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Электроэнергетика Electric power engineering
Original article
DOI: 10.14529/power220301
DESIGN AND SIMULATION OF A SOLAR-WIND STAND-ALONE SYSTEM WITH A SEVEN-LEVEL INVERTER
M.A. Qasim12 mohammed.a.k.qasim@gmail.com, https://orcid.org/0000-0003-0651-5454 V.I. Velkin1, v.i.velkin@urfu.ru, https://orcid.org/0000-0002-4435-4009 S.E. Shcheklein1, s.e.shcheklein@urfu.ru, https://orcid.org/0000-0003-2140-0321 I. Hossain1, hossain.ismail44@yahoo.com, https://orcid.org/0000-0002-7256-8135 Y. Du1, erica002@163.com, https://orcid.org/0000-0001-6563-2621
Ural Federal University named after the first President of Russia B.N. Yeltsin, Ekaterinburg, Russia 2 Ministry of Health, Baghdad, Iraq
Abstract. During an energy conversion process, the total harmonic distortion and losses will increase while its power stability decreases. Multilevel inverter technology can be utilized to alleviate the shortcomings of conventional inverters. These technologies have become recognized as cost-effective solutions for a wide range of industrial applications. Reduced component losses and lower switching losses, as well as improved output voltage and current waveforms are the first advantages of this design. In multilayer inverters, elimination of harmonic components in the inverter output voltage and current is crucial. This paper proposes a system that consists of three different renewable energy sources. Two of them are PV solar systems while the third is wind turbine simulated in MATLAB Simulink. Seven-level inverters based on switch reduction techniques are proposed in this paper. The proposed system design is verified in the absence of PV systems to produce five voltage levels as a contingency in PV systems.
Keywords: seven-level inverters, photovoltaic system, wind turbine, maximum power point tracking, pulse width modulation, permanent magnet synchronous generator (PMSG)
Acknowledgments: The research funding from the Ministry of Science and Higher Education of the Russian Federation (Ural Federal University Program of Development within the Priority-2030 Program) is gratefully acknowledged: Grant number FEUZ-2022-0031.
For citation: Qasim M.A., Velkin V.I., Shcheklein S.E., Hossain I., Du Y. Design and simulation of a solar-wind stand-alone system with a seven-level inverter. Bulletin of the South Ural State University. Ser. Power Engineering. 2022;22(3):5-17. DOI: 10.14529/power220301
Научная статья УДК 621.311
DOI: 10.14529/power220301
ПРОЕКТИРОВАНИЕ И МОДЕЛИРОВАНИЕ АВТОНОМНОЙ СОЛНЕЧНО-ВЕТРОВОЙ СИСТЕМЫ С СЕМИУРОВНЕВЫМ ИНВЕРТОРОМ
М.А.К. Касим12 mohammed.a.k.qasim@gmail.com, https://orcid.org/0000-0003-0651-5454
B.И. Велькин1, v.i.velkin@urfu.ru, https://orcid.org/0000-0002-4435-4009
C.Е. Щеклеин 1, s.e.shcheklein@urfu.ru, https://orcid.org/0000-0003-2140-0321
И. Хоссейн1, hossain.ismail44@yahoo.com, https://orcid.org/0000-0002-7256-8135 Я. Ду1, erica002@163.com, https://orcid.org/0000-0001-6563-2621
Уральский федеральный университет имени первого Президента России Б.Н. Ельцина, Екатеринбург, Россия 2 Министерство здравоохранения, Багдад, Ирак
Аннотация. Во время процесса преобразования энергии в инверторе общие гармонические искажения и потери будут увеличиваться, в то время как его мощность снижается. Многоуровневая инверторная технология может быть использована для устранения недостатков обычных инверторов. Такая технология получила признание в качестве экономически эффективного решения для широкого спектра промышленных применений.
© Касим М.А.К., Велькин В.И., Щеклеин С.Е., Хоссейн И., Ду Я., 2022
Снижение потерь при переключении, а также улучшенные формы сигналов выходного напряжения и тока являются основными преимуществами предлагаемой конструкции. В многослойных инверторах решающее значение имеет устранение гармонических составляющих в выходном напряжении и токе инвертора. В этой статье предлагается система микрогенерации, состоящая из трех различных возобновляемых источников энергии. Две из них представляют собой фотоэлектрические солнечные системы, а третья - ветряная турбина, смоделированная в MATLAB Simulink. В данной публикации предложены семиуровневые инверторы, основанные на методах уменьшения количества переключений. Предлагаемая конструкция системы инвертирования проверяется в отсутствие фотоэлектрических систем для получения пяти уровней напряжения как аналог аварийной ситуации.
