Relay Race

For a fast-track industrial generator protection project, engineers specified a relay and supporting software that allowed the firm to meet a short design cycle while offering the client high performance protection. Relay features allowed engineers to apply a single design for three machines, while appropriately accounting for each generator's individual requirements, saving time—and cos...

09/01/2002


For a fast-track industrial generator protection project, engineers specified a relay and supporting software that allowed the firm to meet a short design cycle while offering the client high performance protection. Relay features allowed engineers to apply a single design for three machines, while appropriately accounting for each generator's individual requirements, saving time—and costs—on all aspects of the project.

The client was a sugar beet processing facility in the Pacific Northwest. A larger expansion project was already under way at the plant—one that would improve plant availability and allow it to sell electric power to the local utility. The overall project included:

  • Upgraded protection for three existing coal-fired generators.

  • A new digital governor for each generator.

  • Additional transformer capacity.

  • A new monitoring and load-shedding scheme.

  • A new recloser and associated relay at the utility tie-point.

Existing Equipment

System engineers were enlisted to design the generator protection upgrade; to calculate and deploy the relay settings; and to support the relay commissioning. Their work was scheduled to be completed in less than two months, prior to the start of the fall beet harvest.

The figure on page 18 shows a simplified single-line diagram of the plant's three existing generators:

  • A 5,000-kW, high-impedance grounded unit connected at 4,160 volts AC.

  • A 2,500-kW, delta-connected unit connected at 480 volts AC.

  • A 1,500-kW, wye-connected, ungrounded unit connected at 480 volts AC.

Coal-fired boilers produce steam for the three turbines and also provide steam for the sugar refining process. The original plant transformer capacity and utility contracts prevented the plant from operating all three generators at full capacity and selling excess power back to the utility. Further, outages on the utility distribution feeder would cause a complete plant shutdown.

Normal in-plant load is greater than the combined capability of the three generators. The new plant monitoring and load-shedding scheme is designed to trip non-critical loads in the event of a feeder trip, allowing the in-plant generators to carry critical loads and maintain plant operation.

Added transformer capacity would make it possible for the plant to sell excess power back to the utility during summer periods if in-plant load is low and electricity prices are profitable. The increased importance of the generators to plant operation suggested an upgrade to their controls and the original protection, which consisted of a few single-function electromechanical relays that were installed when the generators were new.

Symplifying Design

For the protection upgrade, a microprocessor-based generator relay was selected and proved to be beneficial to the project for several reasons:

  • Programmability allowed a single electrical design to be applied for the three individual machines.

  • Common design allowed the majority of relay settings to be the same for all three machines.

  • Setting database software allowed the relay settings to be created quickly and deployed accurately.

The commonality of the designs and settings, along with the step-by-step test instructions, allowed one test technician and an engineer to thoroughly test and commission all three relays in less than two days.

In a protective relay application, the most important output contact function is to trip the circuit breaker, isolating the protected apparatus in the event of an electrical fault or abnormal operating condition. In older generator protection schemes, as many as 12 or more individual relays are grouped together. Each relay is designed and set to detect a particular type of fault or abnormal operating condition. Each relay is equipped with one or more output contacts that must be wired—usually through individual test switch poles—to trip one or more generator lockout relays, which in turn trip the generator main circuit breaker, the field breaker or exciter, the governor or turbine valves, adjacent auxiliary circuit breakers and may also initiate load-shedding or load-transfer control actions.

Due to their electrical complexity, however, these schemes take considerable time to design, construct and test. If a change to the scheme function is required, additional time investments are significant to effect the change to the scheme wiring.

Modern microprocessor-based generator relays are multifunction devices that incorporate the several generator protection functions into a single device that makes one set of electrical measurements. The single relay performs the protection functions that previously were performed by many individual devices. When a fault or abnormal operation condition occurs, the relay trips the generator lockouts through one or two output contacts.

Instead of using extensive DC-control wiring connections typical of the old-fashioned protection scheme, the protection engineer uses programmable logic within the multi-function relay to connect the protection functions together. The integrated protection functions and programmable logic dramatically simplify external wiring.

In the sugar plant application, two output contacts from each relay perform generator breaker and governor tripping for 10 protection elements. Typically, a generator lockout relay contact is connected to prevent the generator breaker from being closed after the generator is shut down due to a protection trip. Because the two 480-volt generators were not equipped with lockout relays, an additional relay output contact was connected to perform the close-inhibit function. Two contact inputs on each relay were connected to monitor the generator breaker position and a trip-inhibit control switch, used by the operator to disable the relay for testing purposes.

The programmable logic of the new relay allowed engineers to design very simple DC connections to be used for all three generators, with the exception of the close-inhibit output, which was unnecessary on the 4,160-volt generator. Differences between the tripping functions used on each machine were instead very efficiently accounted for in the relay setting development.

Reducing Setting Calculation Time

The consulting engineers had applied models of the same relay on earlier projects, in order to protect machines as small as those discussed here to as large as 500 MW. In addition to the standard generator protection functions, the relay adds a number of useful metering, monitoring and reporting capabilities, including recently added support of an external resistance-temperature measuring device that measures up to 12 individual temperatures.

The wide array of features and functions are supported by a large number of settings. The number of these settings would be a burden on the design time, if not for three advantages:

  • The initial relay settings are a list of configuration settings that enable or disable selected protection functions. Unneeded functions are disabled and do not require further settings.

