An Improved DC-Link Voltage Control Strategy for Grid Connected Converters

 ABSTRACT:

This paper presents a robust control strategy to improve dc-link voltage control performances for Grid connected Converters (GcCs). The proposed control strategy is based on an adaptive PI controller and is aimed to ensure fast transient response, low dc-link voltage fluctuations, low grid current THD and good disturbance rejection after sudden changes of the active power drawn by the GcC. The proportional and integral gains of the considered adaptive PI controller are self-tuned so that they are well suited with regard to the operating point of the controlled system and/or its state. Several simulation and experimental results are presented to confirm and validate the effectiveness and feasibility of the proposed dc-link voltage control strategy.

KEYWORDS:

1.      DC-link voltage control

2.      adaptive PI controller

3.      Grid connected Converters

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

 Fig. 1. Commonly used control structure for Grid-connected Converters

 EXPECTED SIMULATION RESULTS

Fig. 2. Simulation results (ξ=0.7, Vdc init=100V, Vdc

*=150, i=0 at t=0s and i=Imax at t=0.5s) (a) Comparison between standard PI control and adaptive PI control  (b) waveform of the selected ωn value for the adaptive PI controller

CONCLUSION:

This paper presented an improved dc-link voltage controller based on an adaptive PI controller with an anti-windup process. The proportional and integral gains of the proposed PI controller are self-tuned so that the following constraints are satisfied: 1) no overshoot after step jumps of the dc-link voltage reference input; 2) fast dynamic response after step jumps of the dc-link voltage reference; 3) fast dynamic response after step jump of the input current i and 4) low grid current THD value during steady state operation. The considered control was experimentally tested on a prototyping platform. The obtained experimental results are quite similar to simulation results and show the effectiveness and reliability of the adopted control strategy.

REFERENCES:

[1] D. Casadei, M. Mengoni, G. Serra, A. Tani, and L. Zarri, “A control scheme with energy saving and dc-link overvoltage rejection for induction motor drives of electric vehicles,” IEEE Trans. Ind. Appl.,vol. 46, no. 4, pp. 1436–1446, Jul./Aug. 2010.

[2] Li, F., Zou, Y.P., Wang, C.Z., Chen, W., Zhang, Y.C., Zhang, J. “Research on AC Electronic Load Based on back to back Single phase PWM Rectifiers,” Applied Power Electronics Conference and Exposition, 2008. APEC 2008. Twenty-Third Annual IEEE. 2008, pp.630-634. 2008.

[3] M. Karimi-Ghartimani, S.A. Khajehoddin, P. Jain, A. Bakhshai, “A systematic approach to dc-bus control design in single phase grid connected renewable converters,” IEEE Trans. Power Electron, vol. 28, no. 7, pp. 3158–3166, July. 2013.

[4] X. Yuan, F. Wang, D. Boroyevich, Y. Li, and R. Burgos, “Dc-link voltage control of a full power converter for wind generator operating in weak-grid systems,” IEEE Trans. Power Electron., vol. 24, no. 9, pp. 2178–2192, Sep. 2009.

[5] C.-S. Lam, W.-H. Choi, M.-C. Wong, and Y.-D. Han, “Adaptive dc link voltage-controlled hybrid active power filters for reactive power compensation,” IEEE Trans. Power Electron., vol. 27, no. 4, pp. 1758– 1772, Apr. 2012.

An f-P/Q Droop Control in Cascaded-Type Microgrid

ABSTRACT:

In cascaded-type microgrid, the synchronization and power balance of distributed generators become two new issues that needs to be addressed urgently. To that end, an f-P/Q droop control is proposed in this letter, and its stability is analyzed as well. This proposed droop control is capable to achieve power balance under both resistive-inductive an resistive-capacitive loads autonomously. Compared with the inverse power factor droop control, an obvious advantage consists in extending the scope of application. Finally, the feasibility of the proposed method is verified by simulation results.

