Some interesting features of a modern power station – Part 2

June 12th, 2014, Published in Articles: Vector

 

From the ICMEESA archive

A closer look at South Africa’s most modern power plant, Arnot, focusing on its coal and ash handling plant; boilers; water plant; generators and control and monitoring, among others.

Part 1 of this article discussed Arnot power station’s coal and ash handling plant; its boilers; circulation system; superheaters and reheaters and water plant, among others (see Vector, May 2014).

Turbo-generators

These units have an output of 350 MW each and are the first reheat turbines ever installed in this country. They are 3-cylinder machines and consist of a high-pressure cylinder, a 2-flow intermediate pressure cylinder and a 4-flow low-pressure cylinder. The turbine is designed for throttle governing.

The advantage of reheat turbines lies in their increased thermal efficiency. The initial capital cost is greater than for non-reheat turbines but considerable fuel economy is afforded over the operational life of the unit.

In operation, live steam flows at a pressure of 162 kPa and at a temperature of 515°C from the high-pressure superheater of the boiler, through the live steam pipes to two stop valves and four governing valves to the inlet branches and corresponding flow channels to the reaction blading. The steam ten leaves the high-pressure cylinder and is led back to the reheat section of the boiler at a pressure of 40,3 kPa and a temperature of 318°C. In the reheat section of the boiler, the steam is increased to 515°C and is then directed through the intermediate pressure turbine and finally expanded in the low-pressure cylinder. The increase in specific volume as the steam is expanded in the three stages is in the ratio of 1 to1100.

Part of the steam is bled during the expansion process at six successive extraction points, and is used for reheating the condensate as well as for supplying the turbine-driven feed pump.

Load distribution over the three turbine cylinders allows the high-pressure turbine to supply 27%. The intermediate pressure turbine supplies 39% while the low-pressure turbine contributes 34%.

Fig. 1: Turbine low pressure rotor.

Fig. 1: Turbine low pressure rotor.

The high-pressure and intermediate pressure cylinders they both have an inner and outer casing, the former holding the blade carriers while the latter acts as a support.

The two halves of the inner casing are held together firmly by means of shrinkrings, a method used where high pressure and temperature differences are present as it distributes the load around the circumference of the inner casing evenly. This reduces heavy stress concentrations which would be present with bolted flanges.

Having two casings allows the pressure and temperature on the outside of the inner casing to be the same as the exhaust pressure for that cylinder. This reduces the pressure and temperature differential. Similarly, the pressure and temperature differential between the inside of the outer casing and atmosphere is reduced. The double casing arrangement reduces the thickness of the casings and the high thermal stresses which would otherwise be induced in the metal.

The outer casings have bolted flanges and tightening of the bolts is done by means of electric heaters fitted in holes along the axis of the bolts. Heating is controlled and the thermal expansion measured and the nut is tightened on attaining the required extension. This produces the correct clamping pressure on removal of the electrical heating.

The low-pressure turbine has a single casing with a rotor shaft length of  9,02 m between bearings and weighs 64,5 kg. The diameter of the final stage blading is 2,7 m, giving a tip speed of 1,3 times the speed of sound and a centrifugal force on the blade root of several tonnes.

The rotor blades are subject to erosion as condensation takes place in the final rows of the low-pressure cylinder. They are therefore made from materials with up to 90% titanium to improve erosion resistance and mechanical strength.

Having passed through the final stage of the low-pressure cylinder, the steam passes into the condenser where it is condensed, creating a partial vacuum. The entrained air in the steam is removed by means of steam ejectors. Once the steam is condensed, it is extracted from the condenser and returned to the de-aerater. From there, it passes to the high-pressure heaters via the feed pumps before being recycled through the boiler. Make-up to replenish the steam lost is achieved by pumping demineralised water into the upstream side of the condenser.

Provision is made for thermal expansion as the various components must expand freely. During the expansion and contraction of rotors and stators, the axis must always be on the same plane and the clearances between fixed and moving parts should always be within safe limits.

