[ieee 2014 ieee innovative smart grid technologies - asia (isgt asia) - kuala lumpur, malaysia...

Synchronous Reference Frame Based Control Technique for Shunt Hybrid Active Power Filter

under Non-ideal Voltage Papan Dey

Power Electronics &Renewable Energy Research Laboratory (PEARL)

Dept. of Electrical Engineering University of Malaya

Kuala Lumpur 50603, Malaysia [email protected]

Saad Mekhilef Power Electronics &Renewable Energy Research

Laboratory (PEARL) Dept. of Electrical Engineering

University of Malaya Kuala Lumpur 50603, Malaysia

[email protected]

Abstract— A hybrid compensator which is a combination of a three phase four wire shunt active power filter and a parallel passive filter has been presented in this paper. The dominant lower order harmonics as well as reactive power can be compensated by passive elements whereas the active part mitigates remaining distortions and improves the power quality. Modified phase lock loop based synchronous reference frame control strategy is adopted here for active filtering system. The proposed three phase hybrid line conditioning system can be quite effective for recompensing harmonics, reactive power & neutral current under unbalanced grid conditions. The simulated results obtained by MATLAB/SIMULINK power system block set are examined in detail for the validity of suggested approach. A laboratory prototype has been built on dSPACE1104 platform to verify the feasibility of the suggested SHAPF controller.

Index Terms — Active power filter (APF); phase locked loop (PLL); harmonics; power quality; synchronous reference frame (SRF)

I. INTRODUCTION

Owing to the wide application of power electronic converter continuity of power flow become polluted due to the burden of high reactive power, contaminated harmonically and unbalancing load currents. Hence to deliver clean power several methods were proposed by the researchers [1].

Passive filters whose work as least impedance path to tuned harmonic frequencies Though simple and less expensive but for several drawbacks like fixed compensation, bulky devices and resonance problem of those L-C filters APF has been developed for complete compensation of distortions [2]. The APFs use power electronics converters whose insert harmonic components to the electrical network

as cancelling out nonlinear load harmonic currents. Added with harmonics compensating ability, APFs are also capable for reactive power reduction and load balancing [3]. Due to no resonance problems in the system, APF draws a significant attention over those tuned passive filters. But it is restricted by high cost and low capacity of switching devices. If we use hybrid configuration it brings down the cost of the active filter significantly and be more practicable in industry applications [4].Because of avoiding complexity and unreliability of series hybrid filtering, a shunt active filter base hybrid filter topology has been considered in this paper for its capability of attenuation and effective operation [5, 6]. Generations of current references using the harmonic extraction method, feedback current controlling technique and the APF inverter circuit behavior in dynamic environment are considered as indispensable role for the APF performance. So, for varying loads, it is essential in active power filters to detect components of harmonics in current with quickly and accurately. Many harmonic revealing approaches were suggested and their dynamic performances were also assessed in papers that can be established in the literature. Among those, control scheme based on SRF is one of the utmost conventional and the most practically applicable method [6]. It performs an excellent job but it requires a PLL circuit for synchronization.

In this paper a new technique constructed on SRF using the adapted PLL algorithm is demonstrated and analyzed its performances for unbalance source condition. Here, Hysteresis current regulator is used to generate switching signals of APF and for maintaining the dc link capacitor voltage a proportion-integral (PI) controller is introduced.

The authors wish to acknowledge the financial support from university ofMalaya HIR-MOHE project UM.C/HIR/MOHE/ENG/24.

2014 IEEE Innovative Smart Grid Technologies - Asia (ISGT ASIA)

978-1-4799-1300-8/14/$31.00 ©2014 IEEE 481

Simulation and technical analysis of those result shows the validation of proposed strategy.

The structure of this paper is as following: The hybrid active power filter is discussed in section II. Section III describes the control algorithm of APF .Simulation results and their brief analyses are presented in section IV. Experimental results are presented in section V. Conclusions are drawn finally in section VI.

Figure 1. Basic structure pattern of SHAPF

II. HYBRID ACTIVE POWER FILTER TOPOLOGY

A. Circuit topology Fig. 1 presents the basic structure of SHAPF. The proposed hybrid filter structure comprises of shunt passive filter and shunt active filter. A 3-leg inverter with split capacitor works as APF. It is designed to be associated in parallel with the single phase and three phase loads that’s considered as a non-linear and unbalance load for a 3-phase 4-wire distribution system [7] while its complex control circuitry and massive dc link capacitors are necessary for perfect operation. The inner point of every branch is attached to the power network through an inductor which is used to filter the ripples of inverter current. The considered LC passive filter at 5th harmonic tuned frequency is connected in shunt to the power line before the load. It provides low impedance trap to harmonic to which the filter is tuned and correspondingly aids in reduction in active filter power rating.

