YASKAWA Page. 1

Understanding Input Harmonics

and Techniques to Mitigate Them

Mahesh M. Swamy

Yaskawa Electric America

YASKAWA Page. 2

Organization

• Introduction

• Why VFDs Generate Harmonics?

• Harmonic Limit Calculations per IEEE 519-

1992

• Harmonic Mitigation Techniques

– Passive and Active Means

• Questions and Conclusions

YASKAWA Page. 3

Motivation

• Harmonics cause unnecessary heat in

equipment connected to harmonic source

• System rich in harmonics is generally

associated with poor power factor and low

efficiency

YASKAWA Page. 4

Motivation - Continued

• Harmonics can overload preexisting power factor

correcting capacitors at plant facility and at utility

distribution points

• Harmonics can initiate system resonance that can

severely disrupt operation

• Hence, control of harmonic current is important

and necessary

YASKAWA Page. 5

Risk of Parallel Resonance

XcXL

ih

ih

Resonance occurs when: Xc = XL

Power Factor Capacitors Relieve Load

Parallel Resonance

Current measured at the capacitor,showing 660Hz, (11th harmonic resonance) Figure 5.2

YASKAWA Page. 6

Introduction

• Non-linear loads – current does not

follow applied voltage waveform

YASKAWA Page. 7

Introduction (Contd.)

• To estimate heating effect due to non-linear

currents flowing through circuit breakers and

transformers, linearization is needed

• Resolving non-linear waveform into sinusoidal

components is Harmonic Analysis

• Ratio of harmonic content to fundamental is

defined as harmonic distortion or THD

YASKAWA Page. 8

Why VFDs Generate Harmonics?

3-phaseinput

DCcapacitor

DC link voltage

Phase voltage

Rectifer input current

• Pulsating current due to dc bus capacitor – main source of non-

linearity in input current

• In weak ac systems, during diode conduction ac voltage is clamped

to dc bus voltage – source of non-linearity in input voltage

YASKAWA Page. 9

Definition of THD• Ratio of the square root of the sum of squares of the rms value of

harmonic component to the rms value of the fundamental component is

defined as Total Harmonic Distortion (THD)

• If the waveform under discussion is current, then the THD definition is

called Current Harmonic Distortion. If the waveform under discussion is

voltage, then the THD definition is called Voltage Harmonic Distortion

1

2

2

I

ITHD

n

nn

I

∑∞=

==1

2

2

V

VTHD

n

nn

V

∑∞=

==

YASKAWA Page. 10

Sample Waveforms

Every Wave shape has Harmonic Distortion!

THD = 78.3%THD = 1.2%

YASKAWA Page. 11

Harmonic Limits Per IEEE 519-1992Table 10.3: Current Distortion Limits for General Distribution Systems

(120 V through 69 kV)

Maximum Harmonic Current Distortion in percent of IL Individual Harmonic Order (Odd Harmonics)

ISC/IL <11 11≤h<17 17≤ h<23 23≤h<35 35≤ h TDD <20 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

Even harmonics are limited to 25% of the odd harmonic limits above.

* All power generation equipment is limited to these values of current distortion, regardless of actual Isc / IL ; where Isc is the maximum short circuit current at PCC and IL is the maximum demand load current (fundamental frequency) at PCC.

YASKAWA Page. 12

Harmonic Limits Per IEEE 519-1992

Table 10.2 Low-Voltage System Classification and Distortion Limits

Special Applications * General System Dedicated

System†

Notch Depth 10% 20% 50%

THD (Voltage) 3% 5% 10%

Notch Area (AN

2)‡ 16,400 22,800 36,500

Note: The value of AN for other than 480V systems should be multiplied by V/480.* Special applications include hospitals and airports.† A dedicated system is exclusively dedicated to the converter load.‡ In volt-microseconds at rated voltage and current.

YASKAWA Page. 13

Definitions• PCC - Point of Common Coupling

• Point where harmonic measurement is to be made

• Typically, where the utility power comes into the business

(commercial building or industrial factory)

• Also defined as the point where non-linear load meets the linear

load within a plant – most popular definition used by Consultants

to enforce Drive Manufacturers to meet IEEE519 at VFD input

• TDD – Total Demand Distortion

• Harmonic current distortion in percent of maximum demand load

current. The maximum demand current interval could be either a

15-minute or a 30-minute interval.

