Case Study: Technical Journal Article
Overview
After learning our company had an opportunity to place an article in a well-respected technical journal, I decided to take extensive notes on a presentation one of our engineers was delivering at our company’s weekly Friday Lunch. Using my notes, I pitched my idea for the article to the engineer and he agreed to collaborate with me on the project. The engineer and other subject matter experts reviewed my draft and requested some changes, which I made. The result was a 5-page article published in the technical journal that positioned our company and our engineers as thought leaders for this particular type of solution.
| Request | Promote a high-profile company solution |
| My Role | Writer |
| Timeline | 2 months |
| Results | Published in the Q4 2014 edition of UTC journal |
Process
- Listen and take notes on engineer’s presentation to the company and its sales reps
- Draft an outline for the article
- Pitch and gain approval of the idea from the engineer and my manager
- Obtain word count limit from Publications manager
- Write the first draft
- Obtain review from the engineer and another SME
- Revise based on feedback
- Obtain final reviews from my manager, SMEs, and company president
- Send to Publications manager for submission to journal
Journal Article Copy
Considerations for Developing Resilient, High-Accuracy Time Synchronization Systems
By Shankar Achanta and Joanna Hofer
Many electric power systems applications depend on precise time. As technologies evolve for time sources and time distribution, most modern power utility intelligent electronic devices (IEDs) use at least one time synchronization method. Time synchronization accuracy requirements for power systems vary by application. For example, to synchronize substation computers, a few milliseconds of accuracy is adequate. Applications like revenue metering, disturbance recording, and end-to-end testing of power system apparatus require time synchronization accuracy at the 1-millisecond level. More demanding applications, like synchrophasors (based on IEEE C37.118), sampled values (based on IEC 61850-9-2), and traveling wave fault location, require time synchronization accuracies of less than 1 µs. When selecting a time synchronization solution, it is important to meet the requirements for your most demanding applications.
Time Synchronization Considerations
There are five elements to consider when designing a dependable and reliable time synchronization system.
Time Sources
One of the most well-known accurate time sources is the atomic clock. Atomic clocks use the natural oscillations of atoms in materials to define time and frequency standards. For example, the cesium-based atomic clock introduced in the 1960s defines the international unit of time based on the natural oscillations of its atoms. This technology continues to improve, and as of early 2014, the latest NIST-based atomic clock can keep accurate time to within 1 second for 300 million years! Although atomic clocks are very accurate, they are also very expensive.
Global navigation satellite systems build on atomic clock technologies and economically provide the same level of accuracies. Several constellations operated by different nations are available today, with U.S.-based global positioning system (GPS) being the most popular system. GPS uses satellites with onboard atomic clocks to broadcast time Information globally using radio frequency signals. The global navigation satellite system (GLONASS) is the Russian equivalent of GPS.
When implementing GPS systems, potential antenna failures, solar flares, GPS jamming, and spoofing should be considered to ensure reliable time distribution. Precise-time equipment manufacturers offer mitigation techniques for these vulnerabilities, such as high-accuracy internal holdover oscillators for clocks (used during GPS signal loss), redundant time sources with source selection logic for failover, multiconstellation signal verification (e.g., comparing GPS and GLONASS signals for verification), and wide-area network-based terrestrial time distribution.
Electric Power System Time Distribution Protocols
Time distribution protocols enable time transport from sources to end devices for use in various applications. Three popular time Protocols are described below.
IRIG-B
IRIG-B is a serial protocol and the de facto timing protocol for high-accuracy time synchronization for electric power systems. It is popular because it is easy to understand, design, characterize, and troubleshoot.
IRIG-B contains a pulse-coded serial bit stream with 100 bits in each frame that gets updated every second. The frame contains top-of-the-second traceability to a global timing standard, providing a precise time reference that can be accurate to less than 100 ns of the global timing standard. The IRIG-B frame carries data that include time-of-day information, such as seconds, minutes, hours, day, and year in binary-coded decimal format. For power systems industry IEEE C37.118 (2005 or 2011), provide definitions for control function bits in the IRIG-B frame that convey the time quality information of the present frame. IEDs use these time quality bits to decide when to accept or reject the time they receive.
Network Time Protocol (NTP)
NTP is the standard message-based time transfer protocol used for synchronizing computers over Ethernet-based networks. A typical NTP server receives time from either an atomic clock or GPS system and distributes the time to downstream computers and network devices. The farther the NTP clients get from the NTP server, the more their accuracy degrades. Typically, the accuracy of NTP is in the milliseconds, so while it is fine for synchronizing computers, it is not recommended for substation applications that require submicrosecond accuracies.
Precision Time Protocol (PTP)
PTP is also a message-based time transfer protocol that runs on packet-based networks. This protocol is based on the IEEE 1588 standard and was developed to enable submicrosecond time synchronization accuracies for devices communicating over packet-based networks.
PTP Operation
PTP synchronizes PTP slave devices as closely as possible to PTP master devices by exchanging accurate time stamps (hardware-assisted time stamps for submicrosecond synchronization) via PTP messages over the network.
IEEE 1588 defines requirements for network devices implementing this protocol and defines device types, which include multiple clock types arranged into a hierarchy. “Ordinary clocks” either serve time or synchronize to time served and communicate on the network using a single PTP port (Ethernet interface). An ordinary clock is a grandmaster clock if it serves time to the entire PTP network and, therefore, is the ultimate source of time for The entire network. An ordinary clock is a slave clock if it synchronizes to another clock serving time. IEDs that run the application algorithms are typically slave-only clocks.
