Sverre Skalleberg Gjerde

Control and Fault Handling in a Modular Series-Connected Converter for a Transformerless 100 kV Low-Weight Offshore Wind Turbine

A transformerless wind power generator concept with the potential to achieve 100 kV dc output is proposed in this paper. In this paper, the modular ac/dc converter system suitable for such a high-voltage generator is analyzed for normal operation and fault-tolerant modes. The control synthesis is addressed, based on an assumption that the control can be designed modularly. Additionally, the investigated control system can facilitate fault-tolerant operation without changing the main structure. The robustness of the module control system is analyzed, and the limits for fault-tolerant operation are investigated. The theoretical studies are supported by transient analysis through simulations of the turbine in EMTDC/PSCAD. Finally, the generator/converter solution is verified experimentally. The laboratory results were obtained using a 45 kW prototype with three generator segments and three converter units in series.

Multilevel converters for a 10 MW, 100 kV transformer-less offshore wind generator system

The nacelle weight reduction is a key design criterion for offshore wind turbines, when these are reaching towards 10 MW. To overcome the weight challenge, a transformer-less concept is under development. This concept employs a special permanent magnet synchronous generator (PMSG) with an innovative system for high insulation level. The generator supplies nine series connected converter modules, which results in a high voltage DC output of 100 kV, reducing the total weight of the system. The work presented here focuses on one of the 11.1 kV converter modules, and identifies the modular multilevel converter (MMC) as the best suitable candidate for the proposed system. The comparison criteria are qualitative: no direct series connection of IGBTs, converter efficiency, voltage quality and redundancy for increased reliability. Simulations in PSCAD/EMTDC are presented, with focus on redundancy, submodule voltage balancing, and implementation of the MMC in full system models. The results show that the MMC performs well in the full system, and is therefore considered as a viable converter for the proposed system.

A Transformerless generator-converter concept making feasible a 100 kV light weight offshore wind turbine: Part I - The generator

A novel HVDC technology which enables lightweight HVDC machines is described in two papers: Part I describes the concept of a segmented HVDC generator and some of its possible designs. Part II addresses the modular AC/DC-converter. In both papers the HVDC concept is applied in an electric drive for an offshore wind turbine. The HVDC technology is a modularized electric drive train in which the connection between VSC converter modules and machine modules will have a great influence on the required insulation thickness in the machine. Two main effects are obtained: Better insulation coordination within the machine and the possibility to exploit the benefits of HVDC insulation. A case study indicates the possibility to make a transformerless 100 kV DC electric drive for offshore wind turbines. The high voltage is achieved at the very first steps in the energy chain. For moderate transmission distances no further voltage transformation steps is necessary, thus minimizing the needed components for offshore energy transmission and increasing the total efficiency of the system.

Fault tolerance of a 10 MW, 100 kV transformerless offshore wind turbine concept with a modular converter system

This paper presents the investigation of fault tolerance for a transformer-less generator/converter concept suitable for 10 MW offshore wind turbines. The fault tolerance is necessary to achieve good availability for the turbine, since the accessibility can be sparse for turbines located far offshore. Based on the identified opportunities, changes to the control of the converter and turbine are proposed to exploit the inherent fault tolerance possibilities in the converter system. The transient response to faults, bypass and continued operation of the turbine was investigated through simulations. An estimate of the availability of the turbine with and without fault tolerance is presented. The estimate indicates that the redundancy will be beneficial, despite increased cost.

The best suitable multilevel converters for offshore wind power generators without transformers

Offshore wind farms are facing challenges which call for redesigning the electrical drive train of future wind turbines. The turbines are expected to increase in power, and plans for 10–15 MW wind turbines have been reported. Hence will a medium-to high voltage converter solution be highly beneficial. Additionally, a DC-distribution grid is discussed for offshore applications. This indicates that the applied converter should have highly controllable DC-output. Also, the converter should include fault redundancy, as the availability for maintenance is limited. Based on the constraints imposed by the application, this paper evaluates which type of converter is the most suitable for offshore wind turbines.

Modelling wind farm with synthetic inertia for power system dynamic studies

The requirement to increase system wind power penetration while providing better frequency quality has necessitated the wind turbines to emulate inertial response of the classical synchronous generators. This paper presents a simulation study done in order to understand the main factors that affect the potential for inertia emulation in a variable speed variable pitch wind turbine. A small power system consisting of three synchronous generators and one wind farm is modelled. Dynamic models are applied for all components in order to gain a more realistic understanding of the underlying phenomenon. The results show how the applied inertia emulation scheme influences the operation of the wind turbine. It is found that while the inertia time constant dominates inertia emulation in an under frequency event, the dynamics of the pitching system dominates inertia emulation in an over frequency event. The study provides an insight into how to design the inertia controller.

Control of direct driven offshore wind turbines in a DC-collection grid within the wind farms

Recent research has been directed towards a DC-collection grid for offshore wind farms with an HVDC-connection to shore. The main motivation is to reduce the of number of conversion steps, and avoid the reactive power problem associated with AC-cables. However, this demands for changes in the control systems for the wind turbines, as DC-grids have different characteristics, both with regards to the grid itself, and the behavior of the grid-connected converters. The control of a direct driven wind turbine connected to a DC-collection grid through a voltage source converter is investigated in this paper. And more precisely, a control scheme for regulating the collection grid voltage is employed and evaluated. The behavior is documented through simulations in EMTDC/PSCAD and the consequences of the operation strategy are discussed.