KARIBA GENERATORS

Reproduced In extract form from Machinery Lloyd and Electrical Engineering (Vol. 37, No.3 - 30th January 1965) with kind permission of the author and the publishers – The Certificated Engineer March 1966.

The Kariba Dam and its underground power station are among the most impressive engineering achievements of recent times. Despite very great difficulties caused by severe flooding, the dam was completed and the generators installed ahead of schedule. Both the waterwheel generators and the 330 kV switchgear at Kariba were supplied by Associated Electrical Industries Ltd., who thus played a major part in the project. The design and construction of the ix 'umbrella' type generators installed in the power station are described in this article.

Turbine

The Francis-type turbines are of orthodox design but are notable for their size and massive construction. On full load each turbine passes 140 ton of water / sec. The size of the turbines is not apparent in the powerhouse, as substantial parts of them are embedded in concrete. The spiral casings each measure 45 ft across, while the height from the bottom of the draft tube to the coupling flange between the turbine and generator is 50 ft. The inlet to the spiral casing has a diameter of 14 ft 6 in, large enough to garage a double-decker bus with room to spare. The steel plates forming the casing are 1 ? in thick and because of the size of the final assembly were welded on site.

The turbine runners which drive the generators at 166.7 rev/min each weigh about 40 ton and are among the largest to have been completely fabricated in Europe. The blades are of stainless steel and are welded by a special technique to a ca t carbon steel hub.

The draft tube arrangement is rather unusual in that it passes under the penstock in the reversed direction so that water enters and leaves the turbines on the same side of the powerhouse.

The draft tubes of two adjacent units connect together into a single tailrace tunnel so that there are three tailrace tunnels to serve the six turbines. These tunnels discharge the water into the river downstream of the dam.

Generators

Examination of the tender specification indicated that the proposed underground installation would lead to a high civil engineering cost and therefore a high capitalisation on the volume of the machines. The prime aim therefore was to minimise the machine volume, particularly the volume not contributing to the direct generation of power. The 'umbrella' arrangement of generator was therefore chosen, in which the thrust bearing is placed below the rotor and there is only one guide bearing, again below the rotor. By comparison with a conventional top bracket machine an 'umbrella' machine can give a space saving of up to 20 per cent, as illustrated in Fig. 1. A half section of one of the Kariba generators is shown in Fig 2.

Rotors

The main problem to be overcome in the design of an umbrella machine is that of mechanical stability. There is in every machine an unbalanced magnetic pull effective at its centre, caused by inequalities in the air gap. There is therefore an overturning moment given by the product of this unbalanced magnetic pull and the distance from the centre line of the guide bearing to the centre of the machine. There is also a restoring moment equal to the dead weight on the thrust bearing times the distance from the axis of the shaft to the centre of pressure of the thrust bearing pads.

The stability constant is the ratio of the restoring moment to the overturning moment; this is an American definition and a typical value is 7.5. The ratio of the Kariba generator as designed was 10.65.

Electrical considerations had fixed the size of the machines in general terms, while the required inertia constant of 5 kW sec/kVA had fixed the approximate relationship between the rotor diameter and its length. The rotor was to be almost 26 ft in diameter and slightly over 6 ft in length.

The rotor body, or rim, which carries the poles and exciting winding, is of the friction, floating type. That is, the rim is built up of thin laminations of high tensile steel overlapped in such a manner as to give the maximum interfrictional efficiency and bolted together with a large number of bolts. The number and size of the bolts is determined by the friction force required to resist slipping at the maximum overspeed, which in the case of Kariba was 320 rev / min or 92 per cent. The driving torque is transmitted from the shaft by a spider, the arms of which have floating keys at their extremities. The arm of the spider assembly have to be strong enough to carry the weight of the rim-about 300 ton and resist their own centrifugal force, but not the centrifugal force of the rim, since this is floating on the driving keys.

The pole are also built of thin laminations clamped between heavy cast steel endplates and are secured to the rim by pairs of ‘T’ heads which fit into slots in the rim. The field coils are square ended and fabricated from rectangular copper strip, with fins provided on the ides and ends to improve the cooling efficiency.

The specification stipulated that the ratio of the sub-transient reactances should not be greater than 1.35, and since the calculated value of this ratio with an open or non-connected cage winding was 1.25 this type was supplied.

The weight of the poles and windings, rim, spider arm and hub is 370 ton. The lifting beam to handle this assembly itself weighed 30 ton, so that a total load of 400 ton made it necessary for two 200-ton cranes to be provided in the machine hall of the powerhouse.

Bearings

There are three principal types of thrust bearings for large loads: the Kingsbury type, the Michell type and the Spring type.

The Kingsbury type bearing has a single spherical pivot and offers unrestricted movement in any direction. The Michell type has essentially a 'line of contact' pivot; the movement is more restricted but the bearing has a potentially greater load capacity and is widely used. The Spring type bearing consists of sets of mattresses of pre-compressed springs with parallel faced pads on top; it combines the flexibility of the Kingsbury with a load capacity equal to or better than that of the Michell and has many other advantages such as its capacity to absorb any shocks from the turbine. The bearings selected for the Kariba machines were of the Spring type.

