A PWM Strategy Based on State Transition for Cascaded H-Bridge Inverter under Unbalanced DC Sources

ABSTRACT:

Cascaded H-bridge converter has been widely used and researched in industry, since it is suitable for the operation under both normal and fault conditions. This paper proposes a novel PWM strategy based on state transition for cascaded H-Bridge inverter with unbalanced DC sources to achieve high quality line-to-line output voltages and maximize the linear modulation range. In this modulation strategy, the duration time of each switching state will be modified directly through the correction value. Ranges of correction value are derived by analyzing the modulation index limitation. Then, proper correction value is added into duration times to transform the switching states and extend modulation index to the maximum value. Meanwhile, balanced AC currents can be obtained under unbalanced DC sources condition, even under larger unbalanced coefficients. Furthermore, a three-phase power control algorithm (PCA) is introduced to achieve the balanced distribution of three-phase power. Compared with the traditional zero-sequence voltage injection method, the proposed strategy is more convenient and effective theoretically, and it can be applied to the higher-level cascaded H-bridge converter. The advantage and effectiveness of the proposed strategy are verified by simulation and experiment results.

KEYWORDS:

  1. State transition
  2. Linear modulation range
  3. Unbalanced DC sources
  4. Power control algorithm

SOFTWARE: MATLAB/SIMULINK

CONCLUSION:

A novel PWM strategy based on state transition for CHBI with unbalanced DC sources has been proposed in this paper. Compared with ZSVIM and NVM, the duration times of each switch states can be modified directly by correction value and the gate signals can be acquired easily through ST-PWM. To acquire the maximum linear modulation index, the reason of the modulation index limitation and the novel modulation strategy based on the state transition are studied. The proposed strategy can achieve high quality line-to-line output voltages and extend the modulation range as high as possible. Besides, the three-phase power control algorithm is introduced to acquire balanced power distribution. The effectiveness has been verified by simulation and experiment results.

In our current work, we incorporate PCA into the ST-PWM strategy, which is a prototype of multi-objective control. Since both modulation index extension and power control are achieved by adjusting ΔT, there is a conflict on the control objectives. That is to say, the control ability of PCA will decrease when the modulation index is extended. However, we have not yet found a strict mathematical relationship between them due to time constraints. And we will do a further research on multi-objective optimal PWM strategy and multi-objective control boundaries under unbalanced dc sources in the future.

REFERENCES:

[1] A. Marzoughi, R. Burgos, D. Boroyevich, and Y. Xue, “Investigation and comparison of cascaded H-bridge and modular multilevel converter topologies for medium-voltage drive application,” in Industrial Electronics Society, IECON 2014 – 40th Annual Conference of the IEEE, 2014, pp. 1562-1568.

[2] S. Kouro, M. Malinowski, K. Gopakumar, J. Pou, L. G. Franquelo, B. Wu, et al., “Recent Advances and Industrial Applications of Multilevel Converters,” IEEE Trans. Ind. Electron., vol. 57, no. 8, pp. 2553-2580, Aug. 2010.

[3] X. Zha, L. Xiong, J. Gong and F. Liu, “Cascaded multilevel converter for medium-voltage motor drive capable of regenerating with part of cells,” IET Power Electronics, vol. 7, no. 5, pp. 1313-1320, May. 2014.

[4] G. Farivar, C. D. Townsend, B. Hredzak, J. Pou, and V. G. Agelidis, “Low-capacitance cascaded H-bridge multilevel StatCom,” IEEE Trans. Power Electron., vol. 32, no. 3, pp. 744-1754, Mar. 2017.

[5] K. D. Teryima, G. Y. Nentawe, and A. O. David, “A Overlapping Carrier Based SPWM for a 5-Level Cascaded H-bridge Multilevel Inverter,” International Journal of Power Electronics and Drive Systems (IJPEDS), vol. 7, no. 2, pp. 349-357, 2016.

A Physical Deterministic Inverse Method for Operational Satellite Remote Sensing: An Application for Sea Surface Temperature Retrievals

ABSTRACT:

We propose a new deterministic approach for remote sensing retrieval, called modified total least squares (MTLS), built upon the total least squares (TLS) technique. MTLS implicitly determines the optimal regularization strength to be applied to the normal equation first-order Newtonian retrieval using all of the noise terms embedded in the residual vector. The TLS technique does not include any constraint to prevent noise enhancement in the state space parameters from the existing noise in measurement space for an inversion with an ill-conditioned Jacobian. To stabilize the noise propagation into parameter space, we introduce an additional empirically derived regularization proportional to the logarithm of the condition number of the Jacobian and inversely proportional to the L2-norm of the residual vector. The derivation, operational advantages and use of the MTLS method are demonstrated by retrieving sea surface temperature from GOES-13 satellite measurements. An analytic equation is derived for the total retrieval error, and is shown to agree well with the observed error. This can also serve as a quality indicator for pixel-level retrievals. We also introduce additional tests from the MTLS solutions to identify contaminated pixels due to residual clouds, error in the water vapor profile and aerosols. Comparison of the performances of our new and other methods, namely, optimal estimation and regression-based retrieval, is performed to understand the relative prospects and problems associated with these methods. This was done using operational match-ups for 42 months of data, and demonstrates a relatively superior temporally consistent performance of the MTLS technique.