Ключевые слова: семиуровневые инверторы, фотоэлектрическая система, ветряная турбина, отслеживание точки максимальной мощности, широтно-импульсная модуляция, синхронный генератор с постоянными магнитами (PMSG)
Благодарности: Исследование выполнено при финансовой поддержке Министерства науки и высшего образования Российской Федерации в рамках Программы развития Уральского федерального университета имени первого Президента России Б.Н. Ельцина в соответствии с программой стратегического академического лидерства «Приоритет-2030»: номер гранта FEUZ-2022-0031.
Для цитирования: Design and simulation of a solar-wind stand-alone system with a seven-level inverter / M.A. Qasim, V.I. Velkin, S.E. Shcheklein et al. // Вестник ЮУрГУ. Серия «Энергетика». 2022. Т. 22, № 3. С. 5-17. DOI: 10.14529/power220301
Introduction
Renewable energy sources (RES) have become very important in recent years owing to their non-degradability, eco-friendliness and self-sufficiency. Systems using RES are reliable and can replace conventional generation techniques. Fossil fuels are de-gradable, non-renewable and contribute to climate change and ecological imbalance. Solar, wind and geothermal energies, as well as other renewable energy resources, are forms of RES. Solar and wind energy are the most commonly used RES [1]. The benefits of a solar photovoltaic systems over wind turbines include their minimal maintenance, lack of moving parts and ease of installation. Wind-turbine systems are less expensive than solar panels, especially when used in large quantities. However, they require a professional staff for operation and maintenance. The two technologies are typically combined using multiple RES so that they may deliver continuous power under various situations [2].
Since the amount of energy provided by renewable systems changes throughout the day, it is critical to maximize the delivered energy [3]. The output power of wind turbines and photovoltaic arrays is influenced by wind speed and solar irradiation. As a result, changes in these parameters must be handled appropriately by system control mechanisms. With variable wind, the turbine speed must be adjusted to optimize the generation of power to ensure that the system runs at its maximum power point (MPP). Similarly, the DC voltage and current at the output of the PV system must be modified to run the PV systems at their MPP [4]. The biggest issue that some RES encounter is they produce DC electrical energy. This requires equipment to convert DC to AC power. Inverters serve this role. However, there are switching components utilized during power conversion that reduce the stability and quality of the electricity [5].
Multilevel Inverter (MLI) technology is used at the DC output terminals of RES to convert the generated electricity into AC power and improve the power
quality and stability. The benefits of MLI include improved voltage and current output waveforms, less electro-magnetic interference (EMI), their small size, and lower total harmonic distortion (THD) [6]. MLI switches are used to interrupt the DC so that it is produced at different levels. They are essential because they determine circuit size, installation dependability, control complexity and cost. In conventional MLIs, the size, cost and complexity of inverters increase with the output voltage level [7]. In this research, MLI switch reduction is used to provide a larger number of output levels with fewer switching components, thereby lowering costs. The proposed MLI circuit reduces the voltage stress on the switches, enhancing protection against overvoltage [8].
There are three main types of reduced switch MLIs that can be used. The first is a reduced switch symmetrical MLI (RSS-MLI) that produce many DC levels with equal magnitude. This method is a lower cost option. A reduced switch symmetrical MLI (RSA-MLI) produces DC of different magnitudes. Typically, this technology is used in a cascaded H-bridge method. Finally, a reduced switch modified MLI (RSM-MLI) does not use a cascaded H-bridge configuration. It cannot be used for high power applications due to its excessive voltage stress on H-bridge switches [9]. Fig. 1 shows the conventional and reduced switch MLIs.
Design and implementation of a single-phase switching reduction MLI in a stand-alone system fed from multiple individual RES are presented in this work. A new logical switching method, pulse width modulation (PWM), is used to create a seven-level output voltage. In this research, two PV-solar systems and one wind-turbine generator with a permanent magnet synchronous generator (PMSG) are offered as distinct energy sources. The proposed method is used to reduce the number of switching devices, thereby decreasing losses and THD.
