AT313_Power System Harmonic Fundamentals

8/10/2019 AT313_Power System Harmonic Fundamentals

1/23

Power System HarmonicFundamental Considerations:

Tips and Tools for ReducingHarmonic Distortion in ElectronicDrive Applications

October 2011/AT313

by

Larry Ray, P.E.

Louis Hapeshis, P.E.

Make the most of your energySM

Revision #1 10/11


2/23

Get connectedto power

Summary

Abstract ...................................................................................................... p 3

Introduction ................................................................................................ p 4

Harmonic Distortion Basics ......................................................................... p 5

Voltage and Current Distortion .................................................................... p 8

Harmonics and Power Factor Displacement and Total .............................. p 11

Harmonic Mitigation Two Passive Techniques ........................................... p 13

Common Harmonic Current Signatures ...................................................... p 15

Harmonic Distortion Simulation Methods ..................................................... p 17

References ................................................................................................. p 22

AT313 | 2

Power system harmonic fundamental considerations


3/23


Abstract

This paper provides an overview of harmonic considerations for designing industrial and commercial electric

power distribution systems. These power systems must serve a combination of loads, many of which produce

non-sinusoidal current when energized from a sinusoidal AC voltage source. While conventional power

distribution systems accommodate a significant amount of non-sinusoidal current, the design engineer can

utilize existing IEEE guidelines and basic software tools to avoid some special circuit and load configurations

that exacerbate harmonic distortion problems.

AT313 | 3



4/23


Introduction

Power system harmonic distortion has existed since the early 1900s, as long as AC power itself has been

available. The earliest harmonic distortion issues were associated with third harmonic currents produced by

saturated iron in machines and transformers, so-called ferromagneticloads. Later,arcingloads, like lighting

and electric arc furnaces, were shown to produce harmonic distortion as well. The final type, electronic

loads, burst onto the power scene in the 1970s and 80s, and has represented the fastest growing category

ever since.

A better understanding of power system harmonic phenomena can be achieved with the consideration of

some fundamental concepts, especially, the nature of non-linear loads, and the interaction of harmonic currents

and voltages within the power system.

AT313 | 4



5/23


Harmonic Distortion Basics

Whats Flowing on the Wire?By definition,harmonic(or non-linear) loads are those devices that naturally produce a non-sinusoidal current

when energized by a sinusoidal voltage source.

Each waveform on the right, for example, represents the variation in instantaneous current over time for two

different loads each energized from a sinusoidal voltage source (not shown on the graph).

Graph 1

For each load, instantaneous current at some point in time (at the start of the graph, for example)is zero.

Its magnitude quickly increases to a maximum value, then decreases until it returns to zero. At this point, the

current direction appears to reverse and the maximum-to-zero-magnitude trend repeats in the negative

direction. This pattern is repeated continuously, as long as the device is energized, creating a set of largely-

identical waveforms that adhere to a common time period.

Both current waveforms were produced by turning on some type of load device. In the case of the current on

the left, this device was probably an electric motor or resistance heater. The current on the right could have

been produced by an electronic variable-speed drive, for example. The devices could be single- or three-

phase, but only one phase current waveform is shown for illustration. The other phases would be similar.

How to Describe Whats Flowing on the Wire?

Fourier SeriesWhile the visual difference in the above waveforms is evident, graphical appearance alone is seldom sufficient

for the power engineer required to analyze the effects of non-sinusoidal loads on the power system. The

degree of non-linearity must be objectively established, and the method of quantifying the harmonic distortion

must also facilitate future analysis and mitigation.

AT313 | 5



6/23

Graph 2

One method of describing the non-sinusoidal waveform is called its Fourier Series. Jean Fourier was a French

mathematician of the early 19thcentury who discovered a special characteristic of periodic waveforms. Periodic

waveforms are those waveforms comprised of identical values that repeat in the same time interval, like thoseshown above.

Fourier discovered that periodic waveforms can be represented by a series of sinusoids summed together. The

frequency of these sinusoids is an integer multiple of the frequency represented by the fundamental periodic

waveform. The waveform on the left above, for example, is described entirely by one sinusoid, the fundamental,

since it contains no harmonic distortion.

