Asset management of gas turbine power stations: combined cycle generators and exciter equipment.

This, part 4 of a four-part overview focuses on the salient asset management (O&M) aspects of generator and exciter plant in both open cycle and combined cycle gas turbine power stations.

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Sep 21, 2017
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Author(s): Douglas Hutchinson

Abstract

This overview was written primarily with graduates, engineers and electrical, mechanical, chemical, instrumentation and control and computer technicians in mind who wish to enter the power generation industry at large and gas turbine power generating plants in particular, since combined cycle gas turbine power plants constitute the largest output and most efficient turbine power generation stations operating today. 

CCGT air & hydrogen cooled generator and exciter units

Essential Basics – Generally around the world, gas turbine and steam turbine generators (GTGs & STGs) use air cooled generators in CCGT plants where generator outputs in the region of 60 MW or less are specified. There are larger air cool generator installations but they are used to a much lesser extent and with specific design considerations in mind. In general, generators rated above 60 MW and in the 100 MW and above range, use hydrogen cooled generators. Again generality demands, in most instances both air cooled and hydrogen cooled generators use air cooled exciter units. In very rare circumstances – specially designed hydrogen cooled exciter units have been used. The author knows of only one such case.

Both gas turbines and steam turbines drive two basic generator/exciter types:

  • (i) Air cooled generators. These machines obviously use air cooled exciter units.
  • (ii) Hydrogen cooled generators which invariably also use air cooled exciters [ 1 ].

Air cooled generator and exciter units

Depending on manufacturer’s designs, many variations on a theme are installed around the world today. The air cooling facilities are erected in and around the generator/exciter units, (generally below the generator/exciter units) and consist of make-up air inlet filters and water coolers with appropriately designed air ducting systems to ensure closed loop air cooling systems.

Cooling air within the generator is propelled by low-pressure generator shaft mounted fans – or in some design cases by external fans or blowers – and air is circulated from the coolers into and through the generator (or exciter) stator and rotor windings. Internal ducting arrangements within the generator (or exciter) casings return heated air back to the water coolers for cooling and re-circulation back into the generator/exciter units.

Cooling air flowing through small cooling passages in generator/exciter windings must be moisture, dirt and oil free. With this aim, closed circuit air cooling systems are the cheapest and most viable cooling option to use, however air cooling does have the disadvantages of being subject to all airborne pollutants of many descriptions and therefore filtration is of the utmost importance.

Ambient air temperature and relative humidity constantly change and over time, relative humidity moisture has a detrimental effect on generator stator and rotor winding insulation values. This comment also covers exciters and because of these detrimental factors, many air cooled generator and exciter designs incorporate substantial electric heaters which are automatically switched on when the generators and exciters come off-line when generator, followed by exciter, main circuit breakers are opened.

Diagrammatic arrangement of air cooled generator and exciter units

(See Fig. 1)

Fig 1: Diagrammatic arrangement of air cooled generator & exciter units

Exciter units are similarly cooled. Many designs are employed around the world by a variety of manufacturers however, the principal layout and equipment used for air cooling exciters is akin to that shown above for air cooled generators, namely,

  • (i) Exciter has closed circuit air cooling system.
  • (ii) Inlet air is filtered.
  • (iii) Water coolers cool the warmed air ejected from the exciter stator and rotor windings. Note: modern AC exciters have rotating diodes mounted on their rotors and these, in conjunction with all windings and terminals are air cooled by either separately mounted air circulating fans or exciter rotor shaft mounted low-pressure fans.

Thus, in the illustration, for brevity and space saving, no closed cooling airflow diagram is shown for the exciter unit. Again note, air cooled exciter units are used in conjunction with hydrogen cooled generators and remain similarly cooled [2].

Note – in older generator designs sliprings were used to convey DC power from the main exciter unit into the generator rotor winding – in such instances, the exciter air cooling systems were used to also provide, through ducting arrangements, cooling air for the generator slipring/brush-gear installation.

