A three-level double-phase IGCT module as used in the PCS6000

Date: Mar 09, 2025 Views: 595

　　Inside the wind-power converter

　　The main building blocks of the converter are the two inverter modules connected by the DC link. The basic circuit diagram also shows auxiliary circuits such as the grid-filter circuit and the edge filter on the generator side. Three-level inverters are commonly used in medium-voltage industrial converters. The transformer can be designed to easily withstand the dv/dt of the switching IGCTs. A grid filter is necessary, however. Its main function is the limitation of harmonic currents to a level that enables IEEE 519-1992 to be fulfilled on even very weak grids. Using an extended filter, the German “VDEW Guideline” can also be met. The grid filter is an LC filter in combination with a special damping circuit for the lowest-order harmonic. On the generator side, a small dv/dt filter limits the rate of voltage rise at the generator terminals.

　　shows the basic circuit diagram of the four-quadrant three-level wind power converter. The power from the inverter unit (INU) is rectified to the DC link and from the DC link with the active rectifier unit (ARU) to the grid. The DC link is protected by a voltage limiter unit (VLU) for smooth uninterrupted operation of the turbine during a fault ride-through situation on the grid. This avoids torque oscillations for the turbine during a grid disturbance.

　　In normal operation, two semiconductor switches in each phase are always in the blocking state. This allows operation at twice the DC-link voltage of a two-level converter using the same elements. Compared to the series connection of elements, the three-level configuration has substantial advantages: The neutral-point diodes guarantee the voltage-sharing between the two blocking IGCTs without the need for special voltage sharing networks. Additionally, the neutral-point potential may also be switched to the output terminals. This results in smaller voltage steps at the output and a lower current ripple. Compared to a twolevel converter with the same average switching frequency of the individual IGCTs, the ripple is four times lower. This very low current ripple significantly reduces the torque ripple on the generator side and therefore the load at the planetary gearbox.

　　Benefits of the PCS6000 4Q-topology

　　The ABB wind power converter for full-scale power conversion is based on the four-quadrant concept, ie, a bi-directional power flow is possible in principle. The application itself doesn’t require a bi-directional power flow, ie, the generator related converter part could be realized as a unidirectional converter. Before the turbine is erected offshore, detailed tests of the gearbox and the generator are performed. The PCS6000 converter starts the generator as a motor, powering the gearbox via the driving collar while at the same time providing reactive current for the generator windings for a heat-run test. During these in-factory tests, all important protection and cooling systems are tested and adjusted. The bi-directional power flow allows the positioning of the turbine rotor to an exact position where the rotor can be locked with the hydraulic brake for investigations of the rotor blades or the pitch system. The upper control system deter mines the desired position for the rotor. It communicates with the converter controller using the PROFIBUS protocol. The PCS 6000 converter can move the rotor to the desired position smoothly and with high accuracy – even during very strong winds.

　　Encoders are known to cause failures. The PCS6000 wind converter operates without encoder to reduce maintenance and ensure high availability. A special feature is the soft-start routine for the main transformer. The DC link of the PCS6000 converter is precharged with a small pre-charge unit to take the voltage up to the nominal DC link level. Then, the grid side converter (ARU) slowly ramps the voltage up and synchronizes the transformer to the grid without causing current inrush. This feature helps connect large transformers to weak grids and prevents voltage dips caused by direct switched transformers in a wind farm. Another main benefit of the full scale power converter is the capability to provide reactive power to the grid. Additional reactive power compensation equipment is not needed, as is the case for traditional wind turbines with double feed induction generators. The PCS6000 wind converter is able to inject and absorb reactive power to control the voltage at the connection point to the grid.

　　Grid codes

　　The PCS6000 wind converter ensures continuous operation even during times of grid faults. The fast dynamic voltage control during balanced and unbalanced grid faults is a function of the PCS6000 wind converter necessary to achieve the grid codes. shows a real measured voltage dip during turbine operation with the prototype of the Multibrid M5000 wind turbine in Bremerhaven, Germany.

　　During extreme grid faults, the fullscale power converter has to provide 100 percent reactive current to support the grid. Therefore, the grid-side converter (ARU) cannot feed the active power from the generator into the grid. This would cause overvoltage in the DC link of the converter system and de-load of the generator because of normal protection functions. The PCS6000 wind converter system is equipped with a voltage limiter unit (brake chopper) that can dissipate the active power during the grid fault in such a way, that the turbine continues to run unaffected . The generator will not see any oscillations in the current (the current being an indicator for the actual load torque).

　　PCS6000 control system

　　The PCS6000 power converter controller receives run/stop signals from the turbine controller (master/slave system) using a digital link via PROFIBUS. Torque commands to the permanent-magnet synchronous generator (PMSG) are based on speed measurements with an update rate of at least 10 ms. A serial link connects the standard industrial programmable logic controller (PLC) master controller to the converter controller AC800 PEC.

PCS6000.pdf

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