HVDC Multi-Terminal Interconnections a viable and optimal solution for India

Presented at Cigré 2000 Conference, Paris, France, Aug/Sept 2000 HVDC Multi-Terminal Interconnections a viable and optimal solution for India K.M.Saxena Dr. Channakeshava Mata Prasad* Dr.R.P.Bhatele Dr. A.S.R.Murty Carl-Gote Carlsson R.S.Moni Madhya Pradesh Central Power Asea Brown Boveri Ltd; Electricity Board, Research Institute, Chandiwala Estate, Vidyut Nagar, Sir C.V.Raman Road, Kalkaji, Jabalpur- 482008 Bangalore-560 094 New Delhi - 110 019 (India) Summary The Western region comprising the states of Madhya Pradesh, Maharashtra, Gujarat and Goa is geographically the largest in India with installed capacity currently around 28000 MW. Peak demand currently around 24000 MW is projected to touch 45000 MW by end of the decade. A large chunk of the increase will have to come from thermal generation located in Eastern Madhya Pradesh and the power will be wheeled to Western Madhya Pradesh, parts of Maharashtra and Gujarat. As the transmission distances involved will be in the range 800-1200 kms special attention is called for in transmission system planning. In an earlier report [1], the benefits of an HVDC pointto-point transmission link from Korba to Indore in Madhya Pradesh, becoming an ideal alternative considering the distances involved was discussed. The study also pointed to further benefits in case of upgrading the point-to-point HVDC transmission to one of higher capacity and to a multi-terminal configuration in a phased manner as power needs to be further extended in to Gujarat, west of Madhya Pradesh. This paper examines the potential benefits and cost-effectiveness of the multi-terminal configuration. Keywords HVDC Converter - HVDC Bipole Bulk Power Transfer - Multi-terminal HVDC - Techno-economic solution Dynamic Stability System losses. 1. Introduction HVDC transmission systems are playing a major role in the power transfer in several countries due to the need for power evacuation in bulk quantities. In the developing world Brazil, China and India are the major countries where the need for bulk power transfer over large distances is very essential and wherein HVDC offers significant economic advantages. In India, the power system consists of five regional grids, namely; Northern, Western, Southern, Eastern and North-Eastern whose combined installed capacity is slated to exceed 100 GW within a couple of years from now. Most of the generation in India consists of coal fired thermal power plants, with more than three quarters of that located within a contiguous area bordering the Northern, Western and Eastern regions. With pit head thermal generation being more economical and major load centres spread across the country particularly in and around the major metropolitan cities, long haul power transmission systems become essential. 2. Western Region Power Network Figure 1.0 shows the Western region network as it exists today. About 85% of the total generation capacity of the region is thermal (coal based) and as a major portion of the coal belt lies in it s Eastern part, particularly within Madhya Pradesh, most thermal plants are located therein. The major load centres of Madhya Pradesh and Maharashtra are however, located over an distance of 800 kms from the generation centres. Gujarat which has a fairly even distribution of load within itself is located at a distance greater than 1000 kms from the coal belt, at the Western end of the region. * Asea Brown Boveri Ltd., HVDC & RPC Division, Chandiwala Estate, Kalkaji, New Delhi 110 019, India 1

