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{{Short description|Turbo generator with gaseous hydrogen as a coolant}}
[[File:Turbinenhalle KSP.jpg|thumb|A power plant turbine generator set: A [[steam turbine]] (blue) drives an [[electrical generator]] (yellow) with an excitation generator (front).]]
[[File:Turbogenerator01.jpg|thumb|A 500&nbsp;MW ''[[Siemens]]'' multi stage [[steam turbine]] with generator set (rear, red)]]

A '''hydrogen-cooled turbo generator''' is a [[turbo generator]] with [[gaseous hydrogen]] as a [[coolant]]. Hydrogen-cooled turbo generators are designed to provide a low-[[drag (physics)|drag]] atmosphere and cooling for single-shaft and [[combined-cycle]] applications in combination with [[steam turbine]]s.<ref>{{cite book|chapter=Development of world's largest hydrogen-cooled turbine generator|first1=S.|title=IEEE Power Engineering Society Summer Meeting|last1=Nagano|first2=T.|last2=Kitajima|first3=K.|last3=Yoshida|first4=Y.|last4=Kazao|first5=Y.|last5=Kabata|first6=D.|last6=Murata|first7=K.|last7=Nagakura|date=1 July 2002|volume=2|pages=657–663 vol.2|via=IEEE Xplore|doi=10.1109/PESS.2002.1043376|isbn=978-0-7803-7518-5|s2cid=37954073 }}</ref> Because of the high [[thermal conductivity]] and other favorable properties of hydrogen gas, this is the most common type in its field today.

==History==
Based on the air-cooled turbo generator, gaseous [[hydrogen]] first went into service as the [[coolant]] in a hydrogen-cooled turbo generator in October 1937, at the [[Dayton Power & Light]] Co. in [[Dayton, Ohio]].<ref>{{cite web|url=https://archive.org/stream/chronologicalhis00natirich/chronologicalhis00natirich_djvu.txt|title=Full text of "A chronological history of electrical development from 600 B.C."|website=archive.org|year=1946 }}</ref>

==Design==
The use of gaseous hydrogen as a coolant is based on its properties, namely low [[density]], high [[Specific heat capacity|specific heat]], and the highest [[thermal conductivity]] (at 0.168&nbsp;W/(m·K)) of all gases; it is 7 to 10 times better at cooling than air.<ref name=HydrogenCoolsWell>{{cite web|url=https://www.power-eng.com/emissions/hydrogen-cools-well-but-safety-is-crucial/|title=Hydrogen Cools Well, but Safety is Crucial|work=[[Power Engineering (magazine)|Power Engineering]]|accessdate=8 October 2017|quote=hydrogen’s low density, high specific heat and thermal conductivity make it a superior coolant for this application ... Hydrogen has one of the best heat transfer properties of any gas, with a specific heat of 3.4 Btu/lb-F at standard conditions. On a mass basis, hydrogen is 14 times more efficient than dry air for removing heat ... Hydrogen, as the lightest gas, has the lowest density of any stable gas. Wind resistance losses are kept to a minimum because the rotor’s wind resistance in a hydrogen-cooled generator is far less than in a similarly sized air-cooled generator }}</ref> Another advantage of hydrogen is its easy detection by [[hydrogen sensor]]s. A hydrogen-cooled generator can be significantly smaller, and therefore less expensive, than an air-cooled one. For stator cooling, water can be used.

[[Helium]] with a thermal-conductivity of 0.142&nbsp;W/(m·K) was considered as coolant as well; however, its high cost hinders its adoption despite its non-flammability.<ref>[http://www2.emersonprocess.com/siteadmincenter/PM%20Rosemount%20Analytical%20Documents/Gas_ADS_Hydro_Cooled_Generators_103-200_200803.pdf]{{dead link|date=January 2018|bot=InternetArchiveBot|fix-attempted=yes}}</ref>

Generally, three cooling approaches are used. For generators up to 60&nbsp;MW, [[air cooling]] can be used. Between 60 and 450&nbsp;MW hydrogen cooling is employed. For the highest power generators, up to 1800&nbsp;MW, hydrogen and [[water cooling]] is used; the rotor is hydrogen-cooled, while the stator [[winding]]s are made of hollow copper tubes cooled by water circulating through them.

The generators produce [[high voltage]]; the choice of voltage depends on the tradeoff between demands of [[electrical insulation]] and handling high electric current. For generators up to 40&nbsp;MVA, the voltage is 6.3&nbsp;kV; large generators with power above 1000&nbsp;MW generate voltages up to 27&nbsp;kV; voltages between 2.3 and 30&nbsp;kV are used depending on the size of the generator. The generated power is sent to a nearby [[Step up transformer|step-up transformer]], where it is converted to the [[electric power transmission]] line voltage (typically between 115 and 1200&nbsp;kV).

