Georgios Stamatiou

Stability Analysis of Two-Terminal VSC-HVDC Systems Using the Net-Damping Criterion

This paper performs a frequency-domain investigation in two-terminal voltage-source-converter-HVDC (VSC-HVDC) systems, focusing on the use of the net-damping criterion to investigate poorly damped conditions and closed-loop stability. The system is represented as a feedback interconnection of two subsystems, defined in this paper as: 1) a dc-grid input impedance and 2) a VSC input admittance. The damping characteristics of the two subsystems are defined at critical frequencies and their separate contribution to the closed-loop stability is assessed by the net-damping criterion. A variety of study cases demonstrates the effectiveness of the latter as a powerful tool for stability investigation, while a correlation has been derived between the net-damping value and the damping factor of the poorly damped poles of the system.

Decentralized converter controller for multiterminal HVDC grids

Multiterminal High Voltage Direct Current (MTDC) grids based on Voltage Source Converters (VSC) are considered ideal for the future integration of large scale and highly distributed renewable power generation, as well as interconnection of remote power systems. The control of such grids is challenging as it is necessary to provide accurate power flow control and multiple voltage controlling stations. Existing control techniques either fail to fulfill those requirements or achieve them at a cost of maintaining a communication channel between the local station controllers and a master central control level. This paper suggests a decentralized controller for MTDC grids that ensures accurate power flow control, provides voltage control capabilities to all stations and requires no need for communication between the stations. The validity of the controller was tested in a five-terminal MTDC model proving its functionality.

Analytical Derivation of the AC-Side Input Admittance of a Modular Multilevel Converter With Open- and Closed-Loop Control Strategies

This paper deals with the derivation of the ac-side input admittance of a grid-connected modular multilevel converter (MMC) under both open- and closed-loop ac-side current control strategies. The contribution of the internal dynamics of the MMC on the input admittance depends on the type of ac-side controllers implemented. For this reason, the input admittance is first derived with the MMC operating in open loop on the ac side. The results are then modified to include various ac-side controllers in order to analyze the impact of the internal dynamics of the MMC on the total input admittance. Based on the findings, recommendations are given on modeling the input admittance of the MMC. Finally, the derived analytical models are verified using a detailed switching model of the MMC in the frequency domain.

A novel decentralized control strategy for MultiTerminal HVDC transmission grids

The concept of Voltage Source Converter based MultiTerminal High Voltage Direct Current (VSC-MTDC) grids, represents both challenges and opportunities for the future of large power transfer and integration of renewable energy sources. For this kind of grids, the control aspect is of great importance, with voltage-droop based methods considered as one of the most attractive solutions. All existing strategies are designed to maintain the level of voltage in the MTDC grid constant during unexpected events, thus sacrificing the power flow. The aim of this paper is to propose a new droop controller structure that maintains the dc-grid voltage close to the nominal values and at the same time tries to preserve the power flow in the dc grid, following events such as faults or disconnection of stations. The control scheme is presented and simulations are carried out in a four-terminal MTDC grid, proving the validity of the concept.

Analytical investigation of poorly damped conditions in VSC-HVDC systems

In this paper, strong AC-grid connected VSC-HVDC systems are studied. Under specific conditions, such systems can suffer from both stability and poor damping related issues, which warrants a stability study. Analytical eigenvalue expressions can directly demonstrate the impact of physical or control parameters on the system stability. However, especially in case of high-order systems, such expressions are challenging to obtain. This paper suggests a method to symbolically represent approximative eigenvalues of two-terminal VSC-HVDC systems, which could also be used to analyze the system dynamics. In addition, by applying symbolic-isolation method, the order of a multi-terminal VSC-HVDC system can be reduced to an equivalent two-terminal VSC-HVDC system, which enables the proposed method to provide symbolic pole expressions. Numerical studies based on Matlab simulations are presented, showing the accuracy of the analytical eigenvalue expressions and providing useful hints on the impact of physical or control parameters.