A boundary clock is a multiport network device that synchronizes to the reference time on one port and serves time on one or more ports. That is, one of the ports is a slave port and the rest are master ports. Switches, bridges, and routers are typical boundary clocks and used at the edges of a network. Boundary clocks can scale PTP networks by servicing requests from slave clocks instead of the grandmaster clock, making it feasible to support a large number of slave clocks (IEDs) in the network.
An end-to-end transparent clock is a multiport network device that measures the length of time a PTP message spends within the device as it is routed from the ingress port to the egress port and then adds that information to a correction field in the message. The end-to-end transparent clock functionality is typically performed by PTP-aware switches.
A peer-to-peer transparent clock is a multiport network device that measures the link delay of each port and adds that information and the residence time to PTP messages traversing the device.
Both types of transparent clocks eliminate network asymmetry and packet delay variations in the device that can impact the overall synchronization accuracy of the slave device. A management node is a network-connected device, typically a computer, used to configure and monitor PTP devices.
PTP operation is conceptually a two-stage process. In the first stage, all the PTP devices in the network are self-organized into a hierarchy using the best master clock algorithm with the grandmaster clock with the highest priority and best quality at the top, boundary/ transparent clocks in the middle, and slave clocks at the bottom. Boundary and transparent clocks exist in the middle of the hierarchy. In the second stage, protocol messages are exchanged to synchronize all clocks ultimately to the grandmaster clock.
PTP has various settings for devices. To simplify implementation, PTP defines profiles that lock down settings to values targeted for specific applications.
The three PTP profiles are “Default,” “Power,” and “Telecom.” Network design is critical when using Ethernet-based time protocols like PTP. PTP provides several settings and Options for flexibility, and this could result in interoperability issues impacting the performance of your network. For example, if the slave PTP clock does not support hardware time-stamping while the rest of the PTP network devices provide hardware time-stamping, submicrosecond accuracy cannot be achieved at the slave clock.
Unlike IRIG-B, PTP is not a simple plug-and-play protocol. For achieving high accuracies, the entire network must be designed with PTP-aware devices that are capable of hardware time-stamping of PTP messages. PTP promises scalability for time distribution; however, migrating from traditional IRIG-B will likely involve making decisions that are a tradeoff between costs, complexity, and synchronization accuracy.
Accuracy and Precision
Accuracy for a time source is defined as how close a set of measurements are to a reference, which is usually the average of a number of measurements. Precision for a time source is how close individual measurements are to each other. It is important to select a time source that is both accurate and precise. There are IEDs that detect variations in time accuracy and precision from sample to sample for time codes they receive and reject the time source if these variations exceed a threshold. Usually these IEDs use precise time for more demanding, mission-critical applications.
Cybersecurity
For strong security, it is important to follow the defense-in- depth principle. As time is an important element for efficient operation of power systems, clocks should provide security features, such as:
User authentication with role-based access: Any user who interfaces with the clock must create and enter a user ID and password that are stored in a centralized authentication server. Or, if there is no centralized server, permissions should be configured on the device by the user or role.
Syslogs: Any important events that happen within the device (i.e., settings changes, losing the synchronization source, etc.) should be logged locally on the device as well as to a remote server with accurate time stamps for traceability.
Reliability
Time synchronization devices like satellite-synchronized clocks operate in the same environments as protective relays in substations. It is important to consider devices that meet or exceed standards detailing environmental and testing requirements for communications networking devices in electric power substations (e.g., IEEE 1613).
Best Practices for Time Synchronization
Understand Your Application Requirements
Consider the following questions. Do your applications need millisecond or microsecond accuracies? Are you planning to implement applications that use synchrophasors, Traveling wave fault location, or sampled values in the future? How long can your application tolerate a loss of its time source?
Pick the Right Time Distribution Protocol
Based on your application needs and your IED compatibility, pick one or all of the protocols (IRIG-B, NTP, and PTP) for your time distribution. For any system, it is important to understand and characterize your system for performance prior to commissioning. For example, when implementing PTP for high accuracy, consider PTP device and profile support, and characterize for time stamp accuracy, network asymmetry, and cybersecurity over Ethernet. For IRIG-B implementations, it’s important to consider the impact of cable delays on time accuracy and the number of IRIG-B enabled end devices on your system.
Design With a Layered Approach and Redundancy
When designing time distribution networks, follow a layered approach for time integrity and redundancy. Each layer in your time distribution can be designed with multiple sources and protocols with devices authenticating these sources. Having redundancy and authentication at each layer of the time distribution system provides protection from single points of failure and other vulnerabilities that may impact your time distribution network.
Conclusion
Modern power systems rely on accurate and precise time for several applications, and there are many factors to consider when designing a time synchronization system that meets your needs. Optimal, economical, resilient, accurate, and secure time synchronization can be accomplished with a thorough examination of the options available. The guidelines described above will help you design a time synchronization system that will provide all the benefits of this technology while maximizing the effectiveness and safety of your power system.
Shankar Achanta received his MSEE from Arizona State University in 2002. He joined Schweitzer Engineering Laboratories, Inc. in 2002 as a hardware engineer, developing electronics for communications devices, data acquisition circuits, and switch mode power supplies. He is currently the R&D manager for precise time and communications at SEL.
Joanna Hofer received her BA in Russian and East European Studies from the University of Texas at Austin in 1992 and her MS in Technical Writing from Portland State University in 2007. She joined SEL in 2008 and currently supervises the Marketing Division’s Technical Editing and Writing group.
Joanna Hofer 