| Abstract |
High Voltage Direct Current (HVDC) systems are used to transport electric energy over large distances or to exchange energy between asynchronous networks. HVDC equipment of modern design is able to work with relatively weak ac systems having a low resonance frequency. The line-commutating thyristor converters of an HVDC system produce characteristic harmonics and possibly, depending on several circumstances, non-characteristic harmonics. This can lead to harmonic instability, if the resonance frequency of the network is close to one of these harmonics. The instability mechanism for the second harmonic is explained and a method is presented, which is able to avoid second harmonic instability by means of control actions. For this purpose, the power frequency component of the direct current is detected by a special filtering system with dc output signals. This new approach allows the use of a PI controller to create a stabilizing signal, which is used to modulate the delay angle of the rectifier side. During field tests on a back-to-back tie, the stabilizing effect of this measure was demonstrated successfully. |
