Harnessing the Power of Giants: The Revolutionary Impact of HVDC Systems

On the first era of Electrical technology development, the electrical power transmission is using direct current (DC) only. The first DC transmission was built in 1882, however when the network is getting larger more constraint is found. For that reason, the Alternating Current (AC) power transmission become more favorable since it was generally proved as an efficient way for long distance electricity transmission. At the present time with the increase of the power sources and its user’s distance plus some introduction of remote power generation source, the AC transmission losses start to become concern and made it increase the CAPEX significantly after some distance. So, the other transmission technology is required to efficiency transmit the power from the generation to the load centers [1]. Due to the condition, electronics industry explore further development on DC transmission for commercial purposed by introducing the HVDC. In 1954, first long distance HVDC transmission are in Gotland Sweden. The system transfers 20 MW over a 98 km long subsea cable between Vasterick on the mainland and Ygne on the island of Gotland with a voltage of 100 KV. With the developments in semiconductor technology, the high-power converters and inverters supporting the further development of HVDC.

The electrical power generation till recent time is based on rotating machines which is inherently alternating in nature.? The current trend in the world is to generate renewable electrical power which is inherently DC in nature.? So far AC power generation and AC distribution system were prevalent in the world. But now with growth of renewables electric energy which is produced in remote location it could be produced and transmitted in DC form. The HVDC technology will enable to integrate the producers of renewable energy with consumers at far distances.

In comparison AC, DC technology provide many advantages and able to make the world to reconsider DC systems. DC lines able to transmit more power per conductor, due to the unavailability of power factor and its reactive power issues. Detail of HVDC advantages in comparison with HVAC system will be further explain in the next article.

With the current domination of HVAC network worldwide, off course it will be more expensive to build new dedicated overhead lines for HVDC systems. This is the main reason why HVDC are not widely adopted since most of the grids are adopting HVAC system [2]. One of the solutions to the situation is by integrating the HVDC system to the HVAC system. Synchronous compensators are required within HVDC links at the receiving end to resolve the integration constraint. It can provide high reliability and regulating capacity which allowed efficient bulk power transmission in particularly from remote location for Renewables energy developments [3].

By 2040, renewable energy sources are expected for more than 50% of the overall generated power. At the last 2019-2020 periods, global renewable energy generation (Solar photovoltaics PV, hydropower, and wind) are increase by 8% in 2021 and more than 6% in 2022 [4]-[5]. According to recent International Energy Agency (IEA) estimate, renewables energy will only be able to meet half of the expected surges in world power demand over the next two years even with this strong growth. Further development to provide effective energy production, transmission and distribution is highly required. Significant renewables resources development which are located remotely from the demand centers are become dependent to the need of efficient long distance bulk energy transmission. For example: Offshore wind farms that increase significantly in the energy market require effective and cost-efficient transmission [6].

As a principal basis process that occurs in an HVDC system is the conversion of electrical current from AC to DC (rectifier) at the transmitting end, and from DC to AC (inverter) at the receiving end. There are three ways of achieving conversion [7]:

·?????? Natural Commutated Converters. Natural commutated converters are most used in the HVDC systems as of today. The component that enables this conversion process is the thyristor, which is a controllable semiconductor that can carry very high currents (4000 A) and is able to block very high voltages (up to 10 kV). By means of connecting the thyristors in series it is possible to build up a thyristor valve, which can operate at very high voltages (several hundred of kV). The thyristor valve is operated at net frequency (50 hz or 60 hz) and by means of a control angle it is possible to change the DC voltage level of the bridge. This ability is the way by which the transmitted power is controlled rapidly and efficiently.

·?????? Capacitor Commutated Converters (CCC). An improvement in the thyristor-based commutation, the CCC concept is characterized using commutation capacitors inserted in series between the converter transformers and the thyristor valves. The commutation capacitors improve the commutation failure performance of the converters when connected to weak networks.