The distorted (non-linear) waveform, however, deserves further scrutiny. This waveform meets the continuous,

periodic requirement established by Fourier. It can be described, therefore, by a series of sinusoids. This

example waveform is represented by only three harmonic components, but some real-world waveforms

(square wave, for example) require hundreds of sinusoidal components to fully describe them. The magnitudeof these sinusoids decreases with increasing frequency, often allowing the power engineer to ignore the effect

of components above about the 50 thharmonic.

The concept that a distorted waveform (even a square wave!) can be represented by a series of sinusoids is

difficult for many engineers. But it is absolutely essential for understanding the harmonic analysis and mitigation

to follow.

Its important for the power engineer to keep in mind a few facts:

The equivalent harmonic components are just a representation the instantaneous current as described by the

distorted waveform is whats actually flowing on the wire.

This representation is necessary because it facilitates analysis of the power system. The effect of sinusoids

on typical power system components (transformers, conductors, capacitors) is much easier to analyze than

distorted signals.

Power engineers comfortable with the concept of harmonics often refer to individual harmonic components

as if each really exists as a separate entity. For example, a load might be described as producing 30 A of

5thharmonic. Whats intended is not that the load under consideration produced 30 A of current at 300 Hz,

but rather that the load produced a distorted (but largely 60 Hz) current, one sinusoidal component of which

has a frequency of 300 Hz with an rms magnitude of 30 A.

AT313 | 6



7/23

The equivalent harmonic components, while imaginary, fully and accurately represent the distorted current.

As one test, try summing the instantaneous current of the harmonic components at any point in time.

Compare this value to the value of the distorted waveform at the same time (see chart below). These values

are the same.

Graph 3

Total Harmonic Distortion

The series of harmonic components that represent a distorted waveform are often described by a single

number, total harmonic distortion. This number is calculated in two different ways, depending somewhat on the

engineers geographic location.

In the United States, total harmonic distortion is calculated as the sum of all the harmonic components (except

the fundamental), divided by the magnitude of the fundamental. This value is represented as THD (all upper

case). Or, in equation form:

Note that the components are summed vectorially, notalgebraically, because they have different phase angles.

For a waveform represented by a fundamental current of 100 A, a 5thcomponent of 20 A, and a 7 thcomponent

of 12 A, for example, Ihwould equal the square root of (202+ 122), or 23 A,not(20 + 12) = 32 A. The THD is,

therefore, 23/100 = 0.23 or 23%.

It is possible for the US-convention THD to exceed 1.0 or 100%, since it is possible for the magnitude of

harmonic current to exceed the magnitude of fundamental current. This is the primary distinction between the

US and European convention. The European convention, thd (all lower case) equals the harmonic components

divided by the total rmscurrent (harmonics plus fundamental). This thd value can never exceed 100%.

AT313 | 7



8/23


Voltage and Current Distortion

Harmonic Current FlowThe current drawn by non-linear loads passes through all of the impedance between the system source and

load. This current produces harmonic voltages for each harmonic as it flows through the system impedance.

These harmonic voltages sum and produce a distorted voltage when combined with the fundamental. The

voltage distortion magnitude is dependent on the source impedance and the harmonic voltages produced.

Figure 4 illustrateshow the distorted voltage is created. As illustrated, non-linear loads are typically modeled

as a source of harmonic current.

Figure 4: Creation of distorted current

With low source impedance the voltage distortion will be low for a given level of harmonic current. If harmoniccurrent increases, however, system impedance changes due to harmonic resonance (discussed below),

voltage distortion can increase significantly.

Circuit Impedance Without Power Factor Correction

While the preceding discussion focused on distorted current waveforms, it is important to note that ac

voltage can also show the effects of harmonic distortion. The degree of distortion is determined by applying

the same techniques as described earlier for current. So, what is the relationship between voltage distortion

and current distortion?

