Hydrogen in power plants

HYDROGEN USAGE IN THERMAL POWER PLANT

1. HYDROGEN GAS UTILIZATION
(Generator Cooling through H2)
IN
THERMAL POWER PLANT

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1. Introduction:
In coal power plants, the turbine generator consists of a series of
steam turbines interconnected to each other and a generator on a common shaft. There is
a high pressure turbine at one end, followed by an intermediate pressure turbine, two low
pressure turbines, and the generator. As steam moves through the system and loses
pressure and thermal energy it expands in volume, requiring increasing diameter and
longer blades at each succeeding stage to extract the remaining energy. The entire rotating
mass may be over 200 to 250 metric tons and 100 feet (30 m) long. It is so heavy that it
must be kept turning slowly even when shut down (at 3 rpm) so that the shaft will not bow
even slightly and become unbalanced. This is so important that it is one of only five
functions of blackout emergency power batteries on site. Other functions are emergency
lighting, communication, station alarms and turbo-generator lube oil.

Superheated steam from the boiler is delivered through 14–16-inch (360–410 mm)
diameter piping to the high pressure turbine where it falls in pressure to 600 psi (4.1 MPa)
and to 600 °F (320 °C) in temperature through the stage. It exits via 24–26-inch (610–
660 mm) diameter cold reheat lines and passes back into the boiler where the steam is
reheated in special reheat pendant tubes back to 1,000 °F (540 °C). The hot reheat steam is
conducted to the intermediate pressure turbine where it falls in
both temperature and pressure and exits directly to the long-bladed low pressure turbines
and finally exits to the condenser.
The generator, 30 feet (9 m) long and 12 feet (3.7 m) in diameter, contains a
stationary stator and a spinning rotor, each containing miles of heavy copper conductor—
no permanent magnets here. In operation it generates up to 21,000 amperes at
24,000 volts AC (504 MWe) as it spins at either 3,000 or 3,600 rpm, synchronized to
the power grid. The rotor spins in a sealed chamber cooled with hydrogen gas, selected
because it has the highest known heat transfer coefficient of any gas and for its
low viscosity which reduces windage losses. This system requires special handling during
start-up, with air in the chamber first displaced by carbon dioxide before filling with
hydrogen. This ensures that the highly explosive hydrogen–oxygen environment is not
created.
2. General generator data
The turbo-generator is designed for the following parameter ranges and duties:

Speed, frequency- 3000 min-1, 50 Hz
Standard rotation -clockwise viewed from driven end
Drive -steam turbine
Load characteristics- base load

The insulation materials are of class F throughout, however design temperatures do not
exceed class B limits. Stator core and rotor winding are cooled directly by hydrogen; the
stator winding is cooled directly by stator winding cooling water. The hydrogen cooling
ensures high efficiency of the generator at different loads. The generator is fitted for
operation with static excitation system.

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3. Structure Of The Generator
The main constituent parts of the generator are:

Stator: comprising casing, core, stator-winding, terminals and hydrogen coolers. The main
casing is a single fabricated-steel cylinder (1), which carries the stator core (2), and to which
are attached the end shields (3), the two cooler-casings with their hydrogen coolers (10) and
the terminal box. The shaft-seal casings (4) are located in the end shields. The stator winding
(8) is fixed into slots in the core, the end winding and its supporting system (6) being fixed by
flexible leaf springs and support rings (7) to the stator casing. The six main terminals (13) are
mounted in the terminal box, located on the under-side of the stator casing. The storage tank
(17) for the stator winding cooling water system is located on the top of the stator casing.



Pic 01: Parts of generator

Rotor: comprising rotor body with coupling flanges, excitation winding, retaining rings, fan,
and current leads. The rotor body (11) is machined from a single-piece forging. The two rigid
couplings (15) and (16) are forged integrally with the rotor. The excitation winding is built from
hollow conductors and is embedded in slots. The retaining rings (9) serve to support the end-
winding against centrifugal force. The main radial-flow ventilator fan (14) at the non-drive-end
(NDE) circulates the cooling gas in the generator.

Bearings: The rotor is supported on two-pedestal bearings (5). The coupled-on slip ring shaft at
the NDE is supported by a third pedestal bearing (5) at the end of the shaft. Both of the NDE
bearings are double insulated.

Shaft Seals: The generator casing is filled with pressurised hydrogen gas, and special seals are
used at the shaft penetrations to prevent leakage. The seal is provided by a floating ring having
close clearance around the shaft. Oil is fed into the gap under pressure higher than hydrogen
pressure in generator. The seals at both ends are double insulated from ground.

Bearing Oil Supply: The generator bearing are fed from the turbine lube-oil system.

Gas Supply: A gas unit, which includes all devices needed to regulate the filling and emptying
of the generator, provides automatic control of gas supply to sustain pressure and purity
during normal operation.

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Deionised Water Supply: A special unit supplies a continuous flow of cooling water to the
stator-winding. It contains the equipment to provide the necessary temperature and quality of
cooling water.
Seal Oil Supply: This function is provided by a seal oil unit, which contains the equipment
necessary to supply oil to the shaft seals at the proper temperature, pressure and purity.



