1. Notes by Kishore Malani Renewable Energy Technology 5-Day course at IIT Bombay, May,12

2. Solar PV Wind Energy Fuel Cells Grid Connected System Islanding, Anti-islanding Micro-Grid Smart Grid Power quality and protection issues STANDARDS related to various aspects of renewable energy systems Demonstrations / Lab-work: Demos of the working models of stand alone and (micro-)grid connected renewable energy systems. (Demo of Micro-grid). Demonstration of Texas Instruments Piccolo C2000 series DSP (microcontroller) applications in power electronics related to renewable energy. Overview 2 3. About the course Course designed nicely: with 2 lecture sessions in morning, and 2 lab sessions in after noon (post lunch)... every day! About 30 participants; mostly faculty from different universities; few were from industry; one had his start-up; and few graduate students from IITB. Course instructor and coordinator : Prof. Vivek Agarwal, Applied Power Electronics Lab, EE dept., IIT Bombay. 3 4. Solar Energy PV Cell Output: Voc (Open Ckt Vtg) = 0.5 to 0.6V Isc (Short Ckt Current) = Few mA's to few Amps Theoretical max. Power = PT = ISC.VOC Actual max. Power = PMAX = IMP.VMP Fill Factor: A measure of quality of Solar Cell. FF = IMP.VMP / ISC.VOC = PMAX/PT Due to so less o/p, you need to connect several of them in series. Still, it is not "appropriate" for 'most' of the loads. Needs 'CONDITIONING'. This is where circuits like inverter and filter come in to picture, before the power is fed to the load. Need for Power Tracking in PV Systems - Non-linear I-V characteristics of PV system. Each cell has its own Max. Power Point (MPP). - Rotation and revolution of earth around the sun. - Changing climatic conditions - PV arrays are expensive; so extract the maximum you can.Typical PV System [1] [1] F. Blaabjerg, Z. Chen and S.B. Kjaer, "Power Electronics as Efficient Interface in Dispersed Power Generation Systems", IEEE Tran. on Power Electronics, Vol. 19, No. 5, pp. 1184-1194. 4 5. Solar Energy (Contd..) Electrical MPP Tracking Methods: Perturb and Observer Method (Hill Climbing) Incremental Conductance Method Ripple Based Method Voltage reference method Current reference method Fuzzy logic control methods Neural network based methods Current sweep methods Converter o/p power maximization A New Fast MPPT Technique There is trade-off between MPP tracking accuracy and tracking time. For MPPT: Load needs to be 'optimized' Equivalent load resistance 'R' should be equal to internal-R of the source PV system installed on EE bldg rooftop: 2kW, installed in yr. 2000. 2000: o/p was 1700 W (on good sunny day) After 10 yrs (in 2010): < 1000 W Types of MPP Tracking (MPPT): (a) Mechanical: Tilt and/or orient the PV array (b) Electrical: Manipulate iPV and vPV Challenges in Electrical MPP Tracking: Location of MPP not known apriori.; MPP not fixed, it changes with temperature & radiation. 5 6. Solar Energy: Grid Connected PV Systems 6 Why Grid Connected PV Systems are Popular? Battery back-up can be avoided Efficient and maximum utilization of PV arrays Can be used for distributed power generation systems Reference: [1] [1] F. Blaabjerg, Z. Chen and S.B. Kjaer, "Power Electronics as Efficient Interface in Dispersed Power Generation Systems", IEEE Tran. on Power Electronics, Vol. 19, No. 5, pp. 1184-1194. 7. Wind Energy Some numbers: Total Installed Capacity In World (2009): 152 GW Denmark : 22% of its total consumption is produced from Wind Energy! Principle: Kinetic Energy (K.E.) Mechanical Energy (M.E.) Electrical Energy (E.E.) K.E. M.E.: using Wind Turbine M.E. E.E. : using Electrical Generator Wind Turbine: Mech. Pwr at shaft of turbine is: P = 0.5 * Rho * A*(Vw)^3 * Cp Rho: Density of air A: Cross-sectional area of turbine Vw: Wind Velocity Cp: Power Coefficient; value: between 0 & 1; Function of Wind Velocity, Angular Velocity, Radius of Rotor, Pitch Angle etc. 7 8. Wind Energy M.P.P. (Max. Power Point): (1) Power vs. Shaft speed plot (2) Cp (Power Coeff.) vs. Lambda (Tip Speed ratio) plot Both plots are the same. Figure 2: Plots at different wind speeds Prediction of Wind speeds (& Solar Radiation... for PV's) Quite a bit of research done. PREDICTIVE ALGORITHMs are used. Types of Electrical Generators (M.E. --> E.E.) (1) Singly Fed Induction Generator (SFIG): Mostly: Squirrel Cage Induction Generator (SCIG). Can be used for both: (1) FIXED SPEED wind turbine; (2) VARIABLE SPEED wind turbine with back to back converter (2) Doubly Fed Induction Generator (DFIG): Supply given to both: (a) stator & (b) rotor. Rotor current components are controlled to optimize the power yield up to the rated