The New Jersey International & Bulk Mail Center (NJI-BMC), one of the largest United States postal facilities, recently faced a dilemma regarding its six aging 300kVAR capacitor banks in its three load centers. Initially, we explored the possibility of replacing all capacitor banks on the system, because one of the cans overheated and subsequently failed. But the solution was as simple as turning them off.
Seven months ago, one of the six 300kVAR capacitor cans at the facility developed a bulge. Within a month, we replaced it. At that time, our maintenance crew noticed indicating lights on the other capacitor banks were lit. We presumed the remaining five banks would fail, and thought the simplest remedy would be to replace all six at once. An engineer quoted us approximately $30,000 for replacement, including labor, material, and testing.
Other Options Our options were to replace the banks now or verify if we could continue operating the facility, without jeopardizing system reliability. After consulting with Gregory Olson and William McCormack of Public Service Electric & Gas (PSE&G) and Rich Bingham and Thurman Bridges of Dranetz-BMI, we decided to connect two Power Platform 4300 power monitors at each end of the double-ended 1000kVA, 4160-480/277V, 3-phase, 60Hz load centers and analyze power flow on the system. The facility is equipped with 17,000 fluorescent T-8 lamps with electronic ballast; 60 150W, 100 250W, 700 400W, and 400 1000W high/low bay HPS fixtures; 1200 motors ranging from fractional horse power to 125hp; 125 adjustable speed drives; 400 PLCs; 300 6-pulse battery chargers; and 30 UPS units. Every one of these devices could distort a sine wave and alter overall harmonic distortion on the system.
Metering With the capacitors turned on, the meters indicated the total harmonic distortion (THD) for current varied from 55% to 63%, whereas the voltage THD varied from 4.2% to 5.7%. When we turned the capacitors off, the current THD dropped to 10% to 12% and voltage THD went down to 1% to 2%. We assumed a major reason for the higher THDs was that in the past five to six years the facility's overall kVA loads had been dramatically reduced through an energy conservation program. This program significantly reduced the electrical ampere load by converting electric chillers to gas chillers, replacing old inefficient lighting fixtures with new energy-efficient lamps and ballasts, and adding adjustable-speed drive motors. In fact, the monitoring equipment indicated the load at one of the load centers varied from 185kVA to 380kVA.
A Surprise Solution At this point, we realized that lower kVA loads coupled with the existing 300kVAR fixed capacitor banks on each side of the facility had caused the overheating and bulging of the capacitor can. Our preliminary calculation indicated the caps were resonating at 7th and 9th harmonics. After testing this load center, we connected power monitoring meters to all remaining load centers. The data revealed the fixed capacitors (if turned on) were creating higher THDs on the system. Later, we learned these fixed cap banks could also damage our supply transformers and equipment. Our solution was to simply turn the cap banks off. By doing this, we prevented a major transformer failure on our system and avoided lost production time.
