a1_107_2012
TRANSCRIPT
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21, rue d Artois, F -75008 PARIS A1-107 CIGRE 2012http : //www.cigre.or
STATOR WATER SYSTEM MONITORING FOR LARGE TURBO-GENERATOR- A USERS PERSPECTIVE
A.K. Gupta Dinkar Devate D.K. chaturvedi*
NTPC Ltd.
Noida, INDIA
SUMMARY
In the developing countries like India, the primary focus of utility is to make the units
available to its maximum and minimising the maintenance duration. Simultaneously,
these units have to run continuously at base load ensuring the high reliability. The
present IEC standards for Electrical Rotating Machines are referred for testing to
establish the performance and quality of the large electrical machine, however they do
not specify limits or trending of various operating parameters of generator StatorWater System, which needs to be continuously monitored. The availability of
different designs of turbo-generators Stator Water systems calls for understanding the
basic ingredients of the system, their function and how the system can perform its
best. The paper covers a brief on the types of Generator Stator Water systems, their
advantages and disadvantages, design requirements & monitoring of critical
parameters along with permissible associated limits. The process of corrosion in
hollow strands, crevice corrosion on clip to strand joint and various steps to check
corrosion right at incipient stage have been discussed. A few designs experienced by
the user for stator water system have also been summarised.
KEY WORDS
Base Load, Trending, Parameters, Monitoring, Turbo-generator, Stator, hollow
Strands, Crevice Corrosion, Incipient.
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DESCRIPTION
Requirements for directly Water cooled generator
The directly cooled generators are provided with roebel bars hollow conductorsconnected to common water header at inlet and outlet of the winding. The conductors
as well as the water headers are generally of copper, however in some cases stainless
steel hollow conductor, cooling water header and piping are being provided. The
direct cooled machine are designed generally above 500MW, to ensure quick removal
of majority of heat by the water flowing in direct contact of the winding conductor, in
addition to the removal of heat by hydrogen from the surface of hot winding
insulation. The winding insulation besides having good dielectric strength, has
excellent thermal conductivity to ensure fast removal of heat. All strands of the roebel
bar are brazed together (with BCuP) at the water box connecting to header (Fig.-1) [1].
The flexible connection between the high voltage (e.g. up to 15kVL-N) conductors to
water header is through Teflon tube. Understanding of the water chemistry is veryimportant for longevity of the machine. Here some important term and the
relationships in the chemical process are described;
Fig-1 Connection of Water Header to Stator Bar
Corrosion in Copper Conductor:The copper does not react with pure water (DO - less than 5ppb, pH= 7). As the water
enters in the winding, the hollow conductor Copper comes in contact with the water
and form cuprous oxide Cu2O /cupric oxide CuO depending on the electrochemical
potential (ECP), which varies with the operating temperature and the DO present. Thecopper oxide forms a stable passive layer on the hollow copper conductor inner
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surface. This stability of passive layer increases with the increase of stator water pH.
As shown in the figure-2, the corrosion rate of copper in contact with the cooling
water mainly depends upon the pH and Dissolved Oxygen (DO).
The makeup water entry in low DO type stator water system, can lead to copper
release in excess of the solubility limits at the operating temperature (say 85 degree
Celsius conductor temperature on rated load). One part of the copper oxides forms acoating on the conductor surface, second part dissolved in water and the other moved
in the circuit, leading to deposit in critical area of the winding. The solubility of
Copper oxides[2] in water does follow V-curve relation (see fig.3) depending upon
the pH of water and the operating temperature. The re-deposition of copper oxides in
the hollow strands also cause uneven blockage.
Figure-2 shows the corrosion rate of copper bar with dissolved oxygen
Fig-3: Solubility of Copper in Water with temperature[2]
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The leakage in stator water system requires frequent makeup, which unless due care is
taken, allows change of DO from low to high and vice versa due to formation of
copper oxides. The air ingress (presence of CO2) at suction side of the water pump
due to aged gaskets can also results in lowering of the pH value to acidic zone[3] from
neutral zone and subsequent increase of corrosion rate.
Deioniser:The process of deionising the water is ensures not only removal of soluble ions, but
also filters the insoluble oxides and other particles. The deioniser controls the
conductivity of water, which should be maintained below 2S/Cm. The conductivity
is determined at reference temperature of 25C, which increases with temperature, 2%
per degree Celsius[4] of temperature rise. The conductivity increases suddenly as
NaOH dosing is done in elevated pH type system. The conductivity depends upon the
presence of free ions in the water system. The deionised pure water has poor
conductivity of 0.055 S/Cm.
The deioniser can be mixed bed type, where water is continuously pass through mixed
bed having both resins in the same vessel. The other system two-bed deioniser
provides larger capacity, where two vessels are used having separate cation and anion
resin beds.
