Research On HVDC Small Signal Modulation And Telephone Interference

This thesis focuses on the application of the fast controllability of HVDC systems to improve the damping of electromechanical mode oscillation in parallel AC/DC power systems and the equivalent disturbing current of HVDC systems. The main works are as follows: (1)A two area single-infeed AC/DC system and a three area dual-infeed AC/DC system are constructed based on the quasi-steady state HVDC converter model and the modularized HVDC controller model using the digital simulation software NETOMAC. Simulation results show that the constructed models satisfy the demands of research on AC/DC electromechanical interactions.(2) A new method is put forward to tune the parameters of an HVDC small signal modulation controller. First, small signal stability of an AC/DC power system is analyzed using the test signal method. Then the frequency of the test signal is scanned at the HVDC rectifier current set point, and the oscillated active power flow in the parallel AC tie line is obtained. Thus, the open-loop transfer function between the current reference signal of the HVDC constant current regulator and the active power flow in the parallel AC inter-tie is identified by Fourier decomposition and curve fitting with time domain simulations using electromechanical transient programs. Finally, parameters of the HVDC small signal modulator are tuned based on the root locus method of the classical control theory. Simulation results show that the proposed parameter tuning method is effective and convenient, and it can be used in a practical large AC/DC power system.(3)There may exist negative interactions among HVDC modulators in multi-infeed HVDC systems, so it is necessary to tune the parameters of the HVDC modulators coordinately. The problem is equivalent to solve a non-linear programming problem provided that the structures of the HVDC modulators are known. In this paper the objective function is selected to contain the active power in the AC inter-ties and the rotor angles of all the generators. The parameters of the HVDC modulators can be optimized simultaneously with an inherent non-linear optimization module of NETOMAC. The presented parameter tuning procedure has itself included the non-linear nature of the AC/DC system and does not need to manually derive the explicit mathematical model of the whole system. Neither system simplification nor linearization is needed. A dual-infeed AC/DC system with two low-damped inter-regionaloscillation modes is constructed to demonstrate the effectiveness of the presented parameter tuning procedure. Simulation results show that the optimized HVDC modulators can improve the damping of overall power oscillations in the dual-infeed AC/DC system. Furthermore, the optimized modulators are robust to changes in system configurations, operating conditions, and fault formats.(4) The equivalent disturbing current, which determines the harmonic interference noise in nearby telephone circuits, is studied systematically. The equivalent disturbing current limitation criteria are identified based on the tolerable noise level in China and assumptions regarding typical "worst case" coupling condition between an HVDC transmission line and a telephone system.