Al Zara: helping to make the super grid a reality19 May 2000
The proposed regional super transmission grid for the Middle East is well under way. Part of the development is to expand the region’s power generation capacity alongside the development of the transmission grid. The Al Zara power plant – a key element in the scheme – is characterised by its use of dry cooling towers, based on the Heller system. David Turner, Mott MacDonald, Brighton, UK.
Syria, the crossing point from Europe to the Middle East, is rapidly expanding its power transmission and generation capacity. It is well placed to be at the centre of the proposed regional tranmission super grid, in which Syria, Turkey, Jordan, Iraq and Egypt will have their power systems interconnected.
The Jordan–Egypt link is complete, the Syria-Turkey link is in the early stages of development, and the Syria–Jordan link is in an advanced stage of construction. Mott MacDonald is the engineering consultant for the Syria–Jordan link.
To meet increasing demand, the Public Establishment for Electricity Generation and Transmission (PEEGT) is building a new power plant at Al Zara, 25 km north of Homs and at the centre of the country’s grid network. The £245 million project is being executed by Mitsubishi Heavy Industries (MHI) and financed by an Overseas Economic Co-operation loan from Japan. The three units are due to enter service during 2000.
The power plant
The Al Zara power plant comprises three 220 MWe units. Provision has been made for a second phase comprising two units of a similar capacity. The plant will run on natural gas. Distillate oil and liquefied petroleum gas (LPG) will be used as start-up fuels while residual oil, available from local refineries at Homs, will act as a standby fuel. Mixed fuel in any ratio will be possible, and change over will not affect the steady state steam production.
An unusual feature of the station is the adoption of dry cooling towers based on the Heller system. The lack of sufficient water required for cooling the turbine condensers was the main consideration for this decision. One tower, 135 m high, will be provided for each generating unit. Wet cooling towers will be used for station auxiliary cooling duties, again minimising the make-up water required. Steam is to be provided via a conventional natural recirculation boiler with reheat.
Maximum main steam flow is 800 tonne/h at 166 bar and 540°C. Maximum reheat steam flow is 707 tonne/h at a pressure of 40 bar and a temperature of 540°C. Boiler feed water, supplied via three 60 per cent electrically driven feed pumps using variable speed hydraulic coupling and discharging at 207 bar, will be preheated via the LP and HP feed heaters to a temperature of 252°C at the boiler economiser inlet.
The cooling systems at Al Zara comprise the main condenser cooling with the closed cycle dry cooling towers and the auxiliary cooling system with the open cycle wet cooling towers for cooling the generator, the turbine lubricating oil system, and other auxiliary cooling duties.
Dry cooling tower system
The indirect dry cooling tower system is unitised and comprises natural draught towers of steel structure covered with aluminium sheeting each 135 m high and 114 m in diameter at the bottom and 59 m in diameter at the throat. Air flow through each tower at 40°C when operating at 100 per cent load with a wind velocity of 5 m/s is 43 100 tonnes/h.
Aluminium fin tube air cooler bundles (deltas) of triangular cross-section are arranged vertically around the base perimeter of the towers. The deltas have motorised automatically-controlled louvres which start to close in the event of low condenser pressure or the CW temperature falling below 15°C. High wind velocities may cause depression in the deltas perpendicular to the wind direction and may result in reverse air flow. For this condition, the louvres are closed for the deltas affected. Cleaning of the cooling delta aluminium plate fins is by 10 bar g pressurised water jets with rotating manifolds, which enable the nozzles to clean the heat exchanger surfaces.
Each delta is connected to a ring vent-pipe and each cooling delta sector is connected to a vertical standpipe. Cooling water is circulated between the cooling tower and condenser by two 50 per cent CW pumps with recovery hydro turbines to reduce running costs. Make-up and excess water facilities maintain water level in the jet condenser and a dump arrangement to drain tanks is provided in the event of the ambient temperature dropping to 2°C or the condenser level being very high.
The dry cooling system is designed to meet heat rejection of the direct contact jet condenser at 100 per cent steam turbine bypass flow (plus water injection) and at an ambient air temperature of 40°C. Two 100 per cent condensate pumps deliver the condensate from the jet condenser back to the feed water cycle of the power plant.
To meet the requirements of the cooling water and condensate/feedwater in the same closed cycle system, it is necessary to maintain two pH levels, one for the boiler and the other for the cooling water system through the aluminium fin tube air cooler bundles.
