12-3.2

IEEE 519, *IEEE Recommended Practices and Requirements for Harmonic*

evaluating harmonic voltages and cur rents. IEEE 519 provides *system level *guidance,

not equipment specific guidance. What this means is that harmonic distortion limits are

established for the facility and the installation of any equipment should not degrade the

system to beyond acceptable levels. Meeting IEEE 519 limits can be easy for a facility

with primarily linear loads and it can be more difficult for a facility with predominantly

nonlinear loads. As an example, some facilities have installed many ASDs for HVAC

system motors in an attempt to reduce energy usage. Unfortunately, each ASD can

generate a large current harmonic distortion and nuisance tripping of other equipment

has occurred at some facilities as a consequence.

12-3.3

IEEE 519 provides the distortion limits for a low voltage system; Table 12-1

summarizes the various limits. Treat IEEE 519 distortion limits as design limits for

interior applications. Hospitals and airports are classified as special applications and

have the most stringent criteria. The criterion of most interest in this table is the voltage

total harmonic distortion (THD) limit of 3 percent for special applications or 5 percent for

general applications. Treat all critical systems as special applications.

Special

General

Dedicated

Applications

System

System

Notch Depth

10%

20%

50%

THD (Voltage)

3%

5%

10%

Notch Area (A N

16,400

22,800

36,500

volt microseconds)

12-4

12-4.1

Excessive harmonic distortion causes higher eddy current losses inside a

transformer, resulting in overheating. ANSI C57.110, *IEEE Recommended Practice for*

states that a transformer should be capable of carrying its rated current provided that

the total harmonic distortion is less than 5 percent. Beyond this amount, derating of the

transformer might be necessary. Newer transformers are often, but not always, already

designed for a higher harmonic distortion environment. Older transformers likely were

not designed for harmonic distortion. Evaluate the effects of harmonic currents on

transformers in accordance with the following subsections.

12-4.2

Whenever significant nonlinear loads are expected in a facility, evaluate the

system to determine if transformer derating will be required. Transformer derating is not

the same as calculating the transformer k-factor. The k -factor is used to match a

harmonic current condition with a k -factor ra ting. For transformers without a k-factor

rating, derating must be used to determine the maximum fundamental load current that

the transformer can maintain with the additional harmonic currents

12-3

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