The Hub Power Plant consists of four generating units each rated at 323 MW gross output, with an oil-fired single re-heat boiler and tandem compound, two cylinder condensing steam turbines directly coupled to a hydrogen cooled generator. The design net available output is exported to WAPDA’s national grid via the power station’s 500kv switchyard. Both the plant configuration and the steam conditions represent conventional design based on proven technology
The Hub Power plant is one of the most efficient oil fired thermal plant and can provide 10+% of country’s electricity demand. The plant is operated in such a way that it is available at a short notice during high demand period and can switch to flexible operations during low demand periods.
The Hub Power Station site has an ongoing proactive approach to safety management. Station’s commitment to safety procedures has already earned it with Royal Society for Prevention of Accidents (RoSPA) Gold Awards consecutively for the last many years.
The Construction Contract was signed with a construction consortium in July 1991, to provide engineering, procurement, construction and commissioning for a 1292 MW power plant on a lump-sum, turnkey basis. The construction consortium included:
- Mitsui and Co. of Japan, consortium and commercial leader;
- Ansaldo ENERGIA of Italy, technical coordinator and designer and supplier of the turbine generators;
- Ishikawajima-Harima Heavy Industries Co. Ltd of Japan, designer and supplier of the boilers,
- Campenon Bernard SGE SNC of France, the civil contractor.
The Plant consists of four generating units which are each rated at 323 MW with an oil-fired single re-heat boiler and tandem compound, two cylinder condensing steam turbines directly connected to a hydrogen cooled generator. The net available output power of 1200 MW is exported to the Water and Power Development Authority (WAPDA) grid by means of the station’s 5OOkV switchyard. Fuel is received by pipeline from Pakistan State Oil (PSO). Load tanker unloading facilities are also available.
The Plant design and engineering are based on proven technology. It operates on the basis of a conventional steam cycle such that the steam produced by the boilers drives the turbine generator sets to make electricity.
The thermodynamic characteristics of the units are as follows:
- Steam pressure and temperature: 170 bar, 540OC;
- single re-heating up to 540OC;
- condensate and feedwater pre-heating, up to 250OC;
- turbine exhaust pressure: 0.087 bar (2.5 in Hg). This is somewhat higher than is normal because of the high average temperature of circulating water available at the plant site.
The plant, when operating at full load converts energy in the residual fuel oil (RFO) at a thermal efficiency of approximately 38.9%.
Activities on the site began with soil investigations in January 1993; two months later, work on site for temporary structures could begin and within eight months, work had started on the permanent structures. The first energisation of the GIS substation and start-up transformer took place in February 1996; the first unit was first operated at full load in June 1996.
The entire plant was fully commissioned on March 31, 1997.
The boilers are drum-type with natural circulation, designed specifically to burn heavy oil. The pressure parts include the economiser, the drum, the down comer and the furnace wall tubes, the superheaters, the reheater and the attemperators for steam temperature control. The combustion air and flue gas system includes the force draft and gas recirculation fans, the air heaters for the heat recovery from the flue gases, air and gas ducts with relevant dampers.
The burners, of steam atomising type, are located in the front and rear walls of the furnace. The oil is heated to 90oC in a heater, using steam as the heating medium. This is secondary steam, generated in a reboiler to prevent the possibility of oil leakage pollution in the mainsteam/water system. In normal operating conditions, the primary steam for the reboiler and the atomising steam for the burners are extracted from the cold re-heat line. An auxiliary boiler which is common to the four units is used to provide heating steam for the oil in “cold start” conditions. Cold light oil can also be used for cold starts.
If the boiler needs to be re-fired shortly after a shut-down, there is the possibility of using primary steam from the main steam system so long as the pressure in the system has not fallen below 30kg/cm2(g).
Each turbine is a two-casing single-shift turbine. It comprises a three cylinder, single reheat condensing machine with steam bled to a regenerative water heat system. It is designed so that superheat steam from the boiler passes through the control valves to the high pressure (HP) turbine inlet, drives the turbine, through the reheater and is then discharged from the HP turbine, through the reheater section of the boiler and back to the intermediate pressure (IP) cylinder inlet. The steam imparts energy to the turbine, flows to the double-flow low pressure (LP) cylinder and is then discharged to the condenser. The turbine drives the alternator at 3,000 rpm.
The Condenser and Feed System
The condenser into which the steam from the LP turbine is discharged is a closed vessel fitted with tubes through which sea water is circulated by the main circulating pumps. The outside of the cold tubes condenses the steam from the turbine back into feed water.
