Multifunctional Grid Connected Inverter Interfaced by Wind Energy Conversion System

ABSTRACT:

This study deals with a three-phase multifunctional grid-connected inverter interfaced with a wind energy conversion system (WECS) is described. The studied system consists of a permanent magnet synchronous generator (PMSG) based wind turbine, a rectifier and a three-phase voltage source inverter connected to the utility at the point of common coupling. To ensure the multifunctional feature, we propose a direct power control (DPC) which is applied to eliminate line current harmonics, compensate reactive power and feeding wind power into the utility. Simulation results are provided to demonstrate the effectiveness of the proposed system. The results show that the control algorithm of system is effective for eliminating harmonic currents, reactive power compensation and inject the active power available from the PMSG wind turbine into the load and/or grid, which allowed us to confirm the robustness of the proposed strategy.

KEYWORDS:

1.      Inverter

2.      Wind energy conversion system

3.      Permanent magnet synchronous generator

4.      Rectifier

5.      DPC

SOFTWARE: MATLAB/SIMULINK

CONCLUSION:

This paper focused on applying direct power control to a three-phase multifunctional grid-connected inverter interfaced with a wind energy conversion system. The proposed control scheme is used in order to achieve harmonics elimination, reactive power compensation, and simultaneously inject the active power available from the PMSG wind turbine into the load and/or grid. The analysis of the simulation results obtained has attested the robustness, the effectiveness and the good performance of proposed system. The DPC method has very good performance in injection of active power produced by PMSG wind turbine to the distribution networks and simultaneously compensating harmonics and reactive power.

REFERENCES:

[1] H.G. Kim, D.C. Lee, J.K. Seok and G.M. Lee, “Stand-alone wind power generation system using vector-controlled cage-type induction generators,” in Proc. of Inter. Conference on Electrical Machines and systems, vol.1, pp.289-292, Nov.2003.

[2] B. Singh and G.K. Kasal, “Voltage and Frequency Controller for a threephase four wire autonomous wind energy conversion systems,” IEEE Trans. Energy Conversion, vol. 23, no.2, pp.509–518, June 2008.

[3] A.Krama, L.Zellouma and B. Rabhi “Improved Control of Shunt Active Power Filter Connected to a Photovoltaic System Using Technique of Direct Power Control,” 8th International Conference on Modelling, Identification and Control (ICMIC-2016).

[4] M.R. Bengourina, M. Rahli, S. Saadi,, L.Hassaine, “Optimization of direct power control of three-phase shunt active power filter by using PSO algorithm”, Leonardo Electronic Journal of Practices and Technologies, vol. 16, no.31, pp. 218-234, Dec 2017.

[5] A.Chaoui., J. Gaubert, F. Karim,, “Power quality improvement using DPC controlled three-phase shunt active filter”, Electric Power Systems Research, vol. 80, pp. 657–666, 2010.

 

Modelling and voltage control of the solar-wind hybrid micro-grid with optimized STATCOM using GA and BFA

ABSTRACT:

Electricity generation from the wind and solar photovoltaic (PV) systems are highly dependent  upon weather conditions. Their intermittent nature leads to fluctuations in their output. Therefore, the need for rapid compensation for energy transmission and distribution systems is increasingly important. Static Synchronous Compensator (STATCOM) can be adopted for reactive power compensation and for decreasing the voltage fluctuation caused by the system and renewable energy sources. This study presents modelling of a Solar PV-Wind Hybrid Micro-grid and the increase of the stable operating limit of the system in case of the incorporation of STATCOM is examined. The major contribution of this paper is the optimization of gain parameters of four PI controllers in STATCOM control circuit based on genetic algorithms (GA) and Bacteria Foraging Algorithm (BFA) and therefore obtaining better responses and voltage stability in terms of nonlinear nature of solar-wind hybrid micro-grid. The Simulink models of the system architecture include a wind turbine model, a solar PV power system model and a STATCOM. It is certified that the voltage fluctuation at the end of the bus bar is reduced by 8% using conventional PI controller, by 10% for GA-based PI controller, and by 15% for BFA based PI controller under variable load. The results obtained by GA and BFA-based optimization of PI controllers are compared with that of the conventional controller and better results attained.

