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DC micro grid with distributed generation for rural electrification


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Paper on DC Microgrids for rural electrification, especially using distributed micro generation using solar or wind energy.

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DC micro grid with distributed generation for rural electrification

  1. 1. DC Micro-Grid with Distributed Generation for Rural Electrification Md Junayed Sarker B. Asare-Bediako J.G Slootweg W. L. Kling B. Alipuria Eindhoven University of Technology Eindhoven University of Technology Eindhoven University of Technology Eindhoven University of Technology Eindhoven University of Technology Abstract- This paper investigates the use of low voltage DC distribution network for rural electrification within an intelligent grid concept. The goal is to provide local communities in sparsely populated areas with electricity supply generated from renewable energy sources. Since these communities subsist with no grid connectivity, they require a concept of micro-grid whereby individual Solar Home Systems (SHS) can be connected. The excess power required by any SHS is supplied from the grid to make the system more reliable. Furthermore, people who cannot afford Solar Home System can connect themselves with the grid as well and get access to the basic need of electricity. In this work a power flow supervision system has been investigated based on domestic and grid level DC electrical systems with MATLAB/Simulink as software supporting platforms. The length of the grid and the number of the Solar Home System deeply affect the load behavior of the network. So it is suggested that a detailed study has to be performed before implementing this new concept Index Terms-- Micro-Grid, Distributed Generation, DC, Solar Home Systems, PV I. INTRODUCTION The world is facing fundamental changes in the energy sectors at a global level. On one hand conventional energy resources are exhausting and becoming expensive to extract; while on the other hand the addition of green house gases especially carbon dioxide in the atmosphere has accelerated the global warming. These two challenges have put the world leadership to devise strategy for adopting renewable energy resources to have clean sustainable energy source and to have energy transformation at point of usage to avoid inefficiencies of energy transport and conversion. One of the sustainable strategies is to create clusters of areas around the rural areas of the world where local renewable energy will be utilized to meet their own energy demand. The major renewable energy source for decentralized generation within rural areas is likely to be small scale solar home system. As a result the integration of such systems in a low voltage DC network will be a noteworthy step towards powering these societies as an efficient approach [1, 2]. Efficiency, protection, power quality and the relative advantages of DC compared with AC are investigated. Although the current technology is not efficient enough to reach to the goal but recent development and attention over the issue suggest that the changes in the energy structure will take place soon. Interestingly, Solar Home Systems (SHS) are already widely in use in the rural areas of South Asia and Africa, where unreliable electricity grid is forcing the policy makers to move into the self reliant renewable energy based electricity generations. So these areas have a lot of potential for new business development relying on this micro-grid concept. II. BACKGROUND From the very early age of electricity, AC has been regarded as the most appropriate option for power transmission and distribution. DC was invented by Thomas Edison and he opted for its use but due to the limitations in voltage transformation and controlling, DC option could not be made feasible in the electricity network. Later Westinghouse made the proposal for AC distribution and it came out really popular. Nikola Tesla developed transformer module to stepping up and down the voltage, which solved the problem of transmission losses in the network. Since then vast advancement has been taken place in the field of AC transmission and distribution. However, in recent days, due to the development in power electronics converters and DC energy sources, loads and storage systems; interest in DC has returned. In the rural areas of developing countries where the inhabitants have no access to public electricity grid, DC based Micro-grid can be developed from locally generated electricity using renewable sources. A. Advantages of DC over AC for Rural electrification a. There will be significant amount of reduction in the energy loss. b. PV can easily be interconnected with the system which is the only source of electricity generation in the rural areas where there is no grid connection. c. Storage system can also be easily coupled with the DC network. d. Good utilization of existing solar home systems of the considered area can be made. e. Existing practice of using DC appliances in the Solar Home systems in the developing countries. B. Optimal Voltage level for DC Micro grid The DC Micro grid is connected with different solar home systems in a particular rural area. The SHSs generate electricity at 24V range for the home appliances. The voltage level is be boosted up for micro-grid connection. So an optimal voltage level has to be decided for the DC micro grid.
