Abstract A novel nonlinear emergency control method for HVDC transmission systems is presented in this paper. First, an observation decoupled state space model of AC and DC interconnected power systems, which allows estimation or computation of the state components from local measurements, is developed. Secondly, the nonlinear control rules of HVDC links are derived using optimal-variable-aim strategy (OVAS), which not only improves the first swing stability, but also effectively damps the subsequent power oscillation. Finally, the simulation results of a three-machine six-bus system show that the proposed method can enough use the capabilities of fast response and short-time overload of HVDC links to improve the transient stability of power systems.

高压直流输电系统的附加控制器可以改善系统的稳定性。本文针对多馈入高压直流输电系统，应用非线性控制系统理论设计了直流系统的附加控制器。所推导出的稳定控制规律考虑到了两条直流系统附加控制的相互影响，与传统控制器相比能在更大的程度上改善系统的稳定性。仿真结果也证明了该控制规律的有效性。

In this paper, a fault current limiter (FCL) with series compensation is proposed. It is composed of a limiting reactor and a compensation capacitor that is parallel with a by-pass reactor controlled by GTO. The FCL can not only control fault current, but also perform series compensation. The good performance of this device is proved by simulation, it will be a practical protection device for power systems in the future.

NETOMAC program is powerful means for science research, system planning and operation in power systems. This paper presents a general overview of digital simulation of the stability model and control for the TCSC in the NETOMAC. It is very convenient to simulate the stability performance of the TCSC, because the TCSC network elements can be modelled by the available variable admittance branch module, and the TCSC control system can be constructed by the evaluation module or FORTRAN subroutine.

To improve the performance of High Voltage Direct Current (HVDC) transmission system, the constant reactive current control is used to take the place of the traditional constant firing angle control at the inverter. This paper presents a method of controller design, which is based on feedback linearization in the nonlinear systems and Variable-Structure control theory in the linear system. It can be testified by simulation that the new controller can not only improve the characteristic of DC system, but also enhance the voltage stability of AC system.