speed. Speed variation over wide range. Used for high power applications (3) Permanent Magnet Synchronous Generator (PMSG) Gives a variable frequency output. Earlier these were used for 'low' power wind turbines. No more true! Siemens recently launched PMSG for 6MW wind turbine!!8 9. Wind Energy Lab Sessions: Demonstrations of: (1) Stand alone system (2) Grid connected system (3) Integrated into MicroGrid GRID INTEGRATION ISSUES: "As the %age of wind power being fed to grid is increasing, its effects on conventional grid are being felt." 1. Wind speed variation 2. Wind Forecasting 3. Stability: Stability of grid with increasing penetration of wind. 9 10. Fuel Cell Technology What is Fuel Cell? An electrochemical device that converts Chemical energy of FUEL & OXIDANT directly into Electrical energy. Like any Electro-Chemical cell, it comprises of: Anode, Cathode, and Electrolyte. Fuel : Anode side, Oxidant : Cathode side (E.g. of Oxidant: Air!!) Efficiency : On field efficiency: ~65% APPLICATIONS : EV's (Electric Vehicles): FC is attractive because it is COMPACT. Why POPULAR? High Efficiency: because no 'thermal step' involved in the energy conversion cycle. Low emissions (eco-friendly) Exhausted Heat Can Be Re-used :-) MODULAR; QUICK Installation MULTIPLE CHOICES On FUEL : Hydrogen/Natural Gas / LPG / Methanol (a recent development) / Naphtha (has been used for FC since quite some time now) Produces energy in the form of Electricity & Heat ... as long as the fuel is supplied Does not require Charging and Re-charging No moving parts providing high durability, long lifetimes, and silent performance . And, electrode materials are NOT consumed in the reaction Unlike internal combustion engines, fuel cells bypass any thermal step during energy conversion, and therefore, they are not limited by the Carnot efficiency, which only permits approximately 40 % of the converted chemical energy to be used for work, depending on the temperatures employed. 10 11. Fuel Cell Technology TYPES OF FUEL CELLS: Based on TYPE of ELECTROLYTE 1. Alkaline FC 2. Proton Exchange Membrane / Solid Polymer Electrolyte Membrane FC 3. Phosphoric Acid FC 4. Solid Oxide FC 5. Molten Carbonate FC 6. Direct Methanol FC ALKALINE FC One of the first modern FC's. Developed around 1960 Electrolyte: aqueous solution of alkaline Potassium Hydroxide soaked in matrix (usually asbestos) Wide range of electro catalysts can be used. e.g. Ni, Ag, metal oxides, spinels, noble metals. Ions transported: Hydroxyl ions, OH-- 11 12. Fuel Cell Technology LAB SESSIONS: 1. FC & UC (Ultra-Capacitor) system.. Type-1: As a 'Stand-alone' system Type-2: Connected to Grid & Local loads Type-3: Supplying power to MicroGrid: FC was 1 of the 4 sources of power. Other 3 were: Wind system, PV system, Conventional generator. 2. Fuel Cell system developed by DRDO Source of fuel: Na-boro oxide. 1 CAN of fuel (2-3 liters) provides 200W-hr Cost: Rs.25/unit (~$0.50/unit)... as against Rs. 4 (~$0.08) for residential areas in India. [Note the numbers are just to give an estimate, and may not accurate.] To provide electricity in the areas where grid lines don't reach. For soldiers to charge batteries/radios/etc. Proton Exchange Membrane / Solid Polymer Electrolyte Membrane FC: Membrane is a thin plastic sheet that allows hydrogen ions to pass thru' it. 12 13. Fuel Cell Technology 13 Stand-alone system Grid-connected system 14. Math Used Currents, Voltages in AC circuits and systems: are sinusoidal quantities Steady state analysis of AC systems involves algebraic functions like: Subtraction, Addition, Division and Multiplication which becomes difficult because current, voltage etc. are sinusoidal quantities. PHASORS: are used to represent the sinusoidal functions. Phasor representation facilitates decoupling of amplitude, phase and frequency information of sine and cosine function. Reference Frame Theory: implemented in CONTROLS. In applications, it involves using Micro-Controllers (mainly DSP) to perform the mathematical transformations. 14 15. Grid connected systems (A) Why grid-connected systems? If you can feed into the grid, you don't need storage (battery) GRID is like an 'Infinite Reservoir' Issues associated with grid-connected systems: 1. If you are feeding power into the grid, you need to be 'paid' for it Reverse METERING system required 2. You can't SIMPLY FEED in to the grid! (a) Challenges: - Frequency of grid - Voltage compatibility - Phase sequence - Harmonics (b) Regulatory Norms - International STANDARDS Example: You can't feed current into grid, if there is more than 5% DISTORTION. 