Harmonics and PFC The love story

By Pol Nisenblat

2

THE FUTURE OF POWER QUALITY

DEFINITIONS

PROBLEMS

“DETUNED” SOLUTIONS

TUNED SOLUTIONS

HOW TO CHOOSE

OPEN DISCUSSION

3

ELECTRICAL POWER SOURCE It is worldwide common practice to assume that utilities

generate a near perfect sine wave voltage Typical electrical energy source is introduced by “voltage”

source

Ideal Voltage Source

Z internal = 0

Z load = infinity

4

DISTRIBUTION TRANSFORMER Typical Internal Impedance is only 2-7% of the fully loaded

Secondary

5

IDEAL ENERGY SOURCE

Simplified power control by

periodical switching (chopping)Nonlinear load operation

generates current

waveform distortion

6

HARMONICS - DEFINITIONS

Harmonics are integral multiples of some fundamental

frequency that, when added together, result in a distorted

waveform

f(x) = sin(x) f(x) = sin(5x)

5

+

f(x) = sin(x) + sin(5x)5

=

7

HARMONICS - DEFINITIONS Harmonic order are MULTIPLES of the fundamental frequency.

Typical Harmonics are the 3rd, 5th, and 7th

Where H3 = 150 Hz, H5 = 250 Hz, H7 = 300 Hz (at 50Hz world)

In fact, any waveform may be constructed from a sine wave and some number of its harmonics like:

8

HARMONICS – MOST IMPORTANT DEFINITIONS

Harmonics are originated at the load side! Harmonics are created in the current!

Harmonic sources are - current sources

9

HARMONIC SOURCE

High internal impedance The harmonic current is being PUSHED towards lowest

external impedance path

10

HARMONIC CURRENT FLOW

Typical harmonic current flow is towards distribution transformer

1 Ohm0.01 Ohm

11

HARMONICS – PROBLEMS?

I2r losses + “skin” effect

Voltage distortions

Iron-core losses

Cables/transformers overheat

Upstream pollutions

But, the real problems are yet to come

12

HARMONIC CURRENT WITH PFC

Where the H5 current should go now?

13

PARALLEL RESONANCE

C

1

X C

LX l

Parallel resonance = Infinity impedanceXX CL

14

PARALLEL RESONANCE High impedance at the resonance frequency The resonance frequency changes with changed number of

capacitor groups (N)

1 2 3 4 6 7

ResonancePoint (7th)

85

ResonancePoint (5th)

k

n

Decreased NIncreased N

15

PARALLEL RESONANCE

0

0.5

1

1.5

2

2.550 100

150

200

250

300

350

400

450

500

550

Frequency

Imp

edan

ce (

Oh

m)

1 Step

2 Steps

3 Steps

4 Steps

5 Steps

6 Steps

16

PARALLEL RESONANCE Harmonic current turns back to the loads Harmonic VOLTAGE raises dramatically Harmonic current circulates (ping pong) between capacitors

and distribution transformer

V

17

PARALLEL RESONANCE – WHAT TO DO? Xl1 reactor in series to the capacitor moving parallel resonance

frequency downstream The new resonance frequency at:

Xc=Xl+Xl1 Locate parallel resonance frequency below lowest

dominant harmonic

18

PARALLEL RESONANCE FREQUENCY –”RULE OF THUMB”

Where hr is the harmonic number of the parallel

resonance

Unfortunately, not accurate enough

hr = kVAsc

kVAC

19

HOW TO CALCULATE PARALLEL RESONANCE FREQUENCY? For those who love formulas:

1

1

1

11

11

|| 1

; ; 1

22

2

22

2

NCLCL

jLCLZ

CjNCLCL

LLC

L

Z

LjLjCjN

LjLjCjN

ZZZZN

Z

LjZLjZCj

Z

TR

TR

TR

TRTR

TR

TR

LTRLC

LTRLTRC

20

HOW TO CALCULATE PARALLEL RESONANCE FREQUENCY? Unfortunately, Xl (Xtr) value which is actually the total

distribution network inductance at the frequency of interest - is unknown

But, we do know that the parallel resonance frequency would always be located downstream to the serial resonance frequency between Xl1-Xc

21

“DETUNED” SOLUTION

22

WHAT FREQUENCIES ARE SAFE?

Even” harmonics are normally not present

“Triple” harmonics are canceled on DELTA connected loads

with balanced 3rd harmonic

On On balanced loads balanced loads and and 3 phase DELTA 3 phase DELTA capacitors cases, capacitors cases,

tuning in H3.5-H4.2 ranges are most popular tuning in H3.5-H4.2 ranges are most popular

)120*3**3sin(*)0*3**3sin(*

)120*3**3sin(*

)0*3**3sin(*

21)3(21

2)3(2

1)3(1

tAtAV

tAV

tAV

H

H

H

V1

V2V3

N120 Deg.

V 1-2

23

DETUNED SOLUTION - DEFINITIONS

Tuned frequency is defined by serial resonance point

Serial connected reactors are defined by % rather then Henries

Reactor’s % defined as impedance at the fundamental

frequency with respect to the capacitor’s impedance at

the same frequencyReactor 50Hz 60Hz Harmonic

7% 189Hz 227Hz H3.78

6% 204Hz 245Hz H4.08

14% 134Hz 160Hz H2.67

5.67% 210Hz 252Hz H4.2

24

7% “DETUNED” SOLUTION

0

0.05

0.1

0.15

0.2

0.2550 100

150

200

250

300

350

400

450

500

550

Frequency

Imp

edan

ce (

Oh

m)

1 Step

2 Steps

3 Steps

4 Steps

5 Steps

6 Steps

25

TUNED SOLUTIONS - FILTERS

Passive filters are “tuned” to just below the harmonic

frequency Care should be taken – not to overload!

Low impedance path at 5th harmonic

Low impedance path at 5th harmonic

26

TOO HIGH HARMONIC LEVELS?

Disconnect ALL PFC capacitors and check again

Significant difference? – Use detuned solution

No difference? – Use tuned solution 2nd and 4th harmonics too high? - Check voltage

converters Weak network? 14% detuned solutions are preferred Balanced 3rd harmonic in current? – Use 5.67-7%

solutions Unbalanced compensation? – Use 14% solutions only Voltage harmonics without current sources?

Don’t use filters! Speak to utility.

Questions?Contact: Asaf Laifer

Alaifer@elspec-ltd.com

972-4-6174127