standalone hybrid generation system

University of South Wales

Master of Sciences Thesis

STANDALONE HYBRID GENERATION SYSTEM FOR THE REMOTE AREA OF THAR, PAKISTAN

Submitted By: Syed Hassan Raza (08011605)

Supervisor: Senior Lecturer Dr Ben Mehenni

Thesis submitted to the Faculty of Computing, Engineering and Sciences in partial fulfillment of the requirements for the degree of Master of Science in Sustainable Power Technology.

October 2014

ABSTRACT This thesis proposes a standalone hybrid generation system in Thar area of Pakistan combining solar and wind energy with provision of a storage bank and controlled using a bidirectional boost DC-DC converter (BDC) to maintain constant dc-link voltage. A three-phase complex vector control scheme voltage source inverter (VSI) is used to control the load side voltage in terms of the frequency and voltage amplitude. The simulation results obtained from Matlab/Simulink show that the overall hybrid framework is capable of working under the variable weather and load conditions.

Standalone hybrid generation system for the remote area of Thar, Pakistan

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ACKNOWLEDGEMENT

First of all, I would like to express my deep sense of respect and gratitude towards my Supervisor Dr Ben Mehenni, Dr Clive Morgan, and Mr. Kang Li who has been the guiding force behind this work. The thesis would not have been accomplished without their readiness to help; their willingness for series of intensive discussions that brought about more valuable suggestions; and their supports are highly appreciated in this regard. Finally, I would like to thank almighty and my parents for giving me strength and supporting me throughout my life.


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LIST OF ABBREVIATION List of abbreviations Description A Ampere AC Alternating Current Ah Ampere-hour BB Battery Bank BC Battery Capacity CC Capital Cost DC Direct Current DCC Daily Charging Capacity DFC Daily Fuel Consumption DG Diesel Generator DOD Depth of Discharge DRT Daily Running Time of the PV-system DSH Daily Sun Hour ECS Energy Conversion System EPP Electrical Power Processing FC Fuel Cost fc fuel consumption HPSs Hybrid Power systems hr Hour KE Kinetic Energy Km Kilo meters KW Kilo Watt KWh Kilo Watt hour KWP Kilo Watt peak LCC Life-cycle Cost LEL Lower Energy Limit of battery MOO Maximum ON/OFF PMS MPPT OG Maximum Power Point Tracker Only OPB Only PV-arrays partly supplying the PMSs Power Management Strategies PPS Peak Power Shaving PSBB Preliminarily Sized Battery Bank pu per unit PV Photovoltaic RBB Resized Battery Bank RC Replacement Cost RESs Renewable Energy Sources SOC State Of Charge V Voltage VC Variable Cost


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WG Wind Generator P&O Perturb and Observe PFC Power Factor Correction PI Proportional integral PID Proportional Integral Derivative PMSG Permanent magnet Synchronous Generator PV Photovoltaic Cell PWM Pulse Width Modulation RES Renewable Energy Systems RMS Root Mean Square RV Recreational vehicle SA Standalone SCR Silicon Controlled Rectifier SOC State of Charge STC Standard Test Condition TSP Tip Speed Ratio UPS Uninterrupted Power Supply VAWT Vertical Axis Wind Turbine VSD Variable Speed Drive WECS Wind Energy Conversion System WRSG Wound Rotor Synchronous Generator


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Table of Contents

ACKNOWLEDGEMENT ................................................................................................................... 2 LIST OF ABBREVIATION ............................................................................................................... 3 Introduction ................................................................................................................................................ 7 Aims ............................................................................................................................................................... 7 Objectives .................................................................................................................................................... 8 System components and Tools ............................................................................................................. 8

CHAPTER 2: Background Research .......................................................................................... 9 2.1. System Location Analysis ............................................................................................................ 11 2.2. Components of the Hybrid generation System .................................................................... 12 2.2.1. Solar Photovoltaic ...................................................................................................................... 12 2.2.2. PV Array System configurations .......................................................................................................... 14 2.2.3. PV Design and Array sizing approaches .......................................................................................... 14 2.2.4. PV-array Implementation ...................................................................................................................... 15 2.2.5. PV Array specification .............................................................................................................................. 16 2.3. Wind Turbine .................................................................................................................................. 17 2.3.1. Wind Energy Conversion System ........................................................................................................ 18 2.3.2. Speed and power relation ...................................................................................................................... 18 2.4. Permanent Magnet Synchronous Generator (PMSG) Model ........................................... 23 2.4.2. PMSG Controller Modeling ..................................................................................................................... 24 2.4.2.1. Optimum power control ...................................................................................................................... 24 2.4.2.2. Efficient operation of PMSG ............................................................................................................... 25 2.5. Energy Storage System ................................................................................................................. 27 2.5.1. Rechargeable battery ............................................................................................................................... 28 2.5.3. Battery sizing ............................................................................................................................................... 32 2.6. Power Electronics .......................................................................................................................... 33 2.6.1. PV Array Power Electronics and control Topology .................................................................... 35 2.6.1.1. Three Phase .............................................................................................................................................. 35 2.6.2. Power Electronics and Control Topology for Wind Generation ............................................ 38 2.6.3. Power Electronics and control Topologies for BES .................................................................... 39 2.6.4. Battery Charging/discharging Control algorithm ....................................................................... 42 2.6.4.1. Charging/Discharging Controlling model ................................................................................... 43 2.7. Design Topologies literature Review. ..................................................................................... 45 2.7.1. Technical configurations for hybrid power systems .................................................................. 45 2.7.2. AC/DC-coupled Hybrid Power Systems ........................................................................................... 45 2.7.3. AC-coupled Hybrid Power Systems ................................................................................................... 46 2.7.4. Centralized AC-coupled Hybrid Power Systems. ......................................................................... 46 2.7.4. Distributed AC-coupled Hybrid Power Systems. ......................................................................... 46 2.7.5. DC-coupled Hybrid Power Systems ................................................................................................... 47 2.7.6. Mixed-coupled Hybrid Power Systems ............................................................................................ 48 2.7.7. Series-Parallel Hybrid Power System ............................................................................................... 48 2.8. Matlab/Simulink User Manual .................................................................................................. 50 2.8.1. Simulink ....................................................................................................................................................... 51 2.8.2. Starting up Simulink ................................................................................................................................. 51 2.8.3. SimPowerSystems ..................................................................................................................................... 53 2.9. Devices Used in the Hybrid Generation System Modeling ............................................... 54 2.9.1. Solar Cell ........................................................................................................................................................ 54 2.9.2. Wind turbine ................................................................................................................................................ 56 2.9.4. Permanent Magnet Synchronous Machine ..................................................................................... 59 2.9.5. MOSFET .......................................................................................................................................................... 61 2.9.6. Insulated gate bipolar transistor (IGBT) ......................................................................................... 64


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2.9.7. Three-Level Bridge .................................................................................................................................... 68 CHAPTER-3: Load Profile And Preliminary Sizing of the HPS Elements. ................... 71 3.1. Load profiles considered for the Thesis ................................................................................. 71 3.2. Renewable power resources ...................................................................................................... 72 3.2.1. Solar irradiation and wind speed ....................................................................................................... 72 3.2.2. Power outputs of a PV-panel and a wind turbine ........................................................................ 73 3.3. Preliminary Sizing of System Elements .................................................................................. 75 3.3.1. Preliminary Sizing of the Battery bank ............................................................................................ 75 3.3.2. Preliminary sizing of PV-arrays ........................................................................................................... 75 3.3.3. Sizing of the renewable energy sources in hybrid systems ..................................................... 77

