Turning on the power10 June 1998
Cleuson-Dixence will go on line in October, achieving time and cost targets. Reaching this point has been a technical and organisational challenge for all involved
Construction and installation work from the Lac des Dix involves considerable civil and electrical infrastructure. Power had to be supplied to several points in the 15km-long water supply tunnel. The three tunnel boring machines, for example, were rated at 2MW but, with associated services, 4-6MW had to be available at each site at all times of the day and night. The electrical system had also to be able to withstand power surges without disturbing other users. The electrical system was designed for 16kV, and the key sites were looped to provide a back-up supply.
At 2000m above sea level the roads are impassable in winter, so the two main sites, Chargeur and Tracouet, each included a heavy-duty cableway powered by connections to the 16kV valley lines passing nearby. Since the area is very steep, the 16kV overhead line on wooden posts, connecting the cableway systems to the plain, was installed by helicopter. Power was also required at Bieudron power plant site.
Finally, plans had to be made to change from temporary supplies to final systems.
ABB Switzerland and the project management discussed several alternatives for the generator design. The final choice — three 423MW generators and five-jet turbines — saved both excavation volume and electro-mechanical equipment, although it caused the generators to be rated at 465MVA (continuous apparent power to 500MVA). This represents a world record for electrical apparent output per pole of 35.7MVA (in fact, for the same reason, the Pelton turbines also represent a world record with an output of 423MW).
In order to save on the height of the cavern, it was decided to design the generators with only two bearings (a combined thrust guide bearing on the non-driven side, and a guide bearing on the turbine side). At Bieudron, this implies that the initial critical speed is between the load-overspeed of 500revs/min and the first runaway speed of 800revs/min. The rigidity of the supports and quality of the concrete must therefore be carefully calculated so that the critical speed is not too close to the load-overspeed.
For these outputs and depending on the number of poles, the manufacturer has to provide for a generator with fully water-cooled windings, in stator and rotor. At Bieudron de-ionised water-cooling is possible. The rotor poles consist of hollow copper turns with a square cross-section. For the stator, six stainless steel cooling ducts are inserted in the copper bundle of a Roebel transposed conductor.
The cooling ducts are treated in the same way. The stator lamination sheets are raw water-cooled. Supplementary losses and windage losses are removed using six air fans distributed around the stator.
The rotating mass of these machines (generator and turbine) is 550t, they are 5.25m in diameter and rotate at 428.6rev/min. If power is lost after the generators start, it takes 1.5hr to shut them down. The bearings of the three generators must therefore be water-cooled by gravity. Under normal circumstances, electrical braking is used to stop these machines, while after de-excitation the generator is short-circuited on the high-voltage side. It takes 3min to stop the generator completely.
Generator static excitation is achieved using four thyristor bridges with redundancy of N-1.
The supplier, Cegelec France, had to guarantee output of 465MVA with a voltage of 21kV, using the dual-bearing design.
Generator-transformer connections are coupled with bus-bars shielded in aluminium for reasons of simplicity, space and maintenance. Since each phase is shielded individually using a 1100mm-diameter aluminium tube, the cross-section is slightly larger than that of the hexagonal aluminium conductor. The current in the trunking therefore flows in the opposite direction to the current in the conductor, reducing external EM fields by 95-98%.
Because it is all-welded there are some benefits: the supports are widely-spaced; live components are shielded from maintenance accidental contact, water and dust on the insulators; phase-to-phase faults are impossible; induced currents are almost eliminated; and in the case of short-circuits there are no stresses between adjacent phase conductors.
Important parameters are: dielectric strength (36kV for Bieudron); continuous current (15,000A for Bieudron); rated short-time current; maximum asymmetric short-circuit current to be withstood.
For this system, the guaranteed losses are 750W/m/phase.
Step-up power transformers
A number of issues concerning the transformers had to be resolved by the customer and the supplier ABB. The first was the choice of single phase or three phase transformers.
The main advantage of single-phase transformers is in transport (in Switzerland weight cannot exceed 280t and the size is limited by the railway tunnels). Another advantage is the simplicity of works tests. But accommodating three single-phase transformers requires recesses at least twice as large as would be needed for a single three-phase transformer because preparing a delta connection is complicated and needs a lot of space.
Economic comparison shows that four three-phase transformers are available for the price of nine single-phase transformers (plus one spare). It was therefore decided to order three of the less expensive three-phase transformers.
