Main Power Blocks
Phase I of the power plant consists of one module of combined cycle unit termed as
- 2GT9341 FA] of 220MW & 1 x STG of 230MW = 670MW) and the Phase II of the plant consists of two more Blocks each of more or less identical configuration
as in Block-1. These combined cycle modules in 2GT9351 FA] of 240MW & 1 x STG of 260MW = 740MW each).
- Apart from the above modules, the plant also has a 34MW open cycle gas turbine unit which would act as a black start facility as well as a peaking capability for the power plant. This open cycle gas turbine unit forms a part of the Phase I.
- Each of the Blocks consists of the following equipment along with all its various subsystems and accessories units:
- Two Gas Turbine (GT) units of GE design
- Two associated Generators (GTG) for the above units
- Two horizontal, natural circulation Heat Recovery Steam Generators (HRSG)
- One tandem flow, reheat Steam Turbine (ST) unit; and
- One associated Generator (STG) for the steam turbine unit.
The initial liquid fuel system (naphtha/distillate) comprises of the off-shore anchored SPM, shore 3 x booster pumps, surge tank, etc., along with emergency shutdown valve and the tank farm consisting of 4 x 48000KL tanks. The liquid send out pumps deliver the fuel from these tanks to the day tanks after fuel treatment (centrifuges). A battery of the fuel forwarding pumps transfers the fuel from the day tanks up to the gas turbine units. There are separate pumps for the burners of the HRSG for supplementary firing.
A few years back, GAIL setup the Dahej Uran-Panvel-Dabhol Pipeline with 12.5MMSCMD capacity to supply natural gas to power, fertilizers and other industrial consumers in Vapi, Valsad, Navsari, Bhiwandi, Kalyan, Patalganga, Khapoli, Pune, Usar, Thal, Mahad, Dabhol, etc. regions including RGPPL.
Cooling Water System
Independent cooling water systems are provided for each of the three power blocks. The steam turbine condenser cooling is with water drawn from the sea, in closed-loop circulation with a blowdown system. Induced draft crossflow cooling towers with 12 cells for Block-1 and 13 cells each for Block-2 & Block-3 are installed for cooling the return water. The make-up sea-water system for the above cooling water systems is common for all and is located at the beachhead.
The auxiliaries are provided with a closed-circuit cooling water system (CCW). DM water is used in this closed circuit which is in turn cooled by circulating raw water in plate-type heat exchangers. Each block has an independent CCW.
In addition, an open-cycle cooling water system (OCW) is provided for generator hydrogen cooling and vacuum pumps seal water cooling. The OCW is provided with a single-cell induced draft cooling tower & OCW pumps. Raw water is used in the OCW system. Each block has an independent OCW.
In Phase II, a glycol-water chilling system was set up for chilling the intake air of GTs in Block-2 & Block-3. This improved the efficiency of the turbines.
Raw Water Supply and Treatment System
Freshwater requirements for service water, DM water, OCW, potable water, etc., are met from water supplied by MIDC. MIDC has laid a 50 kms long, 300mm dia pipeline, tapping the water from Vashishti River at Chiplun for this purpose. The raw water so received is stored in a raw water reservoir and later pretreated through clariflocculators and gravity filter beds. A potable water treatment plant is installed to meet the total potable water requirements of the plant. A dual-compartment sludge pit is provided to facilitate the disposal of solid effluents.
The DM treatment train comprises an ultra-filtration unit (for removal of colloidal silica), activated carbon filter, cation-anion & mixed bed ion exchanger. Backwash and regeneration facilities are also provided including a neutralizing tank.
DM quality water is required for the makeup of the steam-condensate cycle, water injection for NOx emission control while firing liquid fuel and for CCW top-up. Water injection for NOx control accounts for about 40% -50% of total DM water consumption. This is not required when firing natural gas & hence DM water requirement will be significantly lower when the LNG facility is commissioned.
An emergency water supply system has been provided in the power plant complex with a 1.25 lakh m³ reservoir and adequate capacity diesel & motor-driven pumps to meet any eventualities or emergency situations.
