Weak Grid Integration of a Single-Stage Solar Energy Conversion System With Power Quality Improvement Features Under Varied Operating Conditions

ABSTRACT:

A three-phase single-stage solar energy conversion system (SECS) integrated into a weak distribution network is presented. The grid integration and maximum power point operation of the photovoltaic (PV) array are achieved by a voltage source converter. The SECS is capable of feeding distortion-free and balanced grid currents with power factor correction, even at adverse grid side, PV array side, and load side operating conditions. The integration of SECS into the weak grid having distorted, unbalanced, and varying grid voltages is achieved while maintaining the power quality. The dc offset introduced in the sensed grid voltages is also effectively eliminated. For swift system response to changes in load currents, their fundamental weights are swiftly extracted. In the absence of solar irradiance, the power is imported from the utility to supply the local loads, and the system continues to execute its power quality improvement functions. In case of loss of PV power or large voltage deviations, the dc-link voltage is adaptively varied according to the grid voltage changes, increasing system reliability, and reducing operating losses. The efficacy of the SECS is validated through test results at different operating scenarios.

KEYWORDS:

1.      Maximum power point (MPP) tracking

2.      Power quality

3.      Single-stage photovoltaic (PV) system

4.      Weak grid integration

SOFTWARE: MATLAB/SIMULINK

SCHEMATIC DIAGRAM:

Fig. 1. System configuration.

EXPECTED SIMULATION RESULTS:

Fig.2. Performance at grid voltages distortion while (a), (b) SECS is out of operation while (c), (d) SECS is in operation.

 

Fig. 3. DC offset elimination performance of GVP stage.

 

 

Fig. 4. Performance at grid voltages unbalance while (a), (b) SECS is OFF while (c), (d) SECS is in operation.

 

 

Fig. 5. Dynamic response of the system. (a) Irradiance reduction. (b) Irradiance increment.

 

Fig. 6. SECS response at rapid changeover. (a), (c) PV to DSTATCOM operation. (b), (d) DSTATCOM to PV operation.

 

CONCLUSION:

The performance of a single-stage PV system with PV array power delivery directly at the dc-link capacitor of VSC, equipped to operate in weak grid conditions, with resilience against the grid side, PV array side, and load side disturbances, is demonstrated. The presented GVP stage has successfully eliminated the adverse effects of distorted, unbalanced, and varying grid voltages from the grid currents. Furthermore, the dc offset rejection from the acquired grid voltages has been validated. The SECS has also demonstrated the features of grid currents balancing, power factor correction, and harmonics reduction to meet the IEEE 519 standard, at various operating conditions. A DNLMS algorithm has been modified and applied for rapid extraction of the fundamental weights from the load currents. Its accuracy and response speed under sudden load disturbances have been found satisfactory. The MPP tracking has been successfully carried out at various irradiance levels, using an INC-based technique, which has provided the reference value formaintaining the dc-link voltage to the PV array MPP. With the change in the operating conditions, the SECS has demonstrated the smooth transfer of the dc-link voltage regulation from the INC technique to an adaptive strategy, which has generated the reference value according to the grid voltage level. This enabled the optimum operation of the SECS as a DSTATCOM in low-irradiance periods, enhancing the system utilization. The grid currents are demonstrated to vary swiftly at PV power fluctuations and the grid voltage deviations due to effective use of a PVFF term, and hence, dc-link voltage is not disturbed from MPP. The system robustness at weak grid conditions, PV side, and load side fluctuations has been validated by test results.

REFERENCES:

[1] A. J. Waldau, I. Kougias, N. Taylor, and C. Thiel, “How photovoltaics can contribute to GHG emission reductions of 55% in the EU by 2030,” Renewable Sust. Energy Rev., vol. 126, Jul. 2020, Art. no. 109836.

[2] A. A. Almehizia, H. M. K. Al-Masri, and M. Ehsani, “Feasibility study of sustainable energy sources in a fossil fuel rich country,” IEEE Trans. Ind. Appl., vol. 55, no. 5, pp. 4433–4440, Sep./Oct. 2019.

[3] O. M. Akeyo, V. Rallabandi, N. Jewell, and D. M. Ionel, “The design and analysis of large solar PV farm configurations with DC-Connected battery systems,” IEEE Trans. Ind. Appl., vol. 56, no. 3, pp. 2903–2912, May/Jun. 2020.

[4] F.Hafiz, M. A.Awal, A. R. d. Queiroz, and I. Husain, “Real-time stochastic optimization of energy storage management using deep learning-based forecasts for residential PV applications,” IEEE Trans. Ind. Appl., vol. 56, no. 3, pp. 2216–2226, May/Jun. 2020.

