A REPORT ON POWER PLANT VISIT
MODI KHOLA HYDRO POWER
DIMUWA, PARBAT
SUBMITTED BY: Nischal Adhikari ( 062-BEL-325) Rupesh Dhital ( 062-BEL-3 Santosh Bista ( 062-BEL-349 ) Saroj Pyakurel ( 062-BEL-344 ) Suman Lama (062-BEL-345 Tikaram Adhikari ( 062-BEL-346 Udbhav Shrestha (062-BEL-347) Vikash Chhetri (062-BEL-348)
SUBMITTED TO:
DEPARTMENT OF ELECTRICAL ENGINEERING 1
ACKNOWLEDGEMENT
We would like to express our deep gratitude to the department of electrical engineering for giving us special chance to visit the hydropower and study in details. We can’t forget to greet our HOD, Dr. Indraman Tamrakar for boosting up our keen interest . It would be impossible for the visit of the hydropower without the permission of the NEA. Our sincere thanks goes to engineer of Modi Khola HP, Mr. Manoj Kumar Shah as well as other administrative members for their kind co-operation and nostalgic hospitality in our visit.
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ABSTRACT
This report is prepared on the basis of our visit to the hydropower as per our syllabus of Power Plant Equipment. We had choosen Modi Khola HP as the site of the visit which is located in Dimuwa, Parbat. We studied the civil, mechanical and electrical components of the HP. We visited the intake side, power house and the HV switchyard of the hydropower. We also studied about the components of the control room. From the visit, we were able to be familiar with the general components of the hydropower which will be helpful in our study.
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CONTENTS
ACKNOWLEDGEMENT........................................................................................................ 1 ABSTRACT ............................................................................................................................. 3 Introduction: ............................................................................................................................. 5 Civil Structure: ......................................................................................................................... 5 Operational Characteristics: ..................................................................................................... 7 Electrical Equipment: ............................................................................................................. 12 Turbine: .................................................................................................................................. 15 Power Transformers: .............................................................................................................. 19 Auxillaries: ............................................................................................................................. 21 Anciliaries .............................................................................................................................. 23 Generator Neutral Earthing .................................................................................................... 24 HV busbar: ............................................................................................................................. 27 Excitation System: .................................................................................................................. 27 Maintenance ........................................................................................................................... 31 Single line diagram 34
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Introduction:
Modi Khola Hydropower station is a peaking run of the river type with an installed capacity of 14.8 MW. The power house is located at Dimuwa, Parbat district and head work nearly two km from Dimuwa. Salient features: Type Rated net head Design discharge Catchment area Installed Capacity Percentage Exceedance Average annual generation Fountation stone laying Civil Construction start Trial Construction start Construction cost Financed by Run of the river 66.96 m 27.5 m3/ s 510 km2 14.8 MW Q45 92.5 GWh 17 May, 1996 May, 1996 9 Dec., 2000 30,000,000 USD Govt. of Nepal, NEA & EDCF ( Korea ) HMG/ N : NRs. 341,256,000 NEA: NRs. 1,358,344,776 EDCF: KW 12,399,024,230 ( US$ 9,672,641)
Civil Structure:
Components: 1) Weir: The weir is used to divert required amount of water to head work. H= 7.5 m, L= 33m. 2) Flushing Gate: Two set of flushing gates are provided to divert the water as well as to flush the sand as desired. The gates are operated electrically by hoisting motor. The hoisting motors are mounted on the hoisting deck structure. Two set of control panels are provided for individual operation. The gates can also be operated manually.One set of stop log is provided in the flushing gate. The stop log is used during gate maintenance. The stop log is operated by mono rail. 3) Intake Trash Rack ( Fixed Type ): Four sets of trash racks are provided in the headworks to check the floating debris that might damage the turbines. The trash racks are capable to withstand the water impacts, static loads and vibration which are likely to occur due to flow of water through the trash racks. 5