Most of the cylinder expansion is compensated for by shaft expansion but the relative axial displacement between shafts and cylinders must be controlled closely. The rotors and casings must always heat up or cool down simultaneously, so keeping the differential expansion within safe limits. These limits determine the starting time of a turbo-generator set. A practical and economical compromise must established as an increase in the design clearances reduces the efficiency of the turbine. The longitudinal expansion of the complete set amounts to about 44 mm.

On start-up, electrically driven jacking oil pumps supply oil under high pressure to the underside of the shaft at all bearings to lift the shaft off the bearing surface and to introduce the lubricant to the load area. Once the shaft is “jacked”, barring commences at a shaft speed of 60 rpm and the auxiliary oil pumps are switched on. These lubricate all the bearings until the pumps attain sufficient speed to take over when the electric pumps switch off. The control system of the pumps is interlocked so that the pumps start and stop automatically, depending on the speed of the turbine and the oil pressure. Emergency DC oil pumps are also provided to safeguard against major power failure.

The control system for the turbine is hydraulic and operates with the same oil used for the lubricating system. A speed regulator is embodied to control the running-up and loading of the set. It acts on the live steam and reheat steam throttling valves.

The main steam isolating valves and the reheat throttling valves must be closed simultaneously in an emergency as sufficient energy is contained in the steam pipes from the reheater to over-speed the turbine beyond safe limits within seconds.

An accelerometer fitted to the speed controller ensures safe operation of the turbine, should full load suddenly be removed from the generator. Two over-speed devices which can be tested on load protect the set from running above 3300 rpm.

The turbine unit is coupled rigidly to the generator, resulting in a total overall length of 39,65 m. The combined mass of the turbine and generator rotors is some
149 tonnes, and the alignment and balancing of these units during erection must be carried out very accurately to ensure safe operation at the synchronous speed of 3000 rpm.
The generator stator is the heaviest single piece of equipment which must be transported to site and it is therefore built in two sections, the stator core with the windings and steel frame and the outer casing which accommodates the core, hydrogen coolers, hydrogen seals and rotor bearings. These two components are transported to site independently and assembled on the generator foundation.

Fig. 2: 390 MVA15/400kV generator transformer.

Fig. 2: 390 MVA15/400kV generator transformer.

Generators of this rating and larger are subject to double frequency core vibrations due to the magnetic pull of the rotor as it rotates. To prevent the transmission of these vibrations to the foundation, the inner core is spring mounted in the outer casing.

Frictional losses due to windage in large machines are considerable. Running at normal speed, completely de-excited in an atmosphere of air and without any form of cooling, the generator would attain a temperature in excess of the permissible full-load temperature within a few hours.

To reduce this problem, the generator is designed to operate in an atmosphere of hydrogen, at a pressure of 3 kPa. Gas passages are provided in the iron core of both the rotor and stator and hydrogen is circulated through the casing. Heat exchangers are fitted into the gas circuit and effect the cooling of the machine. To increase the thermal efficiency of the stator, the stator windings are formed from hollow section, high conductivity copper bars and through these hollow conductors, cooled demineralised water is circulated. Hydrogen’s heat removal ability is about three times that of air and its relative density is 0,2 that of air. This means the cooling is more efficient and the overall reduction in losses is about 10%. However, water has about 50 times the heat removal ability of air and since water is in direct contact with the copper, the heat transfer does not take place through the winding insulation. High current densities can be used in the stator windings and the overall cross-section of the copper can be reduced.

Since hydrogen is used in the generator, it is necessary to seal the rotor shaft where it emerges from the stator casing. A radial type oil seal is used. The oil becomes saturated with hydrogen and is therefore subjected to vacuum treatment for the removal of the hydrogen before being cooled and re-used.

The seal oil pressure is maintained slightly in excess of the hydrogen operating pressure. The water pressure is maintained at a value lower than that of the hydrogen to prevent leakage of stator cooling water into the generator.

When hydrogen is mixed so that the purity of the hydrogen falls below 78%, an explosive mixture results. The stator casing is therefore constructed of heavy steel section to contain an explosion.

With proper care, however, the handling of the hydrogen is quite safe. When filling the generator, the natural atmosphere of air is completely displaced by a charge of CO2 introduced from the underside of the casing. With this operation complete, the hydrogen is admitted to the top section of the stator casing and the CO2 purged from the bottom until the purity of the hydrogen exceed 95%. The hydrogen is produced on the station by electrolysis.

