3. transmission line through 20kV feeder, Figure-2 bestowing
one line diagram of the system.
Total Harmonic Distortion analysis accomplished at PMSG
and at the PCC to confirm harmonic contents at these
locations. THD analysis package is given in
MATLAB/SIMULINK and can be calculated from equation-1.
Current of PMSG and PCC are demonstrated in Figure 3,
while for THD Table-2 is viewing behavior in the presence of
non-linear load.
Where,
THDi= Total current harmonic distortion
I1= Fundamental current harmonic
I3, I5, I7= Odd current harmonics
In= nth current harmonic
As shown, the THD level of PMSG current is 35.95%,
33.45% and 44.06% of each phase correspondingly. THD
level of PCC current 62.85%, 53.36% and 32.86%
respectively for each phase. As per IEEE-519 THD level
for system is above 69kV rating THD level need be
2.5%. In this research 120kV system is under study so to
keep THD level under specified limits of IEEE-519. Shunt
Active Power Filter (APF) is used to mitigate effect of
these harmonics and maintain acceptable THD level for
current at PCC and at terminals of PMSG.
Phase PMSG THD (%) PCC THD (%)
A 35.95 62.85
B 33.45 53.36
C 44.06 32.86
III. SHUNT ACTIVE POWER FILTER
Among the filters Shunt APF is widely used [11-18].
Shunt APF acts as a harmonic current source that introduces
same magnitude antiphase current to mitigate the load
harmonic and reactive components of the current. Figure-4,
depicting one line diagram of shunt APF. Design of APF
having two important parameters [19-21]. One is the technique
to extract current harmonics (reference current); other is the
current controlling technique. Harmonic current extraction
may be classified as time domain and frequency domain. Fast
Fourier Transform (FFT) is the main principle of frequency
domain whereas instantaneous estimation of reference signal
is the basis of time domain [10]. Response of time domain is
fast and on the other side frequency domain have accurate
response. Predictive current control technique could be the
method to control second parameter of shunt APF [22, 23],
ramp comparison current control, hysteresis current control
method [23]. Current extraction method together with current
control technique drives the overall system technique. Thus in
return be responsible for switching pattern generator [18, 21]
for proper operation of APF.
PMSG Ratings Value
Rated Power 1.5MVA
Stator Voltage 575V
d axis inductance 0.3mH
q axis inductance 0.3mH
DC link Voltage 1150V
No of poles 48
Figure-2. Grid connected WECS
TABLE II. THD LEVEL PMSG AND PCC
TABLE I. SYSTEM PARAMETERS
Figure-1. Grid connected wind turbine with Shunt APF
Figure-3. PMSG and PCC Current
464
4. A. Reference signal extraction techniques
The key parameter which guarantees correct operation of
APF is reference signal extraction method. The detection of
necessary current/voltage signal originates the reference signal
extraction to collect accurate information of system variables.
Depending on these variables of the system, reference signals
estimation in terms of voltage/current levels are estimated in
frequency-domain or time-domain [21] as conferred above. In
[24-28] many theories and methods reported to detect and
measurement of system variables for the reference estimation
techniques. This section presents the techniques which are
used in this research.
B. Instantenous reactive power theory
The p-q theory or instantaneous reactive power theory was
proposed in 1983 [29-31]. The basis of this theory is 0
transformation. It transforms three phase current and voltage
into stationary frame of 0 [32, 33]. Three phase voltage and
current can be expressed as,
And
(3)
Where iLa, iLb, iLc are load currents and vLa, vLb and
vLc are load voltages. Conferring to this theory active power,
reactive power and zero sequence power can be calculated as
[34],
(4)
(5)
(6)
Zero sequence components will not exist in three
phase three wire system [35], therefore, only - will
contribute for the calculation of active and reactive powers.
Powers calculated from above equation (4 and 5) contains AC
and DC components of the system from which i *
and i *
can
be calculated using equations (7) further AC components from
those currents can be extracted by using low pass filter (LPF)
and taking inverse transformation using equation (8) to obtain
reference signal in term of voltage or current [21].
Current reference signal generated for the system
under consideration and given to gate driver control system.
Inside gate driver controller measured current is subtracted
from reference current to pattern proper gate driving signal for
inverter. Conventional PI controller is used here to eliminate
steady state error of DC component of inverter and maintain
constant dc voltage across capacitor.