Since the air cooler deltas of the Heller system are aluminium, conventional water treatment with high alkalinity suitable for ensuring low rate corrosion of steel surfaces is no longer adequate. A pH of 8.5 or less is required for aluminium. In the cooling water system, the dissolved oxygen content is 100-200 ppb and the internal carbon steel surfaces are covered with a thin iron oxide layer (Fe2O3: Haematite) to reduce corrosion. Non volatile alkalising agent sodium phosphate is used for corrosion protection in the boiler section to achieve a pH of 9.5-9.8 and dissolved oxygen at the deaerator outlet is 7 ppb or less. Under these conditions, an iron oxide layer (Fe3O4: Magnetite) will be generated to protect the carbon steel surfaces. While volatile NH3 is not used in the boiler, it is used to control the pH in the condensate and feedwater and is injected prior to the deaerator.
Auxiliary cooling water system
Each unit is equipped with two wet cooling towers (two x 100 per cent) of the mechanical draught evaporative type, and two x 100 per cent circulation pumps capable of fulfilling the cooling requirement at a maximum cooling water temperature of 40°C and a design cooling water temperature after the cooling tower of 30°C at an ambient air temperature of 40°C.
The auxiliary cooling water system is designed for the heat rejection and sealing water for the following systems:
Generator H2 cooler and seal oil cooler
Turbine lubrication oil coolers
Boiler feed pump and booster pump oil coolers
Condensate extraction pump motors
Forced draught and gas recirculating fans
Hydrogen/oxygen generating plant.
Each auxiliary cooling water system is interconnected so that the cooling demand of two units can be supplied by one auxiliary cooling water system.
Design of the Al Zara electrical system encompasses three generator units, plus all auxiliaries and two double bus bar open terminal switchyards at 230 kV and 66 kV. Each 272 MVA generator is a 50 Hz three-phase synchronous machine. Separate hydrogen coolers are provided on each unit with heat dissipated through conventional water cooling. The main unit protection is provided in modular hardware and software design which allows flexible installation and tailoring to PEEGT requirements.
Air insulated isolated phase bus bars (IPB) are provided for the main generator circuit. A digital automatic voltage regulator (AVR) and a static excitation system, which is energised through a transformer tapped into the main generator circuit, allows automatic dynamic control of the generator output.
The generator output at 15 kV is stepped up for each unit through 275 MVA transformers to a grid voltage of 230 kV. Auxiliary power is supplied via a step down 25 MVA unit auxiliary transformer to 6.3 kV. Control of the auxiliary supply voltage is to be achieved via on-load tap changers. Under the normal operating conditions, each unit will support its own auxiliaries via the unit transformers at 6.3 kV, 0.4 kV, 220 V ac uninterruptible power supplies and at 220 V dc.
The switchyard 0.4 kV system is supplied through tappings on the earthing transformer located in the 66 kV switchyard, and is backed up from feeders on the power station emergency board. Separate mosaic displays are provided for the control and indication of the generator circuits, unit and common auxiliaries, 66 kV switchyard and 230 kV switchyard. These will be located in the main control room and in the substation control room. The power flow between the main plant and the 230 kV switchyard uses transmission towers to route the conductors between the dry cooling towers – an arrangement that is determined by the need to minimise losses in the cooling water system by keeping the pipework lengths to a minimum. Two three-winding transformers with on-load tap changers connect the two switchyards and feed power to the 6.3 kV station common auxiliary system. This system is used for start-up and shutdown of each unit when the unit system, used under normal station operation, is not available.
Emergency switchboards are provided for the unit and the common systems, each of which has an associated emergency diesel generator. These emergency diesels and the common systems are interconnected with bus ties. Each diesel engine is used to support one emergency board and is backed up by the other three diesel engines.
Control and instrumentation
The three generator units and the common services are automatically controlled from the central control room through a distributed control system.
Local control rooms are provided at the 66 kV/230 kV substation in order to control the distribution, and at the water treatment plant and dry cooling towers. Unit plant control is automatic between 30 per cent and 100 per cent output and under normal conditions operates in ‘boiler following turbine’ mode. Turbine inlet pressure is controlled at a pre-set value and the system is load responsive. Power production will be determined and controlled from the national power dispatch centre.
The Mitsubishi DIASYS distributed control system (DCS) provides the basis for the plant-wide integrated control scheme. This scheme enables the turbine governor system to be directly connected to the unit data highway of the DCS and it provides a centralised man-machine interface. A suite of VDU workstations and printers provides the operators with graphic displays, alarm handling facilities and printed reports. The DCS – which incorporates a spatially separated, duplicated data highway – comprises the automatic plant control system, burner management system, turbine automatic control system and the plant sequence control system. Sub systems and auxiliary systems for the air compressors and fire fighting are linked to the DCS by the I/O units. The boiler soot blowing equipment is a separate system and functions independently.
Other systems provided and integrated with the DCS are the data logging system, sequence of event monitoring system and energy management system. A suite of self-diagnostics ensures that the operator is alerted to any fault by an alarm, and equipment failures will be pinpointed to card level and displayed on the VDU.