Air cooled generators and exciters – O&M watch and maintain areas

Air cooled generators are inherently very reliable machines. Apart from the myriad of electrical testing to ensure they are electrically fit for service (factory and site acceptance tests, all commissioning tests and annual overhaul maintenance tests) – the following watch and maintain areas usually require O&M supervision and maintenance as follows in Table 1.

Item

Check/S

Actions/comments

Air cooled generator, slipring and brush-gear and exciter casings or frames

Depending on design – generally air cooled up to 60 MW capacity.

Air inlet filters checked and cleaned as required during scheduled shutdown maintenance outage and during annual and any short-term generating unit shutdowns.

Air coolers, generally seawater or other water cooled coolers are checked and routinely cleaned during annual or other suitable shutdown periods. NOTE coolers can be rotated in service and cleaned without generator shut down.

Air cooled generators

Air coolant.

Ambient temperatures and relative humidity will vary on a daily and seasonal basis. Ensure generator/exciter air inlet and air outlet temperatures from coolers is satisfactory and within manufacturer's limits.

Air cooled generators

Sliprings and brush-gear

Definitely a watch and maintain item – can be maintained (with full safety precautions) with the Generator in service OR whenever scheduled or unscheduled maintenance periods allow. Brush-gear coolant is filtered air – circulated by shaft mounted fan/s.

Air Cooled Exciters

Depending on design –old DC generator exciters and newer AC exciters rotating diodes

Old DC generator exciters require frequent maintenance to commutators and brush-gear – maintain whenever scheduled or unscheduled maintenance periods arise.

Newer AC exciters require much less maintenance – rotating diode machines require inspection and diode testing and change-out as necessary.

Exciters being air cooled, require inspection and where possible, as much rotor and stator winding cleaning as possible whenever scheduled or unscheduled maintenance periods allow.

Air cooled generators and exciters

Rotating parts

These machines are large rotating machines hence operational and maintenance will constitute condition and reliability based maintenance strategies – shaft alignments, bearing vibration levels and trends, bearing oil inlet and outlet Plus bearing metal temperatures must be monitored and trended. Inspect and check on annual basis OR during scheduled or unscheduled shutdowns.

Table 1: Air cooled generators and exciters

Hydrogen cooled generator

Essential Basics – When it was decided to increase generator unit sizes to the 100–120 MW mark and beyond, after extensive research, Hydrogen was chosen as the prime generator coolant because:

  • (a) It had a thermal conductivity equivalent to almost seven times that of air – allowing faster heat transfer and an enhanced cooling capability.
  • (b) Hydrogen being lighter than air – reduces generator rotor friction and windage losses.
  • (c) The exclusion of air (moisture/humidity/dirt/atmospheric pollutants) increases the life expectancy of generator stator and rotor insulation materials.
  • (d) Hydrogen will not support corona – this increases the life of generator insulation.
  • (e) Increased hydrogen pressure has been found to suppress partial discharges within the generator thus further reducing maintenance and increasing insulation life expectancy.
  • (f) Pure hydrogen will not support combustion – hence there is a reduced fire risk within the pure hydrogen filled generator. Lubricating oil was selected for hydrogen sealing within large output generators because:
  • (i) The generator shaft exits from the stator frame (at both the front and rear ends) and must be sealed close to, and just inboard of the journal bearings where the shaft diameter is at a minimum – but where the shaft surface speed can be very considerable. This is much too fast for any form of ‘contact or rubbing seal’ – thus, a ‘lube-oil hydrodynamic’ seal was fixed upon.
  • (ii) Lubricating oil was the most obvious fluid available for hydrodynamic sealing.
  • (iii) Seal oil systems use turbo-generator lubricating oil (suitably treated) as the sealing medium.
  • (iv) Large output generators run with hydrogen pressures within the range of 3–5 bar (g) max 43.5–72.5 psig max – (300–500 kPa max) for effective cooling.
  • (v) Seal oil pressures are automatically regulated above these pressures to ensure a seal oil pressure ‘differential’ is created to always be above Hydrogen (H2) pressures. Seal oil pressure to the generator hydrodynamic seals is always > H2 pressure.

Because of hydrogen entrainment in sealing lube-oil – seal oil treatment packages include seal oil filtration, de-watering and hydrogen detraining and removal from the closed loop seal oil system.