KEYWORDS:

1.      Cascaded-type microgrid

2.      Droop control

3.      Power balance

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Structure of islanded cascaded-type microgrid.

CONTROL  SYSTEM:

Fig. 2. The local control diagram of the i-th DG.

EXPECTED SIMULATION RESULTS

Fig.3. Simulation results of case I. (a) Active power. (b) Reactive power

Fig. 4. Simulation results of case II. (a) Active power. (b) Reactive power.

 CONCLUSION:

A bridge modular switched-capacitor-based multilevel inverter with optimized UFD-SPWM control method is proposed in the paper. The switched-capacitor-based stage can obtain high conversion efficiency and multiple voltage levels. Meanwhile, it functions as an active energy buffer, enhancing the power decoupling ability and conducing to cut the total size of the twice-line energy buffering capacitance. Furthermore, voltage multi-level in DC-link reduces the switching loss of inversion stage because turn-off voltage stress of switches changes with phase of output voltage rather than always suffers from one relatively high DC voltage. Most importantly, the control method of UFD-SPWM, doubling equivalent witching frequency, is employed in the inversion stage for a high quality output waveform with reduced harmonic. In addition, the optimized voltage level phase maximizes the fundamental component in output voltage pulses to reduce harmonic backflow as possible. Hence, the comprehensive system efficiency has been promoted and up to peak value of 97.6%. Finally, two conversion stages are controlled independently for promoting reliability and decreasing complexity. In future work, detailed loss discussion, including theoretic calculation and validation of loss breakdown, will be presented.

REFERENCES:

[1] M. Jun, "A new selective loop bias mapping phase disposition PWM with dynamic voltage balance capability for modular multilevel converter," IEEE Trans. Ind. Electron., vol. 61, no. 2, pp. 798-807, Feb. 2014.

[2] N. Mehdi, and G. Moschopoulos, "A novel single-stage multilevel type full-bridge converter," IEEE Trans. Ind. Electron., vol. 60, no. 1, pp. 31-42, Jan. 2013.

[3] E. Ehsan and N. B. Mariun, "Experimental results of 47-level switchladder multilevel inverter," IEEE Trans. Ind. Electron., vol. 60, no. 11, pp. 4960-4967, Nov. 2013.

[4] J. Lai, “Power conditioning circuit topologies,” IEEE Trans. Ind. Electron., vol. 3, no. 2, pp. 24-34, Jun. 2009.

[5] L. He, C. Cheng, “Flying-Capacitor-Clamped Five-Level Inverter Based on Switched-Capacitor Topology,” IEEE Trans. Ind. Electron., vol. 63, no.12, pp. 7814-7822, Sep. 2016.

A Buck & Boost based Grid Connected PV Inverter Maximizing Power Yield from Two PV Arrays in Mismatched Environmental Conditions

ABSTRACT:

A single phase grid connected transformerless photo voltaic (PV) inverter which can operate either in buck or in boost mode, and can extract maximum power simultaneously from two serially connected subarrays while each of the subarray is facing different environmental conditions, is presented in this paper. As the inverter can operate in buck as well as in boost mode depending on the requirement, the constraint on the minimum number of serially connected solar PV modules that is required to form a subarray is greatly reduced. As a result power yield from each of the subarray increases when they are exposed to different environmental conditions. The topological configuration of the inverter and its control strategy are designed so that the high frequency components are not present in the common mode voltage thereby restricting the magnitude of the leakage current associated with the PV arrays within the specified limit. Further, high operating efficiency is achieved throughout its operating range. A detailed analysis of the system leading to the development of its mathematical model is carried out. The viability of the scheme is confirmed by performing detailed simulation studies. A 1.5 kW laboratory prototype is developed, and detailed experimental studies are carried out to corroborate the validity of the scheme.