CHAPTER 4: Implementation, Modeling And Simulation of the Hybrid Generation System. ...................................................................................................................... 78 4.1. Proposed System Design ............................................................................................................. 78 4.2. PV Array Technical Specification .............................................................................................. 79 4.2.1. PV Panel Model ........................................................................................................................................... 79 I-V and P-V characteristics of Single PV panel ........................................................................................... 80 4.2.2. PV Array Model ........................................................................................................................................... 81 4.2.3. Simulation Result ....................................................................................................................................... 82 4.2.4. Boost converter .......................................................................................................................................... 84 4.2.5. Maximum Power Point Tracking (MPPT) modeling ................................................................... 84 4.2.6. VSC converter .............................................................................................................................................. 86 4.3. Wind Turbine .................................................................................................................................. 87 Technical Specification ........................................................................................................................................ 87 4.3.1. Wind Turbine Model ................................................................................................................................. 88 4.3.2. Simulation Result ....................................................................................................................................... 89 4.4. Three Phase Inverter Using Pulse Width Modulation (PWM) ........................................ 92 4.4.1. Simulation Result ....................................................................................................................................... 93 4.5. Hybrid Generation System Model ............................................................................................. 93 4.5.1. Simulation Result ....................................................................................................................................... 94 5. Conclusion ............................................................................................................................................ 96 6. References ............................................................................................................................................ 97

APPENDIX 1 ................................................................................................................................. 100 APPENDIX 2 ................................................................................................................................. 105


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Introduction The rapidly increasing costs of fossil fuel and power line extensions, combined with the desire to reduce carbon dioxide emissions pushed the development of hybrid power system suited for remote locations. Hybrid power systems are designed for the generation and use of electrical power. They are independent of a large, centralized electricity grid and incorporate more than one type of power source. They may range in size from relatively large island grids to individual household power supplies. Especially for remote places where electricity has not reached yet it is recommended that decentralized generation, which is based on renewable energy technologies, is perhaps the only efficient way to provide electricity. For the same reasons that make difficulty of extending the conventional grids, transporting fuel to the remote areas and replacement parts needed for the conventional sources like diesel generators. The energy for Renewable energy sources (RES) is locally available and a properly chosen RESs can reduce the operating periods of the conventional power sources considerably which will result in reduction of the fuel consumption and minimizing the need of maintenance. This in turn will result in the improvement of sustainable power generation. A system, which depends entirely upon RESs, is possible but not reliable for the uninsured availability of the resources for the RES systems. In order to meet the sustained load demands during the varying natural conditions, different renewable energy sources are integrated for extended usage of alternative energy, However, a lot of requirements have to be considered first. It is important to understand all the factors that influence its behavior, in order to get the best of it. The most important factors are location, time and user needs (power). Location associates information about climate, energy sources availability and environment conditions. This information is very important to decide what kind of renewable generators can be chosen [1]. This thesis focuses on the combination of wind, solar, Genset and energy storing systems for sustainable power generation. The wind turbine output power varies with the wind speed at different conditions. The solar energy also varies with the hourly, daily and seasonal variation of solar irradiation. Thus, a generator-set system with a battery bank (energy storage bank) can be integrated with the wind turbine(s) and PV-system to ensure that the system performs under all conditions. In the proposed system, when the wind speed and solar irradiation is insufficient to meet the demand and whenever there is excess supply from the RESs, the energy storage bank stores energy, which will be used at times when there are insufficient supplies from the RESs. If either the available power from the wind turbine or from the solar panels cannot satisfy the load demand, the Genset system can meet the excess power demand.

Aims A combination of different renewable energy sources, like wind generator and PV-system, with conventional energy source, like a diesel generator, is known as hybrid power system. Hybrid systems can provide a steady community-level electricity service, such as village electrification, offering also the possibility to be upgraded through grid connection in the future. The advantages of using renewable energy sources for generating power in remote islands are obvious such as the cost of transported fuel are often prohibitive fossil fuel and that there is increasing concern on the issues of climate change and global warming.


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The project which is presented in this paper is to investigate for the best design layout (connection topology); to size the system elements appropriately; to design different models and simulate for the different power management strategies which are defined for the system of the HPSs, which consists of renewable energy sources (PV-arrays and wind generator), conventional backup energy source (diesel engine generator) and energy storage bank (battery bank) to sustainably and efficiently satisfy the energy demand of remote site.

Objectives The objectives of the project are: -

To investigate for the best design layout (connection topology), model and simulate the selected topology for different power management strategies of the HPSs, which consists of renewable energy sources (PV-arrays and wind generator), conventional backup energy source (diesel engine generator) and energy storage bank (battery bank) to sustainably and efficiently satisfy the energy demand of remote places, where main grid electricity has not reached yet due to many geographical and economic constraints.

Understanding micro-grid concept and understanding of power integration between various renewable energy sources with the micro grid

Understanding and designing of the various power electronics used in integration between generation source and the micro-grid

Designing an improved prototype, demonstrate the tests and analysing the system output. Carrying a series of experiments through simulation to achieve the best connection topology

and analysing the outcome generated. Understand of the MPPT and its principle algorithm. Achieving the project management skills.

System components and Tools The hybrid generation system, described here includes the following main elements: -

Renewable energy sources: PV-system, Wind generator Energy storage bank: Battery bank Backup energy source: Diesel generator set AC-loads Power electronic devices (Rectifier, inverter, PWM) MATLAB simulation software. Simscape systems Simpower systems Simelectronics systems SMA off- grid simulation software Wind farmer simulation software Computer Internet


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CHAPTER 2: Background Research Remote rural areas, especially in developing countries, are in great need of affordable and reliable electricity to achieve development. Likewise, an overview through the most important literature on rural electrification proves that RESs are one of the most suitable and environmentally friendly solutions to provide electricity within rural areas. Autonomous decentralized (off-grid) rural electrification based on the generation of renewable power on the site through the installation of stand-alone power systems in rural households, and the set-up of electricity distribution mini-grids, fed by RES or mixed ones, have been proven for being capable of delivering high quality and reliable electricity for lighting, communication, water supply and motive power, among others. Using RESs have many advantages over conventional sources. Some of the advantages of using RESs are [3]:

- Gain an immediate access to reliable electricity at any time - Avoid long waits for grid extension and permit the connection it comes - Reduce the dependency from oil price fluctuations - Reduce the transportation costs of fuels - Improve health care and reduction in rural areas - Increase economic productivity and create local employment opportunities - Fight climate change and poverty - Allow for a better use of local natural resources

Off-grid renewable energy technologies satisfy energy demand directly and avoid the need for long distribution infrastructures. Hybrid systems can provide a steady community-level electricity service, such as village electrification, offering also the possibility to be upgraded through grid connection in the future. Hybrid systems with a backup Genset run with minimal fuel consumption because the Genset is brought on line only to assist in periods of high loads or low renewable power availability. This in turn results in a large reduction in fuel consumption as compared to a Genset only powered system. Furthermore, due to their high levels of efficiency, reliability and long-term performance, these systems can also be used as an effective backup solution to the public grid in case of blackouts or weak grids, and for professional energy solutions, such as telecommunication stations or emergency rooms at hospitals [3]. The main disadvantage of wind turbines and PV-systems is that naturally variable wind speed and variable solar irradiation cause voltage and power fluctuation problems at the load side. Using appropriate power converters and control strategies can solve these problems. Another significant point is to store the energy generated by wind turbines and PV-systems for future use when no wind and/or no irradiation is available but the user demand exists. For this, an energy storage bank can be incorporated in such a way that the battery stores energy whenever there is excess supply and discharges (or supplies the load) when there is more demand than supply. Successful results have already been obtained with hybrid systems worldwide. Rural communities without hope to be connected to the public grid (at least not in the medium term), lacking resources to keep up with the fuel prices or with unused diesel infrastructures, have found on hybrid systems the most suitable, environmentally friendly and cost competitive solution for power delivery. To mention some [4]: Reference [4, 5] presents a hybrid configuration comprising of a winddiesel system originally developed by the Hydro-Quebec, aimed at reducing the cost of electricity supply in remote areas. The two main blocks of the system presented are, diesel- driven synchronous generator and the wind