The MATLAB Simulink program is used to simulate a seven-level inverter for use with renewable
Fig. 1. Conventional and reduced switch MLIs [9]
energies. RES continue to be more dominant in local power production for stand-alone networks than fossil fuel units. In this research, PV and wind turbine technologies are used to create a self-contained system with high reliability and security that can work under a variety of operational conditions. The contingency of reduced PV solar system performance (due to cloudy winter days, nighttime or fault event) is taken into consideration in this work. Multilevel inverters are an ideal way to employ inverters for stand-alone use in systems that produce just a few kilowatts.
Literature review
Table 1 summarizes the suggested seven-level PWM inverter and contemporary MLI designs using several performance metrics. These metrics include the numbers of semiconductor switches, elements (such as diodes and/or capacitors), transformers and input DC sources, as well as THD before and after incorporation of an LC filter, in addition to contingency analysis. Each MLI design uses a different number of components to achieve the same output-voltage level, as seen in the comparison below.
Proposed system
The proposed system, shown in Fig. 2, is comprised of three distinct RES inputs. Two of them are
Comparison of
PV solar systems and the third is a PMSG-type wind turbine generator. The PV solar systems are both connected to a DC-DC boost converter to increase the output voltages of the two systems to the same level (E/3, and 2E/3). A perturb and observe (P&O) algorithm approach is employed to accomplish maximum power point tracking (MPPT) of the PV solar system and the wind energy conversion system (WECS). The wind turbine output power is AC and thus, an AC-DC rectifier is needed to convert it to DC. A DC-DC boost converter is needed to boost the voltage (E). The wind turbine generator is a three-phase PMSG.
A. PV solar systems
Photovoltaic (PV) systems utilize the photovoltaic effect to convert sunlight into direct current (DC). A combination of P- and N-type semiconductor materials make up the PV cell. As a result, a PV cell can be represented as a diode. When the diode absorbs light, the photovoltaic effect creates current [19]. In most applications, a PV module requires a large number of solar cells connected in parallel or series to provide sufficient voltage and power. An array is a system made up of a large number of PV modules connected on the same panel to generate sufficient power. The MPPT algorithm approach is used to manage the position of the PV system panels so that the maximum
Table 1
MLI topologies
Reference No. No. No. of DC THD before THD after Use RES Contingency
of switches of elements voltage sources LC filter LC filter as inputs analysis
[10] 10 - 3 - 3.53 Not used Verified
[11] 8 - 2 - 1.83 Not used Not verified
[12] 6 11 1 - 3.9 Not used Not verified
[13] 8 - 2 - 2.8 Not used Not verified
[14] 10 3 1 - 1.11 Used Not verified
[15] 12 - 3 19.28 - Not used Not verified
[16] 8 - 2 18.05 - Not used Not verified
[17] 12 - 3 32.1 - Not used Verified
[18] 9 - 3 - 2.38 Not used Not verified
In this 7 - 3 20.51 2.09 Used Verified
work
Fig. 2. The proposed system
generation power of a PV solar system can be obtained [20]. In this study, we will apply the P&O algorithm method to achieve the MPPT.
B. Wind turbine
Three-phase PMSGs powered by adjustable pitch wind turbines are considered the best choice for wind energy producers because of their great efficiency and dependability. The output AC power is converted to DC using a three-phase uncontrolled rectifier. To step up the output voltage and supply a DC bus, a boost converter is also required. Fig. 3 depicts the key components of the WECS [21].
The MPPT of a wind turbine is achieved using the P&O algorithm. Fig. 4 shows the operational principle of this algorithm [22]. C. Switch reduction MLI Inverters are devices that convert DC to AC power. They are often used in household power applications such as motors, UPSs and similar devices. In high-power switching applications, MLIs are becoming more popular. To enhance power quality, high switching frequencies are employed to eliminate ripple in the output voltage or current waveform. Due to switching losses and device rating constraints, switch
Fig. 3. Wind Energy Conversion System WECS
Generator Speed (rad/s)
Fig. 4. Operational principle of the P&O algorithm [22]
reduction MLIs have significant challenges in high-frequency high-power and medium-voltage applications [23].