Higher-frequency (harmonic) components of ac voltage and current follow the same power system rules as

60 Hz voltages and currents Kirchoffs Voltage and Current Laws, Ohms Law, etc. One basic principle is that

voltage and current are related byimpedance. As with ac voltage and current, impedance is a complex term,

consisting of resistance, capacitance, and inductance. While resistance is largely independent of frequency,

impedance associated with capacitance and inductance changes as the frequency of the signal changes.

AT313 | 8



9/23

For the typical industrial power system, the impedance as seen by the loads is dominated by inductance. Since

inductive reactance is directly proportional to the frequency of the current, the system impedance approximates

a straight line, as illustrated below.

Impedance,

Frequency (Hz)

1.50

1.20

0.90

0.60

0.30

0.00

0 300 600 900 1200 1500

Typical system impedance

(without power factor

correction capacitors)

Graph 4

For this typical power system, the impedance encountered by the 300 Hz (5th harmonic) component of

current is approximately five times the impedance encountered by the 60 Hz (fundamental) component. With

this type of power system, the amount of voltage distortion can be estimated by summing the voltage drop

at each harmonic component, as summarized in the following table. The table assumes that the circuit load

is represented by the single harmonic source shown earlier, with a total Irms

= 102.7 A, and ITHD

= 23%, with a

nominal system voltage of 480 Vrms

.

Table 1

Harmonic Current, Irms Impedance,Ohms

Voltage Drop,Vrms

1 100 0.01 1.00

5 20 0.05 1.00

7 12 0.07 0.84

Total Vhdrop 1.3

Resulting VTHD

0.27%

Resulting Vrms

478

Circuit Impedance With Power Factor Correction

Power factor correction capacitors are often utilized in industrial and commercial power systems to reduce

power factor penalties, release circuit capacity, improve voltage regulation and reduce resistive heating losses

in circuit conductors. While power factor correction capacitors do not inject harmonic distortion (that is, PFCs

are linear loads they produce a sinusoidal current waveform when energized from a sinusoidal voltage

source), their presence on a power system dramatically changes the circuit impedance. These impedance

changes can adversely affect power system components, and worsen harmonic distortion concerns.

AT313 | 9



10/23

Graph 5

When power factor correction capacitors are installed, a frequency of high impedance known as the resonance

point results from the new combination of inductive and capacitive reactance. This resonance point is limited

in magnitude only by the amount of resistance in the circuit, and is often many times the value of the inductive

impedance at that frequency. The more capacitance added to the circuit, the lower the frequency at which

this resonance point occurs.

This high-impedance point, coupled with the operation of harmonic-producing loads, can result in much

higher levels of voltage distortion than the circuit without capacitors. Thats why it is so important to closely

evaluate the addition of power factor correction capacitors on a power circuit serving harmonic loads. The

example estimate below shows the distortion estimate associated with the same 103 A, 23% THD

load shown earlier, except that the impedance at the 7thharmonic is assumed to be ten times its non-PFC

value (a resonance point at or near 420 Hz). Note that this change results in nearly a tenfold increase in

voltage distortion.

Table 2

Harmonic Current, Irms Impedance,Ohms

Voltage Drop,Vrms

1 100 0.01 1.00

5 20 0.05 1.00

7 12 0.7 8.40

Total Vhdrop 8.5

Resulting VTHD

1.26%

Resulting Vrms 471

AT313 | 10



11/23


Harmonics and Power Factor Displacement and Total

As discussed, harmonic distortion and power factor correction are seldom considered as separate topics.

This is due to the dramatic effect on system impedance at harmonic frequencies that can result from the

addition of conventional power factor correction capacitors. The relationship, unfortunately, does not end there.

Due to their non-linear nature, the presence of harmonic loads can sometimes fool the power engineer into

considering unnecessary power factor correction in the first place!

Power Factor of a PWM Drive An Extreme Example?

The pulse-width-modulated (PWM) variable frequency drive (VFD) produces a characteristic current waveform

when energized from a sinusoidal voltage source. This three-phase device produces a voltage and current

waveform for one phase that resembles the following graphic:

Graph 6

If the power parameters (real, reactive, and apparent) associated with this PWM device are measured

with a true-rms meter, the typical values would show a relationship of real (kW) to apparent power (kVA) of

approximately 0.6. The engineer might conclude from this knowledge that the power factor of the device is

poor, and that a circuit containing many of these PWM drives (not uncommon) would require power factor

correction capacitors.

