Presented at Cigré Symposium, Kuala Lumpur,Malaysia, September 1999 Viability of a National HVDC Transmission Ring in India Mata Prasad* Rebati Dass S. Chakravorty Carl-Göte Carlsson Göran Isacsson Asea Brown Boveri Ltd., Power System Engineering & ABB Power Systems AB, Business Development, New Delhi, Sweden India Summary The prevailing highest AC and DC transmission system The paper presents the current scenario in India as voltages are 400 kV and ± 500 kV, respectively. The 400 kV AC transmission system has matured enough in regards the EHV AC and DC transmission backbone Northern, Western and Southern regions and will systems. become even stronger in a few years time. In respect of The ultimate goal of the transmission sector is to create HVDC there are a few existing transmission links and an integrated national power system grid for the back-to-back stations, and a few others under effective distribution of electrical energy throughout the construction or in bidding stage, which have been country. detailed in section 4 below. This paper proposes HVDC as an attractive alternative However, there is an ever-increasing demand on more for the transformation of the network from independent and reliable electric power supply from all consumer regional grids to a fully integrated national grid, and sectors. It is estimated that India will need an additional details how the existing, decided and planned HVDC capacity of about 100000 MW over next 15 years for systems could be utilised in such a scheme. ensuring sustained economic growth of about 7 to 8%. A significant part of this capacity is likely to come from A conceptual scheme and control strategy is presented the unexploited vast hydro-potential in the Himalayan to achieve an HVDC national ring-main. Advantages of region, namely; Nepal and the North-Eastern part of such a scheme in terms of its reliability, controllability India. The geological experts put the estimated figures and immunity to different grid disturbances are in the range of 70000-85000 MW. To transmit such enumerated. The paper also outlines the challenges capacity additions would require commensurate bulk involved in terms of algorithms required for control co- power transmission schemes that would include a ordination among different HVDC links and load number of HVDC bipoles. despatch centres for achieving such a system. 2. Need for Integrated Grid Operation Keywords The Indian regional grids are quite different from each National Grid - HVDC Transmission - HVDC other in terms of composition of power generation. Converter - HVDC Bipole - HVDC Back-to-Back - Some have a good mix of hydro and thermal generation HVDC Ring-main - Bulk Power-Control System - while others have predominantly hydro or thermal Communication System - Load Despatch Centre – Optic generation. Due to staggering of monsoon season, even Fibre. the nature of hydro generation is somewhat different as 1. Introduction it peaks at different times of the year in Southern and other regions. The Indian power systems have been developing over the years in the form of integrated independent regional In order to achieve effective and optimal utilisation of grids based on regional self-sufficiency. The entire electrical energy and transmission resources in the national power system has been divided into five country, it is vital that all the five regional power grids regional grids, namely; Northern, Western, Southern, in the country are strengthened and integrated to form a Eastern and North-Eastern. The combined installed national power grid. The main impediments in generating capacity of these Regions in the recent past achieving this, much needed, objective are as follows: (1997) was around 86120 MW with each of the above − Lack of state-of-the-art regional load despatch Regions accounting for 24567, 25064, 20477, 14383 centres (RLDC) and communication facilities. and 1629 MW, respectively. As on date the combined capacity is more than 89000 MW. − Lack of rational tariff structure for inter-regional The state and central governments treat electrical power power exchange. as a concurrent subject. The regulations of power and − Large difference in operating frequency of different overall control are a joint effort and the system is regions due mainly to the lack of grid discipline, undergoing change for optimal control. which can be brought about through state-of-the-art RLDCs. * Asea Brown Boveri Ltd., 4th Floor, 71 Guru Angad Bhawan, Nehru Place, New Delhi, India − Inadequacy of high capacity trunk lines and reliable control systems and/or from RLDCs and national ring-main system(s). load despatch centre (NLDC). 