15 16. Grid connected systems: Grid Tied Inverter (GTI) GTIs are often used to convert direct current produced by many renewable energy sources, such as solar panels or small wind turbines, into the alternating current used to power homes and businesses. During a period of overproduction from the generating source, power is routed into the power grid, thereby being sold to the local power company. (no storage required) During insufficient power production, it allows purchasing of power from the power company. Grid-interactive inverters typically cannot be used in standalone applications where utility power is not available. Various Configurations Centralized inverter (Past) String Inverter (Present) Integrated Module (Immediate Future) Reference: [1] [1] F. Blaabjerg, Z. Chen and S.B. Kjaer, "Power Electronics as Efficient Interface in Dispersed Power Generation Systems", IEEE Tran. on Power Electronics, Vol. 19, No. 5, pp. 1184-1194. 16 17. Grid connected systems: FILTER Filter - Needed Before Feeding Power In To The Grid to reduce/eliminate harmonics Examples: Simple L-filter L-C filter L-C-L filter Fig.: System Configuration of 3-phase grid interconnected inverter with LCL filter Utility Grid Grid Tied Inverter Filter Vdc Passive dampin g filter 17 18. Grid Connected Solar-PV System Major Design Parameters: Circuit Topology Conversion Efficiency MPP Tracking Power Quality Anti-islanding Feature Re-connectivity Feature (Re-synchronization with grid) Cost 18 19. Distributed Power Generation What it means? Generating power near the thickly populated areas (load centers) where it will be consumed Why is it needed? Being able to transmit ELECTRICAL energy over long distances is a very favorable feature; but this is also putting more stress on electrical network due to increasing demand, which is due to increasing standard of living. What will happen in a situation when the generated Power Grid requirement? E.g.: Power generated: 5 MW, Grid requirement 5 MW Instability Voltage rise etc. Grid Src- 1, Region -1 Src- n, Region -n 19 20. Distributed Power Generation: Islanding (d) Challenges / Issues involved: Non false detection of islanded event Repeatability of accurate detection Handling intentional and non-intentional islanded conditions Fast detection to adhere to relevant standards. In case of microgrids, islanding is possible If it is a simple grid connected system (DG), then anti-islanding must be implemented. Tricky: how to detect that the main-grid has turned off?! Islanding (a) An island is a condition where a portion of a grid is energized solely by distributed generators (DGs) while that portion of the grid is electrically separated from the rest of the power system. - IEEE std 1547-2003. (b) According to this standard: An unintentional island is an unplanned island, and the distributed resource (DR) (DG + storage device) shall detect the island and disconnect the DR from the utility grid within 2 seconds of the formation of an island. (c) How does it occur? Utility (Main Grid) detects a fault, and opens the disconnecting device. Accidental opening of the normal utility supply by equipment failure. Intentional disconnect for servicing either at a point on the utility or at the service entrance. Human error or malicious mischief. An act of nature. 20 21. Distributed Power Generation: Anti-islanding What is Anti-islanding? Distributed generators (DG) must detect the un- intentional islanding that has occurred, and immediately stop producing power. For this reason, the inverters that are designed to supply power to the grid are generally required to have some sort of automatic anti-islanding circuitry in them. How to detect un-intentional islanding? Passive scheme: based on voltage (V), frequency (f), f/V, or combination. Active scheme: based on disturbance to observe the system response under grid connected mode. (Islanded condition tends towards system instability.) Communication based schemes: Non-availability of communication signal fed at the grid side and received at micro-grid side. 21 22. Distributed Power Generation: Micro-grid Issues involved in Micro-grids: A risk of out-of-phase reconnection at all open points not designated and designed for synchronized connection. Coordination with load-shedding schemes Voltage and frequency regulation Load (phase) imbalance Load and generation matching Possible damage to equipment due to voltage and frequency aberrations. Next: Grid Synchronization IEEE 1547-4 (2011) IEEE created a new term: multiple DR systems, also referred as Microgrid, to be used for intentional islanding. Potential Benefits of Microgrid: Provide power in case of outage or disturbance in utility supply. Induce islanding, when there is some disturbance locally. This will avoid causing disturbance in utility. Provide isolation in case of power quality issues in utility (voltage distortion, voltage sag, flicker, lightning, transients, etc.) Resolve power-quality issues by reducing total harmonic distortion at the loads. Allows for maintenance on the utility side while keeping islanded customers powered. 