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turbine driven by the asynchronous (induction) generator. The wind turbine block uses a 2-dimensional lookup table to compute the turbine torque output as a function of wind speed and turbine speed. At low wind speeds both the induction generator and the diesel-driven synchronous generator are required to feed the load. When the wind power exceeds the load demand, it is possible to shut down the diesel-generator. In this all-wind mode, the synchronous machine is used as a synchronous condenser and its excitation system controls the grid voltage at its nominal value. A secondary load bank is used to regulate the system frequency by absorbing the wind power exceeding consumer demand. In comparison to the system the system proposed in this report will be more reliable and economical to provide electricity to the remote area of THAR as it includes wind and solar generation sources that will meet must of the load depends it self and in case due to unavailability of the renewable sources due to climate change, diesel generator can be used to meet the demands and also during peak resources availability power can be stored to the storage banks hence use of Genset will be limited making it more economical and environment friendly system and also in the published design the system is using DC coupled HPS which is more complicated and requires more inverters hence make it more costly and less efficient while in system, proposed in this report will be using mixed coupling system to make it more efficient and economical, Below is the block diagram of the published system design(winddiesel system originally developed by the Hydro-Quebec).

Figure 2.1 DC coupled Hybrid generation system [4]

Figure 2.2 Mixed coupled Hybrid generation system [5]

In the literature, most of the models using wind turbines are based on a non-linear relationship between rotor power coefficient and linear tip speed of the rotor blade [6, 7] Muljadi and Butterfield mention the advantages of employing a variable speed wind turbine and present a model of it with pitch control. In his model, during low to medium wind speeds, the generator and the power converter control the wind turbine to maximize the energy capture by maintaining the rotor speed at a


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predetermined optimum value. For high wind speeds the wind turbine is controlled to maintain the aerodynamic power produced by the wind turbine either by pitch control or by generator load control.

The same principle will be adopted in the system proposed in this report for the wind turbine and will also include the Maximum power point tracking (MPPT) for the PV panel to extract the maximum energy falling on the panels, this will be make the system power electronics more complicated but the efficiency of the system can be increased by 30-40% and also in all the new system MPPT technology is used as it significantly increases the efficiency of the system with little increase in the capital cost of the system which in long run make the system economical [6]. A case study reported in [7] describes a cost-effective power supply solution in a remote area in Tunisia. The climate of Tunisia, located in North Africa, well suited to the use of solar energy. There are many small, remote locations in Tunisia that rely on diesel generators for electric power, as grid extension is not economically feasible. The cost of running these generators can be quite expensive when accounting for the transportation costs and efficiency of diesel generators during off-peak periods. As a result, a hybrid solar power with battery storage-based power system was considered for continuous power supply in this area. It has been shown that the power generation cost of hybrid a solar and battery storage system is $240.65 per MWh, whereas a diesel generator based power supply system costs about $289.1 per MWh. In favorable wind conditions, wind turbine-based hybrid power systems can offer a cheaper solution compared with solar energy based hybrid renewable power supply .

2.1. System Location Analysis It is important to understand the importance of the location where system has to be deployed as location associates information about climate, energy sources availability and environment condition which play a major role in the production and that is the reason I choose THAR location which Is located on the coast Pakistan as the location has abundant amount of solar and wind energy, also the energy produced by the conventional sources raises the greenhouse gas emissions, which may be the key source of global warming. Pakistans total GHG emissions in 2012 amounted to 409 million tones (mt) of Carbon dioxide (CO2) equivalent, comprising about 54% CO2, 36% Methane, 9% Nitrous Oxide and 1% other gases. The biggest contributor is the energy sector with 50% share, followed by the agriculture sector (39% share), industrial processes (6% share) and other activities (5% share). Pakistan has also signed the previous Kyoto protocol on reduction of CO2 emission to the atmosphere. For this reason, the Pakistan government is very much concerned about environmental issue and the government wants the overall improvement of the CO2 emission [2] and also THAR is located on the coastal region of Pakistan and coast of Pakistan is about 1,120 kilometers long and It is very expensive to connect small villages to the national electric grid because of the huge infrastructure costs involved and, also Pakistan is facing electricity shortfall since last decade, to overcome this issue a step towards renewable energy is need to be taken. According to the experts, WAPDA (Water and Power Development Authority, Pakistan) at the moment does not have enough money and electricity to supply power to the coastal region. The only way to supply the electricity to the coastal areas of Pakistan is through the use of the renewable resources because abundant natural resources are available nearly all year, below the graphs showing wind and Sun availability throughout the year in the THAR area [2] [4].


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Meteorological conditions of THAR location. (a) Solar irradiation on horizontal plane, (b) wind speed and (c) Ambient temperature.

2.2. Components of the Hybrid generation System The hybrid power system, described here, basically includes the following main elements: -

Renewable energy sources: PV-system, Wind generator Energy storage bank: Battery bank Permanent magnet synchronous generator (PMSG) Power electronics

The following sections give the basic descriptions for the main components used in the proposed system.

2.2.1. Solar Photovoltaic In the generation of energy from solar irradiation, the PV-arrays trap the photons of solar light and convert the light energy into electrical energy. The energy obtained from the PV-systems can be utilized in different applications. DC power is the direct output of PV-arrays and this DC form power can be directly used with DC appliances. For AC appliances, this DC power has to be changed into AC form using power electronic inverters. The building block of the PV array is the solar cell, which is basically a P-N semiconductor junction that directly converts solar radiation into DC current using the photovoltaic effect. PV cells are grouped together in larger units known as PV modules or arrays, which are combined in series and


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parallel to provide the desired output voltage and current. The well-known equivalent circuit of solar cells arranged in NP-parallel and NS-series is shown in Fig. 2.3. It is composed of a light-generated current source, a diode representing the nonlinear impedance of the pen junction, and series and parallel intrinsic resistances. The mathematical model that predicts the power production of the PV generator becomes an algebraically simply model, being the current-voltage relationship defined in Eq. (1) [6,7]

Where: IA: PV array output current VA: PV array output voltage IPh: Solar cell photocurrent

Fig. 2.3 Equivalent circuit of solar cell

Irs: Solar cell reverse saturation current (aka dark current). q: Electron charge, 1.60217733ee19 Cb. A: P-N junction ideality factor, between 1 and 5. k: Boltzmanns constant, 1.380658ee23 J/K. Tc: Solar cell absolute operating temperature, K. Rs: Cell intrinsic series resistance. Rp: Cell intrinsic shunt or parallel resistance. The photocurrent Iph for any operating conditions of the PV array is assumed to be related to the photocurrent at standard test conditions (STC) as follows:

fAMa : Absolute air mass function describing solar spectral influence on the photocurrent IPh. fIA: Incidence angle function describing influence on the photocurrent IPh. ISC: Cell short-circuit current at STC. aIsc: Cell temperature coefficient of the short-circuit current, A/module/diff. temp. (K). TR: Solar cell absolute reference temperature at STC, K. S: Total solar radiation absorbed at the plane-of-array, W/m2. SR: Total solar reference radiation at STC, 1000 W/m2.


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2.2.2. PV Array System configurations In the stand-alone hybrid power systems, PV-array system is used in such a way that loads can be supplied any time in an efficient way. The intensity of solar irradiation varies with time and season. So, the most advisable and efficient way of harvesting the solar energy is by connecting energy storage devices (batteries) in such a way that loads can be supplied any time and the battery can store whenever there is excess supply from the PV-array system. Then, the stored energy in the battery can be used during times where there is less supply or there more demand. Commonly, most appliances and loads are designed to operate on AC. However, the PV-array system only gives a DC power. Thus, an inverter (DC/AC converter) is a must to be used so that AC loads can be supplied from the PV-system.

