3. Stator Water Cooling System- Stage-2 1j3d3j

37

3. GENERATOR STATOR WATER COOLING SYSTEM-STAGE-II DESCRIPTION AND OPERATION 3.1 INTRODUCTION One of the efficient ways of taking away the losses from the windings of any electrical machines is by direct cooling using water. The optimum design of large capacity turbo generator, as a rule envisages water-cooling of stator windings. The 200MW / 210MW / 235MW Turbo generators employ a closed loop circulation of high quality demineralised water through the stator windings made of hollow and solid conductors. The generator is capable of delivering its rated load only when the stator water-cooling system is functioning properly. Therefore it is necessary that highest attention is paid for proper operation and maintenance of all the equipments in this system. This section deals with the principles of working, constructional features and the operation and maintenance procedures of various elements of stator water-cooling system. This manual is prepared keeping in mind that the operating personnel have the necessary basic knowledge of the equipment. Though in this manual, it has been attempted to deal with all the important aspects of operation and maintenance, it should be kept in mind that the final responsibility for operating the unit lies with the operating company personnel and it is advisable to consult BHEL whenever necessary. BHEL reserves the right to change or modify these instructions, in part or full, at a later date, based on the experience gained on similar units in course of time.

3.2 GENERAL DESCRIPTION The heat losses arising in the stator winding, main terminal bushings and phase connectors are removed by demineralised water coming into direct with high voltage winding. The cooling water must have an electrical conductivity of less than 2.5 micro mho/Cm. The Demineralised (DM) water supply system comprises of following main components:  Centrifugal pumps 2 X 100 % duty  Distilled water cooler 2 X 100 % duty  Polishing Unit

38  Mechanical filters 2 X 100 % duty  Magnetic filter 1 X 100 % duty Other components employed in the system are gas trap device, expansion tank, water jet ejector, valves and associated instrumentation. Together with the connecting pipe work these components form the external circuit for the supply of cooling water to generator windings. Refer stator water system diagram. The cooling circuit makes use of either of the following water supplies, free from oxygen. (a) Distilled Water. (b) Fully demineralised water from boiler feed water treatment plant. (c) Condensate. Fully demineralised water from the boiler feed water treatment plant and condensate may only be used if no chemicals, such as ammonia, hydrazine, phosphates etc. are present in the water or condensate. A part of water is byed, and is treated in mixed bed ion exchanger, connected in parallel to the stator winding, magnetic filter and expansion tank and returned into suction side of the water pump, thus maintaining conductivity of closed loop circulating water within permissible limit. The DM water pumps are of single stage type with spiral casing and overhung impeller. The pump is connected to a three phase AC Motor. Failure of the working pump due to fault or power supply failure results in an automatic changeover to standby pump; the criterion for such a changeover is the falling water pressures downstream the pumps. This falling pressure is sensed by means of a pressure switch. The heat absorbed by the DM water in the generator is dissipated to the secondary coolant in the primary water cooler. The water treatment plant (polishing unit) provided across the stator winding, essentially comprises an exchanger tank filled with anion and cation resins in the form of a mixed bed ion exchanger. The base substances of the exchanger resins are normally polymers containing groups of diverse character featuring exchange activity. In this way the synthetic resins are capable of cation and anion exchange. The cation exchanger contains highly acidic groups, while the anion exchanger comprises highly basic groups. The exchanger resins are thus capable of accepting ions from the DM water, while releasing equivalent amounts of other ions (hydrogen