4. Use of Hydrogen in Generator Cooling
While small generators may be cooled by air drawn through filters at the inlet, larger units
generally require special cooling arrangements. Hydrogen gas cooling, in an oil-sealed casing,
is used because it has the highest known heat transfer coefficient of any gas and for its
low viscosity which reduces windage losses. This system requires special handling during start-
up, with air in the generator enclosure first displaced by carbon dioxide before filling with
hydrogen. This ensures that the highly flammable hydrogen does not mix with oxygen in the
air.
The hydrogen pressure inside the casing is maintained slightly higher than atmospheric
pressure to avoid outside air ingress. The hydrogen must be sealed against outward leakage
where the shaft emerges from the casing. Mechanical seals around the shaft are installed with
a very small annular gap to avoid rubbing between the shaft and the seals. Seal oil is used to
prevent the hydrogen gas leakage to atmosphere.
The generator also uses water cooling. Since the generator coils are at a potential of about
22 kV, an insulating barrier such as Teflon is used to interconnect the water line and the
generator high-voltage windings. Demineralized water of low conductivity is used. Its the
importance of generator cooling.

5. Cooling
About 1 % of the power input to the generator is dissipated as heat in the generator itself. This
heat loss is removed by pressurised hydrogen gas. The stator winding is cooled directly by
stator winding cooling water, which removes the winding losses. The rotor winding is intensely
cooled by hydrogen flowing in direct contact with the copper of its conductors. Hydrogen gas
also flows through axial holes to cool the stator core.

Pic 02: Generator Cooling

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The temperature at the inlet into stator bar is regulated at water unit. The temperature of cold
gas may be regulated from the cooling water circuit, which supplies the hydrogen coolers.
The radial flow fan, mounted on the rotor, and the rotation of the rotor itself, drive several
parallel streams of cooling gas through the generator. The general configuration of these
streams is symmetrical from each end toward the center plane. Each of the streams includes
several constituent parallel flows. The cooling gas flows from the fan to the connections area
(1) and through the axial channels in the housing (2) to the end-winding region at the
connections side. The airgap is blanked off, what protects against direct gas flow into the
airgap and enabling fan acting directly onto the rotor cooling system.

The rotor cooling gas (3) enters the rotor at the end winding region between the retaining ring
and the shaft, finding access to the hollow winding conductors through side-ports close to the
point where the conductor enters the slot and to the sub slots located below the main rotor
slots. The first stream of the gas (4) flows through the slot portion of the hollow conductor, to
the end of the first axial section where it leaves through radial exit ports (in the conductors
and wedges) and enters the air gap. The second stream of the gas (4’) flows through the sub
slots towards the center of the rotor (to the end of the second axial section) where it leaves
through radial exits ports and enters the air gap also. The remainder gas stream (5) flows
through the end-winding hollow conductors, finally emerging into the air gap through short
slots at the ends of the rotor, in the pole regions.
The gas cooling stator core flows through axial channels in the core from the slip rings side to
the centre and further through radial ducts outside the core.

The gas (9) leaves the stator yoke at the centre of the stator, goes through the hydrogen
coolers and returns through axial channels (10) to the fan. The stator winding cooling water
flow through the stator winding is forced by a circulating pump (at water unit); it is supplied to
collectors from which it flows to every bar through teflon pipes. The stator conductors are fed
by parallel circuit, the stator winding cooling water leaves the bars by teflon tubes and flows to
collectors at the opposite side of generator. The stator winding cooling water returns to water
unit where it goes through pumps and coolers.

The slip rings and brushes are cooled by air in closed circuit, which includes air coolers,
humidifier and monitoring.

6. Gas Units in Plant
A standard gas supply unit is applied on all generators, which use hydrogen for cooling.
All of the unit components are mounted on a skid and connected by pipework to the gas
supply (cylinder rack or tank) and to the generator. The gas unit includes all necessary
equipment for filling or emptying the generator with hydrogen and also for automatic
maintaining of the required hydrogen pressure and purity during operation. Counter flanges
with weld ends, suitable for the connection of DIN or ANSI (depending on Customer
requirements) pipes are included in the delivery scope.

Three different gaseous media are used in the various filling and emptying stages:
1. CO2:
For displacement of air during filling and displacement of hydrogen during emptying of the
generator. The CO2 is supplied in gaseous form to the gas unit.

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2. Compressed Air:
To displace the CO2 during purging generator. The air is normally supplied from compressed
air systems - e.g. the instrument air.
3. Hydrogen:
For filling the generator and maintaining the operating pressure of hydrogen. The gas is
supplied (e.g. from cylinder racks) at a pressure not exceeding 10 bar (145 psi).


7. Advantages of Hydrogen Cooling
 The use of gaseous hydrogen as a coolant is based on its properties, namely low
density, high specific heat, and the highest thermal conductivity (at 0.168 W/(m·K)) of
all gases.
 It is 7-10 times better at cooling than air.
 The hydrogen cooling medium density is 1/14th of the air.
 Its thermal conductivity is 6.7 times that of the air.
 Hydrogen is its easy detection by hydrogen sensors.
 A hydrogen-cooled generator can be significantly smaller, and therefore less
expensive, than an air-cooled one.
 Windage losses which are proportional to the density of the cooling medium are
drastically reduced, as lower as 10 times compared to the air. Therefore this has the
advantage of increase in the efficiency of the generator by 0.7 to 1%.
 The noise generated by the alternator is reduced considerably due to the lighter
cooling medium and lower friction.
 The cooling surface required for Hydrogen cooling is considerably smaller than that
needed for the air coolers due to high heat transfer rates
 The reliability of the insulation increases and its life span is prolonged. The absence of
oxidation of the insulation and of accumulation of dust and moisture reduces the
maintenance of the machine.
 There is no possibility of the fire hazard in the machine if the failure occurs in the
winding insulation as hydrogen gas does not support combustion. Therefore there is
no needs to incorporate any fire control.
 The corona effects on the conductors in the windings are less harmful in hydrogen
atmosphere compared to air. This also increases the life of the windings.