BoP : Electrical Conversion & Connection - PowerPoint Presentation

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2. BoP: Electrical Conversion & ConnectionDC/DC and DC/AC converters in grid interfacingVesa Väisänen

3. LUT?Lappeenranta University of TechnologyEstablished in 1969Located in Lappeenranta, South Carelia, FinlandFaculty of TechnologyFaculty of Technology ManagementSchool of BusinessNumber of students ~ 5000Number of staff ~ 929

4. Our projectProject started in 20071 professor and 3 researchers Partners in co-operation: ABB, Wärtsilä, VTT

5. ObjectivesFeed the energy from a SOFC stack into electric gridHigh efficiency (>95 %)ReliabilityManufacturability and pricePaying attention to the fuel cell characteristics

6. Prototype testing at VTT, results10 kW Power conversion unit successfully integrated to a SOFC system at VTTOperated over 3000 hGrid connection is done with ABBs grid converterMeasured losses for power electronics (DC/DC + DC/AC) were 1.1kW, corresponding to about 43% of total system losses [1].

7. RequirementsThe requirements for a power conversion unit arise from three major sources:Fuel cell (or any other power source)The supplied load or networkGeneral requirements such as economical constraints, efficiency requirements, expected operating life, standards, patents…

8. Fuel cell requirementsFuel cell voltage drops as a function of current density  need for voltage regulationCurrent reference must be accurately followed to avoid stack overloading  need for accurate current controlLow frequency current ripple must be low to avoid process oscillation and overloading  ripple mitigation by the controllerEffects of long term high frequency (> 10 kHz) ripple still unclear?  the lower the allowed ripple, the more expensive the filter

9. Fuel cell requirementsThe voltage produced by the fuel cell stack can be low (for example 40-60 V), but the DC/AC converter requires a higher input voltage depending on number of phases and the modulation method:One-phase (230 V) Voltage Source Inverter (VSI)  VDC-link > 255 V (for preferred linear modulation ≥ 325 V) Three-phase (400 V) VSI with Space Vector PWM  VDC-link > 628 V (for preferred linear modulation ≥ 693 V) Need for considerable voltage boostFuel cell has high electrical efficiency, so high efficiency is desired also from the power conversion unit to maintain high overall efficiency  converter topology and component selection

10. How to interface a fuel cell?Most DC loads require regulated DC voltage. Therefore the DC/DC converter is typically essential.Galvanic isolation with a transformer is preferred for safety reasons and for voltage boosting.High frequency transformer on DC side is much smaller than a low frequency transformer on AC side.For example a 10 kVA, 50 Hz commercial transformer can weigh 72 kg, while a 50 kHz transformer weighs about 2 kg!

11. Non-isolated DC/DC convertersBoost converter topologySimple, non-isolated topology for voltage step-up.Inductor L1: stores energy and limits the input current rate of changeTransistor S1: acts a switching elementDiode D1: allows inductor current to flow to load while transistor S1 is closed and prevents current flow from load to input.Capacitor C1: feeds energy to load while transistor S1 is conducting.

12. Non-isolated DC/DC convertersBoost converter operating principleIdeal relation between input and output voltage iswhere

13. Non-isolated DC/DC convertersInterleaving of Boost convertersThe basic boost converter is often scaled to higher power levels by paralleling two or more boost converters.The stages are controlled in opposite phases (the transistors do not conduct at the same time), so the total input current ripple is reduced compared to a single converter.

14. Isolated DC/DC convertersPhase-shifted full-bridgeVery common topology capable of zero voltage switching  low switching losses in primary transistors.Suitable for higher input voltages.

15. Isolated DC/DC convertersFull-bridge boostHigh voltage conversion ratioLow input current ripple even without input capacitors.Low inrush current.

16. Isolated DC/DC convertersResonant push-pull boostTwice the voltage conversion ratio compared to full-bridge boostLow input current ripple even without input capacitors.Low inrush current.Near sinusoidal current waveforms and zero current switched secondary.

17. Isolated DC/DC convertersSources of power lossesThe losses in switching converters can be divided into three categories:Conduction lossesSwitching lossesCore losses in magnetic componentsThe dominating loss mechanism depends on the voltage and current as well as converter topology (capability of zero voltage or zero current switching etc.)As a rule of thumb:Low voltage, high current  conduction losses dominateHigh voltage, low current  switching and core losses dominateHigh voltage, high current  depends strongly on the converter design

18. Isolated DC/DC convertersConduction lossesConduction losses are caused by the conductor resistances and the intrinsic resistances in semiconductor junctions.Dissipated power P is the product of resistance R and the current I squared.Example: We have a 10 kW converter and two different stack voltages: 50 V and 250 V. Let us assume that both converters have 3 mΩ of resistance in the primary circuit.50 V  I = 200 A  P = 0.003*2002 = 120 W250 V  I = 40 A  P = 0.003*402 = 4.8 WThere is a 96% reduction in conduction losses, when the input voltage changes from 50 V to 250 V!

