UDE-Based Current Control Strategy for LCCL-Type Grid-Tied Inverters

ABSTRACT:

LCL filter is usually used as an interface between inverters and the grid. However, due to the characteristics of LCL filter and system uncertainties, it is complex to design a controller with proper parameters. In this paper, with LCCL filter, the order of the inverter control system can be reduced from third order to first order, and an uncertainty and disturbance estimator based control strategy for grid-tied inverters with LCCL filter is proposed. Specifically, the proposed control strategy consists of differential feed forward, proportional–integral controller, and grid voltage feed forward. Moreover, with one-sampling computation plus half-sampling pulse width modulation delays considered, a simple and clear tuning algorithm for the proposed control strategy is presented. Finally, the inverter system with the proposed control strategy is investigated, and the effectiveness is supported by the tuning and comparative experiments with a 2-kW inverter.

KEYWORDS:

1.      Current control

2.      Inverter

3.      LCCL filter

4.      Tuning  algorithm

5.      Uncertainty and disturbance estimator (UDE)

 SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. System topology of the grid-tied inverter with LCCL filter.

.

EXPECTED SIMULATION RESULTS

Fig. 2. Result of UDE-based control without grid voltage feed forward. (a) Injected grid current i₂ . (b) Spectrum of the injected grid current.

Fig. 3. Tuning results of UDE-based control with the same α = 10 000 rad/s, β = 5000 rad/s, and different k. (a) k = 10 000 rad/s. (b) k = 9000 rad/s. (c) k = 7000 rad/s.

Fig. 4. Result of UDE-based control under i^*₁₂ (s) = 10 A with α = 10 000 rad/s, β = 5000 rad/s, and k = 8000 rad/s. (a) Injected grid current i₂ . (b) Spectrum of the injected grid current.

Fig. 5. Result of PI control under i^*₁₂ (s) = 10 A with k_p = 17 and k_i  = 14400. (a) Injected grid current i₂ . (b) Spectrum of the injected grid current.

CONCLUSION:

For grid-tied inverter, LCL filter is widely used to attenuate the high switching frequency harmonics caused by PWM. However, due to the characteristic of LCL filter and uncertainty, it is complex to design a controller with proper parameters. In this paper, with LCCL filter, the inverter control system can be degraded from third order to first order. And a UDE-based injected grid current control strategy was built. The proposed strategy unified the system uncertainty and disturbance into the lumped disturbances, and the closed-loop system adjusted by PI regulator approached to the reference model. Meanwhile, the PI controller can be expressed in the error feedback gain, the desired closed-loop bandwidth, and the approximate lumped disturbance bandwidth. Moreover, with one-sampling computation plus half-sampling PWM delays considered, a simple and clear tuning algorithm for the proposed control strategy was provided. Finally, the proposed strategy was verified by the tuning and comparative experiments on a 2-kW inverter.

REFERENCES:

 [1] M. Lindgren and J. Svensson, “Control of a voltage-source converter connected to the grid through an LCL-filter-application to active filtering,” in Proc. IEEE Power Electron. Spec. Conf., May 1998, pp. 229–235.

[2] E. Twining and D. G. Holmes, “Grid current regulation of a three-phase voltage source inverter with an LCL input filter,” IEEE Trans. Power Electron., vol. 18, no. 3, pp. 888–895, May 2003.

[3] G. Shen, D. Xu, L. Cao, and X. Zhu, “An improved control strategy for grid-connected voltage source inverters with an LCL filter,” IEEE Trans. Power Electron., vol. 23, no. 4, pp. 1899–1906, Jul. 2008.

[4] G. Shen, X. Zhu, J. Zhang, and D. Xu, “A new feedback method for PR current control ofLCL-filter-based grid-connected inverter,” IEEE Trans. Ind. Electron., vol. 57, no. 6, pp. 2033–2041, Jun. 2010.