Among the inverter internal voltage control approaches, pulse width modulation (PWM) is one of the most effective control strategies. In essence, pulse width refers to the width of an inverter's output pulse, which is dictated by the conduction period of each switch. This is especially true for bridge inverters, where each switch conducts for the duration of its gate pulse and the output pulse width is directly proportional to the gate pulse period [24]. As a result of changing the gate pulse duration, the output pulse width changes, thus modifying or regulating the voltage. PWM may be classified into two groups based on the approaches for adjusting gate pulse duration. Fig. 5 depicts a multicarrier PWM used to generate seven-voltage levels using a reference sine wave.
Fig. 6 shows the implemented PWM control sys-
tem for a switch reduction MLI of the proposed system. A simple PWM approach is used to regulate gate pulses. The gate pulses for the switches are generated by a PWM pulse generator. Different modulation indices (Ma) are explored under varying output-voltage levels to highlight the influence of various modulation ratios on output-voltage levels. The output voltage has just three values when Ma is less than 0.33 (+E/3, 0, -E/3). The output voltage has five levels with a modulation index of 0.33 to 0.67 (+2E/3, +E/3, 0, -E/3, -2E/3). A seven-level output voltage may be generated with a modulation index of 0.67 to 1 (+E, +2E/3, +E/3, 0, -E/3, -2E/3, -E).
The switching functions are separated into six modes of operation that are linked to the outputvoltage levels considering one output-voltage / reference signal cycle, with the entire switching states based on a comparison of the reference and carrier waveforms, as indicated in Table 2.
Voltage (V)
/ lÉllM'^iÉÉ ШШЯ1М1111ЩРМ À'iMti Hill I'll г 1 iïV и "1 l'il lui Hi UÉÉlÉBiNiil ^ '11 KSgJif и. . и ..'...I PpfllPPi ЖШ ttfiî4i I'M II ...... Ш|.у
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Fig. 5. Multicarrier PWM seven-shifted levels of a sine wave
Fig. 6. Proposed control based on PWM
Table 2
Switching status and output voltages of the proposed MLI
Voltage Level Sm Sp1 Sp2 S1 S2 S3 S4
E 1 0 0 1 0 0 1
2/3E 0 0 1 1 0 0 1
1/3E 0 1 0 1 0 0 1
0 0 0 0 0 1 1 1
-1/3E 0 1 0 0 1 1 0
-2/3E 0 0 1 0 1 1 0
-E 1 0 0 0 1 1 0
The switches (Sp1, Sp2 and St) are connected in parallel with the solar PV system-1, solar PV system-2 and the turbine generator. The pairs of switches (S1, S4) and (S2, S3) operate alternately to provide the MLI of voltages across the load. The three RES are connected to switch reduction MLI consisting of seven switches. The output voltage is equal to turbine voltage E (400 V), while PV source-1 (1/3 • 400 = 133.3 V) and PV solar source-2 (2/3 • 400 = 266.6 V) are the algebraic sum of the two PV sources and is equal to the output voltage of the WECS.
D. Auxiliary circuits
DC-DC step-up and AC-DC rectifier converters are the main auxiliary circuits used with PV and wind turbine systems.
DC-DC step-up converter. One of the most widely used converters to enhance DC output voltage of a PV solar systems to match the needed DC bus voltage is the DC-DC step-up, or boost converter. System losses are reduced as the voltage is increased. Fig. 7 depicts a boost converter circuit diagram. When the switch is "ON" and the diode is an open circuit, the inductor "L" charges the power. However, when the switch is "OFF", the diode conducts while the input and inductor voltages are received at the output side. Capacitor "C" is used in the output waveform [25] to lower the ripple factor.
The following equation gives the duty cycle (D) of a step-up converter [26]:
D = l - (1)
vout
Inductance (L) and capacitance (C) of a step-up converter are given in Equations (2) and (3), respectively [11]:
^min
с =
D (1—D) R ~f ;
m?
(2) (3)
where R is output resistance calculated from the output power and voltage. is the output ripple voltage, normally about 1% to 2%, and f is the switching frequency [10].
