Multilevel converters have been under research and development for more than three decades and have found successful industrial application. However, this is still a technology under development, and many new contributions and new commercial topologies have been reported in the last few years. The aim of this paper is to group and review these recent contributions, in order to establish the current state of the art and trends of the technology, to provide readers with a comprehensive and insightful review of where multilevel converter technology stands and is heading. This paper first presents a brief overview of well-established multilevel converters strongly oriented to their current state in industrial applications to then center the discussion on the new converters that have made their way into the industry. In addition, new promising topologies are discussed. Recent advances made in modulation and control of multilevel converters are also addressed. A great part of this paper is devoted to show nontraditional applications powered by multilevel converters and how multilevel converters are becoming an enabling technology in many industrial sectors. Finally, some future trends and challenges in the further development of this technology are discussed to motivate future contributions that address open problems and explore new possibilities.

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Recent Advances and Industrial Applications of Multilevel Converters

The aim of this paper is to present a review of current control techniques for three-phase voltage-source pulsewidth modulated converters. Various techniques, different in concept, have been described in two main groups: linear and nonlinear. The first includes proportional integral (stationary and synchronous) and state feedback controllers, and predictive techniques with constant switching frequency. The second comprises bang-bang (hysteresis, delta modulation) controllers and predictive controllers with on-line optimization. New trends in current control-neural networks and fuzzy-logic-based controllers-are discussed, as well. Selected oscillograms accompany the presentation in order to illustrate properties of the described controller groups.

Current control techniques for three-phase voltage-source PWM converters: a survey

1. Principles of Renewable Energy 2. Solar Radiation and the Greenhouse Effect 3. Solar Water Heating 4. Other Solar Thermal Applications 5. Photovoltaic Power Technology - PV 6. Hydropower 7. Wind Resource 8. Wind Power Technology 9. Biomass Resources from Photosynthesis 10. Bioenergy Technologies 11. Wave Power 12. Tidal-current and Tidal-range Power 13. Ocean Gradient Energy: OTEC and Osmotic Power 14. Geothermal Energy 15. Energy Systems: Integration, Distribution and Storage 16. Using Energy Efficiently 17. Institutional and Economic Factors Review 1: Electrical Power Review 2: Fluid Dynamics Review 3: Heat Transfer Review 4: Solid State Physics for Photovoltaics Review 5: Units and Conversions: Algebraic Method Appendix A: Units and Conversions Appendix B: Data Appendix C: Some Heat Transfer Formulas Appendix D: Comparisons of Technologies Short Answers to Selected Problems Index

Renewable Energy Resources

The solid-state transformer (SST), which has been regarded as one of the 10 most emerging technologies by Massachusetts Institute of Technology (MIT) Technology Review in 2010, has gained increasing importance in the future power distribution system. This paper presents a systematical technology review essential for the development and application of SST in the distribution system. The state-of-the-art technologies of four critical areas are reviewed, including high-voltage power devices, high-power and high-frequency transformers, ac/ac converter topologies, and applications of SST in the distribution system. In addition, future research directions are presented. It is concluded that the SST is an emerging technology for the future distribution system.

Review of Solid-State Transformer Technologies and Their Application in Power Distribution Systems

The inherent limitations of commanding voltages and currents in a three-phase load with an inverter are examined. An overview of several current controllers described in the literature is presented, and computer simulations are used to compare performance. A switching diagram is developed which reveals some of the operating characteristics of hysteresis controllers. For ramp comparison controllers, a frequency transfer function analysis is used to predict the line currents and provide some insight into the compensation required to reduce the current errors.

