The main challenge associated with the grid-connected inverter (GCI) in distributed generation systems is keeping the harmonic content of the output current below the specified value and compensating for unbalanced loads even when the grid is subject to disturbances such as harmonic distortion and unbalanced loads. To overcome these challenges, a current control strategy for a three-phase GCI under unbalanced and nonlinear load conditions is presented. The proposed control method enables the GCI to inject balanced and clean currents to the grid even when the local loads are unbalanced and/or nonlinear; it also compensates for the harmonic currents and controls the active and reactive powers. The main advantage and objective of this method is to effectively compensate for the harmonic current content of the grid current and microgrid without the use of any compensation devices such as active, passive and LCL filters

Guerrero, J.M.

Hafiz Habibuddin, M.

Mohd. Zin, A.A.

Naderipour, A.

Universiti Teknologi Malaysia Institutional Repository

Harmonic currents compensator GCI at the microgrid

The main challenge associated with the grid-connected inverter in distributed generation (DG) systems is to maintain the harmonic contents in output current below the specified values and compensates for unbalanced loads even when the grid is subject to disturbances such as harmonic distortion and unbalanced loads. To overcome these challenges, a current control strategy for a three-phase grid-connected inverter under unbalanced and nonlinear load conditions is presented. It enables grid-connected inverter by the proposed control method to inject balanced clean currents to the grid even when the local loads are unbalanced and/or nonlinear and also compensate of the harmonic currents and control the active and reactive power. The main advantage and objective of this method is to effectively compensate for the harmonic currents content of the grid current and microgrid without using any compensation devices; such as active, passive and LCL filters

Asuhaimi Mohd Zin, A.

Habibuddin, M.H.

Guerrero, Josep M.