[5] R. P. Alzola, M. Liserre, F. Blaabjerg, R. Sebasti´an, J. Dannehl, and F. W. Fuchs, “Analysis of the passive damping losses in LCL-filter-based grid converters,” IEEE Trans. Power Electron., vol. 28, no. 6, pp. 2642–2646, Jun. 2013.

Dual-function PV-ECS integrated to 3P4W distribution grid using 3M-PLL control for active power transfer and power quality improvement

 ABSTRACT:

This study proposes a single-stage solar photovoltaic energy conversion system (PV-ECS) integrated to a three phase four-wire (3P4W) distribution grid with dual-function capabilities, i.e. active power transfer and power quality (PQ) enhancement at the point of interaction (PoI). The PV-ECS system comprises of a solar photovoltaic array and a voltage source inverter (VSI), supplying active power (during daytime) to the distribution grid and connected single-phase and three-phase loads. Apart from transfer of power, the system also improves the PQ at the PoI by compensating reactive power and neutral current, attenuating harmonics, correcting power factor and balancing grid currents. During night, the VSI acts as a shunt active power filter mitigating PQ issues, thereby increasing the device utilisation factor. A three-phase magnitude-phase locked loop (3M-PLL) method is utilised to extract and estimate fundamental term of load currents and an incremental conductance algorithm is applied for maximum power point tracking. To demonstrate its effectiveness, the system is modelled and its performance is simulated on MATLAB and experiments are performed on a developed prototype in the laboratory.

SOFTWARE: MATLAB/SIMULINK

 CIRCUIT DIAGRAM:

Fig. 1 System configuration and control scheme

(a) Structure diagram of 3P4W grid-connected PV-ECS

EXPECTED SIMULATION RESULTS

Fig. 2 Dynamic behaviour of system at

(a), (b) Unbalanced load, (c) Step increase in irradiance from 700 to 1000 W/m2

CONCLUSION:  

A dual-function single-stage PV-ECS integrated to the 3P4 distribution grid has been proposed here. Two modes of operation of PV-ECS are to supply and transfer active power to the grid and tied loads as well as to improve quality of power at PoI. An In  Cbased approach is utilised here for tracking MPP of solar PV array and a 3M-PLL-based control scheme is utilised for extracting  fundamental components of load current. Simulated and test results have demonstrated the performance of the system under various conditions such as non-linear loading, unbalanced loading and varying irradiance levels. Test results have shown that the system has improved the power quality at the PoI by compensating neutral current and reactive power, correcting power factor and balancing loads on the grid side. The harmonics are reduced to below 5% on grid side, which is within the limits of an IEEE-519 standard. Moreover, test results have indicated that the system has operated suitably during night-time (sunlight unavailability) thereby increasing the utilisation factor of the VSI by two-fold. The single stage structure has decreased the losses in the system and increased the total efficacy of the system.

REFERENCES:

[1] Muro, M., Saha, D.: ‘Why rooftop solar – and full retail feed in tariffs – benefits all consumers’, 30 May 2016. Available at http:// reneweconomy.com.au/2016/rooftop-solar-net-metering-is-a-netbenefit- 28170

[2] Meza, E.: ‘India implements new 40 GW rooftop, small PV plant program’, 20 May 2016. Available at http://www.pv-magazine.com/news/details/beitrag/ india-implements-new-40-gw-rooftop--small-pv-plant-program-_100024678/ #axzz4ADc3MIV6

[3] Deo, S., Jain, C., Singh, B.: ‘A PLL-less scheme for single-phase grid interfaced load compensating solar PV generation system’, IEEE Trans. Ind. Inf., 2015, 11, (3), pp. 692–699

[4] Yang, Y., Blaabjerg, F., Wang, H., et al.: ‘Power control flexibilities for grid connected multi-functional photovoltaic inverters’, IET Renew. Power Gener., 2016, 10, (4), pp. 504–513

[5] Agarwal, R., Hussain, I., Singh, B.: ‘LMF based control algorithm for single stage three-phase grid integrated solar PV system’, IEEE Trans. Sust. Energy, 2016, 7, (4), pp. 1379–1387