THE TYPES OF STATOR COOLING WATER SYSTEM
The stator water systems generally available are of three types [5]:
a) Elevated pH (pH 8.5) low Dissolve Oxygen typeb) High dissolve oxygen (>2000ppb) neutral pH type andc) Low dissolve oxygen (
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Figure-4 : shows a typical Scheme of Elevated pH type System
DEARATED TYPE STATOR WATER SYSTEM
The dearated type stator cooling water system was introduced by GE to eliminate the
issue of crevice corrosion at clip to strand joint of the stator bars. The corrosion
initiated in brazed metal (BCuP-1) due to formation of low pH phosphate solution.
The chemical reaction attacks both the brazing alloy BCuP and Cu3P. The
development of cracks results in leakage of hydrogen into water. The leakage ofhydrogen for long duration allows moisture penetration to the insulation (by capillary
action) causing swelling and severe deterioration.
Figure-5 Showing the configuration of a typical Neutral pH and Low Dissolved
Oxygen Type Stator Water System
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The system is designed for low dissolved oxygen and therefore, it is mandatory to
have following features in this type of system[6]:
Detraining TankThe detraining tank is provided in the scheme to remove the dissolved gases from the
cooling water by passing the primary water on a set of baffles. It is important toensure that the vent pipe is not closed.
Stator Water Storage Tank-Special features
The tank is of stainless steel provided topping over water with hydrogen blanketing or
nitrogen scavenging. The nitrogen scavenging is preferred considering the safety
aspects despite requirement of additional system with nitrogen cylinders. The water in
storage tank has to be continuously stirred by suitable sparging equipment. Hydrogen
blanketing has advantage of effective use of catalyst palladium to control the DO, as
and when it increases to more than 50ppb.
Gasketing Arrangement RequirementsThe gaskets for stator water system operating under low oxygen regime have to be
designed for long life while working in contact with water, hydrogen and air. The
flange joint surfaces shall have smooth profile to ensure proper sealing and no ingress
of air especially at water inlet side of the pump. The hydrogen leakage can also take
place on high pressure side of the water circuit by Venturi effect (where the difference
of water pressure and hydrogen pressure is low).
Concern in Low DO Type system:The initial filling is with neutral pH water having normal value of DO i.e. 1500ppb.
As the water is filled up and passes through the circuit, the dissolved oxygen is
consumed forming cuprous oxide Cu2O layer on the hollow copper conductor inner
surface. The DO consumption results in reduction of DO level to about 10ppb, i.e.
low DO regime. In case of makeup with same water at later stage, the DO suddenly
increases and can cause severe corrosion.
The following precautions may be taken in the low DO type system:
a) Sparger along with nitrogen or hydrogen capping in the stator water storagetank may be installed. Necessary vent open to atmosphere has to be provided.
b) Oxygen removal system may be used in the makeup line. In case of low DO
system catalyst palladium[5] is used in the deioniser to form water quicklyfrom the dissolved oxygen. Such provision shall be done with Hydrogen
blanketing in the stator water storage tank to make the dissolve hydrogen
present in the cooling water.
The pH can also be slightly elevated as emergency measure by NaOH dosing to check
the global corrosion rate, but the solution is to be worked out in next overhaul of the
unit. Keeping elevated pH in old system originally designed for low DO neutral pH
will lead to severe crevice corrosion in clip to strand joints.
The continuous trending of Dissolved Oxygen and conductivity is must for neutral pH
operation.
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HIGH DISSOLVED OXYGEN TYPE SYSTEM[6]
The figure-6 shows a typical scheme of the high dissolved Oxygen type of stator
cooling water scheme.
Figure-6: High Dissolved Oxygen type of Stator Cooling Water Scheme
The high DO system operates at Oxygen Concentration above 2ppm and neutral pH.
The pH of stator water system has to maintain always at 7 considering the risk of
crevice corrosion at clip to strand joint at elevated pH.
Clip to strand joint
The fig-1 shows the water box connection from the water header to the winding bars.On one side of the water box the Teflon tube connection is there with washer and nut
to ensure perfect sealing of the flexible joint with the water box, on the other side the
water box is connected to copper conductor strands[1]. These strands are brazed with
the water box. The brazing is done with phosphorous rich braze alloy. If the brazing is
not proper, water may travel to outside the water box due to capillary action damaging
the insulation.
PRECAUTIONS
The aerated as well as deareated type of system have their own benefits, however
deviation from specified requirement can cause severe damage to the windingconductor as well as the insulation.
All the designs discussed above have their own advantages. However, as seen from
figure-2, the operation close to neutral pH with dissolved oxygen of 200-300ppb can
cause severe plugging of the stator bars requiring Ethylene-Diamine-Tetra acetic Acid
(EDTA) cleaning.