Phase PMSG THD (%) PCC THD (%)
A 3.22 5.44
B 4.23 4.26
C 6.98 3.49
C. Synchronous Reference Frame Technique
Parks Transformation used in this method to convert three
phase system voltage into a synchronous rotating frame [36,
37]. Active and reactive components of load voltage and
current are decomposed to direct and quadrature components
respectively [21]. Breakdown of three phases a-b-c to d-q
reference frame as,
The d and q components of current represent active
and reactive power components of current and are
Figure-4. One line diagram of Shunt APF
Figure-5. PMSG and PCC Current
TABLE III. THD LEVEL OF PMSG AND PCC
465
5. decomposed as in equation (10 and 11). Low pass filter can be
used to extract DC component.
The transformed d-q output signals depend on the
load currents and the phase locked loop (PLL) performance
[38]. The rotation speed of PLL circuit of the rotating
reference frame t set as fundamental frequency component.
Sin and cos is provided by PLL circuit for synchronization.
To filter harmonic contents id-iq current passed through low
pass filter (LPF) and allows only the fundamental frequency
components. Function of PI controller here is also the same as
in p-q theory. The dc capacitor voltage is sensed and
compared with reference voltage to calculate error voltage.
This voltage error is involved with PI gain (KP=0.1 and KI-1)
to regulate capacitance in dynamic conditions. Further the
output of PI controller is subtracted from d-axis current to
eradicate steady state error. The procedure is then developed
to extract reference signal in d-q rotating frame which is
converted back to a-b-c stationery frame. Transformation from
d-q to a-b-c is achieved by following equations.
Harmonic affected system when simulated with SRF
technique, the system THD have been decreased and
satisfying IEEE-519 standard both at PMSG and at PCC.
Figure- 6 and Table IV are depicting their waveforms and
THD.
Phase PMSG THD (%) PCC THD (%)
A 1.11 0.99
B 2.59 1.59
C 1.15 1.86
Utilization of SRF technique giving less THD as compared
to p-q theory, as presented current THD level at PMSG and
PCC have decreased down to limits specified by IEEE-519.
IV. COMPARISON OF BOTH TECHNIQUES
Summarized chart for PMSG and PCC current THD level
without compensation and with the techniques used is
illustrated in Figure-7 and 8.
V. CONCLUSION
In this paper grid connected PMSG centred variable speed
wind turbine system discussed with shunt active power filter
to mitigate harmonic effect of non-linear loads from PMSG
and point of common coupling. Two mostly used techniques
(p-q and SRF) to extract reference signal were used separately
for appropriate operation of shunt APF. Results of both
techniques conferred in this paper and compared in section-IV.
Simulation of the system proved that using shunt APF at WTG
side will decrease the current harmonic to a satisfactory level
vis-à-vis to IEEE-519 at both sides i.e., PMSG and PCC. This
technique further lessened the voltage harmonic at PCC.
In order to have full coverage of system, it would be
interesting if thermal losses in WTG can be considered. It will
also be potential to study the combination of series and shunt
APF for the system under consideration. In this paper, effect
Figure-6. PMSG and PCC Current
TABLE IV. THD LEVEL PMSG AND PCC
Figure-7. Summary for PMSG Current THD
Figure-8. Summary for PCC Current THD
466
6. of atmospheric temperature have been neglected, for advance
investigation the impact of temperature rise and fall, the
requirement of cooling system and influence of temperature
on efficiency of system can be of more concern.
ACKNOWLEDGEMENT
The authors would like to acknowledge the facilities
provided by Universiti Teknologi Malaysia for the
accomplishment of this work and also thankful to Mehran
University of Engineering and Technology Shaheed Zulfiqar
Ali Bhutto Campus & Technology, Pakistan for providing
financial assistance under Faculty Development Program
(FDP).
REFERENCES
[1] A. M. Massoud, S. Ahmed, and A. S. Abdel Khalik, "Active
Power Filter," Power Electronics for Renewable Energy Systems,
Transportation and Industrial Applications, pp. 534-572, 2014.
[2] J. Tsai and K. Tan, "H APF harmonic mitigation technique for
PMSG wind energy conversion system," in Power Engineering
Conference, 2007. AUPEC 2007. Australasian Universities, 2007,
pp. 1-6.
