When changes occur and a system doesn’t work properly anymore, it is often good to go back and review the events leading up to the change to determine the why’s of the past, along with the why’s of the present. In a recent problem at a large package handling facility, capacitor banks that were installed years ago to improve the power factor were failing.
There were six 300kVAR capacitor banks, and one of them had a bulge on the side. While replacing that one, the maintenance crew noticed that the indicating lights on the other capacitor banks were lit and it was presumed the remaining five banks would fail. The estimates came to $30,000 for replacement, including labor, materials, and testing.
This facility had double-ended 1,000kVA, 4160-480/277V, three-phase, 60Hz load centers. Over the past five to six years, overall kVA loads had been dramatically reduced to between 15 and 45 percent of rated capacity through an energy conservation program. The program included converting electric chillers to gas chillers, replacing old inefficient lighting fixtures with new energy-efficient lamps and ballasts, and adding adjustable-speed drive (ASD) motors.
Now, within the large warehouse facility were (17,000) fluorescent T-8 lamps with electronic ballasts; (60) 150W, (100) 250W, (700) 400W, and (400) 1,000W high/low bay high-pressure sodium (HPS) fixtures; (1,200) motors ranging from fractional horse power to 125hp; (125) adjustable speed drives; (400) programmable logic controllers (PLCs); (300) six-pulse battery chargers; and (30) UPS units.
A power quality monitor was placed on each of the transformers for a week to monitor the outputs. This site had previously experienced numerous sags, which caused high-intensity discharge (HID) lights to trip off, but the utility had subsequently fixed a faulty feeder, and it was not thought to be the source of the failed capacitor banks.
With the capacitors turned on, the current total harmonic distortion (THD) varied from 55 to 63 percent, and the voltage THD varied from 4.2 percent to 5.7 percent.
This voltage distortion was well above recommended levels. The power factor was approximately -0.8 leading. Very significant seventh and ninth harmonic levels were measured. As part of the tests, the capacitor banks were switched off, resulting in the current THD dropping to between 10 percent and 12 percent, and the voltage THD going down to between 1 percent and 2 percent. The power factor changed to lagging at around 0.75.
The excessive harmonic distortion was the result of the lower kVA loads coupled with the existing 300kVAR fixed capacitor banks. With the reduced load and harmonic-current-generating devices, such as ASDs now in the facility, the capacitor banks formed a tuned circuit with the inductance of the lines and transformers, resonating around the seventh harmonic. This in turn caused the overheating and bulging of the capacitor can.
The capacitor banks were selected when the total load on the system was much higher. This resonance created excessive voltage levels that could also damage supply transformers and equipment. Tests at the remaining load centers had similar results.
The facility engineer decided that it was better to leave the capacitor banks off than to risk more capacitor failures and other damage. The lagging power factor was within contract limits with the utility company. Utilities generally prefer lagging power factors, rather than leading, as the excess capacitance in leading power factor circuits can cause higher-than-normal distribution voltage levels.
This is not to recommend turning off capacitor banks indiscriminately. Rather, when the system is changed, such as by reducing the load with conservation programs or adding new equipment, the entire system should be re-examined for potentially incompatible equipment. Monitoring before and after gives important clues to why things changed. Resizing capacitor banks may be just one of the actions required when the system load is significantly decreased or increased, or when the types of loads are changed.
BINGHAM, manager of products and technology for Dranetz-BMI in Edison, N.J., can be reached at (732) 287-3680.