III. CONTROL ALGORITHM

A.PLL Operation In order to safe and consistent operation of active power filter under unbalance and distorted grid voltage situation phase and frequency extraction of positive sequence fundamental component of voltage should obtain quickly and accurately[8]. Because of dynamic behavior SRF-PLL performance is dissatisfactory under non ideal voltage mains

[9]. In this study, the improved PLL developed in fig.2 is put forward for determination of positive sequence components with stability and rapidity.

Figure 2. PLL circuit block diagram

The modified PLL circuit use Clarke and park transform for identifying the peak of positive sequence voltage. The three phase unbalance voltages are converted to stationary coordinate system. Through the measured phase angle, the voltages in stationary co-ordination are transformed to DQ. A PI controller forced D-axis component Vd to zero in order to align the mains voltage space vector with Q-axis [10]. The estimated phase angle θ which is attained by integrating the proportion-integral (PI) controller output is in turn used for coordinate transformation process. Here, a second order resonant filter is used for suppress the double fundamental frequency which is created for unbalancing and the gain adaptation block shows convergence of any value of system voltages and creates normalized templates of fundamental [11]. The rate limiter block works as an attenuator against ripple. The reformed PLL can go satisfactorily as long as the gains of PI are adjusted accordingly under highly inaccurate and disturbed system voltages.

For non-ideal mains voltage

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ααβ CosVCosV

SinVSinVVV

V (4)

Performing d-q transform

[ ] αβVTVV

V dqq

ddq =⎥

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=

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)()()()(^^

^^

θθθθθθθθ

CosVCosVSinVSinV (5)

The estimated phase angle= ^θ ; assuming tωθ ≈^ . PLL successfully tracks the phase at −+ == θθθ ^ So,

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)2()2(

^

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θθ

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VV

q

d (6)

Here ^2θ is the double frequency to be eliminated. It is the basic concept of the modified PLL structure. It can provide the positive sequence component by cancellation of 2ω oscillations

B. Reference Current Generation The three phase load currents are measured using hall-effect current sensor and converted into d-q-0 by means of a rotational frame synchronous with the system voltage positive sequence in eqn.1.

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ωst is considered here as the transformation-angle of positive sequence source voltage and it is delivered by the proposed phase lock loop.

Figure 3. SRF based control block diagram

The system under observation is three phase-four wire wherever active component id and oscillating part iq are reflected. After the load currents id and iq are found, they are allowable to pass over a low pass filter to separate ac and dc part where the active and reactive fundamental current components (id-DC, iq-DC) are obtained. The both currents (id-AC, iq-AC) ac parts are related with the responsibility for active and non-active harmonic components. The filters used in the circuit are the 2nd order butter worth type and their cut off frequency is identical to one half of the fundamental frequency. With consideration of the reactive current, Passive filter provides its DC value while VSI delivers an AC voltage to sink the harmonics [12]. The filtered active and non-active currents from eqn.2 are used for generation of the accurate references to the modulator

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As well as providing harmonic currents, the dc voltage of PWM VSI should be maintained for accurate operation. The voltage of capacitor is controlled by regulating the reactive current as shown in fig 3.Then the abc frame reference currents are

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C. DC Link Capacitor Voltage Control The dc voltage can be build up in APF and regulate across dc capacitors by itself. The capacitance is designated such that the voltage ripple is less than 1%. Selection of dc value confirms that the current time derivatives of converter supply must be required for the compensation of selected harmonics [13]. Using this concept the capacitor voltage is chosen from:

max)(...3 ppdc

fdc V

IC

−

=ω

π (10)

In order to control inverter current actively the nominal dc bus voltage Vdc should be larger than or identical to line-line peak voltage [13]. The value of voltage is preferred for a specific non-linear load which is function of rated load power and the compensated maximum distortion. According to the capacity of the system it can be selected as SdcS VVV 2≤< .


483

Figure 4. DC voltage optimization with PI approach

A proportional–integral controller is used to regulate the voltage of DC link and added to active part of the fundamental load current. The obtained capacitor voltage is matched with a set reference value. The loss component is then handled concluded a PI controller which contributes to the zero steady error in tracking the reference current signal and it is shown in fig 4. The supply current peak value is calculated using the output component of PI controller.

