The Future of Time: Evolving Requirements for Precise Time Synchronization in the Electric Power Industry Jackie Peer, Eric Sagen, Shankar Achanta, and Veselin Skendzic Schweitzer Engineering Laboratories, Inc. Presented at the 13th Annual Western Power Delivery Automation Conference Spokane, Washington March 29–31, 2011 1 The Future of Time: Evolving Requirements for Precise Time Synchronization in the Electric Power Industry Jackie Peer, Eric Sagen, Shankar Achanta, and Veselin Skendzic, Schweitzer Engineering Laboratories, Inc. Abstract—Advancements in power system control and be argued that, if present, an independent synchrophasor- analysis methods continue to drive requirements for increasingly based situational awareness network, by itself, may have been precise synchronization of time clocks in control devices and sufficient to prevent this blackout. computers. Increasing varieties of sources for precise time are online or are planned for deployment in the near future. In the aftermath of the Northeast Blackout, we have seen a Activities in IEEE and IEC standards reflect these time- large increase of interest in precise globally coordinated time synchronization requirements, with specifications and synchronization, with some of the applications and recommendations for time-distribution mechanisms within a performance limits becoming specified by international substation or plant. The availability of precise time sources, standards and government regulatory agencies. distribution methods, and devices that can accurately Overall, the availability and reliable dissemination of synchronize time has opened the door to applications that use precise time to coordinate operations between sites for improved precise Coordinated Universal Time (UTC) are becoming voltage control and reduced wear on equipment. Synchronized critical parts of power grid operation. phasor measurement and control applications also drive the need for precise time coordination and holdover. II. TIME-SYNCHRONIZATION ACCURACY NEEDS This paper summarizes present and future sources of precise The needs and accuracy of time synchronization have time, including ground-based radio stations (e.g., WWVB, CHU, and GBZ), enhanced Long Range Navigation (LORAN), Global evolved. With new technologies and better distribution Navigation Satellite Systems (GNSS), and hybrid land-based and methods, most modern intelligent electronic devices (IEDs) satellite systems. include at least one form of time synchronization [1]. Many Time-synchronization requirements are matched to time- devices offer multiple ways to synchronize device time, distribution methods, including IRIG-B, Ethernet Network Time providing different or higher-accuracy synchronization. This Protocol (NTP), Simple Network Time Protocol (SNTP), section discusses some of these time-synchronization needs. IEEE 1588 Precision Time Protocol (PTP), and communications network-based systems, such as a synchronous optical network A. IED Time-Stamp Resolution (SONET). It is important to understand the difference between accuracy and resolution when considering devices with time- I. INTRODUCTION stamped data. “Accuracy is the degree of absolute correctness The need for precise measurement and distribution of time of a measurement device; resolution is the smallest number is an important part of our society, from early civilizations that the device can display or record” [2]. Thus, accuracy is a needing to know when to plant crops, to navigating ships or measure of how close the device time stamp is to an absolute coordinating the first train networks. In the power industry, reference (or how close the device relative time is to the true the need to have globally synchronized time was recognized or absolute time). Resolution refers to the smallest increment very early, with many devices capable of time-tagging events of time allowed by the time stamp. and collecting vast amounts of power system data. The quality IED time-stamp resolution varies from manufacturer to of time synchronization, however, was very sporadic, and the manufacturer and from device to device. Time-stamp United States Northeast Blackout of 2003 exposed many resolution is typically available to the millisecond. This means weaknesses of time synchronization. Following the blackout, that as events occur through inputs, outputs, and logic, they it took several months to parse through and piece together are time-stamped to the millisecond of when the events were numerous event records, where time-stamp quality ranged detected. Another important factor to note is the accuracy of from “not synchronized” to “utterly wrong.” A small number the given time stamp, which is based upon the processing of systems had access to Global Positioning