[5] M. A.Mahmud, T. K. Roy, S. Saha,M. E. Haque, and H. R. Pota, “Robust nonlinear adaptive feedback linearizing decentralized controller design for islanded DC microgrids,” IEEE Trans. Ind. Appl., vol. 55, no. 5, pp. 5343–5352, Sep./Oct. 2019.

Vienna Rectifier Fed Squirrel Cage Induction Generator based Stand-alone Wind Energy Conversion System

ABSTRACT:

 Back to back voltage source converters are normally preferred for interfacing squirrel cage induction generators (SCIG) with loads in stand-alone wind power generation applications as they allow for maximum power point tracking. However, the total converter losses tend to be very high, especially near the rated wind speed. Therefore, to reduce the power converter related losses, a Vienna rectifier is utilized as the machine side converter (MSC) in the present work to interface the SCIG. Owing to the limited reactive power handling capability of the Vienna rectifier a fixed capacitor bank is also required to provide the excitation VAR for the variable speed SCIG. In the present work a new computational method is proposed to calculate the value of this fixed capacitance based on maximizing the yearly energy output from the Wind Energy Conversion System (WECS). A voltage sensor less vector control scheme for the Vienna rectifier fed SCIG with the reference frame oriented along the machine terminal voltage is also proposed which adheres to the operating limits of the Vienna rectifier and gives much better dynamic performance under load and wind speed transients compared to similar Vienna rectifier based VSCF generating systems reported earlier in the literature.

KEYWORDS:

1.      Vienna rectifier

2.      Voltage sensor less

3.      Maximum yearly energy

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

 

 

Fig. 1 Equivalent circuit representation of an induction machine with excitation capacitor and Vienna rectifier

 EXPECTED SIMULATION RESULTS:

Fig. 2 Performance of the proposed control scheme for Vienna rectifier assisted SCIG based WECS during load transient

 

Fig. 3 Performance of the proposed control scheme for Vienna rectifier assisted SCIG based WECS during wind speed variation

 CONCLUSION:

The excitation VAR required by the SCIG in a SCIG based stand-alone VSCF WECS can be provided by a combination of a fixed capacitance and a Vienna rectifier. An algorithm is proposed to choose the value of the fixed capacitance based on maximizing the yearly energy. A control scheme is developed to regulate the active power and reactive power adhering to the operating limits of the Vienna rectifier. The proposed control scheme increases the annual energy capture by 8 % to 10 % compared to operating the Vienna rectifier at unity terminal power factor. The performance of the control scheme is found to be much superior (compared to similar control scheme reported in the literature) under load transients, wind speed transients and with nonlinear/unbalanced loads. However, the LUT used in the present scheme to compute the terminal voltage reference is machine parameter dependent which varies with operating condition and ageing effect. The uncertainty in the values of the machine parameters can be mitigated by computing the terminal voltage reference value from the above method using nominal machine parameters and can then be further refined using online search based methods. The proposed converter scheme extracts about 8 % more electrical power at the rated wind speed compared to the B2B scheme and requires less cut-in wind speed compared to the STATCOM assisted SCIG. Further, the Vienna rectifier based WECS is shown to be capable of generating maximum annual energy output at most locations. Hence, this configuration can be regarded as a “standard topology” for small wind energy conversion systems feeding isolated loads.

REFERENCES:

[1] Z. Alnasir and M. Kazerani, “An analytical literature review of stand-alone wind energy conversion system from generator viewpoint,” Renew. Sustain. Energy Rev., vol. 28, pp. 597–615, 2013, doi: 10.1016/j.rser.2013.08.027.

[2] A. S. Satpathy, D. Kastha, and K. Kishore, “Control of a STATCOM-assisted self-excited induction generator-based WECS feeding non-linear three-phase and single-phase loads,” IET Power Electron., vol. 12, pp. 829–839, 2018, doi: 10.1049/iet-pel.2018.5482.

[3] A. S. Satpathy, N. K. Kishore, D. Kastha, and N. C. Sahoo, “Control Scheme for a Stand-Alone Wind Energy Conversion System,” IEEE Trans. Energy Convers., vol. 29, no. 2, pp. 418–425, 2014, doi: 10.1109/TEC.2014.2303203.

[4] G. K. Singh, “Self-excited induction generator research - A survey,” Electr. Power Syst. Res., vol. 69, no. 2–3, pp. 107–114, 2004, doi: 10.1016/j.epsr.2003.08.004.

