# harmonic distortion - sept 21 2011

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Power Quality Management - Harmonic Distortion

and Variable Frequency Drives

Voltage and Current Harmonic Distortion Cause and Effect

23/09/2011 Marek Farbis, Mirus International Inc. 1

We will focus on

Introduction: the ideal vs. distorted waveform Definition of harmonics Effects of harmonic distortion What is a cause for harmonic voltage distortion? Definition and calculation of THD VFDs and harmonics Standards and recommendations Harmonic mitigation techniques Applications

23/09/2011 Marek Farbis, Mirus International Inc. 2

Introduction Electricity generation is

normally produced at constant frequencies of 50 Hz or 60 Hz and can be considered practically sinusoidal.

Ideally, an electricity supply should invariably show a perfectly sinusoidal voltage signal at every customer location.

In reality however these signals contain many types of disturbances.

-800

-600

-400

-200

0

200

400

600

800

0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018

Vo

lts

Time [sec]

3-Phase, 480V, 60Hz Power Supply

V(A,B) V(B,C) V(C,A)

Marek Farbis, Mirus International Inc.

Introduction

The deviation of the voltage and current waveforms from sinusoidal is described in terms of the waveform distortion, often expressed as harmonic distortion.

In nearly all cases harmonic distortion is produced by a customers equipment (non-linear loads) injecting electrical noise into the power system i.e. Variable Frequency Drives.

Marek Farbis, Mirus International Inc.

Definition of Harmonics In a periodic signal the primary, desired frequency is

the "Fundamental Frequency. The term harmonics was originated in the field of

acoustics, where it was related to the vibration of a string or an air column at a frequency that is a multiple of the base frequency.

A harmonic component in an AC power system is defined as a sinusoidal component of a periodic waveform that has a frequency equal to an integer multiple of the fundamental frequency of the system.

French mathematician Jean Baptiste Joseph Fourier (1768-1830) found that any function of a variable can be expanded in a series of sines of multiples of the variable.

23/09/2011 Marek Farbis, Mirus International Inc. 7

Distorted Waveform

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Harmonics - Components of a Distorted Waveform

Fourier Series f(t) = Ao+A1sin(wt+q1)+A2sin(2wt+q2)+A3sin(3wt+q3) ...

Fundamental - 60 Hz

-1.5

-1

-0.5

0

0.5

1

1.5

5th Harmonic - 300 Hz

-1.5

-1

-0.5

0

0.5

1

1.5

7th Harmonic - 420 Hz

-1.5

-1

-0.5

0

0.5

1

1.5

Resultant Waveform

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Resultant Waveform

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Resultant Waveform

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Harmonic Spectrum

0

20

40

60

80

100

1 3 5 7 9 11 13

Harmonic #

% o

f F

un

dam

en

tal

Harmonic Spectrum

0

20

40

60

80

100

1 3 5 7 9 11 13

Harmonic #

% o

f F

un

dam

en

tal

Harmonic Spectrum

0

20

40

60

80

100

1 3 5 7 9 11 13

Harmonic #

% o

f F

un

dam

en

tal

Time domain

Frequency domain

300 Hz

420 Hz

60 Hz

23/09/2011 Marek Farbis, Mirus International Inc. 8

Why is the harmonic distortion bad?

Effect of penetration in the electrical system affecting adjacent installations.

Thermal effect on electric rotating machines, transformers, capacitors, and cables (extra losses).

Pulsating torques in rotating machines. Neutral conductor overloading. Increased risk of faults from overvoltage conditions

developed on power factor correction capacitors and resonant conditions.

Unexpected Fuse Operation. Abnormal operation of electronic relays. Abnormal operation of solid-state devices. Lower system power factor preventing effective utilization.

23/09/2011 Marek Farbis, Mirus International Inc. 10

What causes a voltage distortion?

Relationship between System Impedance and Voltage Distortion.

23/09/2011 Marek Farbis, Mirus International Inc. 11

Relationship between System Impedance and Voltage Distortion.