[3] H. Sasaki and T. Machida, "A new method to eliminate ac
harmonic currents by magnetic flux compensation-considerations
on basic design," Power Apparatus and Systems, IEEE
Transactions on, pp. 2009-2019, 1971.
[4] M. A. M. Radzi and N. A. Rahim, "Neural network and bandless
hysteresis approach to control switched capacitor active power
filter for reduction of harmonics," Industrial Electronics, IEEE
Transactions on, vol. 56, pp. 1477-1484, 2009.
[5] S. Rahmani, N. Mendalek, and K. Al-Haddad, "Experimental
design of a nonlinear control technique for three-phase shunt active
power filter," Industrial Electronics, IEEE Transactions on, vol.
57, pp. 3364-3375, 2010.
[6] B. Singh and J. Solanki, "An implementation of an adaptive
control algorithm for a three-phase shunt active filter," Industrial
Electronics, IEEE Transactions on, vol. 56, pp. 2811-2820, 2009.
[7] C. Lascu, L. Asiminoaei, I. Boldea, and F. Blaabjerg, "Frequency
response analysis of current controllers for selective harmonic
compensation in active power filters," Industrial Electronics, IEEE
Transactions on, vol. 56, pp. 337-347, 2009.
[8] B. Singh, K. Al-Haddad, and A. Chandra, "A review of active
filters for power quality improvement," Industrial Electronics,
IEEE Transactions on, vol. 46, pp. 960-971, 1999.
[9] F. S. dos Reis, J. Ale, F. Adegas, R. Tonkoski, S. Slan, and K. Tan,
"Active shunt filter for harmonic mitigation in wind turbines
generators," in Power Electronics Specialists Conference, 2006.
PESC'06. 37th IEEE, 2006, pp. 1-6.
[10] A. Hoseinpour, S. Masoud Barakati, and R. Ghazi, "Harmonic
reduction in wind turbine generators using a Shunt Active Filter
based on the proposed modulation technique," International
Journal of Electrical Power & Energy Systems, vol. 43, pp. 1401-
1412, 2012.
[11] M. Aredes and E. H. Watanabe, "New control algorithms for series
and shunt three-phase four-wire active power filters," Power
Delivery, IEEE Transactions on, vol. 10, pp. 1649-1656, 1995.
[12] H. Akagi, "Control strategy and site selection of a shunt active
filter for damping of harmonic propagation in power distribution
systems," Power Delivery, IEEE Transactions on, vol. 12, pp. 354-
363, 1997.
[13] M. I. M. Montero, E. R. Cadaval, and F. B. González,
"Comparison of control strategies for shunt active power filters in
three-phase four-wire systems," Power Electronics, IEEE
Transactions on, vol. 22, pp. 229-236, 2007.
[14] H. Akagi, H. Fujita, and K. Wada, "A shunt active filter based on
voltage detection for harmonic termination of a radial power
distribution line," Industry Applications, IEEE Transactions on,
vol. 35, pp. 638-645, 1999.
[15] M. K. Mishra, A. Joshi, and A. Ghosh, "A new algorithm for active
shunt filters using instantaneous reactive power theory," Power
Engineering Review, IEEE, vol. 20, pp. 56-58, 2000.
[16] A. Chandra, B. Singh, B. Singh, and K. Al-Haddad, "An improved
control algorithm of shunt active filter for voltage regulation,
harmonic elimination, power-factor correction, and balancing of
nonlinear loads," Power Electronics, IEEE Transactions on, vol.
15, pp. 495-507, 2000.
[17] A. M. Al-Zamil and D. A. Torrey, "A passive series, active shunt
filter for high power applications," Power Electronics, IEEE
Transactions on, vol. 16, pp. 101-109, 2001.
[18] M. El-Habrouk, M. Darwish, and P. Mehta, "Active power filters:
A review," IEE Proceedings-Electric Power Applications, vol.
147, pp. 403-413, 2000.
[19] A. Massoud, S. Finney, and B. Williams, "Predictive current
control of a shunt active power filter," in Power Electronics
Specialists Conference, 2004. PESC 04. 2004 IEEE 35th Annual,
2004, pp. 3567-3572.
