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Search on Infobel for other companies in the category Electromechanical Engineering in Abu Dhabi. Alpha Power Electro Mechanical Contracting Llc. 5 StreetHunan Kukai Electromechanical Co. Ltd. Experts in Manufacturing and Exporting SEC E9 Key Cutting Machine, Alpha Key Cutting Machine and 467 more Products. A Verified CN Gold. Business Type Manufacturer, Trading Company.Apply to 33 Alpha Jobs in Gurgaon on Naukri.com, India's No.1 Job Portal. Explore Alpha job. MIS reporting, HVAC, LT panels, electromechanical, Sewage treatment. trading, Capital Markets, R, Valuation, HFT, Trading, SPSS, C#Die Alpha Wertpapierhandels GmbH ist als unabhängiger Wertpapierdienstleister auf den Handel in deutschen Small & MidCap Werten spezialisiert. Free forex demo account for beginners. 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 long distances or between AC power systems of different frequencies.Almost all HVDC converters are inherently bi-directional; they can convert either from AC to DC (rectification) or from DC to AC (inversion).A complete HVDC system always includes at least one converter operating as a rectifier (converting AC to DC) and at least one operating as an inverter (converting DC to AC).
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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). Early HVDC systems, built until the 1930s, 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 1940s have used electronic (static) converters.Electronic converters for HVDC are divided into two main categories. كتابة اعلان تجاري قصير. 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 1970s, use transistors, usually the Insulated-gate bipolar transistor (IGBT).
As of 2012, both 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 power, with one particular type of converter, the Modular Multi-Level Converter (MMC) emerging as a front-runner.As early as the 1880s, the advantages of DC long-distance transmission were starting to become evident and several commercial power transmission systems were put into operation. Mohamed Saad الشخصي على LinkedIn، أكبر شبكة للمحترفين في العالم. لدى Mohamed2 وظيفة مدرجة على الملف الشخصي عرض الملف الشخصي الكامل على.Welcome To Alpha Innovative Industrial Equipment Trading LLC We The Alpha Innovative Group are involved in a wide range of activities in Construction, Marine, Oil & Gas, Heavy Industrial and Light Market in U. A. E as well as abroad.Our linear systems impress with gearboxes, pinions and racks which are optimally adapted to your requirements. WITTENSTEIN alpha is your supplier for linear.
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For this reason, a line-commutated converter for HVDC is also considered as a current-source converter.Because the direction of current cannot be varied, reversal of the direction of power flow (where required) is achieved by reversing the polarity of DC voltage at both stations.The basic LCC configuration for HVDC uses a three-phase Graetz bridge rectifier or six-pulse bridge, containing six electronic switches, each connecting one of the three phases to one of the two DC terminals. [[A complete switching element is usually referred to as a valve, irrespective of its construction.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.The two conducting valves connect two of the three AC phase voltages, in series, to the DC terminals.
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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 this period, the DC output voltage is given by the average of the voltages of phases 1 and 2, minus the voltage of phase 3.
The overlap angle μ (or u) in an HVDC converter increases with the load current, but is typically around 20° at full load. When just valves 1 and 2 are conducting, the DC voltage is formed from two of the three phase voltages.During the overlap period the DC voltage is formed from all three phase voltages.From the foregoing equation, it is clear that as the firing angle increases, the mean DC output voltage decreases. 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 increased: firing angles of up to 90° correspond to rectification and result in positive DC voltages, while firing angles above 90° correspond to inversion and result in negative DC voltages.
However, the firing angle cannot be extended all the way to 180°, for two reasons.Firstly, allowance must be made for the overlap angle μ, and secondly for an additional extinction angle γ which is needed for the valves to recover their ability to withstand positive voltage after conducting current.The extinction angle γ is related to the turn-off time t (in degrees) With a phase change only every 60°, considerable harmonic distortion is produced at both the DC and AC terminals when the six-pulse arrangement is used. An enhancement of the six-pulse bridge arrangement uses 12 valves in a twelve-pulse bridge.A twelve-pulse bridge is effectively two six-pulse bridges connected in series on the DC side and arranged with a phase displacement between their respective AC supplies so that some of the harmonic voltages and currents are cancelled.The phase displacement between the two AC supplies is usually 30° and is realised by using converter transformers with two different secondary windings (or valve windings).
Usually one of the valve windings is star (wye)-connected and the other is delta-connected.With twelve valves connecting each of the two sets of three phases to the two DC rails, there is a phase change every 30°, and harmonics are considerably reduced.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 number of adaptations were needed to make such valves suitable for HVDC, in particular the use of anode voltage grading electrodes to minimise the risk of arc-back at the very high reverse voltages experienced in HVDC.Much of the pioneering work in this area was performed in Sweden by Dr Uno Lamm, widely considered the “Father of HVDC” and in whose name the IEEE introduced the “Uno Lamm Award” for outstanding contributions in the field of HVDC.The very long anode columns needed for high voltage applications limited the current which could safely be carried by each anode, so most mercury-arc valves for HVDC used several (most often, four) anode columns in parallel per valve.