    Aalborg UniversitetHarmonic currents Compensator Grid-Connected Inverter at the MicrogridAsuhaimi Mohd Zin, A.; Naderipour, A. ; Habibuddin, M.H.; Guerrero, Josep M.Published in:Electronics LettersDOI (link to publication from Publisher):10.1049/el.2016.1890Publication date:2016Document VersionEarly version, also known as pre-printLink to publication from Aalborg UniversityCitation for published version (APA):Asuhaimi Mohd Zin, A., Naderipour, A., Habibuddin, M. H., & Guerrero, J. M. (2016). Harmonic currentsCompensator Grid-Connected Inverter at the Microgrid. Electronics Letters, 52(20), 1714 - 1715. DOI:10.1049/el.2016.1890General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            ? Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            ? You may not further distribute the material or use it for any profit-making activity or commercial gain            ? You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access tothe work immediately and investigate your claim.Downloaded from vbn.aau.dk on: May 01, 20171  Harmonic currents Compensator Grid-Connected Inverter at the Microgrid  A. Asuhaimi Mohd Zin, A. Naderipour, M. H. Habibuddin Josep M. Guerrero  The main challenge associated with the grid-connected inverter in distributed generation (DG) systems is to maintain the harmonic contents in output current below the specified values and compensates for unbalanced loads even when the grid is subject to disturbances such as harmonic distortion and unbalanced loads. To overcome these challenges, a current control strategy for a three-phase grid-connected inverter under unbalanced and nonlinear load conditions is presented. It enables grid-connected inverter by the proposed control method to inject balanced clean currents to the grid even when the local loads are unbalanced and/or nonlinear and also compensate of the harmonic currents and control the active and reactive power. The main advantage and objective of this method is to effectively compensate for the harmonic currents content of the grid current and microgrid without using any compensation devices; such as active, passive and LCL filters.   Introduction: The main problem of microgrids (MGs) when exchanged the current with grid consider as source of harmonic distortion in Grid-Connected Inverter (GCI). Conventionally, despite the fact that it is not cost-effective, for elevating power quality problems, active power filters has been used [1]. The GCI controller is able to accomplish with unbalanced current harmonics and current of utility grid. However, compensation for the power quality problem, such as current harmonics, can be achieved through appropriate control strategies. This Letter proposes a new current compensation control method for photovoltaic (PV) GCI. The proposed control strategy consists of  Synchronous Reference Frame (SRF) Phase Locked Loop (PLL) [2]. The ASRF is proposed to control power injection to the grid, and also is used for harmonic current compensation between Point of Common Coupling (PCC) and the MG.   The Proposed Control Method: To enhance grid and MG currents quality, an advanced current control method for the GCI, as shown in Figure 1, is introduced.   Eq. (1)&Eq. (2)PLLia, ib vab, vbc IddqIqVdVqVdVqIq*Id*Id*Iq*P*Q*ϴ  VdcVabcabcdqabcdqabc ωL ωLSPWMS1-S66PIIdIdqVabcIabcIabcVdcdqabcdqabcHarmonic currents compensation control unit Active and reactive power control unit  Fig. 1 Block diagram of the proposed control method      Various control methods are based on the frequency domain or the time domain. The ASRF control is also called the dq control and is used to control the grid-connected inverter in this letter. This method uses a reference frame transformation module, abc dq→ , to transform it into a reference frame that rotates synchronously using the transform of the grid current and the voltage waveforms. The park transformation application to three generic three-phase quantities supplies their components in 0dq coordinates [3]. In general, three phase voltages and currents are transformed into 0dq  coordinates by matrix [L] as follows:  [ ] [ ]     0 0u iu id dA Au L u and i L iq B q Bu iu iC C= =                                                             (1)  [ ]2π 2πsin sin  sin  3 32 2π 2π cos cos  cos  3 3 31 1 12 2 2Lα α αα α α− += − +                                         (2)  The phase angle of voltage and current signals set as reference current which achieves unity power factor while I*q=0. Refer to figure 1 the current controller is under study. The Sinusoidal Pulse Width Modulation (SPWM) voltage frame guaranties. The voltage reference and design PLL synchronize inverter with grid. Hence I*q and I*d as reference current in dq transform re-calculate as:  *0 *V PqP V I V I Iq qd d d Vd== + =→                                        (3) *0 *V QqQ V I V I Iq q qd d Vd== − =→                                         (4)  where Vd and Vq are the grid voltage in dq transform. Furthermore the inverter is able to deliver P* and Q* where are reference active and reactive power, respectively. The inverter inject real power which the flow is controlled by Id and a specified reference current I*q. When the system is fully inject reactive power I*q=0. The other components I*d is extracted of the DC capacitor’s dynamics analysis. Hence the V*d and V*q of inverter gate SPWM are obtaining as following:  * * *( ) ( ) )* * *( ) ( ) )V K I I K I I dt L I Vp qId d d d d f dV K I I K I I dt L I Vq p q q q q qI f dωω= − + − − +∫= − + − − +∫           (5)          The SPWM modulating voltage signals