Application-oriented research in Power Electronics Dr. Wajiha Shireen Professor Department of Engineering Technology, College of Technology And Department of Electrical & Computer Engineering, Cullen College of Engineering University of Houston Overview • Power Electronics Systems • Applications • Research projects in the area: • A DSP Based SVPWM Control For Utility Interactive Inverters Used In Alternate Energy Systems. • Active filtering of input ripple current to obtain efficient and reliable power from fuel cell sources. Power Electronics • Power Electronics deals with the application of solid state electronics for the control, conditioning, and conversion of electric power. • Raw Power (120 V, 60 Hz AC) supplied by the utility in many cases need to be conditioned, controlled or converted for the following reasons: Ø to achieve energy efficiency Ø to improve reliability Ø to match different load requirements (eg. DC, high frequency AC, variable voltage variable frequency AC) • Renewable energy sources like solar and wind power systems are power electronics intensive. • It is estimated that 60% of electric power generated in the U.S flows through some power electronics system. Power conversion circuit • The function of a power conversion circuit is to control the energy flow between a given electrical source and a given load. • A power converter must manipulate energy flow but should not consume energy. • A power converter connected between the source and load also affects system reliability. Hence the converter must be reliable to avoid degrading a system. Primary design objectives in Power Electronics • Efficiency Target→100% • Reliability Target →No failures over application lifetime. The Efficiency Objective—The Switch • Ideally when a switch is ON, it provides vswitch=0, and will carry any current imposed on it. • When a switch is OFF, it blocks the flow of current (iswitch=0), regardless of the voltage across it. The device power, vswitch.iswitch, is zero at all times. • A circuit built from ideal switches will be lossless. Other lossless elements such as capacitors, inductors and conventional transformers might also be used for the conversion. The Reliability Objective-Simplicity and Integration • Power Electronics circuits tend to have few parts specially in the main energy flow paths. Often this means complicated control strategies applied to seemingly simple conversion circuits. • Manufacturers seek ways to package several switching devices, their interconnections, and protection components together as a unit. Power Electronics.vs.Linear Electronics • The main differences are : power handling capability and efficiency requirements. • A small power loss and hence high energy efficiency is important in power electronics systems because of two reasons: the cost of wasted energy and the difficulty in removing the heat generated due to the dissipated energy. • High efficiency and high power cannot be achieved by simply scaling up low power circuits but a different approach to power conversion/control needs to be adopted – power electronics. Power converters • It is a basic module of power electronic systems. • It utilizes power semiconductor devices controlled by signal electronics along with energy storage elements. • Power semiconductor switching devices like BJTs, MOSFETS, SCRs etc. are operated as a switch (cutoff region or saturation region) unlike in linear electronics where they are operated as variable resistors (active region). • Broad categories of power converters: ac to dc dc to ac dc to dc ac to ac • The controls take information from the source, load, and designer, then determine how the switches operate to achieve the desired conversion. • Power Electronics is an interdisciplinary field including power systems, solid state electronics, machines, analog/digital control, signal processing etc. • Power semiconductor devices can be regarded as the muscle and microelectronics as the brain of the system. Interdisciplinary Nature of Power Electronics • UPS systems provide protection against power outage as well as voltage regualtion. They also suppress transients and harmonic disturbances. Application in Adjustable Speed Drives • Conventional drive wastes energy across the throttling valve to adjust flow rate • Using power electronics, motor-pump speed is adjusted efficiently to deliver the required flow rate Variable speed operation of electric motors • The capability of varying the speed of an electric motor to match the load required by their end-use system, optimizes its performance and minimizes energy use. • Tests with pump, fan and compressor applications indicated typical energy savings of 20-50% using motors with variable speed drives. • Switching power converters offer an easy way to regulate both the frequency and magnitude of the voltage applied to a motor for speed control. Variable speed motor drives in a wide range of end equipment u Major Appliance and HVAC Motor Controls—Washers, Blowers, Compressors u Factory Automation Systems— Power Inverters, Industrial Drives u Automotive Systems—Electronic Power Steering, Anti-Lock Brakes, Suspension Control u Office Products—Tape Drives and Magnetic Optical Drives, Copiers, Printers A DSP Based SVPWM Control For Utility Interactive Inverters Used In Alternate Energy Systems - Dr. Wajiha Shireen - Srinivas Vanapalli - Hrishikesh Nene ØAlternate energy systems have increasingly become popular over the last few decades due to their inherent advantages over conventional energy