Kevin J. Dyke

Electrical Oscillations in Wind Farm Systems: Analysis and Insight Based on Detailed Modeling

This paper presents modeling and analysis of electrical oscillations in a wind farm system. The detailed modeling and modal analysis of a wind farm system are presented in this paper. The approach to modeling uses detailed representation of a wind turbine generator and collection system including high-voltage direct-current (HVDC) power converter system control, facilitating a comprehensive analysis of the wind farm system. Various modes are classified according to the frequency of oscillation. The detailed modal analysis is used to characterize the critical modes. Time-domain simulation also confirms the presence of these modes. The effect of wind farm operating conditions and voltage source converter control tuning on critical oscillatory modes are also assessed and discussed in detail.

The use of trapezoid waveforms within converters for HVDC

The Controlled Transition Bridge (CTB) is a converter topology that combines series connected semiconductor “director switches” with chains of switched capacitor modules, chainlink circuits, in such a way that the director switches carry the main current for a significant portion of the period and the chainlink elements provide a controlled traverse of voltage between different director switches conducting. The simplest example of this is where the director switches form a six pulse bridge and the chainlink elements traverse at a constant rate between the upper director switch conducting and the lower director switch conduction etc., so that the output AC waveform is a trapezoid. The use of a trapezoid waveform reduces the level of super harmonics significantly and with a star delta transformer to remove the “triple N” harmonics, the total harmonic distortion is reduced, but not sufficiently for use in HVDC application. The use of filtering is undesirable because of the VARs they introduce and while active filtering can be used there are control difficulties that need to be overcome, so a two slope trapezoid waveform is proposed in which the slope characteristics are chosen specifically to minimise a wide range of harmonics for a given fundamental magnitude. For this a cost function is derived that includes the functions of the harmonics being considered and a search is carried out using standard algorithms such as Newton-Raphson, to minimise its value within a given region. Modelling is used to demonstrate that the resulting primary THD would meet the requirements for VSC HVDC operation.

Cell capacitor sizing in modular multilevel converters and hybrid topologies

This paper presents a method to calculate the minimal size of cell capacitors in multilevel VSCs which meets a maximum voltage deviation criterion under ideal conditions. This method is applied to the Modular Multilevel Converter (MMC), the Alternate Arm Converter (AAC) and the hybrid multilevel converter with ac-side cascaded H-bridge cells (AC-CHB). The results show that the newer VSC topologies exhibits smaller energy deviation in their stacks, leading to an overall smaller volume of cell capacitors for the converter station but often accompanied by some compromises such as higher power losses or degraded DC current waveform quality.

Control of an alternate arm converter connected to a star transformer

This paper details a novel energy management technique that is applied to the Alternate Arm Converter (AAC), to ensure converter stability. The control strategy is modeled in Matlab/Simulink and tested on a HVDC active and reactive power operating envelope. To realize the commercial benefits of the AAC, stability in all normal operating conditions is verified. Simulations run over the PQ envelope are shown and the commercial benefits of the new energy management technique assessed.

The Adequacy of the Present Practice in Dynamic Aggregated Modeling of Wind Farm Systems

Large offshore wind farms are usually composed of several hundred individual wind turbines, each turbine having its own complex set of dynamics. The analysis of the dynamic interaction between wind turbine generators (WTG), interconnecting ac cables, and voltage-source converter (VSC)-based high voltage DC (HVDC) system is difficult because of the complexity and the scale of the entire system. The detailed modeling and modal analysis of a representative wind farm system reveal the presence of several critical resonant modes within the system. Several of these modes have frequencies close to harmonics of the power system frequency with poor damping. From a computational perspective, the aggregation of the physical model is necessary in order to reduce the degree of complexity to a practical level. This paper focuses on the present practices of the aggregation of the WTGs and the collection system, and their influence on the damping and frequency characteristics of the critical oscillatory modes. The effect of aggregation on the critical modes is discussed using modal analysis and dynamic simulation. The adequacy of aggregation method is discussed.

