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Design of Active Filters to reduce Harmonics for Power Qualify Improvement Mr. H. Prasad, M.E
Dr. T. D. Sudhakar, M.E., Ph.D.
Assistant Professor, Department of Electrical and Electronics Engineering st. Joseph's college of engineering Chennai - 119, India
[email protected]
Associate Professor, Department of Electrical and Electronics Engineering st. Joseph's college of engineering Chennai - 119, India
[email protected]
Abstract
-
in the operation of a load and affect its performance badly [12]. Harmonics are generally caused due to non-linear nature of loads which inject harmonic currents in the AC system and increase overall reactive power demanded by the equivalent load [l3]. This increases in power electronic based non-linear loads may cumulatively lead to a state of harmonic pollution that affects the operation of power system since modem day digital electronics and control equipments require high quality of power supply for perfect operation and thus they cannot yield expected results with a system affected by harmonics [5]. A harmonic filter is used to eliminate the harmonics [5]. There are three basic types of harmonics filters namely Passive, Active and Hybrid harmonic filters [14]. A lot of work, surveying and analysis were done so as to design a suitable filter to reduce harmonics for power quality improvement. Primitively used filters were of passive type which consisted of inductors and capacitors. But later it was found that active power filters are advantageous in many respects when compared to passive harmonic filters [2] [ll].Then the advent of technology has paved way for the development of active power filters and hybrid power filters [15].
Delivering high quality of power to
the consumers has become an indispensable issue of concern to power engineers. There are various which
issues
regarding
harmonics
affect
power the
quality
power
of
system
operation and their mitigation should be given paramount importance. Harmonic filters are used to address this issue. Of the various types of harmonic filters, Active harmonic filters are seen as a very much feasible and affordable solution and their commercial implementation is being researched by a lot of people. This paper discusses the simulation of an Active power filter to mitigate harmonics in single phase system using MATLAB / Simulink.
Keywords
-
compensating
Active power filter, RL - load, current,
harmonics,
THD
-
Total Harmonic Distortion.
INTRODUCTION Power Quality (PQ) issues were considered only in the past few years ago. Earlier this problem was only considered by the power engineers. Recently only the community deals about the PQ problems because of their awareness and the standard power stations maintain. Even in the industries only the recent generation people are trained in these power quality areas. There are various PQ issues related in power system. Some of the power quality issues are reactive power balance, voltage imbalance, harmonics, transients and interruption. Of these issues, harmonics play a major role in the PQ. Harmonics generally refer to something that co-exists. In electrical terminology harmonics can be defmed as components that are present with fundamental waveform of voltage or current and that are integer multiples of fundamental frequency. Harmonics cause certain undesirable effects
I.
II. PRINCIPLE OF OPERATION The objective of the active filtering is to solve these problems in a dynamic way rather than using components whose rating is predetermined, along with a much-reduced rating unlike bulky passive components [2]. The idea of active filters is relatively old, but their practical development was made possible with the new developments in
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power electronics and control strategies as well as with reduction in cost of electronic components [3]. Depending on the problem nature, active filters can be implemented in three basic topologies as shunt type, series type, or a combination of shunt and series active filters (shunt-series type) [10]. The shunt active filter, with a dc capacitor or dc voltage source, has a topology similar to that of a static compensator (STATCOM) used for reactive power compensation in power transmission systems [4]. Through power electronic switching, the active filter introduces current or voltage components, which cancel the harmonic components of the nonlinear loads. Shunt active power filters compensate load current harmonics by injecting equal-but opposite harmonic compensating current [4] [10]. In this case the shunt active power filter operates as a current source injecting the harmonic components generated by the load but phase shifted by 180°. Series active power filters act as a voltage regulator and as a harmonic isolator between the nonlinear load and the utility system [6]. The series-connected filter can mitigate inadequate supply- voltage quality. This type of approach is especially recommended for compensation of voltage unbalances and voltage sags from the ac supply. The figures I and 2 shown below depict the placement of single phase shunt active power filter in an electrical network.
signal obtained. If this through properly gated output of VSI is fed back to source side in phase opposition, leads to the phase cancellation of harmonic components. The source current after compensation is given by, Is(t)*
=
s !.n (. w t + ip) + � In si n(nw t + ip) - Ic(t)
lm
(1) (Or) Is(t)*
=
It(t)
+ Ih(t) - Ic(t)
The output of VSI is a voltage and it can be converted to current source by connecting an RL branch in series. The value of the RL branch is determined in accordance with the magnitude of compensating current. The process of active harmonic filtering involves the following steps. • Harmonic component extraction. • Inverter operation to generate compensating current. • To moderate the compensating current by control of gating pulses to the inverter based on reference current mechanism.
A.
