shunt active filter report
description
Transcript of shunt active filter report
ABSTRACT
A Shunt Active Power Filter(APF) is a device that is connected in parallel to group
of loads.APF cancels the reactive and harmonic currents drawn by the load so as to make
supply current sinusoidal. Active Power Filter play a vital role in present day liberalized
energy market. Active Power Filter are explored for executing different power conditioning
function simultaneously along with harmonic elimination due to increase in nonlinear and
unbalanced load, at the point of common coupling. The aim of present dissertation is to
study different control strategies for Active Power Filter. More importantly to study
instantaneous power theory based Shunt Active Power Filter which is predominantly used in
present scenario. The shunt active power filter is investigated through Matlab/Simulink
simulation under different load conditions. Simulation results are discussed in depth. Then
the design issues of Active Power Filter for different load conditions are also discussed.
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CHAPTER 6SIMULATION DESIGN
The p-q theory based shunt APF is implemented for Harmonic compensation and power
factor correction. Logic utilized for shunt APF is discussed in chapter 5 and is summarized
in fig.5.2
6.1 Specification of the design:
Simulation is performed on 2 types of Three phase Balanced Non –Linear Load as
fallows:
System Parameters
Source Voltage V Sa , V Sb ,V Sc 220 V rms(line- line)
System Frequency f 60 Hz
APF
Dc-link voltage V dc 800V
Dc side capacitance C 1100μF
Ac side inductance Lc 3.75mH
Ac side resistance Rc 0.01 Ω
(Rating of APF is generally decided by peak voltage and RMS Current)
Load 1 Thyristor Rectifier (of rating 4 KVA)supplying to DC motor equivalent of 2.5KW
AC side inductance LLac 1mH
AC side resistance RLac 0.01 Ω
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DC side Resistance RLdc 18 Ω
DC side Inductance LLdc 85mH
Load2 Diode rectifier (of rating around 3KVA) supplying to purely resistive load
AC side inductance LLac NA
AC side resistance RLac NA
DC side Resistance RLdc 18 Ω
DC side Inductance LLdc NA
(NOTE: Rating of APF is generally decided by peak voltage and RMS Current[14]
APF rating for Load1 is V peak=312 v∧Irms=3 A will result in rating of
1
√2×312 ×3=0.661 KVA .Thus in practical cases can be assumed to be around 1-1.5KVA}.
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Fig 6.1 p-q theory based control block diagram of three-phase shunt APF system.
220
V rm
s L-L
3-ph
ase
Sour
ce
PI co
ntro
ller
Low
Pass
FIlte
r
Curre
nt m
easu
rem
ent
Volta
ge m
easu
rem
ent
RcLc
Conti
nuou
spo
wer
gui
A B C
non-
linea
r loa
d
curre
nt m
easu
rem
ent1
v+-
v+-
VcVbVa
Valph
a
Vbet
a
pdc
ploss
Isalf
a
Isbe
taIn
1
In2
Out1
Load
Cur
rent
mea
sure
men
t
Isalp
ha
Isbe
ta
Ia*
Ib*
Ic*
Inve
rse T
rans
form
atio
n
Isa*
Isb*
Isc*
Isa
Isb
Isc
A1 A2 B1 B2 C1 C2Hy
steris
is Ba
nd C
urre
nt C
ontro
ller
[Vdc
]Go
to7
[isc]
[Vsc
][V
sb]
[isb]
[isa]
[Vsa
]
[Vdc
]
[isc]
[isb]
[isb][is
a]
[isc]
[Vsb
]
[Vsa
]
[Vsc
]
[isa]
i+
-
i+
-
i+ -
i+ -
i+ -
i+
-
i+
-
i+
-i+
-
800
g11
g12
g21
g22
g31
g32
Vdc
a b cCo
mpe
nsat
or
Vsa
Vsb
Vsc
Isa
Isb
Isc
Valph
a
Vbet
a
p
Clar
ke T
rans
form
atio
n
Capa
citor
volta
ge
butte
r
v+-
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6.2 Clark Transformation:
is done in accordance with section 4.2.2
3
p
2
Vbeta
1
Valpha
Vbeta
Ibeta
Valpha
Ialpha
p
Subsystem5
a
b
c
alpha
beta
Subsystem2
a
b
c
alpha
beta
Subsystem1
6
Isc
5
Isb
4
Isa
3
Vsc
2
Vsb
1
Vsa
Fig 6.2 Block Diagram for Clark Transformation and p calculation
2
beta
1
alpha
Sum ofElements1
Sum ofElements
-K-
K=sqrt(2/3)
-K-
K=sqrt(2/3)
-K-
K=-1/2
-1
1
1
-K-
K=-1/2
3
c
2
b
1
a
Fig 6.3 Clark transformation block diagram for both V ∝ , V β , I α∧I β
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6.3 Calculation of p
According p-q theory real and imaginary power can be separated into two parts:
Real power: p=p+~p
Imaginary power: q=q+~q (from eq)
w h ere p and qare average power due to component iap∧iaq respectively
~p and ~q are oscillating power due to components i~ap∧i~aq respectively.
