Stability and interaction analysis in islanded power systems including VSC-HVDC and LCC-HVDC power converters

Islanded power systems are often connected to larger mainland power systems using HVDC cables. These interconnections are used to import power at lower cost compared to local generation and improve the security of supply. The increase of HVDC interconnectors in islanded systems will allow the reduction of local synchronous generation, which might lead to new interaction and stability problems due to the low inertia and short-circuit power available in the system. Traditionally LCC-HVDC technology has been used to connect island grids, but recently VSCs are presented as an alternative solution that offers more controllability to the islanded grid. Therefore, in order to increase the power transfer to the islands multi-infeed hybrid HVSC systems with VSCs and LCCs might become a common solution. The introduction of VSCs in islanded systems will allow operations in weak grids, but possible interactions with LCCs must be analysed in detail. This paper introduces the potential interactions in multi-infeed HVDC systems with LCCs and VSCs. An initial benchmark model of an islanded power system with a LCC and a VSC-HVDC link is presented to analyse new interaction phenomena between the converters and the islanded AC grid. Simulation results in PSCAD/EMTDC are presented to validate the benchmark model for voltage stability and commutation failure analysis.Postprint (published version

Stability studies of different AC collection network topologies in wind farms connected to weak grids

In this paper, the stability studies for different wind farm collection network topologies have been performed. As the wind farm becomes larger, the inter-array network becomes larger.so that the impedance of the overall system will be increased. This means that the inter-array configuration can impact the stability of system. The dynamic studies results presented in this paper show that the star collection network topology has the ability to be connected to weaker grid followed by radial, double sided and single side ring collection network topology

An alternative current-error based control for VSC integration to weak grid

An enhanced current control strategy is proposed for voltage source converters for the integration to weak grids. The control derives from the current-error based vector control. By imple menting simple close-loop compensations of both angle and magnitude inputs to the pulse width modulation, the damping of vector control in the weak grid can be significantly improved hence able to deliver full rated power to very weak grid. Due to the presence of the current loop, the fault-ride-through capability can be maintained with no need for mode switching. A comprehensive frequency domain model is employed to analyze the stability. Time domain simulations are further carried out to validate its effectiveness and robustness of integrating to the weak grid with fault-ride-through capability

Improved droop control with DC grid resonance damping capability

The concept of the so-called droop control has been widely discussed in the literature as the preferred controller for Voltage Source Converter for High Voltage Direct Current (VSC-HVDC) multiterminal and DC grid schemes. Droop control provides fast dynamic response and power-sharing between converter stations among other advantages but, as the controller is usually implemented as a merely proportional gain, the DC grid resonances damping is often very poor. This is because although the grids under study are DC, a broad range of high-frequency components and transient dynamics are inevitable in DC grids caused by long cables and switching to name a few. Conventional droops based on proportional gain are not able to handle such frequency-dependant issues of the DC grids. To improve the DC dynamic response, this paper presents a new droop controller where a DC resonance mitigation compensator is augmented to the conventional droop control that will result in an improved droop compensation by guaranteeing that power-sharing task will satisfactorily meet the desired damping requirements

Stability limits and tuning recommendation of the classical current control providing inertia support

The drastic increase in renewable energy sources in power grids has raised stability concerns. A particular concern exists in the ability of the converters to preserve frequency stability, due to their inherent lack of inertia provision. Grid forming converters have been presented as a solution to this issue, however the control structure for such converters is significantly different from the vector current control structures utilized by most installed control-converter systems. The classical current controller with a Phase Locked Loop (PLL) can be modified to provide inertia by including an additional control loop that injects active power in the case of a frequency event. This paper presents a detailed stability study, using a small signal model, and presents a set of controller tuning recommendations for the classical current controller with inertia emulation capability. The investigation found that the classical current and PLL tuning decreases the power that can be provided using the inertia emulation loop. Reducing the current loop time constant can allow for stable inertia emulation with classical vector current control