Figure 1.0: Western Region Network of India The prevailing highest AC transmission system voltage in the region is 400 kv which is complimented by a network of 220 and 132 kv transmission lines. The region has one HVDC transmission system, the + 500 kv bipolar line from Chandrapur to Padghe (near Bombay) in Maharashtra and there are two back-toback schemes connecting the region to the Northern and Southern regions. 3. Transmission Alternatives The logistics of power transfer, from the thermal power generating complexes in Eastern Madhya Pradesh to the distant load centres in the Western part of the state, demand long haul high capacity power transmission systems. A preliminary study indicates HVDC transmission as an optimum choice [1]. Among the planned generation, is a thermal generation complex of 2000 MW in the Central Sector at Seepat quite close to the Korba complex from wherein part of the future power requirements of Madhya Pradesh, Gujarat and Maharashtra will be met. The total quantum of power transfer required from the Korba complex will be of the order of 3000 MW to the Western region of Madhya Pradesh as well as Gujarat and parts of North-West Maharashtra. The need to transfer this amount of power, up to Western Madhya Pradesh and Gujarat, involving distances of 800 and 1200 kms respectively would definitely call for large EHVAC transmission systems or an HVDC system. The EHVAC transmission would involve double-circuit transmission lines with intermediate sub-stations. For the HVDC option a direct bipolar line to Gujarat with an intermediate tapping in Madhya Pradesh would be the optimum alternative. 4. Comparison of transmission alternatives 4.1. EHVAC Transmission System Expansion The 765 kv system, examined in the study includes a double-circuit system from the Seepat power station towards Indore and Chhegaon in Western Madhya Pradesh, with an intermediate switching station at Seoni. To control the voltage at least two, intermediate switching stations will be required. As the power output from the Seepat station will also be shared by Gujarat and Maharashtra, the transmission system emanating from Western Madhya Pradesh into Gujarat and Maharashtra will also require strengthening, which could consist of double-circuit 400 kv lines. The line capacity required, would be in the range of 500 1000 MW towards Asoj or Dehgam. 4.2. HVDC Transmission System Options In the HVDC alternative the following option has been considered. a) Korba Indore HVDC link in the first stage with line capacity of 3000 MW b) Extension to a Korba-Indore-Dehgam multiterminal configuration in the following stage The power drop at Indore would be 2000-2500 MW and the remaining drop of 1000-500 MW at Gujarat. Thus the rectifier at Korba would be rated for 3000 MW while the inverters could be rated for the respective tapoff power. The d.c.voltage considered is +/- 500 kv for the HVDC bipolar system. 400 kv interconnections from Seepat to the Korba complex and Seoni (or Koradi in Maharashtra) are also considered in the alternative. 5. Performance of Alternatives 5.1 765 kv EHVAC System Steady-State conditions The load flow analysis establishes the feasibility of the system from a power flow point of view. Line shunt reactors of 110 MVAR have been used for the load flow simulations, which correspond to peak load conditions, however reactors of 220 MVAR would be required at both ends of all the line sections to control the voltage during light load conditions. The flow on each of the circuits is in the range of 625 MW on the Seepat Seoni 765 kv lines and 900 MW and 600 MW towards Indore and Chhegaon respectively, from Seoni. Dynamic Conditions Dynamic stability of the first stage alternative has been determined by considering outage of one of the 765 kv line sections at fault clearing. Figure 2.0 below shows the system behaviour (machine angles) at outage of the Seoni-Indore 765 kv line following a single phase to ground fault. It is seen that the system is not well damped. The simulation shows that although the system is transiently stable (first swing stability) the overall damping at recovery is poor. 2

Additional damping devices, either in the form of SVC s or well co-ordinated Power System Stabilisers on generators are required. together with the power modulation function stabilises the system with very good damping. Figure 3.0: HVDC Alternative Dynamic Performance Figure 2.0: 765 kv Alternative Dynamic Performance 5.2 Multi-Terminal Bipolar HVDC System Steady-State conditions Load flow analysis shows that the basic HVDC scheme with a 3000 MW power transfer capability from Korba to Indore over a distance of 800 kms is not only quite feasible but a very practical transmission option. In addition to the bipolar HVDC transmission system a 400 kv double-circuit transmission system would be required to interconnect the Seepat 2000 MW plant with Seoni and the Korba HVDC terminal. Apart from some strengthening of the downstream 220/132 kv networks in the receiving centres no other EHV (400 kv) transmission lines are required in Madhya Pradesh or Gujarat other than the ones already planned. The load flow pattern observed shows that most of the power is required to flow in the Indore and Nagda regions of Madhya Pradesh. In the load flow analysis the generation schedule for Madhya Pradesh and Gujarat has been selected to give the worst case power flow conditions, called Maximum Eastern Despatch, which refers to the condition of maximised thermal generation in the Korba complex and surrounding areas in Eastern Madhya Pradesh. The total system absorbed load is of the order of 39,500 MW in both the alternatives with losses in the HVDC multi-terminal lower by about 15 MW. Dynamic Conditions An outage of one pole of the bipole from Korba to Indore would constitute the worst possible outage for the HVDC alternative. Figure 3.0 below shows the system behaviour for the single pole outage. The pole power compensation function on the remaining pole 6. Comparing the alternatives 6.1 Technical The foregoing analysis shows that apart from lower losses in the steady-state, the main benefit of the HVDC scheme, is in the dynamic performance the system s inherent fast control capability ensures quick and efficient damping of oscillations during post fault recovery. The HVDC alternative also uses a shorter and compact bipolar line of only 800 kms, while the 765 kv system uses a total line length of 1400 kms in addition to other 400 kv lines. Another significant advantage with the multi-terminal extension to Dehgam in Gujarat is that it would offset the need for any planned a.c. interconnections into Gujarat for transferring it s share of power from other proposed power plants in the Eastern region. Although only 500 MW has been simulated as the dropped power in Dehgam in the above simulations the HVDC terminals have been rated for 1000 MW, bipolar. The advantage in reduced losses, is considering the absence of these planned 400 kv EHVAC lines, which however are required in the 800 kv alternative to transfer power into Gujarat. The HVDC system has many other benefits as well, some which are highlighted below. Other benefits Full control of power flow, which can be exercised either through individual HVDC link control systems and/or from Regional Load Dispatch Centres 3