Fig. 2.4: PV-system supplying AC loads connected at the main DC-bus

Figure 2-4 shows PV-array system, which is a part of the whole system, with battery bank charging or supplying the AC load. When planning for a long-term operation of a stand-alone PV-system, the battery bank is playing important role and it is necessary to avoid the overcharge and deep discharge of the battery to enhance its usable life. Thus, a controller is an important element of the energy storage in the PV-array system. Over and above, it is important to note that a bi-directional converter of DC/DC type is used here allowing the battery to charge at times when there is more generation from the PV-arrays and to discharge (or supply to the load) during times of more demand on load or less supply.

2.2.3. PV Design and Array sizing approaches When designing the PV-array system, many factors have to be considered seriously. These include the technical specification, sizing of individual system components, safety considerations as well as system economics. As it can be referred from Figure 2-4 above, the system comprises basic components: the PV-arrays, inverters and battery. While dealing with the efficiency of the system, the efficiency of the basic components together with the wiring efficiency must be taken into an account. The gross daily energy demand of a load can be given [11] as:

2.3

Where End and Egd are the net and gross energy demands per day, is efficiency with subscripts i and w representing the inverter and wiring respectively. As it is explained in [9], inverters, batteries and the wiring of a well-designed PV-system have typical efficiencies of 85%, 85% and 98% respectively. Once the gross energy demand is calculated, the system voltage can be fixed. As it is suggested in [9],


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system voltage is chosen to be 24V if daily energy demand is greater than 1KWh and 12V if it is less than 1KWh. The system sizing depends on the estimation of the peak-load demands of loads. The total estimated power demand can be found [9, 11]:

2.4

Where Pest = total estimated peak-load power Pli = Peak power of single load, for loads I = 1, 2, 3n. The DCC12 of the PV-arrays can then be determined at this point as [9]:

2.5

2.2.4. PV-array Implementation The most important factors, which have to be considered with the implementation of the PV modules, are [9, 11]:

Daily energy demand Solar insolation at the geographical location specified Methods of mounting the modules

When talking about mounting methods of PV-modules, either fixed mounting or mounting on tracker techniques can be used with the latter yielding about 20% more output than the former option [9]. However, it requires more skilled maintenance. Taking the less familiarity of rural community toward this into consideration, fixed mounting techniques have been advisably used in the rural electrification with HPSs.

Fig. 2.5 series and parallel connection of the PV-modules.

The number of series connected PV-modules can be calculated as follows [12]:

2.5 Where, Ns = the number of series connected PV-module (rounded up to a whole number) Vsys = nominal system voltage Vmod = nominal module voltage.


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The number of parallel panels can be determined as [9, 12]:

2.6 Where, Np = number of parallel connected modules (rounded up to a whole number). LDA = Average daily load (A). A:L = average photovoltaic Ah available-to-average daily load Ah ratio. SL = system loss. Imp = module current at maximum power (A). DSH = the daily sun hours. Isc, PV module = Short circuit current per chosen PV module The average daily PV-to-load Ah ratio can be determined from [12]

2.7 Where DRT is the daily running time of the system and typical A:L values are 1.1-1.2 for non-critical loads and 1.3-1.4 or higher for critical loads, and the month with lowest A:L ratio has to be taken when designing a PV-array system[12]. The total number of PV-modules forming PV-array is, therefore [13]:

2.8

2.2.5. PV Array specification PV cells produces around 0.5V and it is the smallest unit of the solar PV system. Cells are further connected in series or/and parallel combination to form a PV array. The power-voltage and current-voltage characteristics of 330 SunPower modules (SPR-305) PV model are obtained according to the value of the variables ph, o, sh, and . The value of the variables are collected from manufacture datasheet and are shown in the appendices, they usually provide values for PV and PV at open circuit, short circuit, and maximum power point and finally the number of the PV cells. The current-voltage and power-voltage characteristics of a solar PV module operating at a standard temperature of 25 C and different solar irradiance are shown in Figures 2.6 and 2.7. According to solar irradiation or load current, the maximum output power of the PV module varies. Therefore, a proper control system is needed to use the PV model more efficiently as an electric power source by building a MPPT. There are many different MPPT methods discussed in [12,15], among them perturbation and observation method (P&O) is most widely used because it is much simpler and needs fewer measured variables. The 100-kW PV array of the detailed model uses 330 SunPower modules (SPR-305). The array consists of 66 strings of 5 series-connected modules connected in parallel (66*5*305.2 W= 100.7 kW).


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Manufacturer specifications for one module are: - Number of series-connected cells: 96. - Open-circuit voltage: Voc= 64.2 V. - Short-circuit current: Isc = 5.96 A. - Voltage and current at maximum power: Vmp =54.7 V, Imp= 5.58 A. The characteristics of the SunPower-SPR305 array are shown below.

Fig. 2.6 I-V characteristics of the PV- Array

Fig. 2.7 P-V characteristics of the PV- Array

Dot on blue curve indicate module manufacturer specifications (Voc, Isc, Vmp, Imp) under standard test conditions (25 degrees Celsius, 1000 W/m2).

2.3. Wind Turbine Wind turbines are used to generate electricity from kinetic power of the wind i.e. the generation of wind energy primarily depends on the wind speed [22]. To enhance the energy capture, the turbine is mounted on a tall tower. Wind is an intermittent resource; however, wind turbines are not unreliable technology. Modern wind turbines capture more than 90% of available energy from wind, compared with fuel efficiency between 30- 40% for a conventional coal fired station, which typically loses a significant portion of energy through heat loss and pollution. Some advantages of using wind turbines to generate power are - Environmentally friendly. - Produce no pollution. - No traditional fuel required.


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- Requires relatively little maintenance. - Longer life-time (up to 30 years). Disadvantages - Interference with radio/TV signals if located inappropriately. - The wind doesnt blow all the time at required speed. - High initial cost. The main components of a wind turbine are the rotor of the turbine, which is the prime mover, and the induction generator. The rotor is connected to the generator via a gearbox, which matches and enhances the rotational speed. As the wind does not blow all the time nor does the sun shine all the time, solar and wind power alone can be poor power sources for a long time energy supply. Hybridizing solar and wind power sources together with storage batteries to cover the periods of time without sun or wind provides a realistic form of power generation.

2.3.1. Wind Energy Conversion System The proposed system consists of a variable speed wind turbine, an interior type permanent magnet synchronous generator and a PWM controlled rectifier. The variable speed wind turbine captures aerodynamic power from the wind. The interior type synchronous generator converts this wind power to useable electrical power. The PWM controlled rectifier is used to capture optimum aerodynamic power by controlling the generators rotor speed. The structure of the wind energy conversion system is shown below: -

2.3.2. Speed and power relation Air of mass m moving with speed v has a kinetic energy given by [11]:

2.10 The power P in moving air is the flow rate of kinetic energy per second. Thus the mechanical power in the moving air can be given by [11, 22]:

2.11

Where, P = mechanical power in moving air, W = Air density, Kg/m3, Arot = area swept by the rotor blades exposed to the wind, m2 V = wind speed, m/s.


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The swept area, A depends on the dimensions of the rotor. For a horizontal axis turbine of rotor diameter d, the swept area can be given by [11]:

2.12

For a vertical axis turbine of maximum rotor width wr and rotor height hr, the swept area can be approximated by [11]:

2.13

The air density depends on pressure and temperature. It can be expressed as [11]:

2.14 Where P is the air pressure, T is the absolute temperature and R represents for the gas constant. The gas density is about 1.225 kg/m3 at sea level, 1-atm and 289K temperatures; but it has less value at high elevation [11, 22]. The volumetric flow rate is (A.v) and the mass flow rate of the air in Kg/s (.A.v) resulting in the above expression for the mechanical power in the moving air. The potential of wind power at a given site is expressed in terms of wind power per square meter area of the swept rotor blades, referred to as specific power and is given as:

2.15 This power that is in the upstream wind [11] varies with air density linearly and cube of the wind speed. The extracted power by the blades is, therefore, less than the above valued as some power is left in the downstream air that continues to move at reduced speed. The actual mechanical power extracted by the rotor blades is [11]:

2.16 Where v and vo are the upstream and downstream wind velocities at the entrance and exit of rotor blades respectively and the mass flow rate of air, mfr is given as a function of the average speed.