KEYWORDS:

1.      Voltage control

2.      Bacteria foraging algorithm

3.      PV-wind hybrid system

4.      Static synchronous compensator

5.      Genetic algorithm

SOFTWARE: MATLAB/SIMULINK

CONCLUSION:

In this study, the impacts of a 2MWwind power induction generator based wind generation system and a 0.4MW solar power generation system on the grid were investigated. For this hybrid system, it has been pointed out that STATCOM provides reactive power compensation. A solar PV-wind power system with a hybrid structure was designed and the voltage profiles at the output were examined. STATCOM was incorporated to study the voltage profiles in the system according to capacitive and reactive operating states. On this basis, this work pointed out that power instability in large transmission systems can be minimized, and the fluctuations caused by the adoption of renewable energy sources to the system can be diminished. The comparisons of the results showed that the effectiveness of the STATCOM tuned with GA and BFA was improved. By acquiring the best values for PI controller gains, voltage swell occurred due to the change in reactive power has been overcome and a better dynamic response was reached. In future studies, different optimization techniques and different FACTS devices can be used to compare and determine a more effective one.

REFERENCES:

[1] F.H. Gandoman, A. Ahmadi, A.M. Sharaf, P. Siano, J. Pou, B. Hredzak, V.G. Agelidis, Review of FACTS technologies and applications for power quality in smart grids with renewable energy systems, Renew. Sustain. Energy Rev. 8 (2018) 502–514, https://doi.org/10.1016/j.rser.2017.09.062.

[2] A. Mohanty, M. Viswavandya, D.K. Mishra, P.K. Ray, S. Pragyan, Modelling & simulation of a PV based micro grid for enhanced stability, Energy Proc. (2017) 94–101, https://doi.org/10.1016/j.egypro.2017.03.060.

[3] H. Liao, S. Abdelrahman, J.V. Milanovic´ , Zonal mitigation of power quality using FACTS devices for provision of differentiated quality of electricity supply in networks with renewable generation, IEEE Trans. Power Deliv. 23 (2017) 1975–1985, https://doi.org/10.1109/TPWRD.2016.2585882.

[4] A. Saraswathi, P. Sanjeevikumar, S. Shanmugham, F. Blaabjerg, A.H. Ertas, V. Fedák, Analysis of enhancement in available power transfer capacity by STATCOM integrated SMES by numerical simulation studies, Eng. Sci. Technol., Int. J. 19 (2) (2016) 671–675, https://doi.org/10.1016/j.jestch.2015.10.002.

[5] D. Menniti, A. Pinnarelli, N. Sorrentino, An hybrid PV-Wind supply system with D-Statcom interface for a water-lift station, International Symposium on Power Electronics, Electrical Drives, Automation and Motion, 2010. DOI: 10.1109/SPEEDAM.2010.5545070.

Modelling and Voltage Control of the Solar- Wind Hybrid Micro-Grid with Optimized STATCOM

 ABSTRACT:

Electricity generation from the wind and solar photovoltaic (PV) systems are highly dependent upon weather conditions. Their intermittent nature leads to fluctuations in their output. Therefore, the need for rapid compensation for energy transmission and distribution systems is increasingly important. Static Synchronous Compensator (STATCOM) can be adopted for reactive power compensation and for decreasing the voltage fluctuation caused by the system and renewable energy sources. This study presents modelling of a Solar PV-Wind Hybrid Micro-grid and the increase of the stable operating limit of the system in case of the incorporation of STATCOM is examined. The major contribution of this paper is the optimization of gain parameters of four PI controllers in STATCOM based on genetic algorithms (GA) and therefore obtaining better responses and voltage stability in terms of nonlinear nature of solar-wind hybrid micro-grid. The Simulink models of the system architecture include a 2 MW wind turbine model based on a doubly fed induction generator (DFIG), 0.4 MW solar PV power system model and a STATCOM rated at 3 MVAR. It is certified that the voltage fluctuation at the end of the bus bar is reduced by 8 % using conventional PI controller. The results obtained by GA-based optimization of PI controllers are compared with that of the conventional controller and better results attained.

