes to seconds, milliseconds, and . transmitted and then collected now microseconds at one or more locations and aligned in time . applications: . increasingly being used in • SCADA/EMS electrical power systems • disturbance worldwide monitoring/recording • fault location • synchrophasors

3 Time Synchronization in Electrical Power Transmission and Distribution Systems Accuracy requirements

Fault Location Devices: SCADA/EMS (state estimation): . less than about 1.7 . one to three seconds microseconds . travelling wave locators rely Disturbance monitoring/recording: on the difference in clocks at either end of a fault. . within 2 milliseconds of UTC • fault signal travels at • specified by the National approximately the speed of Energy Reliability Corporation light (NERC). – NERC is the U.S. regulatory . 1.7 microseconds equals agency for the energy sector about 500 meter uncertainty in the location of the fault

4 Time Synchronization in Electrical Power Transmission and Distribution Systems Synchrophasors

The reporting rates from the PMU can be from one report per Phasor measurement units second to 120 reports per (PMUs) measure power second. system voltages and currents . and estimate the Typical reporting rates are 10, synchrophasor of each phase 20, 30 or 60 reports per second in 60 Hz power systems; 10, 25 or 50 reports Synchrophasors are a polar or per second in 50 Hz power rectangular representation of systems the power system amplitude and phase compared to a Synchrophasor data from theoretical signal at the power multiple PMUs are aggregated system nominal frequency at remote locations and can synchronized to UTC provide a real-time measurement of the system state. 5 Time Synchronization in Electrical Power Transmission and Distribution Systems Synchrophasor accuracy

Equation 1: Total Vector Error [6] The maximum allowed error for ˆ 2 ˆ 2 PMU measurements is 1 % (X r (n)− X r (n)) + (X i (n)− X i (n)) TVE(n) = 2 2 total vector error (TVE) ((X r (n)) + (Xi(n)) ) . PMU performance limits are specified by IEEE Standard C37.118.1-2011, revised by Figure 1: Graphical depiction of synchrophasor error IEEE C37.118.1a-2014 The total vector error combines both the phase error and magnitude error into a single scalar value which is the magnitude of the vector between the measured synchrophasor value and the “true” value (the actual input to the PMU)

6 Time Synchronization in Electrical Power Transmission and Distribution Systems Effect of time accuracy on synchrophasors

The recommended time accuracy for synchrophasors is 1 µs [4]. . All synchrophasor For a 50 Hz or 60 Hz power measurements at a particular system, a 0.573 ° (0.01 rad) time should be made within 1 error (with 0 magnitude error) µs of each other. would yield a 1 % TVE. . The phase accuracy is a . A 0.573 ° phase error could function of the frequency of be caused by a 26.53 µs time the voltage or current being error for a 60 Hz system or a measured and the time 31.83 µs timing error for a 50 accuracy. Hz system • For a 60 Hz measurement, a 1 . This leaves no room for µs error would yield a 0.377 mrad (21.6 mdeg) error, and magnitude error for a 50 Hz measurement the error would be 0.314 mrad (18 mdeg) 7 Time Synchronization in Electrical Power Transmission and Distribution Systems Synchrophasors’ future

A Department of Energy ARPA-E program is investigating applications for a “micro-PMU” The synchrophasor standard is (µPMU) being revised by a joint . extremely precise time-stamping of IEEE/IEC working group measurements to allow comparisons of . considering new applications voltage phase angle down to small fractions of a degree . expects to publish a joint synchrophasor standard: • may be beneficial in distribution networks where IEEE/IEC 60255-118-1 the phase differences between nodes are smaller sometime before 2018 . µPMU technology is expected to discern . applications which require angle differences to significantly better than ± better than 1 % TVE are 0.05 deg (aiming for ±0.01 deg) being considered . the maximum timing error would need to be less than 0.463 µs for a 60 Hz system or 0.557 µs for a 50 Hz system

8 Time Synchronization in Electrical Power Transmission and Distribution Systems other synchrophasor timing requirements

A time-synchronous measurement device calibration system should be at least 10 times as accurate as the measurement

system under test (20 times Table 1: Phase error and TVE contribution of timing errors more accurate is preferable). timing error phase error (60 Hz) TVE contribution 26.53 µs 0.01 rad (0.573 deg) 1% . A phase accuracy of 87 µrad 1 µs 0.3 mrad (0.022 deg) 0.04% (0.005 deg) is the 0.463 µs 0.17 mrad (0.01 deg) 0.02% 0.231 µs 87 µrad (0.005 deg) 0.01% recommended target accuracy. . The difference between the calibration system’s time and the device-under-test’s time must be small. If so, the time difference with UTC does not matter

