I J E E S V© I
OLUME
4 • NUMBER 1 • JUNE 2012
SCIENCE PRESS
NTERNATIONAL
A Simplified Indirect Control Scheme for Shunt Active Power Filters
M. ANBARASAN1 AND RASHMIRANJANDAS2
Abstract: This paper presents a simplified indirect control scheme for shunt
active power filters for compensating harmonic currents and improving power
factor. The control is based on the simple method for generation of the reference
compensating current. The reference compensating current is obtained by
subtracting load current from reference source current. PI controller is used to
adjust the amplitude of the reference source current that regulates the DC
capacitor voltage. Hysteresis band current controller is used to control the shunt
active power filter output current with less switching ripples. The effectiveness
of the proposed control scheme for the single phase and three phase shunt active
power filters are analyzed through simulations using Matlab/Simulink software.
Index Terms: Active power filter, harmonic currents, reference source current,
hysteresis band current controller
I.
INTRODUCTION
Rapid advancements in the power electronics technology have resulted in the usage
of various power electronics equipments for both industrial and commercial
applications. This widespread use of power electronics equipments pollutes the
power system with harmonic currents due to its nonlinear nature. The harmonic
currents causes many adverse effects such as low power factor, overheating of
power system components,EMI problems[1,2] .Conventionally shunt passive LC
filters are used to compensate harmonic currents and improvement of power factor.
However they offer many disadvantages like large size, possibility of resonance
and fixed compensation characteristics [3,4].
Shunt active power filters are considered as the effective tool for solving
problems caused due to the harmonic currents. Shunt active power filters are
broadly classified into two types namely single phase and three phase shunt active
1
2
PG Student, Power Electronics and Drives Deivision, VIT University, Vellore, India, E-mail:
anbu_vit2010@yahoo.com
Senior Assistant Professor, Control System Division, VIT University, Vellore, India, E-mail:
rashmiranjandas@vit.ac.in
56
M. Anbarasan and Rashmiranjandas
power filters. Single phase shunt active power filters are limited to low power
applications and three phase shunt active power filters are widely used in high
power industrial applications[5]. The control of shunt active power filters are
basically divided into direct current control[6]­[8] and indirect current
control[9]­[11].
For the three phase shunt active power filters “p­q theory” is widely used,but it
fails to work when supply voltage is distorted or unbalanced[15]. In this paper a
simple indirect control scheme is proposed, which can be applied for both single
phase and three phase shunt active power filters topology.The performance of the
proposed control scheme is analyzed through simulations using Matlab/Simulink
software.
Fig. 1. shows the simple representation of shunt active power filter. The
compensation is achieved by injecting the compensating current at the point of
common coupling (PCC). The compensating current is characterized by equal
amplitude but opposite in direction to that of load current harmonics component.
Figure 1: Simple Configuration of Shunt Active Power Filter
A Simplified Indirect Control Scheme for Shunt Active Power Filters
57
Many research papers published for single phase shunt active power
filters[12]­[14] are seems to be complicated.
II. GENERATION OF REFERENCE COMPENSATING CURRENT 1 1
At instance consider single phase shunt active power filter for reference
compensating current generation. In this control, supply voltage is assumed to be
sinusoidal and it is given by
The nonlinear load current is given by
∞
iL(t) = ∑ In sin (nωt + θn )
n =1
(2)
Eqn(2) contains fundamental components and harmonics component.
Therefore, the instantaneous load power can be expressed as
PL (t) = vs(t) iL(t)
= I1Vm sin2(ωt) cos θ1+
I1Vm sin(ωt) cos(ωt) sin θ1 +
∞
∑V
n=2
m
sin(ωt) In sin(nωt + θn )
= pa(t) + pr(t) + ph(t)
(3)
In eqn(3) pa(t) is the active power demanded by the load, pr(t) is the reactive
power demanded by the load and ph(t) is the harmonic power. For compensation
Shunt active power filter should provide only reactive power and suppress harmonic
power.
The active fundamental current component is given by
ia(t) = I1cos θ1sin(ωt)
(4)
Source is expected to supply only active fundamental current component with
compensation.
Reference source current to be generated is given by
i* sm(t) = I* smsin(ωt)
(5)
In eqn(5) I*sm represents the peak value of the active fundamental current
component.
I* sm = I1cos θ1
(6)
For the generation of reference compensating current, initially reference source
current has to be generated. Generation of reference source current is achieved by
maintaining the DC capacitor voltage of the shunt active power filter as constant.
M. Anbarasan and Rashmiranjandas
58
Control of DC capacitor voltage is done by controlling the amplitude of the source
current,this is done by the following method as follows
The Source current with compensation can be given as
is(t) = I* smsin(ωt)
(7)
The active power supplied from the source is given by
ps(t) = vs(t) is(t)
=
1
1
Vsm I * sm − Vsm I * sm cos(2ωt) = Ps + p s (t)
2
2
(8)
In eqn(8). P s is the DC component, and p s (t) is the AC component. The
instantaneous power consumed by the load is given by
pL(t) = vs(t) iL(t) = PL + p L (t)
(9)
Where PL is the DC component and is given by
PL =
Vm I1 cos θ1
2
(10)
p L (t) is the AC component and it is given by
−Vm I1
cos(2ωt + θ1 )
2
∞
V I
+ ∑ m n (cos((n − 1)ωt + θn )
2
n=2
− cos((n + 1)ωt + θn ))
p L (t) =
(11)
The compensating power generated by the shunt active power filter is given by
pc(t) = ps(t) – pL(t)
= Ps + p s (t) − PL − p L (t)
= Pc + p c (t)
(12)
In eqn(12). Pc represents the DC component of the compensating power, and
p c (t) represents the AC component of the compensating power. If the power loss of
the shunt active power filter is assumed to be a constant and it is represented as
Ploss, the magnitude of Pc can be given by
Pc =
1
(Vm I * sm − Vm I1 cos θ1 )
2
(13)
A Simplified Indirect Control Scheme for Shunt Active Power Filters
59
The energy balance in the DC capacitor is given by
1
C ∆ Vc2 = ( Pc − Ploss )∆t
2
(14)
Thus the average voltage of the DC capacitor is given by
∆Vc =
(Vm I * sm − Vm I1 cos θ1 − 2 Ploss )∆t
C
(15)
The average voltage of the DC capacitor can be regulated by adjusting the
amplitude of the reference source current I*sm.