KEYWORDS:

1.      Flexible AC transmission systems

2.      Genetic Algorithm

3.      PV-Wind hybrid system

4.       Static synchronous compensator

5.      Voltage control

SOFTWARE: MATLAB/SIMULINK

CONCLUSION:

In this study, the impacts of a 2 MW wind power induction generator based wind generation system and a 0.4 MW solar power generation system on the grid was investigated. For this hybrid system, it has been pointed out that STATCOM provides reactive power compensation. A solar PV-wind power system with a hybrid structure was designed and the voltage profiles at the output were examined. STATCOM was incorporated to study the voltage profiles in the system according to capacitive and reactive operating states. On this basis, this work pointed out that power instability in large transmission systems can be minimized, and the fluctuations caused by the adoption of renewable energy sources to the system can be diminished. The comparisons of the results showed that the effectiveness of the STATCOM tuned with GA was improved. By acquiring the best values for PI controller gains, voltage swell occurred due to the change in reactive power has been overcome and a better dynamic response was reached. In future studies, different other optimization techniques will be used to determine a more effective one.

REFERENCES:

[1] F. H. Gandoman, A. Ahmadi, A. M. Sharaf, P. Siano, J. Pou, B. Hredzak, V. G. Agelidis, “Review of FACTS technologies and applications for power quality in smart grids with renewable energy systems”, Renewable and Sustainable Energy Reviews, vol. 82,pp. 502–514, 2018. DOI: 10.1016/j.rser.2017.09.062.

[2] A. Mohanty, M. Viswavandya, D. K. Mishra, P. K. Ray, S. Pragyan, “Modelling & simulation of a PV based micro grid for enhanced stability”, Energy Procedia, vol. 109, pp. 94–101, 2017. DOI: 10.1016/j.egypro.2017.03.060.

[3] H. Liao, S. Abdelrahman, J. V. Milanovic, “Zonal mitigation of power quality using FACTS devices for provision of differentiated quality of electricity supply in networks with renewable generation”, IEEE Trans. Power Deliv., vol. 23, pp. 1975–85, 2017. DOI: 10.1109/TPWRD.2016.2585882.

[4] V. Kumar, A. S. Pandey, S. K. Sinha, “Grid integration and power quality issues of wind and solar energy system: A review”, Int. Conf. Emerging Trends in Electrical Electronics & Sustainable Energy Systems (ICETEESES 2016), Sultanpur, India, 2016, pp. 71–80. DOI: 10.1109/ICETEESES.2016.7581355.

[5] D. Menniti, A. Pinnarelli, N. Sorrentino, “An hybrid PV-Wind supply system with D-Statcom interface for a water-lift station”, IEEE Int. Symposium on Power Electronics, Electrical Drives, Automation and Motion, Pisa, Italy, 2010. DOI: 10.1109/SPEEDAM.2010.5545070. 

LMMN Based Adaptive Control for Power Quality Improvement of Grid Intertie Wind-PV System

ABSTRACT:

  A new topology comprising of wind turbine driven synchronous generator (SG) and solar photovoltaic (PV) array for renewable energy harvesting, is proposed in this work. The stochastic inputs for proposed system, are agitated by the nonlinear time dependent parameters such as variable wind speed and changing solar insolation. The speed variations are absorbed using back to back interfaced power electronic converters (PECs) namely synchronous generator side converter (SGC) and utility grid side converter (UGC) with a common DC link where solar PV array is tied directly. The power injection into the utility grid, is levelled by the optimal utilization of PECs. The SGC uses vector control (VC) for speed control of SG and maintains unity power factor (UPF) at stator terminals. UGC acquires its switching pulses with proper application of least mean mixed norm (LMMN) control technique. The new application of LMMN control scheme is used for harmonics compensation and fundamental load component extraction. The DC link voltage is regulated using proportional integral (PI) controller. A prototype is developed and tested under different conditions of sudden changes in load, wind velocity variations as well as under varying solar PV insolation. The power sharing scheme proves to be effective. The power quality (PQ) issues are also addressed and mitigated effectively. The performance is exhibited for the validation of the proposed system and its control.