Four sets of stop logs and one set of monorail are provided in the headworks. 4) Intake Gate: One set of intake gate is provided in the headworks. The gate consist of gate leaf, guide frame and hoist. The gate is operated electrically by hoisting motor. The hoisting motor has a control cabinet located on the hoisting deck. 5) Sand Purging Gates: Two sets of sand purging gates are provided to flush the sand in the desanding basin. The gates have screwed spindle operating mechanism. The gates can be operated electrically or manually. 6) Tunnel Inlet Gate: One set of tunnel inlet gate is provided. The gate consists of gate leaf, guide frame, hoist and roller bearing. The gate is operated electrically and manually. The electrical system has a control cabinet located on the hoisting deck. 7) Tunnel Inlet Trash Rack ( Fixed Type ): Two sets of trash racks are provided downstream side of regulating pondage. The trash racks are used to check the floating debris. 8) Tailrace Gates: Two sets of gates are provided in the tailrace entrance. The gates consist of gate leaf and guide frame. The gates consist of gate leaf and guide frame. The gates operation is achieved by monorail. Tailrace gates are provided to check back flow from river in the event of power plant shutdown. The gates are provided at the draft tube outlet. 9) Steel Penstock: The length of the penstock is 328m. The steel penstock is concrete embedded and consist of straight pipes, bifurcation, reducing pipes, stiffner rings, manhole, etc. The diameter of the penstock pipes are 3.5 ,3.2 and 1.7 m. The thickness of the pipe shells are capable to withstand both the external and internal pressures and other loads. 10) Desanding Basin: Two desanding basins are used to trap sand before entering the pondage. L= 154.80 m, W= 10 m, H= 6.5 m. 11) Spillway: Spillway are situated along the side of desanding basin to release excess of water through the basin if back water force increases the level. Spillway is provided with holes to release the vacuum trapped inside it. 12) Pondage: It is regulating type of reservoir. It’s gross volume is 37900 m3 and as per the information collected the pond can supply load till five hours.
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13) Surge Tank: Surge tank is situated between the two hills and tunnel way is provided for surge tank. It has following specifications: H= 37.96 m, D= 9 m. The water level rises up during the monsoon season. 14) Cut and Cover: Two special type of tunnel between intake and desanding basin is situated underground. Comments: Huge debris were found at trashrack before entering to the tunnel even after using trash rack and purging gate from intake to head race tunnel. Need: The hydro power should be provided with pocket channel so that the power can be generated even if the main intake are in maintenance phase. Remarks: Initially mentioned major components are modified due to geological condition during construction. The waterway of the project was relocated in some portion and modified as required. Thus, the open surface type penstock was changed from cut cover to underground type. Pressure tunnel and vertical shaft were added. Three work audits were introduced to facilitate the construction of underground works. Out of those three adits, adit-1 is constructed as access to the headrace tunnel for repair and maintenance whereas adit-3 is constructed as the ventilation path for surge tank.
Operational Characteristics:
a) Graph of total generation in various month:
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Energy Generation in F/Y 2060/61
Actual Energy Generation (GWH)
Total Generation (MWH)
6000 5000 4000 3000 2000 1000 0
Mangsir
Shrawan
Bhadra
Ashwin
Chaitra
Falgun
Jestha Jestha
Poush
Magh
Months
Energy Generation in F/Y 2061/62
Actual Energy Generation (MWH) Total Forecast (MWH)
Energy Generation (MWH)
6000 5000 4000 3000 2000 1000 0
Mangsir
Shrawan
Bhadra
Magh
Ashwin
Chaitra
Poush
Falgun
Baishakh
Months
Baishakh
Ashadh
Kartik
Ashadh
Kartik
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Energy Generation in F/Y 2062/63
Actual Energy Generation (MWH) Total Forecast (MWH)
Energy Generation (MWH)
6000 5000 4000 3000 2000 1000 0
Mangsir
Shrawan
Bhadra
Ashwin
Chaitra
Falgun
Jestha
Poush
Magh
Months
Energy Generation in F/Y 2063/64
Actual Energy Generation (MWH) Target Forecast (MWH)
Enery Generation (MWH)
10000 8000 6000 4000 2000 0
Shrawan
Falgun
Baishakh
Ashwin
Baishakh
Mangsir
Months
Ashadh
Bhadra
Poush
Chaitra
Jestha
Kartik
Magh
Ashadh
Kartik
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Energy Generation in F/Y 2064/65
Actual Energy Generation (MWH) Target Forecast (MWH)
Energy Generation (MWH)
8000 7000 6000 5000 4000 3000 2000 1000 0
Mangsir
Ashwin
Magh
Baishakh
Shrawan
Months
b) Graph of wet season i.e. Jestha on hourly basis for a day:
Hourly Generation Energy
Hourly Generation Energy (MWH)
Generated Energy (MWH)
8 6 4 2 0
13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00
Time (Hours)
Ashadh
Bhadra
Chaitra
Falgun
Jestha
Kartik
Poush
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c) Graph for dry season i.e. Poush on hourly basis:
Hourly Generation Data
Hourly Generated Energy (MWH)
Generated Energy (MWH)
8 6 4 2 0
13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00
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Time (Hours)
d) Graph of total generation in Magh 2065 on daily basis for a month:
Daily Generation Energy
Generated Energy (MWH)
150 100 50 0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
29
Days
12:00
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Power factor variation: From the control room engineer, we got to know that the machine always runs between 0.95 to 0.99 lagging p.f. whatever be the change in load.