Possibility of forced stabilisation of the regional grid in the event of disturbances through various higher level control functions Limitation of short circuit level thus obviating the need for either replacement of existing 400 kv equipment that are rated for 40 ka or installing new series reactors 6.2 Economic Comparison An economic analysis of the two alternatives has been done comparing essentially the capital investments on the terminal station/sub-station equipment and the transmission lines. The analysis has been presented in the form of graphs. The graph in Figure 4.0 below gives an overall comparison of the capital cost of the 800 kv based HVAC transmission system alternative and the +/- 500 kv, HVDC alternative as a function of the transmission distance in kms. In the case of the HVDC alternative the graph essentially reflects the cost of the two terminal system. It can be seen that the HVDC alternative becomes cheaper for transmission distances beyond 700 kms for the level of transfer considered i.e. 3000 MW. much larger flexibility in terms of operation by way of dynamic variation of the ratio of tapped power and reserve for future. Another aspect that has not been indicated in the costs is the significant gains obtained in the dynamic performance of the multi-terminal system. To obtain an equivalent dynamic performance the AC system will have to consider the cost for the additional damping devices. In case of the transmission distance increasing further, the HVDC alternative would again become cheaper, either as a function of distance or power transferred. Cost of Alternatives 0.25 0.2 0.15 0.1 0.05 0 Cost Comparison 0 250 500 750 1150 Length of Transmission HVAC HVDC Cost Comparison Cost of Alternatives 0.2 0.15 0.1 0.05 0 0 250 500 750 Length of Transmission HVAC HVDC Figure 4.0: Comparison of costs of basic two-terminal HVDC system with EHVAC alternative. When extending the scheme to a multi-terminal, the slope of the cost curves beyond the point of first tapping changes depending on the power transmission capacity beyond the point and the type of systems considered in the HVAC alternative. In the case analysed here although only 500 MW of the 3000 MW flowing has been extended further in to Gujarat, HVDC Terminal capacity of 1000 MW (bipolar) has been included to achieve economies of scale and provide room for future additional power transfer. This has been compared with the amount gained by avoiding a proposed 400 kv double-circuit EHVAC line of around 700 kms. It can be seen from the graph in Figure 5.0 as to how the economics slightly shifts and the basic capital costs reach almost the same value. However the graph does not reflect the higher capacity built into the HVDC terminals which gives a Figure 5.0: Comparison of basic costs of multi-terminal HVDC system with EHVAC alternative. 7. Operation of Multi-Terminal HVDC Schemes Multi-terminal schemes can be operated in different modes depending on the requirements of generation and load flow patterns. High processing speeds possible nowadays have made implementation of even complex control strategies required for multi-terminal schemes quite easy. So far as control of HVDC converters is concerned, the Quebec-New England multi-terminal system with three terminals (designed for five terminal operation) was commissioned successfully and has shown satisfactory operational performance for many years now [4]. In a multi-terminal system, one of the converters is chosen to control the d.c. voltage with the others maintaining the respective current orders. The scheme can be operated in any configuration, the only necessity being that there should be at least one terminal each operating as a rectifier and an inverter. The other terminals can operate in either mode. The sum of the rectifier and inverter power/current orders will always be zero. Co-ordination of the power/current orders is done through communications between the terminals.. The intermediate terminals are usually configured as parallel converters for reasons of reliability and maintainability. 4

As a multi-terminal HVDC system would span across more than one state co-ordination with dispatch centres of the respective states would be required to effect coordinated power control, operational mode changes and enable co-ordinated system higher level control actions. 8. Conclusion The application of an HVDC transmission system for the Indian Western region will facilitate effective utilisation of the available right-of-way, while imparting dynamic stability to the entire region. With the extension of the primary bipole scheme to a multiterminal scheme not only can the overall system losses be further reduced but also the dynamic stability of the entire region can be enhanced. While being economically quite competitive, the tremendous flexibility in operation and the superior dynamic performance is also an inherent advantage which may not be discernible straight away in economic terms but still merits consideration while planning the complete system. In the above instance, the benefits of the multi-terminal system becomes well established in terms of the benefits while still being cost effective. Acknowledgement The authors are thankful to the management of Madhya Pradesh Electricity Board, Jabalpur, Central Power Research Institute, Bangalore and Asea Brown Boveri Ltd, New Delhi for permitting the publication of this paper. The views expressed in the paper are that of the authors and not necessarily that of the organisations. References [1] Mata Prasad et. al, Techno-Economics of HVDC Multi-Terminal Interconnection in the Western Region Network of India (CIGRE SC14, International Colloquium on HVDC, Langkawi, Malaysia, September 1999) [2] Fourth National Power Plan 1997-2012, Central Electricity Authority (CEA), Government of India. [3] Mata Prasad et. al, Optimal Transmission System Upgrade Options for AC-DC Hybrid Regional Network (Cigre SC 14, International Colloquioum on HVDC and FACTS, Paper no.5.12, 29-30 September 1997, Johannesburg, South Africa). [4] Y. Allard, D. Soulier, J. Cochrane, B. Railing; Multiterminal Operations Experience Hydro- Quebec-Nepool Phase-II HVDC (Cigre SC14, International Colloquium on HVDC and FACTS, Paper no. 6.4, September 1995, Montreal, Canada). 5