POWER QUALITY PROBLEMS HARMONICS AND ITS MITIGATION

Nimai Mahapatro Hindalco Industries Limited

Dimensions of stable power

2

Parameters considered for Power Quality

Voltage Magnitude

Frequency

Wave form

3

A perfect power supply must guarantee an uninterruptible service within voltage and frequency tolerance limits with a distortion-free sinusoidal waveform.

Sources of PQ Problems Utility Sources

• Lightning

• Faults

• Switching

End User Sources

• Rectifiers

• PF Correction Equipment

• Uninterruptible Power Supplies (UPS)

• Variable Frequency Drives (VFD)

• Battery Chargers

• Large Motors During Startup

• LEDs, Electronic Dimming Systems, Electronic Ballasts

• Arc Welders, and Other Arc Devices

• Office Equipment and Computers

• Medical Equipment e.g. MRIs and X-Ray Machines

4

Power Quality Issues

Transients

Over-Voltage

Sag

Under-Voltage

Voltage Unbalance

Interruption

Swell

Harmonics

Power Quality Issues:

5

Causes and Solutions of PQ Problems:

PQ Problem Major Sources Solutions

Frequency The Utility

Voltage variations Load changing Sync. loads, Fast PF correction

Voltage flicker Load changing, mainly welding Fast PF correction

Voltage dip Motor and other load startups Motor starters, VFD, Fast PF correction

Interruptions The Utility

Overvoltage Over PF compensation Fast PF correction

Transients Connections, Switching Controlled switches

Unbalance Unbalanced loads, transformer phase shift

Balance the loads

Harmonics Non linear loads, resonance Active/passive filtration, detuned capacitors, Improved VFD operation

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Should we Care about Power Quality?..........Yes Because

Comprehensive Research experience the following:

• Power Factor correction system failures (17%)

• Failures in high load switching (16%)

• Neutral conductor overheating (12%)

• Computer lockouts (20%)

• Light flickering (22%)

• Electronic card failures (18%)

• Unexpected breaker operation (11%)

• Power meters inaccurate readings (6%) And of course excess losses and downtime

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Harmonics

Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at which the supply system is designed to operate (termed the fundamental frequency; usually 50 or 60 Hz).

Harmonic distortion originates in the nonlinear characteristics of devices and loads on the power system.

A nonlinear device is one in which the current is not proportional to the applied voltage

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9

Harmonic spectrum and distortion factor

Ideally, the harmonics produced by the semiconductor converter equipment in steady state condition of operation are called characteristic harmonics of the converter and are expressed as:

h=np±1 where, h = order of harmonics n = an integer 1, 2, 3,…. p = number of pulses per cycle

For a single phase bridge rectifier, the number of pulses p = 2 for one cycle of line frequency and therefore the characteristic harmonics are: h = n .2 ± 1 = 1 (fundamental), 3 , 5 , 7 , 9 , 11 …..

For a three phase bridge rectifier, since the number of pulses p=6 per line frequency cycle, the characteristic or dominant harmonics are: h = n ⋅6 ± 1 = 5, 7, 11, 13, 17, 19, 23, 25, 35, 37… Similarly, the characteristic harmonic currents for a 12-pulse rectifier will be: h = n ⋅12 ± 1 = 11, 13, 23, 25, 35, 37…

How Harmonics are Formed?

10

inverters &

high-frequency

lighting

Switched-mode

power supplies

supply impedance Variable-speed

ac/dc drive

systems

Uninterruptible

power supply

systems

11

Non Linear Loads

LED lights & Arc Furnaces

AC/DC Converters

Non-linear Load

Linear Load

3 February 2017 12 Dr.Parthasarathy/KLNCE/ WESCO-odisa

Supply

Voltage

Linear

Load

Supply

Impedance

Normal Operating Conditions

Supply

Current

PCC

VPCC

+

-

Nonlinear Operating Conditions

Supply

Voltage

Nonlinear

Load

Supply

Impedance PCC

Supply

Current

VPCC

+

-

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Star Point Grounded – Linear Loads

A B C

N

In the star point grounded system, the unbalanced current should return to the substation via the metallic neutral conductor, provided the neutral is of sufficient size and designed to handle the types of current loads it will be required to carry

20

20

20

0

Amps

A B C

N

20

20

20

42

With non-linear loads, the

neutral current can be

225% of the highest phase

conductor.