39 ions from the cation exchanger and hydroxyl ions from the anion exchanger) to the DM water at the same time. This exchange can only take place between equally charged ions. Therefore, cation exchanger exchange cation and anion exchanger exchange anion. This process takes place within a very short time while the DM water is ed through the ion exchanger. A combination of highly acidic cation exchanger and highly basic anion exchanger constitutes a multitude of small demineralization units, resulting in a high purity deionite. The mechanical filter eliminates any foreign particles in the water, which may choke, erode the hollow conductors. Difference of pressure at inlet and outlet of the filter is indicative of the degree of choking. The choked filter may be cleaned after using the standby without affecting the system corking. The magnetic filter prevents any magnetic particles from entering the generator. Any accidental leakage of hydrogen into the main distilled water stream is detected by gas trap device (see description of gas trap device). The water from the outlet of stator winding collects in an overhead expansion tank, which provides a constant level of water during normal running of the system. The hot water enters the tank through perforated pipe in the form of spray thus releasing any entrapped gas. A water jet ejector is connected to expansion tank for creating vacuum for the purpose of removing any traces of Oxygen/Hydrogen Gas which may be present as a result of hydrogen leakage into the DM water stream. Level signaling device provided in the expansion tank monitors the high and low level of distilled water and initiates a tripping command for stator water pump at low level in the expansion tank. Make up DM water to the system is provided at expansion tank through a float operated level regulator. The quantity of DM water flowing through the windings is measured by a system of orifice plate, flow transducers, and flow indicator and recorder. Signalling s are available in flow switch/the indicator/recorder, which are set to annunciate at low flow through the windings, and initiates tripping of the machine at emergency flow on the principle of two out of three. Conductivity cell and suitable indicator and recorder do continuous monitoring of the conductivity of water, which annunciate alarm at conductivity high set value. At very high conductivity the unit is tripped automatically. In addition to above, the system also provides for necessary instrumentation for indicating/Signalling of the

40 temperatures and pressures at various points of the system. Pressure interlocking of the pumps is also provided.

3.3 STATOR WATER PUMP DESCRIPTION 01. DESIGN FEATURE Stator water for cooling the stator winding is circulated in a closed loop. To ensure the continuous generator operation, without any interruption two full capacity pumps are provided. The pumps have equal priority and are alternatively brought into operation, a changeover being possible at any time without interruption. The standby pump is kept ready for service and is automatically started in the event of the failure of stator water supply, which results in fall of the pressure below the set value in pressure switch installed in the circuit. Stator water pumps are of single stage centrifugal type with spiral casing and overhung impeller. The pumps suction is arranged in the axial direction, while the discharge is directed radially upward. The spiral casing of the pump is ed to the bearing housing through a flange. The pump impeller and the spiral are of the stainless steel construction. The point casing is sealed by means of mechanical seals. The pump shaft is ed in oil lubricated antifriction bearing. The pump is connected to a 3 phase 415 Volt A.C. motor by a coupling covered by the coupling guard spacer. Normally the pump-motor set is installed at the zero meter level of the power station.

3.4 WATER COOLER DESCRIPTION The stator water cooler is shell and tube type heat exchanger suitable for vertical mounting, in which the D.M. water flows through the shell. Shell is made of non-magnetic stainless steel. Cooling water flows through the tubes. Tubes are made of stainless steel. The general arrangement of the cooler along with different nozzle connections is shown in the diagram. To have effective heat transfer between the coolent and cooled water, baffles made of brass are provided across the tube nest to guide the water to be cooled in a zigzag fashion. The cooling water is led in and out of the cooler from the flanges provided at the bottom of the cooler. Four numbers of dial type thermometers have been provided for the following purpose: (i)

One thermometer each at inlet and outlet of primary water to measure inlet and outlet temperatures.

41 (ii)

One thermometer each at inlet and outlet of secondary cooling water to measure the inlet and outlet temperature.

3.5 STATOR WATER FILTER DESCRIPTION The filter consists of cylindrical body having separate inlet and outlet chambers. An assembly of 45 filter elements surrounds a perforated cylindrical body having holes all along its length and periphery. The filter elements are kept in position by helical spring, which is kept pressed by the cover. The filter elements consist of sound corrugated brass sheet with a central hole and two sizes of wire mesh aperture, 0.224 mm and 0.5 mm held in one assembly by the inner and outer brass rings. The material of construction of body, cover, and perforated cylinder is stainless steel whereas that of filtering element is brass. The water enters the filters as shown in the drawing. The water after ing through the filtering elements enters the perforated cylinder from where it flows to the outlet chamber. Mechanical impurities get collected on the wire meshes as the liquid es through the mesh holes from the inlet chamber to the delivery chamber through the perforated cylinder. The filter is provided with vent and drain plugs.