CHANGE OF SYSTEM AT LATER STAGE
The change of system from one type to other is not advisable rather the operation has
to be ensured as per OEM recommendation for longevity of the units. NTPC has so
far no experience of change of system from one type to other. However, reports
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suggest that change of system at later stage requires utmost care and understanding of
the associated process, else results in severe damage to the machine. A few cases has
been summarised below:
a) Elevated pH instead of Low Dissolved Oxygen (DO) System
The change of neutral pH Low DO system to elevated pH is good as long as the
process of corrosion is not initiated. This change at later stage can lead to severe
crevice corrosion at clip to strand brazed joints and damage to the winding
subsequently. This occurs if the winding prior to change has experience or initiated
process of crevice corrosion or Global corrosion. It has been experience that NaOH
dosing at later stage can lead to increase in crevice corrosion and subsequent winding
insulation failure.
b) Change from Dearated to Areated
These systems worked under neutral pH. As per EPRI report, a large fleet of EDFunits have been converted from low DO to High DO system[8]. The dearated system
was having nitrogen blanket in the stator water storage tank. EDF have problems of
plugging in all of their dearated systems due to ingress of air from joint to the stator
water. This resulted in dissolved oxygen higher than the desired level leading to
plugging of the tubes.
Even after the conversion to high DO type system, the plugging continued due to:
Operation of the units in Intermediate range DO and
Presence of residual of global corrosion occurred earlier.
After thorough cleaning of the system and taking due precaution the system operated
successfully.
c) Change from Areated to Deareated
One of the nuclear power stations of Europe was having the generator to operate with
high DO level but actually operated as low DO type system. The system was provided
with only conductivity monitoring at inlet & outlet of the deioniser[8]. The clip to
strand leakage resulted in large quantity of dissolved hydrogen in the stator water
system, accumulated at the top of water surface in stator water tank. The capping of
hydrogen also not allowed the air contact to water, which is mandatory to achieve
2000ppb dissolved oxygen level. The dissolved oxygen was consumed in corrosionprocess of copper, leading to operation in low oxygen regime. Due to hydrogen
leakage from clip to strand the DO level reached below 10ppb (instead of >2ppm).
After thorough cleaning of the system, the system was converted to dearated type by
taking suitable measures and thereafter run successfully.
LEAK REPAIR
It has been experienced that even in new machine cracks may appear at clip to strand
joint initiating leakage of hydrogen into the primary water. The cause can be one of
the following:
Severe end winding vibration of effected bar causing development of the crackAny kind of impact the bar subjected during the transportation or installation.
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Development of crevice corrosion and
Poor brazed joint with presence of voids.
One of the following methodologies is adopted by leading manufacturer:
a. Complete bar replacement (for top bar)
b. Epoxy injection repairc. Braze/TIG repair.
In case the leaks are at many places, Global Epoxy Injection or complete[1] stator
rewinding is done.
THERMAL MONITORING OF STATOR COOLING WATER SYSTEM
The thermal profile of cooling water is very important for healthy operation of the
machine. The most critical measurement in this system is the water temperature at the
bar outlet, and hence the same has to be continuously monitored for each bar
individually or common for top & bottom bars. The Embedded temperature Detectors
are placed on the designed hot spot locations and thus have temperature higher thanthe water temperature flowing in the conductors. Considering the availability of
excellent insulation tapes of high electric strength, good thermal conductivity and
coefficient of expansion close to the copper, the temperature at ETD locations shall
not exceed in any case more than 95 degree Celsius[7]. As we know that water boiling
point slightly rises with increase of operating pressure (in this case water pressure in
the conductor) and operation close to those limits shall cause bubble formation and
subsequent blockage of cooling water flow. The hot water temperature at outlet shall
generally be kept below 75 degree Celsius.
COMMON FEATURES
The stator water system should have the following to ensure healthiness of the
system:
a) Monitoring of every single or two(top & bottom) bar water outlet temperatureb) Monitoring of Online Dissolve Oxygenc) Measurement of pHd) Conductivity Monitoring (should be less than 2S/Cm.)e) Differential pressure across stator barsf) Differential pressure across deioniserg) Primary water inlet pressure and
h) Regulated alkali dosing for elevated pH (maintaining the conductivity).
In line samples are also taken periodically to check and monitor the values of b), c)
and d) above. The monitoring of differential pressure across stator winding is on
continuous basis. The trend shall clearly show the corrosion or blocking of the bars.