  • Note 1 – The seal oil system has its own dedicated AC seal oil pumps, back-up DC seal oil pumps and generally there is an additional turbine lubricating oil emergency seal oil supply system with seal oil regulating control valves to always keep seal oil pressures above generator hydrogen pressure within the correct differential pressure ratio.
  • Note 2 – Always – the seal oil pressure must be Above the hydrogen gas pressure in the generator stator frame. As hydrogen pressure is increased to allow generator full power output, so an automatic increase of seal oil pressure to maintain the seal tightness is regulated and maintained.
  • Note 3 – The operation of hydrogen cooled generators requires very careful attention, since hydrogen and oxygen (air) is an explosive mixture – the air initially in the generator is purged with inert carbon-dioxide (CO2) which is then purged with hydrogen.

To perform maintenance anywhere within the pressure parts system of the generator demands the hydrogen be purged with inert CO2 which is then removed by air. Hydrogen and CO2 fixed (catharometer) and back-up portable monitors are then used to ensure it is safe to work on generator seals or other generator internals.

Diagrammatic layout H2 cooled generator seal oil system

(See Fig 2)

Fig 2: Diagrammatic layout H2 cooled generator seal oil system

Seal oil/H2 ‘Diff’ regulating valves always automatically maintain the seal oil pressure above hydrogen pressure [3].

To ensure lubricating oil quality, the turbo-generator lubricating oil installation carries permanent oil purification equipment, permanent oil filtration banks and also magnetic oil filters, thus maintaining as far as possible, the quality of lubricating oil supplied to the generator seal oil system and hydrogen seals.

Diagrammatic layout – hydrogen cooled generator casing or frame

(See Fig 3)

Fig 3: Diagrammatic layout – hydrogen cooled generator casing or frame

Hydrogen cooled generator casing or frame development sought to minimise the quantity of hydrogen coolant used for any given generator electrical output and thus very compact generator frames were designed wherein, multiple hydrogen gas coolers were built into the frames in both the horizontal or vertical positions (see Fig 4).

Horizontal coolers (2 off or 4 off depending on design output) afforded very convenient ‘compact wrap-around’ the stator winding convenience, however, horizontal coolers had the disadvantage of:

  • (i) Length rigidity – both in and out of the generator casing, and whilst being withdrawn and re-installed after maintenance – these lengthy coolers need considerable longitudinal support.
  • (ii) They can only be inserted/withdrawn from the exciter end of the generator and preferably with the main and pilot exciters removed from their bedplates.

Whereas vertically mounted coolers (usually 4 off) afforded very convenient drop-in/lift-out maintenance ease together with self-priming/self-draining ease of operations.

Hydrogen cooler positions within generator casings or frames

(See Fig 4)

Fig 4: Hydrogen cooler positions within generator casings or frames

Hydrogen operations

The operation of hydrogen cooled generators requires very careful attention, since hydrogen and oxygen (air) is an explosive mixture. The air initially in the generator is purged and removed with inert carbon dioxide (CO2) gas which is then purged and removed by hydrogen gas.

The operation of hydrogen cooled generators demands bulk delivery and bulk storage facilities for hydrogen, carbon-dioxide and dirt free, oil free and moisture-free compressed air. Based on the generator manufacturers’ (OEMs) design, when ‘gassing-up’ (filling) a generator is to be undertaken, the OEMs recommendations should be followed to the letter.

Gassing-up’ means taking the machine from being air filled after construction or maintenance at atmospheric pressure and temperature to being thoroughly purged with CO2 and then safely purging out the CO2 gas and replacing it hydrogen. For generator full power output operation – the generator frame is pressurised further to the OEM’s recommended full power output hydrogen gas pressure.

Gassing-down’ a generator – constitutes de-pressurising a hydrogen filled generator wherein the hydrogen is safely purged and replaced with inert carbon dioxide gas (CO2) which is then purged and replaced with moisture free, dirt free and oil free clean compressed air and brought down to atmospheric pressure so that it can be completely opened for man-entry and internal maintenance.

