Arizona utility Salt River Project recently installed new static exciters in seven hydro units. The enhanced exciter capability enables utility personnel to conduct reliability-related equipment tests required by the Western Electricity Coordinating Council, rather than outsourcing the testing.
By John R. Hunter
From 2005 to 2008, Salt River Project (SRP) in Arizona replaced the static exciters for seven turbine-generator units at three of its seven hydro plants. SRP chose these units, which ranged in size from a 10-MW conventional unit to a 115-MW pumped-storage unit, because they were the largest units at its peaking facilities. The remaining SRP units are smaller, at run-of-river projects.
As utility personnel completed work on the first unit, they realized that the new static exciter offered the capability for utility personnel to perform all reliability tests required by the Western Electricity Coordinating Council (WECC), rather than outsourcing the testing as had been done previously. Now, SRP performs all tests required by WECC – including static, generator saturation curve, generator dynamic model, and exciter dynamic model testing, as well as unit inertial model verification and governor model validation. This saves the utility the time and money involved in hiring an outside company to perform reliability testing and allows SRP to perform select tests during commissioning of new equipment.
Installing new static exciters
Before selecting the static exciters to be installed at the plants, SRP polled others in the hydro industry for vendor recommendations. The utility then submitted requests for proposals to the identified vendors. In 2005, the utility selected the EX2100 excitation control system from GE Energy. SRP installed this system in all seven units over three years under three separate contracts. Installation was completed on the first unit in 2006. Based on the success with this installation, SRP contracted to have two more systems installed in 2007 and the remaining four in 2008.
For the two pumped-storage units and one of the larger (40 MW) conventional units, the utility installed EX2100 models that came with a redundant bridge. This provides a backup should one bridge fail. Because of the cost associated with this feature, SRP chose a model with a single bridge for the one 10-MW and three 11-MW units.
Discovering the ability to perform WECC testing
Arizona operates under reliability requirements governed by the Western Electricity Coordinating Council (WECC). As the utility completed the commissioning process for the first unit, personnel recognized that the EX2100’s processing power and interface provided the capability to perform all WECC testing in-house. Before the new static exciters were installed, SRP hired technical support people from outside the company to perform all reliability testing required by WECC. For this, the company had to bring all the hardware necessary to complete testing to the site, figure out where to wire into the old analog exciters, complete all testing, and provide the reports to be submitted to WECC. For each unit tested, the company required at least a day to mobilize to the site and complete setup and another day to complete testing and de-mobilize. In addition, this work had to be coordinated with the operating group and dispatching. The reports would arrive several weeks later.
SRP concluded that using the installed capability of the static exciters to complete all data gathering in-house would save the utility quite a bit of money. It also would allow SRP to more easily schedule the testing around its needs, rather than the schedule limitations of the outside testing company.
Almost all the hardware needed to perform the testing was already in place. All seven units featured 4-20 milliamp (mA) loops for wicket gate position and speed. The only equipment SRP needed to order was an additional analog input card for the EX2100, to allow the utility to record the variables generated during the testing. Installing this card only involved wiring it in and configuring the GE interface software (called Toolbox).
How SRP performs the tests
In 1996, WECC experienced a severe system disturbance. To validate its computerized models of the system, in 1997 WECC required all generator operators connected to its system to complete unit testing. WECC wanted tests to be performed on all units rated 10 MW or greater and required that the tests be performed every five years. In 2006, the North American Electric Reliability Corporation (NERC) passed new regulations that formalized these testing requirements. Tests that must be performed every five years include static testing, generator saturation curve testing, exciter dynamic model testing, generator dynamic model testing, unit inertial model verification, and governor model validation.
The following sections describe how SRP now performs these tests in-house, using the new static exciters.
For static testing, each unit is loaded to various points on the megavolt ampere reactive (MVAR) capability curve. Operations personnel perform this test and record the data manually. SRP developed a recording form for personnel to use to ensure that all required data is obtained. On the form we use to record the static test data, we also record wicket gate position throughout the load range because one test requires wicket gate position versus unit load.
For the first couple of static tests, SRP had a relay engineer on site to assist with the process. This engineer developed a spreadsheet to indicate where we were in relation to the generator protection impedance circle for the under-excited tests. Additionally, it was helpful to provide the operators with a laptop computer hooked into the EX2100 system that showed, in real time, where they were in relation to the underexciter limit and overexciter limit functions. By comparing the data on the spreadsheet with the data on the underexciter limit and overexciter limit functions, we were able to confirm that the exciter protection comes into service before the unit is tripped by the protection system.
Depending on additional unit operations and system demands, the static test probably will take two to four hours to complete.
Generator open circuit saturation curve testing
This test is performed with the unit off line and at speed with an applied field. NERC requires that the voltage applied to the unit be adjusted from 105 percent of rated down to 50 percent of rated in 5 percent increments, to validate the generator saturation curve. To avoid disrupting the test, it may be necessary to defeat the field flashing circuit. Information SRP records includes the required voltage to be applied, the actual voltage achieved, and the exciter field volts and amps. Again, operations personnel perform this test and record the data manually on standard SRP-developed forms. Table 1 shows a sample of data obtained during a generator saturation test.