And i−(i~ap+i~aq) will produces a purely sinusoidal waveform. But in order to
achieve unity power factor APF must compensate for qfrom component iaq. Thus,
i−(i~ap+i~aq+iaq) will produce purely sinusoidal waveform with unity power factor.
Thus, inverse transformation iap will produce reference current iS¿ for each phase.iap
can deduced from p which is filtered out using low pass filter from p.
1
p
Product1
Product
4
Ialpha
3
Valpha
2
Ibeta
1
Vbeta
Fig 6.4 Block diagram for calculation of p
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Fig 6.5 p fromp using Low Pass filter
6.4 DC-Bus Voltage Control
Under a loss free situation, the shunt APF need not provide any active powerto cancel the reactive and harmonic currents from the load. These currents show up as reactive power. Thus, it is indeed possible to make the DC-bus capacitor delivers the reactive power demanded by the proposed shunt APF. As the reactive power comes from the DC-bus capacitor and this reactive energy transfers between the load and the DC-bus capacitor (charging anddischarging of the DC-bus capacitor), the average DC-bus voltage can be maintained at a prescribed value.
However, due to switching loss, capacitor leakage current, etc., the distribution source must provide not only the active power required by the load but also the additional power required by the VSI to maintain the DC-bus voltage constant. Unless these losses are regulated, the DC-bus voltage will drop steadily.
A PI controller used to control the DC-bus voltage is shown in Figure6.6. Its transfer function can be represented as
H (s )=K p+K I
swhere K p is the proportional constant that determines the dynamic response of theDC-bus voltage control, and K I is the integration constant that determines its settling time.
1
ploss
Subtract
PID
PID Controller
2
constant
1
Vdc
Fig 6.6 PI controller for DC-bus voltage control(Note:Kd=0∈above PID cotroller)
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p p
It can be noted that if K p and K I are large, the DC-bus voltage regulation is dominant, and the steady-state DC-bus voltage error is low. On the hand, if K p and K I are small, the real power unbalance give little effect to the transient performance. Therefore, the proper selection of K p and K I is essentially important to satisfy above mentioned two control performances.
6.5Reference Current Calculation:
Reference Currents are calculated from inverse clark transformation.
2
Isbeta
1
Isalfa
Product3
Product2
Product1
Product
Divide1
Divide
4
ploss
3
pdc
2
Vbeta
1
Valpha
Fig 6.7 Block diagram for calculation of I s∝∧I sβ
3
Ic*
2
Ib*
1
Ia*
Subtract
-K-
Gain3
-K-
Gain2
-K-
Gain1
-K-
Gain
2
Isbeta
1
Isalpha
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Fig 6.8 Reference Current calculation I a¿ , I b
¿∧I c¿
6.6 Hysteresis Band Current Controller:
It is introduced in chapter 3 section 3.5.2
6
C2
5
C1
4
B2
3
B1
2
A2
1
A1
Subtract3
Subtract2
Subtract1
Relay3
Relay2
Relay1
NOT
LogicalOperator2
NOT
LogicalOperator1
NOT
LogicalOperator
6 Isc
5 Isb
4 Isa
3
Isc*
2
Isb*
1
Isa*
Fig 6.9 Hysteresis Band Current Controller
Actual source currents (iSa ,iSb , I Sc) are compared with the reference currents iSa¿ ,iSb
¿ , I Sc¿
generated by the control algorithm in the hysteresis-band current controller. Three
hysteresis-band current controllers generate the switching pattern of the VSI. The switching
logic is formulated as follows
If iSa< (iSa¿
HB) higher switch is OFF and lower switch is ON for leg “A” (QA=1)
If iSa> (iSa¿
+ HB) higher switch is ON and lower switch is OFF for leg “A” (QA=0).
The switching functions of QB and QC for legs ‘‘B’’ and ‘‘C’’ are determined similarly,
using corresponding reference and measured currents and hysteresis bandwidth (HB). The
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hysteresis-band current control is the fastest control method with minimum hardware and
software but variable switching frequency is its main drawback
6.7 Compensator:
Switching is done according to gating signals from Hysteresis Band Current Controller.
Capacitor Voltage is continuously measured and fed to PI controller as explained earlier.