The augmented modular multilevel converter

The Controlled Transition Bridge (CTB) is a class of converter topology that combines series connected semiconductor “director valves” with chains of switched capacitor modules, “chainlink circuits”, in such a way that the director valves carry the main current for a significant portion of the period and the chainlink circuits provide a controlled traverse of voltage between different director valves conducting. This combination is applicable to HVDC where efficiency is paramount, since it allows thyristors or diodes to be used for the director valves to reduce the conduction losses, with the chainlink circuits providing commutation and direct control of the rate of change of transition voltage. Since the chainlink portion of the AMMC only has to manage the transition between the upper and lower director valves, the size of the capacitors in the individual sub-modules can be reduced, reducing the converter footprint. Also the trapezoidal waveform that results can be tailored to give reduced harmonic levels, so reducing filtering required for the AC waveform to meet regulations on distortion at the point of common coupling for the converter (PCC). An analysis is presented of an example of this type of converter where a modular multilevel converter (MMC) is combined with a conventional thyristor bridge, the Augmented MMC (AMMC). Various aspects of the operation of the bridge are discussed, including the management of the charge in the chainlink capacitors and the converter losses.

Alternate arm converter operation of the modular multilevel converter

A new operating mode of the Modular Multilevel Converter (MMC) using modified arm current waveforms inspired from the working principle of the Alternate Arm Converter (AAC) is presented in this paper. A reduction in the cell voltage deviation is observed at power factors close to unity at the cost of an increase in power losses, especially when reactive power is required. This gain in voltage margin is then used in further optimizations of the MMC performance, mainly focusing on either increasing the number of redundant cells or improving the overall power efficiency of the converter.

Choice of AC operating voltage in HV DC/AC/DC system

Demand for DC/DC conversion in HV applications is expected to rise because of the increasing number of HVDC links using different DC voltage levels. This paper presents a DC/AC/DC system consisting of two VSCs connected through an inductor. The two VSCs are Alternate Arm Converters (AAC). Since the two AACs share the same AC voltage level, they cannot be operated at their respective “sweet-spots” at the same time. This results in an energy drift in the valves which is tackled by additional balancing currents. However, the choice of the AC voltage level remains critical as it determines the required amount of balancing current, the number of devices and the cell topology, influencing greatly the total efficiency and volume of the obtained DC/DC converter. A study on a scale-down converter highlights the trade-offs affecting the AC voltage choice.

Stability Analysis of a PMSG-Based Large Offshore Wind Farm Connected to a VSC-HVDC

This paper presents modal analysis of a large offshore wind farm using permanent magnet synchronous generator (PMSG)-type wind turbines connected to a voltage source converter HVDC (VSC-HVDC). Multiple resonant frequencies are observed in the ac grid of offshore wind farms. Their control is crucial for the uninterrupted operation of the wind farm system. The characteristics of oscillatory modes are presented using modal analysis and participation factor analysis. Sensitivity of critical modes to wind turbine design parameters and their impact on closed loop stability of the system are discussed. A comparison between a full wind farm model and an aggregated model is presented to show differences in the characteristics of critical modes observed in the models, and implication of using the models for stability studies It is concluded that robust control design is important for reliable operation of the system.

The controlled transition bridge

The Controlled Transition Bridge (CTB) is a converter topology that combines series connected semiconductor “director switches” with chains of switched capacitor modules, “chainlink circuits”, in such a way that the director switches carry the main current for a significant portion of the AC power frequency period and the chainlink elements provide a controlled traverse of voltage between different director switches conducting. The simplest example of this is where the director switches form a six pulse bridge and the chainlink circuits traverse at a constant voltage rate between the upper director switch conducting and the lower director switch conduction etc., so that the output AC waveform is a trapezoid. A modified form of trapezoid waveform has been used to give both AC voltage magnitude control and reduced harmonic level. The paper presents the method of circulating the current through the bridge to give full control over power and reactive power while minimizing the power loss within the chainlink circuits. Aspects of the control of the converter are discussed including the sizing of the submodule capacitors within the chainlink circuits and the method of energy balance. Finally results are presented from a model of a full converter illustrating the different ways in which the current circulates through the converter bridge during the power ramp up.