Student Projects from Vestas

Transcription

1 s from Vestas af January 2003 Edited by Kenneth Krabbe

2 Project Title: Dead time compensation in matrix converters Target Group: M.Sc.E.E. or B.Sc.E.E. graduate students Matrix converters are in a progressing development and seem to be a promising alternative to conventional inverters. Fig. 1 shows the matrix converter topology. Supply grid Input filter sw1 sw2 sw3 Induction motor Switch configuration Matrix converter Fig. 1: The three-phase to three-phase matrix converter topology. However, the matrix converter is quite unexplored in medium and high power applications although properties such as bi-directional power flow and sinusoidal input currents are more crucial in these applications than in low power applications. When entering medium and high power applications where the load typically has a low damping, instability may occur due to harmonics in the output voltage. These harmonics may be introduced due to dead time between switching and due to minimum pulse width problems. Unfortunately, topics such as dead-time compensation and compensation for minimum pulse width are quite unexplored areas for the matrix converter. Compensating for dead time and minimum pulse width is a quite complex task due to the following reasons: The exact switching time between two switch vectors depends on both current direction and the input voltages to be switched. The problem statement for this project is: Investigation and design a dead time compensator for a three-phase to three-phase matrix converter Project Content: Development of an accurate simulation model of the matrix converter system (incl. Motor load). Design of hardware test-set-up. Investigation of different commutation strategies and their influence on the dead time problem. Investigation of different modulation strategies and their influence on the dead time problem. d-space or DSP implementation of the control of the matrix converter. Vestas Contact: Lars Helle

3 Project Tittle: Overmodulation of the three-phase-to-three phase matrix converter Target Group: M.Sc.E.E. or B.Sc.E.E. graduate students In recent years, matrix converters for use in induction motor drives have received considerable attention as a competitor to the normally used pulse width modulated voltage source inverter (PWM-VSI). The matrix converter topology is shown in Fig. 1, where each of the nine switches represents a bidirectional configuration. Among other factors indicating the growing interest for the matrix converter, the semiconductor manufacturer EUPEC has recently introduced a single chip module including all nine bi-directional switches. Compared to the PWM-VSI with diode rectification stage at the input, the matrix converter provides sinusoidal input and output waveforms, bi-directional power flow, controllable input power factor and more compact design. On the other hand, the matrix converter can only be linearly modulated to an output voltage equal to times the input voltage if input current and output voltage are to be sinusoidal. The limited voltage transfer ratio is often stated as the major obstacle for the break through of the matrix converter. Supply grid Input filter sw1 sw2 sw3 Induction motor Matrix converter Switch configuration Fig. 1: The three-phase to three-phase matrix converter topology. In the field of conventional voltage source converters (PWM-VSI), especially in heavy applications where the switching frequency is limited to say below 1 khz, special pulse patterns have been calculated to eliminate certain harmonics in the output voltage. Since the output voltage from this type of modulation basically is a square-wave voltage with some strategic placed voltage notches, a side effect is, that the VSI can be modulated to about 1.18 times the input voltage (depending on the number of harmonics to be eliminated). Adopting this harmonic elimination method for the matrix converter is more or less directly applicable and then the matrix converter can achieve a voltage gain near unity. However since the matrix converter produces the power conversion in one step, it might be advantageous to calculate new pulse patterns in order to achieve a better input current (of course at the expense of the output voltage quality and/or voltage gain). So the problem statement becomes: Calculate optimum pulse patterns for use in the three-phase to three-phase matrix converter with regards to some objective function, weighting both input current quality, output voltage and voltage transfer ratio.

4 Project Content: Hardware design of the matrix converter Development of a simulation model to test different modulation strategies before implementation. Input filter design This topic is a very crucial issue, especially when entering the overmodulation range. Investigation of different modulation strategies, both in the linear modulation range and in over modulation d-space or DSP implementation of the control of the matrix converter. Vestas Contact: Lars Helle