1
Vdc
3
c
2
b
1
a
v+-
Voltage Measurement3
gm
12
gm
12
gm
12
gm
12
gm
12
gm
12
C
6 g32
5 g31
4 g22
3 g21
2 g12
1 g11
Fig 6.10 Compensator
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6.8 Non-Linear Loads
Case:1 Thyristor Converter Supplying to DC motor equivalent
Synchronization Voltages
DC motor equivalent circuit
PI Curent Regulator
LlacRlac
3 C
2 B
1 A
v+-
Vca
v+-
Vbc
v+-
Vab
g
A
B
C
+
-
Thyristor Converter
alpha_deg
AB
BC
CA
Block
pulses
Synchronized6-Pulse Generator
1s
Id_Refence
i+ -
Id
5
100
90
0
Fig6.11 Block Diagram for Thyristor Converter controlled DC motor
Using PI controller DC motor current value is maintained at 20 Amps. PI controller varies
alpha of thyristor until motor current matches reference current. Pulse width is takes as 15°.
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case2:Diode Rectifier supplying to pure resistive load
3 C
2 B
1 A
Diode5
Diode4
Diode3
Diode2
Diode1
Diode
Fig 6.12 Block diagram for Diode rectifier supplying to pure Resistive Load
A pure resistive load is taken in order to APF performance. As in this load phase current
varies in abrupt manner on the contrary to RL load where load phase current is smooth
varying curve.
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CHAPTER 7 SIMULATION RESULTS
7.1 Case 1: Thyristor converter supplying to DC motor Equivalent(R-L Type Load)
FiFig 7.1 Source Voltages and Load Currents with APF(Case 1)
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V Sa
I Lb
I La
V Sb
I Lc
V Sc
Fig 7.2 Harmonic Analysis of Load Current with APF(Case 1)
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I Sa¿
Fig 7.3 Reference Current I Sa¿
(Case 1)
Fig 7.4 Source Current with APF(Case 1)
Fig 7.5 Compensating Current and Load Current(Case 1)
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I CaI La
I Sa
Fig 7.6 Source Voltage and Source Current with APF(Case 1)
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I Sb
V Sb
V Sa
I Sa
V Sc
I Sc
Fig7.7 Harmonic Analysis of Source Current (Case 1)
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Fig7.8 DC Capacitor Voltage for three-phase APF(Case 1)
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V c
7.2 Case 2: Diode Rectifier supplying to pure resistive
Fig 7.9 load Source Voltage & Load Current with APF
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V Sa
I La
V Sb
I Lb
V Sc
I Lc
Fig 7.10 Harmonic Analysis of Load Current
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Fig 7.11 Source Current after Compensation(Case 2)
Fig 7.12 Compensating Current and Load Current(Case 2)
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I Sa
I Ca I La
Fig 7.13 Source Voltages and Source Current(Case 2)
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V Sa
I Sa
V Sb
I Sb
V Sc
I Sc
Fig 7.14 Harmonic analysis of Source Current(Case 2)
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Fig 7.15 DC Capacitor voltage for three-phase APF(Case 2)
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7.3 Simulation Result Discussion:
As the source current and voltage are in phase,also the source current is almost
sinusoidal(very low THD) it can be said that source is providing only active power required
by the circuit. In instantaneous power theory view, source current is providing only average
real power component(p) while remaining components i.e real oscillating power(~p),
imaginary average power(q) and imaginary oscillating power(~q), is being provided by
Shunt APF.(see Discussion in section 6.3 )
From source currents and THD in case1 (RL load) and case 2 (purely resistive
load )it can be said that the effectiveness of the active filter in compensating for harmonic
components of the load current depends on the specific load current waveform involved.
Two different waveforms may have the same rms harmonic content but the active filter may
do a better job of compensating for one of the waveforms because of the waveshapes
involved. Source current has very less THD in case of RL load compared to purely resistive
one. Thus it can be inferred performance of shunt APF with RL load is much better than
purely resistive load.
In general, the current waveform of an ac regulator with resistive load is an example
of the waveshape that poses the severest challenge for an active filter. The problem is the
high di/dt that is required of the filter to compensate for the high di/dt at turn on of the
regulator. The problem is most severe when the regulator is turned on with a firing angle
close to 90 degrees because this is when the available driving voltage stored on the dc
capacitor is at a minimum. The output di/dt capability can be raised either by increasing the
dc voltage setting or by reducing the size of the interfacing inductance. The limiting factor
for increasing the dc voltage is the voltage withstand capability of the IGBT devices. The
limiting factors for reducing the interfacing inductance include the IGBT di/dt withstand
capability, control requirements, the interface passive filter requirement, and overall system
stability. If the interfacing inductance becomes too small, the dc voltage cannot be kept
constant for normal operation.