KEYWORDS:

  1. Condition number of matrix
  2. Ill-conditioned inverse methods
  3. Regularization
  4. Satellite remote sensing
  5. Sea surface temperature (SST)
  6. Total error
  7. Total least squares (TLS)

SOFTWARE: MATLAB/SIMULINK

CONCLUSION:

We have demonstrated in this work the advantage of the MTLS, which is the family of the deterministic inverse methods, for producing SST retrievals compared with other prevailing methods. In addition, it is noteworthy that MTLS does not require additional error information, e.g., well-specified errors in observational and a priori information. This may provide a significant advantage for climate-based applications where retrievals should be as independent of external error sources as possible. The MTLS retrieval is improved by using the newer version of CRTM, which implies that more accurate forward models and ancillary data can further reduce the remaining MTLS error. This package can also calculate a metric relating to the total retrieval error and automatic QI at individual pixel level. Apart from the QI, MTLS is also capable of identifying the most difficult retrievals due to cloud contamination or high WV profile error. The sensitivity analysis confirms that MTLS solution is independent of a priori/IG error. The data driven dynamic regularization property of MTLS regularizes solutions toward the IG when the problem is either highly ill-conditioned or has high observation error or both to keep the solution below the a priori error. It is found that OEM retrieval, at least as implemented for this problem, is worse than the LS solution, and sometimes worse than the a priori error, irrespective of the version of CRTM. OEM is the most popular choice for physically based operational retrievals due to the assumption that a priori based constraining of an ill-posed inversion should still yield reasonable reasonable results under conditions where there may be unaccounted for parameters or unforeseen errors, as may be the case in real-world retrieval problems. However, these results suggest that this view may be based more on perception of idealized Bayesian statistics rather than comparative scientific study with respect to alternative methods. This study has also demonstrated that the sensitivity of OEM retrievals under practical circumstances renders it more vulnerable to noise than MTLS retrievals. Even by employing dynamic error covariance matrices, OEM is unable to produce the best retrieval for a fairly linear and moderately ill-conditioned problem of SST retrieval. Moreover, the estimation of error of the errors, which is a prerequisite for OEM, is rather difficult in practice, which perhaps explains why OEM results do not match the expectation from the theory of adding to/constraining by a priori knowledge. To date, operational SST retrievals are dominated by regression (REGB), which highly simplifies RT physics. Mostly, it does produce reasonable results (SD) due to the fact that the global variance of SST fields itself is not very high (e.g., compared with gaseous distributions) and the atmospheric attenuation for 3.9-μm channel is rather low, but such methods are still subject to biases on a spatial and temporal basis, with seasonal variations, and has no inherent means of correcting for them. This derivation of MTLS is based on linear algebra. However, this paper illustrates that a deterministic classical mathematics approach can produce better retrievals for real-world RT problems compared with more recent probability-based mathematics that solve ill-posed problems using covariance matrices. The MTLS retrievals outperform the OEM retrievals due to the fact that the regularization in MTLS is data driven. As opposed to OEM that uses regularization from user-defined a priori knowledge of measurement error and forward model error, as well as a priori knowledge error of the retrieved target parameter. A reliable estimation of both the errors in an operational environment is very difficult due to the highly dynamic atmosphere, fast forward model error, including NCEP data, as well as error in the measurements. An alternate effort toward error estimation using simulation minus observation (S-O) bias correction leads to further ambiguities and may potentially mislead our fundamental science understanding. With the advent of newer sensors with improved multispectral capabilities (e.g., the Visible and Infrared Imaging Radiometer Suite and the future Advanced Baseline Imager), employing a deterministic physical method for simultaneous retrieval of SST and WV (critical for weather and climate studies), such as the MTLS package, has the potential to provide substantial improvements in the use of satellite data and derived products.

REFERENCES:

[1] P. Le Borgne, H. Roquet, and C. J. Merchant, “Estimation of sea surface temperature from the spinning enhanced visible and infrared imager, improved using numerical weather prediction,” Remote Sens. Environ., vol. 115, no. 1, pp. 55–65, Jan. 2011.

[2] C. D. Rodgers, Inverse Methods for Atmospheric Soundings: Theory and Practice. Singapore: World Scientific, 2000.

[3] S. Twomey, An Introduction to the Mathematics of Inversion in Remote Sensing and Indirect Measurements. New York, NY, USA: Elsevier, 1977.

