Torsional dampers are employed in wind turbines to damp vibrations in the drive-train. The conventional design is based on band-pass filters (BPF); however, its effectiveness can be compromised due to parametric uncertainty. To restore the performance of the damper, it is a common practice to re-tune it during the commissioning of wind turbines. To overcome this shortcoming, a model-based torsional damper was designed and its performance compared to the conventional approach when subjected to model uncertainty. A stability analysis was conducted and simulations were performed in Simulink. A real-time hardware-in-the-loop experiment was carried out, with experimental and simulation results showing good agreement. The proposed model-based torsional vibration damper showed a superior performance over the conventional BPF-based approach. Results also showed that the model-based damper can eliminate the need for retuning procedures associated with BPF-based designs.

A high-voltage direct-current (HVDC) grid protection strategy to suppress dc fault currents and prevent overcurrent in the arms of modular multi-level converters (MMCs) is proposed in this paper. The strategy is based on the coordination of half-bridge (HB) MMCs and hybrid dc circuit breakers (DCCBs). This is achieved by allowing MMC submodules (SMs) to be temporarily bypassed prior to the opening of the DCCBs. Once the fault is isolated by the DCCBs, the MMCs will restore to normal operation. The performance of the proposed method is assessed and compared to when MMCs are blocked and when no corrective action is taken. To achieve this, an algorithm for fault detection and discrimination is used and its impact on MMC bypassing is discussed. To assess its effectiveness, the proposed algorithm is demonstrated in PSCAD/EMTDC using a four-terminal HVDC system. Simulation results show that the coordination of MMCs and DCCBs can significantly reduce dc fault current and the absorbed current energy by more than 70 and 90% respectively, while keeping MMC arm currents small.

Although the probability of occurrence of station internal ac grounding faults in modular multilevel converter (MMC) based high-voltage direct-current systems is low, they may lead to severe consequences that should be considered when designing protection systems. This paper analyzes the characteristics of valve-side single-phase-to-ground (SPG) faults in three configurations of MMC systems. Fault responses for symmetrical monopolar MMCs are first studied. Upper arm overvoltages and ac side non-zero-crossing currents arising from SPG faults in asymmetrical and bipolar configurations are then investigated. DC grounding using an LR parallel circuit is employed to create current zero-crossings which will enable the operation of grid side ac circuit breakers. The theoretical analysis is verified through simulations performed in PSCAD/EMTDC, with simulation results and the theoretical analysis showing a good agreement. The studies in this paper will be valuable for the design of protection systems for station internal ac grounding faults.

Controllers for a pitch and a stall regulated horizontal axial flow, variable-speed tidal stream turbine are developed, and a performance comparison is carried out. Below rated flow speed, both turbines are operated in variable-speed mode so that the optimum tip-speed ratio is maintained. One of the turbines has variable pitch blades, which above rated speed are pitched to feather in order to regulate power. The other turbine has fixed pitch blades and uses speed-assisted stall to regulate power. The control system design behind both strategies is examined in MATLAB, with the performance under turbulent flows, loading and energy yield analysis being evaluated in GH Tidal Bladed. Both strategies provide a satisfactory performance, but the out-of-plane loads on the stall regulated turbine were higher over the entire range of operation. In addition, the dynamic characteristics of the stall regulated turbine require a more complex control design. The results suggest that the pitch regulated turbine would be a more attractive solution for turbine developers.

Bitumen tanks were tested to investigate the capability of industrial heating loads to provide frequency response to an electric power system. A decentralized control algorithm was developed enabling the tanks to alter their power consumption in proportion to the variations of grid frequency. The control maintains the normal operation of tanks and causes little impact on their primary function of storing hot bitumen. Field investigations were undertaken on 76 tanks with power ratings from 17 to 75 kW. A model of a population of controlled tanks was developed. The behavior of the tanks was compared between the simulations and the field tests. The model of controlled tanks was then integrated with a simplified Great Britain power system model. It was shown that the controlled tanks were able to contribute to the grid frequency control in a manner similar to and faster than that provided by frequency-sensitive generation. contribute to the grid frequency control in a manner similar to and faster than that provided by frequency-sensitive generation.

Mathematical modeling of synchronous and induction generators for wind turbines using state-space representations is presented. Emphasis is given to those models suitable for control schemes of variable-speed wind turbines and their application for different power system studies. The state-space representations provide a convenient way to assess different configurations of fixed and variable-speed wind turbines based on synchronous and induction generators. The modeling approach here presented allows both transient and small-signal stability analysis. As a case study, the performance of fixed and variable-speed wind turbines under faults and voltage sags is assessed. The state-space models are used to investigate the capability of different wind turbine technologies to satisfy Grid Code requirements. An eigenanalysis is included to show the flexibility of the models.