Modeling of renewable energy sources for calculation short current currents of local distribution systems of Egypt : dissertation for the degree of candidate of electrical engineering sciences : 05.14.02

Clean and renewable electricity generation is main target of the world using different sources while wind energy conversion has been rapidly deployed worldwide. Although the availability of well-designed wind turbines could reach as high as 98% with assistance of fast field service and the cost of maintenance is high. Most wind turbines are operated in variable speed conditions due to the varying wind speeds, and fail more often than other rotating machines [1]. The permanent magnet synchronous generator (PMSG) is used in wind energy applications because it is simple winding structure, ease of control, and the ease of realization of multi-pole machines for low-speed applications [2]. Technical problems (like frequency variations, voltage fluctuations, short circuit current contribution, overall system stability, necessary updating required for power system, power quality…) are expected to accompany high level penetration of wind energy [3-4].
Grid codes are described by the relation between voltage and time to obtain smallest time to be connected during the disturbance of the system. The low- voltage ride-through (LVRT) is used to keep wind turbine PMSG connected and contributed to the grid as well as to improve the voltage profile during low- voltage transients in case of a disturbance such as a voltage dip as low as 15 % retained voltage. The PMSG wind turbine supplies reactive power to help the grid voltage to recover from the voltage dip. Several solutions have been investigated for the LVRT requirements in PMSG wind power systems. Direct-drive wind power system with permanent magnet synchronous generator (PMSG) has lots of advantages over traditional wind systems where its LVRT technology has been paid more and more attention. Fig. 1 shows the LVRT requirements of Germany (E.ON), Great Britain, and Denmark.
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Egypt with its large coastal strip, cloudless sky almost year-round and small fossil fuel reserves, to the maximum extent satisfies the conditions for the development of distributed generation (DG) on renewable energy sources. Here, renewable resources make it possible to produce electricity not only individually, but also centrally, with high economic efficiency at power plants (farms) with generating units, of sufficiently large unit capacity. The amount of solar energy incidence per square meter varies between 5 and 8 kWh per day and 2300 kWh/m2 per year (for 3000-4000 hours per year). The wind speed on the Red Sea coast exceeds 10 m/s with an energy of 400-800 W/m2 at an altitude of 40 m above ground level. A Zafarana is the one place in Egypt, which has high-speed wind. Zafarana wind farm is the biggest project in Africa and Middle East, which was beginning in 2001 to 2010 by the cooperation protocols between Egypt and other countries (Germany, Denmark, Spain and Japan). The total installed power of this wind farm is 425 MW connected to electric power system. The construction of this wind farm has been realized in few steps (60, 80, 85, 80, 120 MW). Large wind speed value (average 10 m/s at 40 m above ground level) is main advantage of Zafarana location. Table. 1 shows various kinds of wind turbines used in Zafarana wind farm [5].
The motivation of this investigation is that there are around 167000 residents in Egypt according to New and Renewable Energy Authority (NREA) are experiencing need or totally denied from electricity. Such households are distributed around 264 isolated villages in nine towns along Egypt. Among these villages, 211 villages are completely isolated from electricity and the remaining are fed through standalone diesel stations. The vast majority of these isolated villages are situated in desert areas, where rich measure of solar energy and in some places wind energy exists moreover. In Egypt, numerous new projects are being carried out in various areas, for example those in the new valley in the western desert of Egypt and those in the northern coast of Egypt. Worth to

Wind Turbine
Nordex 600kW
ESTAS 660kW
Speed Generator 11 SEIG
15 SEIG
Power
Control Turbine
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mention that over the past four year, Egypt’s electricity sector has confronted it’s most noticeably crisis in production lagging behind consumption resulting in extended periods of rolling blackouts that reaches two to four hours daily in urban areas and sometimes double those in rural areas. The increasing population and increasing comfort demand of households is presenting severe load on the network. One option to handle the electricity crises and diminishing the load on the network is the utilization of both on-grid and off-grid of PV systems. The first integration of renewable sources of energy was in 2011 from the solar thermal part of the power station built in Kuraymat in the south of Egypt with a portion of 20 MW from a total capacity of 140 MW. A 10 MW solar photovoltaic plant has entered service in 2015 in the Siwa Oasis in Western Sahara. Nowadays, Egyptian electricity holding company is planning to find the administrative steps to complete the agreements, which control the process of purchase of the generated energy from the private sector’s projects. The project has a total of capacity of 1000 MW distributed as follow: a wind farm in Suez Golf with an installed capacity of 250 MW, a 200 MW photovoltaic power plant in Aswan (Komombo), and a project in the west of Nile River with a total installed capacity of 550 MW from different renewable sources.
Table. 1. ─ Different types of installed wind turbine
No. of Active Stall 105
GAMESA 15 DFIG 850kW
Opti 117 Slip/Pitch
Pitch 336

