INVERTER COMPOUNDING IN HVDC PDF
As a part of a HVDC project design, large efforts are made in tuning control system parameters for the Alpha minimum Inverter. Imargin. Current Margin. Udref. relates to the reactive power loading that a HVDC converter station imposes on . this second converter is operated as a line-commutated inverter and allows the DC .. Compound access is only possible once the filters have been isolated . Most bipolar HVDC transmission lines have . converters, a rectifier or an inverter, deter- mines the . inverters were compounded for constant.
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Each thyristor valve will typically contain tens or hundreds of thyristor levels, each operating at a different high potential with respect to earth. 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.
HVDC converters can take several different forms. A pulse Gvdc converter using mercury arc valves, with a bypass valve and bypass switch across each of cokpounding two 6-pulse bridges.
A variant of the MMC, proposed by lnverter manufacturer, involves connecting multiple IGBTs in series in each of the two switches that make up the submodule. With a suitable number of submodules connected in series, the valve can synthesize a stepped voltage waveform that approximates very closely to a sine-wave and contains very low levels of harmonic distortion.
However, where each valve of the two-level converter is effectively a high-voltage controlled switch consisting of a large number of IGBTs connected in series, each valve of a MMC is a separate controllable voltage source in its own right. Webarchive template wayback links All articles with dead external links Articles with dead external links from November Articles with permanently dead external cokpounding Articles kn dead external links from January Commons category link is on Wikidata Commons category link is on Wikidata using P Line-commutated ibverter LCC used mercury-arc valves until the s,  or thyristors from the s to the present day.
List of electricity sectors Electric energy consumption. The term line-commutated indicates that the conversion process relies on the line voltage of the AC system to which the converter is connected in order to effect the commutation from one switching device to its neighbour.
HVDC converter – Wikipedia
Retrieved 20 December Because thyristors have breakdown voltages of only a few kilovolts each, HVDC thyristor valves are built using large numbers of thyristors connected in series. Consequently, in practice all LCC HVDC systems use either grid-controlled mercury-arc valves until the s or thyristors to the present day. From Wikipedia, the free encyclopedia. The direct current splits equally into the three phases and the alternating current splits equally into the upper and lower valve of each phase.
In the indirect optical triggering method, the low-voltage control electronics sends light pulses along optical fibres to the high-side control electronics, which derives its power from the voltage across each thyristor.
Consequently the harmonic performance is excellent and usually no filters are needed. This gives an output voltage waveform with fewer, larger, steps than the conventional MMC arrangement. 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. As ofthyristor valves had been used on over HVDC schemes, with many more still under construction or being planned.
Two such converters are provided at each end of the scheme, which is of conventional bipolar construction. However, there are several different configurations of voltage-source converter  and research is continuing to take place into new alternatives. Another type of three-level converter, used in some adjustable-speed drives but never in HVDC, replaces the clamping diode valves by a separate, isolated, flying capacitor connected between the one-quarter and three-quarter points.
For this reason the twelve-pulse system has become standard on almost all line-commutated converter HVDC systems, although HVDC systems built with mercury arc valves usually allowed for temporary operation with one of the two six-pulse groups bypassed. A typical MMC for an HVDC application contains around submodules connected in series in each valve and is therefore equivalent to a level converter.
However, all HVDC systems built since the s have used electronic static converters.
The alternative direct optical triggering method dispenses with most of the high-side electronics, instead using light pulses from the control electronics to compkunding light-triggered thyristors LTTs although a small monitoring electronics unit may still be required for protection of the valve.
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. In an attempt to improve on the poor harmonic performance of the two-level converter, some HVDC systems have been built with three level converters.
The mean DC output voltage of a six-pulse converter is given by: Thus, the DC output voltage at any given instant is given by the series combination of two AC phase voltages.
Archived from the original on The two valves corresponding to one phase must never invefter turned on simultaneously, as this would result in an uncontrolled discharge of the DC capacitor, risking severe damage to the converter equipment. Another alternative replaces the half bridge MMC submodule described above, with a full bridge submodule compoundin four IGBTs in an H bridge arrangement, instead of two.
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 hvdd improve the harmonic distortion of the converter. Normally, two valves in the bridge are conducting at any time: During the overlap period the DC voltage is formed from all three phase voltages.
Three-phase, two-level voltage-source converter for HVDC.
Voltage-source converters are also considerably more compact than line-commutated converters mainly because much less harmonic filtering is needed and are preferable to line-commutated converters in locations where space is at a premium, for example on offshore platforms.
For example, if valves V1 and V2 are initially conducting and then valve Compunding is turned on, conduction passes from V1 to V3 but for a short period both of these valves conduct simultaneously. 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.
The last and most powerful mercury arc system installed was that of the Nelson River DC Transmission System in Canadawhich used six anode columns in parallel per valve and was completed in This page was last edited on 18 Novemberat Early LCC systems nvdc mercury-arc valveswith designs that had evolved from those used on high power industrial rectifiers. 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 invertdr.
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.
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Nvdc thyristor plus its grading circuits and other auxiliary equipment is known as a thyristor level. The two conducting valves connect two of the three AC phase voltages, in series, to the DC inverrter. Electronic converters for HVDC are divided into two main categories. 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.
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