Method for Determining Generator Performance before an Upgrade

When planning for an upgrade of a turbine-generator unit, it is important to establish performance benchmarks. The authors discuss the method they used at the 23-MW Gartshore Generating Station and results received.

By Andrew Punkari and Albert F. Mikhail

Brookfield Renewable is planning an upgrade to the turbine-generator unit at its 23–MW Gartshore Generating Station, which has been operating since 1958. But first the utility needed to establish performance benchmarks for the unit, with the goal of using this data to measure the gains in efficiency that were achieved through the upgrade work.

After considering several options, Brookfield Renewable chose a testing method that combined the velocity area method and the relative index method. This article includes a discussion of the test method, results received, and future plans for using the data.

Background

The Gartshore Generating Station on the Montreal River in Ontario, Canada, is owned and operated by Brookfield Renewable. There are four cascading hydroelectric generating stations on this river, all owned by Brookfield Renewable: 46.9-MW Andrews, 18.5-MW Hogg, Gartshore and 62-MW Mackay. Gartshore contains one vertical Kaplan turbine-generator unit operating under a normal head of 35 m and was declared in-service in 1958.

The water supply passages to the turbine consist of one rectangular reinforced concrete supply intake, a penstock 4.877 m in diameter, and a spiral casing. The operating head of the unit varies from 29 m to 36.23 m. The flow available for power production varies from 0 m3/sec to 87.6 m3/sec.

Brookfield Renewable decided to upgrade the generating unit to gain efficiency and thereby maximize the utilization of the water resource.

Selecting the UICMS to measure performance

Brookfield Renewable and HydroPower Performance Engineering analyzed several test methodologies to select the most appropriate method for the configuration of this particular unit. The test methodologies checked were the pressure–time method (Gibson method), acoustic method and the velocity area using intake current meters.

The Gibson method has limitations, such as the availability of two sets of piezometer taps, at the lower ring and upper ring, with enough distance between them to create a differential pressure that can be reliably measured. Using the Gibson method would also require significant outage time and installing piezometer taps, which are costly in both lost generation and the cost of drilling and installation. Also, the strain on the unit required to carry out 30 to 50 load rejections in the test period was a concern.

The acoustic method requires shutting down the unit and installing the acoustic instruments in the penstock. There is no horizontal section of the penstock at Gartshore to install the acoustic transducers according to the minimum test code requirements. The transducers would have to be installed on the inclined section of the penstock, which requires costly scaffolding.

Thus, it was determined that the velocity-area method using the Universal Intake Current Meter System (UICMS) of measuring unit flow would be the most accurate, economical and repeatable method to measure flow at the Gartshore station according to international standards for all field performance tests.

The UICMS uses a grid of propeller meters to cover the entire intake area. It also utilizes today’s technology in data acquisition and data analysis and allows determining the performance of the field tests with limited interruption of operations.

This method was selected for measuring flow on the following basis:

  • Expected test accuracy (±1.75% to ±2.3%) is acceptable
  • It is economical relative to the other test methods discussed above in terms of unit outage time and cost to set up the test, perform the test, and remove the equipment
  • It is approved by the IEC-60041 test code and is suitable for the unit configuration
  • Gartshore unit geometry is suitable
  • Unit can operate safely while testing

Instrumentation required

For this method, high-precision instruments are used for the constant measurement of water level, turbine entrance pressure, generator output, scroll casing differential pressure, servomotor stroke and gate opening.

The flow measurement instrument used for this method was the propeller Type A Ott current meter (100 mm diameter). The Ott meters used were the V-Arkansas type, which are suitable for use with axial and oblique flows up to 45 degrees. These current meters were calibrated prior to running the performance tests by Environment Canada – Canada Centre for Inland Water, an independent laboratory.

Conducting the testing

The test procedure was developed to ensure accuracy, consistency and compliance with the international test codes. The procedure is as follows:

Test equipment setup

Frames and supports for the intake current meters were assembled to fit the stoplog opening and installed in the stoplog slot. Current meters were installed on the supporting frame, checked and connected to the test control center. Spacing of the current meters was measured and documented. The intake opening dimensions (width and height) were measured and verified. In addition, hoist supports were installed and it was verified that the current meters could be lowered to the bottom of the intake opening.

At this point, the test control center was set up with computers, data acquisitioning, signal cables, and communications between each measuring point and the computer center.

The next step was to install the servomotor stroke, wicket gate angle and runner blade angle measurement instrumentation, then calibrate the instruments and verify measurements.

At this point, the contractor installed the power meter measurement instrument and verified measurements.

Temporary stilling wells were installed to measure the static water level of the generating unit headwater and tailwater elevations. These stilling wells were made of 2-inch-diameter aluminum pipes that were open at the top and capped on the bottom, with a 3/16-inch-diameter hole to minimize water level variations caused by turbulence. The wells were fixed to the concrete wall and temporary benchmarks were established on the top of each well. One stilling well was installed in each intake, while two stilling wells were installed in each of the two tailrace slots.