Thus, since carbon dioxide (CO2) is an inert gas – it is ‘ always used’ as the inter-face gas to purge air out of the generator prior to the admission of hydrogen…….OR…….to purge hydrogen out of the generator prior to the admittance compressed air (Fig 5).

Fig 5: Operational sequence – filling and de-gassing hydrogen filled generators

Individual generator OEMs design manuals/gas quantities/gas purity sampling equipment calibration/gas purity sampling equipment maintenance/and step-by-step safety and operations instructions must necessarily to be followed to the letter.

Hydrogen cooled generator installations require on-site bulk hydrogen and carbon dioxide storage facilities together with clean moisture and oil free compressed air. The various gases have extensive safety-interlocked handling systems which essentially introduce the appropriate gases into the generator casing in the correct safe sequence by way of perforated internal gas manifolds at the top and bottom of the generator casing.

After putting the turbo-generator lubricating oil and generator seal oil systems into service, the entire turbo-generator train shafts are lifted with jacking oil – Note jacking oil shaft lift is taken into account when setting generator seal final clearances. Then the entire turbo-generator shaft train is continually turned using the shaft barring or turning gear.

With lubricating oil, seal oil and barring gear satisfactorily in service, dry, oil and dirt free compressed air is then admitted into the generator casing which is pressurised (see all OEMs recommendations and figures) – and air leakage tightness tests on the entire generator casing, generator terminals, terminal bushings, inspection cover gaskets, all piping joints and connections, and valve glands is conducted. Everything associated with the generator casing must be air tight and thus gas tight.

On satisfactorily completion of air pressure tests, (see OEMs air-test procedures) generator gassing-up operations are commenced using all the generator OEMs safety checks and precautions, operational recommendations and operational figures and calculations. The essential methodology would be:

Gassing-up operations (filling the generator with hydrogen)

  • (i) Generator at atmospheric pressure and temperature with air.
  • (ii) Generator top manifold (on large output machines there may be more than one manifold) is opened and vented to atmosphere.
  • (iii) Heavy and dense CO2 gas admitted into ‘bottom manifold/s’, thereby from the bottom-up, completely purging and removing and expelling all air from the bottom up, out of the very top of the generator casing. Note – generator casing venting systems generally vent to atmosphere at station roof level.
  • (iv) Purity checking % CO2 in air – to OEMs calculations and instructions – for the volumes of carbon-dioxide and air in the generator casing at atmospheric temperature and pressure. Always check the OEMs recommended % CO2 in air is achieved.
  • (v) Hydrogen is then admitted via top manifold/s the heavy and dense CO2 drains out of the generator casing and is purged out – from the top down – bottom gas manifold/s of the generator casing or frame.
  • (vi) Purity checking % Hydrogen (H2) in carbon-dioxide (CO2) – to OEMs calculations and instructions for the generator casing volumes. Always check the OEMs recommended % Hydrogen (H2) in carbon-dioxide (CO2) is achieved.
  • (vii) Hydrogen purity is increased to be operationally 98–99% H2 in air – by increasing the generator casing or frame pressure to the normal hydrogen pressure values required for generator full rated output operations.

De-gassing operations (removing hydrogen from generator)

  1. Generator hydrogen pressure reduced to OEMs recommendations just above atmospheric pressure.
  2. Heavy and dense CO2 gas admitted into bottom manifold/s, thereby from the bottom-up, completely purging, venting and removing (lighter-less dense) hydrogen out from the top generator manifold/s.
  3. When purging generator H2 with CO2 it is typically necessary to achieve the OEMs carbon-dioxide purity level of 90–95% CO2 in H2
  4. When satisfactory carbon-dioxide purity levels are achieved, thus having safely removed hydrogen from the generator casing, – dry, clean and oil free compressed air is admitted to the generator top manifold/s, thereby draining and purging the casing of dense carbon-dioxide via the bottom manifolds and replacing it with air to typically achieve an OEMs 95% air in CO2 gas content.

Hydrogen cooled generators – generator casings O&M watch areas

Apart from the extensive Generator OEM’s and life cycle O&M electrical tests, the pressurised hydrogen filled generator casings O&M watch areas are indicated (Fig 6).

