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Special Report | March 2014

Towards a more energy efficient future

 Being a power deficient country, the need of hour is quick execution and commissioning of large size power plants to meet power demands and faster return on investment (ROI). Taking a journey through L&T's Rajpura Supercritical Thermal Plant, Projects Info realises that for supercritical plants, the accuracy and response time of the DCS is more critical than in sub-critical units.

Coal is expected to be the fuel of choice for the foreseeable future in India mainly because of its abundance and affordability. On the other hand, as carbon consciousness is becoming more prominent, technologies for achieving higher operating efficiencies and reducing emissions from coal-fired plants are becoming more critical. Today's supercritical units can achieve thermal efficiency of more than 45 per cent, compared with a typical sub-critical plant's 30-38 per cent. Therefore, government policies and technology advancement is directing the market towards a supercritical/ ultra-supercritical technology regime and sub-critical plants will be existing as a part of old legacy only.

The contract
L&T, through its subsidiary NABHA Power Limited (NPL), invested for setting up a coal-fired supercritical thermal power plant of 2x700 MW at Rajpura. NPL awarded an EPC contract to L&T Power towards setting up the plant.

L&T Power is an Independent Company (IC) of Larsen & Toubro, with a mandate to provide end-to-end EPC solution to the thermal power sector based on latest technology. L&T Power in turn awarded an order to C&A for Control System package, which included DCS, operator training simulator (OTS), performance analysis, diagnostic and optimisation (PADO), management information system (MIS), computerised maintenance and information management system (CMIMS).

The plant was constructed on a turnkey basis by L&T Power with more than 90 per cent of the equipment sourced from the group companies of L&T. The supercritical boiler and turbine were manufactured by L&T's joint venture companies with Mitsubishi Heavy Industries (MHI) located at Hazira, Gujarat.

Describing the Rajpura Plant as another important milestone in L&T's history, Shailendra Roy, Member of the Board, L&T, said, "We have demonstrated our capability as the leading power plant EPC player in the country with the successful commissioning of the first unit of the Rajpura Plant. The timely completion and stable operations further establishes L&T's ability to build large complex engineering marvels of global standards."

Challenges
Critical design of DCS: For supercritical plants, the accuracy and response time of the DCS is more critical than in sub-critical units. A well-designed control system that provides tight regulation and the ability to reach and maintain set points in shortest possible time span can help utilities to capitalise on the economic and environmental potential that these units offer. Further, to obtain higher efficiencies, these units generally operate in sliding pressure/modified sliding pressure modes wherein even a slight disturbance in any of the critical parameters can lead to a high degree of instability of the plant. Therefore, this mandates for a very fast response from the control system.

Tightly integrated system
Another challenge arises during integration of the various control systems. Traditionally, the boiler and turbine OEMs provide their proprietary control systems for the boiler and turbine protection and control, and the same needs to be seamlessly integrated with the station DCS system, which is supplied by station C&I contractor and who, in turn, is responsible for the overall control of the power plant including boiler, turbine and all associated BOP equipment.

Due to requirement of fast operation, it is better to have a centralised operation of all the three control systems (boiler DCS, rurbine DCS and station DCS) from the same operator station. However, it is challenging to achieve such flexibility of control if these are three different DCS and are integrated over non-time-critical methods of data exchange such as OPC, MODBUS, etc. Moreover, it also requires additional hardware and software for interface.

Due to above, L&T Power decided to opt for uniform control system for boiler integral control (burner management system BMS and boiler protection system BPS), turbine integral control (turbine control system TCS and turbine protection system TPS) and station DCS. With this philosophy in mind, the order was given to L&T C&A to provide a tightly integrated DCS for the complete plant operation and consequently integrated factory acceptance test (FAT) with all these three systems interconnected on a single network was a part of the deliverable of the order.

Solution
The solution was designed to control more than 15,000 I/O's as a part of the unified DCS. C&A was responsible for engineering, procurement, simulation, integrated factory testing, site works and commissioning. C&A provided Diasys Netmation technology platform of MHI for this critical DCS.

The DCS included functions like boiler furnace safety (especially during the start-up and low load operations like Runback). Similarly it included rurbine safety functions like overspeed protection, turbine stress evaluator. It also had the advanced process loops like automatic plant startup, unit master control, main steam temperature & pressure controls, feedwater flow control. The key here was to intelligently design the function groups and choose the controller partitioning accordingly. The system communicates with third party systems like CHP, AHP, electrical distribution systems through OPC/MODBUS and IEC 61850 (for relays).

As a part of the station DCS package, C&A also provided MIS (management information system), PADO (plant analysis diagnostics and optimisation), OTS (operator training station) and CMIMS (computerised maintenance and inventory management system). All these were integrated with the DCS for bi-directional data exchange and intelligent decision making.

For testing the integration, checking the performance under simulated plant condition and tuning the system to achieve near optimum performance C&A designed the EMULATOR system, which included generic simulator and modelling software to simulate various plant conditions. The system simulates real time conditions for coordinated boiler and turbine control, burner management, data acquisition, motor control, and balance-of-plant processes along with various integration interfaces.

A comprehensive testing and FAT using such a facility ensured testing/ tuning of the system in line with the above. This would reduce commissioning time to a large extent and thereby would help taking the plant to the stable maximum load conditions quickly.

The time starts now
Final pre-commercialisation tests involved continuous operation of the plant for 72 hours at full peak load of 700 MW. Subsequently the plant had to undergo two sets of ramp up and ramp down tests. Conducted by an independent engineer, the tests were monitored by teams of experts from Punjab State Power Corporation, Lahmeyer International and L&T Power. The plant demonstrated near 100 per cent stability during trial operations.

Commercial operations of the plant will significantly augment Punjab's generation capacity making the state power surplus, and will enable it to export power to the national grid. Based on supercritical technology, it will be one of the most eco-friendly and efficient coal-based thermal plants in India.

The foundation stone of the green field project was laid by Chief Minister of Punjab Sardar Parkash Singh Badal in presence of L&T's Group Executive Chairman, A M Naik on March 3, 2010. The plant went through 'Light Up' on December 5, 2013 and was dedicated to the state of Punjab by the Chief Minister in the presence of the Deputy Chief Minister Sardar Sukhbir Singh Badal and Shailendra Roy on 8 December 2013.

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