[5] B. Singh, S. S. Murthy, and R. S. R. Chilipi, “STATCOM-based controller for a three-phase SEIG feeding single-phase loads,” IEEE Trans. Energy Convers., vol. 29, no. 2, pp. 320–331, 2014, doi: 10.1109/TEC.2014.2299574.

System Modeling and Stability Analysis of Single- Phase Transformerless UPQC Integrated Input Grid Voltage Regulation

 ABSTRACT:

This paper extends the conventional features of a transformerless unified power quality conditioner (TL-UPQC). An enhanced control methodology is presented to allow exchanging reactive power between the system and the grid to provide input grid voltage regulation. Therefore, both load side and input grid side voltages are regulated with one converter. In this regard, a phase angle is created between the input current and the input voltage. Thereby, the system behavior as a capacitive or an inductive reactance is controlled. An additional ac voltage control loop has been designed. The inner current loop has been reformed to receive reference information from two outer voltage loops. The enhanced control strategy takes action based on local information collected by the TL-UPQC with no requirements of additional sensor circuits. Since the conventional functions of the TL-UPQC system have been extended, aspects related to system modeling and control design should be developed. Small-signal model that characterize the dynamics of the power stage and the controller are presented. Design guidelines considering grid impedance to achieve a desired performance are developed. A 500VA / 120V, 60 Hz prototype has been built to verify the models and the overall system performance. Steady-state and transient experimental results are presented and discussed.

KEYWORDS:

1.      Grid impedance

2.      Modeling

3.      Power quality

4.      Reactive power compensation

5.      Transformerless

6.      UPQC

7.      Voltage control

8.      Voltage regulation

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. A block diagram of power flow in a TL-UPQC system.

EXPECTED SIMULATION RESULTS:

Fig. 2. Simulation results adopting conventional TL-UPQC system under random voltage variations in the network.

 

Fig. 3. Simulation results adopting proposed solution under random voltage variations in the network.

 

CONCLUSION:

The paper expands the control strategy of a TL-UPQC system to be capable of injecting and absorbing reactive power into and from the input grid in low voltage distribution networks. Employing the proposed enhanced control strategy, the TL-UPQC was able to filter out harmonic components generated by nonlinear loads, compensate all voltage fluctuations across sensitive loads with fast dynamic response and improve the voltage profile of the input grid. A detailed stability analysis and control design criteria employing small-signal models were presented. The experimental results validated the capability of the system to provide voltage regulation at the PCC while supplying linear and nonlinear sensitive loads at the same time. The system was able to reduce the total harmonic distortion of the PCC voltage, the load bus voltage and the grid current. The system was competent to deliver a stable voltage with a constant amplitude to the loads connected to the PCC in the event of voltage sags and swells. Experimental results favorably showed good agreement with the theoretical findings. It is to be noted that adopting a half-bridge topology would increase the voltage stress across the semiconductor switches. Safety mechanisms should be considered if the proposed transformerless system is to be adopted in residential applications.

REFERENCES:

 

[1] R. Tonkoski, D. Turcotte and T. H. M. EL-Fouly, "Impact of High PV Penetration on Voltage Profiles in Residential Neighborhoods," IEEE Trans. Sustainable Energy, vol. 3, no. 3, pp. 518-527, July 2012.

[2] T. Aziz and N. Ketjoy, "PV Penetration Limits in Low Voltage Networks and Voltage Variations," IEEE Access, vol. 5, pp. 16784-16792, 2017.

[3] H. R. Esmaeilian and R. Fadaeinedjad, "A Remedy for Mitigating Voltage Fluctuations in Small Remote Wind-Diesel Systems Using Network Theory Concepts," IEEE Trans. Smart Grid, vol. 9, no. 5, pp. 4162-4171, Sept. 2018.

[4] H. F. Bilgin and M. Ermis, "Design and Implementation of a Current- Source Converter for Use in Industry Applications of D-STATCOM," IEEE Trans. Power Electron., vol. 25, no. 8, pp. 1943- 1957, Aug. 2010.

[5] T. Lee, S. Hu and Y. Chan, "D-STATCOM With Positive-Sequence Admittance and Negative-Sequence Conductance to Mitigate Voltage Fluctuations in High-Level Penetration of Distributed-Generation Systems," IEEE Trans. Ind. Electron., vol. 60, no. 4, pp. 1417-1428, Apr. 2013.