To control the [[centrifugal force]]s at high rotational speeds, the rotor diameter typically does not exceed 1.25 meters; the required large size of the coils is achieved by their length and so the generator is mounted horizontally. Two-pole machines typically operate at 3000&nbsp;rpm for 50&nbsp;Hz and 3600&nbsp;rpm for 60&nbsp;Hz systems, half of that for four-pole machines.

The turbogenerator also contains a smaller generator producing [[direct current]] [[Electric generator#Self-excitation|excitation power]] for the rotor coil. Older generators used [[dynamo]]s and [[slip ring]]s for DC injection to the rotor, but the moving mechanical contacts were subject to [[wear]]. Modern generators have the excitation generator on the same shaft as the turbine and main generator; the [[diode]]s needed are located directly on the rotor. The excitation current on larger generators can reach 10&nbsp;kA. The amount of excitation power ranges between 0.5 and 3% of the generator output power.

The rotor usually contains caps or cage made of nonmagnetic material; its role is to provide a low impedance path for [[eddy current]]s which occur when the three phases of the generator are unevenly loaded. In such cases, eddy currents are generated in the rotor, and the resulting [[Joule heating]] could in extreme cases destroy the generator.<ref>{{cite web|url=http://www.freepatentsonline.com/3898491.html|title=Damper winding for turbine generator rotors|website=www.freepatentsonline.com}}</ref>

Hydrogen gas is circulated in a closed loop to remove [[heat]] from the active parts then it is cooled by gas-to-water [[heat exchanger]]s on the [[stator]] frame. The working pressure is up to 6&nbsp;[[Bar (unit)|bar]].

An on-line [[thermal conductivity detector]] (TCD) analyzer is used with three measuring ranges. The first range (80–100% H<sub>2</sub>) is to monitor the [[hydrogen purity]] during normal operation. The second (0–100% H<sub>2</sub>) and third (0–100% CO<sub>2</sub>) measuring ranges allow safe opening of the turbines for maintenance.<ref>{{cite web|url=http://news.thomasnet.com/fullstory/487684|title=Gas Analyzer constantly monitors hydrogen purity|website=news.thomasnet.com}}</ref>

Hydrogen has very low [[viscosity]], a favorable property for reducing [[drag (physics)|drag]] losses in the rotor. These losses can be significant due to the rotor's high rotational speed. A reduction in the purity of the hydrogen coolant increases [[windage]] losses in the turbine due to the associated increase in viscosity and drag. A drop in hydrogen purity from 98% to 95% in a large generator can increase windage losses by 32%; this is equivalent to 685&nbsp;KW for a 907&nbsp;MW generator.<ref name=purity/> Windage losses also increase heat loss in the generator and increase the problem of dealing with the waste heat.<ref>[http://protonenergy.com/backend/arc_contenido/archivo117.pdf]{{dead link|date=January 2018|bot=InternetArchiveBot|fix-attempted=yes}}</ref>

==Operation==

The absence of [[oxygen]] in the atmosphere within significantly reduces damage to the winding insulation from [[corona discharge]]s; these can be problematic as the generators typically operate at [[high voltage]], often 20&nbsp;kV.<ref>{{cite web|url=http://www.gepower.com/prod_serv/products/generators/en/hydrogen_cool.htm|title=Gas Turbines|website=GE Power Generation|access-date=2010-03-10|archive-url=https://web.archive.org/web/20100505085646/http://www.gepower.com/prod_serv/products/generators/en/hydrogen_cool.htm|archive-date=2010-05-05|url-status=dead}}</ref>

===Seal oil system===
The [[bearing (mechanical)|bearings]] have to be leak-tight. A [[hermetic seal]], usually a liquid seal, is employed; a [[turbine oil]] at pressure higher than the hydrogen inside is typically used. A metal, e.g. [[brass]], ring is pressed by [[spring (device)|springs]] onto the generator shaft, the oil is forced under pressure between the ring and the shaft; part of the oil flows into the hydrogen side of the generator, another part to the air side. The oil entrains a small amount of air; as the oil is recirculated, some of the air is carried over into the generator. This causes a gradual air contamination buildup and requires maintaining hydrogen purity.<ref name=":0">{{cite web|url=http://www.control.com/thread/1267097548|title=Generator Hydrogen Cooling System|website=www.control.com}}</ref>