The centre of pressure of each pad in. this type of bearing is offset from the pivot or equivalent pivot point in order to make the pad tilt at its leading edge and cause the oil wedge to form which is essential for the correct operation of any thrust bearing.

The pad are mild steel blocks which are faced with a tin based habbit metal and polished by a special process until the surface finish is of the order of 10μ in CLA.

The total live load on the bearings is 660 ton; there are eight pads, each resting on 92 springs, shown in Fig 3.

The rotating part of the thrust bearing, known as the thrust block or collar, is machined from a high quality steel casting. A high quality casting with a minimum of sand inclusions is essential for this purpose and castings were therefore rigorously tested mechanically, and subjected to ultrasonic or deep penetration X-ray investigations. The finished thrust block for a Kariba generator weighs just 18 ton, is 5 ft tall and ha a diameter of 80 in at it base.

There are several ways of securing a thrust block to a shaft, but the method used for the Kariba machines is quite interesting. The bore of the block was machined with a slight taper-small at the top-and a corresponding taper some 0.030 to 0.40 in bigger in diameter was machined on the shaft. The block was then expanded on to the shaft by the use of oil at a pressure of several tons per square inch and pressed into position. A split ring key holds the block in position on the shaft but no other keys are necessary because of the heavy interference fit.

The bearing surface of the rotating part was formed on a thick steel disc which was bolted and dowelled to the block. This disc was machined with perfectly parallel surfaces prior to assembly and after a rotational check was polished until a surface finish of the order of 5μ in CLA had been obtained.

The guide bearing journal is formed on the neck of the thrust block so that it is as close to the centre of the machine as possible. The guide bearing is of the Michell adjustable pad type, 69 in d and is self lubricated.

Both bearings are fitted with oil cooler consisting of specially shaped tubes through which water flows.

Stators

The stator frames were completely fabricated in four section to facilitate transportation and also to reduce the amount of site winding to a minimum. Each stator core is built up of thin laminations of 4 per cent silicon alloy, low loss steel, 0.014 in thick with a large number of ventilation ducts built into it to allow air to circulate through the core and around the winding.

The bigger a machine, the more difficult it becomes to design it for a particular voltage. The machine designer is usually given a choice of voltage between about 11 kV and 16 kV but for economic reasons the machine voltage at Kariba was fixed at 18 kV.

The best synchronous peed of the set carne out to be 166.7 rev/min so that the generators had to have 36 pole. To obtain a completely balanced winding the number of slots for a 36 pole machine must be divisible by 27. T simplify the core building with an even number of stator part the number of slots must also be divisible by 2. It was finally decided that there should be 378 slots filled by a diamond lap type winding with multi turn preformed and insulated coils. Each coil weighs almost 80 Ib and has Class B insulation applied in the form of continuous overlapping layers of bitumen mica silk tape, vacuum dried and bitumen impregnated.

Almost 1 500 joints had to be made, insulated and inspected minutely before the winding was completed and tested at 37 kV A.C. for one minute,

Each complete stator weighs 168 tons.

Ancillary equipment

Each generator at Kariba ha a mass of ancillary equipment including a direct coupled exciter, rated at 364 kW but with a virtual rating of 1 200 kW, a permanent magnet generator rated at 1.8 kVA for supplying the turbine governor pendulum motor, a complete set of automatic control equipment to enable the machine to be started and stopped by push buttons, and a magnetic amplifier controlled automatic voltage regulator. A complete set of special tools and sling was supplied for machine maintenance purposes.

Tests

One of the contractual obligations associated with the Kariba generators was that the first machine was to be fully tested in the manufacturer's work.

A complete programme of tests was carried out including, among other things, the determination of the generator efficiency in accordance with BS269, a temperature test at 92MV A zero power factor strong field, a series of instantaneous 3-phase short circuits including one at full voltage, a harmonic analysis of the generated voltage and finally a full overspeed test.

The overspeed test was carried out without an extra steady bearing, that is the generator was run up to 320 rev / min (92 per cent overspeed) on just its own thrust and single guide bearing. It took well over two hours to drive the rotor up to the correct speed; in fact It took 15 min for the speed to be increased from 319 rev/min to 320 rev/min and over 4MW was needed for the test plant supplying the machine. Closed circuit television was used to view the generator.

In addition to the tests at the works, AEI played its full part in the important system commissioning programme. This programme was not without its difficulties, the most predominant of which was the fact that a transmission line 275 mile long, from Kariba through Lusaka to Kitwe, had to be commissioned with only one generator available.

The first generator set at Kariba was commissioned on 28 December 1959, three days before the scheduled date, and has been up plying power ever since. Commissioning was achieved despite all the setbacks, and similarly set number 2, 3, 4 and 5 were all commissioned on schedule almost to the day. The sixth set was commissioned in March 1962, one month earlier than originally planned in a programme drawn up five years before.

The estimated cost of stage one of the Kariba Scheme was ￡80 million and the final cost seems likely to be about ￡78 million. Of this, over ￡42 million is for civil works including the Kariba township, ￡22 million for the electrical and mechanical works, ￡4 million for the resettlement of Africans and about ￡8 million for finance charges.

Some provision has already been made for a second powerhouse on the North Bank at Kariba a this was envisage I in the original scheme.