Coal, Gas

GPS-Based Time Synchronization Improves Plant Reliability

Issue 8 and Volume 107.

By: Bob Heinert, Great River Energy

In the power generation industry, accurate time-keeping by supervisory control systems is of utmost importance. Operational failures can propagate rapidly across plant systems and trigger a chain reaction of equipment alarms and shutdowns. Personnel rely on trip logs and alarm print-outs to troubleshoot problems. Events are recorded for a period of time before and after a trip. Alarms are also time-stamped to indicate when equipment failed.

This article describes how Great River Energy achieved accurate time-keeping by plant control systems at its Coal Creek generating station. The Elk River, Minnesota, utility minimized operational failures and improved reliability by implementing an innovative Global Positioning Satellite (GPS)-based time synchronization solution at the facility.

What’s At Stake

Faced with the need to improve operating efficiency and increase generating reliability, electric utilities are seeking ways to synchronize time-keeping in critical plant systems. Indeed, the dynamic nature of many electrical generation applications demands time synchronization down to the ten millisecond range.


Figure 1. Great River Energy’s Coal Creek Station, located near Underwood, N. Dak.
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When power plant systems run on different time clocks, determining the sequential order of process upsets or trips is virtually impossible. The inability to determine when, and how, an upset occurred also increases the odds of a repeat failure, thus compounding the loss of uptime and the expense of equipment repairs.

Problems on the Clock

Inconsistent time-keeping by process control systems was a continuing problem at Great River Energy’s Coal Creek Station, located near Underwood, N. Dak. (Figure 1). Great River Energy, the fourth largest generation and transmission cooperative in the country in terms of assets, provides wholesale power and related services to 29 electric distribution cooperatives in Minnesota and Wisconsin, serving nearly 550,000 member/consumers.

The lignite-fired Coal Creek Station is North Dakota’s largest electrical generation plant. Ground was broken for the facility in the fall of 1974, and it became fully operational in 1981. Coal Creek has two units with a total generation capacity of 1,100 MW.

From the time the Coal Creek Station was commissioned, its three main plant systems – beta annunciation, turbine generator control and boiler control – had operated off separate clock sources. Local time was manually entered on the clocks and broadcast to system nodes for use in time-stamping and displays. But the tendency of the clock sources to drift over time, coupled with inconsistencies in time entry, resulted in wide variations in time-stamped data. This hampered the efforts of site personnel to diagnose trips and other events during the normal course of operation.

GPS Solution

In early 1998, Great River Energy decided to employ GPS technology to synchronize time-keeping for Coal Creek’s supervisory control architecture. The company first installed a GPS clock enhancement on the beta annunciation system. Engineers then connected the turbine generator controls into the GPS clock via a coaxial cable.

Great River turned to its main automation contractor, Honeywell Industry Solutions, for the final phase of the project – tying boiler controls into the new GPS time-keeping method. Honeywell had provided the original data acquisition system for Coal Creek in 1975 when the station was still under construction. The facility was upgraded in 1989 to a Honeywell Distributed Control System (DCS), and again in 1998 with high-performance I/O to replace its original boiler controls.


Figure 2.
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Honeywell supplied an integrated clock synchronization solution whereby all of Coal Creek’s main plant systems, including Human-Machine Interface (HMI) stations and application processing nodes, were synchronized to the Universal Time Coordinated (UTC) standard. UTC is the international time standard broadcast by an array of GPS satellites. The system consisted of a GPS antenna, a power/junction box, and cabling, as well as special, Honeywell-supplied interface boards connecting operator stations and servers to the facility’s Local Control Network (LCN) (Figure 2).

Unlike most commercially available GPS systems, which typically provide synchronization within 300-400 milliseconds, the GPS at Coal Creek enables clock synchronization down to the one millisecond range. The system employs a “smart” GPS antenna which functions as a highly accurate, stable clock source referenced to UTC. The antenna generates a pulse-per-second (PPS) output synchronized to UTC within 50 nanoseconds (one sigma), and outputs a time message packet for each pulse. Cabling from the antenna is used to connect to the DCS location as necessary within a 1,000 ft. radius.

Time link cable between the power/junction box and individual system nodes provides GPS signal reference to a single clock source. This configuration ensures continued system synchronization in the event the GPS signal is lost.

The power/junction box is designed to provide DC power to the antenna receiver, isolate the system from power surges that may be induced in the antenna, and drive the time link signal distribution.