KEYWORDS:

1.      Utility Grid

2.      Wind

3.      SG

4.      Solar PV Array

5.      SGC

6.      UGC

7.      LMMN Control

8.      Load Compensation

9.      Power Quality

SOFTWARE: MATLAB/SIMULINK

CONCLUSION:

A three-phase grid intertie wind-PV system with effective load compensation capability, is proposed and its suitability is justified through hardware validation on a developed prototype in the laboratory under various operating conditions such as changing wind velocity, variations in solar insolation and perturbation in nonlinear load. The parallel operation of solar PV array and wind driven SG, allows a possibility of load sharing. The fundamental extraction from the load currents, is successfully done with the application of LMMN adaptive filtering control. The load current fundamental component is extracted, moreover, the disturbances and harmonic content in grid currents are removed in order to improve the power quality at CPI. The aim of improving the voltage profile and reducing the harmonic content at the CPI, is attained successfully by implementing the LMMN adaptive control. The LMMN adaptive control schemes, leads to fast response and less misadjustments. The maximum power is extracted effectively from solar PV array and wind turbine using P&O algorithm. Sensorless VC for speed control of SG, has resulted in low system cost and increased system reliability. Test results obtained under steady state and dynamic conditions, show the acceptability of control techniques. Moreover, the grid currents under the enforced conditions, have their THD below 5% confirming to the IEEE-519 standard.

REFERENCES:

[1] O. A. Lara, N. Jenkins, J. Ekanayake, P. Cartwright and M. Hughes, “Wind energy Generation Modelling and control”, Wiley-IEEE Press, 2009.

[2] V. A. Suryad, S. Doolla and M. Chandorkar, “Microgrids in India: Possibilities and Challenges,” IEEE Electrif. Magazine, vol. 5, no. 2, pp. 47-55, June 2017.

[3] M. Faisal, M. A. Hannan, P. J. Ker, A. Hussain, M. Mansur and F. Blaabjerg, “Review of energy storage system technologies in microgrid applications: Issues and challenges,” IEEE Access, 2018.

[4] M. G. Molina, “Energy storage and power electronics technologies: a strong combination to empower the transformation to the smart grid,” Proc. IEEE, vol. 105, no. 11, pp. 2191-2219, Nov. 2017.

[5] R. Vijayapriya, P. Raja and M. P. Selvan, “A modified active power control scheme for enhanced operation of PMSG based WGS,” IEEE Trans. Sustain. Ener., Early Access.

 

Irradiance-adaptive PV Module Integrated Converter for High Efficiency and Power Quality in Standalone and DC Microgrid Applications

ABSTRACT:

The strive for efficient and cost-effective photovoltaic systems motivated the power electronic design developed here. The work resulted in a DC-DC converter for module integration and distributed maximum power point tracking (MPPT) with a novel adaptive control scheme. The latter is essential for the combined features of high energy efficiency and high power quality over a wide range of operating conditions. The switching frequency is optimally modulated as a function of solar irradiance for power conversion efficiency maximization. With the rise of irradiance, the frequency is reduced to reach the conversion efficiency target. A search algorithm is developed to determine the optimal switching frequency step. Reducing the switching frequency may, however, compromise MPPT efficiency. Furthermore, it leads to increased ripple content. Therefore, to achieve a uniform high power quality at all conditions, interleaved converter cells are adaptively activated. The overall cost is kept low by selecting components that allow for implementing the functions at low cost. Simulation results show the high value of the module integrated converter for DC standalone and microgrid applications. A 400 W prototype was implemented at 0.14 Euro/W. Testing showed efficiencies above 95% taking into account all losses from power conversion, MPPT, and measurement and control circuitry.
KEYWORDS:

1.      Boost converter

2.      Distributed maximum power point tracking (DMPPT)

3.      Microgrid

4.      Module integrated converter (MIC)

5.      Photovoltaics (PV)

6.      Power optimizer

7.      Power quality      

8.      Solar irradiance

9.      Switching frequency modulation (SFM)

SOFTWARE: MATLAB/SIMULINK

CONCLUSION:

