Stephen Moore* explains how problematic concrete growth was overcome when refurbishing the Owen Falls power plant in Uganda
Refurbishment of a power station where major overhaul of the generating equipment has not been performed before, where detailed maintenance records do not exist, and where structural problems add to the complication, is a time consuming and costly exercise. During the refurbishment of Owen Falls hydroelectric station, located on the Victoria Nile near its source on Lake Victoria in Uganda, many technical and managerial issues had to be addressed.
Owen Falls was commissioned between 1954 and 1968, with an eventual installed capacity of 150MW from ten Kaplan turbine-generator units. The station provides virtually all of Uganda’s electrical power and is intended to provide 30MW of firm power for neighbouring Kenya.
During the 1970s, as a result of political and economic strife in Uganda, the power station and generating equipment suffered from non-availability of spare parts and a consequent lack of regular maintenance. It is to the credit of the Uganda Electricity Board’s (UEB’s) operating and maintenance staff that they were able to keep most of the generators operating throughout this time. Nonetheless, by the time Uganda returned to political stability, the mechanical and electrical plant in the station was in very poor condition and much of it was in need of refurbishment and modernisation.
The consultants Kennedy & Donkin and Sir Alexander Gibb & Partners were therefore engaged in 1983 by the Overseas Development Administration (ODA) of the UK to recommend a programme of uprating and rehabilitation for Owen Falls power station, as well as for Uganda’s transmission and distribution system.
As part of its ongoing programme of poverty alleviation in the developing world, the ODA agreed to fund supervision of the refurbishment work recommended for the power station and transmission system.
In 1988 Kennedy & Donkin and Sir Alexander Gibb & Partners were appointed by the ODA to provide services as engineers to the power station refurbishment contracts.
At the time refurbishment began at Owen Falls, seven of the generators were limited to below 15MW output and two were unserviceable, leaving the station with a maximum output of less than 110MW.
Key features of the refurbishment were to be:
•Restoring all ten generators to service and uprating their output from 15MW to 18MW.
•Formation of work teams consisting of UEB personnel under the supervision of specialist contractor staff, so that all necessary skills for dismantling, repair and re-installation of the turbine generators could be passed on to a new generation of UEB personnel.
•An emphasis on developing technical solutions to fulfil the aim of refurbishing the station for another 25-30 years of service, rather than seeking ‘quick fixes’ to problems.
•Co-operation between UEB, the consultant and the contractors to minimise costs so that the maximum benefit could be obtained within the limits of available funding.
Specifically, the major refurbishments included:
•New stator cores with uprated stator windings.
•Installation of a static excitation system.
•Modernisation of the electrical and control equipment.
•Provision of two new generator transformers and various major items of cable and switchgear.
•Overhaul or replacement of turbine mechanical equipment, including parts of the turbine control and governing system, the cooling and lubrication systems, brakes and jacking equipment, turbine and generator bearings, shaft seals, turbine blades and hub seals, guide vanes and draft tube cones.
•Underwater inspection and repair of the dam, station intakes and draft tubes.
•Refurbishment of the dam sluice, intake and draft tube gates and modification of their control equipment.
•Power station structural repairs.
•Power house re-roofing.
•Repairs to the draft tube platform supports.
•Provision of mobile in-service regeneration equipment for transformer oil.
Owen Falls refurbishment programme
The initial refurbishment programme was based on overlapping unit outages to maximise productivity of the generator and turbine repair crews. However, the loss of generation that resulted, at a time when reliance on Owen Falls was virtually the sole source of power in Uganda, led to the programme being altered so that only one unit at a time was under repair.
Dismantling and repair of the first unit showed that the planned life extension could not be achieved without a significant expansion in the original scope and programme of the electro-mechanical refurbishment contract.
Therefore in January 1993 there was a major review of the project funding, supervision agreements, and contract scope that took the project through to completion in June 1998. The revisions increased the value of the turbine and generator refurbishment contract from the original £9.7M to around £13M, and provided for virtually twice the original amount of consultancy supervision.
