Description:
United Sprint Telephone's main office building, in Leesburg,
Florida, is a high rise building housing all of the company's major
business operations for the Lake County area.
Because of the sensitive nature of the facility's commerce, many of the electrical loads are considered critical and hence protected by UPS' and backup generators.
In October 1994, it was discovered that an electrical service within the building that supported a data processing center, was experiencing difficulties. Neutral conductors were becoming overheated and a UPS system, which was thought to have had excess capacity, was switching into bypass. Because most of the demand was made up of single phase non-linear loads (computer terminals, facsimile machines, printers, modems), harmonic distortion was immediately considered as the primary cause of the malfunctioning. Concerned with the high heat levels of the neutral conductors, the company made arrangements to immediately install new oversized conductors and to then have the problem investigated further. A detailed inspection of the troubled system confirmed high zero-sequence harmonic currents on the neutral conductor and a distortion of the voltage above the IEEE-STD.519-1992 guidelines of 5%.
Goals:
Although Sprint's immediate goal was to correct the
obvious problem as soon as possible, the company also wished to provide
protection for future expansion of loads. In addition, it was felt
by management that, while the IEEE guideline of 5% voltage distortion
was acceptable for general commercial loads, a 3% guideline would
be a more prudent goal for the company's data processing facilities.
Constraints:
While one approach to solving this problem might have
been the installation of a new service utilizing oversized neutrals
and a K-Factor rated transformer (a standard industry solution that
attempts to merely survive harmonics through oversizing both design and equipment),
Sprint was aware of the fact that a K-Factor rated transformer would
not improve power quality, and the company preferred a solution that
would actually reduce the harmonic distortion on the system. Other
constraints that had to be considered were the lack of floor space
within the subject service area and the high cost involved in resizing
the facility's existing wiring.
Solutions:
The ideal solution was to install a Zero-Sequence Harmonic Filter. These
unique transformers are very special in their ability to cancel zero sequence harmonic
currents within their secondary windings. This design feature prevents
the circulation of zero-sequence harmonic currents with the primary
delta winding, while presenting an ultra low zero sequence impedance
to these load generated harmonic currents. The result is low voltage
distortion. In addition the Zero-Sequence Harmonic Filter is no larger than
other conventional transformers.
Because installation of aZero-Sequence Harmonic Filter would afford United Sprint Telephone considerable savings by allowing the company to utilize existing floor space and because this installation would also provide a system of cancellation and removal rather than one of coexistence, the choice became a win-win solution.
Results:
The results of the Zero-Sequence Harmonic Filter-filtering
transformer installation were instantaneous. Total Harmonic Voltage
Distortion was reduced from a high of 5.4% to less than 3%. The Zero-Sequence Harmonic Filter
fit comfortably within the assigned service area.
Contact: Mr. Don Webster
Buildings Operations Mgr.
United Sprint Telephone
Leesburg, FL 34749-0048