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Fig. 1. ─ Limit curves for the voltage generator disconnection
The installation rate of photovoltaic (PV) generated energy into the grid system was increased in worldwide. This increased penetration level offers unique challenges to systems operators. Improving reliability so as to reduce replacement cost during the life of PV array is one of significant challenges [6]. Usually PV plant consists of 3 main parts: large amount of PV units which are combined together to feed inverters. The inverters then are connected to low frequency transformer (medium voltage electric grid) [7].
Converters are main part in PV system to connect with AC loads or electric grid [8]. Nowadays, multilevel converters are widely used and proposed for medium and high voltage applications due to reduce switching losses, low harmonic distortion, high voltage capability and good dynamic response [9]. In general there are six main common multilevel converter topologies; Neutral point Clamped converter (NPC) [10], cascaded H-bridge [11], Y-connected Hybrid Cascaded [12], Capacitor Clamped [13], Z-source [14] and quasi Z-source [15]. The main circuit and control scheme of NPC three-level converter are simple with comparison the conventional two-level topology. The basic benefits of the NPC three-level inverter as follows: 1) reducing the harmonic and the switching

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frequency, so that the system loss is small, 2) rising rate of voltage(dv/dt) is lower than the two-level inverter and the rise rate of current (di/dt) is also reduced, 3) reducing the torque ripple and electromagnetic noise with the increase of the number of levels, each amplitude of level will be reduced relatively, 4) reducing the size of the circuit because the voltage which is linked with the absorption circuit is only half of that, the energy flowing into the absorption circuit is small.
The main technical difficulty in any application of the NPC three-level topology is to maintain the two DC-side capacitors’ voltages equal and at a pre-specified level, which is more significant for the equipment running safely and reliably. Several methods have been used to retain the capacitor voltages at a desired value; the hardware implementation method including separate DC sources, an auxiliary converter to inject a current in the neutral point is used with additional circuit which adds to the equipment the inefficiency, cost and complexity and the software method is realized by modifying the converter-switching mode according to related control algorithms based on the space vector pulse width modulation (SVPWM) or sine pulse width modulation (SPWM) strategy.
The output power of PV array is depend on irradiation which incident on the PV module surface over the time due to climatic conditions; the output power also varies producing several power-voltage curves for several irradiation levels. The maximum power output of PV array is generated by adjusting voltage or current for irradiation and temperature [16].
To increase the efficiency of PV power generation system, Maximum power point tracking (MPPT) techniques are implemented. Several methods are using to get maximum power point of PV. Perturb-and-observe (P&O) method is dominantly used in practical PV system for MPPT control due to its simple implementation. Look-up table would require large memory storage of the micro- controller as large amount of panel data is stored with high accuracy are implemented.

