Abstract—This paper presents a hybrid approach to harmonic

current compensation in a single-phase variable speed ac drive. The passive filtering is aimed to achieve the total current harmonic distortion after compensation to below 20%. The active filtering is aimed to assist the passive filter to compensate for the residue group harmonic. The active filter employs the sliding window Fourier analysis (SWFA) for harmonic identification. Simulation and experimental results are presented to illustrate the effectiveness and the usefulness of the proposed method. The proposed method can cope with the speed range of 72-100% rated speed, and the load torque range of 80-110% rated torque, while the power factors are greater than 0.998 for all cases. Keywords—Harmonic, Hybrid power filter, Contour-based

method, Sliding window Fourier analysis, AC drive.

I. INTRODUCTION ower converters have become widespread in industrial drives, and consumer electronics. Electrical loads with

converters are nonlinear, and introduce a great amount of harmonic currents into power systems [1-6]. Without a proper treatment, such harmonics could cause failure to power, and communication systems. To regulate the harmonic currents, one may follow an international standard such as the IEEE Std 519-1992 [7], for instance.

Elimination of harmonic currents can be achieved by using passive, active, and hybrid power filters. Even though a passive power filter (PPF) is low cost, its components are typically large, and usually the harmonic cannot be completely eliminated. To achieve acceptable harmonic elimination, a few to several tuned PPFs have to be installed, for instance in railway systems [8]. In contrast, an active power filter (APF) is expensive due to its complicated hardware and software requirement. Its common structure is an inverter with a DSP board performing harmonic identification and compensation. As a result, it outperforms PPFs, but at high cost. Recently, a hybrid approach of using both passive, and active power filters has become attractive since a PPF helps to decrease power ratings of an APF, and hence reduce the cost [9-11]. An APF commonly used is parallel type due to its simple installation [12-14]. There are some harmonic identification

The authors are with the School of Electrical Engineering, Suranaree

University of Technology (SUT), Nakhon Ratchasima, Thailand.The financial supports from SUT are greatly acknowledged. Thanks are also due to Synchrotron Light Research Institute of Thailand for supporting PSIM.

Correspondence: kongpol@sut.ac.th

methods available namely dq-frame method [15], instantaneous reactive power method [16], synchronous detection method [17], abc-reference-frame method [18], and dq-axis-with-Fourier (DQF) method [19]. Among those, the DQF method results in the most accurate harmonic information with fast calculation. This is due to the window-based realtime updating process for harmonic calculation of the sliding window Fourier analysis (SWFA) method [20]. Nonetheless, some algorithms may incorporate identification and compensation tasks together, for example an approach of using Walsh transform described in [21].

This paper addresses a technological advancement for eliminating harmonic currents, and improving power factor of a single-phase induction motor drive system. Developments of the PPF, and the APF are described in sections II, and III, respectively. Section IV presents simulated, and experimental results. Conclusion follows in section V.

II. PASSIVE POWER FILTER DEVELOPMENT Figure 1 shows that diagram representing the drive system

incorporated with the developed hybrid power filter (HPF). The HPF composes of a series PPF, and a shunt APF. An accurate method for design of the PPF indicated by Lf and Cf in the figure is the contour map method [22]. The contour generation is achieved by using PSIM, and MATLAB for the drive system consisting of a 1φ, 4 poles, 220Vrms, 5Arms, 50Hz, 1,425rpm induction motor, and a PWM inverter. The operating point is at its rated of 5Arms,3Nm. It is important to note that the ranges of Lf and Cf must not cause resonance at the supply frequency. Correspondingly, the effective ranges

Hybrid Compensation for Harmonic and Power Factor in Single-Phase AC Drive

Kongpol Areerak, Soupagorn Visawa-phatra-dhanadhorn, Sarawut Sujitjorn

P

Fig. 1 Simulation diagram using PSIM of the drive with hybrid

power filtering.

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are 0-250mH and 0-20μF respectively. Simulation by PSIM produces useful numerical data including total harmonic distortion in current and voltage (%THDi, %THDv), power factor (PF), and the dc voltage (VDC) fed to the inverter.