[20] F. Z. Peng, H. Akagi, and A. Nabae, "A new approach to harmonic
compensation in power systems-a combined system of shunt
passive and series active filters," Industry Applications, IEEE
Transactions on, vol. 26, pp. 983-990, 1990.
[21] Z. Salam, P. C. Tan, and A. Jusoh, "Harmonics mitigation using
active power filter: A technological review," Elektrika, vol. 8, pp.
17-26, 2006.
[22] A. Massoud, S. Finney, D. Grant, and B. Williams, "Predictive
current controlled shunt active power filter using three-level
cascaded type inverter," 2006.
[23] N. Mendalek, K. Al-Haddad, F. Fnaiech, and L. Dessaint,
"Nonlinear control technique to enhance dynamic performance of a
shunt active power filter," IEE Proceedings-Electric Power
Applications, vol. 150, pp. 373-379, 2003.
[24] J.-C. Wu and H.-L. Jou, "Simplified control method for the single-
phase active power filter," IEE Proceedings-Electric Power
Applications, vol. 143, pp. 219-224, 1996.
[25] W. M. Grady, M. J. Samotyj, and A. H. Noyola, "Survey of active
power line conditioning methodologies," Power Delivery, IEEE
Transactions on, vol. 5, pp. 1536-1542, 1990.
[26] N. Mariun, A. Alam, S. Mahmod, and H. Hizam, "Review of
control strategies for power quality conditioners," in Power and
Energy Conference, 2004. PECon 2004. Proceedings. National,
2004, pp. 109-115.
[27] D. Chen and S. Xie, "Review of the control strategies applied to
active power filters," in Electric Utility Deregulation,
Restructuring and Power Technologies, 2004.(DRPT 2004).
Proceedings of the 2004 IEEE International Conference on, 2004,
pp. 666-670.
[28] M. El-Habrouk and M. Darwish, "Design and implementation of a
modified Fourier analysis harmonic current computation technique
for power active filters using DSPs," in Electric Power
Applications, IEE Proceedings-, 2001, pp. 21-28.
[29] H. Akagi, Y. Kanazawa, and A. Nabae, "Instantaneous reactive
power compensators comprising switching devices without energy
storage components," Industry Applications, IEEE Transactions
on, pp. 625-630, 1984.
[30] S. Luo and Z. Hou, "An adaptive detecting method for harmonic
and reactive currents," Industrial Electronics, IEEE Transactions
on, vol. 42, pp. 85-89, 1995.
[31] L. P. Ling and N. Azli, "SVM based hysteresis current controller
for a three phase active power filter," in Power and Energy
Conference, 2004. PECon 2004. Proceedings. National, 2004, pp.
132-136.
[32] S. Buso, L. Malesani, P. Mattavelli, and R. Veronese, "Design and
fully digital control of parallel active filters for thyristor rectifiers
to comply with IEC-1000-3-2 standards," Industry Applications,
IEEE Transactions on, vol. 34, pp. 508-517, 1998.
467
7. [33] J.-S. Lai, "Power electronics applications in renewable energy
systems," in Industrial Electronics Society, 2003. IECON'03. The
29th Annual Conference of the IEEE, 2003, pp. 3025-3026.
[34] G. Adam, A. G. Stan, and G. Livint, "A Matlab-Simulink
Approach To Shunt Active Power Filters," in ECMS, 2011, pp.
205-210.
[35] J. L. Afonso, M. Freitas, and J. S. Martins, "pq Theory power
components calculations," in Industrial Electronics, 2003. ISIE'03.
2003 IEEE International Symposium on, 2003, pp. 385-390.
[36] V. Soares, P. Verdelho, and G. D. Marques, "An instantaneous
active and reactive current component method for active filters,"
Power Electronics, IEEE Transactions on, vol. 15, pp. 660-669,
2000.
[37] S.-G. Jeong and M.-H. Woo, "DSP-based active power filter with
predictive current control," Industrial Electronics, IEEE
Transactions on, vol. 44, pp. 329-336, 1997.
[38] A. Sharma and A. Upadhyay, "Harmonic Mitigation Using Inverter
Based Hybrid Shunt Active Power Filter," International Journal of
Electronic and Electrical Engineering, vol. 7, 2014.