Electrical Equipment:
1) Generator: Two sets of generator of 8260 KVA alternating current generators designed and manufactured by ALSTOM, France are installed in the power house. Each unit is completed with stators, rotors, excitation system, surge absorbers, neutral ground resistors and fire protection system. The generator is directly coupled to the hydraulic turbine shaft. 2) Excitation system: Excitation system is brushless type and consists of ac exciter, rotary rectifiers, field circuit breaker and voltage regulating control cubicle, etc. 3) Neutral Grounding Resistor: Two sets of neutral grounding resistors are provided for two generators. The NGR consists of a disconnecting switch, metallic resistors, protective CT, etc. Specifications: Manufacturer Year Type Rated capacity Rated voltage Rated current Power factor Frequency Connection Rated speed Runaway speed Flywheel effect ( GD2 ) Efficiency SCR Insulation class Maximum temperature rise Capacitance of stator winding Stator winding resistance (at 75˚ C) Type of excitation ALSTOM, France 1998 salient pole 8260 KVA 6.6 KV 723 A 0.9 50 Hz. 2Y 428.6 rpm 820 rpm 60 ton m2 97.2% 1 F 140˚ C 0.12 µF to ord 0.033 Ω/Φ Brushless
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Reactances: Direct axis synchronous reactance ( X01) Positive sequence reactance ( X1 ) Negative sequence reactance ( X2 ) Zero sequence reactance ( X0 ) Direct axis transient reactance ( Xd ΄ ) Direct axis sub transient reactance ( Xd ΄΄ ) Quadrature axis reactance ( Xq = Xq΄ ) Quadrature axis sub transient reactance ( Xq ΄΄ ) 1.15 pu 0.1 pu 0.25 pu 0.1 pu 0.32 pu 0.23 pu 0.08 pu 0.27 pu
Losses ( at 0.9 pf ): a) Mechanical b) Armature iron c) Stray d) Armature resistance e) Field iron Weights: Weight of stator frame Weight of rotor Weight of miscellaneous 19793 KN 29389 KN 17435 KN 46 KW 46 KW 20 KW 49 KW 26 KW
Cooling: Closed air circulation with two sets of horizontal water/ air coolers are present. Rotor: Rated current Rated voltage No. of poles Capacitance of field ( Winding to ground ) Winding resistance at 18˚ C 778 A dc 64 V dc 14 0.03µF 0.057
Exciter: Ac exciter has rotating armature and stationary field. One of the field poles has been magnetized at the factory. The exciter is directly coupled on the top of generator. Rated voltage Rated current 64 V dc 792 A dc 13
Capacity Rotating diodes: Rotatory rectifier Configuration type No. of diodes Rated voltage Rated current
50.7 KW
silicon diode type GRAETZ BRIDGE 6 1800 V AC 860 A AC
All the diodes are 300 times size and protected by surge suppressors. Excitation Potential Transformers: Phases Frequency Voltage ratio Capacity 3 50 Hz. 6600/140 V 1500 VA
IGBT Transistor Rectifier: Rated voltage Rated current Excitation Control Cubicle: Manufacturer Model Control Power Source Minimum voltage Ceiling voltage Ceiling current Maximum current Mode of operation Manual set point with follower Digital potentiometer Automatic setting range Manual setting range Voltage accuracy Response time Under excitation limiter Surge current limiter Under frequency limiter Nominal current
