Indirect current controlled single phase shunt active filter

International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME
264
INDIRECT CURRENT CONTROLLED SINGLE PHASE SHUNT ACTIVE
FILTER
Narayan G. Apte1
, Dr. Vishram N. Bapat
1
(Lecturer, Department of Electrical Engineering, Walchand College of Engineering, Sangli,
Maharashtra State, India)
2
(Director, Ganga Instt. Of Technology & Management, 20km, Bahadurgarh-Jajjhar Rd., Kablana,
Dist. Jajjhar, Haryana, India)
ABSTRACT
This paper presents indirect current control scheme for single phase shunt active filter. There
are two control loops, the outer loop for dc bus voltage control and inner loop for tracking current.
The shunt active filter (SAF) reference current, which is the desired instantaneous supply current, is
extracted by using sine signal integrator (SSI) considering the fundamental active current required by
load and that required by SAF to meet losses and maintain dc capacitor voltage. A novel phase
locking technique for single phase based on SSI is also proposed. Simulation results are reported to
validate effectiveness of the proposed technique.
Keywords: Shunt Active Filter (SAF), Sine Signal Integrator (SSI), Instantaneous Reactive Power
(IRP), indirect control, reference current generation, harmonics.
1. INTRODUCTION
Power electronics based systems form a major constituent of today’s power processing used
at transmission and distribution and domestic levels. These devices/equipment draw
chopped/distorted current from the utility which gives rise to presence of unwanted frequency
components (harmonics) in the current drawn. Power quality problems arise due to harmonics. Shunt
active filters, series active filters, and their combination are employed to mitigate power quality
problems. With emergence of distributed generation (DG), which are essentially single phase
networks, there has been significant research and development on single phase active filters.
Harmonic compensation and reactive power support are the principle issues of a single phase system
that need to be handled by a single phase SAF. Fig. 1 shows basic schematic for single phase SAF.
Shunt active filters (SAF) basically works by controlling switching of the inverter switches with an
objective of minimizing harmonic and reactive currents in supply current.
INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &
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ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 4, Issue 4, July-August (2013), pp. 264-273
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265
The critical task of the SAF controller is to generate reference current. The SAF control
consists of two loops. The outer loop controls dc voltage across capacitor. The inner current loop
controls switching of inverter switches in order to track SAF reference current. The accuracy of the
SAF current reference determines performance of SAF. Reference current generation needs
particular attention since the theories are developed for three phase
Figure 1: Basic Schematic block diagram of single phase SAF
applications and the same are not directly applicable to single phase case. Different techniques for
reference current generation for single phase SAF applications are reported in [1-12]. In [3], the
methods are classified in direct and indirect methods. The direct method involves sensing load
current and extracting harmonic and reactive component. The current controller injects these
harmonic and reactive current components with same magnitude but opposite phase. The direct
methods include the Instantaneous Reactive Power (IRP) theory [5-8], the Synchronous Reference
Frame (SRF) theory [12-15] and the Fourier transform method [1]. In indirect method, a sinusoidal
reference is generated by means of grid voltage sensing. The inverter switching is controlled in such
a way that supply current is dictated to follow sinusoidal supply voltage reference. The main
advantages of this method are that it requires only low bandwidth sensor and has a faster transient
response [14]. The indirect methods reported in literature are based on controllers such as
Proportional-Integral (PI) or an Enhanced Phase Locked Loop (EPLL) to find the reference current
[2][3]. Some of these methods are sensitive to voltage distortion, require several control parameters
and introduce delays on the reference current [13]. The IRP and SRF theories are the most sought
after techniques addressed in relevant literature but originally developed for three phase systems. In
three phase case, both IRP and SRF techniques operate in a reference system with two orthogonal
axes (αβ for IRP and dq for SRF). The same logic is not directly applicable for single phase as the
system possesses only one variable. Therefore, it is necessary to have a ‘fictitious’ or imaginary
variable which is phase shifted by 900
at all frequencies. To maintain orthogonality of the original
and imaginary variable at all frequencies poses problem particularly for load current with high
harmonic content. IRP and SRF theories for single phase applications are discussed in [7][15][16]
and for creating orthogonal variable, delay blocks have been adopted. In this paper, SSI based
reference current generation has been used for indirect control of single phase SAF. Fundamental

