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GPS Time Synchronization

Aug. 19, 2011
How precision timing and sequence of events recording will make the Smart Grid even smarter

When the power is out, you need answers — fast. First, you’ll need precise details about what happened and when. The cause could be external or internal, and a single disturbance can initiate a series of actions, adding to the confusion. An electric current travels at nearly the speed of light, and power system events can occur in rapid succession, with just milliseconds in between. Root-cause analysis depends on reconstructing this sequence of events in proper order, with time resolution in milliseconds.

Precision timing is essential for monitoring and control of electrical power networks. Circuit breaker control schemes rely on precise timing for proper operation, and sequence of events recording (SER) systems aid diagnostics and reveal the corrective actions needed. New global positioning system (GPS) technologies simplify implementation and make sub-second timekeeping more affordable.

What good are onboard clocks if they can’t tell time?

Today’s intelligent electrical devices (IEDs) take advantage of electronics, and most have their own clocks (Figs. 1A and 1B)(click here to see Fig. 1A) and (click here to see Fig. 1B). But electronic clocks are subject to drift. Even when updated frequently over a network by application software, their clocks may vary from each other by as much as a second. Because several events can occur during this time frame, modern power systems require more precise coordination to ensure high reliability. The commonly accepted goal is 1 ms resolution, implying an accuracy of at least 500 µsec. Fortunately, most protective relays and power meters include provisions for synchronizing to an accurate external time source, such as a GPS time signal.

Not just for Navigation Anymore

Today, GPS is commonly associated with determining location. GPS technology is embedded in our cell phones and vehicles — even our pets. But GPS technology also provides a highly accurate time reference for devices in an electrical network, ensuring synchronization even across great distances.

The global positioning system consists of at least 24 satellites orbiting the Earth (called the space segment), and a set of receivers on the ground (called the control segment). Each satellite (see Photo) is equipped with a set of four highly accurate atomic clocks and continuously broadcasts its location and time (called its ephemeris) as well as data about the entire GPS system (called the almanac). A GPS antenna is typically mounted on the roof or other external location, allowing line-of-sight access to multiple satellites. At least four satellites are required to calculate initial position and time. A receiver decodes the antenna’s raw time data and broadcasts a precise time signal using one or more protocols. This time reference is expressed in Coordinated Universal Time (UTC), the de-facto global standard for civil time.

Typically, each IED accepts a series of sync pulses via a dedicated, high-speed digital input configured for this purpose. The device then decodes the supported time protocol and adjusts its clock as needed. The most common time protocols (click here to see the Table) include the following, each with its advantages:

  • IRIG-B (unmodulated and modulated)
  • DCF77
  • Sync pulses: such as 1 PPS (one pulse per second) and 1per10 (one pulse every 10 seconds)
  • Network time sync: NTP/SNTP and IEEE-1588 (PTP).

The number of devices to be synchronized, the protocols used, and the distances involved all affect system architecture.

IRIG-B time code

The IRIG-B time code is best known for its wide usage in electric utilities, especially in the United States. The IRIG-B time code uses 100 pulses to transmit an entire time and date string once each second. Thus, the maximum pulse width (i.e., index count interval) is 10 ms.

IRIG-B is typically distributed as a DC level shift signal (i.e., unmodulated IRIG-B) or as an amplitude-modulated signal using a sine wave carrier with a frequency of 1kHz (i.e., modulated IRIG-B). The most common implementation of unmodulated IRIG-B uses TTL-level signals (0VDC to 5VDC) over shielded twisted-pair cable.

Others include IRIG-B over RS-485 or multi-point distribution using 24VDC for both signal and control power.

DCF77 Time Protocol

An international time protocol that originated in Europe, DCF77 was developed by the Physikalisch-Technische Bundesanstalt (PTB), the National Institute for Science and Technology in Germany. DCF77 is both a longwave time signal and a radio station used by the PTB to broadcast this signal.

The DCF77 protocol provides a complete date/time string repeated once every minute using a 24VDC level-shift signal, accurate to 100 µsec in reference to UTC. Each string contains a binary coded decimal (BCD) value for minute, hour, day, day of week, month, and year as well as other control parameters, such as leap second and Daylight Saving Time.

Because DCF77 uses 24VDC, it can be distributed to multiple devices over long distances, making it well-suited to power and automation applications. In addition, thanks to its relatively low bit-rate of 60 pulses per minute (compared to 100 bps for IRIG-B), DCF77 requires less processor overhead, yet offers equivalent accuracy.

Sync Pulses

Sync pulses enable accurate time by indicating the precise start of each interval, or “on-time mark.” However, unlike IRIG-B or DCF77, there is no indication of the date or time of day. An approximate date and time must first be provided to a device by some other means (e.g., supervisory software). Two common sync-pulse protocols are 1 PPS and 1per10 — 1 PPS is commonly used in lab instruments, whereas 1per10 is used by at least one manufacturer’s protective relays.

With 1per10, the rising edge of the first pulse occurs at the exact start of a minute, and subsequent pulses follow at 10-second intervals. Because only the rising edge is used for synchronization, the width (duration) of the pulse is not important. As long as the device clock remains within ±4 seconds of the correct time, the sync pulse can be used to ensure higher accuracy. The disadvantage is that two networks are required — one for setting the approximate date/time and one for the sync pulse.

Network Time Synchronization

All of the methods described above require a separate network for distributing the time reference — perhaps more than one network if different protocols are required. But why can’t device clocks be updated over the same Ethernet network already used for monitoring? The answer is, they can — with some limitations.

Network Time Protocol (NTP) is an Internet protocol used to transmit time data between computers over an Ethernet network or the Internet. However, the accuracy associated with this approach is normally on the order of a second. With special network engineering, accuracies of a few milliseconds are possible — but still not sufficient for the 1-ms precision required in critical power applications.

Simple Network Time Protocol (SNPT) is a subset of NTP that omits certain timing algorithms not always needed for devices on a local area network. Despite its name, SNTP is no less accurate than NTP. It could be combined with a 1 PPS sync pulse to achieve higher accuracy; however, it is normally used with PCs or other devices where millisecond resolution is not required.

A relatively new standard for precision time synchronization is described in IEEE-1588 (also known as Precision Time Protocol, or PTP). Using IEEE-1588 hardware and software at each node, devices can achieve sub-microsecond accuracy over an Ethernet network. However, 1588-compliant Ethernet hardware is required at each node to achieve superior accuracies, and the protocol is not yet widely supported by electrical power devices. In the future, IEEE-1588 is likely to become the preferred method for time synchronization in electrical distribution systems, in conjunction with GPS technology.

In Summary

Precision timing is a foundation of the Smart Grid. What happens in one part of the grid can affect another; therefore, a common time reference is essential. GPS technology satisfies this need and is becoming more affordable and prevalent in specs. Sequence of Events Recording (SER) systems enable
root-cause analysis to answer questions about what happened and when. Such systems are now common in data centers, hospitals, refineries, automation systems, and throughout electric utility networks.

SER systems rely on millisecond precision with accuracy in microseconds. GPS provide a means to synchronize power system devices to UTC; however, several time protocols are sometimes required. Today, synchronization is accomplished using dedicated networks, but an emerging standard, IEEE-1588, promises to enable precision time synchronization of next-generation protective relays, meters, and event recorders over a local Ethernet network. Basically, the Smart Grid is about to get even smarter.

Kennedy, P.E., is a 30-year veteran of Square D/Schneider Electric and is now a director at Cyber Sciences, Inc., Murfreesboro, Tenn. He can be reached at: [email protected].

About the Author

Robert A. Kennedy, P.E., Cyber Sciences, Inc.

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