468
![Comparison of Reference Signal Extraction Methods
for Active Power Filter to Mitigate Load Harmonics
From Wind Turbine Generator
Sajid Hussain Qazi1,2
, Mohd Wazir Bin Mustafa1
, Shakir Soomro2
, Raja Masood Larik1
1
Fakulti Kejuruteraan Elektrik,
Universiti Teknologi Malaysia,
Skudai, Johor Bahru, Johor
Abstract— This paper highlighting on two reference extraction
methods; Synchronous Reference Frame (SRF) and
Instantaneous Reactive Power Theory. The operation of these
two techniques will be analyzed with variable speed grid
connected permanent magnet synchronous generator based wind
turbine system. The simulated results will confirm which method
has better performance and which can maintain total harmonic
distortion THD level of the system at the value specified by IEEE
standard 519 when applied with wind energy conversion system.
Keywords— Permanent Magnet Synchronous Generator, Wind
Turbine Generator, Point of Common Coupling, Synchronous
Reference Frame
I. INTRODUCTION
Added harmonic distortion in the grid affected the
power quality of the main supply primarily due to the
intensified utilization of power electronics-based devices and
equipment. The existence of these equipments have been
increased in our domestic life. For suppose, personal
computers, refrigerators, air conditioning systems all
comprises power electronics devices, in commercial
applications heaters, uninterruptable power supplies (UPS),
elevators. Similarly use in industrial appliances such as
compressors, blowers, variable frequency drives (VFD),
furnaces [1] the main cause of distorted power quality. The
utilization of these equipment upturn the share of nonlinear
loads compared to that of linear loads, and hence vitiates the
issues of grid supply quality. Alongside, all these equipment
are also sensitive to harmonic distortion. Likewise, the
generation of these harmonics tempts additional losses in the
generator, thereby producing more heat. Thus reduces the
efficiency of generator and deteriorating its overall life span
[2]. Likely, in the case of wind turbine generator connected
with power grid, these same foulest effects can be observed at
point of common coupling (PCC) which affect the
performance of wind turbine generator (WTG) and
simultaneously abating the quality of power at (PCC).
Active filter proposed in [3] to reduce effect of these
harmonics. This type of filter is capable of injecting obligatory
2
Department of Electrical Engineering
Mehran UET SZAB Campus,
Khairpur Mir’s, Pakistan
shunt currents or series voltages to reduce harmonics effect
from system. Identification, Modulation and Inverter are the
three main parts of active filter. While designing filter and its
application, the process for extraction of harmonics from line
current and filter reference current determination are quite
important. Actually the accuracy and its speed of response
depends on pointed methods [4, 5]. The two main methods for
extraction of reference current are time domain and frequency
domain [6-8]. For accurate results, frequency domain methods
are used, whereas time-domain methods are quicker. The p–q
methods (active and reactive instantaneous powers theory)
tactics in time domain do not provide a good performance in
the presence of source harmonics. On the other hand
synchronous reference frame (SRF), which is also based on
time domain, can overcome the aforesaid problem.
In this paper current harmonic mitigation due to non-
linear load from grid connected variable speed wind energy
conversion system (WECS) comprises of PMSG wind
generator, AC-DC-AC converter and Shunt APF is presented.
The p-q theory and SRF method are used for reference current
generation aimed at driving proper signal for APF. The
detailed working and comparison of both methods (p-q and
SRF method) is presented using MATLAB/SIMULINK to
substantiate which method is more feasible for reference
current generation and eventually best for harmonic mitigation
when used with variable speed WECS. The paper is structured
as follows, section II dedicated for system under
consideration, discussion on Shunt APF and types of reference
current generation in section III, comparison of SRF and p-q
theory detailed in section IV, conclusion in section V.
II. SYSTEM UNDER CONSIDERATION
Study of harmonics for WECS proposed in [9], in [10]
harmonic analysis carried out on WECS with islanding mode,
same is accomplished in this paper with grid connected mode
as shown in Figure-1. System parameters are given in Table-1.
In this section grid connected PMSG is simulated
with non-linear load to pattern the effect of harmonics on the
voltage and current waveform of PMSG and at the PCC.
Variable wind speed between 8m/s-12m/s is applied to wind
turbine. Wind turbine exports power to 120kV grid via 30km
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