The virtual synchronous machine (VSM) control strategy has been demonstrated as an effective method to improve system stability in recent research. It mimics the dynamic characteristics of traditional synchronous generator (SG), and therefore can provide frequency support under disturbances. However, the integration of multiple VSMs may deteriorate the power system performance by participating in the system oscillations due to their emulations of SG's swing equation. Therefore this paper investigates the system performance with multiple VSMs integrated. Besides, a supplementary controller is designed to enhance system stability. A modified two-area system is utilized to validate the effectiveness of the designed controller in Real-Time Digital Simulator (RTDS) platform.

The direct-drive wave energy conversion (WEC) system adopts direct-drive power takeoff technique and the linear permanent magnet generate (LPMG) is coupled directly to the floater, which leads to simple configuration and high efficiency. This type of device usually has common features in term of structure, model, optimal control strategy, and power conditioning. Taking the AWS as the example, the models of the direct-drive WEC are first presented, and recommendations are also proposed for their applications. To extract maximum power from the wave, the AWS has to be regulated in resonance with the wave. Different types of control strategy using mechanical and electrical devices have been developed. Their features and applications are compared and discussed. An optimal control strategy, which enables AWS extracting maximum power from the irregular wave, is proposed. Since the floater reciprocates, not only the magnitude but also the direction of the translator motion speed changes. In this condition, the frequency, terminal voltage, and output active power of the LPMG vary. These features may not satisfy the requirements of the grid code for the integration of wave generation into power grid. Hence, the power conditioner is very important for the integration of the AWS into the power grid. The power conditioners are developed to integrate the AWS into the power grid, and the application of the energy storage in smoothing the output power of the AWS is also studied.

The articles in this special issue provide a timely and comprehensive review of recent progresses, applications, and challenges spurred by the marine energy generation and transmission technology developments. It is intended for a broad audience interested in gaining comprehensive understanding of technical issues and opportunities that may exist in this important emerging field.

Marine energy generation technologies include offshore wind, wave, and tidal energy generation, which are considered to be the fundamental renewable generation technologies for the long-term aim to reduce our climate's CO2emissions. Hence, marine energy is the key to develop future sustainable energy supply.