266
active component of the load current is extracted from load current signal. Additionally, the outer
voltage control loop decides the active component to be absorbed by SAF. The linear current
regulator tracks the supply current and making SAF to indirectly compensate for the reactive and
harmonic current of the load. An SSI based single phase PLL has also been proposed. Use of SSI for
detecting fundamental component of supply voltage makes the system insensitive to supply voltage
distortion. This paper is organized in following sections. Section 2 describes the basic SSI control
technique. Section 3 proposes a technique for SAF reference current generation. The design of phase
locking system based on SSI is discussed in section 4. Section 5 presents simulation results and
section 6 presents conclusion of the paper.
2. SAF REFERENCE CURRENT GENERATION USING SSI
In a single-phase system, the use of a rotating frame is not possible unless a virtual system is
coupled to the real one in order to simulate a two-axis environment. One common requirement of
single phase IRP and SRF reference current generation technique is necessity to create a fictitious
orthogonal or imaginary signal in which all frequency components are phase shifted through 900
electrical degrees with respect to original variable. Most common techniques to create such
orthogonal component are to use Hilbert Transformation [7] or to use FIR filter. The drawback of
Hilbert transformation is that it leads to non-causal system and cannot be implemented directly. FIR
filter may cause some phase delays in the orthogonal variable.
Alternatively, computation of the SAF reference current can be performed using Sine Signal
Integrator (SSI) with less computational burden [17]. The SSI presents finite gain at the desired
frequencies and adjustable bandwidth through the gain ka. The block diagram of the SSI is shown in
Fig. 2
Figure 2: Block diagram of the SSI
The input signal is xi(t), while the output signal in phase with the input signal is xo(t). The
output quadrature signal is xoq(t). The transfer function for each output correspond to (1) and (2)
respectively and their bode diagrams are as shown in Fig.3. In this diagram, a difference of phases of
2
π
is observed at 0ω ω= . Besides that,
( )
( )
o
i
x s
x s
behaves as a band pass filter and the bandwidth is
determined by value of ka and
oq
i
x s
x s
( )
( )
behaves as a low pass filter.
1 2 2
0
( ) 2
( )
( ) 2 ω
= =
+ +
o a
i a
x s k s
H s
x s s k s
(1)
0
2 2 2
0
( ) 2
( )
( ) 2
ω
ω
= =
+ +
oq a
i a
x s k
H s
x s s k s
(2)

267
In steady state operation the relationship between the phases of the transfer functions and in
frequency domain is,
H s H s1 1( ) ( )
2
π
∠ = ∠ + (3)
The inherent capability to produce quadrature component is utilized in generating
instantaneous fundamental reactive power component of the load.
3. SAF REFERENCE CURRENT GENERATION
As per IRP theory, the instantaneous powers in a single phase system can be expressed as,
( ) ( ) ( )( )
( ) ( ) ( )( )
v t v t i tp t
v t v t i tq t
α β α
β α β
ω ω ωω
ω ω ωω
    
=      −     
(4)
The instantaneous powers can be expressed in terms of dc and ac quantities as,
( ) ( ) ( )
( ) ( ) ( )
p t p t p t
q t q t q t
ω ω ω
ω ω ω
+   
=   +   
%
%
(5)
For indirect control, the quantity of interest is ωp t( ), which is fundamental active power.
The fundamental active power, ωp t( )can be calculated by knowing fundamental voltage
components α ωv t1 ( ), β ωv t1 ( ) and current components α ωi t1 ( ), β ωi t1 ( ) and the resulting SAF
current reference can be expressed as,
α α α β β α
α
α β
ω ω ω ω ω ω
ω
ω ω
 + + =
+
1 1 1 1 1 1*
2 2
1 1
( ) ( ) ( ) ( ) ( ) ( ) ( )
( )
( ) ( )
dcv t v t i t v t i t v t P wt
i t
v t v t
(6)
(a) (b)
Figure 3: Bode Diagrams of (a) H s1( )and (b) H s2 ( )