 

Symmetrical Pole Placement Method-Based Unity Proportional Gain Resonant and Gain Scheduled Proportional (PR-P) Controller With Harmonic Compensator for Single Phase Grid-Connected PV Inverters

ABSTRACT:

In this paper, a symmetrical pole placement Method-based Unity Proportional Gain Resonant and Gain Scheduled Proportional (PR-P) Controller is presented. The proposed PR-P controller resolved the issues that are tracking repeating control input signal with zero steady-state and mitigating of 3^rd order harmonic component injected into the grid associated with the use of PI controller for single-phase PV systems. Additionally, the PR-P controller has overcome the drawbacks of frequency detuning in the grid and increase in the magnitude of odd number harmonics in the system that constitute the common concerns in the implementation of conventional PR controller developed as an alternative to PI controller. Moreover, the application of an unprecedented design process based on changing notch filter dynamics with symmetrical pole placement around resonant frequency overcomes the limitations that are essentially complexity and dependency on the precisely modelled system associated with the use of various controllers such as Adaptive, Predictive and Hysteresis in grid connected PV power generation systems. The proposed PR-P controller was validated employing Photovoltaic emulator (PVE) consisting of a DC-DC Buck power converter, a maximum power point tracking (MPPT) algorithm and a full-bridge grid connected inverter designed using MATLAB/Simulink system platform. Details of the proposed controller, Photovoltaic emulator (PVE) simulations, analysis and test results were presented in the paper.

 KEYWORDS:

1.      Proportional resonant current controller

2.      Harmonic compensator

3.      Buck converter based PV emulator

4.      MPPT

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Figure 1. PVE Based Single Phase Grid-Connected Inverter System.

EXPECTED SIMULATION RESULTS:

Figure 2. The PVE Current For Varying Irradiance.

Figure 3. The PVE Voltage For Varying Irradiance.

Figure 4. System Outputs With The Use Of Proposed PR-P Controller.

Figure 5. Generated Power With Delivered And Reactive Powers.

Figure 6. Closed-Loop Error In Terms Of 3^rd Order Harmonics.

Figure 7. PR-P And PI Controlled Grid Currents With Scaled Grid Voltage.

CONCLUSION:

This paper has presented an alternative unprecedented design process for a Proportional-Resonant (PR) controller with a selective harmonic components (3rd and 5th order) compensator for Photovoltaic Emulator (PVE) supported single phase Grid Connected Inverter (GCI) systems. The design procedure of the proposed controller unity proportional resonant (PR) path is conducted based on notch filter dynamics regulated by symmetrical pole placement methods. Addition of scheduled proportional gain designed by loop shaping method to the resonant path increased the performance of the controller in terms of robustness, achieving better results in the presence of non-linear load and weak grid. The performance of the proposed controller and harmonic compensator is validated employing a PVE consisting of a DC-DC Buck converter, a Maximum Power Point Tracking (MPPT) algorithm and a full-bridge GCI designed using MATLAB/Simulink platforms. Frequency and time domain analysis of the system elements showed satisfactory behaviors. A comparative analysis with different PR controller design techniques used in various papers is performed and resulted in confirming that the proposed technique is robust and simple to implement. The performance of the Proposed PR-P controller with the harmonic compensator is compared with a PI in stationary reference frame and conventional PR current controllers in terms of steady-state error and harmonics mitigation. The simulation results demonstrated that the proposed PR-P controller with harmonic compensator is superior at tracking sinusoidal reference current with zero steady-state error and lower total harmonic distortion with eliminated 3rd and 5th order harmonics. The overall system is under development and experimental results will be presented in the near future.

REFERENCES:

[1] R. Ayop and C. W. Tan, ``A comprehensive review on photovoltaic emulator,'' Renew. Sustain. Energy Rev., vol. 80, pp. 430_452, Dec. 2017.

[2] S. Seyam, I. Dincer, and M. Agelin-Chaab, ``Development of a clean power plant integrated with a solar farm for a sustainable community,'' Energy Convers. Manage., vol. 225, Dec. 2020, Art. no. 113434.

[3] W. Xiao, Photovoltaic Power Systems: Modeling, Design, and Control, 1st ed. Hoboken, NJ, USA: Wiley, 2017.

[4] G. Price, Renewable Power and Energy, 1st ed. New York, NY, USA: Momentum Press, 2018.

[5] B. Carrera and K. Kim, ``Comparison analysis of machine learning techniques for photovoltaic prediction using weather sensor data,'' Sensors, vol. 20, no. 11, p. 3129, Jun. 2020.