  2. 2. It is decided for 120 V after the loss calculation in the network [3]. It is really an important factor to decide for the following system parameters: Overall system Efficiency: The overall system efficiency depends on the power conversion stages between the generation and the load. The less no. of conversion results in a less power loss. In case of DC micro grid there will be no loss in DC to AC conversion. And voltage has to be boosted in the grid end to minimize the power losses in the distribution line. Cost: Cost is another important issue to be considered on. Power electronics converters and voltage distribution cables are expensive. They consumed major portion of system budget. Some optimal voltage level will leverage the system design parameters. Safety: 120V is hazardous to human. Typically, costs of safety and protective devices increase with the increment in voltage level. Safety devices have to be designed based on this issue III. LAB SETUP The principles of modeling the micro-grid are formulated and a model of the grid is developed using MATLAB/ Simulink. SimPowerSystems feature of MATLAB is used to design and build the model to simulate the power grid. The test network is simulated with all the possible scenarios to evaluate the system behavior. After validation and analysis of the system, necessary changes are brought in. In Simulink, a simple model of a solar home system is simulated in Simulink platform. Two voltage levels are considered for the total system, 24V is maintained at the domestic level and 120V is maintained at grid level. The optimal voltage level is decided after examining the load flows and power losses in the micro-grid. The voltage level is boosted up through DC-DC converters before providing it to the grid to connect all the solar home systems of a particular area to form a DC network. A. Solar Irradiance Generator A Simulink model has been developed to provide the accurate irradiance data for a particular area based on the climate data input from 1974 to 2000. It is necessary to use the irradiance data for a specific location over a certain period of time. This model is capable of providing irradiance and temperature data for any location of the world. This model is developed by Adrianus Wilhelmus Maria van Schijndel during his PhD thesis work in 2007. The model is partially used as to adjust with the requirement of the thesis work. B. PV array The PV array block represents a combination of solar cells. A simple PV cell consists of an ideal current source in parallel with an ideal diode. Due to the photons received on PV panels current is generated which is represented by current source. Short circuit current and the open circuit voltage are the two important parameters of PV cell. The current at the terminal of PV cell can be expressed by following equation Where, I= Output current of PV, amp (A) V= Voltage across the PV cell, volt (V) Isc = Iph = Short circuit current/Photon current, (A) Id = Current through intrinsic diode, amp (A) Io= Reverse saturation current of the diode, amp (A) q= Electron charge (1.602 ), coulomb (C) Vd = Voltage across the diode, volt (V) k= Boltzmann’s constant (1.381 ) T= Junction temperature, Kelvin (K) After derivations and applying Newton’s method, The first step is to simulate the system is to get the irradiance data for a particular year. The solar generator block used in the model can generate irradiance and temperature data for a particular location. The Fig 2 shown below is irradiance data for year 2000 in The Netherlands. Fig 1. Equivalent circuit of a simple PV cell Fig 2. Solar Irradiance for a year 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0 200 400 600 800 1000 Time(Year) Irradiance(W/m2)
  3. 3. The same block can generate temperature profile. Solar irradiance and temperature data are used in the PV array block to generate PV current. The Fig 3 shown below is representing the temperature profile for one year. Due to the solar irradiance on the surface of PV panel, power is generated in the PV panel. The following curve shows the power generation trend by the PV panel for three days. During the simulation 120 W PV array is considered. B. Battery The battery is modeled based on a lead-acid battery structure. Lead acid battery cell has two plates; positive and negative plates are deepened in a diluted sulfuric acid solution. Anode and cathode are made of lead dioxide (PbO2) and lead (Pb) respectively. The battery model is designed to have two mode of operation of charging and discharging. The following parameters are used to model the battery block. Where, SOC1 = Initial State of Charge/available, % SOCm=Max. SOC /Max. battery capacity, (Wh) SOC(t)=Current