Thus, the corresponding contour maps are obtained as illustrated by the figures 2(a)-(d). The appropriate values of Lf and Cf can be extracted from the maps by interpolation. It must be borne in the mind that (i) too large Lf would cause a higher drop in VDC, and a higher source current, although lower %THDi and %THDv, (ii) too large Cf would draw a higher rms current, and increase losses, although increase PF. Selection of the LC components of the PPF requires a tradeoff between the two.

Simulation via PSIM for the drive system without any filters, and with some LC-PPFs were firstly conducted such that a suitable PPF could be identified. Table 1 summarizes the results, which include %THDi, and power factor (PF) as effectiveness indicators of the PPF. Five PPFs denoted as Cases II-VI in the table are considered. Among those, Case VI having Lf = 105mH and Cf = 10μF gives the most satisfactory

filtering performance bearing in mind that the filter current (IP) must not be too great.

III. ACTIVE POWER FILTER DEVELOPMENT Referring to figure 1, the diagram shows that the APF used

is parallel type, and requires a harmonic identification unit, and a current source. The harmonic identification unit employs the SWFA method in which the Fourier coefficients A1 and B1 are calculated according to the equations (3)-(4). Only the fundamental coefficients are required because subtraction of the fundamental component from the instantaneous waveform results in the group harmonic as expressed by the equation (1), which can be used directly for compensation.

1( ) ( ) ( )τ τ τ= −h pi k i k i k (1)

Where

1 1 1 1 1( ) cos( ) sin( )τ ω τ ω τ= +i k A k B k (2)

Fig. 2 Design contour maps for a passive power filter, (a) %THDi , (b) %THDv, (c) PF, and (d) VDC.

TABLE I SIMULATION AND EXPERIMENTAL RESULTS ILLUSTRATING THE EFFECTIVENESS OF PASSIVE POWER FILTER

Simulation and Parameters of Effectiveness indicators

experimental PPF Simulation Experimental

cases Lf Cf %THDi PF %THDi PF

CaseI no PPF 122.3 0.61 110.9 0.66

CaseII 55mH 10μF 47.53 0.90 42.40 0.93

CaseIII 55mH 16μF 42.25 0.89 35.50 0.90

CaseIV 100mH 10μF 25.63 0.97 21.20 0.98

CaseV 100mH 16μF 22.48 0.95 19.10 0.97

CaseVI 105mH 10μF 24.37 0.97 17.90 0.98

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0

0

1

1 12 ( )cos( )

N N

pn N

A i n nN

τ ω τ+ −

=

= ∑ (3)

0

0

1

1 12 ( )sin( )

N N

pn N

B i n nN

τ ω τ+ −

=

= ∑ (4)

k = 0, 1, 2, 3…(n-1), and τ = T/N. Calculation can be made according to the diagram in figure 3 representing the process of a block of data sliding through a window. So, at each sampling interval, the coefficients A1 and B1 can be updated by using the equations (5) and (6), respectively.

[ ] [ ]

[ ] [ ]

( ) ( )1 1 0 0 1

0 0 1

2 { ( ) cos ( ) ...

( 1) cos ( 1) }

new oldp

p

A A i N N N NN

i N N

τ ω τ

τ ω τ

= + + +

− − − (5)

[ ] [ ]

[ ] [ ]

( ) ( )1 1 0 0 1

0 0 1

2 { ( ) sin ( ) ...

( 1) sin ( 1) }

new oldp

p

B B i N N N NN

i N N

τ ω τ

τ ω τ

= + + +

− − − (6)

The harmonic identification process can be conducted according to the procedures shown in figure 4.

IV. RESULTS AND DISCUSSION

A. Simulations To ensure that the rms current passing through the PPF is

bounded within 5 Arms (rated load current), simulation of the drive system having the designed PPF was conducted. The simulated current, IP, illustrated in figure 5 ensures this. Simulation of the drive system having a HPF is a little bit more complicated. Even though an ideal current source can be assumed as an APF, it is required to develop a DLL file to execute the SWFA harmonic identification algorithm to be used with PSIM DLL block. The corresponding C codes for this are listed in figure 6. The PPF current, IP, is decomposed into fundamental, and group harmonic. The harmonic information is then sent to the current source to generate the

compensating current injected into the system. Figure 5 illustrates the simulated compensating current, IC2, as well as the source current, IS, after being compensated by the proposed HPF. It can be observed that the simulated steady-state current (IS) is purely sinusoid.

