HK Polytechnic University

gilamonsterbirdsΗλεκτρονική - Συσκευές

24 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

58 εμφανίσεις

Harmonic Reduction Techniques

D. Sutanto

Department of Electrical
Engineering

HK Polytechnic University

Presentation Overview


Introduction


What is a power quality problem?


What are harmonics and triplen harmonics?


What are the major causes of harmonics?


What are typical harmonic current and voltage
distortion?


Why now?


What are the future trend?


What are the effect of harmonics?



Introduction

“Don’t think there are no
crocodiles because the
water is calm”



Malaysian Proverb



Introduction


Power electronics technology has become widely accepted
in industrial, commercial, and municipal facilities.


In high rise buildings, almost every office has a computer,
a printer etc and every floor has a photocopy machine, a
fax machine etc, all driven by power electronics.


With these increasing installed base comes inevitable
problems, like reduced power quality, harmonic distortion
problem with its disruptive effect and low power factor with
its costly consequences.


In this talk, we’ll try to explore these problems and see how
power electronics can actually solve many of these
problems.



What is a power quality problem?



A Power Quality Problem is:


Any

occurrence

manifested

in

voltage,

current

or

frequency

which

results

in

failure

or

mis
-
operation

of

end
-
use

equipment

Why is Power Quality Important


Load Equipment Sensitivity
(microprocessor
-
based)


Process Automation


Networks


Power Electronics


POWER QUALITY MEANS PRODUCTIVITY

Categories of Power Quality


Harmonics


Voltage Variations (Sags) and
Interruptions


Transients


Harmonics


Harmonics are currents or voltages at higher frequencies
than the fundamental power frequency (50Hz)


Most significant harmonics are the low order integral
harmonics, typically from the 2nd to the 31st.


The use of newer devices such as cycloconverter,, slip
recovery drives etc. has produced frequencies which are
non
-
integer multiples of the supply frequency, I.e. non
-
characteristic harmonics with possible side band and
they are often refereed to as ‘inter
-
harmonics’


Harmonics


Harmonics results from the distortion of sine wave caused
naturally by non
-
linear load.


Harmonics distortion can affect voltage or current or both


These disturbances can cause significant problems within a facility


Often harmonic remains an unknown problem until major
disruption occurs


Harmonics can even flow back into the supply line and create
havoc at other locations


Harmonic is a problem that needs to be addressed ideally during
planning stages but certainly as soon as signs of its occurrence is
detected.

Harmonics


“Even if you are on the right track,
you’ll get run over if you just sit
there”






Will Rogers

Harmonics


A classic example, a high rise building in Sydney, Australia


During its design stage, it appears that there are ample capacity in the
transformer to meet the the total demand of the building and the
wiring is perfectly adeaquate to meet the projected peak load with
sufficient margin


A few weeks later when people moves in and everybody adds
computer, fax machine, printer etc to their desk, the transformer that
supplies the building was overloaded and very hot, yet the actual watt
loading in the building is very low.


The heating that was taking place in the transformer is totally due to
the harmonics.

Harmonics


Success comes from good judgment


Good judgment comes from
experience


Experience comes from bad
judgment

Harmonics


In the past, the ideal goal of the the utility is to have the power factor
near unity or around 0.9 and the customer then make business
judgment on the best way to meet this requirement due to his own
load and that’s the end of the problem.


However, now power factor is not only due your own load, it also
depends on the utility’s quality of supply and due to your neighbour’s
load.


With the competitive electricity supply industry, the utility are now
being forced more and more to provide quality service at a reasonable
cost to the customer


The customer’s responsibility is first to minimise his cost and then to
be a good neighbour.

Harmonics


Classifications of harmonics:


Fundamental (50Hz)
-

positive sequence


2nd harmonics
-

negative sequence


3rd harmonics
-

zero sequence


4th harmonics
-

positive sequence


5th harmonics
-

negative sequence


6th harmonics
-

zero sequence


7th harmonics
-

positive sequence etc.


Triplen harmonics are all the odd harmonics which are multiple of
third harmonics. In 3
-
phase 4 wire system, they are in phase with
each other and add numerically in the neutral conductor wire.

Major Causes of Harmonics


Power Electronics Converters


DC power supplies in Computer, TV, Fax machines, printers etc


Uninterruptible Power Supplies (UPS)


Solid State Rectifiers


Electronic Process Control equipment, PLC’s etc


Electronic Lighting Ballasts, including Light Dimmer


Reduced Voltage Motor Controllers


Electrical Traction System


Variable speed ac motor drives


HVDC


Induction heating


Electrochemical and electro
-
metallurgigal process




Major Cause of Harmonics(cont.)