[4] J. T. Houghton, F. W. Taylor, and C. D. Rodgers, Remote Sounding of Atmospheres. Cambridge, U.K.: Cambridge Univ. Press, 1984.

[5] J. L. Mead, “Parameter estimation: A new approach to weighting a priori information,” J. Inv. Ill-Posed Probl., vol. 16, no. 2, pp. 1–21, 2007.

 A Multi-Cell 21-Level Hybrid Multilevel Inverter synthesizes a reduced number of components with Voltage Boosting Property

ABSTRACT:

A multi-cell hybrid 21-Level multilevel inverter is proposed in this paper. The proposed topology includes two-unit; an H-bridge is cascaded with a modified K-type unit to generate an output voltage waveform with 21 levels based only on two unequal DC suppliers. The proposed topology’s advantage lies in the fine and clear output voltage waveforms with high output efficiency. Meanwhile, the high number of output voltage waveform levels generates a low level of distortion and reduces the level of an electromagnetic interface (EMI). Moreover, it reduces the voltage stress on the switching devices and gives it a long lifetime. Also, the reduction in the number of components has a noticeable role in saving size and cost. Regarding the capacitors charging, the proposed topology presents an online method for charging and balancing the capacitor’s voltage without any auxiliary circuits. The proposed topology can upgrade to a high number of output steps through the cascading connection. Undoubtedly this cascading will increase the power level to medium and high levels and reduce the harmonics content to a neglectable rate. The proposed system has been tested through the simulation results, and an experimental prototype based on the controller dSPACE (DS-1103) hardware unit used to support the simulation results.

KEYWORDS:

  1. 21-Level Multilevel Inverter (MLI)
  2. Hybridization
  3. Modified K-type inverter
  4. Online charging
  5. Self-balancing
  6. Voltage boosting inverter
  7. Total Harmonic Distortion (THD)

SOFTWARE: MATLAB/SIMULINK

CONCLUSION:

The work in this paper presented a hybrid multilevel inverter that consisted of a series connection between two units (an HB unit with a modified K-Type unit). This combination generates an output voltage waveform with 21 steps. This high number steps in the output voltage help in reducing the level of noises in the output voltage and reduced the stress in the switching devices, which on the one hand generating fine and clear waveforms and on the other hand reduces the harmonic content in the waveforms to a deficient level (satisfying the harmonics standard IEEE519). Economically, the structure of the proposed topology presented an optimal design in terms of reducing the number of switches and DC sources which in turn enhancing the system reliability by reducing the inverter cost. For the capacitors charging process, the paper presents an online method for charging and balancing the capacitor voltages without any auxiliary circuits for that. This helps in the continuous operation of the charging and discharging process for the capacitor without disturbing the process of generating the output voltage. The proposed topology supports the modularity process in order to maximize the range of output power to the medium and high level, and the paper presented two scenarios for the series connection 2HB+K and HB+2K both the cases raise the level of the output power and enhances the system performance to achieve high efficiency. Due to the dependence on multi DC sources, this topology is suitable for renewable energy applications; DC sources are abundant. The hybrid renewable energy sources application will be more appropriate between all the renewable energy applications because the proposed topology-based mainly on two unequal DC suppliers, which will be available easily in the hybrid renewable energy sources.

REFERENCES:

[1] F. Z. Peng, W. Qian, and D. Cao, “Recent advances in multilevel converter/inverter topologies and applications,” in The 2010 International Power Electronics Conference-ECCE ASIA-, 2010, pp. 492-501.

[2] J. Rodriguez, J.-S. Lai, and F. Z. Peng, “Multilevel inverters: a survey of topologies, controls, and applications,” IEEE Transactions on Industrial Electronics, vol. 49, pp. 724-738, 2002.

[3] L. M. Tolbert and X. Shi, “Multilevel power converters,” in Power Electronics Handbook, ed: Elsevier, 2018, pp. 385-416.

[4] K. K. Gupta, A. Ranjan, P. Bhatnagar, L. K. Sahu, and S. Jain, “Multilevel inverter topologies with reduced device count: A review,” IEEE transactions on power electronics, vol. 31, pp. 135-151, 2015.

[5] P. Omer, J. Kumar, and B. S. Surjan, “A Review on Reduced Switch Count Multilevel Inverter Topologies,” IEEE Access, vol. 8, pp. 22281-22302, 2020.