22 23. Grid Synchronization Two situations: 1. Microgrid operating in islanded mode and we require to switch over to grid connected mode. 2. Grid is available and microgrid energizes taking grid supply as reference (ZCD or PLL based). E: Grid voltage, 50 Hz V: Micro-grid voltage, 50 Hz Voltage across the switch is proportional to vector-difference between E and V. And the magnitude of resulting current flow is proportional to voltage across the switch. Position 2: worst will result in large transients in current. Position 4: safe to close the switch. 23 24. Smart Grid Broadly, Smart Grid architecture could be depicted as a 3-tier framework: 1. Physical Power Layer (Transmission and distribution) 2. Data Transport and Control Layer (Communications and control) 3. Application Layer (applications and services) Standards: IEEE, ISO, IEC, ITU, IETF Smart grid is a term used to denote the modern way of power transmission and distribution of electricity grids by making them intelligent using the advanced techniques in communication and IT. It is supposed to make the grid more reliable, efficient, responsive, quality focussed, resilient, with less cost, and new value added applications. Example: Variable tariff structure for energy consumption during peak (higher rate) and off peak periods (economy rate). This would help flatten the load demand curve by automatically switching off the units (ACs, Refrigerator, heating units, etc) during peak load period and turning them on when the load demand eases.24 25. Thank You! 25 26. Appendix 26 27. [One of the Lab Sessions] Use of Micro- controllers In a lab session, the graduate students demonstrated 6 different applications (5 of them on TI boards with micro-controllers) 1. LED Lighting control (This one was the most visually appealing. There were lot of features to control the lighting of 3-color and white high-wattage LED's) 2. Motor control (Controlling two brush less DC motors) 3. Renewable energy demo (Stand alone, 1-ph, PV system) 4. DC/DC buck converter circuit (Controlling the Duty cycle. Two converters were there on the board; one with R-load, and other had a small bulb as load, whose intensity was varied by changing the duty cycle, thus controlling the current flowing through it.) 5. AC/DC rectifier circuit, used in Telecom applications 6. Characteristics of a small PV board (It behaves as a diode when there is no light) 27 28. Reference [2] [2] S. Jain and V. Agarwal, Comparison of the Performance of Maximum Power Point Tracking Schemes Applied to Single Stage Grid Connected PV Systems, IET Electrical Power Applications, Vol. 1, No. 5, Sept. 2007, pp. 753- 762. 28 29. General MPP Tracking Circuit 29 30. STORAGE Needed in 2 situations 1. When output power is surplus, i.e. more than demand. 2. When power is not available (e.g. For PV, no power at night) No one likes batteries!! E.g. Lead acid batteries In car: life is 2-3 yrs only :( At home, in inverters, life is 2-3 yrs only :( 30 31. DECOUPLING To decouple the fast dynamics (switching) of Power Elec Ckt from the renewable energy source Electrolytic Cap Weakest link!! Many research papers to replace capacitors; on circuit topologies that'll avoid the use of capacitors. Matrix Converter is one way to replace the Decoupling Capacitor. 31 32. Phase Locked Loop (PLL) PLL's are useful when you want to feed power (generated locally from renewable source) into the grid; this is a major task/challenge. PLL is used for synchronization with the grid. 32 33. Solar PV Power Conditioning Systems Connected between renewable source (say, PV modules) and the Grid, to which we want to supply power to. Types: (1) Single stage (2) Two stage (3) Multi-stage (Eg. PV Module - DC/DC - Hi freq DC/AC - Transformer, Small sized - Rectifier - DC/AC, at line freq of say 50Hz - Grid) 33 34. Solar Panels For Residential Installations Polycrystalline solar cell 34 35. Bookmarks IIT Bombay, Dept. of Energy Science and Engineering Desertec Wikipedia Renewable Energy Portal Wikipedia Energy Portal http://kishoremalani.in 35 36. Solar PV Systems: Grid Connected Inverters Current fed inverter Voltage fed inverter 2-stage solar PV system Single-stage inverter (2 stages combined into one) 36