3. Means to Achieve Integrated Grid Operation − Possibility of forced stabilisation of the national grid in the event of disturbances through various The ultimate goal of evolving a national power grid can “defence plans”. Isolation of regional grids will be best be achieved by keeping in view the nature of the possible by safe blocking, in case of proliferation of existing and planned bulk power transmission systems a grid disturbance. in different regions of the country as well as present and near future interconnections between different regions. 3.3 Hybrid Ring Different possible options of the integration are After the different regional grids stabilise in terms of discussed below. their operating parameters, the interconnection through 3.1. EHVAC Ring 400 kV AC lines at the regional boundaries can be done smoothly without creating any problems in the As indicated above, the highest existing EHVAC operation of the concerned grids. Such a regional transmission system consists of a fairly strong network interconnection can co-habit with an HVDC ring-main. of 400 kV lines and associated substations. It is During the period when such an interconnection is not possible, and is being attempted, to interconnect possible, already implemented and in the process of adjoining regional grids by building rather short 400 kV being implemented HVDC back-to-back links will serve lines at the boundaries. At present, it is not possible to the purpose of interconnection on a small-scale power operate regions interconnected in this fashion as exchange basis. uncontrolled power flow takes place on such links due to difference in operating frequency of the 4. Present and Future HVDC Schemes interconnected grids. Since the existing RLDCs are not India is one of the few countries having a large number equipped to bring about effective grid control and of HVDC schemes in operation; under commissioning, discipline, practically it seems impossible to achieve the construction and planning. The HVDC schemes objective. Moreover, due to limited power carrying presently in operation are: capacity of 400 kV lines, it is impossible to realise a reliable ring-main system. − 2x250 MW, 70 kV Vindhyachal back-to-back link There is another possibility of having a national ring- connecting Northern and Western regions. main system comprising 735/765/800 kV lines. As a − 1500 MW, ±500 kV, 814 km long bipole link matter of fact, there are few 765 kV lines under between Rihand and Dadri in Northern region. construction and some of which initially are to be operated at 400 kV. But the extent of 765 kV network − 2x500 MW, 140 kV Chandrapur back-to-back link required to form a kind of national ring-main would be connecting Southern and Western regions. enormous. However, it could be possible to have a kind The links under erection/commissioning are: of ring-main on regional basis. − 1500 MW, ±500 kV, 752 km long bipole link 3.2 HVDC Ring between Chandrapur and Padghe in Western region. There are a number of bulk power HVDC transmission − 500 MW, 140 kV Vizag back-to-back link systems in operation, under construction as well as connecting Southern and Eastern regions. under medium and long term planning. These systems are very strong and involve long intra and inter-regional Those under bidding stage are: distances. One attractive alternative could be to − 2000 MW, ±500 kV, 1376 km long bipole link transform the existing and upcoming HVDC systems between Talcher and Kolar between Eastern and into a backbone national ring-main system by adding a Southern regions. few missing links, modern control and communication systems. − 500 MW, 140 kV Sasaram back-to-back link The HVDC ring will become a kind of overlay EHV connecting Northern and Eastern regions. transmission system for 400 kV AC network. There will There are a couple of HVDC links under active be some definite advantages of an HVDC ring-main consideration for implementation within 3-6 years time system, such as: from now. Most of these are bulk power bipole transmission systems. So far as long term planning is − Short circuit level will be contained thereby concerned, beyond 6 years from now, a large number of obviating the need for either replacement of exiting high capacity long distance bipole schemes are being 400 kV equipment that are rated for 40 kA or considered for evacuation of power from hydro power putting series reactors. stations from North-Eastern part of India . Though at − Full control over power flow, which can be conceptual stage, there is a likelihood of long distance exercised either through individual HVDC link bulk power HVDC bipole lines from the neighbouring country of Nepal . 