state of charge, % ns= No. of 2V series cells K=Charge/discharge battery efficiency D= Battery self discharge rate, Hour-1 (h-1 ) =Terminal voltage of the Battery, Volt (V) =Terminal volt of Battery while charging, (V) =Terminal volt of Battery while discharging,(V) =PV current, amp (A) = Battery internal resistance, (R) = Battery current, (A) = Load current, (A) = Battery resistance while charging, (R) =Battery resistance while discharging, (R) The value of battery current ( ) is positive when battery is in charge mode and negative when it is in discharge mode. So, the battery terminal voltage is: In case of charging: In case of discharging: Now the relationship between SOC and Battery current and voltage, So, the current value of SOC (t), in given time t is found by looping the result in Simulink. The battery current determines the State of Charge (SOC) trend of the battery. When battery is discharging the SOC will decrease and when the battery is charging it will increase. For safe operation and long durability of the battery, SOC limit is set in such a way that it cannot discharge below 50%. C. Charge Controller This is basically a charger block, which is necessary to keep the battery from getting overcharged or under discharged. Fig 7. State of charge (SOC) for three days 0 1 2 3 0 0.7 0.75 0.8 0.85 0.9 0.95 1 Time (Day) SOC Fig 3. Temperature for a Year 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec -10 0 10 20 30 40 Time (Year) Temperatute(degC) Fig 6. Battery current for three days 0 1 2 3 -3 -2 -1 0 1 2 3 4 5 Time (Day) Current(A) Fig 5. Battery model
  4. 4. Generally a deep cycle battery should not exceed the state of charge (SoC) 100% and go below 20%. The charger module designed in the solar home system consists of two switches. TABLE I SWITCHING OPERATION IN CHARGE CONTROLLER Condition A’ A Condition B’ B V>= 25.40 0 0 V<= 22.75 0 0 V>= 25.40 1 0 V<= 22.75 1 0 23.92<V<25.40 0 0 22.75<V<24.03 0 0 23.92<V<25.40 1 1 22.75<V<24.03 1 1 V<=23.92 0 1 V>=24.03 0 1 V<=23.92 1 1 V>=24.03 1 1 From the Fig 19, it can be depicted that switch A is connected between PV array and rest of the system and switch B is connected between load and rest of the system. Switch A in the PV side is opened when battery voltage becomes larger than the desired value and will remain open until the voltage gets dropped down. Whereas switch B is opened when the battery voltage drops down below the desired level and will remain in that stage until it rebounds back to the preferred level. These switching functions are implemented in Simulink. Here the battery voltage is being compared with different voltage levels using ‘compare to constant blocks’ of Simulink which will return 1 if it is true otherwise 0. D. Load The energy demand is very low in the rural areas of developing countries. Their basic electricity demand is for lighting purpose. Recently lot of development has been taken place in energy efficient bulbs. LED has made a huge advancements and it is more efficient than the standard incandescent bulbs. Moreover, most of the electrical appliances use DC power as well. The prices of these home appliances are not very expensive and available in the market. Cell phones have become an important tool in everyday’s life of all sorts of people. So the option of mobile phone charging is really attractive for them. In Simulink, the controllable load is modeled using resistors and switches. E. DC-DC Converter In the model, two types of DC-DC converter are used. The converter shown in Fig 21, only changes the output current of PV array to the current of battery voltage. In the real hardware setup DC-DC converter is used with MPPT to regulate the voltage output of PV. The DC-DC converter block demonstrated in Fig 3, is used to change the voltage from 24 V to 120 V and vice versa. Mathematical modeling approach has been performed here to design the converter. It is Switched-mode Boost (step-up) DC-DC averaged converter model with input current control and efficiency model. In DC-DC converter topologies, the output voltage is dependent on the input voltage. It is also related with the duty cycle of the switch. The relationship among these parameters can be shown as follows: Where, = output voltage of the converter = input voltage of the converter = duty cycle for the converter One problem with DC-DC converters is that, the efficiency tends to fall when operated below the rating. So in the implemented model of DC-DC converter, the efficiency was optimized for the optimal operation. PV Array Battery Load Switch A Switch B Fig 5. Block diagram of a solar home system SW Control Micro Grid = = = = Mic ro Gri d = = Mic ro Gri d = = 120V Grid Line Central Control UnitConsumer Type B Consumer Type A Battery Load PV Array DC-DC Converter DC-DC Converter Battery PV Array SW Control Fig 7. Layout of the DC Micro-grid
