Simon Balbierer, Dr. Asok Mukherjee, Siemens AG, Germany
With a total area of 3.2 million km2, a rapidly growing population that last year passed the 1 billion mark and a growing economy, one major concern of India is how to meet increasing energy demand. In spite of a manifold growth in power generation since independence – the installed capacity stands at 100 000 MW today – India’s power sector is characterized by a constant shortage of supply against demand, with a peaking shortage of 18 per cent and an energy shortage of about 12 per cent.
Figure 1. The East-South Interconnector II project will transmit power over a distance of 1450 km
The challenge is enormous; experts’ reports say that India’s power sector needs to add 58 000 MW by 2003, or 19 000 MW per year, which translates into a required investment of $60 billion in generation capacity alone. In its quest to improve the national energy infrastructure and make electricity available to all regions, the country has to overcome another problem in the power transmission area, namely to build an interconnected national grid out of five asynchronously operating regional networks.
For a secured availability and reliability of power supply, such an interconnection is indispensable. Indian power system planners realised quite early on that the High Voltage Direct Current (HVDC) transmission technique offers an economic and technically feasible solution in this regard and have been successfully pursuing HVDC projects since the early 1980s. Today, with six HVDC projects in operation and two under construction, these asynchronous, or technically dissimilar regional networks are interconnected with each other, making flexible power transfer between the regions possible, and therefore adding to the reliability of supply.
The most recent HVDC long distance transmission project under development is the East-South Interconnector II, that will bring power from the eastern region to the rapidly developing industrial and high-tech area near Bangalore in the south of the country. The total transmission distance is about 1450 km. In February 2000, this contract was awarded to Siemens by Powergrid Corporation of India Limited, India’s biggest power transmission utility. The contract value is nearly a200 million.
The East-South HVDC link is a bipolar long-distance transmission system, with a total rated bipole transmission capacity of 2000 MW. This bulk power will be evacuated from the thermal power plant complex in Talcher in the province of Orissa and brought to the province of Karnataka. Nearly 80 per cent of India’s electricity is generated in thermal facilities using coal or petroleum products. The thermal power stations at Talcher are also coal-fired.
Figure 2. Converter valves are the heart of an HVDC station
For the East-South link, the DC transmission technique was chosen not only because of the asynchronous eastern and southern grids, but also because of the vast transmission distance (approximately 1450 km) between the centres of power generation and consumption in Talcher and Kolar respectively. Over such a long distance, the line losses with DC are only a fraction of the losses of a conventional AC transmission line, because DC transmission entails ohmic losses alone and no losses due to skin effect or the transportation of reactive power. Further advantages of HVDC transmission are:
- Very fast control of power flow with regard to both magnitude and direction, using a fully digitized control system. This characteristic can also be effectively used to stabilize existing AC networks.
- Narrow rights of way, combined with relatively low costs for lines and poles compared to a 3-phase HVAC transmission system
- High reliability resulting from the initial design and built-in reliability for individual key components
The principle of HVDC is an AC to DC conversion at the point of power transmission (rectifier station) and re-conversion from DC to AC again at the power receiving end (inverter station), by using power thyristors. These thyristors are the basic element of converter valves, which build the heart of a converter station. The converter valves consist of modules in which a certain number of thyristors are connected in series. Heat sinks arranged in between the thyristors help carry away the heat generated by means of water, which is used as the cooling medium.
Snubber circuits comprising damping resistors and capacitors help to absorb the oscillations generated in the commutation process.
The use of a modular technique has simplified manufacture, testing, transport and installation of the valves and helps to reduce the costs. In case of an outage, the replacement of faulty individual components within a module is also easy and fast.
For the East-South HVDC project, the total number of thyristors in the converter valves of both stations is about 4000.
Figure 3. A single phase, three winding transformer in the testing field of Siemens, Nuremberg
Converter transformers, another key component of an HVDC system, adjust the AC network voltage at the rectifier station down to a suitable level for the thyristor valves and vice versa at the inverter station. In comparison with normal AC power transformers, the technical requirements of converter transformers are higher, because they are confronted with a high harmonic content in the current and thereby risk localized heating of the windings. Further, the valve side windings are subjected to additional DC voltage stresses, lightning impulse and switching surge voltages.
