A Hysteresis Space Vector PWM for PV Tied Z-Source NPC-MLI With DC-Link Neutral Point Balancing

ABSTRACT:

The Photo-voltaic (PV) tied Z-source Neutral-point clamped multilevel inverter (Z-NPC-MLI) is used in solar grid connected applications due to its single stage conversion and better performance. Though the Z source inverters adaptation is accepted in grid connected technology, the need for suitable controller and PWM scheme are necessary to meet out the performance such as shoot through switching, neutral point balancing, and harmonic reduction. The space vector pulse width modulation (SVPWM) strategy is a prominent modulation technique for Z-source NPC-MLIs due to the flexibility to select the appropriate voltage vector. Previous publications have shown the control of a Z-source MLI using the SVPWM with and without modification of shoot through switching. However, the current controller (CC) based SVPWM is not matured, which is the most essential consideration for the grid connected inverter to provide neutral point balancing, shoot through control for low harmonic distortion and a high quality current. With all these aims, this paper presents a PV tied Z-NPC-MLI grid connected system with a unique hysteresis current control SVPWM (HSVM) strategy with neutral point (NP) balancing control and direct current control in the inverter input side. Also, the proposed HSVM is assuring the grid connection with high quality voltage and current waveforms. This CC based SVPWM for Z-NPC MLI has been validated through simulation and FPGA based experimental investigations. The results are confirmed the feasibility and reliability of the proposed HSVM for the PV tie grid connected Z- Source NPC-MLI.

KEYWORDS:

1.      Z source MLI

2.      Neutral-point clamped inverter

3.      Space vector PWM (SVPWM)

4.      Hysteresis current controller (HCC)

5.      Neutral point balancing

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

 

 

Figure 1. PV Tied Grid Connected Three-Phase Three-Level Z Source Npc-Mli.

 EXPECTED SIMULATION RESULTS:

Figure 2. Simulation Results Of Pv Output.

Figure 3. Simulation Results Of Three Phase Voltage And Current

Waveform Of Z-Npc-Mli

Figure 4. Simulation Results Of Harmonics Spectra For Z-Npc-Mli Atma D 0:9; (A) Voltage Harmonics Spectra,(B)Current Harmonics Spectra.

 

Figure 5. Simulation Results Of Actual Load Current And Reference Current When H Band Is Fixed At 5amps.

Figure 6. Simulation Results Of Z-Npc-Mli Voltage Across Dc-Link Capacitors; (A) Conventional Svm Method, (B) Hsvm.

Figure 7. Simulation Results -Transient Response Of Inverter Output Currents.

CONCLUSION:

In this paper the three-level Z source NPC-MLI has investigated for PV tied grid connected system. Further also developed the PV tied grid connected system with hysteresis current control combined Z source SVPWM is known as HSVM is developed. The proposed HSVM uses minimal ST compares to conventional Z source SVPWM. In addition the proposed HSVM eliminates the low frequency oscillations using suitable ST (Upper and Lower ST), with regular switching events, which ensures the neutral point DC-link capacitors balancing along with current control. The HSVM maintains the volt-second and inverter voltage boosting competence irrespective of the angular location of the reference vector throughout the inverter operation. A 2 kWp solar panels attached three-phase three-level IGBT based Z-NPC- MLI grid connected system is established with Xilinx family FPGA SPARTAN-6 controller. From the results, it shows that the performance of proposed method is superior than compare to conventional Z source SVPWM in terms of Neutral point fluctuation, current control capability and better harmonic performance. This proposed HSVM method well suited for wind tied inverters, industrial and Electrical vehicles motor applications.

REFERENCES:

[1] Z. Dobrotkova, K. Surana, and P. Audinet, ``The price of solar energy: Comparing competitive auctions for utility-scale solar PV in developing countries,'' Energy Policy, vol. 118, pp. 133_148, Jul. 2018.

[2] R. Teichmann and S. Bernet, ``A comparison of three-level converters versus two-level converters for low-voltage drives, traction, and utility applications,'' IEEE Trans. Ind. Appl., vol. 41, no. 3, pp. 855_865, May/Jun. 2005.

[3] F. Z. Peng, ``Z-source inverter,'' IEEE Trans. Ind. Appl., vol. 39, no. 2, pp. 504_510, Mar./Apr. 2003.