KEYWORDS:

1.      Grid connection

2.      Single phase

3.      Transformerless

4.      Buck & Boost based PV inverter

5.      Maximum power point

6.      Mismatched environmental condition

7.      Series connected module

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. Dual Buck & Boost based Inverter (DBBI)

EXPECTED SIMULATION RESULTS

Fig. 2. Simulated waveform: Variation in (a) p_pv1 and p_pv2, (b) v_pv1 and

v_pv2, (c) i_pv1 and i_pv2 during entire range of operation

Fig.3. Simulated waveform: v_g and i_g and their magnified views

Fig. 4. Simulated waveform: i_L1 and i_L2 and their magnified views

Fig. 5. Simulated waveform: v_co1 and v_co2 and their magnified views

CONCLUSION:

A single phase grid connected transformerless buck and boost based PV inverter which can operate two subarrays at their respective MPP was proposed in this paper. The attractive features of this inverter were i) effect of mismatched environmental conditions on the PV array could be dealt with A single phase grid connected  transformerless buck and boost based PV inverter which can operate two subarrays at their respective MPP was proposed in this paper. The attractive features of this inverter were i) effect of mismatched environmental conditions on the PV array could be dealt with in an effective way, ii) operating efficiency achieved, _euro = 97.02% was high, iii) decoupled control of component converters was possible, iv) simple MPPT algorithm was employed to ensure MPP operation for the component converters, v) leakage current associated with the PV arrays was within the limit mentioned in VDE 0126-1-1. Mathematical analysis of the proposed inverter leading to the development of its small signal model was carried out. The criterion to select the values of the output filter components was presented. The scheme was validated by carrying out detailed simulation studies and subsequently the viability of the scheme was ascertained by carrying out thorough experimental studies on a 1.5 kW prototype of the inverter fabricated for the purpose.

REFERENCES:

[1] T. Shimizu, O. Hashimoto, and G. Kimura, “A novel high-performance utility-interactive photovoltaic inverter system,” IEEE Trans. Power Electron., vol. 18, no. 2, pp. 704-711, Mar. 2003.

[2] S. V. Araujo, P. Zacharias, and R. Mallwitz, “Highly efficient singlephase transformerless inverters for grid-connected photovoltaic systems,” IEEE Trans. Ind. Electron., vol. 57, no. 9, pp. 3118-3128, Sep. 2010.

[3] B. Ji, J. Wang, and J. Zhao, “High-efficiency single-phase transformerless PV H6 inverter with hybrid modulation method,” IEEE Trans. Ind.Electron., vol. 60, no. 5, pp. 2104-2115, May 2013.

[4] R. Gonzalez, E. Gubia, J. Lopez, and L. Marroyo, “Transformerless single phase multilevel-based photovoltaic inverter,” IEEE Trans. Ind. Electron., vol. 55, no. 7, pp. 2694-2702, Jul. 2008.

[5] H. Xiao and S. Xie, “Transformerless split-inductor neutral point clamped three-level PV grid-connected inverter,” IEEE Trans. Power Electron., vol. 27, no. 4, pp. 1799-1808, Apr. 2012.

A Grid Connected Single Phase Transformerless Inverter Controlling Two Solar PV Array Operating under Different Atmospheric Conditions

ABSTRACT:

A grid connected single phase transformerless inverter which can operate two serially connected solar photo voltaic (PV) subarrays at their respective maximum power points while each one of them is exposed to different atmospheric conditions is proposed in this paper. As two subarrays are connected in series, the number of serially connected modules within a subarray is reduced to half. Reduction in the number of serially connected PV modules within a subarray leads to an overall improvement in the magnitude of power that can be abstracted from a subarray while the modules of the subarray are exposed to varied atmospheric conditions. The topological structure of the inverter ensures that the common mode voltage does not contain high frequency components, thereby reducing the magnitude of leakage current involved with the solar panels well within the acceptable limit. An in depth analysis of the scheme along with the derivation of its small signal model has been carried out. Detailed simulation studies are performed to verify its effectiveness. A 1 kW laboratory prototype of the scheme has been fabricated. Detailed experimental validations have been carried out utilizing the prototype to confirm the viability of the proposed scheme.