The normal pressure drop, across the filter corresponding

to the rated flow is of the order of 0.1 – 0.2 kg/Cm 2 (g). As the particles get deposited on the wire meshes, the resistance to flow increases and consequently pressure drop across the water filter increases. When the pressure drop increases to 0.4 kg/Cm2 (gauge) cleaning is recommended.

3.6 TECHNICAL PARTICULARS 1. Type

: Wire mesh Disc Type

2. Rated discharge

: 30 M3/hr.

3. Maximum pressure rating

: 12 kg/Cm2

4. Minimum size of particles that can be filtered

: 0.224mm.

5. Pressure drop across filter during normal running : 0.1-0.2kh/Cm2(g) 6. Pressure drop across filter at which cleaning is recommended: 0.4 kg/Cm2 (g) 7. Material of body and perforated cylinder

: Stainless Steel

8. Material of filtering elements

: Brass

9. Weight (Kg) 10.Size of wire mesh (Aperture) mm 0.5

: 71.5 : 0.224 and

42 : 38 kg/Cm2 (G) for 15 minutes.

11.Hydraulic test pressure

3.7 MAGNETIC FILTER DESCRIPTION

The magnetic filter is of permanent magnet type designed to trap ferromagnetic (strong and average magnetic) particles, if present in the water system. The capacity of the magnetic filter is 36 M 3/hr. Maximum strength of magnetic field in the circular air gap is of the order of 4000 gauss. The filter mainly consists of an upper and a lower body, housing the permanent magnet system, comprising of the magnets, pole shoes and pole ring as shown in the figure. The demineralised water before entering the winding es through the magnetic filter, where it successively es through two annular air gaps in which ferromagnetic particles are filtered out because the later are attracted to the pole shoes and pole ring points in the zone of high magnetic field strength. The filter is installed to the vertical position and is connected to the corresponding piping flanges by means of bolts.

3.8 TECHNICAL PARTICULARS 1.

Magnetic filter type

: YΦ – 36

2.

Magnet type

: Permanent magnet

3.

Maximum induction density

: 4000 Gauss

4.

Capacity

: 36 M3/hr.

5.

Weight

: 113 kg

6.

Pressure rating (kg/Cm2)

7.

Type of particles which can be filtered

: Highly & medium magnetic

8.

Hydraulic test pressure

: 16 kg/Cm2 for 10 minutes.

9.

Pressure drop

: Negligible

: Not more than 10

3.9 EXPANSION TANK DESCRIPTION The expansion tank is a hermetically sealed container made of stainless steel (see diagram). The DM water from the stator winding outlet header discharges into a section of the tank through a perforated pipe. The tank is under vacuum of 250-300 mm of mercury under the influence of water jet ejector. In the proce4ss of spraying under vacuum, the entrapped air/gas in DM water, entering the tank, if any, gets liberated and ejected out. The tank is provided with a make up connection from the power station DM water source. The inlet of make up water in to the tank is controlled by means of a float operated level regulator,

43 which opens or closes depending on the level in the tank. The stator water pump draws water from the bottom of the tank for recirculation. The tank is provided with the following instruments/components. (i)

A gauge glass to indicate the level of the water in the tank.

(ii)

Two nos. of transducers for level signalling device to give high and low level alarm signals.

(iii)

A vacuum gauge level regulator vacuum inside the tank.

(iv)

A float operated level regulator to regulate the make up water.

(v)

A drain valve to facilitate draining of the complete tank in case of necessary.