Surveying work was performed from official station benchmark to each headwater and tailwater measurement to verify the gross head and net head measurements. The contractor also established temporary benchmarks at each measuring location, including station headwater and station tailwater measurement locations.

Instrumentation calibration

With the exception of power and flow velocity, all of the instruments were calibrated on-site prior to the start of the test. The power meter and current meters were calibrated off-site by independent laboratories to national standards.

Piezometer taps inspection

A visual site inspection of each installe piezometer was carried out before the field test began to ensure that they were operating in good condition. Additionally, each tap was flushed with water periodically to clear any unseen obstructions or any trapped air that may have hindered the water pressure measurement.

The piezometers were individually measured to verify that the differences between the piezometer taps are within the acceptable practice as well as within the test code.

Actual field tests

Nine index tests and one full UICMS test were carried out on the unit.The index tests determined the best wicket gate opening for each increment of the turbine runner blade angle to verify the cam current cam design. The absolute test (UICMS) determined the flow and efficiency for each combination that was determined from the index tests.

  • Nine index tests were carried out at 3-degree increments of the runner blade opening from zero degree to maximum runner blade angle opening of 21.75 degrees. Each index test was carried out at a fixed runner blade angle while the wicket gate opening was changed in increments of average of 9 gate opening to determine the best efficiency gate opening for each specific runner blade angle. The peak efficiency of the maximum blade angle of 21.75 and 18 degrees were at full stroke (may not be the best efficiency as the efficiency is still rising). Figures 1 and 2 (see pages 6 and 7) show examples of runner blade angle best efficiency. Figure 3 (see page 7) shows a comparison between all runner blade angles results, which represents the best cam design for this turbine.
  • One full current meter test was carried out using the UICMS. The test runs were selected at the best gate opening for each runner blade angle increment that was tested by the index method. Each test run was repeated three times to determine any test uncertainties.

Unit head measurements

Station headwater elevation, unit headwater elevation, station tailwater elevation, unit tailwater elevation, pressure elevation at the scroll case entry, and scroll case differential pressure (Winter-Kennedy) were measured using high-accuracy instrumentation.

Generator measurements

Generator output was measured using watt-hour instrumentation and the station recordings.

Turbine runner blade angle measurements

The runner blade angle was measured using a linear transducer that was calibrated on site, as well as the readings recorded by the station’s supervisory control and data acquisition system.

Turbine servomotor stroke measurements

The servomotor stroke opening was measured using a linear displacement transducer that was installed on the servomotor stroke piston rod.

Turbine wicket gate angle measurements

The wicket gate angle opening was measured using a linear displacement transducer that was mounted on a protractor.

Turbine flow measurements

Turbine flow was measured by the velocity area method using the UICMS as follows:

Flow = Average Flow Velocity x Area

Average flow velocity was determined by measuring the velocity profile using the UICMS. The UICMS consists of several current meters mounted and spaced across a supporting frame. The frame is designed in several modules that can be assembled to fit the stoplog openings between 3 feet to 56 feet openings (1.0 to 17 meters).

The frame is raised and lowered in the stoplog openings of the unit intakes (see Figure 4 on page 8). The current meters measure the point velocities at pre-determined positions along each horizontal level or profile (see Figure 5 on page 10).

The velocity profiles and the average velocity at each level were calculated horizontally using the numerical integration technique (ISO 3354). Using these horizontal averages, the average vertical velocity was calculated by the same numerical integration technique (ISO 3354). This average is the overall average velocity of the intake.

  • Number of measuring points per horizontal profile = 11
  • Number of horizontal measuring profiles(levels) = 11
  • Number of measuring points per intake = 121

The current meters used are suitable for axial and oblique flow up to 45 degrees.

All current meters were calibrated at regular intervals before the test for axial and oblique flow conditions by Environment Canada Hydraulic Studies Laboratory personnel at the Canada Centre for Inland Water (CCIW).

The UICMS performance test consisted of 28 test runs covering a range of wicket gate openings from 20% (speed-no-load) to 100 % of full servomotor stroke. Turbine flow was measured and computed using the velocity-area method and employed the UICMS. The test runs were made at 50-minute intervals. At the beginning of each run, the wicket gates were set at a fixed opening. About five minutes were allowed for conditions to stabilize.

During the remaining 45 minutes, the data acquisition system measured all parameters simultaneously at a rate of 100 scans/minute.

Test conclusions

The present operating characteristics of the turbine at the Gartshore Generating Station and the overall unit performance relationships were determined by this test, using the index method to determine the relative turbine flow and the absolute values of the flow and efficiency using the UICMS. The current cam setup is matching with the test results except at the higher gates. The wicket gate separates from the cam above 90% gate opening. The separation from the optimum condition impacts the efficiency of the unit by -1.7%.

Reference

Punkari, Andrew, and Albert Mikhail, “Performance Testing Experience at the Gartshore G.S.,” Proceedings of HydroVision International 2016, PennWell, Tulsa, Okla., 2016.


Andrew Punkari, P.E., is technical manager for Brookfield Renewable’s Central Asset Management Group. Albert F. Mikhail, P.E., is principal engineer with HydroPower Performance Engineering.

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