Hydropower makes waves on the River Nile12 January 1998
In August 1996, the biggest rehabilitation project in the history of Egypt’s hydroelectric power industry was completed successfully. Paul Haag* describes the challenge of refurbishing the Aswan 1 plant on the River Nile.
AFTER construction of the Aswan 1 hydroelectric power plant in 1960, and of the Aswan high dam in 1967, the state-owned power utility Egyptian Electricity Authority (EEA) went on to build several thermal power stations, thus shifting the main focus of its power generation strategy. However, over the past 15 to 20 years, it has expanded its hydroelectric power capacity and followed a policy of refurbishing its older hydroelectric plants.
Following the completion of detailed studies and surveys by the Swedish consultant SWECO, the Egyptian government decided in the late 1980s that Aswan 1 was in need of rehabilitation. The radical modernization of this hydro plant, which has been in operation for more than 30 years, was to cover the seven turbine-generator units (each rated at 47 MW) the complete electrical equipment, and the water passages.
A special challenge was presented by the need to keep the plant running while work was being carried out on the individual machines. This meant that during the entire rehabilitation project no more than two out of the seven main machines could be shut down.
In 1991, a German/Austrian consortium headed by ABB Kraftwerke AG, Mannheim, and including Sulzer-Hydro, Ravensburg, and Waagner-Biro, Vienna, was awarded the contract to rehabilitate the hydropower plant. The consultant engineers were lahmeyer International GmbH, Frankfurt am Main, Elektrowatt Engineering Services Ltd, Zurich, and Utility Consultants International GmbH, Frankfurt am Main. Unlike the mechanical parts, the electrical equipment and the electronics needed to be replaced in their entirety. The main parts involved were the medium- and low-voltage installations, the entire cabling and the instrumentation and control system, which monitors and controls all of the power plant functions.
At Aswan 1 seven generators produce more than one billion kWh of electricity every year. Through the dam’s top inlets, up to 2.44 million m3 of water an hour pass through seven 30m-long penstocks. After 30 years of operation, these feed channels were exhibiting severe unevenness, thus reducing the water flow velocity - and therefore the output of the power plant - to such an extent that the intake tubes had to be repaired.
The work required for the seven generators was particularly challenging, since it involved re-insulation of the rotor coils and replacement of the stator core, stator winding and the excitation system.
A total of 65t of high-quality, protection-coated steel sheets was re-inserted into each machine using a special stacking procedure. The use of the improved steel sheets reduced the core losses by around 32 per cent.
After the ‘re-coring’, two Roebel bars were inserted using the ABB round-packing method. This technique guarantees that the winding is securely embedded in the stator, and thus extends the machine’s useful lifetime. To protect the new laminated cores and windings of the stators from the dust and dirt produced during sandblasting of the turbines, the shaft between the turbine and generator of each machine had to be covered with a special platform. This structure, which can take a load of up to 40t, was designed and built by staff on site. In addition, the old generator coolers, with six air-water cooler sets (two coolers per set) for each machine, had to be replaced together with all the cooling-water and oil lines.
The original excitation machine, which was flanged onto the generator shaft, was removed and replaced by a static excitation system with a three-phase excitation transformer. The static rectifier features modern thyristor technology and consists of three three-phase thyristor bridges connected in parallel. If one bridge fails, the remaining two will supply the excitation current required for full-load operation of the generator. The voltage is controlled automatically, but there is also the option of manual control, e.g. for test purposes. The rotor current, stator current and load angle are also controlled fully automatically.
The generators are linked to the unit transformer by three-phase metal-enclosed generator busducts and flexible connection strands at the generator terminals. The busducts are dimensioned for the full rated current of the generators and are air-cooled. Feeders lead to the field-circuit transformer. In the case of machines four and six, which were fitted with new three-winding unit transformers, additional generator circuit-breakers were installed to enable the station-service power supply to be maintained should the network or power plants fail completely.
During a routine inspection of the spiral of the first recommissioned machine, cracks were discovered in the corrosion protection and, after further inspections, at the last support blade. More detailed investigations revealed a large number of hairline cracks at the top and bottom ends of the support blade, and a massive crack in the middle of the blade. Support blade cracks had never been encountered in this kind of turbine anywhere in the world before, and were thus completely unforeseeable.