 

Stability Evaluation of AC/DC Hybrid Microgrids Considering Bidirectional Power Flow Through the Interlinking Converters

ABSTRACT:

 The bidirectional power flow through the interlinking converter (IC), in ac/dc hybrid microgrids (HMGs) consisting of distributed generators (DGs) with droop controllers, plays an important role on the stability of such systems during islanding. This paper investigates the impact of the power flow direction on the small-signal stability of islanded droop-based HMGs. Firstly, a linearized state-space model of an HMG is developed. Secondly, eigenvalue analysis is carried out to realize the dominant modes, which are the rightmost eigenvalues. Thirdly, participation factor analysis is performed to identify the system and control parameters that effect stability the most. Lastly, sensitivity analysis is conducted to determine the critical droop gains and stability margin. It is observed from the eigenvalue and sensitivity analysis that the dominant modes of HMGs become more stable as more power flows from dc to ac subgrid. On the contrary, an increase in the power flow from ac to dc subgrid degrades the HMG stability. Additionally, the sensitivity of the dominant modes to changes in ac and dc droop gains is studied under bidirectional power flow through the IC to ascertain their impact on the stability margins. Finally, time-domain simulations, in MATLAB/Simulink, suggest that more generation on the dc subgrid would enhance the overall HMG stability margin during islanding.

KEYWORDS:

1.      Bidirectional power flow

2.      Distributed generator

3.      Droop controller

4.      Ac/dc hybrid microgrid

SOFTWARE: MATLAB/SIMULINK

CONTROL DIAGRAM:

Figure 1. General Converter-Based Dg Control Structure.

EXPECTED SIMULATION RESULTS:

Figure 2. Dynamic Responses For Dc To Ac Power Flow Condition Without Ic Reactive Power Support.

 

 

Figure 3. Dynamic Responses For Dc To Ac Power Flow Condition With Ic Reactive Power Support.

Figure 4. Dynamic Responses For Ac To Dc Power Flow Condition.

 

Figure 5. Dynamic Responses For An Increase In The Ac Power Generation Capacity.

Figure 6. Dynamic Responses For An Increase In The Dc Power Generation Capacity.

 

 

Figure 7. Dynamic Responses For Ac To Dc Power Flow Condition In Case

Of (Pac 􀀀 V ) And (Qac 􀀀 !) Droop Control In The Ac Subgrid. Figure 16. Dynamic Responses For Ac To Dc Power Flow Condition In Case Of (Pac 􀀀 V ) And (Qac 􀀀 !) Droop Control In The Ac Subgrid.

CONCLUSION:

The operating point including the amount and direction of the power flow between ac and dc subgrids in an HMG largely affects the stability. Thus, this paper investigated the impact of the power flow on the stability of HMGs formed by the interconnection of ac and dc subgrids through bidirectional ICs. It is observed that as the power flow from the ac to dc subgrid increases, the stability margin of the HMG may be reduced. This is mainly because when the power is exchanged from the ac to dc subgrid, the dynamics associated with the ac subgrid have greater influence on the HMG stability as compared to those of the dc subgrid. Moreover, an increase in the generation capacity of the ac subgrid increases the power flow from the ac to dc subgrid to supply the dc load power, which could degrade the stability of the HMG. Thus, it is technically advised to design the HMG such that the ac subgrid receives power from the dc subgrid. The stability analysis presented in this paper is not meant to emphasize that the amount and direction of the power transfer could always jeopardize the stability but rather, precaution should be exercised when transferring power from one subgrid to the other.

 REFERENCES:

[1] S. Anand, B. G. Fernandes, and J. Guerrero, ``Distributed control to ensure proportional load sharing and improve voltage regulation in low- voltage DC microgrids,'' IEEE Trans. Power Electron., vol. 28, no. 4, pp. 1900_1913, Apr. 2013.

[2] R. Majumder, ``Some aspects of stability in microgrids,'' IEEE Trans. Power Syst., vol. 28, no. 3, pp. 3243_3252, Aug. 2013.

[3] E. A. A. Coelho, P. C. Cortizo, and P. F. D. Garcia, ``Small-signal stability for parallel-connected inverters in stand-alone AC supply systems,'' IEEE Trans. Ind. Appl., vol. 38, no. 2, pp. 533_542, Aug. 2002.

[4] F. Gao, S. Bozhko, A. Costabeber, C. Patel, P. Wheeler, C. I. Hill, and G. Asher, ``Comparative stability analysis of droop control approaches in voltage-source-converter-based DC microgrids,'' IEEE Trans. Power Electron., vol. 32, no. 3, pp. 2395_2415, Mar. 2017.

[5] J. M. Guerrero, L. GarciadeVicuna, J. Matas, M. Castilla, and J. Miret, ``A wireless controller to enhance dynamic performance of parallel invert- ers in distributed generation systems,'' IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1205_1213, Sep. 2004.