5 Project Title: Modulation of three-level inverter with common-mode voltage elimination and DC-link balancing Target Group: M.Sc.E.E. or B.Sc.E.E. graduate students Conventional two-level voltage source inverters (VSI) generate common-mode voltage within the motor windings, which may cause motor failures due to bearing currents. Further, due to the capacitive coupling between the stator winding and the grounded motor frame, a common mode leakage current will flow, resulting in significant common-mode EMI. By use of a three-level inverter, c.f. Fig 1. and by use of only six of the active switch vectors, the common-mode voltage can be eliminated. (The common-mode voltage elimination is achieved at the expense of a reduction in the voltage transfer ratio, which becomes ) This modulation scheme was proposed by [1] where the voltage levels within the three-level inverter were achieved from independent DC-sources. However, in three-level converter structures, where the voltage levels are obtained by series connected capacitors, a DC-link voltage problem might occur by which an excessive high voltage might be applied to the switching devices (only for the topology in Fig. 1a). Further, the three-level converter might be unable to synthesize the reference voltage if too large voltage unbalance occurs. Hence, besides avoiding the common mode voltage, the modulation of the three-level converter also has to address the voltage unbalance between the upper and the lower switches. s 1A s 1B s 1C s 1A s 1B s 1C D 1A D 1B D 1C s 2A s 2B s 2C C 1 C 1 V DC N s 2A s 2B s 2C V DC N D 1A D 1B D 1C D 2A D 2B D 2C s 3A s 3B s 3C C 2 D 2A D 2B D 2C C 2 s3a s3b s3c s 4A s 4B s 4C s4a s4b s4c A B C A B C a) b) Fig. 1. Three-level neutral point clamped inverters. a) Conventional topology. b) Modified topology. [1] Haoran Zhang and Annette von Jouanne and Alan Vallace, Multilevel inverter modulation schemes to eliminate common-mode voltages, Transaction on industry applications, Vol. 36, No. 6, pp , 2000 Based on the problems listed above, the problem statement for this project becomes: Development of a modulation scheme that addresses both common-mode voltage elimination and DC-link balancing. Project Content: Hardware design of the three-level inverter Development of a simulation model to test different modulation strategies before implementation. d-space or DSP implementation of the modulation/control of the three-level converter. Comparison of the emitted common-mode EMI from conventional modulation schemes and the developed modulation scheme. Analysis of the DC-link unbalance problem. Vestas Contact: Lars Helle

6 Project Title: Discontinuous DC-link Balancing Modulation Strategy for Three-level Inverters Target Group: M.Sc.E.E. or B.Sc.E.E. graduate students Since the introduction of the Neutral Point Clamped (NPC) inverter, c.f. Fig. 1a, this inverter has mainly been applied for high voltage- and low switching frequency power conversion applications. However, progressing advance in computational power processors and in solid state switching devices, such as the IGBT, makes the NPC inverter applicable also in high switching frequency application. Considering low switching frequency applications (f sw < 1 khz), a lot of research has been concerned about calculating optimal switching patterns to eliminate low order harmonics in the output voltage. As the switching frequency increases, research on harmonic voltage elimination recedes while problems like reduction of switching losses becomes more urgent. A simple method to reduce the switching losses of a three-level converter is to employ the discontinuous modulation schemes known from the conventional two-level voltage source inverter (VSI). However, a non-modified adoption of the discontinuous two-level VSI modulation schemes is only functional when the voltage-levels in the threelevel converter is built from separate DC-sources. When series capacitors are used to divide the DClink voltage, three-level inverters (multi-level inverters in general) have a voltage unbalance problem due to the following reasons: Unequal capacitor values due to manufacture tolerances. Unequal loading of the capacitors due to unintended switching delays. Unequal loading of the capacitors due to e.g. non-linear loads containing even order harmonics. Transient operation of the converter, s 1A s 1B s 1C s 1A s 1B s 1C D 1A D 1B D 1C s 2A s 2B s 2C C 1 C 1 V DC N s 2A s 2B s 2C V DC N D 1A D 1B D 1C D 2A D 2B D 2C s 3A s 3B s 3C C 2 D 2A D 2B D 2C C 2 s3a s3b s3c s 4A s 4B s 4C s4a s4b s4c A B C A B C a) b) Fig. 1. Three-level neutral point clamped inverters. a) Conventional topology. b) Modified topology. Based on the problems listed above, the problem statement for this project becomes: Development of a discontinuous modulation scheme that addresses the DC-link unbalance problem. Project Content: Hardware design of the three-level inverter Analysis of the DC-link unbalance problem. Development of a simulation model to test modulation strategies before implementation. d-space or DSP implementation of the control of the three-level converter. Vestas Contact: Lars Helle