A Modified Carrier-Based Advanced Modulation Technique For Improved Switching Performance of Magnetic Linked Medium Voltage Converters

ABSTRACT:

The high-frequency magnetic link is gaining popularity due to its light weight, small volume, and inherent voltage balancing capability. Those features can simplify the utilization of multilevel converter (MLC) for the integration of renewable energy sources to the grid with compact size and exert economic feasibility. The modulation and control of MLC are crucial issues especially for grid connected applications. To support the grid, the converter may need to operate in over-modulation (OVM) region for short periods depending upon the loading conditions. This OVM operation of the converter causes increased harmonic losses and adverse effects on overall system efficiency. On top of that, the size and cost of filtering circuitry become critical to eliminate the unwanted harmonics. In this regard, a modified OVM scheme with phase disposed carriers for grid connected high frequency magnetic link-based cascaded H-bridge (CHB) MLC is proposed for the suppression of harmonics and the reduction of converter loss. Furthermore, with the proposed OVM technique, the voltage gain with modulation index can be increased up to the range which is unlikely to be achieved using the classical ones. Extensive simulations are carried out with a 2.24 MVA permanent magnet synchronous generator-based wind energy conversion system which is connected to the 11 kV ac grid through a high-frequency magnetic link and a 5-level CHB MLC. A scaled down laboratory prototype is implemented to validate the performance of the converter.

KEYWORDS:

  1. Multilevel converter
  2. Over modulation
  3. Grid connection
  4. High-frequency magnetic link
  5. Wind energy

SOFTWARE: MATLAB/SIMULINK

CONCLUSION:

To improve the system performance, a modified OVM technique is presented in this paper with grid connected and islanded operation. With the proposed modified carrier signal based BCPWM techniques, the overall loss and THD are decreased for both the islanded and grid connected modes compared with the traditional OVM techniques. Moreover, the voltage gain can be increased and remains approximately constant in the proposed method, which may not be possible to obtain using the traditional OVM methods. In this paper, a high-frequency magnetic link-based fully-rated CHB converter is developed for wind energy applications and the behavior of the system under rated and overrated load conditions are investigated.The use of magnetic link for the generation of isolated and balanced dc sources of the MLC inherently overcomes the voltage imbalance problem of CHB MLC and hence effectively simplifies the system control complexities. The core loss of high-frequency magnetic link is also measured to identify the overall loss of the system. The effectiveness of the proposed technology is confirmed by the simulation and experimental results.

REFERENCES:

[1] M. R. Islam, Y. G. Guo, J. G. Zhu, H. Lu, and J. X. Jin, “High-frequency magnetic-link medium-voltage converter for superconducting generator-based high-power density wind generation systems,” IEEE Trans. Appl. Supercond., vol. 24, no. 5, pp. 1–5, Oct. 2014.

[2] N. Mendis, K. M. Muttaqi, S. Perera, and S. Kamalasadan, “An effective power management strategy for a wind–diesel–hydrogen-based remote area power Supply System to meet fluctuating demands under generation uncertainty,” IEEE Trans. Ind. Appl., vol. 51, no. 2, pp. 1228–1238, Mar.–Apr. 2015.

[3] B. Jain, S. Jain, and R. K. Nema, “Control strategies of grid interfaced wind energy conversion system: An overview,” Renew. Sustain. Energy Rev., vol. 47, pp. 983–996, Apr. 2015.

[4] Y. Tan, K. M. Muttaqi, P. Ciufo, and L. Meegahapola, “Enhanced frequency response strategy for a PMSG-based wind energy conversion system using ultracapacitor in remote area power supply systems,” IEEE Trans. Ind. Appl., vol. 53, no. 1, pp. 549–558, Jan.–Feb. 2017.

[5] M. R. Islam, Y. G. Guo, and J. G. Zhu, “A multilevel medium-voltage inverter for step-up-transformer-less grid connection of photovoltaic power plants,” IEEE J. Photovolt., vol. 4, no. 3, pp. 881‒889, May 2014.

 

A Dual Control Strategy for Power Sharing Improvement In Islanded Mode of AC Microgrid

ABSTRACT:

Parallel operation of inverter modules is the solution to increase the reliability, efficiency, and redundancy of inverters in microgrids. Load sharing among inverters in distributed generators (DGs) is a key issue. This study investigates the feasibility of power-sharing among parallel DGs using a dual control strategy in islanded mode of a microgrid. PQ control and droop control techniques are established to control the microgrid operation. P-f and Q-E droop control is used to attain real and reactive power sharing. The frequency variation caused by load change is an issue in droop control strategy whereas the tracking error of inverter power in PQ control is also a challenge. To address these issues, two DGs are interfaced with two parallel inverters in an islanded AC microgrid. PQ control is investigated for controlling the output real and reactive power of the DGs by assigning their references. The inverter under enhanced droop control implements power reallocation to restore the frequency among the distributed generators with predefined droop characteristics. A dual control strategy is proposed for the AC microgrid under islanded operation without communication link. Simulation studies are carried out using MATLAB/SIMULINK and the results show the validity and effective power-sharing performance of the system while maintaining a stable operation when the microgrid is in islanding mode.