From harmonics analysis of Source Current it can be seen due to uneven switching
of compensator large number of interharmonics are introduced. But,it should be noted that
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those components have very less magnitude.(Maximum magnitude of interharmonic is 0.11
% in case 1)
Using PI Controller DC capacitor is maintained at reference value. It was seen that
Settling time improved drastically using PI controller.
It is worth to also to note that p-q based APF can be used for complete harmonic
elimination not selective harmonic elimination.
7.4 Future Scope
As p-q theory can be implemented in three-phase with excellent results in terms of
THD, transient response, reference current generation. The work on extending use of p-q
theory in single phase APF is being done[13].
Switching required in APF is very high in order of 10 kHz. Resulting in appreciable
amount of power. Thus, one can further work on to reduce switching frequency and to
switching losses.
One can also work on linear control technique to replace hysteresis band
controller .So, that irregular switching in compensator can be removed.
Study of Control system of APF is also a possibility n order to get lesser steady state
error and improved settling time. Most importantly to study various APF techniques and
comparing them in terms of dynamic response, performance under various type of load, total
harmonic compensation is to be done.
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CHAPTER 8CONCLUSION
The validity in terms of eliminating p-q theory in terms of eliminating harmonics and power
factor improvement is confirmed from low THD source current which is in phase with source
voltage. But p-q theory utilizes large number of sensors and reference current calculation block.
Large number of calculation in p-q theory demands higher processing power. Resulting in utility to
be complex and expensive. The p-q theory base APF is predominantly utilized in three phase circuits
thus can not be used at remote single phase customer. As a result, Harmonics are present in large part
of system. From source currents of the both cases (i.e. RL Load and purely resistive load) it can be
inferred that APF is most effective when the load current waveform does not have abrupt changes.
The overall filtering effectiveness depends significantly on the types of loads being compensated. As
a result, it is very effective for most voltage source inverter-type loads, even when the distortion is
high.
From comparing reference current and source waveforms it can be concluded that hysteresis
band current controller done the compensation at the cost of high switching frequency. Which can
result in high switching losses in practical high power APF applications. PI controller performance is
also validated from the DC-bus capacitor voltage which shows decreased settling time.
In theoretical view p-q theory has some shortcomings which need to be addressed. Like
mathematical expression of instantaneous power does not fallow power conservation and real and
imaginary power needed to be more accurately defined as zero sequence instantaneous power can
not be defined by the theory. In practical approach also it can be noted that p-q theory is incapable of
providing selective harmonic elimination and specific power factor compensation.
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References
1.H. Akagi, Y. Kanazawa and A. Nabae, "Generalized Theory of Instantaneous Reactive
Power and Its Applications," Transactions of he lEE-Japan, Part B, vol. 103, no.7, 1983, pp.
483-490
2.Power Quality C.Sankaran
3.H. Akagi. “New trends in active filters for power conditioning”, IEEE Trans. on Industry
Applications, vol. 32, pp. 1312-1322, (1996).
4. Das, J. C. Passive Filters – Potentialities and Limitations. IEEE Trans. On Industry
Applications. 2004. 40(1): 232-241.
5. Power Electronics Handbook CRC PRESS
6. El-Habrouk, M., Darwish, M. K., and Mehta, P. Active Power Filters: A Review. Proc.
IEE Electric Power Applications. 2000. 147(5): 403-413.
7. Characteristics of Three Phase Active Power
Filter using Extension pq Theory. Proceedings of the IEEE International
Symposium on Industrial Electronics (ISIE). July 7-11, 1997. Guimaraes,
Portugal: IEEE. 1997. 302-307.
8.Chen, C. L., Chen, E. L., and Huang, C. L. An Active Filter for Unbalanced
Three-Phase System using Synchronous Detection Method. Proceedings of
the Power Electronics Specialist Conference (PESC). June 20-25, 1994.
Taipei, Taiwan: IEEE. 1994. 1451-1455.
9.Chen, D. –H. and Xie, S. –J. Review of Control Strategies Applied to Active
Power Filters. Proceedings of the IEEE International Conference on Electric
Utility Deregulation, Restructuring and Power Technologies (DRPT). April
5-8, 2004. Hong Kong: IEEE. 2004. 666-670.
10.Textook of “Modern Power Electronics and AC Drives”, B.K.Bose
11.Instantaneous p-q Power Theory for Compensating Non-sinusoidal Systems
E. H. Watanabe, Senior Member, IEEE, H. Akagi, Fellow, IEEE and M. Aredes, Member,
IEEE
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12.Instanteneous Power Theory and applications to power conditioning, IEEE Press, H.
Akagi, E. H. Watanabe, M. Aredes.
13. M. Tarafdar Haque “SINGLE-PHASE PQ THEORY”, IEEE Trans.
14 “Active filter design and specification for control of harmonics in industrial and commercial facilities”, Mark McGranaghan Electrotek Concepts, Inc.
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