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This paper presents a structured approach for analysing the effect of HVDC transmission system expansion on the small signal dynamics. The methodology decomposes the expansion of the system into a sequence of steps and aims especially at identifying the eigenvalues associated with interactions between the initial system and an added converter terminal. The procedure is illustrated with an example...
This paper presents a small signal eigenvalue analysis applied to a droop-controlled HVDC terminal based on the Modular Multilevel Converter (MMC) topology. The applied linearised model is derived from previous modelling efforts recently proposed in the literature, which rely on the application of three Park transformations at different frequencies (ω, −2ω and 3 ω) applied to associated variables...
This paper presents a protection scheme for multi-terminal Voltage Source Converter (VSC)-based HVDC systems, designed to detect and clear cable-to-ground faults in a range from very low to relatively high impedance. The protection scheme consists of two complementary and integrated layers: a protection layer for low impedance faults without communication between the terminals and a slower layer for...
In order to predict the stability of cable-based offshore Voltage Source Converter High Voltage Direct Current (VSC HVDC) systems, an accurate cable model is needed to represent the characteristics of the transmission system. This paper presents a model consisting of cascaded pi-sections with multiple longitudinal parallel branches, allowing for a detailed representation in the frequency domain. Furthermore,...
The stability range of power transfer in high impedance, weak, grids with Voltage Source Converters (VSC) can be extended a simple modification of the conventional Synchronous Reference Frame Phase Locked Loop (PLL) used for grid synchronization. By introducing an impedance-conditioning term in the voltage used by the PLL, the VSC control system is virtually synchronized to the voltage at a stronger...
The realization of multi-terminal HVDC grids requires availability of adequate fault handling methods. This includes techniques for fault detection and location, and dc-breakers for faults clearing. The characteristics and consequences of dc-side faults are very different compared to ac-system faults, and conventional ac-protection methods are not necessarily suited in HVDC grids. This paper presents...
The main purpose of the paper is to investigate the effect of state-space modelling fidelity on the small signal stability characteristics of HVDC systems based on Voltage Source Converters (VSCs). For this purpose, a small-signal state-space model of a point-to-point VSC-based HVDC connection, including the dynamics of the ac networks and the converter control loops is developed, and verified by...
This work presents a general approach for calculating the circulating current references of Modular Multilevel Converters (MMC) based on mathematical optimization by means of Lagrange Multipliers in the ABC frame. The resulting control method is especially suitable for operation of MMCs in multi-terminal HVDC systems, under both balanced and unbalanced AC grid voltages, since it is designed for eliminating...
This paper presents selected case studies of small-signal stability analysis for power systems consisting of interconnected AC and DC sub-systems. The analysis is based on the Cigré DC-grid test system, which is designed to resemble a generalized future configuration of multi-terminal HVDC (MTDC) systems. The small-signal stability of the system is investigated to identify critical modes associated...
This work presents a generalized and versatile approach for controlling Modular Multilevel Converters (MMC), with three or more phases, by means of Lagrange Multipliers in the ABC frame. The resulting control method is suitable for operation of HVDC converters under both balanced and unbalanced AC grid voltages, since it is designed for eliminating oscillations in the DC power flow independently of...
This work presents a simple approach to eliminate DC power ripple in a three-phase Modular Multilevel Converter (MMC) while operating with balanced (load) currents during unbalanced grid faults. In this study, the MMC is connected to an AC grid on one of its terminals, and to a constant ideal DC source on the other, while a single phase fault takes place. The presented “first” approach is based on...
It will be important for the operation of future multi-terminal HVDC systems that each converter unit is capable of limiting the influence of nearby AC faults on the DC grid. Thus, the local control systems at each terminal should be able to prevent double frequency active power oscillations caused by unbalanced AC faults from propagating to the DC-side of the converter. A simple approach for achieving...
This paper proposes a new and explicitly frequency-adaptive method for Virtual Flux estimation and voltage sensor-less grid synchronization under unbalanced conditions. The proposed system is based on using Second Order Generalized Integrators, arranged to simultaneously fulfill the purposes of frequency-adaptive band-pass filtering, integration and quadrature signal generation. This results in a...
This paper presents the application of space-vector based hysteresis current control in the synchronous reference frame for grid connected voltage source converters. The space-vector based approach allows for systematic application of zero-voltage vectors and prevents high switching frequencies caused by phase interaction. The average switching frequency can be minimized by always selecting the zero-voltage...
This paper investigates the operation of a current controlled Voltage Source Converter (VSC) under weak grid conditions caused by large grid impedance. Three different current controller structures are investigated; the conventional decoupled PI-controller in the synchronously rotating reference frame, Proportional Resonant (PR) controllers in the stationary reference frame, and the phase current...
This paper investigates how different approaches to grid synchronization and reference frame orientation can improve the control performance of a voltage source inverter operating in a weak grid. It is first shown how voltage-sensor-less operation based on Virtual Flux allows for flexibility in how to choose the reference frame orientation of the control system. The different possible orientations...
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