KEYWORDS:

1.      Grid connected single phase transformerless PV inverter

2.      Maximum power extraction

3.      Mismatched operating condition

4.      Serially connected PV subarrays

SOFTWARE: MATLAB/SIMULINK

 PROPOSED CIRCUIT DIAGRAM:

Fig. 1. Combined Half Bridge Inverter with AC Bypass (CHBIAB)

 EXPECTED SIMULATION RESULTS:

Fig.2. Simulated performance: (a) Power output from PV1 and PV2, (b) Voltage output of PV1 and PV2, (c) Output current of PV1 and PV2

Fig. 3. Simulated performance: Grid current and voltage along with their

magnified versions 

Fig. 4. Simulated performance: Output capacitor voltages along with their magnified versions

CONCLUSION:

A grid connected single phase transformerless inverter which can extract maximum power from two subarrays during  mismatched operating condition is presented in this paper. Salient features of the proposed inverter are as follows: i) number of series connected modules is less thereby reducing the effect of shading, ii) two subarrays can be operated at MPP simultaneously, thus it is well suited for PV subarrays operating under mismatched operating condition, iii) decoupled control structure is employed to control the two component half bridges of the inverter, iv) _euro of 96% is achieved which is the highest compared to the topologies dealing with solar PVs experiencing mismatched operating conditions, v) the scheme is realized through single stage of power conversion leading to a considerable reduction in size, weight and volume, vi) simple MPPT algorithm is employed thereby reducing the computational burden of the digital signal processor involved, vii) PV leakage current is limited within the limit specified in the standard, VDE 0126-1-1. The operating principle of the proposed scheme is explained in detail by exploring all the equivalent topological stages. Subsequently the mathematical analysis of the scheme has been carried out and the small signal model of the scheme has been derived. The philosophy of control is described in detail and the configuration of the controller is derived. The design guidelines for selecting the filter components of the inverter are presented. Detailed simulation and experimental studies are carried out to confirm the viability of the proposed scheme.

REFERENCES:

[1] T. Kerekes, R. Teodorescu, P. Rodriguez, G. Vazquez, and E. Aldabas, “A new high-efficiency single-phase transformerless PV inverter topology,” IEEE Trans. Industrial Electronics, vol. 58, no. 1, pp. 184-191, Jan. 2011.

[2] S. V. Araujo, P. Zacharias, and R. Mallwitz, “Highly efficient single phase transformerless inverters for grid-connected photovoltaic systems,” IEEE Trans. Industrial Electronics, vol. 57, no. 9, pp. 3118- 3128, Sep. 2010.

[3] G. M. Masters, Renewable and efficient electric power systems, New Jersey: John Wiley & Sons Inc, 2004, ISBN: 0-471-28060-7.

[4] T. Shimizu, O. Hashimoto, and G. Kimura, “A novel high-performance utility-interactive photovoltaic inverter system,” IEEE Trans. Power Electronics, vol. 18, no. 2, pp. 704-711, Mar. 2003.

[5] T. Shimizu, M. Hirakata, T. Kamezawa, and H. Watanabe, “Generation control circuit for photovoltaic modules,” IEEE Trans. Power Electronics vol. 16, no. 3, pp. 293-300, May 2001.

A New Design Method of an LCL Filter Applied in Active DC-Traction Substations

ABSTRACT:

This paper concentrates on the LCL filter with damping resistance intended to connect the shunt active power filter of an active DC-traction substation to the point of common coupling with the transmission grid. In order to find design conditions and conceive a design algorithm, attention is directed to the transfer functions related to currents and the associated frequency response. The mathematical foundation of the design method is based on the meeting the requirements related to the significant attenuation of the high-frequency switching current, concurrently with the unalterated flow of the current that needs to be compensated by active filtering. It is pointed out that there are practical limitations and a compromise must be made between the two requirements. To quantify the extent to which the harmonics to be compensated are influenced by imposing the magnitude response at both highest harmonic frequency to be compensated and switching frequency, a performance indicator is defined. As an additional design criterion, the damping power losses are taken into consideration. The validity and effectiveness of the proposed method are proved by simulation results and experimental tests on a laboratory test bench of small scale reproducing the specific conditions of a DC-traction substation with six-pulse diode rectifier.