DC-AC rectifier circuit. This is a power electronic circuit that used to convert or transform DC power into AC. This three-phase circuit is made up of three parallel branches. Each branch has two diodes in series. The anode of the upper diode is connected to the cathode of the lower diode. A rectifier circuit is used to convert the generated AC wind turbine power to DC, which is then stepped up by a boost converter. The primary purpose of this conversion is to manage the DC voltage and to achieve the maximum power point [27-29].
A DC link capacitor is used to fix the voltage and reduce the ripple. An LC filter is used at the output of the MLI to reduce the harmonics to acceptable IEEE 519-2014 standard limits.
Simulation and results
Fig. 8 shows the proposed system modeling using MATLAB Simulink.
The system consists of a WECS with an output DC voltage of 400 V and two PV systems with output DC voltages of 133.3 V and 266.6 V, respectively. The temperatures of the two PV systems were assumed to be constant at 25 °C and the irradiance was 1000 W/m2. The output power of each system was 2000 W, and the (I-V) and (P-V) characteristics of
Fig. 7. A schematic of a DC-DC boost converter
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the selected PV panel are given in Fig. 9. Each PV system consists of 10 solar panels connected in parallel, the output voltage of the system is 37.26 V and current is 53.7 A (10 • 5.37 A). The total power is 2 kW for each solar system. Fig. 10 shows the output voltage, current and power of each PV system. An LC filter is used to reduce the harmonics and make the output waveform as sinusoidal as possible so that it is within the IEEE 519-2014 standard.
The output voltages of the DC-DC boost converters are (E/3) 133.3 V and (2E/3) 266.6 V for PV-system-1 and PV-system-2, respectively. The DC-DC
boost converter circuit of the WECS is (E) 400 V DC. Parameters of the step-up converters are given in Table 3. The selected switching frequency is 4 kHz and the ripple voltage is about 1%. Fig. 11 shows the output voltage of the three boost inverters.
The wind speed was assumed to be constant in the current work at 10m/s and the rated output power at this speed is 17 kW. Fig. 12 shows curves of the input/output characteristics of the wind speed, electrical torque, output power, stator current, three-phase stator voltage, and turbine speed in rad/sec as well as pitch angle.
Fig. 9. The (V-I) and (P-V) characteristics of each PV system
Fig. 10. Output voltage and current for each PV system
Table 3
Step-up converter parameters
Converter No. Input voltage (V) Output voltage (V) Power (W) Duty cycle Inductor (mH) Capacitor (uF)
1 37.26 133.3 2000 0.72 0.125 0.02
2 37.26 266.6 2000 0.86 0.15 0.6
3 160 400 17 000 0.6 0.22 0.79
(b) PV Syitim-2
Fig. 11. The output boost voltages of the PV systems
Tim« (sec) Tim« (t«c)
Fig. 12. Characteristics of the WECS
A seven-level voltage waveform without a filter is shown in Fig. 13. The same waveform after connecting an LC filter is shown in Fig. 14.
Harmonics on the low voltage side (below 1 kV) should have a maximum value of less than 5%, according to IEEE519-2014. It is clearly seen in Fig. 15 and 16 that the LC filter reduces the THD, making the output waveforms as sinusoidal as possible. The THD values of the output voltage are 2.09% and 20.51% with and without an LC filter, respectively.
Contingency analysis is important when the system has multi-RES to ensure the reliability of the overall system. When PV solar system-1 is out of service for any reason, the system must still provide the required output voltage. However, the voltage will decrease from seven to five levels and the THD will increased. Fig. 17 shows the output voltage without an LC filter at t = 0.1 sec when the PV solar system-1 is out of service. The THD of the five level inverter, which consists of wind turbine and PV solar system-2,
is increased to 47.87%. When an LC is used, the THD consisting of wind turbine and PV solar system-1, is
is reduced to 5.16%. If PV solar system-2 is out of increased to 31.39% and the THD is reduced to 3.72%
service, the output voltages without an LC filter are as when an LC is used. shown in Fig. 18. The THD of a five level inverter,
Voltage (V) Output Voltage (without LC filter)
a ooz oo-t o.oe o.ob o.i oiz o.id o ie o.ib
Time (sec)
Fig. 13. Output voltage of a seven-level inverter without an LC filter
Fig. 14. Output voltage of a seven-level inverter with an LC filter
0 2 4 6 • Ю 12 M M И 20
Harmonie order
Fig. 15. THD of the output voltage waveform without an LC filter
Fig. 16. THD of the output voltage waveform with an LC filter
Fig. 17. Output voltage transition from seven to five levels (PV system-1 out of service)
Fig. 18. Output voltage transition from seven to five levels (PV system-2 out of service)
Conclusions
In this study, a seven-level inverter fed by solarwind systems was designed and simulated using MATLAB Simulink. Three RES are suggested. Two of them are PV systems with output voltages (E/3
and (2E/3), while the third is a wind turbine with voltage (+E). A multicarrier PWM approach was used to build the control circuit. The THD of the voltage waveform with an LC filter is about 2.09%, meeting the IEEE519-2014 standard. When one of the PV sys-
tems is removed from the system, the voltage level of the output waveform will have five levels. However, the THD will increase. Good design of an LC filter
keeps the THD within standards during contingencies. The system performed well, demonstrating the efficiency of the designed control and power circuits.
