THE NEED TO ADDRESS HARMONICS
One of the two most common problems for industrial power users is striving to meet IEEE Std. 519-1992. IEEE 519 is a standard developed for utility companies and their customers in order to limit harmonic content and provide all users with better power quality. In the future, some utility companies may impose a penalty for users producing harmonics. Some key areas of the standard are as follows in Tables 1 and 2:
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TABLE 1
Voltage Distortion Limits
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Bus Voltage at PCC*
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Individual Voltage Distortion (%)
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Total Voltage Distortion THD (%)
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69 kV and below
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3.0
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5.0
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69.001 kV through 161 kV
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1.5
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2.5
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161.001 kV and above
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1.0
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1.5
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NOTE: High-voltage systems can have up to 2.0% THD where the cause is an HVDC terminal that will attenuate by the time it is tapped for a user.
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*PCC is Point of Common Coupling
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TABLE 2
Current Distortion Limits for General Distribution Systems
(120 V Through 69 000 V)
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Maximum Harmonic Current Distortion in Percent of IL
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Individual Harmonic Order (Odd Harmonics)
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ISC / IL
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<11
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<11≤h<17
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17≤h<23
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23≤h<35
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35≤h
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TDD
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<20*
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4.0
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2.0
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1.5
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0.6
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0.3
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5.0
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20<50
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7.0
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3.5
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2.5
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1.0
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0.5
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8.0
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50<100
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10.0
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4.5
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4.0
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1.5
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0.7
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12.0
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100<1000
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12.0
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5.5
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5.0
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2.0
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1.0
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15.0
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>1000
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15.0
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7.0
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6.0
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2.5
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1.4
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20.0
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Even harmonics are limited to 25% of the odd harmonic limits above.
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Current distortions that result in a dc offset, e.g., half-wave converters, are not allowed.
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*All power generation equipment is limited to these values of current distortion, regardless of actual ISC / IL
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where
ISC = maximum short-circuit current at PCC**
IL = maximum demand load current (fundamental frequency component) at PCC**
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*PCC is Point of Common Coupling
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The other most common problem resulting from harmonics is power factor capacitor failures, or a need to correct power factor in a harmonic environment. In low-voltage systems (600 V or less), capacitors are typically the lowest impedance at harmonic frequencies. Therefore, they experience very high RMS currents and increased heat, which causes them to fail. So, even if meeting IEEE 519 is not the goal, dealing with harmonics may still be required.
When correcting power factor, there is the possibility of creating a resonant circuit. A resonant circuit occurs at the frequency when the impedance of the system (mostly the power transformer) and the impedance of the power factor capacitor are equal. This is called the resonant frequency.
The following formula can be used to find the resonant frequency in terms of harmonic number.
h = resonant frequency in terms of harmonic number
KVAsc = short-circuit capacity at the capacitor
kVAR = rated kVAR of the unswitched capacitance
If there are any sources of current at that frequency, they will be amplified. This will cause high distortion levels and could blow fuses or trip circuit breakers.