2.17 With some algebraic manipulation, the power extracted by the rotor blades can be expressed as a fraction of upstream wind power as follows [11, 22, 23].

2.18


20

Where Cp is the fraction of upstream wind power captured by the rotor blades, also called as power coefficient of the rotor or the rotor efficiency, expressed as:

2.19 Cp depends on vo/v ratio and is a single (theoretical) maximum value function whose value varies between (0.59 0.2) depending on the type of turbines i.e. Cp value of less than 0.5 for high speed, 2-pole turbine and Cp value between 0.2 and 0.4 for low speed with more blades are commonly used [11, 23].

Fig.2.8: Rotor efficiency as a function of vo/v ratio[11]

The total (actual) amount of wind energy that passes area Arot by wind speed V is [22, 23]:

2.20 Where Cp = coefficient of performance (0.59 0.35 depending on the turbine types). g= efficiency of generator. gb = gearbox/bearings efficiency. A general characteristic curve that describes the wind turbine output power variation with steady wind speed is shown in Figure 2.9 below [24].

Fig. 2-9: Typical wind turbine power output with steady wind speed characteristics [24]


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From the above characteristic curve, there are three important points at which much attention is paid for the speeds and the corresponding turbine output powers for every wind turbine. These are the cut-in speed, rated output speed and cut-out speed. The important terms characterizing the turbine power-speed (Figure 2-9) characteristics are described below. Cut-in speed A very low wind normally exerts insufficient torque on the wind turbine blades to make them rotate. When the speed increases, the wind turbine starts rotating and generating electrical power. The speed at which the turbine first starts to rotate is called the cut-in speed and is typically between 3 and 4 m/s [24]. Rated output wind speed When the wind speed increases beyond the cut-in speed, the wind turbine generates electrical power and this increases rapidly as it is cubic function to speed and this can be seen in Figure 2-9. However, depending on the type of turbines, the wind turbine reaches a point where its capability of generating the maximum output power limit is attained at some speed. This limit to the generator output is called the rated power output and the wind speed at which it is reached is called the rated output wind speed. At higher wind speeds, the design of the turbine is arranged to limit the power to this maximum level and there is no further rise in the output power. How this is done varies from design to design but typically with large turbines, it is done by adjusting the blade angles so as to keep the power at the constant level. Cut-out speed As the speed increases above the rate output wind speed, the forces on the turbine structure continue to rise and, at some point, there is a risk of damage to the rotor. As a result, a braking system is employed to bring the rotor to a standstill. This is called the cut-out speed and is usually around 25 m/s [24]. In general, if wind speed is between the rated speed and the furling (cut-out) speed of the wind turbine, the power output will be equal to the rated power of the turbine and if the wind speed is either less than the cut-in speed or greater than the furling speed of the wind turbine, then the output power will be zero [24]. 2.3.3. Variable Speed Wind Turbine Model A variable-speed wind turbine has three main regions of operation as shown in Fig. 2.10. In region 1, the wind speed is below the cut in speed (vo), which is not enough to start a turbine. Region 2 is an operational region of wind turbine where the wind speed remains between the cut in speed (vo) and cut out (vi) region. In region 3, the turbine must limit the captured wind power as the wind speed is above the cut out speed (vi), so as to ensure safe electrical and mechanical operating limits.

Fig. 2.11 steady-state relationship between extracted aerodynamic power and wind speed


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Fig. 2.11 demonstrates the steady-state relationship between extracted aerodynamic power and wind speed. The dotted line represents the power in the unimpeded wind passing through the rotor swept area, while the solid curve represents the power extracted by a typical variable speed wind turbine. Classic control techniques such as proportional, integral and derivative (PID) control of blade pitch are typically used to limit power and speed on both the low and high-speed shafts for turbines operating in region 3, while generator torque control is usually used in region 2. For a variable speed wind turbine operating in region 2, the control objective is to ensure maximum energy capture by operating the wind turbine at the peak of the Cp TSR as shown in Fig. 2.11. The power coefficient Cp (, ) is a function of the tip speed ration (TSR) and the blade pitch . The TSR is defined as [24,25]:

2.21 From (2.16), the rotor aerodynamic power P increases with Cp. As a result, the wind turbine should be operated at the maximum power coefficient Cpmax. The relationship between TSR and blade pitch can be expressed as follows:

2.22 The steady-state power curve of the wind turbine for different wind speeds is given in Fig. 2.12.

The steady-state power curve of the wind turbine for different wind speeds is given in Fig. 2.13.


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2.4. Permanent Magnet Synchronous Generator (PMSG) Model PMSGs are synchronous ac machines. The PMSG consist of 3-phase stator winding similar to the SCIG, while the rotor winding is replaced by the permanent magnets. The advantages of eliminating the rotor field winding are reduced copper losses, higher power density, lower rotor inertia and more robust rotor construction. The demerits are loss of flexibility in field flux control, possible demagnetization/saturation of magnetic material and parameter variation over time. Depending on the magnet placement on the rotor, PMSGs are divided into two categories:

Surface permanent magnet machines (SPM) Interior permanent magnet machines (IPM)

In SPM synchronous machines, the permanent magnets are mounted on the rotor surface as shown in Fig. 2.14(a). The rotor has an iron core that may be solid or made of punched laminations with skewed poles to minimize cogging torque, and the simple design makes it easy to build. This configuration is used for low speed operation, since the magnet may fly during high-speed operation. The permeability of magnetic material approximates air, producing an effectively large air gap. Moreover, the smooth rotor surface design minimizes saliency in the rotor, contributing to a low armature reaction effect due to low magnetization inductance. In IPM synchronous machines, magnets are installed inside the rotor as shown in Fig. 2.14(b). The IPM rotor is difficult to fabricate, although the robust design makes it more suitable for high speed applications. The unequal effective air gap distribution renders it a salient pole machine, where the direct axis inductance is less than quadrature axis inductance (Ld


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Fig. 2.14. Cross sectional view of rotor design of a) SPMSG and b) IPMSG.

2.4.1. Operating Principle of PMSG In permanent magnet synchronous machines, magnets are placed on the rotor as alternate N and S poles. These magnets cause the development of magnetic flux in the air gap. When the stator windings are excited, they develop their own magnetic flux, and the close interaction between rotor and stator magnetic fields produces electromagnetic torque in the rotor. Fig. 2.15 shows a simplified cross-section view of 3-phase, 2-pole PMSG with symmetrical stator windings, displaced from each other at a 120 electrical angle. The relative motion between rotor and stator induces sinusoidal MMF waves on the magnetic axes of the respective phases. The phase difference between rotor magnetic flux and the magnetic axis of stator phase-a winding is known as rotor position angle (r). The rate of change of rotor position angle further calculates the angular rotor speed (r )[27, 28].

Fig. 2.15. Cross-section view of 3-phase, 2-pole PMSG.

2.4.2. PMSG Controller Modeling The primary objective of the PMSG control is to extract optimum power from varying wind and ensures efficient operation of the PMSG

2.4.2.1. Optimum power control The optimum power extraction concept can be defined as the extraction of required power from a wind turbine under varying wind conditions [26]-[29]. In a variable speed wind turbine, the relationship between rotor speed and the output power for a given wind speed is shown in Fig. 2.13.