YASKAWA Page. 14

Definitions - Continued• ISC: Short-circuit current at PCC

• Defines the size of the customer from Utility’s view point – helps

to distinguish between a Seven-Eleven store from a Steel

manufacturing plant

• IL: Maximum demand load current at fundamental frequency• Need not be the rated load current.

YASKAWA Page. 15

Characteristic Harmonics in Rectifiers

•

• h is harmonic order, k is any integer, q is number of pulses at the

dc bus voltage in one period

• For a six-pulse system, h will be:5th, 7th, 11th, 13th, etc.

• For a twelve-pulse system, h will be:11th, 13th, 23rd, 25th, etc.

• Amplitude of harmonics is 1/h for a three-phase ac to dc rectifier

with no dc bus capacitor

• Harmonics of order other than those given above are called non-

characteristics harmonics and are more common than not

1)( ±⋅= qkh

YASKAWA Page. 16

Application Example for Applying IEEE519

• 480V, 100hp VFD fed from a 1500-kVA transformer of 4% impedanceStep 1: Identify PCC – take default to be at VFD terminals

Step 2: Determine ISC from end user. In its absence, use

transformer kVA rating and percent impedance

Step 3: Determine IL from user. In its absence, use NEC Amps for

rated horsepower condition. Here, use 124A

Step 4: Determine ISC/IL. Look up Table 10.3 to determine limit - 15%

105,4504.04803

10001500)100/(%3

1000

=⋅⋅

⋅=

⋅⋅⋅

=

SC

LLSC

I

ZVkVAI

YASKAWA Page. 17

Application Example for Applying IEEE519

• ISC/IL for the present case is 364.

• From Table 10.3, TDD < 15% of Rated Fundamental Current or 18.6A

in this example; Hence IEEE 519 compliance does not mean 5% TDD

• If maximum demand load current is only 45A due to load condition,

and harmonic distortion is 35% at this operating point, spirit of IEEE

519-1992 is still met since 35% of 45A is 15.8A, which is less than the

allowable 18.6A.

• Don’t forget that voltage distortion limits are more important than

current distortion limits due to the fact that voltage is common to all

customers on the same grid, while current is local to a load

YASKAWA Page. 18

Harmonic Mitigation Techniques

• Active Techniques

• Passive Techniques

• Hybrid Techniques – Combination of Active and

Passive Techniques

YASKAWA Page. 19

Active Mitigation Techniques

• Active Front End

• Boost Converter Topology – Inherently regenerative.

Bulky, and expensive. Conducted EMI is of concern

• Non regenerative type: Inject Current from conducting

phase to non-conducting phase using semiconductor

switches

• Shunt type: Monitors load current and injects mirror

image of load current so that harmonics cancel out.

YASKAWA Page. 20

Passive Mitigation Techniques

• AC Line Inductors (Reactors)

• DC Link Chokes or DC Bus Inductor

• Harmonic Filters – Capacitor based

• Multi-pulse Schemes

YASKAWA Page. 21

Passive Mitigation Techniques

• AC Line Inductors (Reactors)

• Makes discontinuous current continuous

• Helps damp transient surges on line due to

lightning and capacitor switching

• Small and inexpensive

• Causes voltage overlap and reduces dc bus

voltage

YASKAWA Page. 22

Passive Mitigation Techniques

• AC Line Inductors (Reactors)

.

.

THD≈80%

THD≈40%

YASKAWA Page. 23

Issues With AC Line Reactors

• DC Bus Voltage Reduces Due to Overlap of Diode Conduction

πμ

πμ

ωωπ

πμ

πμ

)cos(323)cos(23

)()sin(23 )3/2(

)3/(

⋅⋅⋅⋅=

⋅⋅⋅=

=

−−

+

+ −∫

NLLLO

LLO

VVV

tdtVV

322

⋅⋅⋅⋅

=− LL

dc

VILωμ

YASKAWA Page. 24

Passive Mitigation Techniques• DC Link Chokes (DC Bus Inductor)

• Makes discontinuous current continuous

• Small and inexpensive

• Does NOT Cause overlap phenomenon and so

does not reduces dc bus voltage

• Does not help damp transient surges on line

due to lightning and capacitor switching

YASKAWA Page. 25

Passive Mitigation Techniques

• DC Link Choke (DC Bus Inductor)

.