268
The block diagram of the reference current generation technique is shown in Fig. 4. The
feedback for instantaneous frequency ω is taken from a SSI based phase synchronization block
discussed later in this paper. Pdc is the required power to be absorbed by SAF to meet switching
losses and maintain constant dc bus voltage. The current reference α ω*
( )i t goes to linear current
controller.
Figure 4: Block of the reference current generator for indirect control
4. PHASE SYNCHRONIZATION
A rigid phase angle detection mechanism is an integral part of grid tied VSIs. Conventionally,
Phase locked loops (PLL) have been used for the same. The PLL structure is a feedback control
system that automatically adjusts the phase of a locally generated signal to match the phase of an
input signal. Among the various solutions to compute instantaneous phase angle SRF method is more
preferred for single phase systems. SRF based single phase PLLs are presented in [18-21]. In
general, In general, a specific "filtering" techniques has been used in order to deliver a non distorted
signal to the SRF-PLL. In the present work, a single phase PLL based on Sine Signal Integrator (SSI)
has been proposed. Use of supply frequency tuned SSI eliminates the need of additional filters to
generate orthogonal component. The proposed PLL is simple in structure with better performance in
case of distorted supply and frequency variations. The block diagram of the proposed PLL structure
is shown in Fig. 5. SSIV block generates αβ components of the supply voltage. Theαβ components
are further transformed to dq frame by using appropriate transformation. The closed loop control
system tries to maintain qV =0. The dV represents peak of the fundamental supply voltage. The
frequency tracking performance is demonstrated in Fig 6.
Figure 5: Block Diagram of SSI based single phase PLL

269
Figure 6: Frequency tracking performance of SSI based single phase PLL
Simulation of SAF
The general schematic diagram of the simulated SAF for compensation of non-linear loads is
shown in Fig. 7. It consists of a single phase supply, a generic load and SAF unit. The procedure of
the design of SAF is summarized in following steps:
• Collection of system parameters
• Define performance specifications
• Design of power circuit
• Control system parameter setting
Since SAF should meet the IEEE-519 (1992) requirements, the harmonic levels of the supply voltage
and current after compensation should lie below the levels prescribed in the standard. Design of
power circuit is based on following points mentioned below. Table I lists parametric values of the
simulated model.
• Selection of VA rating of SAF.
• Selection of switching frequency.
• Selection of dc bus voltage.
• Selection of coupling inductance.
• Selection of dc bus capacitance.
Figure 7: Schematic diagram of simulated model of single phase SAF