Development of a Bidirectional DC/DC Converter with Dual-Battery Energy Storage for Hybrid Electric Vehicle System

ABSTRACT:

This study develops a newly designed, patented, bidirectional dc/dc converter (BDC) that interfaces a main energy storage (ES1), an auxiliary energy storage (ES2), and dc-bus of different voltage levels, for application in hybrid electric vehicle systems. The proposed converter can operate in a step-up mode (i.e., low-voltage dual-source-powering mode) and a step-down (i.e., high-voltage dc-link energy-regenerating mode), both with bidirectional power flow control. In addition, the model can independently control power flow between any two low-voltage sources (i.e., low-voltage dual-source buck/boost mode). Herein, the circuit configuration, operation, steady-state analysis, and closed-loop control of the proposed BDC are discussed according to its three modes of power transfer. Moreover, the simulation and experimental results for a 1 kW prototype system are provided to validate the proposed converter.

KEYWORDS:

1.      Bidirectional dc/dc converter (BDC)

2.      Dual battery storage

3.      Hybrid electric vehicle

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

Fig. 1. Typical functional diagram for a FCV/HEV power system.

 EXPECTED SIMULATION RESULTS

Fig.2. Measured waveforms for low-voltage dual-source-powering mode: (a) gate signals; (b) output voltage and inductor currents.

Fig.3. Measured waveforms for high-voltage dc-bus energy-regenerating mode: (a) gate signals; (b) output voltage and inductor currents.

Fig. 4. Measured waveforms of gate signals, output voltage and inductor currents for the low-voltage dual-source buck/boost mode: (a) buck mode; (b) boost mode.

Fig. 5. Waveforms of controlled current step change in the low-voltage dual-source-powering mode: (a) by simulation; and (b) by measurement. (iH is changed from 0 to 0.85 A; iL1 is changed from 0 to 2.5 A; Time/Div=20 ms/Div

Fig. 6. Waveforms of controlled current step change in the low-voltage dual-source boost mode: (a) by simulation; and (b) by measurement. (iES1 is changed from 0 to -6 A; iL2 is changed from 0 to 12 A; Time/Div=20 ms/Div)

Fig. 7. Waveforms of controlled current step change in the low-voltage dual- source buck mode: (a) by simulation; and (b) by measurement. (iES1 is changed from 0 to 6 A; iL2 is changed from 0 to 12 A; Time/Div=20 ms/Div)

CONCLUSION:

A new BDC topology was presented to interface dual battery energy sources and high-voltage dc bus of different voltage levels. The circuit configuration, operation principles, analyses, and static voltage gains of the proposed BDC were discussed on the basis of different modes of power transfer. Simulation and experimental waveforms for a 1 kW prototype system highlighted the performance and feasibility of this proposed BDC topology. The highest conversion efficiencies were 97.25%, 95.32%, 95.76%, and 92.67% for the high-voltage dc-bus energy-regenerative buck mode, low-voltage dual-source-powering mode, low-voltage dual-source boost mode (ES2 to ES1), and low-voltage dual-source buck mode (ES1 to ES2), respectively. The results demonstrate that the proposed BDC can be successfully applied in FC/HEV systems to produce hybrid power architecture (has been patented [37]).

REFERENCES:

[1] M. Ehsani, K. M. Rahman, and H. A. Toliyat, "Propulsion system design of electric and hybrid vehicles," IEEE Transactions on industrial electronics, vol. 44, no. 1, pp. 19-27, 1997.

[2] A. Emadi, K. Rajashekara, S. S. Williamson, and S. M. Lukic, "Topological overview of hybrid electric and fuel cell vehicular power system architectures and configurations," IEEE Transactions on Vehicular Technology, vol. 54, no. 3, pp. 763-770, 2005.

[3] A. Emadi, S. S. Williamson, and A. Khaligh, "Power electronics intensive solutions for advanced electric, hybrid electric, and fuel cell vehicular power systems," IEEE Transactions on Power Electronics, vol. 21, no. 3, pp. 567-577, 2006.

