When it comes to undertaking rehabilitation work, risks to hydro plant owners can be reduced if certain conditions are identified in the work contract. Dzintars Ostanevics and Knut Vik report on lessons learned during rehabilitation work on generators at the Daugava river cascade scheme in Latvia

The river Daugava is the largest power source in Latvia, with three power plants making up the Daugava river cascade scheme. Owned by the Latvian state owned power company, Latvenergo, the cascade produces about 40% of the electric energy for the nation.

The oldest hydro power plant in the scheme is Kegums 1, where four vertical Kaplan units are installed. The total rating is 90MVA at 107 rpm by a head of about 13m. Three of these units were commissioned in 1939, but were later severely damaged during WW2. These units, of Swedish origin, were all rehabilitated by the end of the forties. A fourth unit of Russian origin was commissioned in 1952. Since then, only routine maintenance has been carried out, except for the stator winding of the Russian-made unit, which was replaced in 1972. Only minor modifications were made to the instrumentation and controls at this time.

The largest power plant in the scheme is Plavinas. Commissioned in the 1960’s, the plant has a head of 40m, and a large reservoir dimensioned for peaking power over the week. Ten vertical Francis turbines power the generators at the site, each having a rated output of 90MVA, 88 rpm. In the early nineties, an upgrading program was initiated and two units were upgraded to 105MVA, mainly by rewinding. Two more were upgraded shortly afterwards by replacing the complete stator, which was delivered to the site in six sections. This work was performed by the original Russian equipment manufacturer.

A pre-study of further upgrading was initiated in 1993. The result was a decision that all of the Kegums 1 HES should be upgraded to Western European standards, and its residual lifetime should be extended to 40 years. Two units in Plavinas HES also needed to be upgraded. This work was mainly financed by the European Bank for Reconstruction and Development (EBRD), together with suppliers credit and Latvenergo’s own financial capacity.

Upgrading work

The condition of the facilities prior to the upgrading was considerably bad. The stator core and windings of the umbrella type generators in Kegums needed to be replaced, although the mechanical components seemed to be in an acceptable condition. The condition of the combined guide and thrust bearing was unknown, however the operation records were good. The old DC rotating exciter needed to be replaced and all of the instrumentation and controls were in a bad condition, meaning they could not be reused. The generator had an epoxy impregnated stator winding that did however seem to be in an

acceptable condition.

The Plavinas generators had been subject to a hard life. These units, with a stator bore of 12m, were started and stopped frequently. It took less than a minute from when it started to rotate until it was synchronised and produced a maximum power output of 80MW.

Like the Kegums generators, the windings and the stator core were in a bad condition and needed to be replaced. Most of the stator windings problems had occurred through routine high voltage tests. The windings were of the wave type. If a bottom bar in such windings fails, more than double the number of top bars must be removed to take it out, compared to a lap type winding. It was found that the air gap between the poles and the stator core was reduced to about two thirds of its design value at some locations. The DC exciter was also in a bad condition. The generator upper guide bearing was of a conventional type based on babited tilting pads. The thrust bearing was modified nearly 20 years ago by changing out the babit on the tilting pads by Teflon.

Technical challenges

The upgrading of the four Kaplan generators at Kegums was regarded to be a straightforward operation, and did not require any special attention. For the two large diameter peaking power generators at Plavinas it was different. Large diameter units have general problems of deformations, in particular the ones caused by the thermal expansions related to starts and stops. It was specified that the stator core should be stacked at site as one continuous ring. Special actions also needed to be made to keep the thermal stresses and expansion of the stator frame, as well as the rotor, under control. There is no doubt that the reason why the original generators had survived the frequent starting and stopping to and from maximum load, was the excellent cooling of the stator and the tight fit of the overdimensioned asphalt compound insulated stator-winding bars. The airflow through the generators was extremely high and it produced high ventilation losses.

Bidding process

The overall requirements for the project were to upgrade the complete generators, including their auxiliaries, to produce safe and reliable power during the specified lifetime. The work needed to be completed within a given timeframe. If efficiency guarantees and completion time were not fulfilled, liquidated damages were to be applied. These contracts specified the quality for each of the new components, and those components which were reused without any repair. The specification was close to those that would have been applied for new generators.

The contract also defined areas of problems. The contractor was free to reuse or replace the other components. All components which were to be reused were required to undergo a quality check to verify that the requirements in the specification could be fulfilled.

The owner invited the bidders to site inspections as a part of the bidding process. The bidders were given free access to the facilities. They were also given access to all available drawings and documents, including interviews with the owner’s representatives. Only prequalified bidders were invited. To be prequalified, the bidders had to document experiences and qualifications necessary to perform such work, technically as well as financially. A formal quality assurance level was also required. The bidders were free to propose alternative solutions.

Contract requirements

The scope of work was an all-inclusive contract which required a minimum of co-ordination for the owner. The Kegum contract included three parts: turbines and governors; generators and excitation; and control systems for a total of four Kaplan units.

For Plavinas, it was three separate contracts due to the different ways of financing. For these contracts, there was a clause of co-operation between the contractors.

The contractors needed to subcontract quite a number of components. It was specified that the owner should approve essential subcontractors. In most international contracts the work on site will be performed by local subcontractors. This work is critical, particularly the day-to-day co-operation between the contractor’s supervisor and the crew of the subcontractor. Unless they work together as a team, there will also be problems for the owner. The supervisor needs to check all details on the spot and make sure that the quality of the work is as specified. Not doing so can lead to rework and delays.

