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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For this reason, a line-commutated converter for HVDC xompounding also considered as a current-source converter. Various other types of converter have been proposed, combining features of the two-level and Modular Multi-Level Converters.
As a result, IGBTs can be used to make self-commutated converters. Voltage-source converters are also considerably more compact than line-commutated compkunding 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.
Like the two-level converter and the six-pulse line-commutated inverher, a MMC consists of six valves, each connecting one AC terminal to one DC terminal. Views Read Edit View history. 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 ijverter line-commutated and voltage-source technologies are important, with line-commutated converters used mainly where very high capacity and efficiency are invedter, 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-level converter is the simplest type compounxing three-phase voltage-source converter  and can be thought of as a six pulse bridge in which the thyristors have been replaced by IGBTs with inverse-parallel diodes, and the DC smoothing reactors have been replaced by DC smoothing capacitors.
The basic LCC configuration for HVDC uses a three-phase Graetz bridge rectifier or six-pulse bridgecontaining six electronic switches, each connecting one of the three vhdc to one of the two DC terminals.
In contrast to line-commutated HVDC converters, voltage-source converters maintain a constant polarity of DC voltage and power reversal is achieved instead by reversing the direction of current. 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.
Each MMC valve consists of a number of independent converter submoduleseach containing its own storage capacitor.
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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. 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 compouncing of harmonic distortion.
Both the diode-clamped and flying capacitor variants of three-level converter can be hvfc to higher numbers of output levels for example, fivebut the complexity of the circuit increases disproportionately and such circuits have not been considered practical for HVDC applications. The market for voltage-source converter HVDC is growing fast, driven partly by the surge in investment in offshore wind powerwith one particular type of converter, the Modular Multi-Level Converter MMC  emerging as a front-runner.
The two conducting valves connect two of the three AC phase voltages, in series, to the DC terminals.
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. 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.
Secondly, the submodule capacitors themselves are large and bulky. 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.
As ofthyristor valves had been used on over HVDC schemes, with many more still under construction or being planned. Consequently the harmonic performance is excellent and usually no filters are needed.
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. Arc-fault circuit interrupter Earth leakage circuit breaker Residual-current device GFI Power-system protection Protective relay Digital protective relay Sulfur hexafluoride circuit breaker.
In the most common form of the circuit, the half-bridge variant, each submodule contains two IGBTs connected in series across the capacitor, with the midpoint connection and one of the two capacitor terminals brought out as external connections. The DC output voltage of comlounding converter steadily becomes less positive as the firing angle is increased: 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.
HVDC converters can take several different forms. Operating principle of 2-level converter, single-phase representation Voltage in Graph: Although HVDC converters can, in principle, be constructed from diodes, such converters can only be used in rectification mode and the lack of controllability of the Yvdc voltage is a serious disadvantage. In other projects Wikimedia Commons. In fact, with a line-commutated converter, the firing angle represents the only fast way of controlling the converter.
HVDC converter – Wikipedia
Early HVDC systems, built until the s, were effectively rotary converters and used electromechanical conversion with motor – generator sets connected in series on the DC side and in parallel on the AC side. The additional controllability gives many advantages, notably the ability to switch the IGBTs on and off many times per cycle in order to improve the harmonic performance, and the fact that being self-commutated the converter no longer relies on synchronous machines in the AC system for its operation.
Voltage-sourced converters are made with switching devices that can be turned both on and off. The alternative direct optical triggering method dispenses with most of the high-side electronics, instead hcdc light pulses from the control electronics to switch light-triggered thyristors LTTs although a small monitoring electronics unit may still be required for protection of the valve.
Line-commutated converters LCC used mercury-arc valves until the s,  or thyristors from the s to the present day. Because thyristors can only be turned on not off by control action, and rely on the external AC system to effect the turn-off process, the control system only has one degree of freedom — when to turn on the thyristor.
However, there are several different configurations of voltage-source converter  and research is continuing to take place into new alternatives. Usually, each arm of each six-pulse bridge consisted of only one mercury-arc valve, but two projects built in the former Soviet Union used two or three mercury-arc valves in series per arm, without parallel connection of anode columns. This page was last edited on 18 Novemberat Each thyristor valve will typically contain tens or hundreds of thyristor levels, each operating at a different high potential with respect to earth.
HVDC systems based on voltage-source converters normally use the six-pulse connection because the converter produces much less harmonic distortion than a comparable LCC and the twelve-pulse connection is unnecessary. With some other types of semiconductor device such as the insulated-gate bipolar transistor IGBTboth turn-on and turn-off can be controlled, giving a second degree of freedom.
Consequently, in practice all LCC HVDC systems use either grid-controlled mercury-arc valves until the s or thyristors to the present day. Balancing the voltages of each of the submodule capacitors is a significant challenge and requires considerable computing power and high-speed communications between the central control unit and the valve. 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.
Two optical methods are used: