Ralph Feldman

A Hybrid Modular Multilevel Voltage Source Converter for HVDC Power Transmission

HVDC transmission systems are becoming increasingly popular when compared to conventional ac transmission. HVDC voltage source converters (VSCs) can offer advantages over traditional HVDC current source converter topologies, and as such, it is expected that HVDC VSCs will be further exploited with the growth of HVDC transmission. This paper presents a novel modular multilevel converter hybrid VSC intended for the HVDC market. The concept of the converter operation is described based on steady-state ac-dc power balance. Techniques for dynamic voltage control, enabling the active and reactive powers exchanged with the grid to be controlled, are introduced. Simulation results further illustrate the theory of operation of the converter and confirm the viability of the proposed control approaches. Detailed predictions of the semiconductor losses confirm the potential to achieve very high efficiencies with this topology. Experimental results are provided to validate the presented converter operation.

A sub-module capacitor voltage balancing scheme for the Alternate Arm Converter (AAC)

A per-phase sub-module capacitor voltage balancing scheme for the Alternate Arm Converter (AAC) is proposed. Leg balancing regulates the sum of sub-module capacitor voltages in a leg to its set-point. Arm balancing regulates each sub-module capacitor voltage to the nominal sub-module capacitor voltage. Additional converter control requirements and the modulation scheme are also discussed. The control and modulation scheme has been verified by simulation. The leg balancing control loop was designed to be critically damped to primarily limit oscillations in the output variable caused by changes in AC-side power demands; the simulation results indicated this specification was met.

State-space switching model of modular multilevel converters

This paper presents a comprehensive summary and detailed analysis on the modeling of modular multilevel converters for High Voltage Direct Current (HVDC) transmission, and develops a state-space switching model for the now well-known Modular Multilevel Converter (MMC), and also for the more recently developed Alternate Arm Converter (AAC). Various models used for the MMC are firstly discussed, followed by the complete derivation procedure of the state-space switching models of both MMC and AAC. The modeling approach used is based on the premise that only state variable transformations and simple voltage and current equations could be formulated, allowing it to capture all circuital information. The only assumption in the derivation is the ideal nature of the switching devices. The paper presents as well an in-depth analysis of the models developed, comparing the MMC and AAC at their most fundamental level of operation.

A high-power DC-DC converter based dual active bridge for MVDC grids on offshore wind farms

This paper presents the steady-state analysis of a high power step-up DC-DC converter based three-phase dual active bridge (3DAB) for use as a medium voltage (MV) DC-DC collector of offshore wind farms. An optimization procedure for a high-power medium frequency transformer is explained and moreover, a design of an optimal control phase shift angle is explained and verified through simulation. The comparisons with two scenarios are presented: 4MW converters for power conversion from low dc voltage (LVDC) of the wind turbine output terminal to 40kV MVDC grid of offshore wind farms. The proposed 3DAB DC-DC converter is investigated for the given scenarios in terms of losses of the semi-conductors and the magnetic part as well as the quantity of semi-conductors.

Active Stator Variable Speed Drive: 120 kW DC-fed demonstrator

Current requirements in multi-megawatts electric marine propulsion systems and wind power generation, particularly in off-shore plants, ask for drives with increased power densities. The recently proposed Active Stator (AS) Variable Speed Drive is a clear answer to this demand. A power density increase of up to 2–3 times is anticipated compared to the current state-of-the-art equivalent drives. Such a major leap forward is primarily achieved through the use of a machine with trapezoidal distribution of the air-gap flux density and through the integration of the machine power electronics into the machine frame, as is suggested by the expression Active Stator. The AS topology is well suited to both AC and DC supply types. This is an important feature when the previously mentioned applications of the AS topology are considered, as both are seemingly open to both types of supply. This paper describes the AS topology and reports the main results obtained from a DC-fed 120 kW demonstrator purposely built to prove the AS topology.

The Alternate Arm Converter (AAC)—“Short-Overlap” Mode Operation—Analysis and Design Parameter Selection

This paper presents converter operation principles and theoretical analyses for “short-overlap” mode operation of the alternate arm converter (AAC), which is a type of modular multilevel voltage source converter that has been proposed for HVdc transmission applications. Fourier series expressions for the ideal arm current and reference voltage are derived, for the first time, in order to develop an expression for the submodule capacitance required to give a selected peak–peak voltage ripple of the summed submodule capacitor voltages in an arm. The dc converter current contains nonnegligible low-order even harmonics; this is verified by deriving, for the first time, a Fourier series expression for this current. As the dc converter current needs to be filtered to form a smooth dc grid current, a novel dc filter arrangement is proposed, which uses the characteristics of a simplified dc cable model, as well as the capacitance of the dc link and additional dc-link damping resistance, in order to form a passive low-pass filter. Results obtained from a simulation model, which is based on an industrial HVdc demonstrator, are used in order to verify the presented converter operation principles and theoretical analyses.