Personal Sign In. The two conducting valves connect two of the three AC phase voltages, in series, to the DC terminals. Retrieved 20 December However, the full-bridge arrangement requires twice as many IGBTs and has higher power losses than the equivalent half-bridge arrangement. Various other types of converter have been proposed, combining features of the two-level and Modular Multi-Level Converters. Another alternative replaces the half bridge MMC submodule described above, with a full bridge submodule containing four IGBTs in an H bridge arrangement, instead of two. On the AC side, the converter behaves approximately as a current source, injecting both grid-frequency and harmonic currents into the AC network. 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. Mercury arc valves for HVDC were rugged but required high maintenance. Normally, two valves in the bridge are conducting at any time: one on the top row and one from a different phase on the bottom row.
VSC-HVDC Transmission with Cascaded Two-Level Converters.
Bjorn Jacobson, Patrik Karlsson, Gunnar Asplund, Lennart Harnefors, Tomas Jonsson. ABB. This has been put to use in a further development of high-voltage dc (HVDC) transmission employing voltage-source converters (VSCs). A cascaded two-level. VSC - HVDC transmission with cascaded two-level converters. File Size: ,9 KB. Study Committee: B4? Year: Download.
Views Read Edit View history. However, the full-bridge arrangement requires twice as many IGBTs and has higher power losses than the equivalent half-bridge arrangement.
Early LCC systems used mercury-arc valveswith designs that had evolved from those used on high power industrial rectifiers. Modernizing the Electrical grid. Each submodule therefore acts as an independent two-level converter generating a voltage of either 0 or U sm where U sm is the submodule capacitor voltage.
Three-phase, two-level voltage-source converter for HVDC.
Operating principle of 3-level, diode-clamped converter, single-phase representation.
Vsc-hvdc transmission with cascaded two-level converters
|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type of three-phase voltage-source converter  and can be thought of as a six pulse bridge in which the thyristors have been replaced by IGBTs with inverse-parallel diodes, and the DC smoothing reactors have been replaced by DC smoothing capacitors.
Like the two-level converter and the six-pulse line-commutated converter, a MMC consists of six valves, each connecting one AC terminal to one DC terminal. 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 confers additional flexibility in controlling the converter and allows the converter to block the fault current which arises from a short-circuit between the positive and negative DC terminals something which is impossible with any of the preceding types of VSC.
Video: Vsc-hvdc transmission with cascaded two-level converters HVDC MaxSine - The VSC demonstrator
In such schemes, power flow in the non-preferred direction may have a reduced capacity or poorer efficiency. From the s onwards,  extensive research started to take place into static alternatives using gas-filled tubes — principally mercury-arc valves but also thyratrons — which held the promise of significantly higher efficiency.
Jonsson, "VSC-HVDC Transmission with Cascaded Two-level Converters,". Online ISSN: VSC-HVDC transmission with cascaded two-level converters. Jacobson Bjorn, Karlsson Patrik, Asplund Gunnar, Harnefors Lennart,.
Reliability modeling and evaluation of VSCHVDC transmission systems IEEE Conference Publication
An HVDC converter converts electric power from high voltage alternating current (AC) to high-voltage direct current (HVDC), or vice versa. HVDC is used as an alternative to AC for transmitting electrical energy over Almost all HVDC converters are inherently bi-directional; they can convert either from AC to DC.
Because of this, most mercury-arc HVDC systems were built with bypass switchgear across each six-pulse bridge so that the HVDC scheme could be operated in six-pulse mode for short periods of maintenance.
Operating principle of 3-level, diode-clamped converter, single-phase representation. In a refinement of the diode-clamped converter, the so-called active neutral-point clamped converter, the clamping diode valves are replaced by IGBT valves, giving additional controllability.
Three-phase, three-level, diode-clamped voltage-source converter for HVDC. Another alternative replaces the half bridge MMC submodule described above, with a full bridge submodule containing four IGBTs in an H bridge arrangement, instead of two.
As early as the s, the advantages of DC long-distance transmission were starting to become evident and several commercial power transmission systems were put into operation.
Vsc-hvdc transmission with cascaded two-level converters
|As a result, IGBTs can be used to make self-commutated converters.
Voltage-sourced converters are made with switching devices that can be turned both on and off. The thyristor plus its grading circuits and other auxiliary equipment is known as a thyristor level.
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. As ofboth the line-commutated and voltage-source technologies are important, with line-commutated converters used mainly where very high capacity and efficiency are needed, and voltage-source converters used mainly for interconnecting weak AC systems, for connecting large-scale wind power to the grid or for HVDC interconnections that are likely to be expanded to become Multi-terminal HVDC systems in future.
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. As ofthyristor valves had been used on over HVDC schemes, with many more still under construction or being planned.