Catastrophic damage may result if the battery power required to start and run emergency pumps fails when it’s most needed
By Michael P. O’Brien & Bryan Dardar, Nolan Power
Can your generation plant survive an unplanned or emergency trip without sustaining major damage?
Good question, and one your insurance company or investors are likely to ask and want documentation on.
The heart of the safety system that allows for an orderly and safe generating unit shutdown is the station battery and associated DC system. The generating unit will sustain major or even catastrophic damage if the station battery and associated DC system fail to perform as specified in the system design criteria.
How can you determine if your generation plant can safely survive an unplanned or emergency trip without risking plant damage? By conducting properly designed and performed battery capacity tests at the appropriate frequency and by conducting proper battery maintenance in accordance with the appropriate IEEE/ANSI standards. We’ll illustrate why capacity testing of the station battery and associated DC system is essential to safe generation plant operation and how to properly perform such tests.
The station battery and associated DC system provides control power for switchgear and generator control and provides power for emergency lube oil pumps, emergency seal oil pumps and sometimes even jacking oil pumps. Under normal conditions these emergency pumps are never called into operation; however, during an unplanned or emergency trip, these pumps provide all lube oil and seal oil pressure required to bring the generating unit to a safe shutdown. The generating unit will suffer major or even catastrophic damage if the station battery and associated DC system fail to provide the DC power required to start and run these emergency pumps.
Properly conducted capacity tests are an integral part of station battery and associated DC system maintenance and ensure the reliability of the DC system. Capacity tests are the only method of determining and quantifying station battery performance. Impedance or internal resistance measurements, while valuable, do not measure battery/cell capacity and are not a replacement for capacity tests. Properly designed and performed capacity tests will prove that the battery can, or cannot, support a worst-case trip condition, as well as determine how much useable life is left in a battery system, thus allowing for planning and budgeting station battery replacement. Capacity testing also provides baseline performance data, aids in identifying manufacturing defects, installation deficiencies and incipient problems not detectable by other means.
Designing the Capacity Test
The capacity test design is crucial to prove the battery will support worst-case emergency loads and measure battery capacity. This is because batteries can produce different measured capacities at different discharge rates. This effect becomes more pronounced as the battery ages. We need to design a test that encompasses the battery duty cycle to prove the battery will, or won’t, support the connected emergency loads and measure the battery capacity. In essence, we want to design a test that objectively and repeatably measures battery capacity as it is used in a specific plant or generating unit.
A capacity test is basically a constant load discharge of the battery to a specified end voltage. Unfortunately, generation plants don’t have a constant DC load during an emergency; they have a complex DC load profile. So, a “Modified Performance Test” is designed, as defined in IEEE/ANSI Std. 450-2005.
As shown in Table 1, the highest loads occur during the first minute of an outage as the emergency pumps start, circuit breakers trip and other devices operate. The load drops after the first minute as the battery has to run only the emergency pumps and various controls over a time period that usually stretches for hours.
The most critical portion of this load profile is the first minute. That’s because all of the emergency pumps start during these 60 seconds. It won’t matter how long the battery can run the pumps if the battery can’t supply the energy required to start the emergency pumps. Therefore, the test must require the battery to supply the first minute’s currents, but only for one minute. A battery capable of supporting this load profile, (EnerSys GC-25) is capable of supporting 1,200 amps for 58 minutes to an end voltage of 1.75 volts per cell. Why not just test the battery at this rate? Because this would be more severe than the duty cycle and plant personnel likely would replace the battery before required (80 percent capacity) because high rate capacity generally falls off earlier in battery life than low rate capacity.
This means the first minute of the test will require the battery to supply 1,200 amps. This proves the battery can start the emergency pumps while supporting other loads. We prove the battery can run the emergency pumps and other loads for the required time and also measure the battery capacity by examining the load profile and the battery manufacturer’s discharge ratings. We then find a discharge rate that envelopes the remainder of the duty cycle. In this case, the next-highest current is 500 amps and the total duty cycle is four hours. The battery manufacturer’s published discharge ratings show that this battery is rated to support a load of 504 amps for four hours to an end voltage of 1.75 volts per cell. This means the “Modified Performance Test” will require the battery to supply 1,200 amps for one minute and then transition to the battery manufacturer’s published four-hour discharge rate to 1.75 volts per cell (504 amps). This test covers the entire battery duty cycle, proving that the battery can, or cannot, support the worst case emergency loads and measures battery capacity.