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The converter current is included high order harmonic that flows into the grid causing harmonic pollution. Filters are used to mitigate these harmonics. The most common filter is L-filter while its inductance must be high value to decrease the current ripple. The LC-filter can mitigate these harmonics but it is expensive for medium and high power application and the operation of filter is not better because of uncertainty of grid impedance. Due to the large naturally cut-off frequency and robust powerful capability of LCL filter in low-frequency power systems, the LCL filter is commonly implemented in the inverter. The operation of three-phase inverters of PV system connected to electric network is examined the inductance, the relation of two inductances, choosing the filter capacitor and resonance resistance. The proposal form of LCL filter is having a significant role in the entire system and plays vital role for stability of the system. The designing of LCL filter is necessary to take exceptional attention where the grid impedance plays important role in influence on the performance of the system while any change of the grid impedance occurs the change of the resonance frequency of LCL filter i.e. stiffness of the grid. To mitigate high frequency current harmonics produced from switching of inverter, the electromagnetic interference filter (EMI) is used. In the PV system connected to electric network, L-filter or LC- filter is usually used. From aforementioned cases, the inductance is large value, bulky and costly. In the case of use LCL filter as an EMI filter with two different modes, inductance and differential inductance.
During the past decades, the electric power industry has undergone significant changes in response to the rising concerns of global climate change and volatile fossil fuel prices. For more efficient, reliable, and environmentally friendly energy production, it is critical to increase the deployment of distribution generation (DG). This trend has evolved into the concept of a “microgrid” which can be described as a cluster of distributed energy resources, energy storage managed by an active energy management system. Microgrid (MG) can work

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whichever in grid-connected or islanding method of operation. Islanded MGs are viewed as an incredible decision for some applications when the connection to the main network is absurd or not best. For example, islanded MGs are the answer for remote networks situated far from the main grid. AC microgrids are very popular because of easy modeling, simple design and efficient performance
MGs have been significantly deployed during last few years and are anticipated to grow even more in the near future, where future power grids can be pictured as systems of interconnected MGs. The MG is a number loads and renewable energy sources connected together under electrical constraints with only manageable object depending on the grid where it can connect or disconnect from the grid.
In remote and rural area which many people use the electricity service of electric grid, MG is attractive to supply domestic loads as well as many applications like irrigation system. MG is combined distribution system containing small distributed renewables sources and loads capable of operating in islanded as well as grid connected mode [17]. MG is the practical solution to the challenges produced by high DG penetration and makes the large-scale application of DG system possible. The Point of Common Coupling (PCC) is the bus connection between MG and electric network where MG was able to import and export energy from and to the grid flexibly by regulatory of active and reactive power flow. Like all power systems, MGs are not globally stable, which presents unique control challenges. An effective technique is using static synchronous compensator (STATCOM) to increase the stability of AC MG by injection reactive power to support voltage stability. Power electronics, which are used to interface distributed generators and loads to the network, offer new opportunities for decentralized and autonomous control of power supply and demand. They draw constant power from the network to regulate their outputs [18]. The active and reactive power compensation techniques, besides a PID controller

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implantation, are used to maintain the constant terminal voltage of constant power load (CPL) by using current injection method from the storage system to increase the stability scenario of AC MG [19]. To decrease the size and weight of major system components of AC MG, such as DGs and transformers as well as harmonic filters, High frequency AC MGs are used by multiple high speed generators connected in parallel. They can be lighter, smaller and cheaper than those operating at 50 or 60 Hz [20]. An MG stability controller with energy storage equipment is used to help MG switch operation mode steadily and flexibly and improve transients’ characteristics of MG during the fault in distributed network [21].
In contrast with AC MG, the DC MG has drawn many advantages such as higher efficiency, an absence of reactive power and harmonics. The DC MG is already implemented in industrial system, data center, telecommunication system, marine power system and residential applications. DC MGs are very popular in recent time because increasing DC appliances in MGs. The control strategy is essential for addressing the disturbances to maintain DC and AC MGs voltages in grid- connected and standalone operating modes.
Superconducting fault current limiters (SFCL) is one of the applications of superconductors due to the superconducting characteristics. The working of SFCL depends on the unexpected change from the superconducting state to the regular state by surpassing the basic current Ic of the material. An electromagnetic equivalent of resistive SFCL depends on E-J power law. To limit the short circuit current in conventional electric power and wind farm networks, The SFCL is implemented in an electric network during the fault [22]. Various strategies for restricting short circuit current by SFCL are studied and the results showed the capability of SFCL to minimize the short circuit current in the range of 20–50% in electric power system. There are many benefits of the SFCL:

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• Minimum power loss and voltage drop due to small impedance system in normal state.
• Large equivalent impedance to limit short circuit current during the fault.
• High response to change from normal to emergency operation.
The SFCL is not only used to limit short circuit current, but it is also used to increase temporary stability of generators in the electric network and subsequently the general stability of the electric power system [23].
The DG during the fault makes the equivalent circuit of grouping system not similar to the equivalent of the conventional electric network. In view of the control system and the proportional model of inverter integrated DG under fault, DG can be taken as voltage source or current source in sequence-network. To analyze fault characteristics of DGs with different control schemes and deliver elaborative comparisons among them.
Various approaches have been familiar with controlling islanded MGs. Droop control is generally utilized which is a sort of ward control by which active and reactive powers be shared among DGs. Virtual impedance is used to improve the exhibition of droop control. The adaptive virtual impedance used to improve traditional virtual impedance procedures. The estimation of virtual impedance is not consistent in the adaptive virtual impedance system and fluctuates at any moment by the method which expels impedance imbalance between various DGs. The control innovation of MG is a key to the operation of the MG. Develop control innovations can improve the adaptability of MG activity and improve control quality. The issue of fault current computation in distribution network containing inverter-based distributed generation is underlined by the fault ride- through prerequisite declared in grid codes.
The influence and the potential risk when clean the faults using the relay protection system on power supply in distribution systems are given increasingly

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more consideration. The impacts of continuous faults, clean distraction, and absence of reaction of the relay protection systems are analyzed on low electrical energy fed electrical load.
A probability computation method of power source risk incidence because cleaned faults using relay protection system is a probability function of fault of the relay protection systems, the frequency of the cleaned faults happening in operation time and the drop power fed to load. The conventional power distribution grid has recently changed; expanding the portion of distributed power generation systems (DGs) makes it a functioning player in electric power production and supply. Although an expanding DGs penetration might be advantageous, power produced by those devices is influenced by high uncertainty, in this manner may increase system risk, system non-linearity, and overall complexity and propose several challenges for the future grid. Static security risk calculation is one of the important problems in the distribution network due to the influence of surprising and inevitable faults and failures, in which many types of research give more attention.
The degree of development of the research topic. There is a fairly large number of scientific publications devoted to the DG on renewable energy sources. Basically, they are published in foreign publications and are devoted to the issues of conceptual development and organization of microgrid; the design of individual units of generating devices; simulation of the work of sources of DG in normal and emergency modes; analysis of transients arising from failures in the case of parallel operation with the network or during isolated operation of the DG; stability analysis of generating devices; development of protection schemes when the network operates on direct and alternating current at various operating modes of microgrid. At the same time, there are many publications on the calculation of short circuit currents in the most famous foreign journals.

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Among Russian scientists, a significant contribution to the development of control theory and the increase in the effectiveness of DGs was made by permanently acting in Ural Federal University under the guidance of Ерохина П.М., Паздерина А.В., Ерошенко С.А. и Самойленко В.О seminar “Problems of connecting and operating small generation in parallel with the electrical power system of Russia”, as well as scientists: Воропай Н.И., Дьяков А.Ф, Илюшин П.В., Тарасенко, В.В., Фишов, А.Г. Среди, foreign researchers: Xiaohang Jin, Liyan Qu, J. Yang, J.E., Fletcher, Md, Michael Negnevitsky, Julia A. Belk, Konstantin Turitsyn, Jiahui Zhua, Xiaodong Zhenga, Ming Qiua, Zhipeng Zhang, Mazen Abdel-Salam, Khairy Sayed, Canbing Li, etc.
The objects of study are microgrid with a DG based on renewable energy sources; Wind turbine PMSG; PV connected to a three-phase three-level converter with neutral point; hybrid local electric power stations operating in the climate zone of Egypt, with the allocation of subsystems for AC and DC microgrid.
Purpose and goal of work. The purpose of this study is to develop a methodology and algorithm for calculating short circuit currents in local electrical systems with distributed generation on renewable energy sources.
To achieve this goal, the following tasks were solved:
1. Analysis of the specific properties of wind turbines with converters for their consideration in the calculation of short circuit currents.
2. Development of a mathematical model for maximum power point tracking of the PV system under conditions of grid-connected and stand-alone.
3. Testing the feasibility and necessity of using the Arduino Nano microcontroller to analyze the PV control system for short circuit in an external circuit.
4. Development of a Simulink model for modeling generating devices with converters and their control systems with the aim of identifying significant