3.10 VACUUM PUMP DESCRIPTION Liquid Ring Vacuum Pump is used to create vacuum in expansion tank. It also builds up vacuum in stator water-cooling system during starting. The pump consists of a ‘SHAFT’ mounted multibladed impeller located eccentrically in the ‘STATOR’. Process gas is sucked through in the ‘INLET CONTROL DISC’ via other ‘INLET PUMO CASING’ compressed in stator and expelled through a groove in ‘OUTLET CONTROL DISC’ into the ‘OUTLET PUMP CASING’ to the atmosphere. Service liquid is inserted into the pump by means of a control value provided. Due to rotation of the eccentrically mounted multibladed impeller, service liquid is thrown towards the periphery of the stator thus forming a sealant. Process gas is trapped in the space between the impeller blades, compressed and expelled. Thus a continuous suction cycle is initiated. The service liquid functions as a sealant as well as coolant.

3.11 TECHNICAL DATA 01. MATERIAL OF CONSTRUCTION *

PUMP CASING

:

SS316

*

SHAFT

*

IMPELLER

:

SS316

*

STATOR

:

SS316

*

CONTROL DISCS

*

INTERMEDIATE DISC

:

SS316

*

BEARING HOUSING

:

CAST IRON

*

SHAFT SEAL

:

SS

N/B FLANGE (TABLE D) BS 10

:

1”

:

:

SS316

SS316

44 SERVICE LIQUID CONNECTION COUPLING

: :

SERVICE LIQUID USED

½” BSP

FLEXIBLE :

8 LPM AT 10 Psig

0.2 PUMP DETAILS MODEL

:

2T – 30

VACUUM

:

720 mm Hg Col. (Max.)

DISPLACEMENT

:

51 M3/HR

R.P.M SEALANT REQD.

: :

3HP AT 415V 3Φ 50 C/S

8 LPM

03. MOTOR DETAILS MAKE

:

BHARAT BIJLEE

TYPE

:

TEFC SQUIRREL CAGE

H.P.

:

3

R.P.M VOLTAGE

: :

2850

415 VOLTS

3.12 POLISHING UNIT DESCRIPTION The polishing unit serves to maintain a low electrical conductivity of the primary water. The water treatment system is connected in parallel to the main circuit and contains a mixed-bed ion exchanger with series connected fine filter, an integrating flow meter and a conductivity transmitter.

3.13 MIXED-BED ION EXCHANGER The mixed-bed ion exchanger consists of a tank filled with anion and cation exchanger resins. Nozzle trays in the upper and lower part of heat exchanger tank prevent the escape of the resins into the piping system. A fine filter after the ion exchanger retains any resins particles. An alarm is initiated when the filter is contaminated. The water flow ing through the ion exchanger is measured by means of an integrating flow meter after the filter. After the ion exchanger part of the flow is ed through a conductivity transmitter for checking the resin activity.

3.14 ION EXCHANGER RESINS The ion exchangers consist of chemically durable and highly active synthetic resins. The base substances of the exchanger resins are polymers.

45 The polymer in the cation exchanger contains highly acid groups, while the polymer in the anion exchanger is composed of highly basic groups. The exchanger resins are thus capable of accepting ions from the primary water, while simultaneously releasing equivalent amounts of other ions (hydrogen ions from the cation exchanger and hydroxyl ions from the anion exchanger) to the primary water. This process takes place within a very short time while the primary water is ed through the ion exchanger. The combination of highly acid cation exchangers and highly basic cation exchangers forms a multitude of small demineralization units, resulting in a high-purity de-ionate. The number of ions it can exchange limits the capacity of the mixed-bed of the ion exchange. This capacity is primarily determined by the type of exchanger used, but also depends on the quantity of reactivating agent, the rate of flow and the water temperature. When the resins are exhausted, new resins must replace them. After removal from the primary water treatment system, the resins can be reactivated.

3.15 SUPERVISION OF STATOR WATER COOLING SYSTEM The most essential measuring and supervisory devices in the primary water supply system are: (1)Specific Resistivity / Conductivity measuring device (2)Level meter Device (3)Flow measuring device (4)Pressure gauges/Pressure switches. (5)Temperature detectors and Indicators or Recorders.

