The document discusses the importance of stator cooling water systems in generators, detailing types of cooling systems, potential problems, and chemistry regimes for maintaining optimal operation. It emphasizes the need for proactive measures and quality control in monitoring water chemistry parameters to prevent issues such as copper corrosion and water leakage. Maintenance strategies, including the replacement of ion exchange resins and regular inspections, are also highlighted to ensure effective cooling and efficiency of the generators.

1. Generator stator cooling
water Chemistry
Sudhanshu Sekhar Dash
Executive – Chemist
Haldia Energy Limited
+91 9674222631

2. Content Of Slide:-
1. Introduction
2. Types of Generator cooling system
3. Introduction on stator cooling water system
4. Problems in stator cooling systems
5. Pro-activeness & Preventive action
6. Quality control: Different types of chemistry regime for stator water
7. Monitoring stator water parameters
8. Maintenance of Stator water
9. Thank You

3. Introduction:
Generator is a key component of a unit from its availability point of view.
Kinetic energy(rotational) from a Turbine is converted to electrical
energy by a Generator.
When mechanical energy do the work against electromagnetic force and
produce Electricity, during this process high current flow through Stator
coils heat is generated in the Generator stator coils.
The stator cooling water system is used to remove heat from generator
armature bars.
To maintain efficient operation and energy conversion, the coils must be
cooled to keep the stator within specified operating limits
The cooling medium is demineralized water – typically with a
conductivity of 0.5μS/cm.

4. Types of generator cooling systems:
Typically there are three types of Generator cooling system
Water & Hydrogen cooled
Hydrogen Cooled
Air Cooled
Hydrogen/Water
cooled
2/4 Pole Steam
Turbine Generator
*H2/Water cooled
Above 400MVA
Hydrogen cooled
Gas/Steam Turbine
Generator
*H2 cooled
150-400 MVA
Air cooled
Gas/Steam Turbine
Generator
*Air cooled Below
150 MVA

5. Water & Hydrogen cooling system
 Our system is water & hydrogen cooling system. Water for generator stator &
hydrogen gas for rotor.
 Water cooling was introduced in 1956 by AEI (UK) with a view to accommodate
larger generator power outputs
 Generator with above 400 MVA require a more efficient cooling method. So that
adopted the Hydrogen-Water cooling system.
 The stator windings are directly water cooled by Deionized water. Supplied by a
closed loop cooling system.
 Deionized water flows through hollow copper strands located in the stator
windings.

6. Introduction:
Briefly the schematics for stator water system is :

7. Water & Hydrogen cooling system

8. Simplified Schematic diagram
P
P
PHE-A
PHE-B
Generator
Storage tank
Deionizer
SCW pumps
Plate heat
exchanger
Main filter
Make-up filter
Make-up line
FCV
Drain
Vent at Roof

9. Stator water cooling system

10. Five points of stator water cooling system
 To provide deionize water for cooling to the Generator stator hollow
windings at a rated pressure and rated flow.
 To detect and alarm if the conductivity of DM water goes up to
unsafe level.
 To provide filtration to remove any particulates which could plug the
very small bores of the stator tubes.
 To provide venting to atmosphere for any H2 gas that becomes
entrained in the stator coolant.
 To provide for addition of DM coolant to make-up for any loss due to
leaks or evaporation from the stator cooling system.

11. Problems on Stator water coils and solutions
 Copper Corrosion & Plugging in copper hallow conductors
 Water leakage of stator windings
 Clip-to-strand leak due to crevice corrosion.
 End-winding vibration
 Increase in conductivity of water.
 Deposits on insulating hoses.

12. Problems on Stator water coils and solutions
Copper Corrosion:
 As the Stator water chemistry copper corrosion is not a normally problem firstly but if
the poor chemistry maintenance can lead to another problems if it excess or less.
 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 (Red Colour) /cupric oxide CuO (Black colour) depending on
the electrochemical potential (ECP), which varies with the operating temperature,pH
and the DO present. In generators, a mix of these oxides is always found, predominantly
Cu2O with low concentrations of dissolved oxygen, and predominantly CuO with higher
levels of oxygen
 The copper oxide forms a stable passive layer on the hollow copper conductor inner
surface. This stability of passive layer increases with the increase of stator water pH. As
shown in the figure, the corrosion rate of copper in contact with the cooling water mainly
depends upon the pH and Dissolved Oxygen (DO).