D. Harmonic current generator For generation of commanded compensating current switching strategy is developed widely. A hysteresis comparator is used here for quick response and appropriate no.sinusoidal current tracking capability. Moreover it is easy for implementation than other switching methods. The active filter switching patterns are decided by the current controller [13]. The desired current, Ila (t), and the injected inverter current, Ila*(t) are compared with one another. The logic of switching is as followed: If Ila < (Ila*-hb) upper switch S1 is OFF & lower switch S2 is ON in leg of ‘a’ of active power filter. If Ila > (Ila*-hb) upper switchS1 is ON & lower switch S2 is OFF in leg of ‘a’ of active power filter. Correspondingly in the legs’b’ & ‘c’ the switches are initiated. Here ‘hb’ is hysteresis band and determine the

variation of load current harmonics and switching frequency of the devices.

IV. SIMULATION RESULTS

Performance of 3 legs- 4 wire Hybrid active power filter for filtering current distortions, compensation of neutral current, load current balancing and reactive power mitigation have been examined under unbalance source-unbalance load condition. The goal of this case study is to demonstrate the validity and evaluation of proposed approach, if the source is greatly unbalanced. Using power system blockset in matlab-simulink for three phases four wire power network with shunt active and shunt passive filter the presented simulation results are obtained. The results are specified before compensation, compensation using only passive filter and after the operation of hybrid filter in the following simulation studies. Three and single phase diode rectifier nonlinear loads are coupled in power system for producing unbalance, reactive current and harmonics in load current as well as neutral current. Single phase diode rectifier with RL load in DC side is connected in ‘c’ phase for evaluation of the dynamic performance. The widespread simulation outcomes are discussed below.

A. Unbalance Source-Unbalance Load Condition When 3 phase source voltages are not balanced properly, actual values of phase voltages are not identical and negative sequence fundamental voltage component will be present in system voltage. The three phase’s unbalanced voltages are considered as: Va= 100 sin ωt,Vb= 80 sin(ωt-120) andVc= 50 sin(ωt+120) Simulation results with passive filter and hybrid active filter for eliminated harmonic current and load current balancing under this condition are shown in fig.6

d

i

a b c

e f

g

h


484

Figure 6. Simulation results for SHAPF operational performance a) Source voltage b) load current c)THD before compensation d)passive filtering load current e)THD after passive filtering f)Load current after hybrid filtering g) THD after hybrid filtering h)injecting current i) DC link voltage j)instantaneous reactive power k) instantaneous active power l)Source neutral current

Before filtering the non-linear load current THD level is 21.20%. With passive filter alone the three phase obtained currents are not pure sinusoidal. As mains voltage negative sequence part is eliminated with hybrid filter, balanced and sinusoidal load currents are achieved after compensation.

Compensation of reactive current, neutral current elimination and dc link voltage is maintained properly with the proposed method. Comprehensive compensation of load currents and their harmonics level are shown in table1.under unbalance source & unbalance load condition.

TABLE I. SIMULATION RESULTS FOR UNBALANCE SOURCE

Load current Before filter Passive filter HAPFTHD (%) A-phase 21.20 14.69 1.98 B-phase 48.24 26.59 1.83 C-phase 9.87 6.17 1.14 Neutral 61.25 - - RMS(A) A-phase 1.58 2.31 7.02 B-phase 0.88 1.42 6.8 C-phase 3.12 3.43 7.05

Neutral 5.23 - 0.82

V. EXPERIMENTAL RESULTS The performance of the power inverter is first verified under balanced grid condition. Before turning on the inverter source voltage is sinusoidal but load current is highly nonlinear in nature and its frequency spectrum measured by Fluke power quality analyzer is shown and its THD is 28% for phase ‘a’. After using the 5th harmonic tuned passive filter the value of load current THD is reduced with 11.2%. But when APF is connected in shunt with line and passive filter, the harmonic contains in load current is supplied by APF and load current approaches to sinusoidal. The THD of source current of phase A after compensation is reduced to 5% which is under IEEE standard limit.

Figure 7..Experimental results for unbalance source a) Supply voltage (25v/div) b) Load current before compensation (0.8A/div) c) %THD before compensation d) Load current after passive filtering e) %THD after passive filter f) DC link voltage and load current after compensation g) %THD after SHAPF compensation h) injected filter current i) PLL output (sinwt, coswt vector)

j k l

a b c

d e f

g h i


485

TABLE II. SIMULATION PARAMETERS

Parameters of the SHAPF system used in this study are listed in table II. In addition a comparison between proposed topology and [3], [9] is shown in table III in terms of control method, THD% and balancing the load current according different conditions. The harmonic extraction algorithms are built on same concept with slight modification and DC link voltage control is maintained through considering the active or reactive part of the filtered ac current. Regarding their performance it is evaluated and the proposed technique formed better compensation. TABLE III.COMPARISON BETWEEN RECOMMENDED TOPOLOGY AND [3], [9]

Topology description Property

Hybrid filter(shunt

active +shunt passive)

proposed

Shunt active (Zaveri et al.,

2012)

