Some Topics On Novel HVDC And Its Related Technology

With the development of power electronic technology, it has become possible to use voltage source converter (VSC) in HVDC. In this dissertation, the operation and control of novel HVDC systems (Voltage source converter based HVDC, or HVDC-VSC) as well as some related technology are studied.By analyzing the basic principle of Voltage Space Vector PWM, a method is proposed to choose the zero vectors dynamically according to the power factor angle of the load and the concept of the lag angle of non-switching sectors. With this method, the minimum switching losses can be reached, and the maximum switching current can be reduced effectively. So this method can meet the requirement of HVDC-VSC where lower switch loss is emphasized.On the basis of natural-sampled PWM and regular-sampled PWM, the concept of asymmetric regular-sampled PWM is developed. By applying this concept in voltage space vector PWM (VSVPWM), the asymmetrical voltage space vector PWM (AVSVPWM) is proposed, which is suitable for lower switching frequency and can be used in GTO based VSC.From the perspective of minimum transmission losses, an instantaneous reactive power theory for systems with any number of phases and with or without neutral line is developed. Under the new definition, each quantity has its own physical meaning, and the zero sequence current can be decomposed into active and reactive components. Furthermore, a generalized compensation method for zero sequence current is developed. The relationship and difference between the instantaneous reactive power and the conventional reactive power based on average values are also revealed. The theory can be served as the basis for studying reactive power control.By selecting the amplitude and phase angle of the VSC output voltage as control variables, the steady-state model of HVDC-VSC is developed. According to this model, the corresponding relationship between the two controlling and the two controlled variables of the VSC is determined. Furthermore, a nonlinear control strategy combining an inverse model controller with a PI controller is proposed. The control strategy of the VSC which supplying a passive network is also proposed, and the designing of the passive high-pass filter for HVDC-VSC is briefly discussed.By selecting the voltage space vector of the voltage across the equivalent converter impedance as the control variables, the steady-state model of HVDC-VSC is developed. On this basis, a steady-state feedback linearization strategy is proposed, then, linear and decoupled control is realized using PI controllers and feedforward compensation.Based on the concept of space vector, an open loop detecting method for the fundamental negative and positive sequence components as well as the harmonic components is developed, which combining the conventional detecting method based on filters and the concept of space vector. This method can overcome the defaults of the conventional detecting method, and can meet the requirement of the asymmetrical control of HVDC-VSC.Based on the adaptive interference canceling theory and the concept of space vector, an adaptive fundamental positive/negative sequence component and harmonic component detecting method is developed. Due to the close loop structure, this method has quite good robustness, and can meet the requirement of the asymmetrical control of HVDC-VSC well.According to the transient mathematical model of HVDC-VSC, an equivalent simulation model for HVDC-VSC is developed. This model expresses the dynamic characteristics of HVDC-VSC well, and with this model, the simulation is simplified greatly, the simulation time is reduced to 1/20 or less, thus the study of HVDC-VSC become much simpler.A hybrid simulation method for symmetrical system is presented, combining the hybrid simulation method with the proposed equivalent simulation model, a hybrid simulation method for HVDC-VSC is presented, which is useful for studying the action and reaction between the ac and dc systems.