Each generator at Arnot is equipped with an AC main exciter and a permanent magnet generator. The latter supplies DC for the main exciter field which is regulated by the voltage regulator. The output from the main exciter is rectified through a bank of stationary semi-conductor rectifiers from which the generator derives its excitation.

Generator busbars and transformers

Busbars

Metallic phase barriers are installed at the generator terminals and flexible connections are made to the busbars. Phase isolated busbars are used for safety and as this would obviate interphase faults while the high mechanical stresses experienced between busbars of the non-segregated type are eliminated.

The neutral terminals of the generator are enclosed in an aluminium cubicle and are connected to the star point, which is then earthed through a neutral earthing transformer. The generator metering, protection and test current transformers are also housed in this cubicle.

The length of the extruded sections is limited due to the physical size of the bars and the sections must be jointed. Welded joints are made between sections of the bars and enclosures.

The rating of the bars is 15 kA at 15 kV with a permissible temperature rise on full load of 50°C over an ambient 40°C.

The main bars consist of two semi-octagonal sections 0,5 m across the flats, arranged with a slot top and bottom, which allows air to circulate. The interior of the enclosures are painted matt black for heat absorption while the exterior is painted light grey inside the building, where it is protected from the sun. The sections outside the building are left bright and unpainted. The sectional area of the busbars is 25 000 mm2, while that of the enclosure is 13 000 mm2. The enclosures are made airtight and are kept under slight pressure to prevent moisture ingress.

The busbars do not have to be force-cooled but the limit of temperature has been experienced in the current transformer cubicle due to the additional heat liberated by the current transformers.

Forced air cooling will be needed to reduce the size of the bars, for busbars rated  20 kA and above. The space beneath the generator terminals is very limited and difficulty was experienced in designing the installation, ensuring adequate clearance, while retaining a design which could be installed physically.

Generator transformers

These units are rated 390 MVA with a voltage ratio of 15/400 kV. They are double wound transformers with offload tap changers and are arranged for forced oil circulation and forced air cooling.

The heaviest single piece for transportation is the tank containing the core and windings, which weighs 170 tonnes. Here again with the excessive mass and physical dimensions, transportation, site erection and commissioning problems are encountered which always prove so interesting.

Once the unit was finally positioned, all the fittings, bushings and coolers were prepared carefully for erection. The tank was then opened and bushings and isolating valves were fitted. The tank was then sealed, a vacuum was drawn and oil filling was carried out. The oil is then filtered and samples are taken and tested periodically. Filtration continues until the desired dielectric strength of the oil is obtained.

Station and unit transformer

Power is obtained from the station transformer to start the steam raising plant and the turbo-generators. The station transformer is also used for supplying non-unit equipment which includes the coal and ash handling and water-purification plant, and power for the residential areas.

The unit transformers are connected to the generator busbars and supply the load of all the auxiliary equipment associated with a single boiler and turbo-generator set. The ratings of these transformers are  20 MVA at 15/11 kV.

Control and monitoring

General

For turbo-generators and boilers with ratings up to 60 MW, the instrumentation and supervisory gear were confined to essentials only, and the satisfactory operation of the plant depended upon the skill and judgment of the operator. With the adoption of the larger machines, however, it became necessary to provide a greater degree of instrumentation to indicate and record incidences.

Control of the plant is carried out from unit control rooms which house the control, indicating, and alarm devices.

The high voltage yard

The high voltage yard is the focal point of the station’s entire output. It is divided into two sections, one at 400 kV and the other at 275 kV. Four generators are connected to the 400 kV sections while two generators are connected to the  275 kV section. Double busbars are used throughout the yard with an additional transfer bus incorporated in the 400 kV section to facilitate switching and testing.

Acknowledgements

The author thanks the management of the Electricity Supply Commission for permission to publish this paper.
This article was originally published in the Journal of the Institution of Certificated Mechanical and Electrical Engineers, South Africa, March 1974, and is republished here with permission.

Contact Mariana Jacobs, ICMEESA, Tel 011 615-4304, icmeesa@icmeesa.org.za

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