UPQC,(Kesler &Ozdemir, 2011)

Control Method Modified SRF,Vdc=220 v

Idq control

SRF, Vdc=700v

PLL Pre-filter+ SRF PLL

- Modified PLL

RMS current Unbalance source

unbalance load Phase a-7.02A

Balance load Phase a-6.2A

Balance load Phase a-14.3A

THD%

Unbalance source

Phase a-1.98 Unbalance load

Phase a-4.18 Balance load

Phase a-3.3 Balance load

VI. CONCLUSION

A new SRF control technique based on advanced PLL used in SHAPF has been successfully alleviated the load current harmonics along with reactive power compensation, reduction of excessive neutral current and the load current balancing under non ideal supply and unbalance load condition. It can also be seen that regulation of DC voltage is stable, free from overshoot and no steady state error. The computer simulation and experimental verification proves its effectiveness and this will be really useful for the distribution system. As compared to simulation results, the experimental THD is slightly more due to accuracy limit of sensors and sampling time limit of DSP in dSPACE.

REFERENCES

[1] S. Rahmani, A. Hamadi, K. Al-Haddad, and A. Alolah, "A DSP-based implementation of an instantaneous current control for a three-phase shunt hybrid power filter," Mathematics and Computers in Simulation, 2012.

[2] J. Tian, Q. Chen, and B. Xie, "Series hybrid active power filter based on controllable harmonic impedance," IET Power Electronics, vol. 5, pp. 142-148, 2012.

[3] N. Zaveri and A. Chudasama, "Control strategies for harmonic mitigation and power factor correction using shunt active filter under various source voltage conditions," International Journal of Electrical Power & Energy Systems, vol. 42, pp. 661-671, 2012.

[4] C.-S. Lam, M.-C. Wong and Y.-D. Han, "Hysteresis current control of hybrid active power filters," IET Power Electronics, vol. 5, pp. 1175-1187, 2012.

[5] T. Messikh, S. Mekhilef, and N. A. Rahim, “ Adaptive Notch Filter for Harmonic Current Mitigation”, International Journal of Electrical and Electronics Engineering, Vol. 1 No. 4 2008, pp. 240-246.

[6] Y. Suresh, A. Panda, and M. Suresh, "Real-time implementation of adaptive fuzzy hysteresis-band current control technique for shunt active power filter," IET Power Electronics,vol. 5, pp. 1188-1195, 2012.

[7] P. Salmeron and S. P. Litrán, "A control strategy for hybrid power filter to compensate four-wires three-phase systems," IEEE Transactions on Power Electronics, , vol. 25, pp. 1923-1931, 2010.

[8] R. Belaidi, A. Haddouche, and H. Guendouz, "Fuzzy Logic Controller Based Three-Phase Shunt Active Power Filter for Compensating Harmonics and Reactive Power under Unbalanced Mains Voltages," Energy Procedia, vol. 18, pp. 560-570, 2012.

[9] M. Kesler and E. Ozdemir, "Synchronous-Reference-Frame-Based Control Method for UPQC under Unbalanced and Distorted Load Conditions," IEEE Transactions on Industrial Electronics, vol. 58, pp. 3967-3975, 2011.

[10] R. K. Sinha and P. Sensarma, "A pre-filter based PLL for three-phase grid connected applications," Electric Power Systems Research, vol. 81, pp. 129-137, 2011.

[11] V. F. Corasaniti, M. B. Barbieri, P. L. Arnera, and M. I. Valla, "Hybrid power filter to enhance power quality in a medium-voltage distribution network," IEEE Transactions on Industrial Electronics, vol. 56, pp. 2885-2893, 2009.

[12] Mekhilef, S.; Abdul Kadir, M. N.; Salam, Z., “ Digital Control of Three Phase Three-Stage Hybrid Multilevel Inverter”, IEEE Transactions on Industrial Informatics, Volume 9, Issue: 2, pp. 719 - 727 (2013)

[13] S Mekhilef, M Tarek,’’ Single-phase Hybrid Active Power Filter withAdaptive Notch Filter for Harmonic Current Estimation’’, IETE Journal of Research, vol 57, pp.20-28, 2011

Parameters Values Source impedance R=0.01ohm,L=2mH Load impedance 3-phase diode rectifier R=80Ω,L=60mH

1-phase diode rectifier R=0.1Ω,L=1 mH

Passive Filter 5th harmonic tuned, R=0.2Ω,L=5mh,C=40μFShunt APF R=0.001ohm,L=2mH,TwoDClink

capacitor=2100μF, Vdc=220 V,HB=±2A Source voltage 100V(RMS),50 HZ


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