Impacts of AC Filter Tripping at the Songo Rectifier Station in Southern Africa

Large over-frequencies at the Songo Rectifier Station can be reached due to the loss of active power balance during an HVDC disturbance. Large over-frequencies cause the tripping of the sharply tuned ac harmonic filters together with the tripping of the Bus Coupler (BC), resulting in power reduction to Eskom, South Africa and/or to ZESA, Zimbabwe. During over-frequency events, the sharply tuned ac harmonic filters no longer provide any useful harmonic filtering at their tuned frequency and are tripped to avoid parallel resonance between the raised harmonic frequencies and the next higher tuned harmonic filter (e.g., raised 11<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> harmonic frequency and the 13<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> tuned harmonic filter). The ac filter tripping has a negative impact on the 220 kV three-phase ac bus, where the converter bridges are connected (converter bus). It also has a negative impact on the HVDC line voltage. Despite this, the ac filter tripping does not have a negative impact on the 220 kV three-phase ac bus, where the ac transmission lines are connected (transmission bus). This paper focuses primarily on analyzing the impact of ac filters tripping, following an over-frequency event, on the 220 kV three-phase voltage waveform, on the bus where the converter bridges are connected (converter bus).