The GPS antenna is connected to the interface boards that, in turn, are plugged into PCs which serve as the operator interface and Application Processing Platform (APP). The interface boards are a key component of the GPS system, with dedicated hardware and firmware that distributes the GPS time signal over the communication network and synchronizes all controllers and devices on the system (Figure 3).

With implementation of the GPS clock sync solution, system nodes throughout the Coal Creek site now run on an identical clock time, and local time-stamps are synchronized to a single, accurate reference. Furthermore, the need to manually check and adjust separate clock sources, reconcile time errors in historical data, and manually synchronize time across the LCN has been eliminated.

Easy Installation, Low Project Cost

Great River Energy found that installing the new GPS time synchronization system was a relatively simple task, requiring no outside assistance. This factor played a significant part in the low overall cost of the project.

An important initial step in the installation process was upgrading all of the system nodes with the latest processor and I/O circuit boards. Engineers also updated network, HMI and APP software to the most recent versions.


Figure 3. “Smart” GPS antenna provides a stable clock source referenced to UTC
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Suitable cable lengths were determined for connecting the roof-top GPS antenna to the power/junction box, and for connecting the J-box to the HMI and APP nodes. The power/junction box was mounted in a weatherproof enclosure in close proximity to a dedicated 120 VAC power source. Engineers ran GPS cable through conduit from the antenna to the junction box, and then routed time link cable from the J-box to interface connections for the various nodes (Figure 4).

Finally, monitor software was downloaded to PC workstations linked to the J-box connector. After configuring the proper Windows NT settings, the system was operational (Figure 5).


Figure 4. Power/junction box drives time link signal distribution
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Big Benefits

Great River Energy’s implementation of GPS technology for time synchronization has significantly improved troubleshooting and maintenance of fixed assets at the Coal Creek Station. Local time-stamps are now synchronized to the globally-accepted UTC reference. This enables site personnel to collect accurate, consistent process data, as the GPS accuracy is sub-millisecond in real-time.


Figure 5. GPS system monitor display
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With all systems operating from the same internal clock, operators can quickly identify the cause of process disturbances and trips. The cost associated with equipment failures has been significantly reduced through improved diagnosis of faults and easier predictive maintenance. Expenses involved in manually setting LCN time clocks, analyzing inconsistencies in time-stamped data, and synchronizing clock sources on different networks have also been eliminated.

Installation of the GPS system was fast, painless, and caused little disruption to normal operations. Site personnel were able to install and start-up the system with no outside help from the automation contractor.

Best of all, the total price for the GPS system was about $15,000 — a relatively low cost solution to a long-term problem.


The Perils of Inaccurate Time-Keeping

By Paul Griem, Honeywell Industry Solutions

It’s a fact that events can happen without warning in a power plant – often, just milliseconds apart. Accurate time synchronization of each system, therefore, is critical in determining the actual order in which the events occurred.

For example, the trip mechanism on a turbine generator may include an electrical trip system and a hydraulic system, which, if it loses pressure, will shut down the turbine. Several possible events can precipitate a trip, such as turbine overspeed, generator protective relay/solenoid failure, high/low lube oil temperature, high vibration, high bearing temperature, low fuel/steam pressure, or a manual trip.

The common causes of a turbine overspeed trip include:

Sticking control or intercept valves

  • Failure of the governor drive
  • Failure of control levers/links or gears/LVDTs (linear variable differential transformers)
  • Failure or sticking of a servomotor, pilot valve, or other component of the hydraulic system

In the event of a turbine overspeed trip, several of these subsidiary causes may occur in rapid succession. If more than one monitoring system is recording these events, it is essential that their time clocks be accurately synchronized so that the correct sequence of events can be determined in order to identify the root cause of the problem.

Every second, the GPS-based time-keeping solution in Honeywell’s GPS system synchronizes the internal time of each node on the local control network within 400 microseconds of UTC. This enables events logged by the system to be accurately correlated with events logged by other systems so that a valid cause-effect analysis can be performed.

Author

Bob Heinert is the Controls Network Specialist at Great River Energy’s Coal Creek Station. His primary responsibilities include system administration and project management for process control systems. Heinert has been with Great River Energy at the Coal Creek Station for more than 25 years. He is a member of the IEEE Computer Society and Honeywell’s Service Advisory Board. Heinert holds an associates degree in electronics from the North Dakota College of Science and a BS degree in computer information systems from the University of Mary.