19. Isolated DC/DC convertersTransistor switching lossesSwitching losses arise from two major sources:Overlapping of current and voltage during switchingCharging/discharging of parasitic capacitances in componentsIn ZVS the transistor body diode conducts before gate voltage is applied.Voltage across the transistor is limited to body diode forward voltage during diode conduction.There is no Miller plateau in the gate-source voltage and thus the gate drive losses are also decreased.

20. Isolated DC/DC convertersTransistor switching lossesSwitching with ideal MOSFET No EMI, minimal lossesSwitching with unideal MOSFET Increased EMI and losses!

21. Isolated DC/DC convertersDiode switching lossesSwitching losses in diodes are caused by forward recovery and reverse recovery phenomena, as a diode requires a finite time to switch from conducting state to non-conducting state and vice versa.Forward recovery loss is typically small compared to reverse recovery loss.Charge Qrr must be swept away from the junction during the recovery time trr.Voltage VR and current IRM behavior during the off-transition defines the switching losses.Silicon Carbide (SiC) diodes do not experience reverse recovery effects.Voltage dependent junction capacitance Cj causes additional switching losses also in SiC diodes.[2]

22. Isolated DC/DC convertersMagnetic component core lossesMagnetic field strength H is related on current I flowing through N turns of conductor surrounded by a magnetic core having a magnetic path length of lm.The flux density B in a magnetic material depends on the material permeability µ and the magnetic field strength H.Flux density B can be plotted as a function of H to form a hysteresis loop.The loop shape depends on the core material.The area inside the loop is the energy dissipated in the core material.

23. Isolated DC/DC convertersMagnetic component core lossesCore losses depend on the difference between the maximum and minimum flux density (ac flux). The larger the ac flux, the larger the losses.The higher the operating frequency, the higher the core loss at certain ac flux.In transformers there is a trade-off between the number of turns (conduction losses) and the core losses. An optimal design is found near the point where winding losses and core losses intersect.

24. Isolated DC/DC convertersExamples of loss distributionsExample loss distributions are given for a 3 kW full-bridge boost [3] and a 10 kW resonant push-pull converter [4].The component stresses are dependent on the input/output parameters, selected topology and component optimization!

25. Isolated DC/DC convertersExamples of prototype costsIn modular converters the cost of auxiliary components may be higher in proportion than in single unit converters.Magnetic components can be smaller and cheaper in modular systems, but it is easier to achieve higher efficiency with larger components.Semiconductor efficiency is typically much better in modular converters due to smaller currents.

26. DC/DC convertersBidirectional convertersProcess control backup powering is often implemented with UPS systems connected to the grid side.In emergency shutdown the excess stack power is dissipated in resistors.Bidirectional DC/DC converters can interface the fuel cell to battery packs, that act as small time constant energy storages.Some of the stack energy could be recovered also during shutdown.

27. DC/DC convertersSummaryA DC/DC converter is an essential component in the power supply chain, unless the voltage levels between the power source and the load are directly compatible.It is more efficient to transfer certain power with high voltage and low current than vice versa.If galvanic isolation is not needed for safety or voltage step-up, the conversion efficiency is likely to increase and less complex converter topologies can be used.If the fuel cell output has a high tolerance for high frequency ripple (> 10 kHz) the DC/DC converter input filter requirements can be less stringent  smaller, cheaper and more efficient components.Higher efficiency often results in higher initial costs, so the total cost efficiency is dependent on the projected system life time.

28. DC/AC convertersSingle phase topologiesHalf-bridge inverterSimple structure and controlOutput peak voltage is ma * Vd/2, where ma is the modulation index (ma ≤ 1 in the linear region) [5]Full-bridge inverterOutput peak voltage is ma * Vd, where ma is the modulation index (ma ≤ 1 in the linear region)Bit more complex than the one-leg inverterThere are lots of other variants too especially in wind and solar applications!

29. DC/AC convertersSingle phase modulation methodsBipolar PWM [5]Half-bridge and full-bridge inverterUnipolar PWM [5]Only full-bridge inverterLower harmonic content

30. DC/AC convertersThree phase topologiesAble to supply all three phase-loads such as motors or electric grid.Can be implemented either as voltage source inverter (VSI) or current source inverter (CSI).CSI converters are able to boost voltage from input to output.Input inductor in CSI reduces the ripple current taken from the source. VSICSI

31. DC/AC convertersThree-phase modulation methodsThree-phase PWM for VSITriangular wave is compared with sinusoidal waveforms that are 120° out of phase.With linear modulation (ma ≤ 1) the maximum line-to-line rms voltage isThe maximum obtainable line-to-line rms voltage with overmodulation is[5]

32. DC/AC convertersThree-phase modulation methodsSpace vector PWM for VSI Eight discrete voltage vectors based on the logic states of power switches.Other voltage vectors in a sector can be produced by using the active vectors and zero vectors for a certain time during the switching period Ts.Maximum radius of the red circle (linear region) isTheoretical maximum output voltage is[6]

33. DC/AC convertersMultilevel convertersIn two-level inverters the available voltages at output are Vd and –Vd.By adding levels to the inverter, more output voltages can be produced (diode-clamp multilevel converter).A three-level inverter could provide also the neutral voltage N.Additional voltage levels reduce the harmonic distortion, so a filter could be omitted.Other types of multilevel converters are flying capacitor converters and cascaded converters with separate DC sources [7].