sources. ØIn order to make a noteworthy impact, these systems must be utility interactive. ØA typical system consists of a DC-DC converter followed by a DC-AC converter. A Typical Alternate Energy System DC Bus Load / Alternate DC-DC DC-AC Electric Energy Converter Inverter Utility Source DC Bus which is not DC! • Above system assumes a constant ripple free DC bus. • Alternate energy sources themselves do not always produce voltages that are constant. • In any practical converter, it is difficult to realize a constant ripple free DC bus. Effect • This ripple on the DC bus causes lower order harmonics to appear at the output of the inverter, deteriorating the output voltage quality. • The magnitude of the fundamental component of the inverter output may not remain constant. Conventional Solution Normally large DC Capacitors are employed to get a stiff DC bus. However, • They are bulky and occupy too much space. • Contribute to slow response and increased cost. So is there a way to get a quality output without those bulky capacitors? Block diagram of the DSP- based interface. PWM Inverter DC voltage from Utility AC alternate energy System sources MOSFET drivers & isolation PWM Voltage sensing & DSP conditioning circuit Controller Pulse Width Modulation (PWM) • The switching power converter (inverter) consists of power semiconductor switching devices. • The ON and OFF periods of the switching devices are controlled by pulse width modulated (PWM) control signals. • Space Vector Pulse Width Modulation (SVPWM) is a special switching sequence used in AC Induction motor and PM motor drives. Advantages of DSP control over analog techniques • Precision with stability • More reliable • High Speeds • Implementation of complicated Mathematics. • Smaller Size • Low cost • Ability to make changes in software instead of re- designing equivalent analog circuitry Laboratory set-up of Inverter and DSP Board • A single phase variable ac source (Variac). • A three phase inverter Board (Spectrum Digital DMC 1500 drive platform) • A DSP Controller EVM Board (TMS320F240 EVM platform) • A Personal computer with the DSP software (an IBM compatible PC with Code Composer Studio installed). Hardware setup for a DSP controlled three phase AC Induction motor drive Emulator AC Motor Inverter DSP EVM Variac 30 % Ripple introduced Manual variation of the using 47 uF capacitor DC bus voltage over a wider range Without using the Output Voltage mag. in V 14 Without using the Algorithm 36 Output tVoltage m a g . in V V 13.5 Algorithm O u t p u V o lt a g e mag. in . 13 12.5 31 12 11.5 26 Using the 11 Algorithm Using the 10.5 21 Algorithm 10 9.5 16 55 65 75 85 95 25 27 29 31 32 35 34 2 7 2 10 10 11 DC Bus Voltage in V. DC Bus Voltage in V DC Bus Voltage in V Active Filtering of Input Ripple Current to Obtain Efficient and Reliable Power from Fuel Cell Sources Dr. Wajiha Shireen Hrishikesh Nene Block Diagram of a Typical Fuel Cell System h1 PES Inverter stage è Introduces a 2nd harmonic (120 Hz) current h2 ripple into the system. h3 This low order current ripple, has been found to be detrimental to the performance, efficiency and life of the fuel cell. Slide 37 h1 Explain - DC-DC boost need and inverter need. hrnene, 7/16/2006 h2 It can reduce the fuel cell available output power by as much as 6 %, causing internal losses and increasing distortion of its terminal voltage hrnene, 7/16/2006 h3 Current ripple is boosted from inverter to the fuel cell. hrnene, 7/16/2006 Proposed System The goal of this research is to prevent this 120 Hz ripple current from circulating through the fuel cell source so as to obtain a more reliable and efficient system. To achieve this, an active filter is connected across the DC-DC converter. The active filter provides an alternate path for the 2nd harmonic current and saves the fuel cell. No external energy storing device is required. Proposed System Block Diagram Fuel DC-DC DC-AC Cell Converter Inverter LOAD Voltage Sense Active EzDSP F2808 Filter DSP Controller Current Sense DSP Controller • It provides switching signals for the boost, the inverter and the active filter switches. • It senses the 120 Hz current ripple generated by the inverter. • It calculates the amplitude and phase of the cancellation current and adjusts the PWM pulse widths and phase for the active filter switches accordingly. • It can also modify inverter switching signals and/or boost switching signals to compensate for any variations on the DC bus. Side - by – Side Comparison Time Domain Time Domain DC DC 120 Hz 240 Hz 240 Hz 120 Hz Frequency Domain Frequency Domain Fuel Cell Current for Fuel Cell Current for a Conventional System the Proposed System Conventional System Vs Proposed system (Experimental Results) Quantity Conventional Proposed System System Vdc (Bus Vltg) 25 V 25 V Ifuel-cell@ 0 Hz 2A 1.9 A h4 (DC current drawn) Ifuel-cell @ 120 Hz 1.2 A 0.6 A (120 Hz ripple current imposed on the fuel cell) Ripple factor due to 50 % 29.5 % 120 Hz current Total Ripple factor >55.7 % < 36 % Slide 42 h4 Reduction in the DC current drawn hrnene, 7/16/2006 Conclusions • Total elimination of the ripple should be possible by fine tuning the phase and amplitude of the active filter generated current • The proposed system, after its performance optimization, can contribute significantly in increasing the life, efficiency and reliability of fuel cell systems. • Such a system can be targeted at low to medium power residential applications.
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