9 Time Synchronization in Electrical Power Transmission and Distribution Systems Reliability requirements

NERC publishes reliability standards for the bulk electric systems of North America At this time, there are no NERC . numerous standards which are required to requirements for redundancy in be met by power system designers and time-synchronization facilities operators. . Notably, requirement R2 of the NERC cyber security . For many systems considered essential to standards states: the reliability of the bulk power system, redundant systems are required. “The (standards drafting team) understands that there may be some Cyber Assets where • For example, in Standard COM-0011.1 – automated monitoring may not be possible Telecommunications: (such as a GPS time clock). For that reason, “Each reliability coordinator, transmission automated technical monitoring was not operator, and balancing authority is responsible explicitly required and a responsible Entity to provide adequate and reliable telecommunications facilities for the exchange of may choose to accomplish this requirement interconnection and operating information. through manual procedural controls” Where applicable, these facilities shall be redundant and diversely routed”

10 Time Synchronization in Electrical Power Transmission and Distribution Systems Time reception

GPS as Today’s Only Option In the United States, NERC creates standards for power system reliability, and power . PMUs should maintain 1 µs accurate system operators must comply with the standards. synchronization with UTC and may . most of the mission critical power be located almost anywhere in the system components are required to be world. redundant and diversely routed. . PMUs today are not considered . satellite-based time synchronization mission critical components of the is the only option available today, and power system. GPS is the only system presently • however, demands on the system are being used for timing. Other satellite changing with new loads such as electric vehicle charging and synchronous DC systems are being developed. These motors. include GLONASS, Galileo, and • furthermore, new energy sources such as BeiDou. wind and solar power are becoming distributed across transmission systems and even across distribution systems . research is needed into the which were not originally designed for bi- plausibility of using other sources of 1 directional power flow. µs accurate UTC time to provide a • based on synchronized measurements, PMUs are widely expected to become redundant, diversely routed source of mission critical over the next 5 to 10 years.

11 time signal. Time Synchronization in Electrical Power Transmission and Distribution Systems

Time distribution

Depending on the manufacturer GPS Antennas and Splitters

and model of the device, the . Many devices containing time-synchronizing source PMU functions include their input may be: own GPS receivers.

. GPS antenna input, . input is often a coaxial cable

. Inter Range Instrumentation directly from a remotely

Group (IRIG) standard 200-04 located GPS antenna or a

[11] input (sometimes also GPS antenna splitter.

requiring a digital pulse per • EMI, impedance matching, and second (PPS) input) cable delay are issues that can

contribute to time . or for some newer equipment, synchronization errors or loss an IEEE 1588 Precision Time of lock. Protocol (PTP) delivered via

Ethernet as specified by the

Power Profile

12 Time Synchronization in Electrical Power Transmission and Distribution Systems

Risks

Today’s Risk is Low Tomorrow’s Risk is Uncertain . today’s applications of time- . time-synchronized measurement applications are being researched by power system synchronized power system operators, utilities and government measurements are not considered laboratories. mission critical. . include: • real-time system protection applications, . applications in use today include: • real-time generator control, • real-time frequency and voltage control, • power system base-lining • load shedding, • event analysis, • intentional islanding control, • many others. • model verification, . which of these applications may be • transient analysis, implemented in the electrical system and the • fault location, time frame is uncertain at this time using time-synchronized measurements for the • above applications could: • restoration, – prevent large system outages • modal (oscillation) analysis, and state – restore the system faster if outages do happen. estimate verification. . these applications depend on PMUs: . None of these applications is solely • failure or sabotage of the time- dependent on PMU data; as a synchronizing source signal could introduce vulnerabilities that have the consequence, a loss of PMU data is potential to degrade the grid operation or highly unlikely to contribute directly to damage equipment. a system outage or blackout. 13 Time Synchronization in Electrical Power Transmission and Distribution Systems

Conclusion Before time-synchronized measurement becomes mission critical on the power system, it is likely that measurement systems will be required to have redundant and diversely routed synchronizing time sources.

Thank you Allen Goldstein NIST Synchrometrology Lab Gaithersburg, MD [email protected]

14 Time Synchronization in Electrical Power Transmission and Distribution Systems