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characteristics for calculating short circuit currents; checking the need to install filters of harmonic components and a selection of their parameters; checks of all developed computational procedures.
5. Developmentofcomputationalproceduresforcomputer-basedcalculation of short circuit currents in microgrid with DG based on renewable energy sources.
6. Performing test calculations of short circuit currents with the aim of debugging the proposed computational procedures.
7. Analysis of the balance reliability of subsystems formed as a result of dividing microgrid when short circuits are disconnected.
Scientific novelty:
1. A methodology has been developed for taking into account the current limiting properties of renewable energy generating devices in the calculation of short-circuit currents in local electric power systems.
2. A structural mathematical model of renewable energy sources with converters and control systems for analyzing their operation in the short circuit mode is proposed.
3. A mathematical model of microgrid with distributed generation based on renewable energy sources has been developed.
4. A mathematical model has been developed for maximum power point tracking as the determining function of the PV simulator.
Theoretical and practical significance of the work. The work is fully
oriented to solve the main technical problems caused by connection of renewable energy based DGs in the Egyptian energy system. This work was carried out as part of the target program for the development of the electric power industry in Egypt. The developed software procedures for calculating short circuit currents can be used in the educational process and become the basis for the development

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of a software package for calculating normal and emergency modes in local EPS with DG based on renewable energy sources
Methodology and research methods of dissertation. During the study, the theoretical foundations of electrical engineering, optimization methods and non-linear programming, as well as various methods for solving system linear equations were used. The C ++ language for the software implementation of programing Microcontroller Arduino Nano is using to detect the irradiation value and control the SPWM. To verify the obtained calculation results, models are created to test networks in an interactive environment for Simulation MATLAB Simulink and MATLAB code.
The main provisions of the dissertation research submitted to the defense:
1. ModelingofDGbasedonrenewableenergysourceswiththeallocationof subsystems that determine short circuit currents.
2. AmathematicalmodelforofthemaximumpowerpointtrackingofthePV system for grid-connected and stand-alone modes.
3. A modified method for calculating short circuit currents.
4. Assessment of the balance reliability indicators of microgrid based on
random divisions of the system into subsystems in post-fault conditions.
The author’s personal contribution is the development of mathematical
modeling of DGs (Wind, PV) for steady state and transient analysis of the electrical power system. Design the protection scheme devices for wind turbine PMSG as well as a simple protection scheme of the grid-connected wind farm. In addition, it is development of digital control strategy of SPWM MPPT of PV system is designing for two cases; stand-alone and grid-connected using implementation of the cheap and fast digital microcontroller named Microcontroller Arduino Nano 3x is using to detect the irradiation value and control the SPWM. A SFCL investigated to protect the AC and DC Microgrid from high fault current. A method is proposed for the fault current for grid-

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connected and isolated microgrids with the comparison with the virtual impedance method. The static security risk and load shedding are calculated during the fault in different fault locations using an operation scenario in which the distribution system will divide into small subsystems.
The reliability of scientific results. The proposed algorithms and methods in the dissertation were used in the calculations of steady-state and transient analyses of PMSG wind turbine connected to grid, PV system for two modes (stand-alone and grid-connected) and hybrid wind-PV-battery bank AC and DC microgrids as well as the fault current calculation of the IEEE 33-bus with 5 DGs distribution system for grid-connected and isolated microgrids. A comparison of the results of the work of these methods with data obtained using other programs and other scientists allows us to judge the proper operation of the developed methods and algorithms, as well as the possibility of their application to solving practical problems within the network of the established modeling conditions.
Testing the results of work. The main provisions of the work were reported and discussed at 8 conferences.
Publications. Based on the research results, 8 works were published, including one in Russian-language publications from the list of the Higher Attestation Commission (“Electricity”, “Electrical Systems and Complexes”), 3 articles in publications, indexed in international abstract citation databases Scopus and Web of Science and 4 in other publications.
The structure and scope of work. The dissertation consists of introduction, three chapters, conclusion, and two appendices. It contains 164 pages, includes 95 figures and 20 tables.