Raw Water Supply and Treatment System
A common fire-fighting system is provided for Phase I & II of the power plant. The firewater piping covers the plant area in three loops for equal pressure availability. The system is serviced by 2 x outdoor type diesel engine driven horizontal pumps taking water from the raw water reservoir. The system is kept pressurized by a jockey pump. All the fire-fighting equipment (hydrants, deluge valves, sprinklers and foam) draw water from the above ring header.
Deluge valve systems are provided for protection of transformers and oil tanks, while foam system is provided for fighting the fire in naphtha/distillate tanks. Sprinklers are provided in some areas while CO2 flooding is provided for generators. Mobile fire-fighting equipment includes fire tenders and fire extinguishers. Most areas are provided with fire, smoke, heat & gas detection and alarm systems.
Compressed Air System
Each Block has its own independent compressed air system. The system comprises one instrument air compressor, an identical oil-free service air compressor which will also provide backup to the instrument air compressor, one number air dryer and receiver. A centralized compressed air system is also provided for some of the auxiliaries.
Other Mechanical Systems
Various miscellaneous mechanical systems comprising the bulk gas system, lube oil system, HVAC system including desiccant type dehumidifiers, sewage system, pollution monitoring/control/disposal systems, etc. as well as laboratories & workshops have been set up to facilitate the smooth operation of the power plant.
As discussed earlier, the electrical power at Dabhol Power Station is generated by 9 main generators in Power Blocks 1, 2 & 3. The output of each main generator is connected to a step-up transformer (15KV/420KV) using an isolated phase bus and further evacuated by 4 x 400KV double circuit transmission lines via the 400KV Switchyard. The bocks had been originally designed to generate about 2150MW power.
The start-up power is supplied either by MSEB through the station service transformers, or by the 34MW black-start GTG. The unit auxiliary transformers of each block supply power to the auxiliary loads of GTGs and STGs as well as the common utility load. Each power block also has 250V & 125V DC systems complete with battery chargers. In addition, each block has UPS systems to supply power to critical loads. There are also around 11 x 415V diesel generating sets ranging from 50KVA to 1250KVA for providing emergency power at various locations in the complex.
The electrical distribution system has three voltage levels -11KV, 6.6KV and 415V. Electric motors are greater that 200KW operate on 6.6KV or 11KV voltage, powered through vacuum contactors or breakers and controlled through the DCS or respective process controllers. Motors less than 200KW as well as other electrical appliances operating on 415V, are powered from associated LT MCCs.
400KV Switch Yard
The two switchyards are interconnected to provide flexibility in operation. The yard equipment comprises of isolator switches, SF6 circuit breakers, current/ voltage transformers, surge arresters, supporting lattice structure and the bus system.
Each of the switchyards has a control room that houses protection/relay panels, revenue metering, MSEB relay and communication equipment, etc. Each switchyard also has four bays and two 400KV line connections to the MSEB grid system. For start-up, MSEB can supply power to this Power Plant via the 400KV Dabhol to New Koyna Line #1.
The main power blocks are monitored & controlled by Invensys-Foxboro DCS systems. These systems use field-mounted processing and I/O modules with a centralized operator control, interlocks, AVR, excitation system, etc. The control of the block is possible from the MCR only. Local panels are generally of the monitoring type. Electrostatic discharge protection systems are provided to protect the electronic circuitry.
PLC-based control systems are provided for the burner interlock and shutdown system for the HRSGs, DM plants, IA and SA compressors, CW system, etc. These communicate with the operator stations at MCR over serial ports. In general, these plants are monitored from MCR but operated from the respective local panels.
The communication in the power plant area is through a PABX plus telephony system and a public address system which covers most of the plant.
Scaling Down of the Power Plant
The power project was originally planned with a generation capacity of 2150MW; however, this capacity has now been scaled down to 1967MW because of problems with the equipment. Thus Block-1 has been scaled down from 670MW to 640MW, while Block 2 & 3 has been scaled down from 740MW to 663.5MW each.