Metallurgical analyses dated the inception of these cracks, which could be rendered visible only by means of special procedures using contrast media and UV light, to several decades ago. Detailed examinations of the phenomenon revealed that the cracks had probably originated after initial commissioning during the early years of operation, and had been triggered by Karman vortexes.
The blades were repaired using appropriate heat-treatment processes. In addition, the shaping of the end edges of all the support blades were optimized.
All the rehabilitated machines had to prove their operational reliability in a precisely specified, seven-day trial run before being handed over to the customer. Index measurements also had to be carried out before and after rehabilitation. They all showed a significant improvement in the efficiency of the machines after rehabilitation.
Instrumentation and control
The entire power plant process is monitored and controlled by the ABB Master hydropower plant process control system. This system has a hierarchical structure and controls the operation of the power plant on three process levels:
• Level 1: Individual control cubicles for the operation of individual drives and apparatus, e.g. for maintenance and testing.
• Level 2: Machine control cubicles of the seven turbine-generator units, in which all of the incoming and outgoing information is monitored and processed.
• Level 3: Central control room, from which the entire power plant is controlled.
Each machine has its own process station which acquires and processes the incoming signals and data and also sends commands to the peripherals. The process visualization is provided by a local monitor. All the individual process stations are linked over the station data bus to the central control room of the power plant, where the ongoing process can be viewed and controlled at an operator station.
Besides this fixed operator station, there is also a mobile operation station (a so-called trolley version). The mobile version can be connected when required to any local process computer in a machine control cubicle using plug-type connectors. This arrangement had already proved to be an advantage for rehabilitating machines while the other machines were still running.
The functions of the operator stations are as follows:
• Displaying information in the form of mimic diagrams, group displays and trend curves.
• Acquiring and listing events and disturbance messages.
• Dialogue functions involving the process, e.g. specification of setpoint values, commands for switching circuit-breakers.
• Creating event logs.
The process is observed by means of both a mosaic-type display board and a redundant MMC (man-machine communication) system. There is provision for future expansion - the highest level envisaged being managed by a regional and/or national load dispatching centre which would receive the relevant information from the power plant, and control the individual machines as required.
The backbone, so to speak, through which all the system’s ‘nerve-paths’ run, is the redundant databus (LAN network), consisting of two coaxial cables with a data transmission rate of 10 Mbit/s inside the hydroelectric power plant and two glass-fibre cables to the 132kV outdoor substation. This databus links all the process computer systems to each other and to the MMC systems.
Central control room
The central control room was designed by ABB using state-of-the-art human engineering principles, and was successfully executed by EEA with help from local contractors.
The operating staff can conveniently monitor the ongoing power plant process from the control room console, video displays and a mosaic-type display board, enabling them to intervene manually if necessary.
For the power plant auxiliaries, ABB Arab installed a new 11kV substation in place of the old installation, which had oil circuit-breakers. The new metal-enclosed substation is fitted with withdrawable SF6 circuit-breakers. It is fed by the two station-service turbines, each rated at 11.5MW, and by machines four and six.
In addition, the station-services transformers and a part of the 132kV outdoor substation were replaced.
All the 400V substations were replaced by modern substations in MNS withdrawable-module design (supplied and installed by ABB Arab). Also replaced were the DC systems (i.e., accumulators, rectifiers and DC substations) and the public address system.
Before being handed over to the customer, all the rehabilitated machines were put through a seven-day, pre-defined trial run to prove their operational efficiency. Index measurements were also taken before and after the rehabilitation. These showed a significant improvement in efficiency for all the machines. After 58 months, in August 1996 ABB Kraftwerke AG concluded the biggest rehabilitation project so far in the history of Egypt’s hydroelectric industry.
As a result, the Aswan 1 hydroelectric power plant has become one of the most advanced facilities for power generation in Egypt, and will be supplying electricity to an energy-hungry nation for at least another thirty years to come.
|Table1: Technical specifications of the generators|
|Power rating 49.5MVA Rated voltage 11kV Rated power factor 00.95 Rated frequency 50Hz Rated speed 100rev/min Runaway speed 345rev/min Flywheel effect(GD2) 15 000tm2 Stator bore 7500mm Rotor weight 485t|