3.16 SPECIFIC RESISTIVITY/CONDUCTIVITY MEASURING DEVICE: Within the primary water system the conductivity of the primary water is supervised on: (a) Down stream of the ion exchanger and (b)Upstream of generator inlet. The measuring point downstream of the ion exchanger serves for checking the ion exchange for proper performance. The measuring point on the primary water inlet to the generator permits the conductivity of the entire cooling circuit to be assessed. Both measuring devices are equipped for indication and alarm.

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3.17 LEVEL METER DEVICE: A level-signalling device senses the water level in the expansion tank, with a high or low water level initiating an alarm. A local water level gauge is arranged in parallel to the electrical meter system.

3.18 FLOW MEASURING SYSTEM: The amount of water flowing through the stator winding is measured by flow measuring system.

3.19 PRESSURE GAUGE/PRESSURE SWITCHES: Pressures are sensed at the following measuring points in the stator water-cooling circuit. (1)Pressure downstream of stator water pump ‘A’. (2)Pressure downstream of stator water pumps ‘B’. (3)Pressure before and after water filters. (4)Pressure switches and pressure gauge at the inlet of stator winding. (5)Pressure gauge at the outlet of stator winding. (6)Vacuum pressure gauge at the expansion tank.

3.20 TEMPERATURE DETECTORS/INDICATORS OR RECORDER: The following temperatures are measured locally by means of thermometers. (a) Temperature of inlet and outlet of water coolers (b)Temperature of the water at the inlet and outlet of stator winding with alarm s. (c) Resistance thermometers at the inlet and outlet of the stator windings. These resistance thermometers are connected to the indicators or temperature recorder on UCB.

3.21 CONTROL DESCRIPTION There are three control cabinets for stator water-cooling system namely: i)

Stator water-cooling system instrumentation cabinet.

ii)

Starting cabinet for stator water-cooling pump motors.

iii)

Annunciation cabinet.

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3.22 GENERAL DESCRIPTION The control cabinets are freestanding sheet steel enclosures; dust and vermin proof, provided with mesh covered louvers on front and rear doors for natural ventilation. The control s are provided with: 

Earthing point.



Adequate terminal station to facilitate interconnections (external).



Removable bottom cover which can be suitably drilled for fixing of cable glands and pipe entry.



Lockable servicing door(s).



Lifting arrangement through eye bolts at the top.

3.23 EQUIPMENTS MOUNTED IN CONTROL CABINETS The following components are mounted in the stator water-cooling systems’ signalling cabinet: 

Conductivity measuring indicators.



Flow indicator.



Audio-visual annunciation system equipment.



Multiplier relays, control devices etc.



Power supply units.



Alarm units.



Level switches.



D.O.L starter and control equipments for both working and standby pump motors.

3.24 GAS TRAP DEVICE DESCRIPTION Gas trap device is used for detecting the presence of hydrogen gas in the water employed for cooling stator windings. Appearance of hydrogen in the water stream is possible due to loose/leak of ts, as the water pressure in the winding is kept lower than the hydrogen pressure in the generator casing. This device is to be used continuously. The main features of this device are illustrated in figure 1. It consists of a lower cover (1), which has two drain tubes and three inlet connections along with valves. The drain tube (2) is push fitted in disc (3) positioned 180mm from the lower cover. The sealing between tube and disc and between disc and the transparent cylinder (4) is attained by 0-rings. This

48 drainpipe consists of a plug (5), which has a central hole and a slot, which connects the space at the upper chamber. The hole at the centre in the plug faces a small hole in the drainpipe, which is having small loop at the bottom. The loop remains filled with water during normal operation thus isolating the atmospheric gases/air from the upper chamber. The cylinder is kept in position by four bolts (7) between upper and lower covers. Two sampling points are provided along with valves in the upper chamber of cylinder for gas sampling. *-*-*-*-*-*-*-*-*-*-*-*-*-*-*-* *-*-*-*-*-*-*-*-*-*-*-*-*