page_title: HVDC converter
section_title: Two-level converter
hierarchical_section_title: HVDC converter / Voltage-source converters / Two-level converter
caption_attribution_description: English: Three-phase, two-level voltage-source converter for HVDC transmission, using series-connected IGBTs as the switching elements.
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: From the very first VSC-HVDC scheme installed (the Hellsjön experimental link commissioned in Sweden in 1997) until 2012, most of the VSC HVDC systems built were based on the two level converter.  The two-level converter is the simplest type of three-phase voltage-source converter and can be thought of as a six pulse bridge in which the thyristors have been replaced by IGBTs with inverse-parallel diodes, and the DC smoothing reactors have been replaced by DC smoothing capacitors.  Such converters derive their name from the fact that the voltage at the AC output of each phase is switched between two discrete voltage levels, corresponding to the electrical potentials of the positive and negative DC terminals. When the upper of the two valves in a phase is turned on, the AC output terminal is connected to the positive DC terminal, resulting in an output voltage of +½ Ud with respect to the midpoint potential of the converter.  Conversely when the lower valve in a phase is turned on, the AC output terminal is connected to the negative DC terminal, resulting in an output voltage of -½ Ud.  The two valves corresponding to one phase must never be turned on simultaneously, as this would result in an uncontrolled discharge of the DC capacitor, risking severe damage to the converter equipment.
  The simplest (and also, the highest-amplitude) waveform that can be produced by a two-level converter is a square wave; however this would produce unacceptable levels of harmonic distortion, so some form of Pulse-width modulation (PWM) is always used to improve the harmonic distortion of the converter. As a result of the PWM, the IGBTs are switched on and off many times (typically 20) in each mains cycle. This results in high switching losses in the IGBTs and reduces the overall transmission efficiency.  Several different PWM strategies are possible for HVDC but in all cases the efficiency of the two-level converter is significantly poorer than that of a LCC because of the higher switching losses. A typical LCC HVDC converter station has power losses of around 0.7% at full load (per end, excluding the HVDC line or cable) while with 2-level voltage-source converters the equivalent figure is 2-3% per end.
  Another disadvantage of the two-level converter is that, in order to achieve the very high operating voltages required for an HVDC scheme, several hundred IGBTs have to be connected in series and switched simultaneously in each valve. This requires specialised types of IGBT with sophisticated gate drive circuits, and can lead to very high levels of electromagnetic interference.
caption: Three-phase, two-level voltage-source converter for HVDC
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