5. Feasibility of Integrating HVDC Schemes Dadri Dehang After having so many HVDC links, it is prudent to extract maximum benefits from the huge investment, Ballabhgarh Rihand versatility and superior technical features of these links. It is interesting to note that the following factors favour, even more, the integration of the HVDC links into an Jaipur Talcher HVDC ring-main system: Vadodra − The existing HVDC transmission lines have capacity Ib Valley to carry more than 2500 MW power continuously as against their present utilisation of 1650 MW (low Padghe Chandrapur ambient). − In view of the available transmission line capacity, it is possible to augment HVDC converter stations, Kolar e.g. by addition of new converters in parallel, in future to create more power transmission capacity. − Too strict environmental regulations will allow very Mangalore Madras restricted availability of new transmission corridors, even adjacent to the existing ones. Existing/under construction Under planning/bidding − All the major regions will have HVDC bipoles in Likely to be considered in future operation in near future, 3-6 years from now, on Figure 1: Conceptual HVDC ring-main scheme either intra or inter regional basis. In fact, this scheme has two levels of interconnections, − In progress asynchronous inter-regional connections namely, one in the form of an HVDC ring-main and through HVDC back-to-back links will provide another in the form of inter-regional asynchronous second line of defence against grid disturbances. In interconnections through HVDC back-to-back links. fact, to be more effective for optimal operation, all This two level interconnection is shown in Figure 2. the inter-regional back-to-back links should be of There are four HVDC back-to-back links with the 2x500 MW rating. names and capacities as indicated above. These are Conceptually, it is possible to achieve an integration of shown abbreviated in Figure 2 as VDBTB (Vindhyachal the existing and future HVDC links in the country. back-to-back), CHBTB (Chandrapur back-to-back), Figure 1 shows such a conceptual scheme. As per this VZBTB (Vizag back-to-back) and SRBTB (Sasaram scheme the following power equipment, control and back-to-back). communication system would have to be added to the combined present and future HVDC bipole systems: Dadri Dehang − One converter each at Vadodra, Mangalore and Madras. SRBTB NER Ballabhgarh Rihand − Transmission lines between Ballabhgarh-Vadodra, NR ER Vadodra-Padghe, Padghe-Mangalore, Mangalore- VDBTB Madras, Madras-Kolar, Chandrapur-IbValley and Jaipur Talcher Talcher-Rihand. Vadodra − Integrated control and communication systems. Ib Valley Depending on the power flow requirements and WR Padghe Chandrapur capacity of the HVDC lines, converters can be added in parallel. The present design of the HVDC lines will VZBTB CH provide a ring, which will be as strong as more than BT 2500 MW. B Kolar While carrying out a feasibility study, a large number of SR alternative schemes will have to be considered with detailed techno-economic analysis of each one of them. Mangalore Madras As a matter of fact, by the time the planners consider implementation of an HVDC ring, very useful Existing/under construction development in the area of power system control, e.g. Under planning/bidding fast computers with enormous capability and capacity of Likely to be considered in future data processing, would have taken place and that will Figure 2: Conceptual scheme with back-to-back links come very handy in its implementation. 5.2.2 Reliable Communication System 5.1 Status of Present Day Technology To be able to realise the controls with the gigantic and complex tasks as indicated above, very fast and reliable Over the last few years, there has been a big leap state-of-the-art communication systems for transfer of forward in terms of speed of computers to perform huge amount of data and control signals among HVDC complex calculations, signal processing and capability links, RLDCs and NLDC will be required. Such a of handling big volume of data necessary to control any communication system may rely on optic fibre and complex system. So far as control of HVDC converters communication satellite systems. As it looks today, is concerned, Quebec-New England multiterminal satellite communication may be difficult due to system with three terminals (designed for five terminal transmission delays. It shall be possible to make use of operation) was commissioned successfully and has the HVDC or EHVAC transmission lines for laying shown satisfactory operational performance . optic fibre systems. In terms of load despatch centres, the present day Though present day communication technology technology, by and large, is able to provide all those possesses the required potential, it needs to be tailored control and supervision features which are necessary for to suit the requirements as mentioned above. the operation of a big and complex power grid. 5.2.3 Control Co-ordination with RLDCs 5.2 Technological Challenges The control co-ordination with RLDCs can conceptually In spite of the fact that the control system technology be divided into two categories, namely, one involving has taken a giant leap forward, there are still a few co-ordination at intra-regional and second at inter- challenges that should be met in order to realise the type regional level as shown in Figure 3. In this figure, of control and co-ordination, which will be required for NRLDC, SRLDC, ERLDC and WRLDC refer to proper implementation of an HVDC ring-main system. Northern, Southern, Eastern and Western load despatch Some of the control features needed for the ring-main centres, respectively. control are discussed below. A conceptual control schematic with different layers of controls envisaged for The North-Eastern region is not separately shown as it an effective control of the different constituents of the likely to be merged and synchronised with the Eastern ring-main system is also presented. region. This is due to mainly to the fact that it is much smaller in terms of the installed capacity at present. 