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ANSI/IEEE Standard 519-1992 states, “Within an industrial plant, the PCC is the point between the nonlinear load and other loads.” Many consulting engineers have interpreted this to mean that THD₁ is to be measured at VFD input-power connections (PCC2, instead of PCC1, in Figure 2). This misapplication of ANSI/IEEE Standard 519-1992 has contributed to the overuse of multipulse drives in the HVAC industry. Many millions of facility-equipment dollars have been squandered through the specification and installation of 12- and 18-pulse drives in commercial office buildings and other environments in which a standard six-pulse drive would have done the same job for substantially less upfront cost.

Also unfortunate is the fact ANSI/IEEE Standard 519-1992 has five different levels of acceptable maximum TDD, which depend on the ratio of maximum short-circuit current (I_SC) to maximum I_L at a PCC. The I_SC-to-I_L ratios in Table 1 are functions of the strength of a utility's feed to a facility and the size of the substation transformer.

CURRENT SITUATION

Many specifications simply state, “VFDs shall meet ANSI/IEEE Standard 519.” Such a statement is meaningless without the information needed to perform harmonic calculations:

• Transformer kilovolt-amperes and percent impedance.

• Total linear connected-load amperage or total expected linear connected amperage.

• The number and sizes of VFDs.

• Utility I_SC available.

Calculations are even more accurate when manufacturers have additional information, such as facility total current, existing harmonic content, and wire sizes and lengths.

Some engineers have taken to writing hardware specifications based on horsepower size requirements. For example: “All VFDs 100 hp and up shall be 18-pulse designs.” At 100 hp, an 18-pulse drive easily can cost four times as much as a six-pulse drive with no improvement in energy savings.