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Information about the authors
Mohammed A. Qasim, Postgraduate Student, Nuclear Power Plants and Renewable Energy Sources Department, Ural Federal University named after the first President of Russia B.N. Yeltsin, Ekaterinburg, Russia; Ministry of Health, Baghdad, Iraq; mohammed.a.k.qasim@gmail.com.
Vladimir I. Velkin, Dr. Sci. (Eng.), Prof., Nuclear Power Plants and Renewable Energy Sources Department, Ural Federal University named after the first President of Russia B.N. Yeltsin, Ekaterinburg, Russia; v.i.velkin@urfu.ru.
Sergey E. Shcheklein, Dr. Sci. (Eng.), Prof., Nuclear Power Plants and Renewable Energy Sources Department, Ural Federal University named after the first President of Russia B.N. Yeltsin, Ekaterinburg, Russia; s.e.shcheklein@urfu.ru.
Ismail Hossain, Cand. Sci. (Eng.), Nuclear Power Plants and Renewable Energy Sources Department, Ural Federal University named after the first President of Russia B.N. Yeltsin, Ekaterinburg, Russia; hossain.ismail44@yahoo.com.
Yang Du, Postgraduate Student, Nuclear Power Plants and Renewable Energy Sources Department, Ural Federal University named after the first President of Russia B.N. Yeltsin, Ekaterinburg, Russia; erica002@163.com.
Информация об авторах
Касим Мухаммед Абдулхалик Касим, аспирант, кафедра атомных станций и возобновляемых источников энергии, Уральский федеральный университет имени первого Президента России Б.Н. Ельцина, Екатеринбург, Россия; Министерство здравоохранения, Багдад, Ирак; mkasim@urfu.ru, mohammed.a.k.qasim@gmail.com.
Велькин Владимир Иванович, д-р техн. наук, проф., кафедра атомных станций и возобновляемых источников энергии, Уральский федеральный университет имени первого Президента России Б.Н. Ельцина, Екатеринбург, Россия; v.i.velkin@urfu.ru.
Щеклеин Сергей Евгеньевич, д-р техн. наук, проф., кафедра атомных станций и возобновляемых источников энергии, Уральский федеральный университет имени первого Президента России Б.Н. Ельцина, Екатеринбург, Россия; s.e.shcheklein@urfu.ru.
Хоссейн Исмаил, канд. техн. наук, кафедра атомных станций и возобновляемых источников энергии, Уральский федеральный университет имени первого Президента России Б.Н. Ельцина, Екатеринбург, Россия; hossain.ismail44@yahoo.com.
Ду Ян, аспирант, кафедра атомных станций и возобновляемых источников энергии, Уральский федеральный университет имени первого Президента России Б.Н. Ельцина, Екатеринбург, Россия; erica002@ 163.com.
Conflict of Interest. The authors declare that they have no conflicts of interest that could have influenced the design and conduct of the study, collection and interpretation of data, or presentation of the results.
Конфликт интересов. Авторы заявляют об отсутствии конфликта интересов, который мог бы повлиять на дизайн и проведение исследования, сбор и интерпретацию данных или представление результатов.
The article was submitted 29.04.2022; approved after reviewing 11.05.2022; accepted for publication 11.05.2022.
Статья поступила в редакцию 29.04.2022; одобрена после рецензирования 11.05.2022; принята к публикации 11.05.2022.