A novel PV module integrated converter (MIC) suitable for boosting voltages for DC standalone and DC microgrid applications was designed, implemented, and tested. The proposed switching frequency modulation (SFM) selects an irradiance adapted switching frequency that is always high enough to avoid operation in discontinuous conduction mode. At a high irradiance, the switching frequency modulation sets a lower value for the frequency, guided by the strive for high efficiency through low switching losses. The proposed automated procedure has shown to be effective in searching for the optimal number and values of switching frequencies. Furthermore, an interleaved boost cell is activated at high irradiance to retain a high level of power quality. Hysteresis functions support the transitions between different discrete switching frequencies as the irradiance changes. The adaptive MIC control scheme is complemented by an MPPT designed for fast tracking. Thus, by combining the SFM with the adaptive usage of the boost converter interleaved cells and a fast MPPT, targets of efficiency and power quality are reached. The efficiency for the entire MIC including all power conversion and control functions was measured at around 95% or higher for irradiance levels ranging from 0.3 kW=m2 to 1.0 kW=m2. The voltage ripple remained below 0.7% during testing. The prototype was rated at 400 W to make the design well suited for integrating photovoltaics in DC microgrids or solar homes. Distributed maximum power point tracking is implicitly supported through the module integration. The prototype’s cost of parts amounted to 0.14 Euro/W when ordering parts individually in the year 2015. Scale effects will allow for further cost reductions. Together with the convincing technical performance, the cost effectiveness makes this MIC design a compelling candidate for renewable solutions of DC microgrids, DC buses, and solar home applications.

REFERENCES:

 [1] REN21. 2016, “Renewables 2016 Global Status Report,” Renewable Energy Policy Network for the 21st Century, Paris, Tech. Rep., 2016.

[2] E. Romero-Cadaval, G. Spagnuolo, L. G. Franquelo, C.-Andr´es Ramos- Paja, T. Suntio, and W.-Michael Xiao, “Grid-Connected Photovoltaic Generation Plants: Components and Operation,” IEEE Ind. Electron. Mag., vol. 7, no. 3, pp. 6–20, Sep. 2013.

[3] M. Das and V. Agarwal, “Design and Analysis of a High-Efficiency DCDC Converter With Soft Switching Capability for Renewable Energy Applications Requiring High Voltage Gain,” IEEE Trans. Ind. Electron., vol. 63, no. 5, pp. 2936–2944, May 2016.

[4] F. Wang, F. Zhuo, F. C. Lee, T. Zhu, and H. Yi, “Analysis of Existence- Judging Criteria for Optimal Power Regions in DMPPT PV Systems,” IEEE Trans. Energy Convers., vol. 31, no. 4, pp. 1433–1441, Dec. 2016.

[5] O. Khan, W. Xiao, and M. S. E. Moursi, “A New PV System Configuration Based on Submodule Integrated Converters,” IEEE Trans. Power Electron., vol. 32, no. 5, pp. 3278–3284, May 2017.

Intelligent Power Sharing of DC Isolated Microgrid Based on Fuzzy Sliding Mode Droop Control

ABSTRACT:

Linear droop control can realize power sharing among generators in DC microgrid without relying on critical communication links. However, the droop relationship between output power and voltage magnitude of renewable power generate system is nonlinear with uncertainties and disturbances from renewable sources and loads in practical DC microgrid. A novel droop scheme is proposed for an isolated DC microgrid to solve the nonlinear problem. The control strategy is proposed by using the Takagi-Sugeno (T-S) fuzzy model and sliding mode algorithm. The nonlinear droop characteristics can be represented by T-S model through taking advantage of locally measured output variables. The sliding mode droop controller is designed for compensating the uncertainties and disturbances to derive accurate power sharing based on T-S fuzzy model. The proposed scheme is proved to be effective under variable operating conditions through PSIM/Matlab simulation.

KEYWORDS

1.      Droop control

2.      Autonomous power sharing

3.      DC microgrid

4.      T-S fuzzy model

5.      Sliding mode control (SMC)

SOFTWARE: MATLAB/SIMULINK

 CONCLUSION:

The novel droop control strategy is proposed for accurate power sharing considering system parameters uncertainties and load disturbances. The technique is designed by using sliding mode controller based on T-S fuzzy model of the DC MG. The overall system stability can be assured. The conclusion is drawn that load changes of the DC MG can be regulated more adaptively. Meanwhile, the proportional load power sharing can be accurately achieved without any communication. The proposed method is verified in PSIM/Matlab simulation. Future extensions of the method can include nonlinear sliding mode droop control of multiple batteries or in AC/DC hybrid MG.

REFERENCES:

[1]. R. Lasseter, “Microgrids” in Proc. IEEE Power Eng. Soc. Winter Meet.,2002, pp. 305–308.

[2]. S. K. Mazumder, M. Tahir and K. Acharya, “Master – slave current-sharing control of a parallel DC-DC converter system over an RF communication interface”, IEEE Trans. Ind. Electron., vol. 55, no. 1, pp. 59-66, Jan. 2008.