Scavenging systems are used for this purpose; gas (mixture of entrained air and hydrogen, released from the oil) is collected in the holding tank for the sealing oil, and released into the atmosphere; the hydrogen losses have to be replenished, either from [[gas cylinder]]s or from on-site hydrogen generators. Degradation of bearings leads to higher oil leaks, which increases the amount of air transferred into the generator. Increased oil consumption can be detected by a [[flow meter]] for each bearing.<ref name=":0" />

===Drying===
Presence of [[water]] in hydrogen has to be avoided, as it causes deterioration of hydrogen's cooling properties, [[corrosion]] of the generator parts, and [[electric arc|arcing]] in the high voltage windings, and reduces the lifetime of the generator. A [[desiccant]]-based dryer is usually included in the gas circulation loop, typically with a moisture probe in the dryer's outlet, sometimes also in its inlet. Presence of moisture is also indirect evidence of air leaking into the generator compartment.<ref name=purity>{{cite web|title=Hydrogen Purity in Hydrogen-Cooled Generators|url=http://www.gesensing.com/downloads/hygrometers/downloads/910-170B1.pdf|archive-url=https://web.archive.org/web/20081116181811/http://www.gesensing.com/products/resources/application_notes/930_087A.pdf|url-status=dead|archive-date=2008-11-16|website=www.gesensing.com}}</ref> Another option is optimizing the hydrogen scavenging, so the [[dew point]] is kept within the generator's specifications. The water is usually introduced into the generator atmosphere as an impurity in the turbine oil; another route is via leaks in water cooling systems.<ref>{{cite web|url=http://www.powergenworldwide.com/index/display/articledisplay/183731/articles/power-engineering/volume-107/issue-8/features/water-contamination-in-hydrogen-cooled-generators-lurks-as-serious-operational-threat.html|title=Water Contamination in Hydrogen-Cooled Generators Lurks as Serious Operational Threat|website=www.powergenworldwide.com}}</ref>

===Purging===
The [[flammability limit]]s (4–75% of hydrogen in air at normal temperature, wider at high temperatures,<ref>{{cite web|url=https://www.energy.gov/sites/prod/files/2014/03/f12/fcm01r0.pdf|title=Module 1: Hydrogen Properties}}</ref>), its [[autoignition temperature]] at 571&nbsp;°C, its very low [[minimum ignition energy]], and its tendency to form explosive mixtures with air, require provisions to be made for maintaining the hydrogen content within the generator above the upper or below the lower flammability limit at all times, and other [[hydrogen safety]] measures. When the generator is filled with hydrogen, overpressure has to be maintained as inflow of air into the generator could cause a dangerous [[explosion]] in its confined space. 

The generator enclosure is purged before opening it for maintenance, and before refilling the generator with hydrogen. During shutdown, hydrogen is purged by an inert gas, and then the inert gas is replaced by air; the opposite sequence is used before startup. [[Carbon dioxide]] or [[nitrogen]] can be used for this purpose, as they do not form combustible mixtures with hydrogen and are inexpensive. Gas purity sensors are used to indicate the end of the purging cycle, which shortens the startup and shutdown times and reduces consumption of the purging gas. 

Carbon dioxide is favored as due to the very high density difference it easily displaces the hydrogen.  The carbon dioxide is admitted to the bottom of the generator first, pushing the hydrogen out at the top. Then air is admitted to the top, pushing the carbon dioxide out at the bottom. Purging is best done with the generator stopped.  If it is done during slow-speed unloaded rotation, the generator fans will mix the gases, greatly increasing the time required to achieve purity.

===Make-up===
Hydrogen is often produced on-site using a plant consisting of an array of [[Electrolysis of water|electrolysis]] cells, compressors and storage vessels. This reduces the need for storing [[compressed hydrogen]] and allows storage in lower pressure tanks, with associated safety benefits and lower costs. Some gaseous hydrogen has to be kept for refilling the generator but it can be also generated on-site.

As technology evolves materials not susceptible to [[hydrogen embrittlement]] are used in generator designs. Not doing so can lead to equipment failure from hydrogen embrittlement.<ref>{{cite web|url=http://h2incidents.org/incident.asp?inc=102&cat=2&val=10|title=Lessons Learned - Hydrogen Tools|website=h2incidents.org}}</ref>

==See also==
*[[Timeline of hydrogen technologies]]
*[[Precooled jet engine]]

==References==
{{reflist}}

==External links==
*[https://web.archive.org/web/20100821180100/http://www.labplan.ufsc.br/congressos/PowerTech07/papers/664.pdf The turbogenerator – A continuous engineering challenge]

{{DEFAULTSORT:Hydrogen-Cooled Turbogenerator}}
[[Category:Electric power]]
[[Category:Hydrogen technologies]]
[[Category:Turbo generators]]

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