KEYWORDS:

1.      DC-traction substations

2.      LCL filter

3.      Passive damping

4.      Regeneration

5.      Shunt active power filters

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Block diagram of the active DC-traction substation.

EXPECTED SIMULATION RESULTS:

Fig. 2. Voltages and currents in the TT’s primary in traction regime

Fig. 3. LCL filter input current.

 Fig. 4. Current flowing through the capacitor of the interface filter.

Fig. 5. Harmonic spectra of the LCL filter input current (black bars) and output current (yellow bars) for harmonic order k[1, 37].

Fig. 6. Voltages and currents upstream of PCC during the operation in traction regime.

Fig. 7. Succesive traction (filtering) and braking (regeneration) regimes: (a)

phase voltage (blue line) and supply current (green line); (b) DC-capacitor

voltage (black line) and DC-line voltage (red line).

CONCLUSION:

A new design method of an LCL filter with damping resistance intended to couple the three-phase VSI of an active DC-traction substation to the power supply has been proposed in this paper. The following elements of originality are outlined.

1) The theoretical substantiation is based on the frequency response from transfer functions related to currents, taking into account the existence of the series damping resistances.

2) The expressed amplitude response and resonance frequency highlight their dependence on only pairs L2Cf and RdCf, It is a very important finding for the conceived design algorithm.

3) The expression of the power losses in the damping resistances is highlighted and an equivalent resistance is introduced as a quantitative indicator of them.

4) By considering the switching frequency as main parameter and taking into consideration the frequency of the highest order harmonic to be compensated, the design algorithm is based on the imposition of the associated attenuations.

5) In the substantiation of the design algorithm, a detailed analysis is performed on the existence of physical-sense solutions, providing the domain in which the values of the parameters must be located.

6) As a large number of parameters values sets can be obtained, a new performance indicator (MPI) is proposed, to quantify the extent to which the harmonics to be compensated are influenced.

The analysis and the simulation results achieved for an active DC-traction substation with six-pulse diode rectifier and LCL coupling filter have indicated that the proposed method is valid and effective. The experimental tests conducted in a laboratory test bench of small scale reproducing the specific conditions of a DC-traction substation illustrate good performance of the system for active filtering and regeneration connected to the power supply by the passive damped LCL filter.

The design proposal can be applied in any three-phase LCL-filter-based shunt active power filter.

REFERENCES:

[1] A. Ghoshal and V. John, “Active damping of LCL filter at low switching to resonance frequency ratio,” IET Power Electron., vol. 8, no. 4, pp. 574–582, 2015.

[2] G. E. Mejia Ruiz, N. Munoz, and J. B. Cano, “Modeling, analysis and design procedure of LCL filter for grid connected converters,” in Proc. 2015 IEEE Workshop Power Electron. and Power Quality Appl.  (PEPQA), pp. 1–6.

[3] M. Hanif, V. Khadkikar, W. Xiao, and J. L. Kirtley, “Two degrees of freedom active damping technique for filter-based grid connected PV systems,” IEEE Trans. Ind. Electron., vol. 61, no. 6, pp. 2795–2803, June 2014.

[4] X. Wang, F. Blaabjerg, and P. C. Loh, “Grid-current-feedback active damping for LCL resonance in grid-connected voltage source converters,” IEEE Trans. Power Electron., vol. 31, pp. 213–223, 2016.

[5] W. Xia, J. Kang, “Stability of LCL-filtered grid-connected inverters with capacitor current feedback active damping considering controller time delays,” J. Mod. Power Syst. Clean Energy, vol. 5, no. 4, pp. 584– 598, July 2017.