Unfortunately, this line of reasoning is incorrect and can lead to disastrous results. While the kW/kVA

relationship indicated above is accurate, 0.6 is not the power factor of the device. At least, it is not thecomplete picture of the power factor.

Further measurements would reveal that the displacement angle between voltage and current for this device

is 0. That is, the current and voltage are in phase with each other. Or, more accurately, the fundamental (60 Hz)

component of voltage and the fundamental (60 Hz) component of current are in phase, as shown below.

AT313 | 11



12/23

Graph 7

Since harmonic loads like PWM drives are able to consume power in a non-linear fashion; that is, by turningon and off in a manner not proportional to the applied instantaneous voltage, their kW/kVA relationship is not

equal to the phase angle between fundamental voltage and current.

This peculiarity, in fact, has required the establishment of two power factor definitions. These two power

factors are equal for undistorted (sinusoidal) voltages and currents.

Displacement Power Factor (dPF) Cosine of the phase angle between fundamental voltage and

fundamental current.

Total (sometimes referred to as True) Power Factor (tPF) Real power (kW) divided by apparent

power (kVA).

Power factor correction capacitors primarily affect the displacement power factor for a circuit. If PFCs are

applied on a circuit that already has a high dPF, then the fundamental current component could be shifted

into a leading relationship to fundamental voltage. This situation can result in voltage regulation and distortion

problems for the circuit.

In addition, the addition of large PFCs on a PWM circuit can also increase the likelihood of harmonic resonance

problems, and the resulting excessive voltage distortion issues introduced earlier.

AT313 | 12



13/23


Harmonic Mitigation TwoPassive Techniques

Harmonics AttenuationThe earlier voltage and current distortion discussion, and voltage distortion estimates assume that the current

distortion remains unchanged regardless of the circuit impedance, but this is not entirely true. Harmonic current

distortion is affected by the amount of circuit impedance. In fact, an engineer will discover that placing the

same harmonic producing load at two different nodes in a power system will result in two different levels of load

current distortion.

Power system designers can utilize this effect, called attenuation, as one method of passive harmonic

mitigation. The current waveforms below show the effects of introducing a series line reactor (choke) at the

terminals of a 100 hp pulse-width-modulated (PWM) adjustable-speed drive (ASD). The current total harmonic

distortion associated with the ASD drops from about 81% to 38%.

Graph 8

Graph 9

AT313 | 13



14/23

This attenuation effect is often employed to reduce the harmonic distortion associated with three-phase ASDs.

The ASD operation is not adversely affected, provided the line reactor chosen for the application does not

exceed about 5% impedance (relative to the drive base).

Harmonics Cancellation

In addition to attenuation, harmonic current distortion can be reduced by cancellation. Cancellation occurs

because individual harmonic components of a distorted current are affected differently when passing through

normal power system transformers. The magnitude of harmonic currents, like the 60 Hz component, increases

or decreases consistent with the transformer turns ratio.

The phase angle of harmonic components, however, is influenced by the type of connection of the three

phase transformer. The 5thand 7thcomponents, for example, experience a 30 phase angle shift through a

power system transformer connected delta-wye, as compared with the same current components transmitted

through a wye-wye or delta-delta connected transformer.

This phase-angle effect can be used with multiple ASDs to reduce the current distortion on the circuit feeding

the drives. As demonstrated in the diagram below, the alternating combination of delta-wye and wye-wye

connections can produce much lower harmonic distortion for similarly-sized and similarly-loaded drives.

The combination of line reactors and delta-wye transformers produces a similar cancellation effect.

Graph 10

AT313 | 14



15/23


Common HarmonicCurrent Signatures

IEEE 519a (Draft) TableDespite the preponderance of electronic loads, there are surprisingly few categories required to characterize

the major harmonic-producing devices in industrial and commercial facilities. Electronic machines that

share similar rectifier configurations create similar characteristic harmonic current signatures, as the table

below demonstrates.

