Recent Technical Development in High Voltage Direct Current (HVDC) Technology, a special Emphasis to Indian projects

Title:

Recent Technical Development in High Voltage Direct Current (HVDC) Technology, a special Emphasis to Indian projects.

Abstract

Apart from transmission of bulk power High Voltage Direct Current (HVDC) technology is also used for various other purposes. Since its invention several research and development activities are going on in this field. The HVDC can give Grid connectivity to weak AC networks, can interconnect asynchronous AC networks. New HVDC technology has features of High dynamic performance, Independent control of active and reactive power and black start capability. The evolution of HVDC technology has given solution to many problems. As well this technology has improved the power transmission situation across the world. The HVDC technology can be classified based on application, feasibility, technology, etc. The most important component of HVDC system is the converters, which convert AC supply to DC and vice versa. These converters can be current source converters or Voltage source converters.

The research and development activities are going on in both of these converter technologies. Engineers and scientists are always trying to optimize the HVDC technology for minimizing capital expenditure, transmission losses and space requirement. Now bulk power transmission can be possible over long distances upto several thousand kilometers.

Whether it is reduction in CO2 emission or transmission of power from offshore wind firms and other renewable sources or providing stability to the grid during evacuation of renewable power, HVDC technology has contributed a lot in the present endeavour to provide Green energy.

This paper provides information on the recent technical development in High Voltage Direct Current (HVDC) Technology utilized in several recent projects. This paper will give a special emphasis on the HVDC projects under operation and under construction in India.

Keywords:     HVDC, VSC, Thyristor, IGBT, Valves, Converter, MMC, LCC

Author: Abhijit Roy

Date : 05-Nov-2018

Technical Development of HVDC System:

HVDC system controllers based on line-commutated converter technology (LCC) have a long and successful history. Since the invention of mercury arc rectifiers in the nineteen-thirties the line-commutated current sourced converter technology has evolved as one of the reliable and mature technology for HVDC transmission. The replacement of mercury arc valves by thyristor valves was a major development. The thyristor valves were put into operation in HVDC projects during late nineteen-seventies. Thyristors have been the key components of this converter topology and have reached a high degree of maturity due to their robust technology and their high reliability. The outdoor valves for Cahora Bassa were designed with oil-immersed thyristors with parallel/series connection of thyristors and an electromagnetic firing system. Further development went via air-insulated air-cooled valves to the air- insulated water-cooled design, which is still state of the art in HVDC valve design. The development of thyristors with higher current and voltage ratings has eliminated the need for parallel connection and reduced the number of series-connected thyristors per valve. The development of light-triggered thyristors has further reduced the overall number of components and thus contributed to increased reliability. Now-a-days HVDC converters with LCC topology are usually built as 12-pulse circuits. This is achieved by using thyristor valves and 3 phase transformer. Innovations in almost every other area of HVDC have been constantly adding to the reliability of this technology with economic benefits for users throughout the world.

Fig.1-1 shows a typical cost comparison curve between AC and DC transmission. The DC curve is not as steep as the AC curve due to considerably lower line costs per kilometre. Typically the break-even distance is in the range of 500 to 800 km depending on a number of factors.

Fig: 1.1 : Cost comparison between AC & DC transmission system

500kV DC transmission system with Line commuted converter topology are being used extensively for long distance upto about 1000km

Recent trend is for Ultra High Voltage DC (UHVDC) transmission by using line-commutated converters for long distance power transmission. Line-commuted Thyristor technology is the preferred solution for bulk power transmission through UHVDC due to the low losses. UHVDC technology with advanced control is particularly suitable for countries like China, India and Brazil, where consumption centers are usually far from available power sources. China is the leader in constructing 800kV HVDC installation. Several 800kV HVDC links are under construction in China for transferring hydro electric power from the western region of the country to the load centers at eastern and southern region over a distance of 1500km to 2000km with typical power rating of 6000 MW and above. There are four UHVDC projects under construction in India. By increasing the voltage level of the transmission, considerable advantages for the environment are gained, such as lower transmission losses and smaller transmission line right-of-ways.

The world’s first 1100 kV DC UHVDC transmission link is under construction between Changji and Guquan in China. This is the biggest HVDC project in the world with a power transfer capacity of 12000 MW and the length of the link is 3,284 kilometers.

HVDC with LCC use power electronic components and conventional equipment which  can be combined in different configurations to switch or control reactive power, and to convert the active power. Conventional equipment (e.g. breakers, tap-changing transformers) has very low losses, but the switching speed is relatively low. Power electronics can provide high switching frequencies up to several kHz which, however, leads to an increase in losses.

It is, however, worth mentioning that line-commutated converters have some technical restrictions in a way that the commutation within the converter requires proper conditions of the connected AC system, such as a minimum short-circuit power.