34. DC/AC convertersLosses in a VSI inverterLoss example of a 10 kW application with Vd = 700 V and fsw = 6 kHz [8].IGBTs having larger rated current exhibit smaller conduction losses (smaller junction resistance) but larger switching losses (slower switching).Typical VSI power losses range between 1-2% of rated power (depending on the operating point).Galvanic isolation or grid filter cause additional losses (typically few percent of rated power).

35. DC/AC convertersSummaryDC/AC converter converts DC voltage to grid frequency AC voltage.The required DC link voltage depends on the converter topology and the modulation method.Linear modulation requires higher DC link voltage than overmodulation, but with linear modulation the output voltage has less harmonics and thus the waveform is closer to pure sine.The better the voltage quality, the smaller and more efficient filters can be used.DC link voltage and switching frequency can often be adjusted in commercial inverters. The selection is a trade-off between voltage quality and switching losses.

36. System interconnectionProcess signalingCase LUT & VTTIf electrical grid is OK, inverter charges the DC link.DC/DC initializes and activates PCU OK signal.If DC/DC is OK  current reference is set  PCU ON signal is activated.Inverter active signal is activated  inverter running signal is received.

37. System interconnectionControl of DC/DC converterReference current is given from the fuel cell plant controller.Actual current is measured from the converter input.The error between the reference and the measurement is fed to a current controller.The current controller increases or decreases the converter duty cycle in order to force the current error to zero.Attention is paid to mitigation of the 150 Hz grid harmonic. [9]

38. System interconnectionControl of DC/AC converterOuter control loop controls the DC link voltage to maintain the power balance of the system.Voltage controller gives a d-axis current reference to the current controller.Current controller compares the current reference to measured values and forces the error to zero.The output of the current controller is a d-q voltage reference.The d-q voltage reference is transformed into α-β reference and given to the modulator together with phase angle.The modulator produces the switching vectors for the DC/AC power stage. [9]

39. System interconnectionCoordinate transformsThree phase grid voltages and currents are transformed into 2-dimensional rotating coordinates (d-q) through Clarke and Park transforms.[10]

40. System interconnectionControl overviewDC/DC controller controls only the input current with as small low frequency ripple and steady-state error as possible.DC/AC converter maintains power balance by keeping the DC link voltage constant.[9]

41. References[1] Halinen, M., et al. (2011). Performance of a 10 kW SOFC demonstration unit. ECS Transactions, 35, pp. 113-120.[2] Walters, K. (n.d.). Rectifier reverse switching performance. MicroNote Series 302, Tech. Rep. Microsemi.[3] Nymand, M. and Andersen, M.A.E. (2009). New primary-parallel boost converter for high-power high-gain applications. In: Applied Power Electronics Conference (APEC), 2009, pp. 35-39.[4] Väisänen, V., Riipinen, T., Hiltunen, J., and Silventoinen, P. (2011). Design of 10 kW resonant push-pull DC-DC converter for solid oxide fuel cell applications. In: Proceedings of the 14th European Conference on Power Electronics and Applications (EPE 2011).[5] Mohan, N., Robbins, W.P., and Undeland, T.M. (2003). Power Electronics: Converters, Applications and Design, Media Enhanced Third Edition, 3rd ed. John Wiley & Sons.[6] Sarén, H. (2005). Analysis of the voltage source inverter with small dc-link capacitor. Lappeenranta University of Technology.[7] Lai, J.-S. and Peng, F.Z. (1996). Multilevel converters – a new breed of power converters. IEEE Transactions on Industry Applications, 32(3), pp. 509-517.[8] Semikron SemiSel thermal calculator and simulator. url: http://www.semikron.com.[9] Riipinen, T. (2012). Modeling and control of the power conversion unit in a solid oxide fuel cell environment, D.Sc. thesis. Lappeenranta: Acta Universitatis Lappeenrantaensis. In peer review.[10] Ross, D., Theys, J., and Bowling, S. (2007). Using the dsPIC30F for vector control of an ACIM. Application note AN908. Microchip Technology Inc. url: http://ww1.microchip.com/downloads/en/AppNotes/00908B.pdf

42. Thank you! Any questions?