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.

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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

πis observed at 0ω ω= . Besides that,

( )

( )o

i

x s

x sbehaves 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 20

( ) 2( )

( ) 2 ω= =

+ +o a

i a

x s k sH s

x s s k s (1)

02 2 2

0

( ) 2( )

( ) 2

ω

ω= =

+ +

oq a

i a

x s kH 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 ωω

ω ω

+ + =+

1 1 1 1 1 1*

2 21 1

( ) ( ) ( ) ( ) ( ) ( ) ( )( )

( ) ( )

dcv t v t i t v t i t v t P wti 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 15 Ω

Diode bridge rectifier

with R-L load.

Ri2

Li2

RL2

L2

0.1Ω

0.8mH

13 Ω

107mH

Diode bridge rectifier

with R-C load.

Ri3

Li3

RL3

C3

0.25Ω

0.7mH

70 Ω

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 5 Ω

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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International Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 1,

2013, pp. 162 - 170, ISSN Print : 0976-6545, ISSN Online: 0976-6553,

[24] Dr. R Prakash and Kiran R, “Modelling & Simulation of Active Shunt Filter for

Compensation of System Harmonics”, International Journal of Electrical Engineering &

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ISSN Online: 0976-6553,

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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