An energy recovery filter for HVDC systems

The thesis investigates the use of a novel filter arrangement called the Energy Recovery filter (ER-filter) for HVDC systems.;Several operating problems can occur with HVDC schemes operating into weak AC systems. Most of the problems are closely associated to the harmonic resonance and instability between AC and DC systems. The harmonic instability is usually mitigated by either modifying the AC/DC system impedance characteristics or adding extra control circuits to damp the oscillations. The majority of existing approaches deal with eliminating the specific harmonic of the most concern. Sometimes a broadband solution is more desirable because resonance frequencies may change with the system conditions. The filter used for this purpose should have a low quality factor for coving a wide frequency range. However, the use of a conventional low Q filter involves large power losses in the fllter resistor. The ER-filter introduced in this thesis will recover the power losses in the filter.;The ER-filter is implemented by replacing the resistors in a conventional filter with a low power back-to-back DC link which takes power from the filter and returns it to the system. The link acts as a resistor to the filter, but does not consume real power.;The studies also show that for certain system conditions a low Q filter is desirable and for other conditions a high Q is better. This inspired the concept of a "Variable Q filter" in which its quality factor is controllable by electronic means. The Variable Q filter provides low Q factor in the steady state to reduce the steady state oscillations, and a high Q factor during fault recovery to help eliminate harmonic currents injected from the saturated transformer. The ER-filter is able to operate as a Variable Q filter by modifying the firing angle control in the inverter of its DC link.;The investigations in this thesis are conducted through digital computer simulation using an electromagnetic transient simulation program PSCAD/EMTDC. The CIGRE benchmark model, which experiences both steady state and core saturation harmonic instabilities, is selected as the base system. The harmonic instabilities in the CIGRE system are first mitigated by installing a conventional filter on AC bus. Later, the conventional filter is replaced by an ER-filter in order to reduce the high losses in the filter resistor. Finally, the ER-filter is modified to a Variable Q filter to provide an improved system recovery from disturbances.;The results of the studies confirm that installing a conventional low Q filter on the AC bus solves the harmonic instability problems. The ER-filter has a similar performance as the conventional filter but with much reduced losses. And the Variable Q filter gives better performance during system recovery from faults.