Fig 6: Generator maintenance watch areas

Note – H2 generator casing section 1 – generator casing, terminals & bushings leak-tightness

Note – H2 generator casing section 2 – generator coolers leak-tightness

Note – H2 generator casing section 3 – bearings & seals

(See Table 2)

Item

Check/S

Actions/Comments

2 cooled generator

Prior to Operation – air tightness tests

Thorough air pressure test checks throughout – seals/bearings area, all stator inspection doors, all cooler joints, all generator seams, joints, pipework flanges, valves, generator terminal bushings – soap leak testing, also hold air pressure for OEMs specified time checking no drop in pressure per OEMs recommendations recording all results for future reference.

2 cooled generator

Hydrogen Coolers

Usually tested before generator air pressure test with coolers out of the generator – hydraulic test, hold pressure noting pressure drop (if any) – hold hydraulic test for OEMs recommended time without any pressure drop.

Table 2: Generator casing, terminals & bushings leak-tightness

Hydrogen cooled generators – seal oil O&M watch areas

(See Fig 7)

Fig 7: Hydrogen cooled generators – seal oil O&M watch areas

Note – generator seal oil section 1 – generator seals

Note – H2 generator seal oil section 2 – generator bearings

Note – generator seal oil section 3 – air-side seal oil system

Note – generator seal oil section 4 – hydrogen-side seal oil system

Note – generator seal oil section 5 – hydrogen fans & coolers

Note – generator seal oil section 6 – seal oil pumps & control valves

See Table 3 – generator bearings and seals

Item

Check/S

Actions/Comments

2 cooled generator bearings.

Bearing Manufacturer to check

Manufacturer will open, inspect, NDE check white metal thickness, surface cracks, condition, and bearing clearances – and will then realign and box up.

This work done during construction/scheduled annual/major work outages when generator is de-gassed and completely opened for inspection and repairs.

2 cooled generator seals.

Seal Manufacturer to check

Depending on OEM and the type of seal being used (ring or face type) OEM will set seal clearances.

This work can only be done during construction/scheduled annual/or major work outages when generator is de-gassed and completely opened for inspection and repairs.

Table 3: Generator bearings & seals

See tables 3 & 4 – generator air-side and H2 side seal oil system

Item

Check/S

ACTIONS/Comments

2 cooled generator.

Air-side seal oil system

Seal oil drained, centrifuge purified, all water, dirt, sludge contaminants removed. All seal oil tanks, receptacles drained and cleaned before readmission of centrifuge clean oil re-supply.

This work can only be done during construction/scheduled annual/or major work outages when generator is de-gassed and completely opened for inspection and repairs.

2 cooled generator.

Hydrogen -side seal oil system

Seal oil drained, centrifuge purified, all water, dirt, sludge contaminants removed. All hydrogen side seal oil detraining tanks, vacuum treatment H 2 tanks, vacuum pumps and receptacles drained and cleaned before readmission of purified clean oil re-supply.

Table 4: Air side and hydrogen side seal oil

See Table 5 – generator hydrogen fans & coolers

Item

Check/S

Actions/Comments

2 cooled generator

Hydrogen Fans or blowers

Fans or blowers are part of generator rotor – blades NDE checked for cracks – profile measurements, and blade root security – all work done during generator rotor inspections.

2 cooled generator

Generator Hydrogen coolers

Dismantle cooler water pipework, remove coolers, inspect and hydro-test to OEMs recommended pressures and ensure cooler leak-tightness and holds pressure for recommended times.

Table 5: Hydrogen fans & coolers

See Table 6 – generator seal oil pumps & control valves

Item

Check/S

Actions/comments

H2 cooled generator

Seal Oil Pumps

Drive motor strip-downs – bearing changes, lube grease changes, all electrical tests + IR tests – realignment with pumps

H2 cooled generator

Control Valves

Strip downs, valve spindles straightness, plus valve seats inspected and ground to ensure leak tightness. Valves re-calibrated to ensure seal oil/H2 pressure differential’ control

Table 6: Seal oil pumps & control valves

See Table 7 – generator bearings & seals

H2 cooled generator

Stator and rotors assemblies

These machines are very large electrical rotating machines hence operational and maintenance will constitute extensive in-situ and/or external [rotor withdrawn from generator] stator and rotor visual inspection and electrical test condition data.

