A REVIEW OF ACTIVE FILTERS FOR POWER QUALITY IMPROVEMENT

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A REVIEW OF ACTIVE FILTERS FOR POWER QUALITY
IMPROVEMENT








LAXMI S.PATTANSHETTI

PREETI N.PATIL








VIII SEM

ELECTRICAL AND ELECTRONICS DEPARTMENT

KARNATAKA LAW SOCIETY’S

GOGTE INSTITUTE OF TECHNOLOGY

UDYMBAG BELGA
UM
-
590008






















ABSTRACT



Active filtering of electric power has now become a mature technology for
harmonic and reactive power compensation in single phase (2 wire), three phase without
neutral (3 wire) and three phase with neutral (4 wire
) ac power networks with nonlinear
loads. This paper presents a comprehensive review of active filter configuration control
strategies, selection of components, other related economic and technical considerations
and their selection for specific applicatio
ns. It also includes comparison between LC
passive filters and active harmonic conditioners. The various implications of poor power
quality, their effects and solutions are proposed with respect to present nonlinear load
patterns of the power system.

























1.INTRODUCTION


Solid state control of ac power using tyristors and other semiconductor switches is
widely employed to feed controlled electric power to electric loads, such as adjustable
speed drives (ASD’s), furnaces, computer power su
pplies etc.

Particularly the increasing penetration of power electronic based loads is creating
a growing concern for harmonic distortion in the ac supply system. Unfortunately there
are some problems associated with these new power electronic circuits and

devices.
Unlike conventional loads they control the flow of power by chopping, flattening or
shaping the otherwise sinusoidal power system voltages and currents. These waveforms
distortion can cause problems for neighboring loads, and they tend to have an

overall
detrimental effect on the quality of the electric power provided to their neighbors. The
resulting electrical pollution whether it is produced by large single sources or by
cumulative effect of many small loads, often propagates for miles along di
stribution
feeds.

Common symptoms of sever distortion include nuisance tripping of computers or
computer controlled industrial processes and medical equipment, excessive heating in
transformers and equipment failure.

Nonlinear lodes give rise to troubles
and serious problems to their utility as well
as to the customer equipment. Bursting of capacitors, blown fuses, insulation failure and
over heating of power equipment such as transformers, cables and motors can exist due to
harmonic distortion, especially

voltage distortion.


One possible solution is to place uninterruptible power supplies (UPS) between
critical loads and the power system. However UPS systems are quite expensive, and
while they do a good job in protecting their own load, they are major pow
er system
polluters and often cause problems for neighboring loads.

Thus, there exists the need for new and innovative circuits that can be placed at
end user facilities and on distortion to reduce distortion levels, cancel the effect of
transient phenomen
a, and provide much of the protection for end users that is now
achievable with UPS systems.

One innovative concept is the APLC also known as AF’s. it appears to be an
attractive, viable method for reducing voltage and current harmonic distortion, voltag
e
spikes , transients, and flickers. It injects equal bur opposite harmonic there by canceling
the original problem and improving power quality on connected power system.

2.TYPES OF ACTIVE FILTERS:

Active filters can be classified based on converter type,
topology and supply system
(number of phases).

The converter type can be of two types namely

(i) Current Source Inverters (CSI). (ii) Voltage Sources Inverters (VSI).


Topology type can be of three types namely

(i) Shunt topology (ii) Series topo
logy or (iii) Combination of both


Based on number of phases there can be three types namely

(i) Single phase (2 wire) (ii) Three phase without neutral (3 wire)

(iii) Three phase with neutral (4 wire)


A]. CONVERTER BASED CLASSIFICATIONS:

DOMAIN BASED CLASSIFICATION:

Two fundamental approaches for improving power quality with APLC’s are

(i) Correction in time domain (ii) Correction in freq
uency domain


Either of these can be used in conjunction with CSI and VSI.

(I) CORRECTION IN TIME DOMAIN
:

Correction in time domain based on the principle of holding the instantaneous
voltage or current within some reasonable tolerance of sine wave.

TIME D
OMAIN CORRECTION TECHNIQUES
:

1.Triangular wave 2.Hysteresis 3.Deadbeat

Triangular wave: This method is the easiest to implement it can be used to
generate either a two state or three state switching functions. A two state switching
function consist o
f dc source that can be connected either to positive or negative output as
shown figure.


Fig1






Fig2





A three state
switching function can be +ve,
-
ve or zero (Off) therefore the
inverter is always On when two state switching function is used but it can be Off when
three state switching function is used



Hysteresis: The most commonly proposed time domain correction tec
hnique
preset upper and lower tolerance limits are compared to the extracted error signal. As
long as error is within the tolerance band, no switching action is taken. Switching action
occurs whenever the error leaves the tolerance band .


Deadbeat: The co
ntrol based switching functions have been proposed for inverter
switching circuit but not yet been used with APLC’s.