Harmonic extraction methods
There are various methods of extraction of harmonic components from the fundamental component. This is done in order to separate the harmonics from the supply waveform mathematically [8]. Some of the harmonic extraction methods surveyed [3] [8] are given as follows. • PQ theory and modified PQ theory • DQ axis with fourier method • Synchronous Reference frame method • Synchronous detection method • Sine multiplication method. E.
Figure 1 APF Installations.
Voltage Source Inverter
Most of the active power filter topologies in use today use a voltage source inverter for their operation. It can operate in all four quadrants i.e supply and absorb both real as well as reactive power. Self commutation is possible in VSI due to IGBTs and MOSFETs. The advantages of the VSC are: • Rapid control of active as well as reactive power [9], • It provides a high level of power quality. [9] • Low power (less than 250 MW) HVDC transmission (commercially referred to as "HVDC Light") [9],
Figure 2 APF control schematic
The compensating current obtained in the the operation will be Ic (t) equal to the error
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inverter consist of chopping the dc bus voltage to produce an ac voltage of an arbitrary waveform. There are a large number of PWM techniques available to synthesize sinusoidal patterns or any arbitrary pattern of which the Triangular PWM technique is the most prefered which is discussed [7]. PWM works by means of one of the simplest method namely the triangular carrier technique. It forces the output voltage over a switching cycle, defined by the carrier period, to be equal to the average amplitude of the modulating wave Vref. Though there are various carriers used, the triangular carrier is most widely used owing to its effectiveness. As for active filter implementation is concerned, this method compares the output current error with a fixed amplitude and fixed triangular wave: the triangular carrier. Thus the compensating current generated for a shunt active power filter. The schematic representation of PWM technique is shown in figure 4 as follows.
VAR Computation (SVC and STATCOM), and Active Power Filters [9], • VSCs utilize self-commutating switches (e.g. GTOs, IGBTs) which can be turned-on or off at will. This is in contrast to the conventional CSCs which operate with line commutated thyristor switches [9]. Commutation in a force commutated VSC valve can occur many times per cycle, whereas in a line commutated CSC it can happen only once per cycle. This feature allows the voltage/current in a VSC to be modulated to produce a nearly sinusoidal output and control the power factor as well. Thus VSls are an obvious choice for the implementation of active filters [9], •
"':.-1::· ::: lnHIn IOOrmI�JH
Figure 3 Single phase VSI topology
This topology, shown in figure 3 converts a dc voltage into an ac voltage by gating the appropriately power semiconductor switches. Voltage source converters are preferred over current source converter because it is higher in efficiency and lower initial cost than the current source converters. C.
-Va
--
--
-- --
--
-- --
-- --
--
-- - - -
-
-- --
-- --
Figure 4. Triangular PWM technique.
Gating Pulses generation{or APF
III.
SIMULATION AND RESULTS Consider a typical single phase 230 V 50 Hz AC system feeding an RL load through a converter and inverter setup. Converters and inverters are essential in the new era of control methodologies using power electronic switches. But the problem of harmonics associated with them needs to be mitigated in order to ensure better power quality. The system consists of diode bridge rectifier and Inverter with MOSFET switches. Separate pulse arrangement is provided for MOSFET switches which decide the frequency of AC output of the inverter. A schematic of the system is given below in figure 5.
This is a very important process which demands high accuracy for the desired operation of APF. Gating is also the most complex process in control of APF which needs utmost concern. The vanous techniques for gating pulse generation are listed below. • External pulse generation based on frequency. • Sine PWM. [7] • Space Vector PWM. [7] • Triangular PWM. [7] For most applications requiring dynamic performance, triangular pulse width modulation (PWM) is the most commonly used technique for gating today. PWM techniques applied to a voltage source
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Figure 5 Schematic of Test System
signal to a PWM block with a triangular carrier where it is compared with the same and switching pulses are generated. These switching pulses are to be given as gating signals to the VSI of the Active filter setup. The circuit shown in figure 6 is the simulation circuit of APF incorporated system. A breaker is used to connect the APF to the source side as and when required. The APF topology consists of a voltage source inverter for which the gating pulse is provided by a PWM generator which uses the extracted harmonic component from the source current as the modulating waveform. Based on these pulses, the output of the APF will vary. The amplitude of output of APF is moderated by the DC source input to the bridge. The APF RL branch is used to convert the constant voltage output of the VSI to current in order to cancel out current harmonics of various orders.
