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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HVDC is used as an alternative to AC for transmitting electrical energy over long distances or between AC power systems of different frequencies.
Some HVDC systems take full advantage of this bi-directional property for example, those designed for cross-border power trading, such as the Cross-Channel link between England and France. In such schemes, power flow in the non-preferred direction may have a reduced capacity or poorer efficiency. HVDC converters can take several different forms.
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.
However, all HVDC systems built since the s have used electronic static converters. Electronic converters for HVDC are divided into two main categories.
Line-commutated converters HVDC classic are made with electronic switches that can only be turned on. Voltage-sourced converters are made with switching devices that can be turned both on and off.
Line-commutated converters LCC used mercury-arc valves until the s,  or thyristors from the s to the present day.
Inerter 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 market for voltage-source converter HVDC is growing fast, driven partly by the surge in investment in offshore wind invertdrwith one particular type of converter, the Modular Multi-Level Converter MMC  emerging as a front-runner.
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. From the s onwards,  innverter 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.
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. 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 DC voltage is a serious disadvantage.
Consequently, in practice all LCC HVDC systems use either grid-controlled mercury-arc valves until the s or thyristors to the present day.
In a line-commutated converter, the DC current does not change direction; it flows through a large inductance and can be considered almost constant. On the AC side, the converter behaves approximately as a current source, injecting both grid-frequency and harmonic currents into the AC network.
For this reason, a line-commutated converter for HVDC is also considered as a current-source converter. 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 phases to one of the two DC terminals. Normally, two valves in the bridge are conducting at any time: 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 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. The mean DC output voltage of a six-pulse converter is given by: 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 invertrr An enhancement of the six-pulse bridge arrangement uses 12 valves in a twelve-pulse bridge.
Usually one of the valve windings is star wye -connected and the other is delta-connected. 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 inverted operation with one of the two six-pulse groups bypassed.
Early LCC systems used mercury-arc valveswith designs that had evolved from those used on high power industrial rectifiers. 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 conpounding anode columns.
Mercury arc valves for HVDC were rugged but required high maintenance.
HVDC converter – Wikipedia
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. 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 Mercury arc valves were also used on the following HVDC projects: Because thyristors have breakdown voltages of only a few kilovolts each, HVDC thyristor valves are built using large numbers of thyristors connected in series.
Additional passive components such as grading capacitors and resistors need to be connected in parallel with each thyristor in order to ensure that the voltage across the valve is shared uniformly between the thyristors. The thyristor plus its grading circuits and other auxiliary equipment is known as a thyristor level. Each thyristor valve will typically contain tens or hundreds of thyristor levels, each operating at a different high potential with respect to earth.
The isolation method can be magnetic using pulse transformers but is usually optical. Two optical methods are used: 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. The alternative direct optical triggering method dispenses with most of the high-side electronics, instead using 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.
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. 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. 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.
As a result, IGBTs can be used to make self-commutated converters. In such converters, the polarity of DC voltage is usually fixed and the DC voltage, being smoothed by a large capacitance, can be considered constant. 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-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.
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. 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.
This simplifies the construction of the converter transformer. However, there are several different configurations of voltage-source converter  and research is continuing to take place into new alternatives. 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. 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. 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.
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. 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.
Three-level converters can synthesize three instead of only two discrete voltage levels at the AC innverter of each phase: A common type of three-level converter is the diode-clamped or neutral-point-clamped converter, where each phase contains four IGBT valves, each rated at half of the DC line to line voltage, along with two clamping diode valves. In this latter state, the two clamping diode valves complete the current path through the phase. 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.
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. Both the diode-clamped and flying capacitor variants of three-level converter can be extended to higher numbers of output levels for example, fivebut the complexity of the circuit increases hvddc and such circuits have not been considered practical for HVDC applications.
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. 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. Each MMC valve consists of a number of independent converter submoduleseach containing its own storage capacitor.
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. Each submodule therefore acts as an independent two-level converter generating a voltage of either 0 or U sm invertet U sm is the submodule capacitor voltage.
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. The MMC nvdc from other types of compoounding in that current flows continuously in all six valves of the converter throughout the mains-frequency cycle. The direct current splits equally into the three phases and the alternating current splits equally into the compouunding and lower valve of each phase.
A typical MMC for an HVDC application contains around submodules connected in series in each valve and is therefore equivalent to a level converter. Consequently the harmonic performance is excellent and usually no filters are needed. The MMC has two principal disadvantages. Firstly, the control is much more complex than that of a 2-level converter. 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.
Secondly, the submodule capacitors themselves are large and bulky. A variant of the MMC, proposed by one manufacturer, involves connecting multiple IGBTs in series in each of the two switches that make up the submodule. This gives an output voltage waveform with fewer, larger, steps than the conventional MMC arrangement.
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. 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.
However, the full-bridge arrangement requires twice as many IGBTs compoundibg 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. From Wikipedia, the free encyclopedia. Archived from the original on Retrieved 20 December Arc-fault circuit interrupter Earth leakage circuit breaker Residual-current device GFI Power-system protection Protective relay Digital protective relay Sulfur hexafluoride circuit breaker.