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The detailed relationship between the rotor speed and the output power for a given wind speed is discussed in the variable speed wind turbine model section. From (2.13) and (2.14), the applied torque or the extracted power from the wind can be controlled by regulating the rotor speed. By rearranging, the relationship between the applied torque and the rotor speed can be defined as follows:

2.23 where Kopt is given by

2.24

The optimum power can be as follows: -

2.25

The rotor speed at optimum power point can be expressed as follows:

2.26 Optimum power can be extracted by controlling the rotor speed. Fig. 2.16 demonstrates the power generated by a turbine as a function of the rotor speed for different wind speeds. As an example, for a particular wind speed (v6), the optimum power (PWopt) can be generated by keeping the rotor speed either equal to 1 or 3. However, as 3 is higher than the base rotor speed, the control system must choose the rotor speed 1. If the wind speed drops to v5 from v6, the control system sets the rotor speed to 2 to extract the required power.

Fig. 2.16. Power generation of wind turbine in different rotor and wind speeds.

2.4.2.2. Efficient operation of PMSG Optimum power extraction algorithms can be implemented in wind energy conversion stages in different ways. An unregulated two-level rectifier with a boost or a buck-boost converter is used to


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regulate the dc-link voltage or rotor speed. This arrangement causes high harmonic distortion, which reduces generator efficiency [30]. A regulated two-level rectifier can improve these distortions [29, 30]. The primary objective of the controller is to regulate d- and q- axis components of the stator current. The reference optimum value of d- and q- axis current determines the operational loss of the IPMSG. The losses of a PMSG can be divided into four components: stator copper loss, core loss, and mechanical loss and stray-load loss. Only the stator copper and core losses are explicitly dependent on the fundamental components of the stator currents. Therefore, optimum reference values of d- and q- axis components of stator current have to be calculated to reduce the operation loss. An algorithm is developed to obtain the optimum reference value of d- and q- axis current to ensure minimum operational loss of IPMSG as discussed in the following. The q-axis stator current component (iq) for constant torque can be expressed as a function of the d-axis stator current component (id):

2.27 The maximum efficiency of the IPM synchronous generator operation can be achieved by minimizing copper and core losses. The copper (PCu) and core (PCore) loss for the IPM synchronous generator can be determined as follows [31].

2.28 Where Rc is the core loss component. The output power from the generator can be given as:

2.29

The optimum value of id can be determined from the output power (Pout) vs d-axis stator current (id) curve based on (2.28) as shown in Fig. 2.9. From Fig. 2.17, optimum value of the d- axis current component is chosen where the output power from IPMSG is maximum. The controller is shown in Fig. 2.18.


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Fig. 2.17. The d-axis current vs output electric power.

Fig. 2.18. Machine side controller.

2.5. Energy Storage System There is currently considerable interest in electrical energy storage technologies, for a variety of reasons. These include changes in the worldwide utility regulatory environment, an ever-increasing reliance on electricity in industry, commerce and the home, the growth of renewable energy sources to meet the growing demand for electricity, and all combined with ever more stringent environmental requirements. The need of energy storage is to transfer the excess power during weak loads or excess supply from RESs to the peak periods [14, 15]. The energy from the RESs has to be transformed into a storable energy form first and then transformed when storing is needed. This can be shown by the following block diagram.

Figure 2-19: Energy storage with distributed energy generation system [14]

The energy storage techniques with hybrid power systems applied to the electrification of remote rural areas will be discussed in this section. The HPSs in this thesis consists of PV-array system, wind turbines and diesel Genset. The solar irradiation varies with time and so does the wind speed, throughout the day. Thus, in a HPS, both the RESs and the load are fluctuating throughout the day.


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These fluctuations may result in imbalances in power distribution (energy sources are not equal to energy sinks). As a result, the voltage and frequency in the power system will be affected. The addition of energy storage will assist balancing the distribution of power in the power network. The energy storage behaves like a large buffer to accommodate the unequal instantaneous energy in the power system. The energy storage element can act as a load or a generator depending on the supply to demand ratio. Ideally, at any instant of time, there should be a zero net exchange between the energy sources and the energy sinks (both real and reactive power) [14-16]. If this balance is not achieved, the voltage and frequency of the system changes to maintain equilibrium. There are many energy storage techniques. To mention some [6, 14]: - Battery Energy Storage System (BESS) - Pumped Hydroelectric Storage System (PHSS) - Superconducting Magnetic Energy Storage System (SMESS) - Ultra-capacitors - Flywheel Energy Storage Systems (FESS), etc. The different energy storage techniques have different applications in power systems. Some of them are [14]: i. Rapid reserve, ii. Area control and frequency responsive reserve, iii. Commodity storage iv. Transmission system stability, v. Transmission voltage regulation, vi. Transmission and Distribution facility deferral, vii. Renewable energy management, viii. Customer energy management, ix. Power quality and reliability. Each technology has its own particular strengths and operational characteristics. In this thesis, the battery bank as energy storage device is only considered.

2.5.1. Rechargeable battery Battery bank is an electrochemical device that uses electrochemical reactions to store electricity in the form of potential chemical energy. The energy storing batteries used with HPSs are rechargeable in a sense that they can charge when there is enough supply from the RESs and discharge when there is larger load demand than there is supply. They are sometimes also called as secondary batteries. The charging time for the battery bank [17] is:

2.30


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Rechargeable batteries have standard electric potential, which is the potential difference between the cathode and anode [6, 17]. A simplified equivalent circuit of a battery at its steady state is given in [11] and is shown below (Figure 2-20) working as a voltage source with an internal resistance.

Fig. 2.20 Equivalent circuit of a battery

Assuming that the open circuit voltage and resistance when the battery was fully charged are E0 and R0, then the open circuit voltage and internal resistance at any time when battery is discharging can be given as [11]:

2.31 The open circuit voltage decreases and the internal resistance increases with the Ah discharge, Qd. The constants K1 and K2 are found from curve-fitting test data [11]. The internal resistance is a function of SOC, battery capacity and operating temperature. With higher battery capacity, the electrodes will be larger and the internal resistance will be lower [11]. Figure 2-21 shows simple circuit model of the simplified electric circuit with the conducting resistance, Rohm included.

Fig. 2.21 General electric circuit model for a rechargeable battery [11]

The simple electric circuit model shown above includes an internal resistance, Ri, conductor resistance, Rohm and a standard cell potential E0. Applying the maximum power transfer theory, the peak power, delivered from the battery in the above model is:

2.32 Since Ei and Ri depend on SOC, the maximum power that can be delivered to the load by the battery also depends on SOC. The efficiency at any SOC can also be derived into the following form where it depends on SOC [11].

2.33


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The energy storage is dependent on the amount of active material in the battery cell [12]. Thus, for an active material of mass, m (in gram) and a cell volume, V (in liters), the specific power (in W/g) and peak power density (in W/l) can be determined from the peak power.

2.34 Best batteries have high standard cell potential, which results in high peak power capacity and high theoretical charge capacity [6, 17, 18]. 2.5.2. Battery characteristics Battery capacity: This is a measure of how much energy the battery can store. The amount of energy that can be extracted from a fully charged battery basically depends on temperature, rate of discharge, battery age and battery type. The three main ratings to specify the capacity of a battery are [17, 18]: - Ampere-hour (Ah): the current at which a battery can discharge at a constant rate over a fixed interval of time. - Reserve capacity: the length of time (in minutes) that a battery can produce a specified level of discharge. - KWh capacity: a measure of energy required to fully charge a depleted battery. A depleted battery is not usually a fully discharged battery. Battery Voltage The battery voltage is that of a fully charged battery. It depends up on the number of cells and voltage per cell. The battery voltage decreases when the battery starts discharging. Cycle depth Fully discharging batteries can facilitate the damage or totally destroy the battery life. Deep-cycle batteries can discharge up to 15%-20% of their capacity [17]. This gives a depth of discharge of 85% - 80%. Energy density/Specific energy: is a measure of how much energy can be extracted from a battery per unit of battery weight or volume. By default, deep-cycle batteries provide the potential for higher energy densities than non-deep-cycle varieties since more of the energy in the battery can be extracted (e.g. larger acceptable DOD). Power density/Specific power: is a measure of how much power can be extracted from a battery per unit of battery weight or volume. Autonomy: This variable is defined by the ratio of restorable energy capacity to maximum power discharge [17]. It refers to the maximum amount of time the system can continuously release energy. The autonomy of a system depends on the type of storage and the type of application.

2.35 Durability (cycling capacity): Energy storage system is designed to release the energy stored after each recharge in a fixed time. The number of times the energy storage can release the energy level it was designed for after each recharge is referred to as durability or cycling capacity. It is expressed in


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number of cycles, N-cycles. The cycling capacity mainly depends on the depth of discharge. Figure 2-22 below shows the number of cycles versus the DOD of VRLA14 batteries.

Figure 2.22: Cycling capacity Vs depth of discharge for lead-acid battery [19, 20]

The higher the DOD, the lower will be the cycles and the lifetime of the batteries (can be seen from Figure 2-22). The battery cycles depend on the DOD and EOL [6].

2.36 The smaller the DOD, the more cycles can be provided by the battery but not in a linear relationship because of the EOL criterion. The EOL, in general, is a decrease of 80% of its capacity [6, 17]. The normal capacity needed can then be given [6] as:

2.37 Self-discharge: This refers to the portion of energy which was initially stored and dissipated in a given non-use period of time. There are different types of batteries, but the most commonly used rechargeable batteries are [6, 11, 17]: - Lead acid battery - Nickel cadmium (NiCad) battery - Nickel metal hydride (NiMH) battery - Lithium ion battery - Lithium-polymer battery - Zinc-air battery The main properties with their main application, the advantages and disadvantages of each these batteries are explained in [6, 11].


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2.5.3. Battery sizing Batteries are the main components of the PV-wind system used to store energy at times when there is surplus of supply, which can be used to supply the load at times of more demand from the load. Many factors affect the sizing of the battery system and the most important are [11, 12]: - Daily energy demand - Number of days of storage - Maximum depth of discharge The battery voltage is that of a fully charged battery. It depends up on the number of cells and voltage per cell [11].

2.38 Ncells = the number of cells in series V/cell = voltage per cell The battery capacity is the total Ampere-hour required to support the load requirement and is given by [11, 12]:

2.39

Where, BC = Battery capacity (Ah). Ebat= energy required from the battery per discharge. disch =efficiency of discharge path (including inverters, wires, diodes, etc). Ncell = Number of series cells in one battery. Vdisch = average cell voltage during discharge. DOD= maximum allowed Depth of Discharge for required life cycle. Npar = number of parallel batteries. Batteries are generally not allowed to be discharged fully. The minimum value to which a battery can be discharged before it is recharged is called maximum depth of discharge, DOD. Deep discharge batteries can discharge up to 30- 40% of their state of charge, SOC [11]. The total energy required from the battery to supply the required loads is the sum of energies consumed by each load, which is the product of the duty ratio and peak energy consumption. It is given as follows [11, 12]:

2.40

Ebat = Wh energy of battery during discharge when all loads (i=1, 2n) are connected Ebi = Wh energy of battery during discharge when load-i is connected Dbi= Duty ratio for load-i during battery discharge Pbi = Peak power consumption by load-i during battery discharge in watts Tri = repetition period for load-i in hrs.


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The parallel strings, Nbat are used to meet design requirements like [11, 17]: - To increase the existing battery capacity - To provide redundancy - To provide battery reserve in case of string disconnection for maintenance To ensure an optimal and safe cell recharge voltage, calculation of the number of series cells is important. The number of series cells can be determined from the battery window voltage limits and voltage per each cell, as given [20] below.

2.41 Where, Ncell = number of series cells. Vbat, max/min = maximum or minimum nominal battery voltage. Vcell,max/min = maximum or minimum cell voltage at Vbat,max or Vbat, min respectivel.y The lowest maximum voltage, Vmax and the highest minimum voltage, Vmin define the voltage window within which all loads in the system operate properly. In general, it is economical and recommended [20] to use fewer cells of higher capacity than more cells of lower capacity while sizing the battery system. The total number of batteries required in series will be [11]:

2.42 And the total batteries required in the system are:

2.43

2.6. Power Electronics A generalized block diagram representation of power electronics interface associated with DE systems is shown in Figure. The power electronics interface accepts power from the distributed energy source and converts it to power at the required voltage and frequency. For the storage systems, bidirectional flow of power between the storages and the utility is required. Figure shown below illustrates a design approach to organize the interface into modules, each of which can be designed to accommodate a range of DE systems and/or storages.


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Figure 2.23 General block diagram of typical DE power electronic systems

The design of the input converter module depends on the specific energy source or storage application. The DE systems that generate AC output, often with variable frequencies, such as wind, micro-turbine, IC engine, or flywheel storage needs an AC-DC converter. For DC output systems like PV, fuel cells, or batteries, a DC-DC converter is typically needed to change the DC voltage level. The DC-AC inverter module is the most generic of the modules and converts a DC source to grid-compatible AC power. The output interface module filters the AC output from the inverter and the monitoring and control module operates the interface, containing protection for the DE and utility point-of-common-coupling (PCC). The monitoring and control module also contains protective functions for the DE system and the local electric power system that permit paralleling and disconnection from the electric power system. Monitoring functions typically include real-power, reactive power, and voltage monitoring at the point of the DE connection with the utility at the PCC. These functions are necessary because, in order to synchronize the DE system, its output must have the same voltage magnitude, frequency, phase rotation, and phase angle as the utility. Synchronization is the act of checking that these four variables are within an acceptable range before paralleling two energy sources. In general, the term power electronics refers to the device switches (e.g., IGBT and SCR), and the various modules that they comprise. In power applications, these devices are most often used to convert electrical energy from one form to a more usable form. Benefits of power electronic devices include increased efficiency, lower cost, and reduced packaging size. A rectifier is a power electronics topology that converts AC to DC. Rectifier circuits are generally used to generate a controlled DC voltage from either an uncontrolled AC source (i.e., micro-turbine, wind turbine) or a controlled AC source (i.e., utility supply) (Kroposki et al. 2006). When converting from a utility supply, a rectifier application is usually for linking DC systems or providing DC voltage for specific load applications such as battery regulators and variable frequency drive (VFD) inputs. Some DE systems like photovoltaic and fuel cells produce DC power. In order to make this power useful for other applications, it must be converted to AC; therefore, inverters are used to convert DC to AC. Inverter circuits generate a regulated AC supply from a DC input. They are commonly found in systems providing standalone AC power, utility-connected DE systems, and on the motor side of a VFD. There are a number of applications for DC-to-DC systems. These systems are used to convert the DC voltage magnitude from one level to another with or without galvanic isolation. They take an uncontrolled, unregulated input DC voltage and condition it for the specific load application. An


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example for such topology can be found in PV applications, where the dedicated DC-DC units are often designed to extract the maximum power output of the PV array. AC-to-AC converters can be used convert the AC source voltage magnitude and frequency to a fixed amplitude and frequency, making it compatible with the utility grid. The AC-to-AC converters are not typically used in modern DE applications due to some inherent disadvantages. A summary of the different power converters that are used for DE applications are given in Table 1 (Shepherd et al. 2004; DeBlasio et al. 2006).

Fig. 2.24 Power electronics systems for different power conversions

2.6.1. PV Array Power Electronics and control Topology The power electronics topologies for the PV systems can be categorized on the basis of the number of power processing stages, the location of power decoupling capacitors, utilization transformers, and the types of grid interfaces (Carrasco et al. 2006). The basic classifications based on the number of inverter output phases are

Single phase Single- multiple phase Three phase

In the report I am going emphasis only on the three-phase interface as the system required three phase supply to the micro-grid.