The first column of the table describes the type of electronic device. A single-phase power supply, for

example, indicates the typical switch-mode power supply inside a conventional personal computer. The

second column shows the typical current signature, or waveform, that the device produces when energized

from a low-impedance, sinusoidal voltage source. For three phase loads, only one phase current is shown.

The other phases are similar in form, and separated in phase angle by 120.

The third column shows the typical ITHD

associated with the waveform. Note that third and fourth entries

represent the PWM drive with and without line reactor discussed earlier.

The fourth and final column requires a bit more explanation. This column represents a weighting factor

associated with each load type intended to facilitate simple harmonic assessments. The weighting factor for

individual loads can be used to estimate the total weighted power requirement of all the harmonic loads in the

facility. This weighted power requirement could then be compared against the system short circuit capacity to

evaluate the likelihood of adverse effects associated with harmonic distortion.

If the weighted power requirement of the harmonic loads, for example, could be shown to be less than 0.1% of

the short-circuit capacity, then the likelihood of harmonic problems is low. A fuller discussion of this weighting

factor is included in IEEE 519a, and in IEC Standard 61000-3-6.

AT313 | 15



16/23

Table 3

Type of Load Typical Waveform CurrentDistortion

WeighingFactor (W

i)

Single-phase power supply 80% (high 3rd) 2.5

Semiconverter High 2nd, 3rd, 4that partial loads

2.5

Six pulse converter, capacitivesmoothing, no series inductance

80% 2

Six pulse converter, capacitivesmoothing, with seriesinductance > 3%, or DC drive

40% 1

Six pulse converter with largeinductor for current smoothing

28% 0.8

12 pulse converter 15% 0.5

AC voltage regulator Varies withfiring angle 0.7

Fluorescent lighting 0.05 0.5

AT313 | 16



17/23


Harmonic DistortionSimulation Methods

The weighting factor method for evaluating the likelihood of harmonic problems is applicable to only a small

power system with few harmonic loads and no power factor correction capacitors. Most power systems do not

fall into this category, so other, more sophisticated methods must be employed to evaluate harmonic concerns.

Computer Techniques

Simple radial networks can sometimes be analyzed utilizing hand calculations (as performed earlier for the

small 103-A harmonic load). In most cases, however, these calculations quickly become tedious as the circuit

size increases beyond a few nodes and devices. The most common computer techniques are based on nodal

admittance equations for the network. These equations are usually stated in the form: I = Y * E, where I is the

injected current at each node, Y is the circuit admittance matrix, and E are the node voltages.

This Y matrix is built at each frequency, and the resulting equations are solved for the node voltages, E. The

solution is carried out by either matrix inversion, Gaussian Elimination, or some other technique. Computers are

especially suited for this solution task.

Commercially-available software tools, like the HI_WAVE module of Power*Tools for Windows(SKM Systems

Analysis, Inc.; www.skm.com), facilitate harmonic analysis of complex systems and loads. These tools provide

graphical interface to build a variety of circuit types, including radial, loop systems, and multiple independent

systems of different voltage levels. They also contain a large library of conductor, transformer, capacitor, motor,

and harmonic load types. The library eliminates the need to enter individual harmonic waveforms, for example,

by offering the ability to use characteristic models already listed.

Variable-Frequency Drive Applications

Another software tool, called HarmCalc,has been developed to facilitate a common harmonic evaluation task:

Application of variable frequency drives (VFD) to an existing low-voltage radial power system. While this tool is

not accurate for complex systems, or for systems with power factor correction capacitors or harmonic filters,

it is widely applicable for evaluating VFD applications. Many consulting engineers and end-users who specify

VFD designs accept this tool as a suitable means of estimating VFD effect on a power system, and as a means

of evaluating certain mitigating devices like line reactors and drive-isolation transformers, delta-wye transformer

connections and broadband filters.

VFDs and IEEE 519

IEEE Standard 519 is frequently quoted in consulting engineer specifications associated with VFD installations.

The gist of these specifications is that the VFD vendor assumes responsibility for supplying VFDs that comply

with IEEE 519. This specification requirement is clearly outside the original intent of the standard, and often

unnecessarily increases the cost of a VFD installation.