In view of their weight and dimensions, which were the maximum admissible for transportation, the decision not to include regulation on these units proved sensible and also increased their reliability. To meet size conditions, the magnetic circuits were designed with five columns (two unwound return columns).
It was also necessary to satisfy opposing constraints on impedance voltage: it should be high to reduce stresses due to the short circuit current of the high-voltage network; but as low as possible to satisfy stability constraints. At 410kV an impedance voltage value of 13.5% was chosen, somewhat lower than the conventional 15%.
The maximum and minimum limits of the network were 375kV and 435kV, respectively, so the transformation ratio was stipulated as 21/410kV. The transformers must withstand extreme frequencies — 47.5 Hz for 30 min — in the event of a sudden power loss on the network.
In view of the extremely high cost of transport, which represents 20% of the price, and the time needed to order the road trailer and railway train, moving the transformers to the installation site (where two 250t travelling cranes are available) using air cushions was considered. This would require epoxy-coated floors and powerful equipment, since the air flow rates involved in this transfer mode are extremely high. The units would be transported 120km by railway and then by road using a trailer with 192 wheels on 24 axles, which would be 75m long, and would require road closures.
Dry cables manufactured by Alcatel Cables Switzerland transmit power 500m across the Rhone. They have XLPE-insulation, and a copper cross section of 800mm. They were manufactured by the horizontal method, and the internal semi-conductor, insulating material and external semi-conductor were extruded simultaneously.
For a three-phase system on full load, the total losses are 65W/m. To limit the losses in the metallic trunking, it is only earthed at the sub-station side, and the power transformer end is protected by low-voltage lightning arrestors (4.6kV).
From the electrical viewpoint, the Bieudron power plant is considered as three separate generators with a unit output of 500MVA, plus one generator constituting the general services. Each generator is supplied by a 16kV cable crossing the Rhone and terminating at four resin-encapsulated three-phase transformers with an output of 16/0.4kV, 2000kVA. To increase reliability, the outgoing line at the Chamoson sub-station includes two sets of 16kV bus-bars and the cables are routed in pairs. The possibility of using a single transformer for the operation of the auxiliary services of two generators, which would further increase the reliability of these generators, was also envisaged. This explains the output of 2000kVA per transformer, and the fact that they are rated to accept a continuous 15% overload.
Using a single three-phase transformer to supply the auxiliary services of two generators is a degraded mode of operation. Nevertheless, it permits operation on full power, albeit with some restrictions, in particular on electrical braking.
In fact, the power required for two simultaneous braking operations would exceed the output of a transformer. However, the electrical shut-down of one of the generators may be delayed or omitted, since these machines shut themselves down naturally in an hour and a half due to their own losses.
The electrical braking system is considered as a service and emergency brake. It is obvious that emergency braking is valid for any mechanical problems. On the other hand, this mode must not be used for certain electrical alarms as it could cause any internal short-circuits to be fed.
The electrical supplies are decentralised and separated into essential and non-essential services. In addition, there is an area clean of electrical interference through the power station on the valve gallery side. This area includes all control panels and the 110V DC supplies. The power and lighting systems are backed up on one phase by an inverter and 110V batteries. As an additional safety measure, none of the wiring and components used include halogen.
The protective system is similar to that of other hydroelectric power plants, but for the 500MVA output it must ensure maximum availability and reliability.
The protective system is divided into two groups arranged in separate cubicles and providing total redundancy using either duplicate identical systems or protective devices of a different design but providing the same protection. These systems are digital and are connected via optical fibres or copper conductors.
The inclusion of electrical braking imposes some additional constraints. Short-circuiting is achieved by way of an earth switch on the 400kV line at the Chamoson sub-station. Since this switch complies with the iec 1129 standard, it is rated for a frequency of 50Hz and not for opening or closing with a variable-frequency current. It was therefore necessary to devise a method for ensuring that the current is almost zero at very low frequencies before an instruction is given to operate the switch. The same test equipment should indicate the braking current. These instrument transformers must therefore measure 1-1000A at frequencies of 0-50Hz.
The solution adopted was to select an LEM SA (Geneva) measuring system based on Hall effect current sensors in a closed loop (with compensation or zero flux). The sensor selected has a measuring range of 2000A with a ratio of 1:5000, and the offset drift as a function of temperature is 0.4mA between 0C° and 70°C.