Pierluigi Tenca

Diode-clamped multilevel converters: a practicable way to balance DC-link voltages

The converter topologies identified as diode-clamped multilevel (DCM) or, equivalently, as multipoint clamped (MPC), are rarely used in industrial applications, owing to some serious drawbacks involving mainly the stacked bank of capacitors that constitutes their multilevel DC link. The balance of the capacitor voltages is not possible in all operating conditions when the MPC converter possesses a passive front end. On the other hand, in AC/DC/AC power conversion, the back-to-back connection of a multilevel rectifier with a multilevel inverter allows the balance of the DC-link capacitor voltages and, at the same time, it offers the power-factor-correction capability at the mains AC input. An effective balancing strategy suitable for MPC conversion systems with any number of DC-link capacitors is presented here. The strategy has been carefully studied to optimize the converter efficiency. The simulation results related to a high-power conversion system (up to 10 MW) characterized by four intermediate DC-link capacitors are shown.

Current Source Topology for Wind Turbines With Decreased Mains Current Harmonics, Further Reducible via Functional Minimization

The paper presents a current-source inverter topology tailored for large multi-megawatt wind turbines. The proposed topology can inherently benefit from the distance between the generator and the mains because the consequent length and possible layout of the power cables may enable the realization of a significant portion of the dc-link inductance. In order to improve the efficiency and to allow the possible utilization of rugged inexpensive thyristors, pulsewidth modulation (PWM) modulation is not used. Unity fundamental power factor at the mains is guaranteed at any load condition while the fifth and seventh harmonics of the mains line currents can be reduced by proper system design at a desired turbine speed, considered most suitable for its operation. Further harmonic reduction is achievable through an active filter controlled via a newly proposed PWM methodology that does not belong either to a carrier-based or to a classical space vector modulation approach. Such a controller relies on a real-time minimization of a proper functional and is capable of implementing true-feedback current regulation. Experimental results from a 10 kW prototype are presented and validate the developed analytical computations.

Wind Turbine Current-Source Converter Providing Reactive Power Control and Reduced Harmonics

This paper presents a current-source inverter topology that is suitable for multi-megawatt wind turbines. The proposed scheme utilizes two series-connected three-phase inverters that employ fully controllable switches and a proper interconnection transformer with the mains. In order to improve the efficiency and to allow the use of high-power devices, the inverters are switched at the mains frequency. The axial-flux permanent-magnet (PM) generator is directly coupled to the turbine (gearless solution), and its design reduces the dependence of the output voltage on the load current. The overall control technique allows to independently impose two desired quantities that can be selected out of the set of three composed of: 1) the total average voltage at the dc side of the inverters, which is directly related to the turbine speed; 2) the fundamental power factor at the mains interconnection point, which can be chosen unitary, leading, or lagging; and 3) the amplitude of one desired component of the spectrum of the mains line currents. The two chosen quantities univocally determine the third one. At specific operating points of the turbine, a significant reduction of the fifth and seventh harmonics can already be achieved without additional filters and/or active harmonic compensation. Nevertheless, the introduction of an active harmonic compensator is necessary to provide the required harmonic reduction (also up to higher orders) more independently and on a wider range of operating conditions. The almost independent regulation of the dc-link current allows further control of the average generator torque. Experimental results that are obtained from a 10-kW prototype with an axial-flux PM generator are presented.

About the DC-link capacitors voltage balance in multi-point clamped converters

The multi-point clamped converter (MPCC) represents an interesting technical solution to overcome the limitations in voltage and power posed by the existing semiconductor devices. However, if real power must be delivered to the load, DC-link capacitors exhibit a problematic voltage unbalance, that can be faced in both active or passive ways, that is, by using or not using additional components. Both approaches can be used together, in order to optimize the system performances. The authors have analyzed this problem from an analytical point of view and have developed a modulation control strategy that exploits the inherent redundancy in switching configurations to achieve a correct voltage sharing, in the case of a MPCC with four DC-link capacitors. The results obtained are presented and discussed in a critical way and they are correlated to the case where an active rectifier is used as an input stage to achieve near unity power factor and capacitors voltage balance.

Reduced cost current-source topology improving the harmonic spectrum through on-line functional minimization

The paper presents a current-source inverter (CSI) topology tailored for large multi-megawatt wind turbine applications. The cable distance between the generator and the mains enables the realization of a significant portion of the DC-link inductance. In order to improve the efficiency and to allow the possible utilization of rugged inexpensive thyristors, PWM modulation is not used in the main conversion chain. Unity fundamental power factor at the mains is guaranteed at any load condition while the 5/sup th/ and 7/sup th/ harmonics of the mains line currents are reduced by choosing a proper nominal operating point for the turbine. Further harmonic reduction is achieved through an active filter controlled via a newly proposed methodology suitable for digital signal processor (DSP) implementation. Such a controller relies on a real-time minimization of a proper functional and is capable of implementing true-feedback current regulation. A part of design simulation results, aimed at constructing a 10 kW prototype, are presented.