Power electronics with self-commutated converters can cope with the limitations in LCC and provide additional technical features. In many applications, the VSC has become a standard of self-commutated converters and will be used more often in transmission and distribution systems in the future. Some of the advantages of Voltage source self-commutated converters are independent control of active and reactive power, the capability to supply weak or even passive networks, the black start requirement, lower space requirements and high dynamic performance. Voltage-Sourced Converters do not require any “driving” system voltage; they can build up a 3-phase AC voltage using the DC voltage. This kind of converter uses power semiconductors with turn-off capability such as IGBTs (Insulated Gate Bipolar Transistors).

Some VSC converters for HVDC applications have been based on two or three-level technology which enables switching two or three different voltage levels to the AC terminal of the converter. The converter voltage created by the PWM (Pulse-Width Modulation) in two or three level technology needs AC filters to achieve an acceptable smooth waveform.

The latest technology uses Modular Multilevel Converter (MMC) approach for Voltage source converters.(VSC), which gives Small Converter AC Voltage Steps, Low Rate of Voltage Rise, Low Generation of Harmonics, Low HF Noise and Low Switching Losses. There is no requirement of AC filters in MMC technology. In the worst case there may be negligible need for AC voltage filtering to achieve an acceptable smooth waveform.

When renewable energies, such as large wind farms, have to be integrated into the system, particularly when the connecting AC links are weak and when sufficient reserve capacity in the neighboring systems is not available , VSC HVDC link can help to solve this problem. This can substantially help reduce CO2 emissions. The oil platforms can be supplied from the coast via VSC HVDC link, so that gas turbines or other local power generation on the platform can be avoided. Furthermore, with its technical performance and its space-saving design VSC HVDC is tomorrow’s Solution for the supply of megacities.

Recent Innovation in HVDC:

Many point-to-point HVDC links have been installed in different part of the world. Now the research and development activities are going on to connect point-to-point HVDC links and make a reliable network. This will facilitate for decreasing transmission losses, load balancing, integration of intermittent renewable plants and trading of energy. One of the major challenges for HVDC grid is the absence of HVDC circuit breaker for interrupting current and isolating fault very fast. Typical HVDC circuit breakers are too slow in controlling DC current for ideal and reliable grid operation. This is very challenging to design and manufacture HVDC circuit breaker due to the requirement for installing additional passive components.

Recently developed hybrid DC breaker by ABB combines ultrafast mechanical actuators and power electronics switching, which is capable of blocking and breaking DC currents very fast at thousands of amperes and several hundred thousands of volts.

Recently Siemens MRTB (Metallic Return Transfer Breaker) DC commutation breaker with a rated current of 5000 ampere has been successfully tested at its Xiluodu-Zhejiang HVDC transmission system in China.

HVDC System in India:

The requirement of power generation in India has increased with the rapid increase in industrialization due to economic liberalization since 1991. Most of power generation was from the coal thermal stations and these stations are located in the central and eastern India. Industrialization in India happened near the big cities, like Delhi, Mumbai, Bangalore. There was urgent need to bring power nearby these big cities. The HVDC transmission system has facilitated this.

All the long distance HVDC transmission systems in India were constructed to transfer power from the thermal power stations, except NER-Agra HVDC transmission system. The NER-Agra transmission system was planned for transmission of bulk power from the hydro-electric power stations in North-East India. Five bipole HVDC systems were installed in India till June’2012, Viz, Chandrapur – Padghe (1500 MW, +/- 500 kV DC), Rihand – Dadri (1500 MW, +/- 500 kV DC), Talcher – Kolar(2500MW, +/- 500 kV DC), Balia – Bhiwadi(2500 MW, +/- 500 kV DC) and Mundra – Mohindergarh(2500 MW, +/- 500 kV DC).

The first Bipole HVDC transmission system was commissioned in India between Rihand in Uttar Pradesh state and Dadri in the same state of the country . The Rihand-Dadri HVDC link was commissioined in 1990. The converters of Chandrapur – Padghe and Rihand – Dadri Bipole HVDC system were supplied by ABB, Sweden. The converters of Talcher – Kolar , Balia – Bhiwadi and Mundra – Mohindergarh Bipole HVDC system were supplied by Siemens, Germany . Thyristors were used as the converters in all these five HVDC projects. These Bipole HVDC links can be used as monopolar link also by changing the DC circuit scheme. All these HVDC transmission systems have used Line commutated converters (LCC) based on thyristor valves .

The Ballia-Bhiwadi project was the first HVDC project in India using the modern state of art technology of direct light-triggered thyristors (LTT) as converters. The LTT technology gives high reliability, as well as little maintenance is required due to the compact and economic design of thyristor valves. Same converter technology was used in Balia-Bhiwadi and Mundra – Mohindergarh, whereas Electrically triggered thyristros (ETT) were used in Chandrapur – Padghe, Rihand – Dadri and Talcher – Kolar projects.

