Harrbach run-of-river power station Generator overhaul

Run-of-river power stations normally feature slow turning synchronous generators with salient pole rotors whereby the shaft is in a vertical arrangement directly above the turbines. “Slow runners” of this type frequently feature rotor diameters in excess of 20 m.

Over the last 120 years, the output available from salient pole machines has been multiplied, thanks to development, by more than 100. The largest machines today run at approximately 750 – 850 MVA output. In the 1st decades of the 20th century, however, it was the peak of engineering to succeed in achieving an approximate output of 6 MVA from any machine.

The category of umbrella-type alternators to some extent represents a variation of salient pole rotor systems. Generators of this type are only single-bearing, hence more economical to manufacture: the shaft supports not only the turbine rotor but also the generator rotor. Between the generator and the turbine, there is only a guide bearing, such that the construction resembles an umbrella.

In order to service or to repair plant of this size, where rotor diameters come to several metres, the whole system has to be dismantled. The weight and the diameters involved are so large as to preclude the possibility of transporting the whole plant in one.

Clean, safe power generation

Many German run-of-river power stations featured systems of this type and were in operation in the first half of the last century. They still stand as a classic form of sustainable energy sources and are still contributing to the production of clean and safe power generation.

One such plant is the Harrbach run-of-river power station that was commissioned in 1940. It is operated by Uniper Wasserkraft GmbH and is located on the River Main, at a point which is 219 km from the source, between the municipal districts of Gmünden and Karlstadt.
The hydroelectric generating set, whose rated output is 4000 kVA, is driven via a tubular turbine. The overall weight of the generator is approximately 80 metric tonnes. Of that weight, the three-part stator on its own, whose 8600 mm diameter is almost as impressive now as it was back in the day, accounts for 28 tonnes.
It was in August 2018 that Uniper decided to overhaul the generator, prompted by the fact of damage to the stator plate laminations combined with unsatisfactory insulation values of the stator and of the rotor.

Let’s continue with an account of how the lamination pack, the stator winding and the magnetic field windings were re-insulated.

Cost-effectiveness and know-how: convincing factors

The final contract award was negotiated via an internet auction. Bidders could propose only such repair measures as had been awarded preference during the pre-selection phase where the bidder had put in a convincing bid, highlighting its technical expertise and relevant reference projects on record.
It was judged to be in BENNING’s favour that it had already repaired run-of-river generators for other power station operators.
Not only that, but Uniper was also impressed by the excellence of service which it had experienced in connection with the retrofit of the four generators at the Kachlet run-of-river power station (please refer to our previous article in POWER news 04/2015). BENNING shone through in terms of its flexibility: its capacity to achieve what it achieves with no sacrifice in terms of industrial safety or quality and also the increased output achieved from the generators.

The end result was that the online auction that took place on 8 January 2019 saw BENNING being awarded this valuable project by virtue of having put in the most cost-effective bid.
Since the 1930s, BENNING’s electrical machinery department (commonly abbreviated to “BeM”) has specialised in repairing generators and motors.
The department boasts decades of experience: with references in the building, rebuilding and repairing of electrical machinery for the widest range of sectors.

Tight window of opportunity

The objective was for the plant to be dismantled, repaired and recommissioned as quickly as possible, so as to restore normal power generation, minimising the period for which no valuable electrical power would be produced.
The window of opportunity was indeed tight, the deadline for disassembly was 10th June 2019. Re-assembly and recommissioning had to be completed no later than by the end of September. The rotor, weighing 52 tonnes, had to be reconditioned within the power station. The work that had to be done to the stator, whose diameter is in excess of 8 m, had to be carried out at BENNING’s repair centre in Bocholt.

It was something of a logistical challenge to transport these components, because nobody had ever considered, in the decades that followed the original commissioning of the power station, the possible value of planning the infrastructure such as to facilitate the eventual need for the generator to be transported. In the meantime, road layouts had been altered, and a new bridge over the railway lines had been constructed.
Specifically in order to avoid overloading the railway bridge, the maximum permissible weight of the transporters required extremely precise calculation. On that basis, the loading consignments for the stator, which can be divided up into 3 sections, were distributed over several transport vehicles.

Quality control from the outset

BENNING’s machinery team had a few quality control measures of their own up their sleeve. Even before they started on the task of dismantling. The machine’s origins went back so far that there was little documentation available. This prompted BENNING to start by taking a wide range of measurements on site. Thanks to adopting that precaution, it was possible to assess operating characteristics and to project definitive curves. The temperature curves associated with a range of different operating points was placed on record, with the aid of modern thermography cameras, and certain assessments were entered into on that basis.
As the stator arrived at the repair centre in Bocholt, it was met by a team ready to place further measurements and readings on record. The original winding had to be taken out in order to determine the conductor sizing and the stator plate geometry. They had to completely remove the old stator lamination pack that was also going to be replaced. In parallel with cleaning the ,now completely empty, stator casing and repainting it with base coats, work could now start on manufacturing the set of approximately 24,500 new laminations. Despite the enormous quantity of sheets involved, and the tremendous time pressure in the background, a production tolerance of no more than a few hundredths of a millimetre had to be adhered to. The high requirements associated with the precision sheets that had to be lasered out meant that a suitable new material had to be used.
They succeeded in starting on the process of laminating the new stator pack.

Windings based on Roebel bars

The generator winding exhibited some unusual features. For example, it did not consist of individual bars as electrical conductors, but of a quantity of approximately 800 Roebel bars.

The principle of the Roebel bar was developed as long ago as 1912. The electrical conductor for a Roebel bar is divided up into several parallel sub-conductors.
These sub-conductors are insulated from each other. They are specifically coated and they are made with a twist. The manufacturing procedure is very labour-intensive, and it entails relatively high costs. For that reason, Roebel bars are normally used only when it comes to large-sized electrical machines. They improve efficiency and enhance the power output.

A customer-specified test program was applied to each of the 792 Roebel bars required for the new winding, before it came to be installed.
As a means of gauging the limits of the system, some of the finished bars were subjected to specific overvoltage tests where the loading was extended to material destruction, in a procedure to which the customer was specifically invited.
Because the circuitry for the Roebel bars had to be embodied with threaded fasteners, BENNING started by carrying out thermographic investigations. The benefit of this is that it enables the thermal characteristics of the connections to be analysed in the most accurate way possible. Accordingly, it was possible to confirm that no unforeseen transition resistances would arise.

Following the successful completion of further high voltage and partial discharge readings in the works, the 3 stator segments were packed off back to the power station. The components of the generator had to be loaded onto specialised transport trailers with the aid of a mobile crane, if they were to fit into the machine shop.

Once the stator was inside the machine shop, work began on reconditioning it fit for duty in the power station. By now, it had been fitted with freshly insulated pole coils, drawing on modern developments in insulating materials. These assembly works were followed up with quality control procedures. Using assorted testing and measurement methods, a range of installation values was placed on record.

author/contact: Matthias Loerwink
telephone.: +49 2871 93 318
e-Mail: m.loerwink@benning.de