Figure 2: Generation of Reference Source Current
Fig. (2) shows the generation of reference source current. In this method the
reference sinusoidal signal is obtained by sensing the supply voltage and dividing
it by the peak value of the supply voltage. The difference in source and load active
power is reflected in terms of the average voltage across the capacitor,based on the
difference in actual and reference voltage (referred as voltage error), PI controller
gives output thereby adjusting magnitude of the reference source current. I *sm is
obtained by PI controller as
I * sm = K p (vcref − vc (t)) + K i ∫ (vcref − vc (t))dt
(16)
The reference compensating current is obtained by subtracting the load current
from the reference source current,and it is given by
i*c(t) = i*sm(t) – iL(t)
(17)
III. HYSTERESIS BAND CURRENT CONTROLLER
The power circuit of the shunt active power filter consist of voltage source
inverter(VSI) operated in current mode. Hysteresis band current controller provides
the switching pulses for the switches of the shunt active power filter.The reference
compensating current i*c(t) and actual compensating ic(t) are compared within the
M. Anbarasan and Rashmiranjandas
60
hysteresis band.Hysteresis band current controller reduces the ripples in the output
waveform.Hysteresis band current controller helps the shunt active power filter to
produce compensating current that exactly tracks the reference compensating
current.
Figure 3: Hysteresis band Current Controller
Switching logic is formulate as follows:
If ic > (i* c – HB) upper switch is OFF and lower switch is ON for the same leg.
If ic > (i* c + HB) upper switch is ON and lower switch is OFF for the same leg.
HB is the hysteresis bandwidth
IV. MATLAB/SIMULINK MODELS
Figure 4: Proposed Indirect Control Scheme for Single Phase Shunt APF
A Simplified Indirect Control Scheme for Shunt Active Power Filters
Figure 5: Simulink Model for Single Phase Shunt APF with Control
Figure 6: Proposed Indirect Control Scheme for three Phase Shunt APF
61
62
M. Anbarasan and Rashmiranjandas
As seen from the Fig. (6) for the three phase shunt active power filter all this
procedures are applied individually to their phases, but only one PI controller is
enough for producing ‘ism’ corresponding to each phase.
Figure 7: Simulation Results and Analysis
A Simplified Indirect Control Scheme for Shunt Active Power Filters
63
Table 1
System Parameter Values for Single Phase APF
Sytem parameters
Values
Supply peak voltage
Ls, Rs
Lc
Ll,Rl
APFcapacitor
DC capacitor voltage
Proportional constant (kp)
integral constant (ki)
Hysteresis band (HB)
325V
1mH,0.1ohm
2mH
10mH,5ohm
4000 microfarad
500V
0.1
20
1
Table II
System Parameter Values for three Phase APF
Sytem parameters
Values
Supply ph­ph rms voltage
Ll,Rl
Lca=Lcb=Lcc
APF capacitor
DC capacitor voltage
Proportional constant (kp)
integral constant(ki)
Hysteresis band(HB)
380 V
10 mH, 30 Ohm
3.5 mH
2200 microfarad
700 V
0.08
0.5
1
Figure 8: Simulated Waveforms of Single Phase System with APF
64
M. Anbarasan and Rashmiranjandas
Fig. 8. shows the simulated waveforms of source current after compensation,
compensating current and load current. It is known that source current is same as
that load current before compensation.
Figure 9: Harmonic Spectrum of Source Current without Compensation
Fig. 9. shows the source current without compensation has the THD of 29.51%.
It is clear that odd harmonic currents are present in the source current without
compensation.
Figure 10: Harmonic Spectrum of Source Current
with Compensation
Fig. 10. shows after compensation the source current THD is reduced to 2.34%
and power factor is improvedtounity.
A Simplified Indirect Control Scheme for Shunt Active Power Filters
65
Figure 11: Simulated Wave form of Source Current Showing Phase a, b, c
without Compensation
Fig. 11. shows the source current before compensation, which is also same as
that of load current.
Figure 12: Simulated Wave form of Source Currents Showing Phase a, b, c
with Compensation
Figure 13: Simulated Wave form of Compensating Currents
M. Anbarasan and Rashmiranjandas
66
Figure 14: Harmonic Spectrum of Source Current without Compensation
Fig. 14. shows the source current without compensation has the THD of 27.38%.
Figure 15: Harmonic Spectrum of Source Current with Compensation
Fig. 15. shows after compensation the source current THD is reduced to 3.58%
and power factorisim proved to unity.
VI. CONCLUSION
The proposed indirect control scheme can be applied effectively for both single
phase and three phase shunt active power filters. The proposed scheme is easy for
the implementation.The performance of the proposed control is analysed through
simulations and founded that the proposed control scheme is very much effective
in compensating the harmonic currents present in the supply current and improves
the power factor near to unity
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A Simplified Indirect Control Scheme for Shunt Active Power Filters
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