The test equipment should be a computer-based battery discharge test system with automatic data logging. It should be capable of conducting single or multiple steps, constant current or constant power (kilowatt) discharge tests. It should include a video display of all pertinent data relating to a discharge test such as individual cell voltages, load current, overall battery voltage, average cell voltage, minimum cell voltage, maximum cell voltage, elapsed test time, program step number and program step elapsed time, battery location and battery identification number. Individual cell voltages should be displayed in a bar graph format, allowing the test engineer/technician to spot at a glance failing cells or abnormal conditions. All individual cell voltage changes, overall battery voltage and test current should be automatically recorded for later retrieval.
The load banks should be DC voltage rated, single phase, air-cooled load banks. The load current should be adjustable in one amp steps throughout the range of the load unit.
Station battery testing should be performed during a scheduled outage and after the generating units have sufficiently cooled and are off turning gear. The station battery will be isolated from the DC system during testing so a temporary battery will be required to support the switchgear and any operating control circuits. Do not allow the battery charger to serve the DC system without a battery connected.
- Ensure the battery has received an equalization charge at least three days, but no more than 30 days, before the test
- Measure and record the electrolyte temperature of at least 10 percent of the cells and calculate the average temperature.
- Measure and record cell float voltages.
- Parallel the portable battery plant with the station battery.
- Isolate the station battery from the DC system.
- Connect test equipment to battery.
- Apply the temperature correction factor to discharge load and program test equipment.
- Initiate the capacity test. The load must be maintained at the specified level throughout the test until the battery terminal voltage drops below the specified end voltage. Note that capacity tests are not stopped after reaching a specified discharge time. Capacity tests are only ended when the battery terminal voltage drops below the specified end voltage. For example, the end voltage per cell multiplied by the number of cells in series equals the battery terminal end voltage; 1.75 (per cell end voltage) x 60 (cells in series) = 105.0 VDC (battery terminal end voltage). The capacity test would end when the battery terminal voltage dropped below 105.0 VDC.
- Check for overheated connections during the test. If the temperature rise across any connection becomes unacceptable, terminate the test. Note here, too, that high-resistance connections will show as low-voltage cells during the test. This is because the cell voltage measurements include the voltage drop across the cell’s associated intercell connector. Normally (but not always) a cell showing low voltage during the first 30 to 60 seconds of a capacity test is not truly a low voltage cell but a high resistance connection.
- If the voltage of any cell approaches polarity reversal (less than one volt), pause the test, jumper around the cell and resume the test to the new end voltage. The test pause time should be no more than 10 percent of the specified test time. Be careful to isolate the cell to be jumpered before connecting the jumper. Do not short-circuit the cell being jumpered. And be careful that cells not be allowed to go into polarity reversal as this causes irreparable damage to the cell and can pose a safety hazard.
- At the conclusion of the test, record the elapsed test time and disconnect all test equipment.
- Parallel the station battery with the portable battery plant.
- Monitor the initial recharge and, if necessary, adjust the charger output current limit.
- Disconnect the portable battery plant.
How often should plant personnel perform a “Modified Performance Test”? The answer depends on the type of battery in use, the schedule of major outages and whether or not the battery room is temperature controlled.
Annual tests are required for valve regulated lead-acid (VLRA), batteries and should be conducted on batteries in hot environments. Battery life can drop dramatically in high temperature environments; therefore, additional testing may be required to insure reliability.
Flooded lead-acid batteries in reasonable temperature environments should be tested upon installation, after two years in service and every five years after that until the battery reaches 85 percent of its rated design life or shows signs of degradation. Degradation is a drop in capacity of 10 percent or more between capacity tests or when the battery has less than 90 percent of rated capacity. Annual testing should be conducted when the battery reaches 85 percent of its rated design life or shows signs of degradation.
No substitute exists for properly designed, instrumented and conducted battery capacity tests. These tests are the only scientific method of proving that a station battery will support the connected load and the only scientific method of determining when to replace a station battery. Tests go a long way toward helping determine if your generation plant can survive an unplanned or emergency trip without sustaining major damage.
Authors: Michael P. O’Brien is technical services manager and Bryan Dardar is test services manager for NOLAN Power Group.