[3]. M. N. Iyer, and M. C. Chandorkar, “A generalized computational method to determine stability of a multi-inverter microgrid,”IEEE Trans. Power. Electron., vol. 25, no. 9, pp. 2420-2432, Sept. 2010.

[4]. R. Majumder, B. Chaudhuri, A. Ghosh, and F. Zare, “Improvement of stability and load sharing in an autonomous microgrid using supplementary droop control loop,” IEEE Trans. Power Syst., vol. 25, no. 2, pp. 796-808, May. 2010.

[5]. P. C. Loh, D. Li, Y. K. Chai and F. Blaabjerg, “Autonomous operation of hybrid microgrid with AC and DC subgrids”, IEEE Trans. Power Electron., vol. 28, no. 5, pp. 2214-2223, May. 2013.

Improved secondary control for optimal total harmonic distortion compensation of parallel connected DGs in islanded microgrids

ABSTRACT:

This study proposes a two-layer hierarchical control to actualize optimal total harmonic distortion (THD) compensation in different buses of parallel-connected inverters in islanded microgrids which had not been studied so far. The proposed secondary layer is used to realize THD compensation of sensitive load bus (SLB) and make distributed generators (DGs) distribute the compensating efforts between them according to their rated capacity. It is noteworthy that improving THD at the SLB can lead to an increase in THD at local buses and/or DG terminals. Although the THD limitations of these buses are not as strict as the THD limitation of SLB, it is necessary to control them within their allowed range. This important problem is not well studied in the literature. A novel complementary part is designed and added to the secondary control to tune the compensation portion of each DG while the THD limitations in DG terminals and local buses are considered. The proposed method actualizes a multi-level voltage quality control in multi-bus islanded microgrids with parallel DGs through a simple yet effective solution. Furthermore, considering the DGs peak current limitation is added to the controller and a method for calculating this peak value is proposed.

SOFTWARE: MATLAB/SIMULINK

CONCLUSION:

A new two-level control hierarchy structure ,which had not been studied before, is proposed in this study to actualise the optimal compensation of voltage harmonics in islanded microgrids without any additional equipment installation. The primary layer controls the voltage and frequency and also the FPS power sharing. The secondary level controls the THD of SLB. Improving THD at the SLB can lead to an increase in THD at local buses and/or DG terminals. Although the THD limitations of these buses are not as strict as the THD limitation of SLB, it is necessary to control them within their allowed range. A complementary part is designed and added to the secondary control in order to tune the compensation efforts of each DG unit while considering the THD limitations in DG terminals and local buses. Furthermore, considering the DGs peak current limitation is added to the controller and a method for calculating this peak value is proposed. The proposed control structure realises a multi-power-quality level control for islanded microgrids with multi-bus and parallel DGs through a simple yet effective solution.

The design of the proposed control is very simple and it is not needed to have the microgrid parameters and structure. Therefore, it would be more functional and also the plug & play ability of DGs would be reserved as well. An example system is modeled and simulated. Simulation and comparison results are presented to demonstrate its effectiveness.

REFERENCES:

[1] Khodabakhshian, A., Andishgar, M.H.: ‘Simultaneous placement and sizing of DGs and shunt capacitors in distribution systems by using IMDE algorithm’, Int. J. Electr. Power Energy Syst., 2016, 82, pp. 599–607

[2] Andishgar, M.H., Fereidunian, A., Lesani, H.: ‘Healer reinforcement for smart grid using discrete event models of FLISR in distribution automation’, J. Intell. Fuzzy Syst., 2016, 30, pp. 2939–2951

[3] Micallef, A., Apap, M., Spiteri-Staines, C., et al.: ‘Reactive power sharing and voltage harmonic distortion compensation of droop controlled single phase islanded microgrids’, IEEE Trans. Smart Grid, 2014, 5, (3), pp. 1149– 1158

[4] Zeng, Z., Zhao, R., Yang, H.: ‘Coordinated control of multi-functional grid tied inverters using conductance and susceptance limitation’, IET Power Electron., 2014, 7, (7), pp. 1821–1831

[5] George, S., Agarwal, V.: ‘A DSP based optimal algorithm for shunt active filter under nonsinusoidal supply and unbalanced load conditions’, IEEE Trans. Power Electron., 2007, 22, (2), pp. 593–601