  5. 5. F. Central Control Unit In the network, there are two kinds of consumers. One type of consumer can generate their own electricity from Solar Home Systems but in case of additional need of electricity or during the night time when the consumption is high, they can consume the electricity from the micro-grid. These type of consumers are defined as ‘Type-A’ consumers. Another type of consumer has no generation system of his own. They consume electricity from the grid all through the day. And they are ‘Type-B’ consumers. So it is quite difficult to bring the balance in between production and consumption and maintain a stable voltage in the grid. A control unit having a large battery bank with PV can ensure the production of electricity and make the system balance. DC- DC converter is used to boost the voltage level and send the energy to the grid. At the type- A consumer end, when thebatery voltage of local battery will go down below some certain level then the system will switch all the loads from its own system to grid. G. Grid Connection Every Solar Home system will be connected to the grid. The grid network can be star or mesh or loop configured. Since the grid will be installed in rural areas in a small region encompassing 10-15 homes so loop network is considered to be the best. The main focus was to design a solar home system and then connect them in the network. Type B consumer can easily consume electricity from the grid whereas the type A, solar home system users will only consume the electricity when they don’t have enough storage in their local system. Basically, the existing solar home system users are having very few number of low power home appliances in their houses as they generate very less energy by their system. If they want to consume more energy then they will have to buy bigger system for their houses. The bigger system needs bigger storage capacity which increases the system price drastically. The proposed DC micro-grid will allow them to get the benefit of having the facility of consuming more energy without spending much amount of money for buying the new system. They can just get the electricity from the micro-grid operators and pay them monthly. As mentioned in the earlier section, a central control unit is considered in the grid which is basically consists of PV panels, DC-DC converters and batteries. The central control system will calculate the demand in the micro-grid and install PV arrays and batteries as per the requirement. IV. CASE STUDY PV systems are being encouraged all over the world and the concept of LVDC grid would open up the door to a number of possible business opportunities. Moreover a number of under developed countries in Asia, Africa and South America are heavily suffering with the problem of unreliable electricity supply. Hence introducing of LVDC grid in the rural area can alleviate the problem of these regions to a great extent. The rural electrification system should be designed in such a way that they are very energy efficient and reliable. Also, already existing structure which are less efficient in energy usage are needed to be retrofitted to reduce the amount of energy consumed by them. Doing this requires engineering knowledge and skills from various disciplines; energy engineering, electrical engineering, mechanical engineering, architectural engineering etc. Fig 7. Power demand for a particular variable load 0 1 2 0 10 20 30 40 50 60 70 80 90 Time (Day) Power(W) Fig 9. Power supplied to the variable load by local system 0 1 2 -20 0 20 40 60 80 100 Time (Days) Power(W) Fig 7. Grid voltage during a day 0 2 4 6 8 10 12 14 16 18 20 22 24 0 60 70 80 90 100 110 120 130 140 150 X: 8.46e+004 Y: 116.6 Time (Hrs) V(Volt) X: 5.65e+004 Y: 117.1 X: 2.91e+004 Y: 117.9 X: 100 Y: 118.1 Fig 9. Power supplied to the variable load by central control unit 0 1 2 -10 0 10 20 30 40 50 60 70 80 Time (Day) Power(W)