13. Rate of corrosion at different pH & DO level

14. Water leakage of stator windings:
Leaks in stator winding cooling system mainly caused by problems with
brazed connections
Water box leakage, combined with strand-to-strand leakage generates a
sponge effect
Water penetrates the bar insulation and increases the risk of electrical
failure
Small leaks in these locations will not result in winding damage during
normal operation as H2 gas Pressure is maintained well above stator
cooling water pressure.
But small leaks pose problem during outage when generator is de-gassed
and stator water system is left in operation. Under those circumstance s
the Press. Diff. force the leaking water into ground insulation.
Problems on Stator water coils and solutions

15. Leakage in Stator Hydraulic components and connections
First let us have a closer look at the stator end winding configuration

16. Clip-to-strand leak due to crevice corrosion.
A leak process initiates in the braze alloy at the inner surface by Crevice
corrosion mechanism. Under right conditions the leak can change to
corrosive penetration of adjacent copper tubes.
Crevice corrosion :
Due to spongy nature of braze, water makes its way to small voids at braze
surface and stagnates. If the void size is large enough, sizable amount of
water comes in and start attacking the phosphorous rich phase of braze
alloy. As the material starts , void size goes on increasing and water
chemistry changes to phosphoric acid.
This process continues till the crevice reaches critical volume, thereby
precipitating out Phosphate salt( Cu3P)on the surface of the braze which
stops further corrosion.

17. If the void size is good enough and the phosphoric acid inside this void
come in contact with Copper strand, the attack by Phosphoric acid to
Copper is a preferred reaction as compared to that of braze and also the
rate of reaction is also high.
The depth of copper is limited and constant( approx. by 0.015 inch) partly
by precipitation of Phosphate salt on Copper and partly by volume of
liquid too large to maintain critical acid conc.
This leads to attack of Phosphorous phase of braze material leading
to formation of Phosphoric acid which again starts reacting with
Copper.
This mechanism switches back and forth from Crevice to acid attack
of copper as the leak path makes its way to the strand.
Crevice Corrosion Continues…..

18. Crevice Corrosion

19. Pro activeness & Preventives
Addressing the plugging issues EPRI guidelines suggest following proactive
measures to detect flow restrictions
(i) Monitoring pressure drop, total stator water flow and individual bar flow.
(ii) Analysis of spent resins from SPU gives a fair indication of the quantity of Cu
release from the system.
Preventing Pluggage :
(a) On line ECP and DO Monitoring
(b) Off line Cu trend analysis From Spent SPU resins
(c) For High oxygen regime : A large diameter vent to provide air access to the
storage tank and forced aeration to the water tank
(d) During shut down when stator water system is drained then it needs to be
dried out with Nitrogen (Grade-1 ) quality

20. Types of Water Chemistry regime used in Stator Water
A . Low dissolved Oxygen
(<10ppb) & Neutral pH
thin layer of passive cuprous
Oxide.(50 % power plant
Used)
B . High dissolved Oxygen
(>2000 ppb) & Neutral pH
CuO (Cupric Oxide) is
formed on the copper.(40
% power plant used)
C . Low dissolved
Oxygen(<10ppb)
&
Alkaline pH ( 8-9),
D . High dissolved Oxygen
(>2000
ppb) & alkaline pH (8-9)
Not suitable due to clip
corrosion

21. A . Low Dissolved Oxygen and Neutral pH
Description :
1. In this type of chemistry regime a thin layer of passive cuprous (Cu3O) oxide layer formed.
2.This treatment option is found in about 50% of the stator cooling systems in the power industry.
3. The water is fully oxygenated when the system is first filled. As the water circulates, it reacts with the copper in the
system, the oxygen is consumed, and the dissolved oxygen gradually approaches zero. The dissolved oxygen is likely to
remain at <10 ppb as long as no water is added to the system.
4.As the water loses and every time water make up to the system it receives little of Dissolved oxygen to the system but if
If, due to leaks in the stator cooling water system, makeup rates are significant, the stator cooling water can swing back
and forth between low- and high-oxygen conditions. These transient conditions release oxides into the system.
5. Air ingress can also cause significant increases in the system’s dissolved oxygen. Putting a nitrogen cap on the stator
cooling water head tank can minimize air in-leakage.
6.Carbon dioxide can enter the system via the makeup water or along with the air. As carbon dioxide is absorbed into the
water, it drops the pH to acidic levels, increasing the corrosion rate of copper. Carbon dioxide can form bicarbonate and
carbonate in the water and exhaust the mixed bed polisher. If the polisher is not changed when it is exhausted, the
released carbonate can form insoluble copper carbonate in the stator.
7. To prevent dissolved oxygen contamination resulting from additions of makeup water, some utilities have turned to
oxygen removal systems. The system, which uses only nitrogen purge gas to sweep out the oxygen that permeates the
membrane, is capable of reducing the dissolved oxygen of the makeup water to about 3 ppb.

22. A . Low Dissolved Oxygen and Neutral pH
Showing the configuration of a typical Neutral pH and Low Dissolved Oxygen Type Stator Water
System

23. B . High Dissolved Oxygen and Neutral pH
Description:
1.This is possibly can be regarded as the simplest form of stator water treatment, and is put into practice
where oxygen cannot be excluded from the system. In this treatment regime, the objective is to form evenly
distribution stable oxide layers (CuO). It will tightly adhere to the surface and create a passive layer on the
metal. This layer tends to be thicker than the Cu2O formed under low-oxygen conditions. It is estimated that
40% of water-based stator cooling systems operate with high–dissolved oxygen and neutral water chemistry
2. There is always some dissolution of the oxide layer, but the dissolved copper is trapped by the ion
exchanger. However, the risk of forming thick oxide layers cannot be excluded. In order to achieve a
stable oxide layer, oxygen levels must be maintained high (e.g. forced air flow into the water tank) at all
times. By introducing air, carbon dioxide is also added to the system. However, it may still be susceptible to
low-pH corrosion from carbon dioxide and carbonates if these are not removed by the mixed bed polisher. So
in this case mix bed polisher will be large.
3. If there is a hydrogen leak into the stator cooling water system, the hydrogen can replace the dissolved
oxygen and create low–dissolved oxygen transients in the system, causing oxides to be released.