  • The similarity of the designs for the three generators allowed for development of a single setting template. The template leaves a small number of settings that must be adjusted to accommodate the differences that exist between machines, further reducing the setting burden.

  • Relay setting database software allowed the template to be quickly copied and edited, speeding the task of customizing the settings for each machine.

The table on page 17 shows the type and number of relay settings offered and illustrates the settings-count reduction—and associated design-time reduction—offered by the use of a well-designed setting template.

Software Tools

The software acts as a relay setting development, storage and deployment tool. The user can: develop a relay setting template; copy the template to individual records for several relays; customize the database entries for the relays as needed; and quickly download settings to the target relays.

For the sugar plant project, engineers first developed the general relay settings template on paper, then entered the template settings into a database record. After performing the calculations for each generator's customized settings, they copied the template to a new database record named for an individual generator. Finally, they entered the customized settings for a generator into the database record for that machine. Repeating these steps yielded a complete database record for each generator relay. The software saved time at this step. Only the 29 customized relay settings for each generator needed to be entered for a given machine because the template contains the settings that are identical among the relays. Including the data entry for the original template development, only 381 settings were entered to tabulate nearly 1,000 settings used by the three relays.

The software offered an additional time savings when it was time to load the settings into the relays at the plant. Rather than manually retyping the 1,000 individual settings into the three relays, the software downloaded the settings through a direct serial-cable connection. Settings were loaded into all three relays in less than 45 minutes, including setup time. The software also supports relay setting download using a dial-up connection and-with the latest software and additional hardware-using an Ethernet connection from a desktop PC.

In addition to saving time in the field, downloading settings from the software removes the possibility that a typographical error can occur during setting entry. Applying a setting template also reduces the potential for error at the database-creation end of the process. These factors contribute to a higher quality deliverable for the clients.

Reducing Commissioning Cost

The design was complete, setting databases created, and the relays were installed on schedule. An engineer visited the site to deliver the relay settings and support the plant technician who would perform the commissioning tests on the new relays. After downloading the relay settings, the team made the necessary electrical checks and connections, and then, they referred to the instruction manual section on relay commissioning. Several factors allowed the work to be performed quickly:

  • The high degree of similarity between the protection schemes meant there were few performance differences to sort out from relay to relay.

  • The relay reporting capabilities made AC and DC connection checks simple.

  • Detailed test procedures meant there was little time spent on test operator errors.

  • High quality workmanship by the plant electricians who installed the relays meant there were few errors to detect and correct.

Improving Reporting

The generator protection was required to trip sooner than anyone expected. In November, with the plant in full operation, the relay connected to the utility tie recloser detected the start-up of a large induction motor and incorrectly tripped the tie, separating the plant from the utility. Unfortunately, the new load-shedding scheme did not operate as planned to clear non-critical loads. Since the generators were overloaded, their operating frequency began to drop, reaching 56 Hz within a few seconds. Six seconds after the tie recloser opened, all three relays correctly tripped their respective generator circuit breakers due to the low frequency.

Initially, plant personnel believed that the generator relays had misoperated, incorrectly shutting down the plant. Within a few hours, analysis of the event reports and sequence-of-events reports provided by the generator relays and the utility relay controlling the tie recloser showed the actual cause of the trips, and that there was no electrical fault present within the plant. The information in the reports provided by the relays gave plant management the confidence to quickly restart production.

Multifunction generator relays—and the software tools that support them—allow engineering firms to deliver high-quality projects quickly and at reduced cost. In addition, high performance microprocessor-based relays measure more accurately, serve with higher availability and report their activities in greater detail than is possible using older technology. Continuing development by the relay manufacturers and by third-party providers allow multifunction relays to be active components of plant and supervisory control and data acquisition (SCADA) systems. All these factors combine to make microprocessor-based relays the best choice for any protection application.

From Pure Power, Fall 2002

Relay Settings Counts

Relay Setting Category

Total, All Possible Settings

Defined by Template

Customized per Generator*

*The total settings count required by the relay in each category is the sum of template and custom settings counts.
Table courtesy of Schweitzer Engineering Laboratories, Inc., Pullman, Wash.

Protection Element Settings

263

98

28

Global Settings

142

135

1

Sequential Events Reporting Settings

44

25

0

Serial Port Settings (time four ports)

12

9

0

Total

497

294

29

Percent of Possible

100

59

6



Speaking of Generators—CEUs for Performance Specifying

A new outreach program focused on industrial standby-power generation is a nationwide effort to expand the professional engineer's knowledge of power-generation equipment and its various capabilities. A joint effort of the Milwaukee School of Engineering (MSOE) and a major power-systems manufacturer, the program's inaugural offering—a one-day course entitled, "Designing and Specifying Emergency Power Systems"—focuses on key criteria in properly specifying standby systems.

While the seminars are largely focused on standby generators, other topics include: electrical equipment requirements, facility load analysis, uninterruptible power supplies (UPS), generator sizing, potential misapplications, transfer switches, multiple source paralleling, UL listing requirements, electrical codes and safety standards. Participants who successfully complete the course will earn eight professional development hours and one continuing education unit (CEU).

What makes these seminars particularly valuable is that the presentation is never product-specific. The intend is to teach engineers how to develop an "open" performance-based specification for this type of equipment.



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