THD≈80%

THD≈37%

.

YASKAWA Page. 26

Waveform With DC Link Choke

• No overlap of Diode Conduction

Ldc

ππμ

ωωπ

πμ

πμ

NLLLO

LLO

VVV

tdtVV

−−

+

+ −

⋅⋅⋅=

⋅⋅⋅=

= ∫323)cos(23

)()sin(23 )3/2(

)3/(

dcmmcr

mmavgphmcr

ITV

itVL

VVVVtiL

6/33

33

⋅⋅−

=ΔΔ⋅⋅

−=

⋅−=−=

ΔΔ⋅ −−

ππ

ππ

π

YASKAWA Page. 27

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7 8AC equivalent Z (% Impedance)

Inpu

t Cur

rent

TH

D (%

)

AC reactor with DCreactor of 2.75mH

AC reactor only

DC reactor only(with ZSC=0.1%)x

Δ

40.00

-30.00

0

-20.00

-10.00

10.00

20.00

30.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=92.5%Z=1%

40.00

-30.00

0

-20.00

-10.00

10.00

20.00

30.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=92.4%Z=1%

30.00

-25.00

0

-20.00

-10.00

10.00

20.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=44%Z=1%

30.00

-20.00

0

-10.00

10.00

20.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=59.6%Z=3%

30.00

-20.00

0

-10.00

10.00

20.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=50.6%Z=3%

25.00

-20.00

0

-15.00

-10.00

-5.00

5.00

10.00

15.00

20.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=37.3%Z=3%

20.00

-15.00

0

-10.00

-5.00

5.00

10.00

15.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=33.8%Z=5%

25.00

-20.00

0

-15.00

-10.00

-5.00

5.00

10.00

15.00

20.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=43.9%Z=5% 25.00

-25.00

0

-20.00

-10.00

10.00

20.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=37.7%Z=5%

AC Line Reactor vs DC Link Choke

0

20

40

60

80

100

120

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Inductance (mH)

Inpu

t Cur

rent

TH

D (%

)

AC reactor with DCreactor of 2.75mH

AC reactor only

DC reactor only(with LIN=47μH)x

Δ

40.00

-30.00

0

-20.00

-10.00

10.00

20.00

30.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00THD=92.5%Lac=0.47mH

40.00

-30.00

0

-20.00

-10.00

10.00

20.00

30.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=92.4%Ldc=0.47mH

30.00

-25.00

0

-20.00

-10.00

10.00

20.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=44%Lac=0.47mH

30.00

-20.00

0

-10.00

10.00

20.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=59.6%Lac=1.4mH

25.00

-20.00

0

-15.00

-10.00

-5.00

5.00

10.00

15.00

20.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=37.3%Lac=1.4mH

20.00

-15.00

0

-10.00

-5.00

5.00

10.00

15.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=33.8%Lac=2.35mH

25.00

-20.00

0

-15.00

-10.00

-5.00

5.00

10.00

15.00

20.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=43.9%Lac=2.35mH

40.00

-30.00

0

-20.00

-10.00

10.00

20.00

30.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=82.9%Ldc=1.4mH

30.00

-20.00

0

-10.00

10.00

20.00

4.98 5.004.99 4.99 4.99 4.99 4.99 5.00 5.00

THD=58.5%Ldc=2.35mH

YASKAWA Page. 28

AC Line Reactor vs DC Link Choke

500

520

540

560

580

600

620

640

0 1 2 3 4 5

Inductance (mH)

Avg

. DC

Bus

Vol

tage

(V)

AC reactor only

xΔ

DC reactor only(with LIN=47μH)