Generator stator maintenance will constitute extensive internal inspection and stator electrical test condition data together with recorded stator winding slot and tooth recorded temperatures. Rotating machines also require condition plus reliability based maintenance strategies – shaft alignment, bearing vibration levels and trends, bearing plus seals visual and NDE inspections together with lube-oil inlet, outlet magnetic strainers plus bearing metal temperature recorded data and trend information.

Inspect and check on scheduled annual basis OR during major suitable scheduled shutdowns.

Air cooled exciters

Stator and rotors assemblies

These machines are also large electrical rotating machines hence operation and maintenance will suitably follow Generator in-situ and/or external visual inspection and electrical test condition data.

Table 7: Generator bearings and seals

Overall CCGT power plant overview summary

Since power industry de-regulation, there has been a revolution in asset management technical and commercial thinking applied to the way generation/transmission/and distribution assets should be designed, constructed and commercially and competitively operated on a 24/7 – 365 basis.

The advent of the combined cycle gas turbine further revolutionised asset management design, construction and on-going operations and maintenance. CCGT asset managers live at the leading-edge of highly technical national power industries.

The design and continuous development of software to run these complex plants, in world-wide locations, in all seasonal conditions, and 24/7 in the thermally ‘hot’ ‘warm’ and ‘cold’ conditions has been an outstanding achievement and has led to a ‘new’ grade of engineering employee – that of ‘power generation computer specialist.’

To suggest since all is computerised, no operational expertise is required is dangerous, erroneous and totally incorrect. All gas turbine generator (GTG) and combined cycle STG operation requires a very great deal of knowledge, operational experience and ‘judgment’ when starting GTG/HRSG/STG combinations in the hot, warm or cold conditions along with all their associated supporting balance of plant systems.

Acquiring the necessary operational and maintenance experience and judgement requires considerable training commencing with evaluating initial education and educational background of potential employees together with the complete assimilation of the principal technologies involved, the plant design and physical detailed layout. The degree of computerisation and the automation employed to operate and control all CCGT machinery is now vast.

CCGT plants located in various countries now requires extra training effort, longer training times, and greater training resources (e.g. language labs) because of differences in education and technical background in schools and/or universities and because of language differences/barriers.

As CCGT plants are started, synchronised and loaded, for and during all thermal conditions, constant surveillance, computer trending, judgement and decision making is constantly required, back and forth, between gas turbine (GT), HRSG and combined cycle STG units and all associated balance of plant.

Engineers, graduates and all technician grades wishing to move into the power industry and particularly into the gas turbine asset management industry sector, are destined a resounding life-long learning curve along with and extensive commercial and technical experience.

References

  1. ‘Steam Turbine Generator Units’, in CEGB Modern Power Station Practice vol 3 (UK, Pergamon, 1963).
  2. ‘Generators & Electrical Plant’, in CEGB Modern Power Station Practice vol 4 (UK, Pergamon, 1971).
  3. ‘Chemistry & Metallurgy – Lubricating Oil’, in CEGB Modern Power Station Practice vol 5 (UK, Pergamon, 1964).
Go to the profile of Doug Hutchinson

Doug Hutchinson

Director, Power Generation Services Pty Ltd

Career achievement, - working as Power Company Operations Manager on the US$ 540 million, 700 MW, joint venture, Shajiao 'B' power generation project in Guangdong Province PRC, the World Bank / IFC came to study the project and Doug was later asked to write and present a paper to The World Bank / USAID organisations for their " Private Sector Power in Asia" conference held in Kuala Lumpur, Malaysia, on 27 - 29 October 1992 covering the private power experience in China. Author – Central Electricity Generating Board – A Method Study Approach to Power Station Operation. Author - IET Eng/Ref - Overview, Asset Management, Gas Turbine Power Stations. STEM Ambassador – UK

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