India is taking a major step towards creating a national electricity grid with the construction of the Sasaram link. The project involves the interconnection of the north and east, allowing these two regions to share resources.
The Sasaram project is the fifth in a series of projects linking different regional electrical grids using high voltage direct current (HVDC) back-to-back connectors, which allow the hydroelectric north and south regions to connect with the thermal-powered regions of the east and west. Sasaram will complete the loop between the north and east, east and south, south and west and west and north.
In the past, India’s power network has regularly been unable to meet its maximum demand. Despite having a large installed capacity of almost 110 GW, typically only three-quarters of the capacity is available at any one time to meet the load requirements because of breakdowns and maintenance downtime.
Each grid has developed independently, spreading out from the centre in a star-shaped pattern. This means that one region could be experiencing a power shortage while another regional power station, just a short distance away, could be shut down.
Figure 1. PowerGrid was established in 1989 to develop a national grid
In 1989, the Indian government decided to establish the PowerGrid Corporation of India and charge it with establishing a national grid. At this time, only one inter-regional HVDC link existed, between the north and west regions.
The north and south regions of India have a healthy supply of hydroelectric resources, and the central and eastern areas of the country have considerable coal reserves. Consequently power generation in the four regions takes advantage of local resources, with the north and south relying heavily on hydroelectric and the east and west using coal fired power stations.
The main driving factor behind creating a national grid is the need to minimize the problems of energy shortages by the sharing of resources. Energy shortages in the hydroelectric-reliant regions lead to water being drawn off reservoirs to shrink below desirable levels. Even more critically, in periods of drought, the hydraulic resources literally evaporate, causing disastrous shortages. Conversely, during monsoon periods of high run-off, the inadequacies of transmission infrastructure prevent full exploitation of surplus energy available. In the thermal regions, power stations are being run inefficiently, where power supply is constantly switched on or off according to mass demand, rather than running at base load 24 hours a day. Thus, linking the regions also has both environmental and energy-efficiency benefits.
Figure 2. The Sasaram project will link India’s north and east regions
It is not possible to connect these regions synchronously because of their disparate frequency control regimes and for reasons of stability. In India, the frequency in all four regions is nominally 50 Hz, but there are sustained variations in frequency between 47 Hz and 52 Hz on a daily basis, which compares with a fluctuation of 0.5 Hz maximum in western Europe.
This is primarily because there is no adequate national regulation of the power supply system. With a multiplicity of producers, state and regional transmission authorities and local distributors of electrical energy and the absence of a national tariff structure, there are no incentives for good, disciplined management of the network. For instance, there is a growing number of independent power generators which continue to receive payment to supply the grid at periods of light system load even when there is no need for the power.
As the load reduces the frequency increases and excess generation exacerbates the problem. Conversely, the frequency decreases as the load increases, but there is little incentive to increase frequency and efficiency by load-shedding on the part of the distributors when this reduces their revenue. Thus frequency management provides another good reason for more accurately matching supply and demand through the provision of a national grid.
As a result of the differences in frequencies, the only solution is to base the grid on asynchronous HDVC interconnections between the regions. This method of connection permits the flow of power without the transfer of disturbances. The interconnections are of zero distance, with the AC-DC-AC converters located in the same station in back-to-back configuration.
Historically, voltage profile experience on the Indian power system has lacked performance, for example, 400 kV grid voltages have varied between 420 and 350 kV within a single day. The back-to-back converters improve the situation because its considerable reactive power resource provides the ability to adjust voltage dynamically.
Figure 3. The links will help the connected regions avoid the construction of new generation capacity
DC has the important attribute that power can be precisely controlled and, whatever the operating frequencies, power can be directed as required. With a DC link, a fault in one region does not affect the other except that the link itself may suffer a temporary power flow interruption.
The HVDC transmission links not only help each region avoid power shortages, but also avoid the need to build new generating capacity. The reserve capacity needed to meet peak loads can be halved if two regions are linked. A 500 MW transmission link can deliver the power produced by a typical generator, and at around one fifth of the investment.
Closing the loop
The Sasaram project, linking the north and east regions, began at the end of 1999 and is due for completion this year. The first 500 MW link was built in Vindhyachal in 1989, linking the north and west regions. Since the creation of the PowerGrid Corporation, Alstom’s transmission and distribution (T&D) sector supplied two 500 MW links at Chandrapur between south and west regions, in 1997 and 1998. It followed this in 1999 with the 500 MW link at Visakhapatnam (Vizag), linking the south and east.
These four Alstom links are essentially of a standard design, which makes them almost off-the-shelf from a production point of view. This was a key factor in reducing costs for the subsequent units and in capturing the repeat business for Alstom. The additional advantages to the client, PowerGrid, are the commonality of spares, the standardization in operation and maintenance procedures.
“The key thing about Sasaram is that it closes the loop. It is the completion of the first phase of the national grid,” said John Loughran, marketing manager with Power Electronic Systems at Alstom T&D. “PowerGrid is now looking to reinforce the first loop with a second Vizag link, which should begin [in middle of this year], and a second Sasaram link following that.” Each of these links will be of the now standard 500 MW capacity.
At Sasaram, Alstom is supplying, on a turnkey basis, a back-to-back link comprising a 500 MW pole operating at 205 kV DC, 2475 A together with conventional switch yards at each end of the link. At the heart of the link are the thyristor valves, which control the level and direction of the power flow between the two regions. To operate at the required voltage and current levels, it is necessary to connect 54 thyristors (including two redundant) to each of 5.2 kV in series to form a valve and four valves stacked vertically to form a ‘quadrivalve’ tower.
The 100 mm diameter silicon wafer thyristors are clamped in pairs between high efficiency water-cooled heat sinks and, together with their local voltage grading and thyristor triggering circuits, form the basic building block of the valve, the thyristor level.
The quadrivalves are arranged in the valve hall with space around them for maintenance access, electrical clearance and connections. The valve hall is designed to provide a temperature and humidity-controlled environment and the screened walls contain the radio frequency interference generated by the valve.
One of the main items to be considered at Sasaram was the converter transformer that connects the AC network on either side of the link. A three-phase unit, weighing some 500 t, would be appropriate in the UK or western Europe because of good transport infrastructure.
But in India, where the link is situated a long way from the coast; the transport infrastructure considerations are such that three smaller, single-phase units are used, each weighing 160 t in transport configuration and 230 t fully installed. This is an expensive option, but with three units on each side, it allows Alstom to use just one extra single-phase unit as a spare for both sides of the link, rather than three.