YASKAWA Page. 29

Optimal Solution

• AC Line Inductor Alone Not Optimal because

of Voltage Drop

• DC Link Inductor Alone Does Not Provide

Surge Protection

• Optimal Solution is a Combination of the two –

1% AC Input Inductor + Standard DC Link

Choke

YASKAWA Page. 30

Harmonic Filters

• Capacitor Based Harmonic Filters

• Series Filter – tuned to offer high impedance

to select frequencies

• Shunt Filter – tuned to shunt select

frequencies

• Hybrid Filters – combination of above

• Large, bulky, expensive, and often ineffective

YASKAWA Page. 31

Series Harmonic Filter

• Series Filter

• Designed to handle rated load current

• More often found in single-phase applications

to impede 3rd harmonic current

Lf

Cf

YASKAWA Page. 32

Shunt Harmonic Filter

• Shunt Filter

• Designed to shunt select frequencies

• Draws fundamental frequency current

resulting in leading VA operation

• Need multiple section to be effective

• Does not distinguish between

intended load and other loads

Lf

Cf

YASKAWA Page. 33

Modified Shunt Harmonic Filter• Shunt Filter with Series Impedance

• Add a series inductance to restrict import of harmonics

• MTE’s and Mirus International’s Filter Structure

U

V

W

Lf

Cf

IM

5% ACReactor

5% ACReactor

YASKAWA Page. 34

Hybrid (Broad Band) Harmonic Filter

• Combination of shunt and series filter

• Series inductance and Shunt

Capacitor – over-voltage problem

• Autotransformer used to solve this

• Bulky, expensive

• Capacitor switching needed

Lf

Cf

Auto-transformer

YASKAWA Page. 35

Issues With Harmonic Filters

• All capacitor based shunt type filters draw

leading current and cause over-voltage

• Power Loss, and Higher Stresses on DC Bus

Capacitors – avoid using this

• Generally multiple sections needed

• Bulky and Expensive

• Can cause system resonance

YASKAWA Page. 36

Risk of Parallel Resonance

XcXL

ih

ih

Resonance occurs when: Xc = XL

Power Factor Capacitors Relieve Load

Parallel Resonance

Current measured at the capacitor,showing 660Hz, (11th harmonic resonance) Figure 36.2

YASKAWA Page. 37

Multi-pulse Harmonic Mitigation Technique

• 12-pulse Techniques

• Three-winding isolation transformer

• Hybrid 12-pulse

• Autotransformer based 12-pulse scheme

YASKAWA Page. 38

Three Winding 12-pulse Scheme

• Rated for full power operation – bulky but

ONLY option when input is medium

voltage and drive is of low voltage rating

X1

X2

X3Y1

Y2 Y3

H1

H3H2

L1

L21L31

L11

3-winding isolation transformer

H1

H2

H3

Ldc

VDC IMX1X2X3

U

V

WL2L3

Y1Y2Y3

YASKAWA Page. 39

Three Winding 12-pulse Waveforms

YASKAWA Page. 40

Hybrid 12-pulse Scheme

• Transformer rated for half power – attractive option

X1

X2

X3

L2L3

L1

L21L31

L11

Half power phase-shiftingisolation transformer

H1

H2

H3

Ldc

VDC IMX1X2X3

U

V

W

H1

H3H2

Matching Inductor(half-rated current)

Optional inputinductor, Lin ILM

IxfmrIin

YASKAWA Page. 41

Hybrid 12-pulse Waveforms

Iin

ILmIxfmr

48

1216

Harmonic Order

% D

isto

rtion

With no Lin

With Lin

THD 5 7 11 13 17 19

THD= 6.7% with 5% input reactor;8.8% with no input AC reactor.

YASKAWA Page. 42

Autotransformer 12-pulse Scheme

• Autotransformer configuration

• Needs IPT and ZSBT – bulky and costly

B

A C

b b

a

a

c

c

L2L3

L1

L21

L31

L11

C ited asprior art inUS patent#4,255,784

Inter-phaseTransformer

Inter-phaseTransformer

3-ph acsupply

zero-sequenceblocking

Transformer

CommonCore

YASKAWA Page. 43

Autotransformer 12-pulse Scheme

• Autotransformer configuration

• If loads are isolated and fairly balanced, this is very attractive

3-Ph ACsupply

B

A C

VFD #1 V Motor 1U

W

VFD #2 V Motor 2U

W

b' b

a

a'

c'

c

S'R'

T'