·?????? Forced Commutated Converters. This type of converters introduces a spectrum of advantages, For example: feed of passive networks (without generation), independent control of active and reactive power, power quality. The valves of these converters are built up with semiconductors with the ability not only to turn-on but also to turn-off. They are known as VSC (Voltage Source Converters). Two types of semiconductors are normally used in the voltage source converters: the GTO (Gate Turn-Off Thyristor) or the IGBT (Insulated Gate Bipolar Transistor). Both have been in frequent use in industrial applications since early eighties. The VSC commutates with high frequency (not with the net frequency). The operation of the converter is achieved by Pulse Width Modulation (PWM). With PWM it is possible to create any phase angle and/or amplitude (up to a certain limit) by changing the PWM pattern, which can be done almost instantaneously. Thus, PWM offers the possibility to control both active and reactive power independently. This makes the PWM Voltage Source Converter a close to ideal component in the transmission network. From a transmission network viewpoint, it acts as a motor or generator without mass that can control active and reactive power almost instantaneously.

The components of an HVDC transmission system are the converter station at the transmission and receiving ends, the transmission medium, and the electrodes [7].

The converter stations at each end are replicas of each other and therefore consists of all the needed equipment for going from AC to DC or vice versa. The main component of a converter station are Thyristor valves, VSC valves, Transformers, AC Filters and Capacitor Banks, DC filters.?

For bulk power transmission over land, the most frequent transmission medium used is the overhead line. This overhead line is normally bipolar [7]?

The transmission technology is critical enabling factor for the renewable energy power development. HVDC transmission are providing the viable option bulk power transmission in the cost-efficient manner. Solar and wind farm renewable generated in the remote area can be transmitted to load center across the country or continent through HVDC

?HVDC is a mature technology that has been in use for more than 50 years. During the first 30 years, it was more of a specialized segment technology, with a limited application. With the changes in demands due to evolving environmental needs, HVDC has become a common tool in the design of future global transmission grids. Key factors for this have been the recent developments within HVDC, with the step-up in voltage up to 800 kV as well as the VSC technology. In addition, hybrid arrangement development between HVDC with HVAC gird provide instant solution for power transmission demand. With these developments, remote sources of renewable energy can now be tapped that were previously inaccessible. This approach will enable the world to meet energy demand through renewable resources and reduce impact on environment.

In conclusion, HVDC technology stands as a beacon of innovation in the realm of electrical power transmission. With its unparalleled efficiency and reliability, it not only overcomes the limitations of traditional AC systems but also opens new horizons for renewable energy integration and long-distance power delivery. As we move towards a more interconnected and sustainable future, HVDC technology will undoubtedly play a pivotal role in shaping the energy landscapes of tomorrow. Its continued evolution is a testament to human ingenuity and our relentless pursuit of technological advancement for a better world.

List of Reference:

[1] J. Smede, C. G. Johansson, O. Winroth. “Design of HVDC converter stations with respect to audible noise requirements.” IEEE Transactions on Power Delivery, 10, No.2(1995) 747–758.

[2] D. M. Larruskain, I. Zamora, O. Abarrategu. “Conversion of AC distribution lines into DC lines to upgrade transmission capacity.” Electric Power Systems Research 81 (2011) 1341–1348

[3] F. H. Gandoman, A. Ahmadi, J. Pou, V. G. Agelidis “Review of FACTS technologies and applications for power quality in smart grids with renewable energy systems”.? Renewable and Sustainable Energy Reviews, 82(2018) 502–514.

[4] IEA.Electricity market? report? -? July? 2021.? [Online].? Available: https:// www.iea.org/ reports/ electricity-market-report-july-2021

[5] IEA.? Electricity? information:? overview.? IEA,? Paris.? (2021). [Online].? Available:? https: // www.iea.org/ reports/ electricity-information-overview

[7] R.Rudervall, J.P Charpentier, R. Sharma. High Voltage Direct Current (HVDC) Transmission System Technology Review Paper. Energy Week 2000 presentation