In the late 1980s, the Egyptian government decided to modernize the Aswan 1 hydroelectric power plant. Now, after 58 months of work, the power plant is one of the most advanced facilities for power generation in Egypt.
The Aswan 1 modernization project was instigated in 1991, when a German/Austrian consortium including ABB Kraftwerke AG, Sulzer Hydro and Waagner-Bir was awarded the contract to carry out the rehabilitation work. The contract covered the seven 47 MW turbine-generator units, the electrical equipment and the water passages. By August 1996, ABB Kraftwerke AG had concluded what was the biggest rehabilitation project so far in the history of Egypt’s hydroelectric industry. The consultant engineers were Lahmeyer International GmbH, Elektrowatt Engineering Services Ltd. and Utility Consultants International GmbH.
The Aswan 1 intake dam is 36 m high and 330 m long, and consists of granite rubble masonry protected by waterproof, steel-reinforced concrete. Immediately behind the dam is the power house equipped with seven conventional turbine-generators installed in a concrete housing. Prior to the rehabilitation, the plant had been in service for over 30 years.
With up to 2.44 million m3/h of water passing through the top inlets into the seven 30 m penstocks, a great deal of erosion had occurred over the plant’s lifetime. The feed channels were exhibiting severe unevenness to the extent that the output of the plant was affected, and so the intake tubes had to be repaired.
In addition, the electrical equipment and the electronics needed to be replaced, including the medium and low voltage installations, the entire cabling and the instrumentation and control system. However, it was necessary to keep the plant running while the work was being carried out and so no more than two of the units could be shut down at any time.
Rehabilitation
The rehabilitation of the generators required re-insulation of the rotor coils and replacement of the stator core, winding and the excitation system. A total of 59 t of coated steel sheets were reinserted in each machine using a special stacking procedure. The use of the improved steel reduced core losses by around 32 per cent. After the ‘recoring’, two winding bars were inserted in each slot. The winding bars were inserted using the ABB ’round packing’ method. In addition, the old generator coolers had to be replaced together with all the cooling-water and oil lines.
Only the frame of the stator was reused with the steel sections for mounting the laminations remeasured and aligned. The laminated core of the stator consists of low loss steel sheets which are covered with a special insulating enamel and then compressed by means of stator bolts and compression plates. The stator windings are designed as two layer Roebel bar windings with class F Micadur insulation. A special corona protection varnish was applied to the winding bars over their entire lengths in order to achieve uniform potential distribution.
The rotors consist of a flange shaft having a through bore of 400 mm, a length of 9 m and a weight of 36 t. A pair of ten armed wheel centres for supporting the gear rims were shrunk onto the shaft with the brake ring attached to the bottom wheel centre. The 60 rotor poles are fitted into the gear rims and secured in position. The flat copper coils of the poles were re-insulated with ‘Nomex’ strengthened synthetic resin. The generator bearings were fitted with new white-metal coated pads, and an automatic high pressure oil system for lubrication was installed and the monitoring instruments replaced.
The thrust bearing has to carry the entire weight, approximately 585 t, of the rotating parts of the generator and the turbine plus the water force of around 855 t.
The bearing consists of a total of 18 pads, which can be individually adjusted, with a carrying capacity of 1440 t. All the machines received new thrust bearings and were fitted with a high pressure oil lifting system, activated automatically on start up, to ensure the required lubrication.
The original excitation machine was replaced by a static excitation system with three-phase excitation transformer. The static rectifier features modern thyristor technology and consists of three thyristor bridges connected in parallel. If one bridge fails, the remaining two will supply the excitation current required for full load operation.
The voltage is controlled automatically as are the rotor current, stator current and load angle. The generators are linked to the unit transformer by three-phase generator busducts and flexible connection strands at the generator terminals. The busducts are rated for the full current of the generators and are air cooled. Feeders lead to the field-circuit transformer.
New control
The entire 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 from individual control of apparatus to the central control room, from which the entire power plant is controlled.
Each machine has its own process station which acquires and processes incoming data and sends commands to the peripherals. All the individual process stations are linked over the station data bus to the central control room.
In addition to this fixed operator station, there is also a mobile operator station which can be connected when required to any local process computer. The process is observed by means of a mosaic-type display board.
The backbone, through which all the system’s ‘nerve paths’ run, is the redundant databus (LAN network) and two glass fibre cables to the 132 kV outdoor substation. This databus links all the process computer systems to each other.
The functions of the control system include the automatic starting and stopping of the machines, acquisition of information from the process, output of signals, commands and disturbance messages and the generation of event logs and trend curves.
The central control room was designed by ABB and was executed by EEA. 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 replacement 11 kV substation. The new metal-enclosed substation is fitted with withdrawable SF6 circuit-breakers. It is fed by the two station service turbines, each of which is rated at 11.5 MW, and also by two of the generators. In addition, the station services transformers and part of the 132 kV outdoor substation were replaced.
All the 400 V substations were replaced by modern substations (MNS withdrawable module design supplied and installed by ABB Arab) as were the DC systems (accumulators, rectifiers and DC substations) in addition to the public address system.
Performance tests
Before being handed over to the customer, all the rehabilitated machines were put through a seven day, predefined trial run to prove their operational efficiency. Index measurements were also taken before and after the rehabilitation which showed significant efficiency improvements for all the machines.
During a routine inspection of the spiral of the first recommissioned machine, cracks were discovered in the corrosion protection and 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. Support blade cracks had never been encountered in this kind of turbine before and were therefore completely unforeseen.
Metallurgical analyses dated the inception of these cracks 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 vortices. The blades were repaired using heat treatment and the shaping of the end edges of all the support blades was optimized.
In August 1996, after a total of 58 months of work, ABB Kraftwerke AG completed the rehabilitation project of the Aswan 1 hydroelectric power plant on schedule. On completion, the power plant became one of the most advanced facilities for power generation in Egypt and should reliably supply electricity for yet another thirty years to come.
TablesTable 1. Technical data for the Aswan 1 turbine-generator units