The condenser is a single-pass surface type, with divided water-boxes to facilitate inspection and maintenance. The pressure in the condenser is 0.084 bar when the circulating water enters at a temperature of 33ºC.
The condensate and feed heating system includes seven heating stages; the heating medium is based on the steam extracted from the turbine.
Four surface-type LP heaters are provided for heating the condensate. These receive steam extracted from the LP section of the turbine. A direct-contact heater is provided to deaerate the feedwater and to be used as suction-tank for the feedwater pumps; this heater receives steam from the turbine IP section exhaust.
The boiler feed pump sets are provided to supply about half of the feedwater needed by the boiler at full load and each pump set has a constant speed motor and booster pump, a hyraulic speed changer with step-up gear and a variable speed high pressure main feed pump.
Two surface type HP heaters are provided for the heating of the feedwater. HP1 receives steam extracted from the IP section of the turbine whereas HP2 gets steam from the cold reheat line.
The cooling water used in the condensers and for the auxiliary plant is drawn from the Arabian Sea through submerged concrete inlet channel about 300m long to a siltation setting basin and than through an open inlet canal some 650m long to the circulating water pump house. The sea water flows from the circulating water intake through screens to remove debris and is pumped by the main sea water circulating pumps through the intake piping and valves to the condensers. The water is discharged through further piping and valves into a culvert and seal pit before flowing into the Hub River estuary through a 1,000m discharge canal.
The remainder of the auxiliary plant and systems are cooled by a closed circuit water system, using demineralised water.
There are four generators which develop a power of 380 MVA each (power factor 0.85) and are of the windings direct cooling type.
The stator winding is cooled by the circulation of deionised water which passes through the hollow conductor which forms part of the semi-coils. The rotor winding is cooled directly by hydrogen.
The generated power is despatched to WAPDA’s grid through two overhead 500 kV lines through a half-breaker scheme, SF insulated substation. There are also two start-up transformers. Station power supply and unit auxiliary power supply are provided to minimise any effects of equipment failure or maintenance on continuity of service.
The electrical auxiliary power system rests on three major sub-systems:
- AC power system
- DC power system
- AC emergency power system
The station transformers performs the start-up and shut-down of the plant and supplies the common services boards.
The emergency ac source is ensured by emergency diesel and 110V inverters, supplying the loads necessary for the safe shut-down of the power plant.
The Control System
A microprocessor control system (DCS) is used to control and monitor the performance throughout the plant.
A Bailey INFI90 DCS supports a decentralised hierarchical structure which allows for maximum mobility and flexibility to different plant locations. Thus control functions and data processing can be undertaken at the detailed plant area while at the same time retaining the ability to operate all control functions from the centre. It is sufficiently flexible to serve a wide range of requirements at loop and component level without any changes to hardware.
The DCS includes:
- The process control unit
- The data communication system
- The operator interface
A dedicated engineering workstation (EWS) provides for fast off-line development of control strategies.
The automation system design operates on a principle of high availability level.
The DCS which is in place ensures a safe, reliable, available, economical and simple operation of the boiler turbine generator units, together with their associated auxiliary plants.
In addition to the main electricity production and fuel storage facilities, the plant has auxiliary plant or sub-plant facilities which include:
- A desalination plant to convert sea water into feed, demineralised and potable water
- Water and waste water treatment plant
- RFO and lubricating oil processing plant
- Safety equipment
- A fire fighting system
- Hydrogen gas generation facilities
- An auxiliary boiler house
Arrangements for Despatch
The Plant is operated within prudent limits in supplying electricity to the WAPDA grid. In accordance with the Power Purchase Agreement, there are:
- limits on the maximum number of starts per unit during the effectiveness of the Power Purchase Agreement and also on a yearly basis
- limits on the rates at which energy output can be increased from a unit
- minimum specified electrical frequencies and voltage functions
Under the Power Purchase Agreement, WAPDA provides Hubco with estimates of its requirements for power from the Plant for a year, quarter, month and day ahead.
Direct telemetering and telecommunication links have been established between the Plant and WAPDA’s grid control centre.
Connections to the WAPDA Grid
Each generator is connected by a 21.5 kV/500 kV step-up transformer to a gas insulated switchgear outdoor sbstation which in turn is connected to two overhead export lines designed by WAPDA and constructed by Saudi Cable of Saudi Arabia. Two station transformers suplied from the 500 kV substation provide power for start up of the Plant and for common services.