AT313 | 17



18/23

The original intention of IEEE Standard 519 was to introduce harmonic current and voltage distortion guidelines

for electric utilities and their customers. The objective was to establish acceptable levels of current distortion

that an individual customer could generate without adversely affecting other electric utility customers sharing

the same distribution system. Further, this standard provided recommended limits for electric utility control of

voltage distortion that could result from customer harmonic current injection

Despite the misapplication, this specification requirement has become so widespread that IEEE 519 has

effectively become the consensus equipment standard for VFD applications. Given that backdrop, use of

HarmCalc and other tools in evaluating VFD installations requires further exploration, and definition of terms

associated with IEEE 519.

Point-of-Common Coupling

As discussed, the circuit node at which harmonic current and voltage limits were to be evaluated was that

point on the electric utility system at which other customers could be served. This so-calledpoint-of-common-

coupling, or PCC is described graphically, as shown above. Often, VFD specifications that require 519

compliance will also designate the PCC. Generally, the closer this PCC is to the VFD terminals, the more

costly the compliance requirements will be.

Graph 11

Voltage Distortion Limits

The voltage distortion limits in IEEE 519 are fairly straightforward, as reproduced below. There are only three

levels recommended, the first intending to address sub-transmission and distribution circuits on the electric

utility system. The voltage distortion, by the way, is calculated using the nominal voltage as the fundamental

component (in the denominator of the THD calculation.

Table 4

Bus Voltage at PCC (Vn) Individual Harmonic

Voltage Distortion (%)Total VoltageDistortion THD

Vn(%)

Vn 69kV 3.0 5.0

69 kV< Vn 161kV 1.5 2.5

Vn> 161kV 1.0 1.5

AT313 | 18



19/23

The assumptions behind establishment of the voltage distortion limits may be useful in determining whether

or not to invest in harmonic mitigation at low-voltage circuits. The 519 Working Group established 5% voltage

distortion as the limit at electric utility distribution circuits under the assumption that customer harmonic loads

would drive the VTHD higher at customer low voltage busses. The 5% THD limit was selected to allow low-

voltage busses to be maintained at 8% VTHD

; a value that is acceptable to most linear and non-linear loads.

Current Distortion Limits

Determining current distortion limits is a more involved than voltage. This is due to the fact that the degree to

which one customers harmonic loads might affect anothers is dependent on the utilitys system impedance at

the PCC node. A relatively weak source impedance point in a utilitys system would reach the voltage distortion

limit at a much lower harmonic current injection level than a stiffer point.

Thats why the current distortion table requires a little more information in order to apply it correctly. Specifically,

the short-circuit value at the PCC, ISC

, needs to be determined. For PCCs on the electric utility circuit, the utility

usually provides this number. The second required value is the estimated or measured demand current of the

customers total load, IL

.

TDD A Valuable Quantity to Learn and Apply

Another useful term needs to be defined at this juncture TDD, or, more accurately, ITDD

. Note that the current

distortion limits below are given in TDD, not THD. ITDD

is similar to the definition of ITHD

introduced earlier, with

one important difference: The denominator is IL; that is, the total customer 60 Hz demand current. Recall that

the ITHD

denominator is the fundamental current of the distorted waveform, whether small or large relative to

other loads on the circuit.

The difference between the two quantities boils down to this: ITDD

more accurately reflects the amount of

harmonic current a power system can absorb than ITHD

. Since ITDD

is the amount of harmonic current compared

to the customers totalfundamental load current, it is a better predictor of the circuits ability to transmit the

harmonic current without adverse effects.

TDD is also an extremely valuable term to utilize throughout harmonic distortion evaluation and analysis, not

just IEEE 519 assessment. ITHD

, on the other hand, is often a misleading value.

Suppose, for example, that a harmonic measurement indicates ITHD

= 47%. Will this circuit experience harmonic

problems? The ITHD value alone cannot be used to answer this question. If the 47% THD measurement

is actually 0.47 A of harmonic current and 1.0 A of fundamental current, and the circuit in question has an

ampacity of 1600 A, harmonic problems are unlikely.

