Q7Power System - I
Question
Q.7. Discuss line commutated converters with the help of schematic diagram.
Answer
Line-commutated converters (LCC) use naturally-commutated thyristor valves, relying on the AC system voltage itself to force commutation (turn-off) of the conducting thyristor as the next phase voltage becomes more positive, and are configured as 6-pulse (or 12-pulse) bridge circuits, forming the traditional basis of HVDC converter stations.
A line-commutated converter (LCC) is a type of AC-DC converter built using thyristor (SCR) valves, in which the natural (uncontrolled) commutation of current from one conducting thyristor to the next relies entirely on the AC system's own voltage waveform, rather than on any active turn-off capability of the switching device itself (thyristors, unlike IGBTs/GTOs, cannot be turned off by a gate signal — they turn off only when the current through them falls to zero and a reverse voltage is applied).
The most common LCC configuration used in HVDC transmission is the 6-pulse (Graetz) bridge, consisting of six thyristor valves connected in a bridge arrangement across the three phases of the AC supply (via a converter transformer), with two valves conducting at any instant. Commutation from one valve to the next occurs naturally at the crossover point of successive line-to-line voltage waveforms, when the incoming phase's voltage becomes more positive than the outgoing phase's voltage (for the rectifying valve group) or more negative (for the inverting valve group), so the AC system voltage itself forces the current to transfer and the outgoing thyristor to turn off — hence the term 'line commutated.'
The DC output voltage is controlled by varying the firing angle α (delay angle from the natural commutation point at which the gate trigger pulse is applied), following Vdc = (3√2/π)·VLL·cos(α) for an ideal 6-pulse bridge (neglecting commutation overlap), where firing angles 0°<α<90° give rectifier operation and 90°<α<180° give inverter operation. For higher-power HVDC schemes, two 6-pulse bridges are connected in series with their AC supplies phase-shifted by 30° (via star-star and star-delta converter transformer windings) to form a 12-pulse converter, which substantially reduces the characteristic AC-side current harmonics (5th, 7th) and DC-side voltage ripple (6th harmonic) compared to a single 6-pulse bridge. LCC-based HVDC requires a relatively strong AC system at both ends (to provide the commutation voltage and reactive power support, since thyristors always draw lagging reactive power regardless of active power direction) and needs reactive power compensation equipment (filters, capacitor banks or synchronous condensers) at each converter station.