Low Voltage Ride-Through Capability for Wind Turbines based on Current Source Inverter Topologies

This paper proposes a circuital solution, aimed at a class of current-source inverter topologies, that provides the ability to ride-through temporary low voltage, as well as open circuit conditions at the mains without opening the mains circuit breakers. Although the proposed solution has broader applications, it is described in the framework of a previously published current-source inverter topology for modern wind turbines. For such systems, the low voltage ride-through capability is becoming a necessary feature. The current source topology considered exploits the cable length between the nacelle and the ground to provide a significant portion of the DC-link inductance. Prior work discussed the control strategy, flexibility, design rules, protection and experimental results of this topology and acknowledged the need for low-voltage ride through capability. One possible solution, derived from an already proposed protection scheme, is presented together with selected simulation and experimental results.

Current-Source Topology for Wind Turbines Capable of Providing Leading Power Factor While Reducing Line Current Harmonics

The paper presents a current-source inverter topology conceived for multi-megawatt wind turbines. The proposed scheme utilizes two series-connected three-phase inverters employing fully controllable switches and a proper interconnection transformer with the mains. In order to improve the efficiency and allow the use of high power devices, the inverters are switched at mains frequency. The axial flux permanent magnet generator is directly coupled to the turbine (gearless solution) and possesses very low leakage inductance. The overall control technique allows to impose independently two desired quantities selected out of the set of three composed by: a) The total average voltage at the DC input side of the inverters; b) The power factor at the mains interconnection point, which can be chosen unitary leading or lagging; c) The amount of the 5th and 7th harmonics in the mains line currents. In specific operating points of the turbine a significant reduction of two such harmonics can be already achieved without additional filters and/or active harmonic compensation. Nevertheless the introduction of an active harmonic compensator can achieve proper harmonic reduction - also up to higher orders - more independently and on a wider range of operating conditions. Experimental results, obtained from a 10 kW prototype with an axial flux permanent magnet generator are presented

Proper Metrological Methodologies to Avoid Severe Systematic Errors When Characterizing High-Power IGBTs on the Installation Field: An Introduction

The exigency of characterizing high-power insulated-gate bipolar transistors (IGBTs) directly on-site increasingly emerges in field installations hosting multilevel converters that employ such devices by the dozen, when not by hundreds, as it is already the reality for modern modular voltage-source HVDC and flexible ac transmission systems (FACTS). Nonetheless, differently from the actualities in permanent laboratories, the typology of the instrumentation rapidly obtainable on the field is often rather restricted by the aspects of logistic, necessary timing of intervention, and ruggedness against the usually harsh environments. Commonly, the only specimens realistically expectable at short notice on a field installation are portable curve tracers (CTs), portable field oscilloscopes with nonisolated channels, passive voltage probes, current probes based on Rogowski coils, portable multimeters, and variable dc power supplies. This paper describes metrological methodologies devised to diagnose and characterize high-power IGBTs by employing solely such a limited set of instruments. The severely erroneous conclusions that possibly descend from the utilization of CTs only are highlighted, both by experiments and by a dynamic model deduced from the nature of the specific metrological context. It is shown how the additional presence of properly connected oscilloscopes and probes becomes mandatory in order to attain a proper characterization. The role of the CTs should be strictly limited to mere stimuli generators only, also because their compensation techniques against the systematic measurement errors occurring with signal and low-power transistors rarely suffice in the case of high-power devices. Additionally, the equivalent differential capacitance between the collector and emitter terminals of a high-power IGBT-diode module is characterized too. It is often insufficiently documented in the datasheets, although its values and evolution are important for the dynamics of the commutation. Experimental observations from an ABB 5SNA 1200G450300 IGBT module (4.5 kV and 1.2 kA) are discussed throughout the different sections in order to elucidate the details of the presented methodologies.

Conversion Topology for Reducing Failure Rate and Life-cycle Costs of High-Power Wind Turbines

The paper presents a Current-Source Inverter (CSI) topology tailored for large multimegawatt wind turbine applications. The cable distance be tween the generator and the mains enables the realization of a significant portion of the DC-link inductance. In order to improve the efficiency and to allow the possible utili zation of rugged inexpensive Thyristors, PWM modulation is not used in the main conversion chain. Unity fundamental power factor at the mains is guaranteed at any load condition while the 5t h and 7th harmonics of the mains line currents are greatly reduced by choosing a pr oper nominal operating point for the turbine. Further harmonic reduction is achieved through an active filter controlled via a newly proposed methodology suitable for Digital Signal Processor (DSP) implementation. Such a controller relies on the real-time minimization of a proper functional and is capable of implementing true-feedback current regulation. A par t of design simulation results, aimed at building a 10 kW prototype are presented together with the description of its experimental setup currently in construction.