The simulation system parameters and results are given in table L The parameters of the active filter topology along with its RL values are also given in the same table for reference, Active power filters are always compared with passive filters in terms of performance, ease of implementation ability to operate in a generalized manner. The active filtering concept aims at the derivation of compensating current by separating out the harmonic component from the fundamental component The source current in a system feeding a non-linear load will contain all the harmonic components along with the fundamental component It is given by [set) =
I sin(.wt
+
rp)
+
L' �'= Insin(1Uo)t
+
rp)
_
(2) denotes the Where I sin(.wt + rp) fundamental component while the other term �' lm .sin(nwt + rp) consists of harmonic components up to nth order. This term containing the n order harmonic components will be the error signal. This is the component that has to be compensated to achieve better power quality. This is being determined by subtracting the entire source current from the ideal sinusoidal reference current which is exactly in magnitude and phase with the fundamental component I sin(.wt + rp) which is the reference. This ideal sinusoidal reference current can be obtained using a mathematical model in simulink or a low pass filter to extract the fundamental component or more precisely a band pass filter can be used. Thus the error signal obtained is now given as a modulating
TABLE I. Parameters of Single phase system with APF
Parameters
Values
Voltage
230 VRMS
Frequency
50 Hz
DC link RL load
15 A, 50 Hz
Carrier frequency
2 kHz
Sample time
0.5J.lseconds
Frequency Vdc_comp APF RL BRANCH
0340
mH
Sine wave ref
Inverter switching
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Figure 6 Simulation of single phase circuit APF.
The results of the simulation with and without the incorporation of filters can be analyzed using the table 3 which compares the results in both cases, It can be seen evidently that incorporation of Active filters reduces harmonics of various orders.
�) .
DC component Fundamental
14.64
Harmonics
3RD ORDER 5 TH ORDER 7TH ORDER 9 TH ORDER 11TH ORDER 13TH ORDER
II \
With APF
I)
6.55
7.66
6.07
5.32
4.68
2.51
2.39
1.20
0.89
1.31
0.85
29.67
15.86
I
.,
�III �I\
,I
i
I�
,I�
L
\,\/
15.7
\ \ \J �III�I �IIII�I ..
;iE -l1:nSe::rn
Figure 7 Source current without APF.
The distortion is more when filters are not incorporated. Current wave form reaches a peak of 15 amperes in this case. When APF is included in the circuit operation, the distortion reduces considerably and the waveform tends to be more like an ideal sine wave which is desirable. This is shown in figure 8. For the same peak at around 15 amperes, the waveform is less distorted. There is an approximately 13 % reduction in THO of source current waveform as observed from table 2.
TOTAL HARMONIC
�.
0.6277
15.99
,I
I)·
TABLE 2 Results with and without APF
Without Filter 1.11
/ 1\
,
DISTORTION
FIgure 6 and 7 show the source current waveform without and with APF installations. It can be observed that the distortion significantly reduces after the incorporating the APF into the circuit.
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Industrial Applications
Vol.
32(6): 1312-
1322, 1996. [2].
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Figure 8 Source current with APF
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CONCLUSION
& Bharti Dwivedi "Power
S.Khalid
Quality Issues, Problems, standards & their effects in industry with correctivemeans", International
Thus this chapter aims at studying the implementation of active power filter to the single phase system and the impact and role of active power filters in mitigation of harmonics. Active power filters separate the harmonic from fundamental component there by it can mitigate the entire order harmonics unlike passive filters where n units are needed to mitigate n order harmonics. The concept of Active power filters involves various harmonic extraction and various control strategies which can be surveyed to suggest a better solution. Thus an active filter posses the quality of dynamism, robustness and doesn't cause electrical resonance or isn't made up of large or bulky equipments. They are considered to be advantageous in such regards when compared to conventional passive filters. But active power filter operation involves highly complex control strategies and complicated compensation current generation techniques due to which engineers might face difficulty in implementation. Thus a compromise IS arrived at by combining the operational merits of both active and passive filters in order to reduce the complexity of Active power filters which has paved way for modern hybrid filters. Active filters can be used with conventional passive filters as Hybrid filters in order to achieve better performance. A hybrid filter is a combined topology of APFs and PPFs which aims at compensating for each other's disadvantages. Active power filters have various other applications in power system domain like reactive power compensation, voltage sag mitigation, flicker compensation and various other applications which can also be implemented. REFERENCES [I].
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BIBLIOGRAPHY H
Prasad
his
received
Bachelors degree in the faculty of Electrical and Electronics engineering
Anna
from
University
Chennai
and
his
Masters degree in the faculty of Power
systems
from
engineering
Anna
Chennai.
University,
He
is
currently
working as Assistant Professor in the Department of Electrical Engineering
at
St.
of
College
Joseph's
Engineering,
Chennai. His areas of interest include
Power
Quality, and
filters
Harmonic application
to
renewable
energy systems.
T. D. Sudhakar received the Degree
in
&
Electrical
Electronics Engineering (B. E.) from
Madras
University,
Chennai, India, in 200 I, the M.E.
degree
and
the
Ph.D.
degree from Anna university, India
in
2004
and
2012
respectively. He is currently an Associate
Professor
Electrical
in
the
Engineering
Department
at
st.
of
College
Joseph's
Engineering,
Chennai, India. He has more than
35
Journal
and
Conference at International and National level publications. His technical interests are in the field power
of
power quality
systems and
performance
&
their
evaluation
through simulations.
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