are transformed from dq0 to abc frame. The PI controller are the main methods to control dq due to proper regulation of DC variables. Equation (6) gives the matrix transfer function in dq coordinates:  ( )0( )  0KiK pdq sG sPI KiK ps+=+                                         (6) where Kp and Ki are the controller’s proportional and integral gain, respectively. In order to provide the phase information of the grid voltage, which is required to generate the current reference iref and also is designed to make the inverter synchronism with grid, The PLL technique can be used. Using a PLL system, the three current references are created, each with the error going into the controller, and the corresponding measured current can be compared. The switches of GCI are controlled by the output of PLL system. The duty cycles for the SPWM pattern is generated by modulator which is controlled by three PI units. The PLL schematic is illustrated in Figure 2. 2  Vdc* ω ωffθ PILoop FilterVCOPLL ControllerTransformation Moduledqαβ abcαβ VaVbVcVqVd1/s Fig. 2 PLL structure of the three-phase  The dq rotating frame converts back to abc stationery frame uses inverse Park′s transformation Eq. 7, 8 and 9, by extracting reference signal. ( ) ( )* sin cosi i t tsa d ω ω= +                                               (7) 2 2* sin cos3 3i i t tsb dπ πω ω= − + −                                          (8) 2 2* sin cos3 3i i t tsc dπ πω ω= + + +                                           (9) The unit of control compensation of harmonic currents uses DC quantities. The design compensation current is injected by appropriate gate plus for inverter which is transferred by gate driver controller to extract reference signal.  Simulation results: The basic MG in this letter includes the MT, the fuel cell, wind turbine and PV array which are connected to the grid by the interface inverter. The proposed control methods is applied to the PV. Furthermore, a fuel cell has an output of 50 kW and a grid-connected PV array has an output of 100 kW and also a 9 MW wind farm and 25MW MT are connected to the grid by AC/DA/AC converter. Another side of this system consists of two non-linear loads, which produced the distorted waveform. In the grid and MG the voltage is assumed sinusoidal. In the simulation, two case study are taken into account.  A. Case study I The resulting system waveforms consist of grid, PV currents are shown in Figure 3 without any compensation devices.   a   B  Fig. 3 System, DG units current waveforms without any compensation: (a) System currents; (b) PV currents B. Case study II This case study has an improved power quality with the absence of compensation devices such as passive, active and LCL filters in the MG. Figures 4 (a) and (b) show the effective compensation values of the harmonic current for the grid and the PV. When all of the loads and DGs are connected, the Total Harmonic Distortion (THD) current without any compensation was 13.5%. As shown in Figure 4 (a), THD is reduced to 1.69% in the system with the proposed control method. The Current and THD value of study system is given in Tables 1.    a  b Fig. 4 System and DG unit current waveforms with propose control method; (a) System currents; (b) PV currents  Table 1: Current and THD Result  Before Compensation After Propose Control Method Current (A) THD % Current (A) THD % System 207.6 13.55 1175 1.69 PV 250 11.43 339.6 0.55  Conclusion: This Letter proposes a new control strategy for harmonic current compensation for photovoltaic inverter betwen PCC and MG and also is responsible for controlling the power injection to the grid, and compensating unbalanced load. The presented simulation results show that the PCC harmonic currents due to unbalanced load, NLL and DGs are compensated to the desired value. This strategy can be used for single-phase and three-phase systems.   Acknowledgments: This work supported by Malaysian Ministry of Education (MOE) under vote 10H58.    A. Asuhaimi Mohd Zin, A. Naderipour, M. H. Habibuddin (Faculty of Electrical Engineering, Universiti Teknologi Malaysia, Johor, Malaysia) E-mail: asuhaimi@utm.my Josep M. Guerrero (Department of Energy Technology, Aalborg University, DK-9220 Aalborg East, Denmark)  References 1  Chen, Z., Blaabjerg, F., Pedersen, JK.; ‘Hybrid compensation arrangement in dispersed generation systems’, Power Deliv IEEE Trans. IEEE, 2005, 20, (2), pp. 1719–27  2 Panda, SK., Chitti Babu, B.; ‘Improved phase detection technique for grid synchronization of DG systems during grid abnormalities’, In: Engineering and Systems (SCES), 2013 Students Conference on. IEEE, 2013, pp. 1–6  3  Hui, P., Zi-ping, L., Ling, L., Chun-ming, P.; ‘Hybrid compensation for harmonic, reactive power and unbalance under dq0 coordinates’, In: Electrical Machines and Systems, ICEMS 2008 International Conference on. IEEE, 2008, pp. 2004–7.   0.1 0.11 0.12 0.13 0.14 0.15-2000200System Currents (A) Time (s)Ia207.6 (A)THD= 13.5%0.36 0.37 0.38 0.39 0.4 0.41-2000200 PV Currents (A) Time (s)IaTHD= 11.4%250 (A)0.42 0.43 0.44 0.45 0.46 0.47-1000-50005001000System Currents (A) Time (s)1175 (A) IaTHD= 1.69%0.39 0.4 0.41 0.42 0.43 0.44-400-2000200400 PV Currents (A) Time (s)IaTHD= 0.55%339.6 (A)

Harmonic currents Compensator Grid-Connected Inverter at the Microgrid

A.A. Mohd Zin

A. Naderipour

M. Hafiz Habibuddin

J.M. Guerrero

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Electronics Letters

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Harmonic currents compensator GCI at the microgrid

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