KEYWORDS:

  1. Microgrid
  2. Inverter parallel operation control strategy
  3. Droop control strategy
  4. Frequency restore
  5. Power sharing

SOFTWARE: MATLAB/SIMULINK

CONCLUSION:

In this paper, the enhanced droop and PQ control strategies for controlling parallel DGs in islanding mode of AC micro-grids were investigated to achieve flexible power regulation. The main advantage of this dual control strategy is to enable operation without any communication between the parallel DGs. The power tracking error for PQ control based inverters was investigated and the enhanced droop control implemented with predefined droop characteristics for power reallocation was proposed. To improve and restore the frequency, a frequency restoration scheme (FRS) implemented among the distributed generators was developed. The proposed droop controller provides stable operating under different control strategies in islanded operation and the DG voltage can quickly respond to the required voltage demand. The PQ controller can effectively track the active and reactive power and the droop control provides voltage control in islanded mode. The simulation results obtained from MATLAB/SIMULINK verified the stability of the load voltage and frequency.

REFERENCES:

  1. Parhizi, S., et al. (2015). State of the art in research on micro-grids: A review. IEEE Access, 3, 890–925.
  2. Lopes, J. A. P., Moreira, C. L., & Madureira, A. G. (2006). Defining control strategies for micro-grids islanded operation. IEEE Transactions on Power Apparatus and Systems, 21(2), 916–924.
  3. Ahmed, M. N., et al. (2015). An overview on microgrid control strategies. International Journal of Engineering and Advanced Technology (IJEAT), 4(5), 93–98.
  4. Lasseter, R. H. (2002). Micro-grids. In 2002 IEEE Power Engineering Society Winter Meeting. Conference Proceedings (Cat. No.02CH37309) (Vol. 1, pp. 305–308).
  5. Basak, P., Saha, A. K., Chowdhury, S., & Chowdhury, S. P. (2009). Micro-grid: Control techniques and modeling. 44th International Universities Power Engineering Conference (UPEC), Glasgow IEEE. (pp. 1–5).

A Flexible Power Control for PV-Battery-HybridSystem Using Cascaded H-Bridge Converters

ABSTRACT:

The cascaded H-Bridge (CHB) is a good candidate to integrate multiple PV arrays into the power grid. However, due to the internal uncertain power supply of renewable sources, it is difficult to meet the power grid power command with only PV arrays. To overcome this limitation, a CHB converter with both PV arrays and energy storage units in the DC rails is proposed in this paper. Firstly, a two-layer hierarchical control is developed for independent PQ control and power distribution among each CHB cell, while meeting with the grid PQ reference command at the same time. Then, a modified power management method is developed to adaptively modify the current power points for PV panels from their maximum power points to solve the potential over-modulation problem caused by frequent battery charging and discharging. With the proposed approach, a good harvesting of PV power can be ensured in various situations. Verification results are provided to validate the performance of the proposed system.

 KEYWORDS:

  1. Cascaded H-bridge
  2. Power control
  3. Coordinated control
  4. PV-Battery system

 SOFTWARE: MATLAB/SIMULINK

CONCLUSION:

In this paper, a compact single-phase PV-battery-hybrid system is developed with a single stage CHB converter, where both PV panels and battery are connected to separate DC rails of the converter. To achieve proper MPPT of each PV array and at the same time, avoid conflicts caused by the difference between PV output power and the grid power demand, a hierarchical control power control scheme is developed. The central controller is responsible for the regulation of grid current to meet the grid demand and the local controller of each H-bridge cell is responsible for MPPT of PV arrays. The battery in the proposed system acts as a buffer to compensate the gap between PV output power and grid demand. To overcome the potential over-modulation problems when the battery is absorbing power with reference voltage angle opposite to that of the PV cells, a modified power management method which can slightly changes PV array operation point in an online manner, is also developed. This method can be used to obtain the maximum allowable output power of PV while ensure an accurate control of power transfer to the grid mains. Since this paper only gave a roughly sketched control scheme of the CHB-based PV-battery-hybrid system, there are still some limitations at present. E.g., firstly, to fit the system PV power capacity, the capacity of the battery may be fairly large as well, thus it may exceed the limitation of system construction investment. However, it should be noted that even if the capacity of the battery is limited, it is still able for this CHB-based system to provide a schedulable output power and participate in the grid power flow regulation in an acceptable region, which will still be effective to improve the energy utilization in a limited range, and help reduce the PV discarding rate. The system planning issue was beyond this topic and not discussed in detail. Secondly, In general, distributed power cells are always required to be able to plug-and-play. While this series system is highly dependent on the communication system, the plug-and-play implementation issue should also consider the communication system design in each controller, making it not only a control issue but also a communication issue. The plug-and-play operation remains a rather interesting topic to be studied in our future work.

REFERENCES:

[1] S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg. “A Review of Single-Phase Grid-Connected Inverters for Photovoltaic Modules,” IEEE Trans. Ind. Appl., vol. 41, no. 5, pp. 1292–1306, Sep./Oct. 2005.

[2] F. Blaabjerg, Y. Yang, D. Yang, X. Wang. “Distributed Power-Generation Systems and Protection,” Proc. IEEE, vol. 105, no. 7, pp. 1311–1331, July. 2017.