B. Experiments The setup drive system as shown in figure 7 consists of an

induction motor (1φ, 4 poles, 220Vrms, 5Arms, 1425rpm), a PWM inverter, and a controllable load unit. The mechanical load is kept constant at 3Nm (5Arms, rated) by the load unit. The PPF composes of Lf = 105mH and Cf = 10μF. The APF composes of a 750-W linear amplifier, and a harmonic identification unit. The identification algorithm, SWFA, is implemented using C++ on a DSP board (eZdspTM F2812). With a clock rate of 150MHz, and a sampling rate of 20kHz, the algorithm can be executed within 5μs. This leaves 45μs available for data acquisition using onboard 12-bit ADCs, and

Fig. 3 Calculation process for the coefficients A1 and B1.

Fig. 4 Harmonic identification procedures.

Fig. 5 Simulated current waveforms obtained from PSIM, where

IP = PPF current, IC2 = APF compensating current, and IS = source current.

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16-bit DACs. The compensating current provided by the identification unit is amplified, and injected into the drive system. Measurements are made using an oscilloscope (Tektronix TDS 420A), and a power quality analyzer (FLUKE 434).

The measured voltage and current waveforms are shown in

figure 8. The current IS before compensation is shown in figure 8(a), while the compensated current IS in figure 8(b). The compensating current IC2 being injected into the drive system is also shown in figure 8(b). The current IP in figure

8(b) is the compensated current achieved by using the PPF alone. It can be noticed that harmonic compensation by the proposed HPF provides the source current with harmonic contents well below the requirements by the IEEE Std 519-1992 for low power rated systems. The numerical values of the %THDi are summarized by the tables 2, and 3 for the PPF, and HPF cases, respectively.

Referring to figure 8(b) and table 2, the current compensated by the PPF alone (IP) has a trapezoidal shape containing harmonic details as shown in the table with %THDi=17.9 much lower than 110.9% before compensation. The measured %THDv is 2.3. The current compensated by the HPF (IS) is almost sinusoidal. Its harmonic contents are disclosed in table 3 with %THDi=4.4 complied with the IEEE Std 519-1992. Additionally, after compensation by the proposed HPF, %THDv=1.6, %THDi=4.4, DPF=1, and PF=0.998 can be achieved.

To further investigate the usefulness and the effectiveness

of our proposed HPF for an AC drive, experiments have been extended to cover more operating points rather than the rated one without any redesign. These include (i) fixed load torque at 3 Nm with motor speed at 86 and 72% of the rated speed as the results summarized by table 4, and (ii) variable speed at the load torque of 80 and 110% of the rated torque as the results summarized by table 5. As indicated by the data in these tables, highly satisfactory results can be achieved practically in terms of %THDi, %THDv, and PF, without the

Fig. 6 SWFA harmonic identification algorithm coded in C to be used with PSIM DLL block.

Fig. 7 Experimental setup.

Fig. 8 (a) Voltage and current waveforms measured at PCC before

compensation, and (b) currents after compensation.

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need for any redesign, and no excessive current drawn by the system.

V. CONCLUSIONS To compensate for harmonic currents in a variable speed

AC drive, a hybrid power filtering has been proposed. With the solution, the passive power filter can be designed by the contour map method. The active power filter consists of a

current amplifier, and a harmonic identification unit, which employs the sliding window Fourier analysis method to provide harmonic information in real-time. The concepts have been investigated by simulations using PSIM, and experiments. Results indicate that the proposed solution for a low power system is very effective subjected to the IEEE Std 519-1992. The practicality covers operating ranges of 72-100% rated speed, and 80-110% rated load torque with excellent results in terms of %THDs, PF, and the current drawn by the compensated system.