[4] N. Yadaiah, A. S. Kumar, and Y. M. Reddy, ``DSP based control of constant frequency and average current mode of boost converter for power factor correction (PFC),'' in Proc. Int. Conf. Adv. Power Convers. Energy Technol. (APCET), Aug. 2012, pp. 1_6.

[5] W. Liu, Y. Yang, T. Kerekes, and F. Blaabjerg, ``Generalized space vector modulation for ripple current reduction in quasi-Z-source inverters,'' IEEE Trans. Power Electron., vol. 36, no. 2, pp. 1730_1741, Feb. 2021.

Z-Source Converter Integrated Dc Electric Spring For Power Quality Improvement In DC Microgrid

 ABSTRACT:

Increasing penetration of renewable energy sources reveals the concept of dc microgrid which has the advantages of low cost and low losses because of the elimination of the AC/DC conversion processes.The most frequently encountered power quality problem in DC microgrid is voltage fluctuations due to the intermittent nature of renewable energy sources. Direct current (DC) electric spring (DCES), which is an emerging power quality device in DC microgrids, is employed in order to mitigate the effect of the related problem. In this paper, z source converter integrated DCES topology (zDCES) is proposed to provide a wide compensation voltage range with lower duty cycle range and a remarkable decrease in the battery nominal voltage in comparison with conventional systems. The proposed system composed of full-bridge converter, z source converter and battery pack. zDCES provides high voltage gain by using z source converter with passive components without any need for additional switches. The shoot through control, which is used to achieve high gain in z source converter, is implemented using existing full-bridge switches. The performance of the proposed system is compared with the traditional DCES system. The performance of the zDCES is validated with a case study with different voltage fluctuation states.

KEYWORDS:

1.      DC electric spring

2.      High gain

3.      Z source converter

4.      DC microgrid

5.      Power quality

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

 

Fig. 1. Equivalent circuit of zDCES.

EXPECTED SIMULATION RESULTS:

Fig. 2. Duty cycle variations during source voltage fluctuations.

 

Fig. 3. PCC voltages during source voltage fluctuations.

 

Fig. 4. Performance waveforms of the proposed zDCES..

 

CONCLUSION:

In this paper, a z source converter integrated full-bridge converter based DCES topology and control method have been proposed. The main superior aspects of the proposed Zdces topology are reduced battery voltage and lower duty cycle range as well as providing a wider compensation voltage range. Also, a wide range bipolar voltage at the output ports of the zDCES is achieved by z source converter without additional switches rather than relatively high voltage battery packs or HFT conversion rate methods used in traditional DCES topologies. Thus, a low cost solution has been developed to alleviate the voltage fluctuation problem. In order to verify the effectiveness of the proposed zDCES system, a case study that includes different dynamic operational changes has been conducted. Besides, to show the superiority of the zDCES system is compared with conventional DCES. Performance results show that the proposed system can keep the busbar voltage constant with a lower duty cycle range while the other system requires a higher duty cycle range. Hence, a wide voltage range can be provided in the input port of the fullbridge converter by z source converter in contrast to other systems. Besides, the zDCES can keep the voltage fluctuation in a lower range when compared with conventional DCES. As a result, zDCES shows better and more flexible mitigation performance for all operational conditions.

REFERENCES:

[1] Y. Yang, S. Tan, S.Y.R. Hui, Mitigating distribution power loss of DC microgrids with DC electric springs, IEEE Trans. Smart Grid 9 (2018) 5897–5906.

[2] M. Wang, S. Yan, S. Tan, S.Y. Hui, Hybrid-DC electric springs for DC voltage regulation and harmonic cancellation in DC microgrids, IEEE Trans. Power Electron. 33 (2018) 1167–1177.

[3] K. Mok, M. Wang, S. Tan, S.Y.R. Hui, DC electric springs—A technology for stabilizing DC power distribution systems, IEEE Trans. Power Electron. 32 (2017) 1088–1105

[4] H. Zhao, M. Hong, W. Lin, K.A. Loparo, Voltage and frequency regulation of microgrid with battery energy storage systems, IEEE Trans. Smart Grid 10 (1) (2019) 414–424.

[5] D. Kumar, F. Zare, A. Ghosh, DC microgrid technology: system architectures, AC grid interfaces, grounding schemes, power quality, communication networks, applications, and standardizations aspects, IEEE Access 5 (2017) 12230–12256.

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.