[3] S. V. Araújo, P. Zacharias, and R. Mallwitz, “Highly Efficient Single-Phase Transformerless Inverters for Grid-Connected Photovoltaic Systems,” IEEE Trans. Ind. Electron., vol. 57, no. 9, pp. 3118-3128, Sep. 2010.

[4] B. Yang, Wuhua Li, Y. Zhao, X. He. “Design and Analysis of a Grid-Connected Photovoltaic Power System,” IEEE Trans. Power Electron., vol. 25, no. 4, pp. 992-1000, Apr. 2010.

[5] J. He and Y. W. Li, “Analysis, design and implementation of virtual impedance for power electronics interfaced distributed generation,” IEEE Trans. on Ind. Appl., vol. 47, pp. 2525-2538, Nov/Dec. 2011.

Development of High-Performance Grid-Connected Wind Energy Conversion System for Optimum Utilization of Variable Speed Wind Turbines

ABSTRACT:

This paper presents an improvement technique for the power quality of the electrical part of a wind generation system with a self-excited induction generator (SEIG) which aims to optimize the utilization of wind power injected into weak grids. To realize this goal, an uncontrolled rectifier-digitally controlled inverter system is proposed. The advantage of the proposed system is its simplicity due to fewer controlled switches which leads to less control complexity. It also provides full control of active and reactive power injected into the grid using a voltage source inverter (VSI) as a dynamic volt ampere reactive (VAR) compensator. A voltage oriented control (VOC) scheme is presented in order to control the energy to be injected into the grid. In an attempt to minimize the harmonics in the inverter current and voltage and to avoid poor power quality of the wind energy conversion system (WECS), an filter is inserted between VOC VSI and the grid. The proposed technique is implemented by a digital signal processor (DSP TMS320F240) to verify the validity of the proposed model and show its practical superiority in renewable energy applications.

KEYWORDS:

  1. Grid connected systems
  2. Self-excited induction generator (SEIG)
  3. Voltage oriented control (VOC)
  4. Voltage source inverter (VSI)
  5. Wind energy conversion systems (WECSs)

 SOFTWARE: MATLAB/SIMULINK

CONCLUSION:

In this paper, the SEIG-based WECS dynamic model has been derived. The VOC grid connected VSI has been investigated for high performance control operation. The test results showed how the control scheme succeeded in injecting the wind power as active or reactive power in order to compensate the weak grid power state. An filter is inserted between VOC VSI and grid to obtain a clean voltage and current waveform with negligible harmonic content and improve the power quality. Also, this technique achieved unity power factor grid operation (average above 0.975), very fast transient response within a fraction of a second (0.4 s) under different possible conditions (wind speed variation and load variation), and high efficiency due to a reduced number of components (average above 90%) has been achieved. Besides the improvement in the converter efficiency, reduced mechanical and electrical stresses in the generator are expected, which improves the overall system performance. The experimental results obtained from a prototype rated at 250 W showed that the current and voltage THD (2.67%, 0.12%), respectively, for the proposed WECS with filter is less than 5% limit imposed by IEEE-519 standard. All results obtained confirm the effectiveness of the proposed system feasible for small-scale WECSs connected to weak grids.

REFERENCES:

[1] V. Kumar, R. R. Joshi, and R. C. Bansal, “Optimal control of matrix-converter-based WECS for performance enhancement and efficiency optimization,” IEEE Trans. Energy Convers., vol. 24, no. 1, pp. 264–272, Mar. 2009.

[2] Y. Zhou, P. Bauer, J. A. Ferreira, and J. Pierik, “Operation of grid connected DFIG under unbalanced grid voltage,” IEEE Trans. Energy Convers., vol. 24, no. 1, pp. 240–246, Mar. 2009.

[3] S. M. Dehghan, M.Mohamadian, and A. Y. Varjani, “A new variable speed wind energy conversion system using permanent-magnet synchronous generator and z-source inverter,” IEEE Trans Energy Convers., vol. 24, no. 3, pp. 714–724, Sep. 2009.

[4] K. Tan and S. Islam, “Optimum control strategies for grid-connected wind energy conversion system without mechanical sensors,” WSEAS Trans. Syst. Control, vol. 3, no. 7, pp. 644–653, Jul. 2008, 1991-8763.

[5] B. C. Rabelo, W. Hofmann, J. L. da Silva, R. G. de Oliveira, and S. R. Silva, “Reactive power control design in doubly fed induction generators for wind turbines,” IEEE Trans. Ind. Elect., vol. 56, no. 10, pp. 4154–4162, Oct. 2009.