If, on the other hand, the 47% ITHD

represents 470 A of harmonic and 1000 A of fundamental current on the

same 1600 A circuit, harmonic problems are highly probable and should be investigated at once.

For this reason, many power engineers prefer to calculate ITDD

for a circuit in question, even if IEEE 519

compliance is not an issue. For low-voltage circuits within a facility, for example, they will often utilize the circuit

ampacity, or some other reasonable estimate of the circuits capacity, for the denominator.

AT313 | 19



20/23

HarmCalc

Background

To facilitate VFD application into power systems that utilize IEEE 519 guidelines, Square D and Electrotek

Concepts co-developed an executable software program called HarmCalc. This software tool is available free

of charge at Square Ds Design Resource Center; follow the Consulting Engineer link under the Customers

and Markets listing at www.squared.com/us to software tools and download the file.

The HarmCalc tool includes an electronic tutorial and help documentation, but the following example can

illustrate its usefulness in evaluating VFD applications and some basic mitigating options. Note that HarmCalc

also includes capabilities for exporting the solution information to two third-party software tools intended

to facilitate additional analysis. Web links to these tools, SuperHarmand TOP,are available with the

HarmCalc installation.

Example

The simple power system shown below was configured to serve as an example of the program and its use.

There is a 1000 kVA transformer serving an MCC at 480 V. The electric utility system has been defined by its

available-fault-current and X/R ratio (values typically available from the utility). The MCC already serves about

300 hp in induction motor load (linear load). Two Square DVFDs are being added, and IEEE 519 compliance

has been specified.

The VFDs specified are identical Square D model Altivar 66 VT, 100 hp each. These VFDs (as described in

the Help screen associated with VFD selection) are variable-torque devices, meaning that they are intended

for application on variable-torque motor loads like centrifugal pumps or fans. The VFDs chosen here are

configured with 6-pulse diode rectifiers, although these VFDs are also available in 12- and 18-pulse versions.

Three options have been considered; as summarized in the table below. Note that the unmitigated addition

of the VFDs would result in an ITDD

slightly above the IEEE 519 current distortion limit, although the VTHD

value

is slightly below 5%. The 519 limits can be readily met if the VFDs are equipped with 3%-impedance line

reactors. While the delta-wye configuration for one drive results in the lowest harmonic distortion, this option is

likely to be less cost effective. In addition, line reactors provide other benefits, including reducing the tendency

of nuisance VFD tripping on electric utility power factor correction capacitor switching transients.

Table 5

Harmonic Mitigation VTHD

at PCC2ITHDat PCC2

IEEE 519Compliance?

None 4.85% 8.92% No3% line reactors only 2.23% 4.06% Yes

Delta-wye/delta-delta only 1.97% 1.69% Yes

AT313 | 20



21/23

Screen Capture 1

ReactiVar AccuSinePCS

Another useful software tool for the harmonic-mitigation designer is available on the Square D website. This

spreadsheet-based tool facilitates sizing and application of the Square D ReactiVar AccuSine active harmonic

filter. The active filter is the state-of-the-art method for harmonic current mitigation. It works by sensing the

amount and type of harmonic current being required by the loads (through the CT, or current transformer,

shown below), then injects harmonic current of the proper magnitude and frequency to cancel the load

harmonics. The resulting current returning to the source is low in current distortion. This and other harmonic

mitigation, power quality and power factor correction products and services are also available.

Graph 12

AT313 | 21



22/23


References

1. IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems,

ANSI/IEEE Std. 519-1992.

2. IEEE Guide for Applying Harmonic Limits on Power Systems Unpublished Draft , IEEE Std P519.1/D9a,

January, 2004.

3. Electrical Power System Harmonics Design Guide, R.C. Dugan, M.F. McGranaghan, E.W. Gunther,

Electrotek Concepts, Inc., Knoxville, TN, 1992.

AT313 | 22



23/23

Schneider Electric USA, Inc.

Data Center Solutions

1010 Airpark Center Drive

Nashville, TN 37217

615 844 8300

AT313_Power System Harmonic Fundamentals

Documents

Transcript of AT313_Power System Harmonic Fundamentals