2.6.1.1. Three Phase For larger systems over 10kW, three-phase inverters are most often used. All the configurations for single-phase utility connection can be used for the three phase. But, using a line-frequency transformer or a high-frequency transformer can do the isolation from the grid either. In the later case, an additional converter is required for converting DC from PV into high-frequency AC. Figure 8 shows a typical topology for a three-phase PV inverter utilizing a line-frequency three-phase transformer. The DC output of the PV array is connected across a filter capacitor. The output of the capacitor connects to the input of a voltage-source three-phase inverter. The output of each phase of the converters is connected to an inductor and capacitor to limit the high-frequency harmonics injected into the AC system. A synthesized AC output voltage is produced by appropriately controlling the switches. A three-phase transformer is then used to connect the power to the utility.


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Fig. 2.25 Three-phase PV topology with line-frequency transformer

Modern inverters tend to use a high-frequency transformer for galvanic isolation. This technology results in entirely new designs, such as the printed circuit board (PCB) integrated magnetic components (Kjaer et al. 2005). One such design is shown in Figure 9, where the transformers are embedded in high-frequency DC-DC converters. Such a topology is also very useful for multi-string configurations, where each of the strings can be connected to a common DC bus and then converted to grid compatible AC by using a single DC-AC inverter.

Fig. 2.26. Multi-string PV topology with high-frequency transformer-based isolation

The most generalized form of power electronics topology for the PV application is the DC-DC converter with embedded high-frequency transformer, along with the DC-AC inverter as shown in Figure 9. In general, the DC-DC converter controller does the MPPT and voltage boost. The power flow control to the utility and the sinusoidal unity power factor current-injection to the utility are produced by the DC-AC inverter controller. A simplified block diagram of the PV system with the power electronics and control is given in Figure 10.


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Fig. 2.27 Generalized power electronics and control of a PV system

The power electronics circuits shown in Figure 10 consist of a DC-DC converter and a three-phase inverter. The DC-DC converter is based on current-source full-bridge inverter with an embedded high-frequency transformer and rectifier. The current-source input stage is beneficial since it reduces the requirement for the filter capacitor in parallel with the PV strings. Furthermore, the diodes included in the rectifiers are current-commutated, involving low-reverse recovery of the diodes and low voltage stress (Kjaer et al. 2005). The voltage from the PV string is first converted into a high-frequency AC; The transformer secondary voltage is then rectified using a full-bridge diode rectifier. The rectified DC is then converted into micro-grid compatible AC and connected to the utility by a three-phase voltage-source inverter. Tracking the maximum power point (MPP) of a PV array is usually an essential part of a PV system. Over the years, many MPPT methods have been developed and implemented. These methods vary in complexity, required sensors, convergence speed, cost, range of effectiveness, implementation hardware, popularity, etc. The names of some of these methods are hill climbing, perturb and observe, incremental conductance, fractional open-circuit voltage, fractional short-circuit current, fuzzy logic and neural network control, ripple correlation control, current sweep, DC-link capacitor droop control, load-current or load-voltage maximization, and dP/dV or dP/dI feedback control. The detailed overview of these MPPT methods can be found in T. Esram and P. L. Chapmans Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques. In Figure 10, a simple but effective method for the MPPT is shown. By measuring the string voltage and current, the PV array output power is calculated and compared to the actual PV array output power. Depending on the result of the comparison, the duty cycle is changed to control the input current for the current-source inverter, accordingly. This process is repeated until the maximum power point has been reached. Other types of MPPT controllers can also be developed within the same controller framework. Furthermore, additional controllers can be designed to control the amplitude of the high-frequency AC voltage at the primary of the transformer. There are two basic control modes for the grid-connected inverters. One is constant current control; the other is constant power control. It is still debatable if an inverter should be allowed to regulate voltage during grid-connected operation. The current IEEE 1547 standard does not allow distributed generation to actively regulate voltage, while some people in the industry suggest that voltage regulation may have some positive impact on the grid (Ye et al. 2006). Control of the utility-connected inverter is shown with constant power control (see Figure 10). Many functions to manage


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practical issues are not shown in the diagram, such as the negative sequence regulation, d-q decoupling, etc. The inner central loop regulates current, and the other control loop regulates power. In some cases, the reactive power reference, Qref, could be a power factor reference. By controlling this reference, the injected current to the utility can be maintained at unity power factor. Also, a variation of the constant power control can be implemented to maintain the inverter DC bus voltage at a constant value. In that case, the active power Pref, can be replaced with Vdref, and Pout can be replaced with Vdout so the DC voltage is regulated to the set-point Vdref. The output of this outer control loop is the set point I*d, which is used as a reference signal to the current regulator. When DC bus voltage is increasing, meaning the power from the primary source is increasing; it is charging the DC capacitor. In order to maintain the DC bus voltage, the i*d will be increased so that the power can be transferred to the inverter output.

2.6.2. Power Electronics and Control Topology for Wind Generation A typical power electronics topology that is used for a permanent magnet synchronous generator is shown in Figure 16. The three-phase variable voltage, variable frequency output from the wind turbine is rectified using a diode bridge. With the change in the speed of the synchronous generator, the voltage on the DC side of the diode rectifier changes. To maintain a constant DC-link voltage of the inverter, a step-up chopper is used to adapt the rectifier voltage. As viewed from the DC inputs to the inverter, the generator/rectifier system is then modeled as an ideal current source. This rectified output signal from the diode bride is filtered into a smooth DC waveform using a large capacitor (Carrasco et al. 2006). The DC signal is then inverted through the use of semiconductor switches into a three-phase, 50 Hz waveform. This waveform can then be scaled using a transformer to voltage levels required by the utilitys AC system. The generator is decoupled from the grid by a voltage-sourced DC-link; therefore, this PE interface provides excellent controllable characteristics for the wind energy system. The power converter to the micro-grid enables a fast control of active and reactive power. However, the negative side is a more complex system where more sensitive power electronic parts are required.

Fig. 2.28 Synchronous generator power electronics topology

The most generalized form of power electronics topology for the wind energy application is the back-to-back rectifier/inverter connection, which provides the improved power flow control as well as increased efficiency. The voltage-fed converter scheme used in such systems is shown in Figure 18. A PWM-based IGBT bridge rectifies the variable-frequency variable-voltage power from the wind generator. The rectifier also supplies the excitation needs for the induction generator. The inverter topology is identical to that of the rectifier, and it supplies the generated power at 50 Hz to the utility grid (Simoes and Farret 2004).


39

Fig. 2.29 Generalized power electronics converter for wind systems

This general structure is suitable for any type of wind energy systems. For the PM synchronous generator, as discussed in Figure 16, the rectifier and boost converter can be replaced by the PWM rectifier to achieve the same level of control. Based on the control design for the back-to-back PWM converter system, various advantages can be obtained such as (Simoes and Farret 2004):

The line-side power factor is unity with no harmonic current injection (satisfies IEEE 519). Wind generator output current is sinusoidal. There are no harmonic copper losses. Continuous power generation from zero to the highest turbine speed is possible. Islanded operation of the system is possible with a start-up capacitor charging the

battery.

Fig. 2.30 Control block diagram of PWM converter with PMSG

2.6.3. Power Electronics and control Topologies for BES The power electronics topologies for the battery energy storage systems can be of different types based on the number of cascaded stages in the conversion systems, types of converters, presence of galvanic isolation, and utility connection. The most fundamental topology for utility connection of the BESS is the single-stage, self-commutated voltage-source inverter as shown in Figure 49. While the DC output of the battery system is connected across a filter capacitor, the capacitor itself is used to limit the harmonic currents in the battery. The output of the capacitor conn

standalone hybrid generation system

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