The table-1 shows the suggested values of parameters of stator watersystem:S.No. Parameter Elevated pH,
Low DOLow DO,Neutral pH
High DO,Neutral pH
1 pH 8.5 to 9.0 7.0 7.0
2 Dissolve Oxygen < 50 ppb 2000ppb3 Conductivity to 2.0 -10S/Cm*
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change
resin(generally to
be done with
increasing trend)
4 Conductivity Max.
(alarm)
10.0 S/Cm
5 Conductor temp. 95C
6 Water pressure at
inlet to conductor
3.5kg/sqcm.
7 Max. Permissible
diff. Pr. (DP)
Across Conductor
1.5Kg/Cm2
8 Recommended Hot
water temperature
75C (max.90C)
9 Differential with
Hydrogen pressure
1-2 Kg/Cm2
10 Hot Gas Max.Temperature
75C
11 Cold Gas Max.
temperature
46C
12 Cooling Water
Max. temperature
40C
EPRI report[5] suggest change of resin at (a)conductivity of 0.5S/Cm (b)
every 18 to 20 months interval or (c) pressure drop across deioniser resin
exceeds 15psi.
NTPC experience is change of water as conductivity reaches max. 10S/Cm
(BHEL, ALstom, Elektrosila suggest max. 10S/Cm for alarm)
NTPC EXPERIENCE
The NTPC has more than 50 nos. water cooled generator of 200MW and 500MW
ratings in operation for more than 5 years. The available designs provide us with
experience of stator water system of following types:
- Low Dissolved Oxygen and Elevated pH (500MW units)- Low Dissolved Oxygen and Neutral pH (500MW units) and- High Dissolved Oxygen and Neutral pH (200MW units).
The experience of these units is generally satisfactory and suggests:
- Due care shall be taken to maintain desired Dissolved Oxygen level byhaving On-Line monitoring.
- Conductivity shall also be ensured within limits, establishing machinesafety and healthiness of deionizer.
- pH should be maintained at the designed level to check initiation ofcrevice corrosion.
- The parameters of stator water system in NTPC generally comply to table-1 above. In two of NTPC units with low DO and neutral pH, severe
corrosion was experience requiring ETDA cleaning of the winding. Aslarge numbers of units in NTPC are with Elevated pH type system, even at
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the same project, the criticality of Maintaining Dissolve Oxygen was not
rightly accessed initially. The initiation of corrosion process in the winding
lead to review the cause. Necessary corrective actions (as mentioned
above) have been taken to maintain the DO well within 20ppb along with
neutral pH.
- The resin is generally changed in duration not exceeding six (6) months.The alarm and trip value have been specified as 13 and 20 S/Cm for high
DO type 200MW (15.75kV machines) and 10 S/Cm as alarm setting has
been specified in 24kV 500MW machines. No tripping has been
considered based on stator water conductivity in 500MW units. As
practice, samples of stator water are taken to monitor the conductivity at
specified interval (normal value 2.0 S/Cm). In case of increasing trend of
stator water conductivity value, the resin is to be replaced.
CONCLUSION
As discussed above maintaining various parameters of stator water as per systemdesign is of utmost importance. Any variation in DO, pH, conductivity, DP and bar
outlet water temperature can damage the machine severely. The increase in DO in low
DO neutral pH system and decrease of DO in neutral pH high DO system can cause
severe corrosion and subsequent deposition in critical areas like clip to strand joint.
The operation under such condition for long can result in insulation failure and
subsequent ground fault. Change from aerated to deareated and vice versa shall be
done only after carrying out a careful detailed analysis. The change from deareated to
elevated pH by implementing ammonia dosing can be done only as short term
measure necessitating implementation of long term measures for running the system
as low-DO type, in next overhaul of the unit. It is suggested that the parameters
tabulated above for stator water system can be reviewed for inclusion in relevant
international standards.
REFERENCES:
1. Understanding, Diagnosing, and Repairing Leaks in Water-CooledGenerator Stator Windings- GE Power Systems
2. Behaviour of Copper in Generator Stator Cooling Water System byRobort Svoboda and Donald A. Palmer, Alstom Power
3. Forgotten Water: Stator Water Chemistry by David G Daniels
4. Deionised Water ApplicationMyron L Company5. Generator Cooling System Operating Guidelines - EPRI6. Handbook of Large turbo-generator Operation and Maintenance by
Geoff Klempner
7. IEEE 50.13 Requirement for Cylindrical Rotor SynchronousGenerators
8. Conversion to Dearated Stator Cooling Water in Generators PreviouslyCooled Areated WaterEPRI
9. Electrochemical Corrosion Potential (ECP) of Hollow Copper Strandsin Water Cooled GeneratorsEPRI
10.Design and Experience Feedback of Large turbo-Generator Upgrade &
RetrofitPhilippe Machard, Paul Lapotre, Wynand Pienaar, MarcPeltier