7 Project Title: Variable Speed Wind Turbine equipped with Synchronous Generator Target Group: M.Sc.E.E. or B.Sc.E.E. graduate students Previously, wind turbines were sited on an individual basis or in small concentrations making it most economical to operate each turbine as a single unit. To day and in the future, wind turbines will be sited in remote areas (including off shore sites) and in large concentrations counting up to several hundreds of MW installed power. This opens up new technical opportunities for designing and controlling the wind turbines but at the same time increasing the demands to reliability, availability and grid impact. One promising solution for operating a large concentration of wind turbines is shown in the figure below: HVDC Main Converter Grid Each turbine generator is a synchronous generator (either wound or equipped with permanent magnets). The generator is loaded with a three-phase diode rectifier. To control the power flow from each turbine and to obtain a certain speed range, the voltage from the diode-bridge is boosted to the voltagelevel of the common DC-link. The system in this project proposal has the following features compared to constant speed solutions: Each wind turbine can be controlled individually, thereby tracking the point of maximum power extraction. Transient power fluctuations caused by e.g. tower shadow can be reduced by using the opportunity to store energy in the rotating parts of the wind turbine. Impacts on the grid during connection of the park and grid failures is very low and is controlled by the main converter. The power factor of the whole park is controllable and controlled by the main converter. Each turbine can be built without a gearbox by use of a multi pole generator. The problem statement of this project is: Design and control of the rectifier part (incl. Boost converter) for one turbine unit.

8 Project Content: Control of the main inverter. Development of simulation tool usable for designing the rectification part. Simulation of a wind turbine park based on the present concept. Vestas Contact: Lars Helle

9 Project Title: Intelligent IGBT driver Target Group: M.Sc. E.E. or B.Sc.E.E. graduate students Today frequency converters are commonly used in wind turbines to control the speed of rotor in order to optimise the energy production and control the load of the mechanical parts. A typical converter configuration is shown in the following figure. The converters are typically based on IGBT modules and due to the high power (several MW) the power modules have to bee connected in parallel or in series. The way to guarantee load share between the individual power modules is typical to add passive components say snuppers or inductors to guarantee equal share of voltage or current. This is a very expensive and space consuming solution and therefore it would be an advantage to solve this load share problem on low power level in the driver. Apart from sharing load the driver should also provide protection functionality e.g. protection against over voltage, over current and over temperature and make a controlled shut down within safe operating area and provide the microprocessor based control unit with a error telegram over a digital interface e.g. CAN bus. The driver could also provide the control unit with measured values of voltage, current and temperature over this digital interface. Based on the problems listed above, the problem statement for this project becomes: Development of an intelligent driver for IGBT modules that addresses load sharing problems, protection issues and provide the control unit with the necessary measured values. Project Content: System analysis and specification of driver functionality. Circuitry design. Simulations of the essential functionally in SPICE or SABER. Test of protection functionality and measurements in the laboratory. Vestas Contact: Kenneth Krabbe / Lars Rasmussen

10 Project Title: Design of grid inverter for wind turbine application Target Group: M.Sc. E.E. or B.Sc.E.E. graduate students Today frequency converters are commonly used in wind turbines to control the speed of rotor in order to optimise the energy production and control the load of the mechanical parts. A typical converter configuration is shown in the following figure. A large variations in produced power due to variations in wind speed causes disturbances on the grid and therefore there is a demand for control of the power flow in the wind turbine. Heavy mechanical load is also a side effect by uncontrolled power flow and therefore there is a potential to optimise the mechanical construction e.g. the gearbox if the power flow can bee controlled. The control scheme of a wind turbine is divided in to two control loops. An outer loop controlling the speed of the wind turbine by adjusting the pitch angel and an inner loop that controls the power production by adjusting the rotor currents of the generator and controlling the voltage on the grid inverter. The control scheme will optimise the produced power by adjusting the speed of the wind turbine until the winds peed reach the level around 14m/s and in case of increasing wind speed it will keep the power constant on the nominal value. Based on the problems listed above, the problem statement for this project becomes: Develop a down scale grid inverter with control scheme for a wind turbine and control the power flow to the grid by adjusting the voltage of the grid inverter. The Grid converter should full-fill the following demands: Input: 3 phase, 400V. Output: 800V dc. Power rating: 10kW continuously. Performance: 2 quadrant Rise time in DC-link voltage control loop: 10ms Max DC link voltage swing 100% load jump: -+50V Efficiency: >96% Power factor: >0,99

11 Project Content: System analysis and simulations. Circuitry design. Design of control scheme. Test in laboratory of the inverter and control the scheme. Vestas Contact: Claus Esbensen / Kenneth Krabbe