This paper describes the two test systems for voltage stability studies set up by the IEEE PES Task Force on "Test Systems for Voltage Stability Analysis and Security Assessment" under the auspices of the Power System Stability Subcommittee of the Power System Dynamic Performance Committee. These systems are based on previous test systems, making them more representative of voltage stability constraints. A set of representative results are provided for both systems, with emphasis on dynamic simulation. They illustrate various aspects such as longterm dynamics, voltage security assessment, real-time detection, and corrective control of instabilities. The value for educators, researchers and practitioners are emphasized.

Abstract-This paper focuses on simplified models of the Modular Multilevel Converter suitable for large-scale dynamic studies, in particular simulations under the phasor approximation. Compared to the existing literature, this paper does not a priori adopt the modeling approach followed for the original twolevel or three-level Voltage Source Converter. On the contrary, a model is derived following a physical analysis that preserves its average internal dynamic behavior. An equivalent control structure is proposed and various alternatives are highlighted. The proposed model with its controllers has been implemented in a phasor simulation software and its response has been validated against a detailed Electromagnetic Transient model. Finally, an illustrative example is presented with the application of the proposed model on a large grid consisting of AC areas interconnected with a multi-terminal DC grid.

Abstract-The subject of this paper is the investigation of the stability of a Voltage Source Converter (VSC) undergoing a sudden decrease of the Short Circuit Capacity of the AC system to which it is connected. A case study is reported on a simple system including an HVDC terminal and a Thévenin equivalent. First, a small-signal stability analysis is performed showing that the PLL-based vector control may become unstable at powers lower than the theoretical static stability limit. Dynamic simulations demonstrate that the stability limit may further decrease depending on the disturbance. The need for a fast instability detection method is highlighted.

This paper presents a novel control scheme for exchange of frequency support between asynchronous AC systems through a High Voltage Direct Current link or grid. The proposed controller bears the spirit of an emergency scheme. Using only locally available measurements, each converter can identify emergency situations that could potentially lead to unacceptable frequency values. Then appropriate control actions are taken to restrain the frequency decline and prevent it from reaching the thresholds of load shedding relays. Inspired of Model Predictive Control, the method uses simplified models of the AC and DC sides of the converter, and can incorporate various constraints. The effectiveness of the method is demonstrated on a test system consisting of two asynchronous AC areas interconnected through a five-terminal HVDC grid.

This paper proposes a novel control scheme for provision of frequency support among asynchronous AC areas through HVDC grids. It is based on local controllers, each acting on a voltage source converter, using local measurements only, and supporting frequency of the adjacent AC area after a significant disturbance. The new discrete control is combined with the existing DC voltage droop technique. The formulation, inspired of Receding Horizon Control, enables providing to the AC area the desired frequency support, while at the same time taking into account various constraints, such as maintaining the DC voltage between secure operating limits. Examples obtained from a test system with a five-terminal DC network connecting two asynchronous areas demonstrate the effectiveness and robustness of the proposed control scheme in various scenarios, with emphasis on component failures.

Abstract-A coordinated supervisory control scheme for future multi-terminal High-Voltage Direct-Current (HVDC) grids is proposed. The purpose is to supervise the grid and take appropriate actions to ensure power balance and prevent or remove voltage or current limit violations. First, using DC current and voltage measurements, the power references of the various Voltage Sources Converters (VSC) are updated according to participation factors. Next, the setpoints of the converters are smoothly adjusted to track those power references, while avoiding or correcting limit violations. The latter function resorts to Model Predictive Control and a sensitivity model of the system. The efficiency of the proposed scheme has been tested through dynamic simulations of a five-terminal HVDC grid interconnecting two asynchronous AC areas and a wind farm.

This paper proposes a new control scheme for the Voltage Source Converters (VSCs) of a High Voltage Direct Current (HVDC) grid connecting asynchronous AC systems, so that the latter can mutually support their frequencies after a disturbance. The scheme involves no communication between VSCs. It is activated upon detection of a significant frequency deviation, with the possibility to limit the support requested from other AC systems. It relies on an integral control enforcing the frequency droop characteristics (of classical speed governors), in addition to the voltage-droop control of VSCs. The controller is tuned to emulate the response of a power plant, while avoiding excessive transients in the HVDC system, and limiting the impact on the other AC systems.

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