Although the IEEE standard recommends limits for measured at part load values will also be acceptable.

both voltage distortion and current distortion, To use our rock analogy, the full load fundamental
specifications that reference a 5% harmonic current is the size of our pond and the harmonic
limitation are generally referring to current current is the size of our rock. (See Table D.)
distortion. In most cases, if the current distortion
Table D
falls within IEEE-519 requirements, the voltage Comparison of TDD and THD(I)
distortion will also be acceptable.
Fundamental Harmonic THD(I) TDD
Determining compliance with IEEE-519 requires an Current (rms) Current (rms )
actual measurement of the system during operation. 1000 50 5% 5%
Predicting compliance in advance often requires a 800 43.8 5.4% 4.4%
system study that accounts for all electrical 600 36.3 6.1% 3.6%
equipment (transformers, wires, motors, VFDs, 400 29.7 7.4% 3.0%
etc.) in the system.
200 20.0 10% 2%

Introduction To Harmonic Terms 100 13.4 13.4% 1.3%

TDD - Total Demand Distortion
Total Harmonic Voltage Distortion - THD (V) THD(I) - Total Harmonic Current Distortion

As harmonic currents flow through devices with Short Circuit Ratio

reactance or resistance, a voltage drop is developed.
Short circuit ratio is the short circuit current value of
These harmonic voltages cause voltage distortion of
the electrical system divided by its maximum load
the fundamental voltage wave form. The total
current. Standard IEEE-519 Table 2 defines
magnitude of the voltage distortion is the THD (V).
different acceptance levels of TDD depending on
The IEEE-519 standard recommends less than 5%
the short circuit ratio in the system. Systems with
THD (V) at the point of common coupling for
small short circuit ratios have lower TDD
general systems 69 kV and under.
requirements than systems with larger short circuit
Total Harmonic Current Distortion - THD (I) ratios. This difference accounts for the fact that
electrical systems with low short circuit ratios tend
This value (sometimes written as THID) represents to have high impedances, creating larger voltage
the total harmonic current distortion of the wave distortion for equivalent harmonic current levels.
form at the particular moment when the (See Table E.)
measurement is taken. It is the ratio of the harmonic
current to the fundamental (non-harmonic) current Mitigating Harmonics
measured for that load point. Note that the
Some utilities now impose penalties for introducing
denominator used in this ratio changes with load.
harmonics onto their grid, providing incentives for
Total Demand Distortion - TDD owners to reduce harmonics. In addition, reducing
harmonic levels can prevent potential damage to
Total Demand Distortion (TDD) is the ratio of the sensitive equipment residing on the same system.
measured harmonic current to the full load There are many approaches to mitigating harmonics.
fundamental current. The full load fundamental Several commonly used methods are discussed here.
current is the total amount of non-harmonic current
consumed by all of the loads on the system when the Line Reactors
system is at peak demand. The denominator used in Line reactors add reactance and impedance to the
this ratio does not change with load. Although TDD circuit. Reactance and impedance act to lower the
can be measured at any operating point (full or part current magnitude of harmonics in the system and
load), the worst case TDD will occur at full load. If thereby lower the TDD. Line reactors also protect
the full load TDD is acceptable, then the TDD devices from large current spikes with short rise
8
 Table E
Table 2 - Current distortion limits for systems rated 120 V through 69 kV

Maximum harmonic current distortion in percent of IL

Individual harmonic order (odd harmonics)a,b

Isc/IL 3≤h<11 11≤h<17 17≤h<23 23≤h<35 35≤h<50 TDD
<20c 4.0 2.0 1.5 0.6 0.3 5.0
20<50 7.0 3.5 2.5 1.0 0.5 8.0
50<100 10.0 4.5 4.0 1.5 0.7 12.0
100<1000 12.0 5.5 5.0 2.0 1.0 15.0
>1000 15.0 7.0 6.0 2.5 1.4 20.0
a Even harmonics are limited to 25% of the odd harmonic limits above.
b Current distortions that result in a dc offset, e.g., half-wave converters, are not allowed.
c All power generation equipment is limited to these values of current distortion, regardless of actual I /I where
sc L
Isc = maximum short-circuit current at PCC
IL = maximum demand load current (fundamental frequency component) at the PCC under normal load operating conditions.

