Sidney Gierschner

A competitive medium frequency AC distribution grid for offshore wind farms using HVDC

HVDC transmission becomes more important for wind farms located over hundred kilometres from the shore. For offshore wind farms decoupled by HVDC transmission, frequency is a degree of freedom. State of the art for offshore wind farms is a 50Hz AC distribution grid. In this paper the effects of a higher line frequency on the main parts of an offshore wind farm are investigated. The main advantages and drawbacks are revealed and compared to a wind farm topology using a DC distribution grid. A loss calculation for a whole wind farm was done to highlight the opportunities and restrictions. Figure 1 shows an offshore wind farm with HVDC transmission.

Current-direction detection for static MOS-control of the BIGT in the three-level neutral-point-clamped converter

The gate-emitter voltage VGE affects the on-state characteristics of the BIGT in diode forward-conduction mode. While for conventional IGBT with antiparallel free-wheeling diode VGE is at 15 V regardless of current direction, it has to be adapted for the BIGT. The gate-emitter voltage VGE has to be below threshold voltage, in order to operate at low static losses in diode forward-conduction mode. This paper proposes an optimised gate control based on the direction of the load current for a three-phase three-level converter.

Lifetime estimation for three level converter for wind energy application

The rated power of wind energy plants increases but generators and generator side converters still operate at low voltage level. This leads to high currents and so to large cable cross-sections and high cable losses. The change to medium voltage level overcomes these drawbacks but will make the use of multilevel converters necessary. The Neutral-Point-Clamped three level converter is a common multilevel converter. Important aspects about the converter are the power cycling stress and the lifetime analysis of the power semiconductors. This paper investigates the power cycling stress and the lifetime of the power semiconductors of three level converter topologies for wind energy application. For the investigation a real wind speed over time profile measured at FINO 1 station in the German North Sea has been used. Due to this it is possible to consider the influence of power cycling stress caused by the fundamental frequency of the generator as well as the power cycling stress caused by variations of the wind speed on the lifetime of the power semiconductors. For counting the total number of cycles, which includes the small fundamental frequency cycles and the large mission profile cycles, the Rainflow algorithm is used. For the fundamental frequency cycles only the maximum and the minimum temperature within a period of the fundamental frequency are extracted from the temperature over time profile. The counted cycles are used to estimate lifetime consumption due to fundamental frequency cycles and mission profile cycles.

High-inductive short-circuit Type IV in multi-level converter protection schemes

To build a redundant medium-voltage converter, the semiconductors must be able to turn OFF different short circuits. The most challenging one is a hard turn OFF of a diode which is called short-circuit type IV. Without any protection measures this short circuit destroys the high-voltage diode. Therefore, a novel three-level converter with an increased short-circuit inductance is used. In this paper several short-circuit measurements on a 6.5 kV diode are presented which explain the effect of the protection measures. Moreover, the limits of the protection scheme are presented.

Comparison of different control techniques for grid side VSC in terms of losses and current harmonics

More and more wind power plants are built which use voltage source converters (VSC) to feed energy into grid. This paper compares two different control techniques and shows how virtual flux direct power control (VFDPC) reduces filtering requirements at the output of the converter and helps to save up to one quarter of converter losses. Furthermore the influence of a reduced, variable dc link voltage of the wind power plant itself and its converter control is discussed.

Potential of RC-IGBTs in three level converters for wind energy application

At medium voltage level the three level NPC converter is commonly used. As generator side converter in wind energy applications the diodes are most stressed and limit operation. The BIGT combines the functionality of the IGBT and the free-wheeling diode in one chip. The larger usable chip area in both IGBT and diode mode decreases the maximum junction temperature as well as the junction temperature ripple compared to a conventional IGBT/diode module at the same output power. This leads to reduced power cycling stress and improved lifetime. A lower maximum junction temperature enables an increase in output power. The potential of the BIGT in the NPC converter for wind energy applications is investigated and compared to the conventional IGBT/diode module for a real wind speed over time profile measured at FINO 1 station in the German North Sea.

Active rectifier with extended operating range

To overcome the drawbacks of diode rectifiers in applications without regenerative functionality active rectifiers can be used. Three level rectifiers are already known, but operating range is limited. A new four level rectifier topology that has an extended operating range and avoids all drawbacks of diode rectifiers is presented.