Viability of a National HVDC Transmission Ring in India by fiw10869

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									                                                                                    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 [1]. 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 [2].
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 [3].
                                                             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
                                                                [1]   Fourth National Power Plan 1997-2012, Central
exchange, etc.                                                        Electricity Authority (CEA), Government of India.
The control concept shown in Figure 3 considers the             [2]   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                [3]   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.

								
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