SR

T

YASKAWA Page. 44

Four Winding 18-pulse Scheme

• Transformer is rated for full power operation - bulky and expensive• Cost effective method if primary is MV• Attenuates conducted EMI effectively

X1

X2

X3

H1

H3

H2

L2L3

L1

L21L31

L11

4-winding isolationtransformer

H1

H2

H3

Ldc

VDC IM

X1X2X3

Y1Y2Y3

U

V

W

+20deg

-20deg

Y1

Y2

Y3

Z1

Z2Z3

L22L32

L12Z1Z2Z3

YASKAWA Page. 45

Autotransformer 18-pulse Scheme

• Autotransformer configuration

• Patented by D. Paice – only two US manufacturers licensed at present

H1

H3 H2

L2L3

L1

L21L31

L11

18-pulse autotransformer

Ldc

VDC IM

123

456

U

V

W

L22L32

L12789

12

3

4

56

7

8

9

H1

H2

H3

Input inductor,Lin

I1

I3

I5

Iin

YASKAWA Page. 46

Autotransformer 18-pulse Scheme

• Needs three diode bridges – Yaskawa uses

external diode bridges – makes it expensive

• Needs 7% input reactor for achieving THD levels

of 5% and below – increases cost and space

• With no input reactor, THD observed is about

8.8%

YASKAWA Page. 47

Autotransformer 18-pulse Waveform

IinI1

I3 I5

48

1216

With no Lin

Harmonic Order

% D

isto

rtion With Lin

THD 5 7 11 13 17 19

THD= 5.5% with 5% input reactor;8.7% with no input AC reactor.

YASKAWA Page. 48

Yaskawa’s own 18-pulse Scheme

• Less Complicated Structure

• Low Cost because of standard configuration

• Needs only 1.5% input reactor to bring THD

level to less than 5%

• With no input reactor, THD level observed to be

6.5%

YASKAWA Page. 49

Hybrid 18-pulse Scheme by Yaskawa

• Power rating of isolation transformer is 2/3 of rated output power

• Current through matching inductor is 1/3 of rated input current

• Patent Pending

X1

X2

X3

H1

H3

H2

L2L3

L1

L21L31

L11

2/3 rated phase-shiftingisolation transformer

Ldc

VDC IM

X1X2X3

Y1Y2Y3

U

V

W

-20deg

Y1

Y2Y3

L22L32

L12

+20deg

H1

H2

H3

Input inductor,Lin

Matching Inductor(1/3-rated current)

YASKAWA Page. 50

Hybrid 18-pulse Waveform

V LN

Iin

48

1216

With no Lin

Harmonic Order

% D

isto

r tion With Lin

THD 5 7 11 13 17 19

THD= 4.5% with 1.5% input reactor;6.5% with no input AC reactor.

YASKAWA Page. 51

Power Factor and Harmonics

• Two Definitions of Power Factor Exists

• Displacement Power Factor: Cosine of the

angle between the fundamental voltage and

fundamental current waveform

• For VFDs, this value is almost always unity

(0.99)

YASKAWA Page. 52

Power Factor and Harmonics• True Power Factor

• Ratio of True Power to Total Volt-Ampere

Demanded by Load

• Total Volt-Ampere includes VA demanded by

Harmonic Content in Waveform

kVAkWpf =

2

21

221

1

1

THDdpfpf

THDIkVkW

IIkVkW

IkVkWpf

ntotal

+=

+⋅⋅=

+⋅=

⋅=

∑

YASKAWA Page. 53

Power Factor and Harmonics

• True Power Factor is poor:

.

THD≈80%

THD≈37%

.

78.08.01 2

=+

=dpfpf

937.037.01 2

=+

=dpfpf

YASKAWA Page. 54

Conclusions

• IEEE 519 does not mean THD < 5%

• Find out PCC, ISC, and IL, Apply the

Spirit of IEEE 519 correctly

YASKAWA Page. 55

Conclusions

• Avoid capacitor based harmonic filter

• 12-pulse techniques can achieve low TDD at drive input

– Hybrid 12-pulse is attractive, less bulky and cost-effective

– Isolation transformer based method is best when input is MV

• Use 18-pulse only when Customer demands (Less than

5% TDD)

– Hybrid 18-pulse is attractive

– Isolation transformer based method is best when input is MV