HYDROGEN COOLED AND HYDROGEN AND STATOR WATER TURBOGENERATORS


Ansaldo Energia hydrogen cooled turbogenerators, with or without direct cooling of the stator winding, comply with PED and ATEX regulations to ensure safer operation in the presence of H2 gas, in addition to the other generally applicable regulations.
 

 

Design features and manufacture
 

Stator

 The stator frame is a welded steel fabrication with sufficient strength to contain the internal pressure exerted by the hydrogen gas.
The stator core is fabricated from low-loss silicon alloy magnetic steel sheet laminations.
Individual segments are punched, de-burred and coated on both sides with heat resistant insulating varnish.
The core is pressed at intervals during stacking and finally consolidated to ensure that individual laminations don’t loosen during service. All generators with water-cooled stator windings are fitted with laminated pressplates to reduce losses and eliminate hotspots. Two pole generators are fitted with radially flexible intermediate members which decouple the housing and foundation from double-frequency core vibrations. The core is cooled by hydrogen gas flowing in radial ducts. In generators with water-cooled stator windings the core is cooled axially. The standard stator winding design is the double-layer Roebel
bar. The two bars in each slot are separated by an insulating spacer and firmly braced in the slot by wedges. Slot forces during operation and transients, forces due to thermal expansion and the long-term behavior of the insulation are factored into the design and construction processes.
Resin Rich tape is used to insulate the main bar in accordance with class F requirements.

Forces generated during sudden short circuits are restrained by heavy rings used as the basic support. This guarantees uniform stiffness of the end winding throughout operating life. Differential thermal  expansion is accommodated by means of an elastic end winding suspension.
This makes these generators particularly suited to variable load operation or frequent starting and stopping.


Rotor

The rotor body is a single heat-treated forging with high strength and high magnetic permeability.
The coupling flanges at each end are integral parts of the forging.
The centrifugal forces caused by the rotor winding and slot wedges are borne by the rotor teeth in the active part of the rotor.
Centrifugal forces from the end winding in the end part of the rotor are borne by its retaining rings.
The rings are made of high-strength non-magnetic steel (18Mn18Cr), which offers the best solution to stress corrosion cracking. The rotor winding is manufactured from silver alloy copper which has a higher yield strength than normal non-alloyed
copper. It also has the fatigue strength needed to withstand the high mechanical stress to which the rotor winding is subjected.
The slot walls are insulated using continuous U profiles with high mechanical, thermal and dielectric  strength.
The winding in the end zone is insulated from the rotor retaining ring by several layers of insulation (aramid paper).
All insulation materials comply with class F requirements.
Generators are provided with the damper winding system designed to damp out rotor swings after sudden load changes. It also prevents excessive rotor surface heating when the three phases are asymmetrical.
The rotor winding is directly cooled by coolant gas flowing inside the rotor conductors. Cooling gas flow through the machine is provided by fans shrunk onto the shaft. Two axial fans, one on each rotor end, are used in hydrogen-cooled machines, while in the largest generator (watercooled  stator winding) one radial fan is foreseen.


Auxiliary systems


A number of auxiliary systems are needed to condition and circulate the water and hydrogen used for cooling and keep the shaft seals supplied with oil.
The gas plant conditions and monitors the coolant under all operating conditions. Its main functions are:

  • to drive out air with an inert gas (CO2)
  • to drive out inert gas with hydrogen
  • to add hydrogen to maintain the correct pressure
  • to drive out hydrogen with inert gas
  • to drive out inert gas with air

 

Larger generators have water-cooled stator windings,terminal bushings and winding connections.
The stator cooling-water plant conditions, circulates and monitors the cooling water (neutral water with very low oxygen content and electrical conductivity). It is therefore provided with oxygen removal and deionization equipment plus all the necessary pumps and filters.
When hydrogen is used as coolant, shaft seals are implemented by oil flowing into the gap between the shaft and suitable rings.
The seal oil plant comprises the equipment needed to supply oil to the shaft seals at the right temperature, pressure and purity.


Excitation system 

 

To deliver stable power supply when operating on a network and maintain generator voltage constant during no load operation or station servicing, large generators need a fast-response excitation system capable of adapting the air-gap flux rapidly to load conditions.
Static excitation units are particularly suited to the purpose.
 

Testing
 

 Specific tests are performed:

  • hydraulic pressure test and leakage check on stator frame
  • gas-tightness of lead bars and studs

 

Running tests can be performed on request, includingopen circuit tests, steady and sudden short circuit tests, temperature and losses evaluation, wave form and harmonic content analysis.

 

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