High Voltage Direct Current Transmission

Introduction

High-voltage electrical energy (HVDC) technology offers many blessings compared to electrical energy transmission systems. as an example, it permits for additional economical bulk power transfer over long distances. However, the value is a vital variable within the equation. Once put in, HVDC transmission systems square measure Associate in Nursing integral a part of the electric power system, rising stability, dependability, and transmission capability.

Typical utility-scale power plants generate electrical energy (AC) electricity, and most electrical hundreds run on AC power. Thus, the bulk of transmission lines carrying power around the world square measure of the AC sort. However, there square measure instances once high-voltage electrical energy (HVDC) transmission systems provide vital edges.

“One massive advantage to HVDC is that the potency of power transmission over long distances,” Saint George card player, vp of power delivery markets for HDR, told POWER. “If the line route is longer than concerning three hundred miles, DC may be a higher possibility as a result of AC lines have additional line losses than DC for bulk power transfer.

The components of an HVDC transmission system

To assist the designers of transmission systems, the elements that comprise the HVDC system, and the options accessible in these elements, ar conferred and mentioned. The 3 main components of Associate in Nursing HVDC system are the converter station at the transmission and receiving ends, the transmission medium, and the electrodes.

The converter stations at every finish area unit replicas of very different and thus consists
of all the required instrumentation for going from AC to DC or contrariwise. the most part of a converter.

station are:

Transformers: The converter transformers adapt the AC voltage level to the DC voltage level and they contribute to the commutation electrical phenomenon. Usually, they’re of the one section 3 winding type however counting on the transportation necessities and therefore the rated power, they’ll be organized in different ways in which 

AC Filters and capacitance Banks: On the AC facet of a 12-pulse HVDC converter, current harmonics of the order of eleven, 13, 23, twenty-five and better area unit generated. Filters area unit put in so as to limit the number of harmonics to the extent needed by the network. within the conversion method, the converter consumes reactive power that is paid partly by the filter banks and therefore the rest by capacitance banks.

In the case of the three hundred, the reactive power is paid by the series capacitors put in in series between the converter valves and therefore the converter electrical device. The elimination of switched reactive power compensation instrumentality modify the AC switchyard and minimize the number of circuit-breakers required, which can scale back the realm needed for associate degree HVDC station engineered with three hundred. With VSC converters there’s no have to be compelled to compensate any reactive power consumed by the converter itself and therefore the current harmonics on the AC facet area unit connected on to the PWM frequency. so the number of filters during this kind of converters is reduced dramatically compared with natural commutated converters. 

DC filters: HVDC converters produce harmonics all told operational modes. Such armonics will create disturbances in telecommunication systems. Therefore, specially designed DC filters area unit used in order to scale back the disturbances. Usually, no filters area unit required for pure cable transmissions as well as for the consecutive HVDC stations. However, it’s necessary to put in DC filters if the associate degree OH line is employed part or all the transmission.

The filters required to require care of the harmonics generated on the DC finish area unit typically significantly smaller and fewer pricey than the filters on the AC facet. the fashionable DC filters area unit the Active DC filters. In these filters, the passive half is reduced to a minimum and trendy power natural philosophy is used to live, invert and re-inject the harmonics, therefore rendering the filtering terribly effective.

Converting from AC to DC

The challenge, however, is that to transmit via HVDC, 2 converter stations area unit required. First, the AC power should be regenerate to DC to start the transmission method, so once it gets to the required linkup destination, the DC power should be regenerate back to AC to be used on the grid.

Conversion technology is well-established. Electrical pioneers were functioning on the building blocks for HVDC-links back within the late 1800s. standard HVDC converter technology is predicated on the employment of line-commutated or phase-commutated converters. In 1954, ASEA, the forerunner of FTO, used this classic technology utilizing mercury arc valves to construct the world’s 1st business HVDC link between Västervik, on the geographic region of the Scandinavian nation, and Ygne, on the island of Gotland within the Baltic Sea. the initial Gotland link might transfer twenty MW over a 98-kilometer (km)-long submarine cable with a voltage of a hundred kV. The service was re-engineered in 1970, increasing capability to thirty MW at a voltage of one hundred fifty kV through the addition of a thyristor valve bridge.

ASEA continuing to push boundaries, developing new HVDC systems throughout the decades that followed. In 1997, FTO commissioned the world’s 1st HVDC demonstration project victimization voltage supply converters (VSCs). VSC technology uses gate turn-off switch devices, like insulated-gate bipolar transistors (IGBTs), to perform the conversion. AN IGBTs high switch frequency capability permits more-precise VSC management and less-complex circuit configuration through the employment of pulse breadth modulation techniques. FTO named its new VSC-based product HVDC lightweight.

VSC technology was additionally improved once Siemens introduced a standard structure converter (MMC). The Trans Bay Cable project, that runs between the point of entry and Pittsburg, California, was completed in 2010, victimization Siemens’ HVDC and system. MMC technology offers wonderful harmonic performance and reduced power losses compared to previous VSCs. All HVDC makers area unit applying MMC technology in VSCs nowadays.

Transmission medium

For bulk power transmission over land, the foremost frequent transmission medium used is that the overhead line.This overhead line is generally bipolar, i.e. 2 conductors with completely different polarity.

HVDC cables are normally used for submarine transmission. the foremost common varieties of cables are solid and therefore the oil-filled ones. The solid kind is in several cases the foremost economic one. Its insulation consists of paper tapes impregnated with a high viscousness oil. No length limitation exists for this sort and styles are these days available for depths of concerning one thousand m. The self –contained oil-filled cable is totally crammed with an occasional viscosity oil and perpetually works besieged. the most length for this cable kind looks to be around 60 km.

The development of latest power line technologies has accelerated in recent years and these days a replacement HVDC cable is obtainable for HVDC underground or submarine power transmissions. This new HVDC cable is made of extruded polyethylene and is employed in VSC primarily based HVDC systems

Advantages of HVDC Systems

Modern HVDC systems mix the great expertise of the recent installations with recently developed technologies and materials. The result’s a really competitive, versatile and economical method of transmitting electrical energy with a really low environmental impact. It is vital to remark that associate HVDC system not solely transmit wattage from one purpose to
another, however, it additionally includes a heap valuable else that ought to are necessary to unravel by another suggests that inthe case of employing a typical AC transmission