The Mundra – Mohindergarh project was the first HVDC project in India constructed by any private company and till date this is the only HVDC project in private sector. This HVDC system is running almost on full load capacity round the year. The reliability and availability of this HVDC transmission system is more than 99% This HVDC system is used for transmission of bulk power generated in Western (Gujarat) region of India to Northern (Haryana) region.

The first Back-to-Back HVDC project in India was commissioned in 1989 at Vindhachal in central India, which has a rating of 500 MW. There are another three Back-to-Back HVDC stations installed in India, viz Chandrapur, Gazuwaka (Vizag) and Sasaram.

One R&D HVDC project was taken up in 1985 with the objective to build indigenous know-how for the design, manufacture and supply of various hardware components of a HVDC system. This R&D HVDC transmission link between Barsur and Lower Sileru inside Indian state of Andhra Pradesh was commissioned in August,1989 with a rated power flow of 100 MW. This is considered as the first HVDC project of India. This HVDC link was designed for 100 kV DC voltage in Monopolar Ground return operation for a distance of 196km. This was upgraded in 1999 for 200 MW and 200kV DC voltage. But this is not in operation since 2014.

UHVDC System in India:

All the initial five Bipole HVDC transmission systems had voltage level of ±500 kV DC. During the year 2003 Government of India initiated schemes for generating 50GW power by utilizing the huge untapped hydro electric potential of North-Eastern region. This was also planned to transfer this hydro electric power to the Northern grid in India. As a result India’s first UHVDC transmission system with the voltage level of ±800 kV DC and a length of 1,728 km was planned between Biswanath Chariali in North-Eastern region and Agra in Northern Region. The capacity of this UHVDC transmission system is 6000MW. This transmission system is a multi terminal UHVDC system, whose third intermediate station is located at Alipurduar in Eastern Region of India. This multi-terminal UHVDC link will have three converter stations – two “sending” stations will convert power from AC to DC for transmission over a single power line and deliver it to a “receiving” station in Agra, where it will be converted back into AC. This is the only multi terminal HVDC system in India as on date and this is also the first multi-terminal HVDC link in the world commissioned by ABB, Sweden. This project is being constructed by a joint venture between state run BHEL and ABB, Sweden.

The 1,365 km long 2nd and 3rd UHVDC link with rated transmission voltage of ±800 kV DC and individual capacity of 3000MW between Champa and Kurushetra is under construction since 2012. It will transfer bulk power from the Chhattisgarh state of India, a hub of private sector Power Producers of coal thermal power, to the load centre in the northern region of the country. These two UHVDC links will run in parallel from Champa to Kurushetra with a total capacity of 6000MW. These UHVDC links were awarded to by Alstom, France by India’s largest transmission utility Power Grid Corporation of India Ltd. The first Bipole is already in operation.

Another UHVDC link will connect Raigarh in Central India to Pugalur in the southern state of Tamil Nadu , which is one of the longest transmission link in the world. The length of this UHVDC link is 1,830 km and capacity is 6000 MW. This project is being constructed by a joint venture between state run BHEL and ABB, Sweden.

Voltage Source Converter (VSC) HVDC in India:

All the HVDC and UHVDC transmission systems in India have used current source converters for converting AC to DC and vice versa. The HVDC connection of about 200km between Pugalur (Tamil Nadu) in southern region to Trichur (Kerala) also in Southern region will be India’s first HVDC link featuring voltage-sourced converter (VSC) technology. The transmission voltage level is ±320 kV DC and the total capacity is 2000 MW. The Trichur converter station will be connected via underground XLPE HVDC cable to a intermediate transition station at a distance of 32 km. The bulk power will be transmitted via an overhead line from the converter station at Pugalur to the intermediate transition station. Siemens, Germany will be supplying two converter stations with two parallel converters, each rated 1000 MW featuring its MMC VSC HVDC technology, while Sumitomo Electric, Japan will be responsible for XLPE HVDC cable system in the DC circuit. The HVDC underground cable is also being used for the first time in India.

Cross border HVDC project in India:

Two Back-to-Back HVDC stations of 500 MW each have been commissioned at Behramara in Bangladesh for transmission of Power from Eastern Region of India to Bangladesh. Both the HVDC stations are based on the Thyristor valve converter technology with LCC topology and have been commissioned by Siemens, Germany. The light-triggered thyristors (LTT) have been used in the B-t-B stations.

The 3rd back-to back HVDC station of capacity 500kV is planned at Comilla, Bangladesh for transmission of power from North-East Region of India to Bangladesh through Tripura.

Potential for such future HVDC also exists between India-Sri Lanka, India - Myanmar and India Pakistan. These are in various stages of planning.

REFERENCES

[1]   Siemens AG , Energy Sector, Freyeslebenstrasse 1 , 91058 Erlangen, Germany, “High Voltage Direct Current Transmission –Proven Technology for Power Exchange,” Book

[2]    M. Davies, M. Dommaschk, J. Dorn, J. Lang, D. Retzmann, D. Soerangr , “HVDC PLUS – Basics and Principle of Operation” , Technical article