An LCC station also presents a harmonic impedance to pre-existing AC harmonics. It depends on the converter transformer reactance, valve behaviour, smoothing reactor, DC line or cable impedance, DC filters, the ground-return or leakage path, the operating point and control action — but at most orders it is often dominated by the AC filters. So the AC filters are the dominant station impedance at many orders, while the converter internal impedance may be important at low orders; the converter detail required depends on the study objective and frequency range.
For a new LCC connection, compliance has two parts that can be analysed separately by superposition. The new-emission assessment treats the converter as a harmonic source, \(V_{h,new}=Z_{eq,h}I_{LCC,h}\). The background-amplification assessment treats the station as a harmonic impedance interacting with background voltage distortion — the total being a function of \(V_{h,background}\), \(Z_{LCC,h}\) and \(Z_{system,h}\). The rule: HVDC compliance is not only about converter emission; it must also consider background distortion amplification.
For new distortion, the simplified model includes the LCC harmonic current source, the AC filter impedance and the external AC system impedance, with the source representing harmonic order, magnitude, phase angle (if available), operating point, monopolar or bipolar operation and filter configuration; the external impedance is usually a harmonic impedance envelope. The study should find the combination of operating point, filter status and network impedance that gives the highest PCC distortion — and maximum distortion may not occur at rated DC power: at low DC power the harmonic content can be proportionally higher while fewer filters are connected for reactive balance. For background distortion, the station is represented mainly as a passive harmonic impedance (AC filters, converter transformer, converter impedance, external AC system); the converter and transformer impedance may be neglected if the filters dominate, but this may be inaccurate for low-order harmonics. So a filter-only model may suffice for general studies, but include converter impedance when low-order accuracy is required. For system-wide studies the LCC model should include the harmonic current source, AC filters, converter transformer, converter impedance if required, shunt compensation and the operating point and filter status — matched to the selected DC transfer level, across minimum, intermediate and maximum DC power, monopolar and bipolar operation and credible switching states. Model the LCC operating range, not only the nameplate condition.