The winding insulation therefore has to withstand combined AC and DC voltage stresses. Special design and manufacturing skills based on a great deal of experience are therefore essential in this area.
For the East-South link, seven converter transformers (including 1 spare) of single-phase three-winding design will be in use at each of the converter stations.
Conversion of AC to DC and vice versa also involves the generation of harmonics. Seen from the AC side, converters can be regarded as a source of current harmonics and from the DC side, a source of voltage harmonics. These harmonics, if allowed to infiltrate unhindered into the interconnected AC systems and DC line respectively, will cause voltage distortion and telephone interference.
Adequately designed harmonic filters to absorb these harmonics are therefore necessary in order to minimize such interaction between HVDC converters and the AC/DC systems. The AC filters also serve another important purpose, namely that they meet the reactive power demand of the converters resulting from the commutation process. The DC side harmonics are mainly damped, by the smoothing reactor. For a higher damping DC filters similar to AC ones have to be installed, as is the case with the East-South link.
Harmonic filter design should however also take into consideration – side by side with reactive power and harmonic performance requirements – the economic aspects and reflect a proper balance between the two.
Details of the AC and DC filter banks for the East-South transmission link are shown in Table 1, as is the rating of the air-core smoothing reactor.
If the converter valves are the heart of an HVDC converter station, the control and protection system can be rightly termed as its brain for intelligent operation. Power should flow from one station to the other, i.e. in a definite direction at the desired level, and should be quickly controllable.
It should also be possible to start up and shut down the converters at will and at an optimal rate (MW/s). Depending on the momentary DC power transmission and reactive power demand of the converters, harmonic filters must be automatically switched on and off. Proper interfaces must be provided for a possible remote control of the HVDC system. Independent of the control system, the protection devices must be able to detect faults and initiate steps to limit damage to individual components and to reduce the down time.
The various levels of control and protection hierarchy that are needed to ensure the above, and must be conceived such that no conflicts arise, the systems are still easy to handle and can guarantee a high degree of reliability and availability for HVDC transmission.
For the East-South bipolar transmission scheme, a state-of-the-art HVDC control and protection system will be implemented with the following salient features:
- A fully digital, redundant design
- Parallel wiring for increased reliability
- The use of the most modern standard technologies
- An innovative hybrid-optical DC measuring system
- Standardized function blocks for control and protection systems
- Fully graphical design and documentation
- A system functionally tested off-site, by using Real Time Digital Simulators (RTDS)
The East-South HVDC contract came into effect on 14 March, 2000. The project schedule foresees a stage by stage completion of the two poles. The commercial operation of pole 1 and then pole 2 is planned for 33 and 39 months respectively after the effectual contract date.
The project is at present in the final design stage, with some key components having already entered the manufacturing phase after a series of detailed technical clarifications. For the equipment to be procured locally as well as services to be rendered by local partners, an efficient local project team has been set up by Siemens Limited India (SLI), who are responsible by virtue of separate contracts for the complete local portion. The overall project management is being carried out by Siemens, Germany.
For the successful completion of a project of this magnitude, the regular and continuous exchange of information between Siemens Germany and SLI in close cooperation with Powergrid is essential. This communication process is functioning extremely well, one very positive aspect being the HVDC expertise of Powergrid personnel, who have been successfully operating several HVDC links in the country and are therefore making use of their invaluable experience for new projects.
The project is being financed by the KfW-Bank of the German government, except for the local portion of the project (approximately 20 per cent of the total contract value), for which Powergrid itself has taken responsibility.
Figure 4. AC filters absorb the AC harmonics and supply reactive power
With the completion of this East-South HVDC link project, one further milestone will be added to the power map of India. This indicates also how the power system planners of this country have been able to skilfully bridge the gap between dissimilar regional networks of the subcontinent by using modern and innovative technology. System planners will continually build up an interconnected national grid, adding to both the reliability and the stability of power supply. This makes flexible transfer of power from one region to the other possible.