The problem was to what extent should representatives of the owner witness the tests at works and at site? For the generators, the quality was acceptable for new components and there may be little to gain to invest too much money in checking these. For subcontracted components it may be worthwhile for the owner to check progress from time to time with the contractors and subcontractors.

The quality control of reused parts was mainly performed by the contractor’s supervisor. Representatives of the owner did not always participate.

Dismantling

Dismantling is a rather critical operation. It will confirm if the assumptions of the conditions of the facilities were good, and reveal any problems. The most challenging aspect of a project is to make decisions of whether to reuse or replace a component. To avoid any time-consuming discussions it is important to have adequate specifications.

During the dismantling it was revealed that the condition of the rotor spider of one unit was not acceptable. The defects were something the contractor could not have reasonably foreseen before the contract was signed. The damage was caused during the war in 1945 and no information of that event was included in the contract or demonstrated by the site inspections.

This problem was found during the early stages of dismantling and the damaged spider was replaced, without causing any delays. The contractor also revealed a number of casting defects causing oil leaks in the bearing tank. Casting defects of this type made in the late thirties is however not unusual.

The contract did not specify the oil coolers to be replaced. For the first unit they were thoroughly checked, and it was decided to replace the tubes and other essential parts. However, leaks were found during the pressure tests just before assembling two of the units. One leak was due to a metallurgical defect in the new tubes in the cooler, despite the fact that the tubes were reported to have passed an eddy current test.

The contract specified that the telephone harmonic factor should be classified to iec 34. However, measurement of one of the units revealed that this factor was found to be much higher. After some investigation, it was revealed that the un-symmetrical poles were arranged in a random way.

The contractor applied for a change order for the additional work of rearranging the poles, but this was rejected by the owner. It was a direct contract specification, and the contractor had not commented to that. It is also well known that the Swedish manufacturer and other manufacturers sometimes use this design to improve the magnetic circuit, and therefore it could not be claimed to be something unforeseen.

A substantial oil leak from the thrust bearing contaminated most of the stator and its winding of the unit. This occurred between the time of the signing of the contract and the start of the rehabilitation work. The insulation resistance tests of the stator winding showed very bad values. This test was performed several months after the unit had been in operation.

Heating to keep the winding dry was not applied. The contractor attempted to use this situation to refuse to reuse the stator winding and sell a new stator under a change order. An insulation resistance measurement made on a contaminated and humid surface will very often show low values due to the reduced surface resistance. On that basis, the owner refused the request for a change. The winding was cleaned and the brittle bandage based on linen cords was replaced as a separate operation by the owner. The insulation resistance after the cleaning was measured and found acceptable and the winding passed the high voltage test.

The contractor measured the stator bore of the unit. It was not cylindrical, and did not meet the specified values. The contractor claimed this would create problems of vibrations. However, vibration measurements were made prior to the dismantling and did not show any harmful vibrations. Therefore the owner assumed a very little risk and refused the contractor’s request for a change order of a new stator.

A problem of rotor circularity and rotor ring growth was also identified in the tender documents for the two larger units. A possible cause of the problem was mentioned, and special correction procedures were recommended. Dimension tolerances were also specified. During the site visit, all information was made available to the bidders, including an interview with the design engineer of the generator. The facilities were also open for any inspections and drawings were made available. The contractor undertook extensive research and identified the problem to be a design problem. He proposed a solution which was approved by the owner.

Co-ordination

As mentioned earlier, one of the upgrading projects involved three contracts. Except for one occasion, this special type of co-ordination did not create many problems. The generator hub was supposed to be bolted on a new turbine shaft. The turbine contractor allowed this shaft to be finish machined before having access to the generator hub. Its dimensions were based on the drawings from the original equipment manufacturer. There is no doubt that control measurements could have been made before finish machining and still be able to deliver the shaft to the site on time. The result was that the diameter of the spigot was too large. In addition, it proved to be rather difficult to check the circularity, the flatness and perpendicularly to the shaft line of the contact surface of the hub towards the turbine shaft. These conditions led to substantial delays of the alignment process. Because of this it become clear that it should be stated in any contract that the contractor is responsible for making sure that new components or repaired components must fit into the allocated positions.

Lessons learned

To reduce owners risks during rehabilitation, the Daugava river cascade scheme demonstrated that the following factors need to be included in work contracts:

• The scope of supply must be clearly defined.

• A very specific functional requirement for each part should be given.

• All parts must be included.

• All interfacing and the contractor’s related obligations should be clearly defined.

• The bidders must be given access to the facilities, and should be given as much relevant information as possible prior to the contract signature.

• The contractor should be allowed to confirm their approach and technical solutions at the contract negotiation stage.

• A detailed quality program should be defined and approved.

• Critical subcontractors and the related quality assurance aspects should be approved by the owner.

• The owner should have the right to force the contractor to terminate any subcontractor if they fails to execute their works or there are indications that they will do so.

• The contractor should only be allowed to get compensation for something which could not be reasonably foreseen by an experienced contractor prior to the contract signature.

• Progress payment should be tied up to engineering, ordering of essential materials and deliveries.

• Liquidated damages should also be related to engineering and deliveries.