Amps

Star Point Grounded – NonLinear Loads

A B C

N

20

20

20

12

Majority of

current flows

uncontrolled

over the earth.

Amps

Star Point Grounded – Non Linear Loads

18

Effects of Harmonics Overheating of the power distribution system leading to equipment failure

Electrical equipment consume 15 % higher energy

Table 1: Voltage Distortion Limits according to IEEE Standard 519 -20104

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Table 2: Current Distortion Limits according to IEEE Standard 519 -20104

20

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Harmonics in Power Distribution system

4 or 5 Rectifier transformers connected at different phase shifts feed the DC to pot line

Harmonics in Aluminium Smelter pot line

Effective pulses comes to either 48 or 60 and AC power feeding this DC load becomes fairly sinusoidal.

Harmonic distortion occurs at the point of coupling mainly due to

Loading and rating of each of the RT may not be uniform.

Outage of one of the RT may reduce the total number of pulses to 36 from 48

Unbalance between loading of the two secondary windings of the same RT

Usage of standby RTs for normal operations to meet the production

Grouped tap position change of one PL can change system impedance significantly shifting the resonance coinciding with one of the prevailing order of harmonics.

Changes in the high voltage network configuration can shift the resonance frequency (due to changes in system impedance).

• Multi pulse AC/DC converters with phase shifting transformers

• Installing filters on AC side

How to Eliminate Harmonics

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Passive Harmonic Filter: • Basic elements of passive filter

Resistor (R), Inductor (L) and Capacitor

• Passive filter creates low impedance path for the particular harmonic frequency and diverts that harmonic current to ground.

• It improves the quality of distorted waveform.

Fig:

Types of

passive

harmonic filters

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Practical considerations of PHF: • Tuning: The filter is tuned slightly below the harmonic frequency generated by the load.

This will allow for tolerance in the filter components and variations in system impedance.

• Switching: Filters provide fundamental frequency reactive power (VAR). Portions of the filter can be switched off at times of minimum load to limit overvoltage.

• Tolerances: Capacitors and inductors must be designed such that the combination of ratings (L and C) does not result in resonance at an undesired frequency. In other words, there should not be positive peaks in the filter impedance curves to correspond with harmonic frequencies.

• Use active switching components (SCR,IGBT,MOSFET and etc.,)

• Only one filter needed to eliminate all the unwanted harmonics

• Used for power factor correction

Active Harmonic Filters:

Fig:

Active Filter

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Nonlinear

Load

Low Pass

Filter

Basic Idea of Active Filters

C. T.

1 3 5 7 9

3 5 7 9

1 3 5 7 9

3 5 7 9

1 3 5 7 9

+

- Switching Power

Amplifier

27

Active Filter with an R-L load

(c)

(a) (b)

Supply

Voltage

Supply

Current

Supply

Voltage

Reference

Supply

Current

Load

Current

Supply

Voltage

Filter

Current

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AHF

• Lower switching frequency

• Smaller sizes of components

• Cheaper Solutions

• Basis for further improvement

PHF

• Reliable operation

• Easy design procedure

• Act as reactive power compensators

• Cheap configurations per harmonic

• Banks of filters used for other harmonics

• Bulky

• Depend on system impedance

• Tuned for a certain loading condition

• Parallel and series resonance may occur for certain harmonics

Comparison of PHF & AHF

Hybrid Power Filter: • Hybrid power filter is nothing but the combination of both passive filter and active

filter.

• The hybrid filters are more attractive in harmonic filtering than the pure filters from both viability and economical points of view, particularly for high- power applications.

• Hybrid power filters consists of power electronic switching devices like MOSFET, IGBT, Thyristors, GTO and passive components such as capacitors, inductors and/or resistors.

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Fig: Hybrid Power Filter

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Harmonic Mitigation with the use of Filters

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