24. B. High Dissolved Oxygen and Neutral pH
Showing the configuration of a typical Neutral pH and High Dissolved Oxygen Type Stator Water System

25. C . Low Dissolved Oxygen and High pH
Description :
1.This treatment has the basic features of a low oxygen operation. With alkaline treatment, the water is
alkalized by adding a small concentration of sodium hydroxide, in order to lower copper oxide solubility
and thus the chance of migration of oxides. The quantity of oxides produced however is as with neutral
treatment.
2. The same requirements as mentioned above for low oxygen/neutral pH treatment apply. In order to
achieve a constant low solubility, it is very important to keep the pH value stable.
3. Alkalization can be achieved in two different ways:
 injection of a diluted NaOH solution via an injection pump, controlled by conductivity [4].
 constant leaching of NaOH from an ion exchanger
4. One method is the use of a dual bed demineralizer, consisting of a mixed bed filter followed in
series by a weakly acidic cation exchanger, loaded with a measured quantity of NaOH. The
cation exchanger provides constant high purity water, that leaches an equilibrium concentration
of NaOH from the cation exchanger [4].
5. Another method is the use of a dual bed demineralizer, consisting of a mixed bed filter in
parallel to a strongly acidic cation exchanger, loaded with NaOH. The Na concentration is
controlled by suitable adjustment of the water flow through these two ion exchange beds [14].
Possible ingress of carbon dioxide traces are buffered by the alkalizer and therefore have smaller
influence on pH than with neutral treatment.

26. C . Low Dissolved Oxygen and High pH
Showing the configuration of a typical High pH and Low Dissolved Oxygen Type Stator Water System

27. Monitoring Chemistry Parameters
Our system is neutral pH & high DO system. So accordingly these below
chemistry parameters to be monitored round the clock.
 a) Monitoring of Online Dissolve Oxygen
 b) Measurement of pH (Should be 6.0-7.0 neutral pH)
 c) Online conductivity Monitoring (should be less than 1 μS/Cm.)
 d)Electro Chemical Potential (ECP)
 d) Copper content of stator water (Should be less than 40 ppb)

28. Monitoring Chemistry Parameters
a)Dissolve Oxygen:
 Measurement of DO is required for maintaining as per the respective treatment
system.
 Measurement of DO in stator water also affords valuable information about
hydrogen leakage through stator
 Tiny H2 leakage- non-detectable through H2 gas trap or consumption pattern-
leads to lowering of DO content of Stator water
b) pH:
 Measurement of pH required to maintain it according to respective treatment
regime. (N.B. It may be noted that measurement of pH of in high purity water is
difficult and may be unreliable. Conductivity is commonly used to indirectly
monitor the pH).
 For elevated pH system, caustic/ammonia dosing system adopted.

29. Chemistry Parameters
c) Conductivity:
It’s the measure of water’s ability to pass on a current of electricity.
Free ions presence in 1 centimeter water sample is known as water
conductivity.
Conductivity is temperature dependent.
e.g. Conductivity of pure DM water at 25 Deg C= 0.054 us/cm, while that at 60
deg C= 0.25 us/cm
d) Electro Chemical Potential ( ECP):
Research project developed by EPRI has concluded that ECP as a significant
parameters for copper release and deposition
It shows that change in oxidation state of Copper (+1 to +2 and vice versa)
induces stresses which is driving factor for particulate release and deposition

30. Chemistry Parameters
 Range of ECP as established by EPRI for two operating regimes are as follows :
Parameter ECP ECP
(Normal Operating values) (Short term action level)
Low Oxygen regime <223 mv >266 mv
High oxygen regime >315 mv <305 mv
e) Copper:
 The monitoring of copper is a essential part of stator water chemistry control.
 Regular copper analysis give reference values for trending. The value should be
less than 40 ppb to be in safer zone.

31. Operational Parameters
In addition to the chemistry parameters, some operational parameters to be
monitored periodically .
Stator water flow and stator winding pressure drop
Stator water inlet and outlet temperature, together with stator water flow
and generator load
Individual bar temperatures (outlet water hoses), together with generator
load
Gas-to-water pressure
Hydrogen leakage rate
Make-up water consumption

32. Maintenance of Stator cooling system
Apart from mechanical & electrical maintenance of stator cooling system,
some chemical maintenance to be taken care.
Replacement of ion exchange resins when exhausted
Cleaning/Replacing filters
Maintenance & Calibration of online sensors
Inspection of the stator coolant bars for corrosion products
Mechanical and / or chemical cleaning