[4] E. Schaltz, A. Khaligh, and P. O. Rasmussen, "Influence of battery/ultracapacitor energy-storage sizing on battery lifetime in a fuel cell hybrid electric vehicle," IEEE Transactions on Vehicular Technology, vol. 58, no. 8, pp. 3882-3891, 2009.

[5] P. Thounthong, V. Chunkag, P. Sethakul, B. Davat, and M. Hinaje, "Comparative study of fuel-cell vehicle hybridization with battery or supercapacitor storage device," IEEE transactions on vehicular technology, vol. 58, no. 8, pp. 3892-3904, 2009

Combination Analysis and Switching Method of a Cascaded H-Bridge Multilevel Inverter Based on Transformers With the Different Turns Ratio for Increasing the Voltage Level

 ABSTRACT:

This paper analyzes the combination in a cascaded H-bridge multilevel inverter (CHBI) based on transformers with the different turn ratios for increasing the voltage level and proposes the switching method for achieving the output voltage distribution among H-bridge cells (HBCs). The transformers used in this paper are connected to the output of the respective HBCs, and the secondary sides of all the transformers are connected in series for generating the final output voltage. Only one of the transformers, in particular, has a different turn ratio for increasing the output voltage level. In this paper, the possible turn ratio of the special transformer with a different turn ratio is discussed in detail, and a switching method based on the level-shifted switching method for the topology used in this paper is proposed. To verify the effectiveness of the proposed method, a three-phase 21-level CHBI is experimentally tested.

KEYWORDS:

1.      Cascaded H-bridge inverter (CHBI)

2.      Cascaded multilevel

3.      Level-shifted switching method

4.      Multilevel inverter

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. Transformer-based CHBI topology used in this paper.

EXPECTED SIMULATION RESULTS

Fig. 2. Simulation results of the proposed switching method with the

balanced voltage distribution at Mi = 1.

Fig. 3. Simulation results of the proposed switching method atMi =1. (a) With and (b) without the balanced voltage and power distributions.

CONCLUSION:

Transformer-based CHBI topology was introduced in this paper. The comparison analysis between topologies was shown in Table XI. The theoretical analysis regarding the selection of the turns ratio of the subtransformer was presented. In addition, a switching method based on the level-shifted switching method with the balanced voltage and power distributions was proposed for the transformer-based CHBI topology. Several requirements related to the decision of switching devices and the design of transformer were suggested.A21-level CHBIwas used to determine the feasibility and effectiveness of the proposed switching method.

When using the proposed switching method, two issues are to be noted: 1) to use the proposed switching method, the configuration of transformer-based CHBI topology should follow that of Table II. It guarantees that the minimum variation (dVx ) of the voltage level is always the same. 2) If the system operates in the wide voltage range (wideMi ), Table VII should be changed as that explained in this paper to guarantee a balanced voltage distribution for Mi range required. Table VI guarantees a balanced voltage distribution for 0.8<Mi < 1. Consequently, this configuration can be applied for the main power supply system generating the ac voltage in grid.

REFERENCES:

[1] J. Rodr´ıguez, S. Bernet, B. Wu, J. O. Pontt, and S. Kouro, “Multilevel voltage-source-converter topologies for industrial medium-voltage drives,” IEEE Trans. Ind. Electron., vol. 54, no. 6, pp. 2930–2945, Dec. 2007.

[2] J. S. Lee and K. B. Lee, “New modulation techniques for a leakage current reduction and a neutral-point voltage balance in transformerless photovoltaic systems using a three-level inverter,” IEEE Trans. Power Electron., vol. 29, no. 4, pp. 1720–1732, Apr. 2014.

[3] C. H. Ng, M. A. Parker, L. Ran, P. J. Tavner, J. R. Bumby, and E. Spooner, “A multilevel modular converter for a large, light weight wind turbine generator,” IEEE Trans. Power Electron., vol. 23, no. 3, pp. 1062–1074, May 2008.