An Efficient UPF Rectifier for a Stand-AloneWind Energy Conversion System

ABSTRACT:

In this paper, a near-unity-power-factor front-end rectifier employing two current control methods, namely, average current control and hysteresis current control, is considered. This rectifier is interfaced with a fixed-pitch wind turbine driving a permanent-magnet synchronous generator. A traditional diode-bridge rectifier without any current control is used to compare the performance with the proposed converter. Two constant wind speed conditions and a varying wind speed profile are used to study the performance of this converter for a rated stand-alone load. The parameters under study are the input power factor and total harmonic distortion of the input currents to the converter. The wind turbine generator–power electronic converter is modeled in PSIM, and the simulation results verify the efficacy of the system in delivering satisfactory performance for the conditions discussed. The efficacy of the control techniques is validated with a 1.5-kW laboratory prototype, and the experimental results are presented.

 KEYWORDS:

  1. Average current control (ACC)
  2. Hysteresis current control (HCC)
  3. Permanent-magnet synchronous generator (PMSG)
  4. Unity-power-factor (UPF) converter

 SOFTWARE: MATLAB/SIMULINK

CONCLUSION:

In this paper, a WECS interfaced with a UPF converter feeding a stand-alone load has been investigated. The use of simple bidirectional switches in the three-phase converter results in near-UPF operation. Two current control methods, i.e., ACC and HCC, have been employed to perform active input line current shaping, and their performances have been compared for different wind speed conditions. The quality of the line currents at the input of the converter is good, and the harmonic distortions are within the prescribed limits according to the IEEE 519 standard for a stand-alone system. A high power factor is achieved at the input of the converter, and the voltage maintained at the dc bus link shows excellent voltage balance. The proposed method yields better performance compared to a traditional uncontrolled diode bridge rectifier system typically employed in wind systems as the front-end converter. Finally, a laboratory prototype of the UPF converter driving a stand-alone load has been developed, and the ACC and HCC current control methods have been tested for comparison. The HCC current control technique was found to be superior and  has better voltage balancing ability. It can thus be an excellent front-end converter in a WECS for stand-alone loads or grid connection.

REFERENCES:

[1] C. E. A. Silva, D. S. Oliveira, L. H. S. C. Barreto, and R. P. T. Bascope, “A novel three-phase rectifier with high power factor for wind energy conversion systems,” in Proc. COBEP, Bonito-Mato Grosso do Sul, Brazil, 2009, pp. 985–992.

[2] Online. Available: http://en.wikipedia.org/wiki/Wind_energy

[3] M. Druga, C. Nichita, G. Barakat, B. Dakyo, and E. Ceanga, “A peak power tracking wind system operating with a controlled load structure for stand-alone applications,” in Proc. 13th EPE, 2009, pp. 1–9.

[4] S. Kim, P. Enjeti, D. Rendusara, and I. J. Pitel, “A new method to improve THD and reduce harmonics generated by a three phase diode rectifier type utility interface,” in Conf. Rec. IEEE IAS Annu. Meeting, 1994, vol. 2, pp. 1071–1077.

[5] A. I. Maswood and L. Fangrui, “A novel unity power factor input stage for AC drive application,” IEEE Trans. Power Electron., vol. 20, no. 4, pp. 839–846, Jul. 2005.

A Novel Control Strategy for a Variable Speed Wind Turbine with a Permanent Magnet Synchronous Generator

ABSTRACT:

This paper presents a novel control strategy for the operation of a direct drive permanent magnet synchronous generator (PMSG) based stand alone variable speed wind turbine. The control strategy for the generator side converter with maximum power extraction is discussed. The stand alone control is featured with output voltage and frequency controller capable of handling variable load. The potential excess of power is dissipated in the damp resistor with the chopper control and the dc link voltage is maintained. Dynamic representation of dc bus and small signal analysis are presented. Simulation results show that the controllers can extract maximum power and regulate the voltage and frequency under varying wind and load conditions. The controller shows very good dynamic and steady state performance.

KEYWORDS:

  1. Permanent magnet synchronous generator
  2. Maximum power extraction
  3. Switch-mode rectifier
  4. Variable speed
  5. Wind turbine
  6. Voltage and frequency control

 SOFTWARE: MATLAB/SIMULINK

 CONCLUSION:

Control strategy for a direct drive stand alone variable speed wind turbine with a PMSG is presented in this paper. A simple control strategy for the generator side converter to extract maximum power is discussed and implemented using Simpower dynamic system simulation software. The controller is capable to maximize output of the variable speed wind turbine under fluctuating wind. The load side PWM inverter is controlled using vector control scheme to maintain the amplitude and frequency of the inverter output voltage. It is seen that the controller can maintain the load voltage and frequency quite well at constant load and under varying load condition. The generating system with the proposed control strategy is suitable for a small scale standalone variable speed wind turbine installation for remote area power supply. The simulation results demonstrate that the controller works very well and shows very good dynamic and steady state performance.