NOTATION LIST

1i = time-domain fundamental current (A)

hi = time-domain harmonic current (A)

pi = time-domain current flowing into the PPF (A)

,k n = semi-positive integers ( ≥ 0)

1 1,A B = Fourier coefficients of fundamental DPF = displacement power factor

1CI = passive power filter compensating current (A)

2CI = active power filter compensating current (A)

LI = nonlinear load current (A)

PI = passive power filter current (A)

SI = source current (A)

1I = magnitude of fundamental current (Arms)

SI = magnitude of source current (Arms) N = number of samples in a T -second period

0N = first datum of N samples PF = power factor 1T

= supply frequency (Hz) with period T (sec)

THD = total harmonic distortion DCV = DC voltage fed to the inverter (V)

1ω = fundamental radian frequency (rad/sec) ωm = motor speed (rpm) τ = sampling interval (sec) τ m = mechanical load torque (Nm)

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[3] V.E. Wagner, J.C. Balda, D.C. Griffith, A. McEachern, T.M. Barnes, D.P. Hartmann, D.J. Phileggi, A.E. Emannuel, W.F. Horton, W.E. Reid, R.J. Ferraro and W.T. Jewell, “Effects of harmonics on equipment”, IEEE Trans. on Power Delivery, vol, 8, no. 2, pp. 672-680, 1993.

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TABLE II

COMPARISONS OF %THDI BEFORE AND AFTER THE USE OF A PPF

Individual harmonic currents at PCC as percent of fundamental and %THDi Harmonic order

Before After 3 85.0 17.4 5 59.6 3.4 7 31.4 2.0 9 8.0 1.3 11 8.7 0.5 13 12.9 0.2 15 10.0 0.4

%THDi 110.9 17.9

Lf = 105mH and Cf = 10μF

TABLE III COMPARISONS OF %THDI BEFORE AND AFTER THE USE OF A HPF

Individual harmonic currents at PCC as percent of fundamental and %THDi Harmonic order

Before After 3 85.0 3.9 5 59.6 1.3 7 31.4 0.8 9 8.0 1.0 11 8.7 0.4 13 12.9 0.2 15 10.0 0.3

%THDi 110.9 4.4

TABLE IV

EXPERIMENTAL RESULTS

τm (Nm) ωm (rpm) %THDi %THDv |IS| (Arms) |I1| (Arms) PF DPF

3 1,425 4.40 1.60 4.30 4.30 0.998 1.00

3 1,225 4.20 1.90 3.40 3.40 0.998 1.00

3 1,025 3.70 1.90 3.30 3.30 0.999 1.00

Motor speed at 86 and 72% of rated speed, fixed load.

TABLE V EXPERIMENTAL RESULTS

τm (Nm) ωm (rpm) %THDi %THDv |IS| (Arms) |I1| (Arms) PF DPF

3.3 1,400 2.90 1.80 4.20 4.20 0.999 1.00

3 1,425 4.40 1.60 4.30 4.30 0.998 1.00

2.4 1,438 3.80 1.80 3.20 3.20 0.999 1.00

Motor speed at 80 and 110% of rated speed, fixed load.

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[7] IEEE Std. 519-1992-IEEE Recommended practice and requirements for harmonic control in electrical power system, 1993.

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[19] S. Sujitjorn, K-L. Areerak and T. Kulworawanichpong, “The DQ axis with Fourier (DQF) method for harmonic identification”, IEEE Trans. on Power Delivery, vol. 22, no. 1, pp. 737-739, 2007.

[20] M. El-Habrouk and M.K. Darwish, “Design and implementation of a modified Fourier analysis harmonic current computation technique for power active filter using DSPs”, IEE Proc. of Electric Power Applications, vol. 148, no. 1, pp. 21-28, 2001.

[21] Y. Zhao, “A novel selective harmonic elimination pulse-width modulation technique for the inverter in dynamic voltage restorer”, WSEAS Trans. on Circuits and Systems, vol. 6, no. 2, pp. 201-207, 2007.

[22] C.S. Moo, H.L. Cheng and S.J. Guo, “Designing passive LC filters with contour maps (for diode bridge rectifiers)”, Proc. Int. Conf. on Power Electronics and Drive System, vol. 2, pp. 834-838, 1997.

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