Research On The SSO Damping Characteristics Of HVDC Systems

Torsional interaction exists between HVDC converters and turbine-generators, and negative electrical damping provided by HVDC may cause subsynchronous oscillation (SSO) that can lead to turbine-generator shaft failure and electrical instability at oscillation frequencies lower than the fundamental system frequency. From the viewpoint of electrical damping, this dissertation focus on the damping analysis of subsynchronous oscillation for different systems, such as HVDC system, hybrid AC/DC system and multiple-converter HVDC system. The main works are organized as follows:Based on the switching function method and time domain simulations, the small disturbance transfer a characteristic through the converter is analyzed. Because of the modulation effect of the converter, the small disturbance in the shaft of turbine generator will induce AC voltage component with the same frequency as the disturbance on the DC side. And the resulted AC current on the DC side will have the similar frequency shift when transfers to the stator of the generator. All these changes, which are of importance to subsynchronous oscillation analysis, cannot be described with the Quasi Steady State (QSS) model any more. Therefore, the deficiency of QSS-model-based eigenvalue method is pointed out and a novel method named complex torque coefficient-test signal method is adopted to analyze the subsynchronous oscillation caused by HVDC or FACTS devices.The torsional interaction from the electrical couplings among the generating sets is presented and the influence of the natural torsional frequency of the generating sets on the torsional oscillation characteristics is described. The torsional dynamics between identical parallel turbine-generators are deduced as well as that between non-identical parallel turbine-generators. Then the basic principle of equivalent simplification of generating units for different operational condition is summarized. This simplification principle is the foundation of the following analysis.Using the test signal method, a detailed analysis of subsynchronous oscillation for a HVDC system is performed. The electrical damping within the whole subsynchronous frequency range is calculated. The impacts of the unit interaction factor between HVDC and turbine generator, the DC power level, the firing delay angle and the parameter settings of HVDC controller on thehave the potential danger of subsynchronous oscillation. On the contrary, because of the effect of positive damping provided by the inverter, the unit near a inveter will have no subsynchronous oscillation risk.Based on the same method described above, a damping study is carried out for a hybrid AC/DC system, which include a HVDC link and an AC transmission line in parallel. The characteristics of damping shows that the uncompensated parallel AC transmission line can release the coupling between the HVDC and generator and improve the damping of the system, hence the possibility of subsynchronous oscillation decrease. In such a system scheme, the HVDC system dominates the changes of damping due to the small magnitude of the damping provided by AC line. When the AC transmission line is compensated with a fixed series capacitor, the situation will be totally different. The large negative damping induced by the series capacitor will impose subsynchronous oscillation danger not only to the generator near a rectifier, but also the generator close to an inverter. The compensated AC line will dominate the system electrical damping.A novel index named comprehensive unit interaction factor is proposed to quantify the torsional interaction in an AC system with multiple HVDC converters. In an AC system with multiple HVDC converters, turbine generators can interact adversely with all the converters through electrical connection, which makes the subsynchronous oscillation more complicated. The new index provided in this dissertation can be used as a screening tool to estimate the torsional interaction and to identify units and system contingencies requiring det...