Page 199 - HVAC Pump Handbook

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Pump Drivers and Variable-Speed Drives

196 HVAC Pumps and Their Performance

2047

Voltage

0

Current

2040
Figure 7.14 Output voltage and current of a series-cell multilevel VFD. (From
Richard H. Osman, Medium–Voltage Variable Frequency Drives for Induction and
Synchronous Motors.)

Harmonics were addressed by the IEEE, which, in 1981, issued
Standard 519-1981 establishing limits on the allowable voltage dis-
tortion on a feeder common to several facilities. In most cases, the
total harmonic voltage distortion was limited to 5 percent. The use of
a current-source or voltage-source thyristor inverter often required
the installation of a high-pass filter to control the distortion. In addi-
tion to increasing the cost of the installation, the filter introduced
losses that reduced operating efficiency. Nonetheless, these filters did
the required job, and thousands are presently in service.
In 1992, an IEEE committee revised the standard and issued
Standard 519-1992, IEEE Recommended Practices and Requirements
for Harmonic Control in Electrical Power System. This document
spelled out, for the first time, the allowable levels of harmonic cur-
rents injected into the utility system. The limit values depend on how
“stiff” the supply system is and on the particular harmonic involved.
In general, the limits are much more severe and difficult to meet than
were the previous limits on voltage distortion only.
Harmonic distortion is of great concern to electric utilities and
building or plant operators. The projected harmonic distortion can be
computed for a new or existing installation of variable-frequency drives.
Figure 7.15 is a form that can be used by the variable-frequency drive
manufacturer to estimate the harmonic distortion for most installa-
tions. The following information is required:

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