[4] V.Yaramasu and B.Wu, “Predictive control of a three-level boost converter and an NPC inverter for high-power PMSG-based medium voltage wind energy conversion systems,” IEEE Trans. Power Electron., vol. 29, no. 10, pp. 5308–5322, Oct. 2014.

[5] J. Mei, B. Xiao, K. Shen, L. M. Tolbert, and J. Y. Zheng, “Modular multilevel inverter with new modulation method and its application to photovoltaic grid-connected generator,” IEEE Trans. Power Electron., vol. 28, no. 11, pp. 5063–5073, Nov. 2013.

Autonomous Power Management for Interlinked AC-DC Microgrids

ABSTRACT:

The existing power management schemes for interlinked AC-DC microgrids have several operational drawbacks. Some of the existing control schemes are designed with the main objective of sharing power among the interlinked microgrids based on their loading conditions, while other schemes regulate the voltage of the interlinked microgrids without considering the specific loading conditions. However, the existing schemes cannot achieve both objectives efficiently. To address these issues, an autonomous power management scheme is proposed, which explicitly considers the specific loading condition of the DC microgrid before importing power from the interlinked AC microgrid. This strategy enables voltage regulation in the DC microgrid, and also reduces the number of converters in operation. The proposed scheme is fully autonomous while it retains the plug-nplay features for generators and tie-converters. The performance of the proposed control scheme has been validated under different operating scenarios. The results demonstrate the effectiveness of the proposed scheme in managing the power deficit in the DC microgrid efficiently and autonomously while maintaining the better voltage regulation in the DC microgrid.

KEYWORDS:

1.      Autonomous control

2.      Distributed control

3.      Droop control

4.      Hybrid microgrids

5.      Interlinked microgrids

6.      Power management

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Interlinked AC-DC microgrids and their control strategy.

EXPECTED SIMULATION RESULTS

Fig. 2 Scenario 1: Results showing (a) generators and tie-converter power, (b) DC microgrid voltage and (c) tie-converter control signals for four different load operating conditions.

Fig. 3. Scenario 2: Results showing (a) DC microgrid load demand, (b) generators and tie-converter power, (c) DC microgrid voltage and (d) tie-converter control signals at varying solar PV and load operating conditions.

CONCLUSION:

An autonomous power management scheme has been presented for interlinked AC-DC microgrids having different configurations. The proposed scheme manages the power deficit in the DC microgrid efficiently and autonomously. The number of tie-converters in operation has been reduced with the proposed prioritization to avoid unnecessary operational losses. The scheme has demonstrated better voltage regulation in the DC microgrid. The performance and robustness of the proposed scheme have been validated for two different scenarios of the DC microgrid at variable load conditions.

REFERENCES:

[1] J. Rocabert, A. Luna, F. Blaabjerg, and P. Rodr´ıguez, “Control of power converters in AC microgrids,” IEEE Transactions on Power Electronics, vol. 27, no. 11, pp. 4734–4749, Nov. 2012.

[2] M. Liserre, T. Sauter, and J. Y. Hung, “Future energy systems: integrating renewable energy sources into the smart power grid through industrial electronics,” IEEE Industrial Electronics Magazine, vol.4. no. 1, pp. 18–37, Mar. 2010.

[3] M. Tsili and S. Papathanassiou, “A review of grid code technical requirements for wind farms,” IET Renewable Power Generation, vol. 3, no. 3, pp. 308–332, Sep. 2009.

[4] T. Strasser, F. Andr´en, J. Kathan, C. Cecati, C. Buccella, P. Siano, P. Leit˜ao, G. Zhabelova, V. Vyatkin, P. Vrba, and V. Maˇr´ık, “A review of architectures and concepts for intelligence in future electric energy systems,” IEEE Transactions on Industrial Electronics, vol. 62, no. 4,pp. 2424–2438, Apr. 2015.

[5] A. Kwasinski, “Quantitative evaluation of dc microgrids availability: Effects of system architecture and converter topology design choices,” IEEE Transactions on Power Electronics, vol. 26, no. 3, pp. 835–851, Mar. 2011.