page_title: HVDC converter
section_title: Line Commutated Six-pulse bridge
hierarchical_section_title: HVDC converter / Line-commutated converters / Line Commutated Six-pulse bridge
caption_attribution_description: English: Six-pulse bridge at two different stages in the cycle. On the left, valves V1 and V2 only are conducting and the DC voltage is formed by the voltage of phase 1 minus that of phase 3. On the right, valve 1 is in the process of transferring its current to valve 3. The DC voltage is the average of the voltages of phases 1 and 2, minus that of phase 3,
context_page_description: An HVDC converter converts electric power from high voltage alternating current to high-voltage direct current, or vice versa. HVDC is used as an alternative to AC for transmitting electrical energy over long distances or between AC power systems of different frequencies. HVDC converters capable of converting up to two gigawatts and with voltage ratings of up to 1,100 kilovolts have been built, and even higher ratings are technically feasible. A complete converter station may contain several such converters in series and/or parallel.
  Almost all HVDC converters are inherently bi-directional; they can convert either from AC to DC or from DC to AC. A complete HVDC system always includes at least one converter operating as a rectifier and at least one operating as an inverter. Some HVDC systems take full advantage of this bi-directional property. Others, for example those designed to export power from a remote power station such as the Itaipu scheme in Brazil, may be optimised for power flow in only one preferred direction. In such schemes, power flow in the non-preferred direction may have a reduced capacity or poorer efficiency.
context_section_description: The basic LCC configuration for HVDC uses a three-phase Graetz bridge rectifier or six-pulse bridge, containing six electronic switches, each connecting one of the three phases to one of the two DC terminals. A complete switching element is usually referred to as a valve, irrespective of its construction.  Normally, two valves in the bridge are conducting at any time: one to a phase on the top row and one (from a different phase) on the bottom row. The two conducting valves connect two of the three AC phase voltages, in series, to the DC terminals.  Thus, the DC output voltage at any given instant is given by the series combination of two AC phase voltages. For example, if valves V1 and V2 are conducting, the DC output voltage is given by the voltage of phase 1 minus the voltage of phase 3.
  Because of the unavoidable (but beneficial) inductance in the AC supply, the transition from one pair of conducting valves to the next does not happen instantly.  Rather, there is a short overlap period when two valves on the same row of the bridge are conducting simultaneously. For example, if valves V1 and V2 are initially conducting and then valve V3 is turned on, conduction passes from V1 to V3 but for a short period both of these valves conduct simultaneously. During this period, the DC output voltage is given by the average of the voltages of phases 1 and 2, minus the voltage of phase 3.  The overlap angle μ (or u) in an HVDC converter increases with the load current, but is typically around 20° at full load.
  During the overlap period, the output DC voltage is lower than it would otherwise be and the overlap period produces a visible notch in the DC voltage.  An important effect of this is that the mean DC output voltage decreases as the overlap period increases; hence the mean DC voltage falls with increasing DC current.
  The mean DC output voltage of a six-pulse converter is given by:
  Where:
  VLLpeak - the peak value of the line to line input voltage (on the converter side of the converter transformer),
  α - the firing angle of the thyristor
  Lc - the commutating inductance per phase
  Id - the direct current
  The firing angle α represents the time delay from the point at which the voltage across a valve becomes positive (at which point a diode would start to conduct) and the thyristors being turned on. From the foregoing equation, it is clear that as the firing angle increases, the mean DC output voltage decreases.  In fact, with a line-commutated converter, the firing angle represents the only fast way of controlling the converter.  Firing angle control is used to regulate the DC voltages of both ends of the HVDC system continuously in order to obtain the desired level of power transfer.
  The DC output voltage of the converter steadily becomes less positive as the firing angle is increased:  firing angles of up to 90° correspond to rectification and result in positive DC voltages, while firing angles above 90° correspond to inversion and result in negative DC voltages. However, the firing angle cannot be extended all the way to 180°, for two reasons.  Firstly, allowance must be made for the overlap angle μ, and secondly for an additional extinction angle γ which is needed for the valves to recover their ability to withstand positive voltage after conducting current.  The extinction angle γ is related to the turn-off time tq of the thyristors.  A typical value of γ is 15°. α, γ and μ are inter-related thus:
   (in degrees)
caption: Commutation process explained.  When just valves 1 and 2 are conducting, the DC voltage is formed from two of the three phase voltages.  During the overlap period the DC voltage is formed from all three phase voltages.
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