REFERENCES:

[1] Müller, S., Deicke, M., and De Doncker, Rik W.: ‘Doubly fed induction genertaor system for wind turbines’, IEEE Industry Applications Magazine, May/June, 2002, pp. 26-33.

[2] Polinder H., Van der Pijl F. F. A, de Vilder G. J., Tavner P. J.:  “Comparison of direct-drive and geared generator concepts for wind turbines,” IEEE Trans. on energy conversion, 2006, . 21, (3), pp. 725- 733.

[3] Chan T. F., and Lai L. L., “Permanenet-magnet machines for distributed generation: a review,” Proc. IEEE power engineering annual meeting, 2007, pp. 1-6.

[4] De Broe M., Drouilhet S., and Gevorgian V.: “A peak power tracker for small wind turbines in battery charging applications,” IEEE Trans. Energy Convers. 1999, 14, (4), pp. 1630–1635.

[5] Datta R., and Ranganathan V. T.: “A method of tracking the peak power points for a variable speed wind energy conversion system,” IEEE Trans. Energy Convers., 1999, 18, (1), pp. 163–168.

A Single-Phase Buck Matrix Converter with High-Frequency Transformer Isolation and Reduced Switch Count

ABSTRACT:

In this paper, a new type of matrix converter also called a single-phase high-frequency transformer  isolated (HFTI) buck matrix converter (MC) is proposed. The proposed converter can provide step-down operation of the input voltage with various types of output voltages such as; in-phase and out-of-phase output voltages, rectified (or positive) output voltage, and output voltage with step-changed frequency. By incorporating HFT isolation, the proposed MC saves an extra bulky line frequency transformer, which is required for the conventional MCs to provide electrical isolation and safety, when used in application such as dynamic voltage restorers (DVRs), etc. Two different circuit variations of the proposed HFTI MC are presented with and without  continuous output currents, with the latter having less passive components. The safe-commutation strategy is also employed for the proposed HFTI MC to provide current path for the inductor during dead-time, which avoids switch voltage spikes without adding any snubber circuits. The operation principle and circuit analysis of the  proposed MC are presented, and switching strategies are also developed to obtain various output voltages. Moreover, a prototype of the proposed MC is fabricated, and experiments are performed to produce in-phase/out-of-phase and rectified output voltages, and output voltage with step-changed frequency.

KEYWORDS:

  1. High-frequency transformer
  2. In-phase and out-of-phase operations
  3. Rectified output
  4. Single-phase matrix converter
  5. Step-changed frequency

SOFTWARE: MATLAB/SIMULINK

 CONCLUSION:

In this paper, a buck MC is proposed with HFT isolation. The proposed MC is capable of providing various types of output voltages, such as in-phase, out-of-phase and rectified output voltages. Moreover, the frequency of the output voltage can be changed in steps, so that it is integer multiple or integer fraction of the input voltage frequency. The use of HFT isolation in the proposed MC for electrical isolation and safety benefits in that it removes the need for extra bulky line frequency transformer, which is added with conventional non-isolated MCs for applications as DVRs.

Two different secondary side structures of the proposed HFTI buck MC are proposed, with one having continuous output current, and the other having discontinuous out current but with one inductor and capacitor less. The soft-commutation strategy is suggested for the proposed MC, which avoids switch voltage spikes without using any snubber circuits. The operation principle and circuit analysis of the proposed converter are presented and switching strategies are also developed to obtain various output voltages. Moreover, a 200 W laboratory prototype of the proposed MC is fabricated, and experiments are performed to produce in-phase/out-of-phase and rectified output voltages, and output voltage with step-changed frequency.

REFERENCES:

[1] F. Z. Peng, L. Chen, and F. Zhang, “Simple topologies of PWM ac-ac converters,” IEEE Power Electron. Letters, vol. 1, no. 1, pp. 10– 13, Mar. 2003.

[2] T. B. Lazzarin, R. L. Andersen, and I. Barbi, “A switched-capacitor three-phase ac-ac converter,” IEEE Trans. Ind. Electron., vol. 62, no. 2, pp. 735–745, Feb. 2015.

[3] H. F. Ahmed, H. Cha, A. A. Khan, and H.-G. Kim, “A family of high-frequency isolated single-phase Z-source ac-ac converters with safe-commutation strategy,” IEEE Trans. Power Electron., vol. 31, no. 11, pp. 7522–7533, Nov. 2016.

[4] C. Liu, B. Wu, N. R. Zargari, D. Xu and J. Wang, “A novel three-phase three-leg ac-ac converter using nine IGBTs,” IEEE Trans. Power Electron., vol. 24, no. 5, pp. 1151–1160, May. 2009.

[5] C. B. Jacobina, I. S. d. Freitas, E. R. C. d. Silva, A. M. N. Lima, and R. L. d. A. Riberio, “Reduced switch count dc-link ac-ac five-leg converter,” IEEE Trans. Power Electron., vol. 21, no. 5, pp. 1301–1310, Sep. 2006.