5.2.1 HVDC Controls Though in future with the development of hydro Though it is possible to effectively implement and potential it will become quite large yet by that time the operate a multiterminal HVDC system with a number of dream of a national power grid would have been converter stations, there are certain issues, which need realised. to be addressed before realising a successful control In this figure, it is assumed that individual link control system for a complex HVDC multiterminal system (ILC) of the links within a region will interact with the participating in a national HVDC grid. Some of these corresponding RLDC. An inter-regional link control issues are: would have to interact with the load despatch centres of (a) Hierarchical formulation and implementation of both the regions. control algorithms which will take care of ; − Dedicated objectives of the individual links, ILC e.g. evacuation of power from a power generating plant, frequency control under NRLDC generator or power plant islanded conditions. ILC ILC − Regional objectives of the link(s), e.g. to maintain a proper load generation balance, optimal power flow on lines and grid ILC WRLDC NLDC ERLDC ILC stabilisation functions. − National objectives of the links, e.g. regional interchange of energy for optimal utilisation ILC ILC and conservation of resources on national SRLDC basis. − Co-ordination among all the HVDC links ILC constituting the ring-main system. − Co-ordination with different RLDCs. (b) Development of control hardware and software to realise all the functions listed under a) above. Figure 3: Conceptual control co-ordination system 5.2.4 Co-ordination between RLDCs and NLDC A national HVDC ring-main is conceptually viable subject to resolution of issues related with controls and The NLDC will interact with all the regional load communication systems. despatch centres and take charge of the national grid control functions such as global load generation References balance, optimal load despatch, inter-regional energy  Fourth National Power Plan 1997-2012, Central exchange, etc. Electricity Authority (CEA), Government of India. The control concept shown in Figure 3 considers the  Mata Prasad, S.K. Deb, Rebati Dass, Goran Isacsson, control and supervision for the ring-main comprising Andreas Borsos; Imminent Challenges of Bulk Power only bulk power HVDC transmission links. There could Transmission from the Himalayan Region for India be another level of control, which can be added by (Cigre SC 14, International Colloquioum on HVDC and taking into account all the back-to-back schemes with a FACTS, Paper no.5.1.1, 29-30 September 1997, view to controlling and supervising inter-regional Johannesburg, South Africa). energy exchange with a supplementary function of  Y. Allard, D. Soulier, J. Cochrane, B. Railing; enhancing national grid stability. Figure 4 shows the Multiterminal Operations Experience – Hydro-Quebec- inclusion of such a control function. This, of course, Nepool Phase-II HVDC (Cigre SC14, International adds to the control complexity. In the actual Colloquioum on HVDC and FACTS, Paper no. 6.4, implementation, there would be a need to develop a September 1995, Montreal, Canada). proper control and supervision hierarchy. Acknowledgement The NRLDCs and NLDC will encompass the control The authors are thankful to the management of Asea and supervision system not only for HVDC systems but Brown Boveri Ltd and ABB Power Systems AB for also for EHVAC system. As mentioned in section 3.3 permitting the publication of this paper. The views above, there could be two tier ring-mains, namely, expressed in the paper are that of the authors and not HVDC and EHVAC. necessarily that of the companies. ILC NRLDC ILC ILC ILC ILC ILC WRLDC NLDC ERLDC ILC ILC ILC ILC ILC SRLDC ILC Figure 4: Conceptual control with back-to-back links 6. Conclusion The realisation of an HVDC ring-main system will facilitate effective utilisation of the built-in overcapacity of the HVDC lines, will provide versatile control features and integration with the regional as well as national load despatch centres.
Pages to are hidden for
"Viability of a National HVDC Transmission Ring in India"Please download to view full document