FREQUENCY DOMAIN CORRECTION TECHNIQUES
:


Correction in frequency domain is based on the principle of Fourier analysis &
periodicity of th
e distorted voltage or current waveforms to be corrected. This concept is
illustrated in the figure. While one early reference suggests the use of predetermined
harmonic injection for situation where there are few predominant and fixed harmonic
present, mo
re recent references use Fourier transform to determine the harmonics to be
injected. Once the Fourier transform and an inverter switching function is computed to
produce the distortion canceling the output. This can be accomplished with either two
state o
r three state switching function . The inverter switching frequency must be more
then twice the highest compensating harmonic frequency.


Fig 3



COMPARISION OF FREQUENCY &TIME DO
MAIN:

Time Domain

Frequency Domain

1.Fast response to changes in the power system Slow response compared to



time domain.

2.Time correction techniques take measurement Frequency domain correction

at only one point in the power system. They are can handle single node generally
limited to signal node appli
cations& are not well problem & can also be extended

suited for overall network connection. to minimize harmonic distortion


through

the network

3.Easy to implement & has little computational Increased computational

burden. Burden

CONVERTER TYPES:


The voltage waveform at a power system bus is af
fected by the
current injected at that bus. A stiff system is one, in which the voltage is rather insensitive
to current, while the voltage at weak system bus is quite sensitive. Therefore providing
that a system is not too stiff, a non
-
sinusoidal voltage
waveform at a bus can be corrected
to sinusoidal by injecting the proper current magnitude & waveform. This is the basic
operating principle of an APLC’s.


Fig4 Fig5




The two basic types of inverter as shown in figure 4 & 5. The dc source of a
voltage invert
er consists of a capacitor that resist s voltage changes, while that of current
inverter consists of an inductor that resists current changes .In both the cases the dc
source receives its power from the ac power system. The choice of current type APLC’s
or

voltage type APLC’s depends on source of distortion at the specified bus, equipment
cost& amount of correction desired.


Voltage type converters have an advantage they can be expanded in parallel to
increase their rating. Their combined switch
ing rate can be increased if they are carefully
controlled so that their individual switching times do not coincide

higher order
harmonics can be eliminated by using parallel voltage type converters without increasing
individual converter switching rates.
The main drawback of voltage type converter lies in
the increased complexity of their control system. For systems with several converters
connected in parallel, this complexity is greatly increased.

COMPARISION BETWEEN VOLTAGE TYPE AND CURRENT TYPE:

Voltag
e type

Current type

1 Lighter Heavier

2 Less expensive More expensive

3 M
ore complex control system Simpler & more reliable

4 Losses are less compared to current type Losses are more compared to voltage



type

5 Easily expandable due to parallel Cannot be easily expanded.


Connection



TOPOLOGY BASED CLASSIFICATION
:

1.Shunt filter 2.Series filter 3.Unified power quality conditioner (combination of both)


4.Hybrid

Shunt filte
r: The filter circuit comprising a combination of R, L and C is connected
between line & earth (phase). The filter is subjected to full line to earth voltage of main
circuit & does not carry main circuit current .It passes harmonic of certain frequency to
earth. Active shunt filter which is the most widely used to eliminate the current
harmonics, Reactive power compensation (also known as STATCON), and balancing and
unbalanced currents.

Active series filter: It is connected before the load in series with th
e mains, using a
matching transformer, to eliminate voltage harmonics, & to balance & regulate the
terminal voltage of the load or line. It has been used to reduce negative sequence voltage
and regulate the voltage on the three phase systems. It can be ins
talled by electric utilities
to compensate voltage harmonics and to damp out harmonic propagation caused by
resonance with line impedances and passive shunt compensators.

Fig 6




Unif
ied power quality conditioner: This is a combination of active shunt & active series
filters. The dc link storage element or a dc bus capacitor is shared between two current
source voltage source bridges operating as active series & active shunt compensato
rs. It is
used in single as well as three phase configuration. It is considered an ideal AF which
eliminates voltage & current harmonics & is capable of giving clean power to critical and
harmonic prone loads.

Fig 7


Hybrid Filter: This is the combination of an active series filter & passive shunt filter. It is
quite popular because the solid
-
state devices used in active series part can be of reduced
size & cost and major part of the hy
brid filter is made of the passive shunt LC filter used
to eliminate lower order harmonics. It has the capability of reducing voltage and current
harmonics at reasonable cost.

Fig 8



COMPARSION BETWEEN LC PASSIVE FILTER & ACTIVE HARMONIC
CONDITIONER:


LC

AHC

1.Harmonic current control:

Requires filter for Simultaneously monitors



each frequency several frequency

2.Influence of modification:

Risk of resonance No effect


in the impedance

3.Influence of frequency:

Reduced effectiveness No effec
t


variation

4.Influence of increase in:

Risk of over load No risk of overload


current

5 Harmonic control by order very difficult Possible by personalization

6 Dimensions Large

Small

7 Weight High Low

8 Losses Average Average


SUPPLY SYSTEM BASED CLASSIFICATIO
N
:

1. Single phase AF 2.Three phase 3
-
wire AF 3.Three phase 4 wire AF

Single phase (2 wire) AF: Are used in all all three modes as active series, active shunt
and a combination of the both unified line condititioners, both converter configuration
(volta
ge and current). The series AF’s is normally used to eliminate voltage harmonics,
spikes, sags, notches etc.