270
Table I: Single phase SAF parameters
The computer simulation is performed using MATLAB/SIMULINK power system block set.
SAF reference current generation is based on the structure shown in Fig. 4. Various parameters,
listed in Table I, are used for simulation. The VSI switches are modeled as IGBTs with anti-parallel
diodes. The IGBTs are assumed ideal except their on-state resistance which amount to switching loss
in the device.
The current ripple generated by the VSI of the SAF power circuitry can spread to the power
line through the PCC where the SAF system is connected to the power system. High frequency
switching harmonics create noise problems for other loads connected to the same PCC. To filter the
high frequency switching ripple currents due to the switching of the inverter, passive switching
ripple filters are placed at the PCC as an integral part of the SAF. Linear current regulators generate
regular PWM ripples around the switching frequency and its multiples and sidebands over the
frequency spectrum. The M-PWM switching technique ca [22] uses the switching ripple to be at 2fsw.
By shifting the switching frequency harmonics to 2fsw, it becomes easy to design switching ripple
filter. A broad-band tuned type switching ripple filter has been used to filter switching ripple as
shown in Fig. 7. The LF, CF branch is a series resonant branch which is tuned to frequency fsw. Rd is a
damping resistor which limits the filter current.
Supply parameters
Source voltage Vs 240V
Nominal Frequency f 50Hz
Source impedance Zs 0.1 + j 0.01256 Ω
Load Parameters
Solid state relay RL1 15Ω
Diode bridge rectifier
with R-L load.
Ri2
Li2
RL2
L2
0.1Ω
0.8mH
13Ω
107mH
Diode bridge rectifier
with R-C load.
Ri3
Li3
RL3
C3
0.25Ω
0.7mH
70Ω
2000 Fµ
Inductive Load QL 10 kVAr
SAF power circuit
parameters
SAF VA rating SSAF 12.5 kVA
Switching frequency fsw 5kHz
Dc bus voltage Vdc 700V
Coupling inductor Lac 2.5mH (0.2Ω )
Dc capacitor Cdc 3500 Fµ
Switching ripple
filter
Filter resistance Rf 5Ω
Filter inductor Lf 0.1266 mH
Filter capacitor Cf 20 Fµ
DC voltage
controller gains
Proportional gain
Integral gain
Sampling time
Kp
Ki
Ts
4e-03 5 %/%
13e-03 %s-1
4e-06 s
Current controller Proportional gain
Integral gain
Kpi
Kii
7 %/%
1e05 %s-1
Sine signal
integrator
Proportional gain
Proportional gain
Kvv
Kvi
10
10

271
The responses of the system using indirect current control are shown in Fig.7. It also exhibits
start-up behavior of the SAF. The harmonic spectrum of source current before and after
compensation is shown Fig. 9 and Fig. 10. It shows significant improvement on the THD of supply
current from 71% to 1% with SAF compensation. The reactive power of the load is completely
compensated by SAF resulting in near unity pf (0.996). Dc bus regulator performance is shown in
Fig. 11. Performance of SAF under distorted utility conditions is shown in Fig. 8. Here the supply
voltage THD is 12 %. The proposed SAF effectively compensates for the load harmonics in
presence of distortion in supply voltage.
Figure 10: Frequency spectrum of supply
current after compensation
Figure 9: Frequency spectrum of load current
Figure 7: Time response of SAF
Figure 8: SAF performance in distorted
supply voltage conditions Figure 11: DC bus voltage controller
response

272
5. CONCLUSION
In this paper, an indirect controlled single phase SAF has been presented. The effectiveness
of the SSI based reference current generation and performance of the SAF has been tested by
computer simulation. A novel phase and frequency detection technique is also proposed. The results
of computer simulations show significant improvement in frequency spectrum of mains current after
SAF compensation which satisfies IEEE519-1992 standard requirements. The proposed SSI based
reference current generation technique also tested under distorted utility conditions and the time
response of the same indicate that the SAF compensation is insensitive to supply voltage distortion.
6. REFERENCES
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7. BIOGRAPHIES
N. G. Apte, received B.E (Electrical) and M.E. (Electrical) in 1993 and 1995
respectively from Walchand College of Engineering, Sangli, Inidia. He is working
as senior lecturer in Electrical Engineering department for last 13 years. His areas of
interest include embedded systems, power electronics applications to power
systems.
Dr. V.N. Bapat, received B.E. (Electrical) and M.E.(Electrical) 1983 and 1985
respectively from Walchand College of Engineering, Sangli, India. He received
Ph.D. from Indian Institute of Technology, Kharagpur in 1993. He has 30 years of
teaching experience. Presently, he is working as Director, Ganga Instt. Of
Technology, Dist. Jajjhar, Hariyana, India. His areas of interest are control systems,
electrical machine design, power quality.

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