Current Control of VSI-PWM Inverters

1,261,838. Static converters. GENERAL ELECTRIC CO. 9 April, 1969 [16 April, 1968], No. 18197/69. Addition to 1,261,392. Heading H2F. [Also in Division G3] In a power converter as claimed in parent Specification having a control circuit for selectively rendering conductive the solid state switching means in primary and secondary current paths at a switching rate frequency which is high compared with the supply frequency, there is further provided a phase shift circuit coupled to the control circuit to render conductive the primary and secondary current paths at a selected phase angle one with respect to the other, whereby to control the output voltage. The switches may be transistors as shown, Fig. 3, or Gate-turn-off thyristors (Fig. 10, not shown). Converter circuit, Fig. 3.-As in the parent Specification the load voltage e 2 is at the same low. frequency as the input side voltage e 1 , the input current being chopped by transistor switches and applied in opposite directions alternately through a ferrite core transformer 14 at a high frequency switching rate, e.g. 5-10 kHz. The h.f. current at the secondary side is reformed into a low frequency waveform by the secondary side transistor switches also operating at the h.f. rate. Reversibility of power flow through the converter is possible. Each switch means is bidirectional and as shown, Fig. 3, comprises an anti-parallel pair of' transistors Q1-Q2, Q3-Q4, Q5-Q6, Q7-Q8 respectively. Both transistors in a pair preferably receive base current from a control circuit 52 simultaneously and the circuit conditions determine which conducts or alternatively the base signals may be applied via logic circuits. Thus during the positive half cycles of low frequency input, Q2 conducts via diode 29 and in negative half cycles thereof, Q1 conducts via diode 28 and the transformer primary. In normal operation (i.e. 0 degree phase shift between input and output side switches) Q1, Q2 receive base signals at the same time as Q5, Q6 the transistors Q3, Q4, Q7 and Q8 being OFF. During the next h.f. half-cycles Q3, Q4, Q7, Q8 receive base signals and the others are OFF, and so on. D.C. supply.-The connection of the converter to a D.C. source is envisaged wherein the phase shift control is modulated to obtain a desired output waveform to a load. To obtain a sine wave the phase shift is changed between 0 and 180 degrees and then between 180 and 0 degrees according to a sine wave function. Alternatively the output may be D.C. Control circuit, Fig. 9.-An oscillator 58 supplies clock pulses at twice the h.f. switching rate to drive a flip-flop 59 connected to an amplifier 61 which provides base signals for the input side transistors Q1 to Q4. The oscillator pulses are also fed to a phase shifter 62 which actuates a second flip-flop 63 connected to a base signal drive amplifier 64 for the output side transistors. The output drive 63 is thus synchronized with the input drive 59 but is appropriately phase delayed by the shifter 62. Voltage regulation.-The output voltage is sensed by a transformer 48, is rectified and compared in a differential amplifier 68 with a pre-set level VR eg and fed to the phase shifter 62 which causes the output side transistor to change conduction states at a different time from the input side transistors, in either leading or lagging sense. Fig. 4 (not shown) illustrates the output voltage and transformer voltage waveforms for different degrees of phase difference between input and output transistor switches. For 90 degrees phase difference for instance the output voltage is zero. Current regulation.-Load current is sensed by a C.T. 49 and compared with a desired value IR eg in a circuit 71 and a difference value output operates the phase shifter 62 to adjust the output voltage and hence load current. Alternatively the load current signal is compared with a desired current limit value I Lim in circuit 71, the phase shifter operating to reduce the output voltage and current, overriding the voltage regulator. Overcurrent protection.-If overcurrent sensed persists for a time determined by a timer 73 (e.g. 20 low frequency input cycles) a base signal modifying circuit 74 sends a signal to drive amplifiers 61, 64 to cut off base signals to all the transistor switches, or to the input side transistors only, preferably at load current zero to minimize reactive current. Alternatively, circuit. 71 may operate the phase shifter 62 if excess current persists for a pre-set time, to give a full phase shift of 90 degrees between input and output side switching causing zero output voltage. After a time delay to allow reactive current to die all the transistors are turned OFF.

Power converter circuits having a high frequency link

In power transistor switching circuits, shunt snubbers (dv/dt limiting capacitors) are often used to reduce the turn-off switching loss or prevent reverse-biased second breakdown. Similarly, series snubbers (di/dt limiting inductors) are used to reduce the turn-on switching loss or prevent forward-biased second breakdown. In both cases energy is stored in the reactive element of the snubber and is dissipated during its discharge. If the circuit includes a transformer, a voltage clamp across the transistor may be needed to absorb the energy trapped in the leakage inductance. The action of these typical snubber and clamp arrangements is analyzed and applied to optimize the design of a flyback converter used as a battery charger.

Selection of Snubbers and Clamps to Optimize the Design of Transistor Switching Converters

It is generally necessary to connect an RC snubber across a power rectifier or thyristor to absorb the energy associated with the recovery current of the device and limit the resulting voltage spike and rate of rise dv/dt. For a given snubber capacitance, it is shown that there is an optimum damping resistance which minimizes the peak voltage, but a lower resistance is required to minimize the average dv/dt to the peak. Design procedures are derived for selecting the capacitance and optimum resistance to limit the peak voltage or dv/dt to specified values. The device recovery current is trapped in circuit inductance, and its energy must be dissipated, while the snubber produces additional losses as the price of performing its limiting function.

Optimum Snubbers for Power Semiconductors

To obtain fast response to a cyclical AC or variable DC input reference signal by a stepped-wave power converter circuit comprising a plurality of equal or unequal inverter modules connected in series or parallel so that the module output voltages add algebraically, the composite instantaneous output stepped-wave voltage is compared with the instantaneous voltage values of the limits of a control band placed about the reference signal. One or more inverter modules are switched to produce a step change whenever the composite output voltage is outside the control band limits. An application is a fast response switching power amplifier for driving sonar transducers.

Fast response stepped-wave switching power converter circuit

The two gate turn-off thyristors in the phase leg of a voltage-source inverter are provided with a series center-tapped inductance snubber and shunt capacitor snubbers having a single discharge resistor and only two auxiliary diodes. This arrangement minimizes the number of components and allows reduction of capacitor size, thereby reducing the snubber losses. The resistor may be replaced by a transformer which conserves a substantial part of the energy trapped in both the series and shunt snubbers.

William McMurray

Papers

Power converter circuits having a high frequency link

Selection of Snubbers and Clamps to Optimize the Design of Transistor Switching Converters

Optimum Snubbers for Power Semiconductors

Fast response stepped-wave switching power converter circuit

Efficient Snubbers for Voltage-Source GTO Inverters