The powerhouse had structural problems that were eventually confirmed to be the result of alkali-silicate reaction (ASR) in the concrete. This ‘slow-late’ form of alkali-aggregate reaction (AAR) occurs between the alkalis in the cement and silicate particles in the coarse aggregate, as discussed by P J Mason in his paper on the effect of concrete expansion at Owen Falls (presented at the Institution of Civil Engineers in May 1998).
The ASR phenomenon was only just becoming widely understood when, after some ten years of operation, the affected concrete at Owen Falls had expanded to the extent that cracks appeared in the powerhouse.
The concomitant relative movement between the supporting structures of the turbines and generators affected the alignment of several units. Among other undesirable effects, the concrete around the steel spiral casing had expanded vertically, and the relative positions of the thrust bearing bracket sole plates and stator sole plates had altered. Changes in the vertical axis of some units meant that their generators were effectively tilting upstream of the turbines.
It was known that the concrete growth could be expected to continue throughout the life of the station, so as part of the refurbishment it was vital to devise a practical means of realigning the turbines and generators, perhaps as often as every three to five years.
A number of options were considered and rejected on grounds of unacceptable cost, programme length, or risk to the structural integrity of the powerhouse. The technique eventually adopted included raising the generator stator and thrust bearing brackets by packing their sole plates to return the rotor to its design position relative to the stator.
The packing inserted between the stator and thrust bearing brackets and their supporting pedestals was calculated to compensate for both existing and future relative movement between the stator and thrust bearing.
In this way the thickness of the plates and shims may in future be progressively reduced to compensate for concrete growth as it occurs, keeping the stator, rotor and thrust bearing in the correct relationship.
So that the thrust bearing and stator brackets could be raised by the amount necessary without upsetting the relationship of the runner and the discharge ring, a tailor-made spacer was fitted between the turbine and generator shafts.
In the worst affected unit a spacer of 90mm was fitted between the generator and turbine shaft flanges. The average thickness of packing placed under the thrust bearings was 75mm, and under the stators 70mm.
By these means periodic realignment of the units is possible at a fraction of the cost of alternatives involving major modification of machinery or replacement of concrete.
Apart from the modifications to allow for concrete growth, many of the bearing surfaces in the turbine control, governing, and bearing systems had to be re-metalled or replaced, the most extensive re-metalling being on the turbine guide bearings.
There was also a need for repairs to cavitation erosion on some turbine blades, in the runner chambers and on the draft tube cones.
Most re-metalling and precision machining was performed on parts returned to the UK, while welding and grinding of runner blades and draft tube cones were performed on site.
To avoid the cost of returning the runner hubs and turbine shafts to the UK, specialist equipment was developed for machining these on site. To keep costs down cracked bearing spiders were, wherever possible, repaired using a proprietary metal locking system.
The dam sluice gates, the headgates and draft tube gates were all removed, sandblasted, repaired and repainted on site before being re-installed into new guide rails with new lifting chains.
A completely new control desk was installed, and modifications were made to the control equipment of the turbine, generator and headgates. One generating unit was modified to demonstrate a capability for semi-automatic starting from the control room.
The control and generator power circuits were completely re-cabled, and systems renewed or refurbished included the protection systems, 33kV generator breakers, and the 48V and 240V DC battery and charger systems.
Two spare generator transformers, one at 33kV and one at 132kV were supplied as spares and these were tested in service to ensure that all the necessary accessories and connections were available.
A number of challenging problems had to be overcome on the uprated generators. These included corona discharge in the stator windings, fretting of the stator core joints, and repeated failures of the steel plates that form part of the arrangement for holding the stator laminations in the frame. The contractor’s initial attempts to develop solutions to these problems were not always successful, and a stator defects remedial programme, which consists of various modifications to increase the stiffness of the core, is scheduled for completion late in 1998.