Table 1: Data Collected during a Generator Saturation Test
The saturation curve test should take less than one hour to complete.
Exciter dynamic model testing
With the unit on line at low load and underexcited (or bucking) MVAR and the automatic voltage regulator in service, the generator breaker is tripped open. Information recorded includes generator volts and field volts and amps.
SRP performs this test, as well as the generator dynamic model testing, unit inertial model verification, and governor model validation, using the EX2100 Toolbox software to record the data. Figure 1 shows an example of how the test results are displayed on the screen using the Toolbox software.
Collected data is exported to a .csv file, which can be opened to chart the data using Microsoft Excel. It is best to use either an Excel or .csv file because most modeling software can read these file formats. SRP sends all the resulting Excel files to GE, where personnel plug the data into modeling software and generate reports that illustrate the results of the testing.
It is convenient to run the tests from the control room with a communication cable to the exciter where test personnel are located. The first step is to put the exciter on line. To perform an exciter dynamic model test, SRP’s procedure is to:
- ) Start and load the unit to about 10 percent load and bucking MVAR.
- ) Set up the Toolbox to record generator volts, exciter volts, and exciter amps.
- ) Remove any voltage setpoint control system from service.
- ) Start recording.
- ) Trip the generator breaker.
- ) Record the values for 20 to 30 seconds.
- ) Stop recording.
- ) Return unit operation to normal.
It does not take very long to complete all of the trip tests. SRP has performed all the required tests, including the static tests, in a single morning.
Generator dynamic model testing
With the unit on line at low load and bucking MVAR and the automatic voltage regulator out of service, the generator breaker is tripped open. Information recorded includes generator volts and field volts and amps.
Unit inertial model verification
With the unit running at moderate output, the rotor speed and wicket gate position are recorded as the generator breaker is forced open.
Governor model validation
This test is also called the on-line speed reference step test. With the unit at 80 percent load, a frequency step change is induced into the control system to validate governor response. Information recorded includes unit load and wicket gate/blade position.
Regular WECC testing
For WECC, SRP must complete some of this testing every five years. If initial model validation was completed and no major maintenance has been performed (i.e., replaced a turbine or governor, replaced an exciter, or re-wound a generator), required tests may be limited to:
– Static tests;
– Exciter dynamic model verification or a step injection test (unless there is a recorded system voltage excursion event that proves the automatic voltage regulator response was appropriate); and
– Governor model validation (unless there is a recorded system frequency event of sufficient magnitude to prove governor response was appropriate).
The static tests are designed to confirm that the unit can achieve the MVAR indicated in the model and ensure that the overexcitation or underexcitation limiters do not interfere with the capability.
For the exciter and governor tests, sample rates vary with use of a voltage or frequency excursion recording to validate the model. For a voltage excursion, WECC requires a sampling rate of 20 per second or faster. Although it may be possible to set the exciter up to record an event automatically, this can be problematic. It is almost simpler to just perform the test. For the governor test, a sampling rate of one every three to five seconds is sufficient. This sampling rate is within the range of most of SRP’s local energy management system/supervisory control and data acquisition (SCADA) historians, making them useful in validation.
Because the exciter step injection test is required during commissioning of a new exciter, it is provided for in the GE software. To perform this test, SRP simply sets the software for the size of the voltage step (about 2 to 3 percent), the time to wait before returning to normal, and the values to record. The unit should be at 80 percent load. Users must make sure to take any voltage setpoint controllers out of service and have the unit loaded such that the overexcitation or underexcitation limiters will not come into service.
Once the test is complete, the user opens the Toolbox and selects Edit, then Configure, then “Block Collected.” The user browses to the AVR/PSS_Test capture buffer, a feature of EX2100 that is designed specifically for this test. He or she then will select the quantities to trend when downloading the file.
SRP actually performs two tests at the same time. SRP has power system stabilizers on all exciters and a bit of local mode oscillation. SRP does one exciter step injection test with the power system stabilizer out of service and one with it in service. This confirms graphically that both the automatic voltage regulator and power system stabilizer are healthy.
As a backup to the capture buffer, the above tests can be recorded with the Toolbox trend recorder.
The ability to periodically test exciter performance can be extremely useful, saving time and money spent troubleshooting problems with old analog exciters. Many hydro plants are undergoing similar exciter upgrades, and most exciter manufacturers likely have similar interfaces and capabilities. Learning to use and maximize the capability of this new equipment can result in time and cost savings.
John Hunter is operations and maintenance supervisor, instrumentation controls and electrical, for the hydro generation division at Salt River Project.
This article has been evaluated and edited in accordance with reviews conducted by two or more professionals who have relevant expertise. These peer reviewers judge manuscripts for technical accuracy, usefulness, and overall importance within the hydroelectric industry.