  6. 6. V. FAULTS CHARACTERISTICS IN A DC NETWORK Another important aspect of modeling of a LVDC grid is to design its protection system. In most cases, this system use grid connected breakers and current limiting capability during DC faults. The DC system has different fault characteristics than an AC system. The protection system has to be designed based on these characteristics. There are basically two methods to clear up the fault current, the first is to take some action to detect and clear the fault before the system gets affected-first tripping of the circuit and the other one is to delay the tripping till the transient gets passed away. There is number of potential devices available in the market which are capable of giving protection to the DC network. Fuse is the simplest of all protection devices. It is very much suitable at the residential end. The mostly used protection devices are Circuit breaker but due to their zero crossing of the current issue they are not suitable for DC [8]. And circuit breaker for DC system is not commercially available. Another important protection device can be solid state switches. The advantage of such devices is, it can operate in micro-second range and already in use in water vehicles. But recent development in the power electronics and DC distribution systems create a great hope for this technology to come alive in mass use. VI. BUSINESS OPPORTUNITIES PV systems are being encouraged all over the world and the concept of LVDC grid would open up the door to a number of possible business opportunities. Moreover a number of under developed countries in Asia, Africa and South America are heavily suffering with the problem of unreliable electricity supply. Hence introducing of LVDC grid in the rural area can alleviate the problem of these regions to a great extent [9]. The financial plan of the entrepreneurship depends on the market sizing and then capturing the share of the market. Since the potential market size for solar home system in the Bangladesh is 500000. For the analysis of financial plan, only this customer segment is considered. Inflation rate and interest rate are considered as 9.15% and 13.7%. The tax rate is assumed as 35%. VI. CONCLUSION In this paper, a LVDC micro-grid is presented to meet the basic energy need of the people of rural areas, which integrates the existing individual renewable energy and storage modules into a single grid. There were some difficulties in delivering the data in the simulink model as maximum accuracy was tried to achieve. Despite these facts the model performs nicely and helps to get the feasibility status of the proposed concept. Some components suggested in the model are not currently available in the market which is another drawback of the whole idea. There are a variety of possible improvements that could be made to the model and the proposed concept. Finally, this grid can provide more efficiency to the system and quality power to the 24V DC loads. Concept and idea of the low voltage DC grid is discussed and simulated, which proves the validity of the proposed idea. REFERENCES [1] Y. Ito, Z. Yang, and H. Akagi, “DC micro-grid based distribution power generation system”, in Proc. International Power Electronics and Motion Control Conference (IPEMC'04), vol. 3, Aug. 14–16 2004, pp. 1740–1745. [2] A. Agustoni, E. Borioli, P. Ferrari, A. Mariscotti, E. Picco, P. Pinceti, and G. Simioli, “LV dc networks for distributed energy resources”,,in Proc. Cigré Symposium on Power System with Dispersed Generation, Athens, Greece, Apr. 13–16 2005. [3] S.Anand, B.G. Fernandes, “ Optimal Voltage Level for DC Microgrids”, in IECON 2010 36th Annual Conference on IEEE Industrial Electronics Society, pp.3034 –3039 November 2010. [4] Gow, J.A. and C.D. Manning. "Development of a Photovoltaic Array Model for Use in Power-Electronics Simulation Studies." IEE Proceedings of Electric Power Applications, Vol. 146, No. 2, pp. 193– 200, March 1999. [5] M. Starke, Li Fangxing; L.M. Tolbert, B. Ozpineci, “AC vs. DC distribution: Maximum transfer capability” in Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century, 2008 IEEE Electr. Eng. & Comput. Sci., Univ. of Tennessee, Knoxville, TN [6] C. M. Shepherd, "Design of Primary and Secondary Cells - Part 2. An equation describing battery discharge," Journal of Electrochemical Society, Volume 112, Jul. 1965, pp. 657-664 [7] S. Rahman, M. Pipattanasomporn and Y. Teklu, “Intelligent Distributed Autonomous Power Systems (IDAPS)”, in Proc. 2007 the IEEE PES Annual General Meeting, Tampa, Florida, 8pp. [8] D. Salomonsson, L. Soder, A. Sannino, “Protection of Low-Voltage DC Microgrids,” Power Delivery, IEEE Transactions on, vol.24, no.3, pp.1045-1053, July 2009. [9] D. C. Barua, Grameen Shakti, “An integrated and sustainable model for bringing light, income, health, and affordable climate friendly energy to therural people”, published by Grameen Shakti, Dhaka, Bangladesh, January2008. Fig 8. Simulink model of the micro-grid Continuous powergui Scope2 Ics Ics2 Ihs2 Ics1 Ihs1 Conn2 Conn4 Consumer 7-Type B Conn2 Conn4 Consumer 6-Type B Conn2 Conn4 Consumer 5-Type B Conn2 Conn4 Consumer 4-Type B Tac G Conn2 Conn4 Consumer 3- Type A Tac G Conn2 Conn4 Consumer 2- Type A Tac G Conn2 Conn4 Consumer 1-Type A Tac G Climate Unit Tac G Conn2 Conn4 Central Control Unit