System- interval of the product. Depending on if it is a relay, synchronized (GPS-synchronized) clocks, which simplified programmable automation controller (PAC), or supervisory the overall data analysis. control and data acquisition (SCADA) device, the accuracy of The Northeast Blackout also exposed the value of the 1-millisecond resolution time stamp can vary by precision time-based wide-area data collection technologies, ±1 millisecond to ±4 milliseconds. When viewing time-tagged such as synchronized phasor measurements (also called data from multiple sources, great care should be taken to use synchrophasors), which, at the time, were mostly deployed in the accuracy of the device and not the resolution. North the western part of the North American electrical grid. It can 2 American Electric Reliability Corporation (NERC) protection and control devices located on the same network. PRC-018-1 currently requires that all internal clocks in Because process bus inputs are sampled at high rates disturbance and monitoring equipment used by transmission (typically 4 to 16 kHz) with independent digitizers distributed and generator system owners be synchronized to within throughout the substation, time synchronization becomes ±2 milliseconds of UTC [3]. critical for all applications that require data from multiple locations (for example, bus differential protection). B. Power System Characteristics Because precise time synchronization of process bus Because ac power systems are sinusoidal and operate at a measurements is as important as the measurement values more or less fixed frequency (typically 50 or 60 Hz), some themselves, a mechanism must be implemented to deal with timing accuracy needs are fairly concrete. In a 60 Hz system, a system startup, network component failures, maintenance- single power cycle is approximately 16 milliseconds in length, related shutdown, and other events that may affect data with zero crossings occurring approximately every delivery and time synchronization. 8 milliseconds. Power system events, such as arc extinction, IEC 61850-5 recognizes this fact and defines the are inherently discretized at the 8-millisecond level. synchronization performance classes, as listed in Table I. Thus, time synchronization needs to be accurate to a level less than 8 milliseconds. Most power system IEDs process TABLE I SYNCHRONIZATION PERFORMANCE CLASSES IN IEC 61850-5 analog signals every one-quarter of a cycle or faster. This Performance aligns well with the typical 1-millisecond time-stamp Accuracy Application resolution. Class Critical process bus and TS5 ±1 µs C. IEEE C37.118 Synchrophasors synchrophasor applications Synchrophasors provide a precise absolute time-based TS4 ±4 µs Process bus, synchrophasors measurement technology that enables users to collect power system state information in real time, monitor system stability, TS3 ±25 µs Miscellaneous Point-on-wave switching, zero implement system-wide control, and perform precise post- TS2 ±100 µs event analysis. The IEDs that produce this information are crossing, synchronism check called phasor measurement units (PMUs). These devices take TS1 ±1 ms Event time tags (1 ms) real-time measurements of the power system currents and TS0 ±10 ms Event time tags (10 ms) voltages and time-synchronize these data across a wide geographic area. Required time accuracy is normally in the IEC 61850-5 recommends that time synchronization be order of 1 microsecond. In order to maintain this accuracy, a implemented over the same communications infrastructure time-synchronization source must provide synchronization to used for data exchange. In practice, this means the preference ±500 nanoseconds. The most widely used technology that can is given to Ethernet-based synchronization methods, such as provide that level of accuracy is GPS clocks that send out Simple Network Time Protocol (SNTP) for ±10 milliseconds IRIG-B for time distribution. (class TS0) and Precision Time Protocol (PTP) for the remaining accuracy classes. Because IEEE C37.238 Standard D. Time-Synchronized Control Profile for Use of IEEE Std. 1588 Precision Time Protocol in Time-synchronized control is an emerging application Power System Applications is currently going through the where multiple control actions are scheduled, synchronized to final approval process, practically all existing process bus each other, and executed in a coordinated fashion [4]. applications currently use alternate time-synchronization Controlled devices include breakers, load tap changers, and methods, such as IRIG-B or one pulse per second (PPS). This capacitor banks. Being able to operate multiple devices at the results in the need for two physical networks (communication same time provides better power system stability and and