Arcing Devices


Discharge Lighting, e.g. Fluoro, Sodium or Mercury Vapour


Arc Furnaces


Welding Equipment


Ferromagnetic Devices


Transformer operating near saturation level


Magnetic Ballast (saturated iron core)


Reactors


Motors


Static Var Compensator


Slip Recovery Drives

Typical Harmonic Current and VoltageDeviation



Standard Personal Computers

Signs of Harmonic Problems


Telephone Interference


Customer capacitor or transformer failure


Capacitor fuse blowing


Transformer overheating at less than full load


Motor Overheating


Clocks running fast


PLC and Computer Failures


Faulty Operation of relays and meters


Hard disk data corruption


Switch mode power supply failure


High Neutral Currents

Signs of Harmonic Problems


These disturbances can cause significant problems
within a facility


Often harmonic remains an unknown problem until
major disruption occurs


Harmonics can even flow back into the supply line and
create havoc at other locations


Harmonic is a problem that needs to be addressed ideally
during planning stages but ceratinly as soon as signs of
its occurrence is detected.

Electronic Loads Cause Excessive Neutral Current



Why harmonics are now being recognised as a a major
technical issue?



Exponential growth of electronics loads in recent years


Commercial


Industrial


Residential


More intelligent buildings over last decade


Information super highway and Internet


Lack of awareness and understanding


Numerous reported problems on harmonics and power quality
from various existing electrical installations in different major
cities


Harmonics now form an integral part of the mandatory EMC
regulations in USA, European community, Australia, etc.


Why harmonics are the future trends in power system
harmonics?



More prevalent because:


The application of sophisticated electronic controlled equipment would
dominate over a wide area


Future buildings will be more intelligent especially the high rise premises.
Many countries are constructing or planning to build the world tallest smart
buildings.


21st century information super highway with next generationcomputers
with speed of 10
-
100 times faster than the current technology and space age
communication establishment.


The non
-
linear loads in USA has increased from 5% in1960 to
30% in 1990. It is estimated that it will reach 60% by the year
2000.

Symptoms of Voltage Variation Problems



Tripping of sensitive loads
(PLC, motor controls, process
controls, machine tools etc.)


Arc Lighting drops out

Remote Transmission Fault

Causes Sensitive Loads to trip



Symptoms of Interruptions


Temporary
loss of loads
(unprotected)


Multiple trips (reclosing operations)


Process shutdowns

Symptoms of Transient Problems


ASD’s & process loads tripping frequently


Equipment failures during lightning storms


Distribution transformers fail during storm


Customer capacitor failures/fuse blowing


Equipment failures on the network


TV failures with cable connection

Utility Capacitors Switching

causes drives to trip



Why is equipment sensitive


Low signal voltage levels


Fast processing speed


Electronic circuit design without shielding or
isolation


Lack of Energy Storage in power supplies


Sensitive Components

Summary


Source of the problems can be anywher
e


Transmission system


Distribution system


Customer system


Equipment sensitivity determines

ext
ent of problems


Solutions must be developed using SYSTEM
approach



System Approach to solving Power Quality
problems



General Approach


Identify the Problem


Characterise the problem


What is the Source?


What are the possible solutions?


Equipment


Customer system


Utility System


Cost of the
Problems


Cost of the Solutions


Cost/Benefit Analysis

Active Filter


Active filters are proving to be a viable option
for controlling harmonic distortion levels in
many applications.


The application of passive tuned filters creates
new system resonances which are dependent
on specific system conditions.


Also, passive filters often need to be
significantly overrated to account for possible
harmonic absorption from the power system.

Active Filter


Passive filter ratings must be coordinated with reactive
power requirements of the loads and it is often difficult
to design the filters to avoid leading power factor
operation for some load conditions.


Active filters have the advantage of being able to
compensate for harmonics without fundamental
frequency reactive power concerns.


This means that the rating of the active power can be
less than a comparable passive filter for the same
nonlinear load and the active filter will not introduce
system resonances that can move a harmonic problem
from one frequency to another.

Active Filter


The active filter concept uses power electronics to
produce harmonic components which cancel the
harmonic components from the nonlinear loads.


These active filters are relatively new and a number
of different topologies are being proposed.


Within each topology, there are issues of required
component ratings and methods of rating the
overall filter for the loads to be compensated.

Active Filter Configuration



One leg of the active Filter



System model



System Model


Source strength at transmission supply point = 200 MVA 138/13.8
kV Transformer: 10 MVA, 7% impedance



Substation capacitor bank size = 3.0 Mvar (switched)


Equivalent load for parallel feeders = 3.0 MW


Modeled feeder circuit: 3.0 miles to example customer


Feeder capacitor bank on 13.8 kV side at example customer:
different sizes evaluated


Customer low voltage capacitor bank: varied


Customer service transformer: 1500 kVA, 6% impedance


Customer load = 1.0 MW


Active Filter size = 400 Vrms, 30 Arms


Nonlinear load: different loads evaluated