~

ZSh

ZTh ZCh

Ih

Harmonic Current Source

Sinusoidal Voltage Source

VS VT VL

Where:

ZSh impedance of the source at harmonic h,

ZTh impedance of the transformer at harmonic h,

ZCh impedance of cables at harmonic h,

VS = Ih x ZSh

Total Harmonic Distortion

Fundamental Current refers to the current carried in the fundamental frequency, Ih1 (60 Hz).

current Total Harmonic Distortion refers to the ratio of all harmonic currents to the fundamental current.

%100

1

2

2max

h

h

h

h

I

I

iTHD

Ratio of the root-sum-square (RSS) value of the harmonic content of the current to the RMS value of the fundamental current.

23/09/2011 Marek Farbis, Mirus International Inc. 13

Standard Variable Frequency Drive (PWM)

IGBT S = FAST KNIFE SWITCHES IGBT = Insulated-Gate Bipolar Transistor

CONTROL VOLTAGE & FREQUENCY

DIODE BRIDGE

23/09/2011 Marek Farbis, Mirus International Inc. 14

VFD, 6-Pulse Rectifier Current Waveform

-300.0

-200.0

-100.0

0.0

100.0

200.0

300.0

0.017 0.022 0.027 0.032

Cu

rre

nt

[Am

ps]

time [msec]

VFD input current

23/09/2011 Marek Farbis, Mirus International Inc. 15

VFD, 6-Pulse Rectifier and Harmonics

For simple diode bridge rectifiers:

When, p = 6

h = n 6 1 h = -- 5,7,--,11,13,--,17,19...

0

20

40

60

80

100

1 3 5 7 9 11 13 15 17 19 21 23 25

harmonic

% F

und.

. ia

Current Waveform and Spectrum

h = n p 1 h = harmonic number p = # of pulses in rectification scheme n = any integer (1, 2, 3, etc.)

23/09/2011 Marek Farbis, Mirus International Inc. 16

Harmonic distortion limits

IEEE Standard 519 1992

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

IEEE Standard C57.110 1986

IEEE Recommended Practice for Establishing Transformer Capability When Supplying Non-sinusoidal Load Currents.

23/09/2011 Marek Farbis, Mirus International Inc. 17

IEEE Standard 519 General Overview

Introduced in 1981 (Latest revision 1992)

Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems o Sets limits for voltage and current distortion at Point

of Common Coupling.

o Recognizes responsibility of both User and Utility.

Widely adopted in N. America Becoming more common globally

23/09/2011 Marek Farbis, Mirus International Inc. 18

Definition of Terms

Point of Common Coupling (PCC)

A point of metering, or any point as long as both the utility and the consumer can either access the point for direct measurement of the harmonic indices meaningful to both or can estimate the harmonic indices at point of interference.

Within an industrial plant the PCC is the point between the nonlinear load and the other loads.

XFMR

XFMR

ZTh

UTILITY

MOTOR1

ZCh3 ZCh4

CUSTOMER/UTILITY

VFD2

ZCh2

MOTOR2

MOTOR3

ZCh1

ZSh

VFD1

23/09/2011 Marek Farbis, Mirus International Inc. 19

IEEE Standard 519-1992, Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems

Definition of Terms

Point of Common Coupling (PCC)

A point of metering, or any point as long as both the utility and the consumer can either access the point for direct measurement of the harmonic indices meaningful to both or can estimate the harmonic indices at point of interference. Within an industrial plant the

PCC is the point between the nonlinear load and the other loads.

XFMR

XFMR

ZTh

UTILITY

MOTOR1

ZCh3 ZCh4

CUSTOMER/UTILITY

VFD2

ZCh2

MOTOR2

MOTOR3

ZCh1

ZSh

VFD1

Rarely

convenient to

measure on

Utility Side.

23/09/2011 Marek Farbis, Mirus International Inc. 20

IEEE Standard 519-1992, Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems

Definition of Terms

Short-Circuit Ratio (ISC/IL): Ratio of the short-circuit current (ISC) available at the PCC to the maximum fundamental load current (IL).

Maximum Load Current (IL): Recommended to be the average current of the maximum demand for the preceding 12 months.

XFMR

XFMR

ZTh

UTILITY

MOTOR1

ZCh3 ZCh4

CUSTOMER/UTILITY

VFD2

ZCh2

MOTOR2

MOTOR3

ZCh1

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