Three phase 3 wire AF: Three phase three wire nonlinear loads, such as ASDS’s are
major applications of solid state power converters and lately, ma
ny ASDS’s etc,
incorporate AF’s in there front end design.

Three phase 4 wire AF: A large number of single
-
phase lodes may be supplied from 3
phase mains with neutral conductor. They cause excessive neutral current, harmonic and
reactive power burden and
unbalance. To reduce these problems four wire AF’s are
developed. They have been developed as

1 Active shunt mode with current fed voltage fed.

2 Active series mode

3 Hybrid form with active series and shunt mode.


Fig 9

Fig10







Fig 11



The first configuration of 4 wire shunt AF is known as capacitor midpoint type, used in
smaller ratings. Here, the entire neutral current flows through D.C bus capacitors which
are of large

value. As shown in Fig 9. Fig 10 shows another configuration known as 4
pole switch type, in which the 4 th pole is used stabilize the neutral of the AF.Fig 11

Shows the three single phase bridge configuration it is quite common and this version
allows pr
oper voltage matching for solid state devices and enhance the reliability of the
AF system.


3.SELECTION OF COMPONENTS AND ADDITIONAL FEATURES OF AF’s:


The selection of components of the AF’s is an important factor to achieve
improved performance. The mai
n component of the AF is the solid
-
state device. In the
earlier days, BJT’s followed by MOSFET’s were used in small ratings and GTO’s
Nowadays, the IGBT is an ideal choice up to medium ratings, and GTO’s are used in
higher ratings. A series inductor (Lc) a
t the input of a VSI bridge working as an AF is
normally used as the buffer between supply terminal voltage and PWM voltage generated
by the AF’s.


A number of configuration discussed earlier have been investigated, but could
not be developed commercially
because of cost and complexity considerations. Initially
reported configurations were quite general and the rating of solid
-
state devices involved
was substantial, which resulted in high cost.


Later on the rating of active filtering without deter
iorating the overall filter
performance. More over modern AF’s are capable of compensating quite high order of
harmonics (typically 25
th
) dynamically. However as high order harmonics are small, they
are compensated using passive ripple filter.

4.IMPLICATIO
NS OF POOR POWER QUALITY

Major concerns Effects Remedies

1. Poor load p.f Improper voltage profile Proper reactive power control






Increased losses






Reduced li
fe of a feeder

2. Harmonic contents in loads Increased losses Use of STATCOM or UPFC.






Deterioration in p.f






Manufacturing of


electronic circuits.



Heating of Induction motor



Premature failure of motors

3.

Notching in load voltage Error in zero crossing Separate transformer for notch









sensitive devices

4. D.C offsets in loads



Offsets the flux,


Non usage of loads producing






excursions

in



dc offset currents.



a distribution transformer


Excessive heating of core


Can involve current



through the earth.


Hence more corrosion







5. Unbalanced loads


Unbalance 3 phase voltages Use of harmonic filters,





Reduction in net torque in SVCs





Induction m
otor



Following certain standards





Overloading & for NPS voltage at PCC





Excessive heating

6. Disturbances in


Relay tripping




Interconnection


supply voltage Substantial loss of revenue

of grids


( Interruptions,


due to outages Proper control of reactive


distortion, Reduced life of



power.


Sag/swell)




consumer loads Effective and efficient






Stalling of motors protection system






Flickering of lamps Use of DVR.






CONCLUSION

An extensive review of AF’s has been presented to provide a clear perspective on
various aspects of AF.The substantial in
crease in the use of solid power control results in
harmonic pollution above the tolerable limits. Utilities are finding it difficult to maintain
power quality at consumer end. Consumers are paying the penalties indirectly in the form
of increased plant do
wntimes etc. At present AF technology is well developed and many
fabricating AF’s with a large capacity. The utilities in the long run will induce the
consumer with nonlinear loads to use the AF’s for maintaining the power quality at
accepectable levels. A

large number AF configuration are available to compensate
harmonic current, reactive power, neutral current, unbalanced current & harmonics. The
consumer can select the AF with the required features. It is hoped that the survey on AF’s
will be useful refe
rence to the users & manufactures.

REFERENCES:

1.Bhim Singh, Kamal Al
-
Haddad, and Ambrish Chandra: A Review of Active Filters for
Power Quality Improvement.IEEE Transaction on Industrial Electronics, Vol 46,No. 5
Oct 1999.

2.W.N.Grady, M.J.Samotyj, A.H. No
yola: Survey of Active Power Line Conditioning
Methodologies.IEEE Transaction on Power Delivery, Vol 5,and No.3 July 1990.

3.J.Nastran, R.Cajhen, M.Seliger, P.Jereb: Active Power Filter for Non Linear AC Loads

IEEE Transaction Power Electronics, Vol 9,Page

90
-
96,Jan 1994.

4. The conference on power quality held in Maduray in 2004