Analysis of subsynchronous oscillations at capacitor commutated converters

When interconnecting two large asynchronous power systems using HVDC technology with a high transmission capacity, the short-circuit power of the connected networks can be low compared to the rated dc power. For this reason stability problems might occur during the operation of the HVDC system. Especially when operated at weak inverter side networks ac faults can result in severe disturbances of the HVDC system. Therefore a new type of HVDC converter, the capacitor commutated converter (CCC) can be used. Furthermore if the network on the rectifier side has high impedance, there can also occur stability problems. In this paper the torsional interaction between the rectifier controller of the HVDC and the mechanical turbine-generator system of a nearby power station is modeled and analyzed for different SCR of the rectifier side network. The design of a subsynchronous damping controller (SSDC) is presented for counteracting emerging subsynchronous oscillations that are insufficiently damped. The enhanced damping effect of the SSDC is shown and the improvement of the stability of the system is indicated especially when operated at low SCR of the rectifier side network.