times. A line reactor placed between the VFD and upstream of the filter. These types of filters generally
the motor would help protect the motor from current have excellent harmonic mitigation characteristics.
spikes. A line reactor placed between the supply and Active filters may reduce generator size
VFD would help protect the supply from current requirements.
spikes. Line reactors are typically only used VFDs Using Active Front End Technology (AFE)
between the VFD and the motor when a
freestanding VFD is mounted more than fifty feet Some VFDs are manufactured with IGBT rectifiers.
from the motor. This is done to protect the motor The unique attributes of IGBTs allow the VFD to
windings from voltage peaks with extremely quick actively control the power input, thereby lowering
rise times. harmonics, increasing power factor and making the
VFD far more tolerant of supply side disturbances.
Passive Filters The AFE VFDs have ultra low harmonics capable of
meeting IEEE-519 standards without any external
Trap Filters are devices that include an electrical filters or line reactors. This significantly reduces
circuit consisting of inductors, reactors, and installation cost and generator size requirements.
capacitors designed to provide a low impedance path An AFE drive provides the best way to take
to ground at the targeted frequency. Since current advantage of VFD benefits and minimize harmonics.
will travel through the lowest impedance path, this
prevents the harmonic current at the targeted Multi-Pulse VFDs (Cancellation)
frequency from propagating through the system.
Filters can be mounted inside the drive cabinet or as There are a minimum of six rectifiers for a three-
free standing devices. Trap filters are typically phase AC VFD. There can be more, however.
quoted to meet a THD(I) value that would result in Manufacturers offer 12, 18, 24, and 30 pulse drives.
compliance with IEEE-519 requirements if the A standard six-pulse drive has six rectifiers, a 12-
system were otherwise already in compliance. pulse drive has two sets of six rectifiers, an 18-pulse
drive has three sets of six rectifiers and so on. If the
Active Filters power connected to each set of rectifiers is phase
shifted, then some of the harmonics produced by one
Some devices measure harmonic currents and set of rectifiers will be opposite in polarity from the
quickly create opposite current harmonic wave harmonics produced by the other set of rectifiers.
forms. The two wave forms then cancel out,
preventing harmonic currents from being observed
9
The two wave forms effectively cancel each REFERENCES
other out. In order to use phase shifting, a IEEE Standard 519-2014. “IEEE Recommended
special transformer with multiple secondary Practices and Requirements of Harmonic
windings must be used. For example, with a Control in Electrical Power Systems.”
12-pulse VFD, a Delta/Delta-Wye transformer
with each of the secondary phases shifted by Rockwell Automation. “Dealing with line
30 degrees would be used. harmonics from PWM variable frequency
drives.”
CONCLUSION John F. Hibbard, Michael Z. Lowenstein.
“Meeting IEEE 519-1992 Harmonic
• VFDs provide the most energy efficient
Limits Using Harmonic Guard Passive
means of capacity control.
Filters”
• VFDs have the lowest starting current of (TRANS-COIL, INC)
any starter type.
Tony Hoevenaars, P. Eng, Kurt LeDoux, P.E.,
• VFDs reduce thermal and mechanical Matt Colosina. 2003. “Interpreting
stresses on motors and belts. IEEE Std 519 and Meeting its
• VFD installation is as simple as connecting Harmonic Limits in VFD
the power supply to the VFD. Applications.”
(IEEE paper No. PCIC-2003-XX).
• VFDs with AFE technology can meet even
the most stringent harmonic standards and Gary Rockis, Glen Mazur, American Technical
reduce backup generator sizing. Publishers, Inc. 1997. “Electrical Motor
Controls.”
• VFDs provide high power factor,
eliminating the need for external power Richard H. Smith, P.E., Pure Power. 1999.
factor correction capacitors. “Power Quality Vista Looks Good
Thanks to IGBTs.”
• VFDs provide lower KVA, helping
alleviate voltage sags and power
outages. FURTHER READING FROM CARRIER
NOTE: It is the responsibility of the user to Carrier. 1993. Harmonics: A Brief Introduction.
evaluate the accuracy, completeness or Carrier. 1999. 19XRV Marketing Guide.
usefulness of any content in this paper. Neither
Carrier nor its affiliates make any Carrier. 2005. Carrier Introduces Rotary
representations or warranties regarding the Chillers with Liquiflo2 Variable Speed Drive.
content contained in this paper. Neither Carrier Carrier. 2005. Carrier Variable Speed Screw
nor its affiliates will be liable to any user or White Paper
anyone else for any inaccuracy, error or omission,
regardless of cause, or for any damages resulting
from any use, reliance or reference to the content
in this paper.

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