600 V AC 150 A AC
ACEO, France R630 230 V ac / 24 V dc 0.3 Vn 1.6 Vn 1.6 In 20 A 35 A for 10 sec. p.f. and voltage 90% to 110% 60% to 110% 0.5% under steady state less than 30 ms
20 A
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Generator Bearings: Guide Bearing ( Upper and Lower ) Type Radial load Bore diameter Bore length Pad width Oil film thickness Pad number Diameter clearance Cooling oil flow Cooling water flow Average bearing temperature Peripheral speed Radial loss Babit radial loss
Oscillating Pads 50000 N 475 mm 125 mm 135 mm 0.015 mm 8 0.38 mm 30.341/ min 71/ min 57˚ C 10.5 m/ sec 3.58 KW 0.41 KW
Thrust Bearing Radial load Average thrust diameter Pad diameter Pad number Oil film thickness Maximum allowable temperature Relief depth Tangential speed Taper value Cold oil flow Thrust part losses Axial babit losses
54600 N 520 mm 125 mm 12 0.015 mm 90˚ C 0.021 mm 11.7 m/sec 1/6008 10.661/min 12.27 KW 1.26 KW
Turbine:
Two sets of 7.6 MW hydraulic turbine designed and manufactured by Kossler, Austria are installed in the powerhouse. Each turbine is complete with governors, inlet valves, pressure oil supply system, spiral casing, cooling water system, runner, guide vanes, servometer, etc.
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Technical Data: Type Gross head at 25 m3/s ( two unit operation ) Gross head at 12.5 m3/s ( one unit operation ) Net head at 25 m3/s ( one unit operation ) Net head at 12.5 m3/s ( one unit operation ) Rated discharge Rated output at net head ( two unit operation ) Rated output at net head ( one unit operation ) Rated speed Axial thrust at net head Radial force at net head Elevation of turbine C.L. Tail race water level ( two unit operation ) Tail race water level (one unit operation) Diameter of spiral inlet Mass of complete turbine Minimum guide vane closing time Minimum guide vane opening time Runaway speed
FSV 12.5/66 72.5 m 72.5 m 66.96 m 70.42 m 12.5 m3/s 7260 KW 7648 KW 428.6 rpm 246 KN 34 KN EL 863.7 m as L EL 863.8 m as L EL 863.5 m as L 1600 mm 30000 kg (approx.) 7 sec 8 sec 820 rpm
Runner: Manufacturer Direction of rotation Inlet diameter Outlet diameter Material No. of runner vanes Wicket Gate: No. of servometer Bore (diameter) Rod (diameter) Stroke Oil volume Operating pressure Head cover: Material No. of parts Out side diameter
Kossler, Austria clockwise 1125 mm 1250 mm A VESTA 2485 V4 GX5 Cr Ni 13.4 16
1 (single arm) 160 mm 90 mm 200 mm 100 bar
RST 37.2 2 pcs 1890 mm
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Spiral Casing and Stay Ring: Spiral casing and stay ring are an integral part of welded steel plate construction. Spiral Casing: Type Material Plate thickness Maximum design pressure Test pressure No. of sections
FSV 12.51666 RST 37.2 8, 10, 12 mm 9.6 bar 14.4 bar two
Stay Ring: No. of sections No. of vanes Material
one set 14 RST 37-2
Draft Tube: Type Material Plate thickness No. of sections Dimensions: Bend: Height Width Length
Elbow + Bend RST 37.2 12 mm Elbow+Bend
2140 mm 3000 mm 4065 mm
Cone: Height Φmin Φmax Outlet section dimension Turbine Shaft: Material Diameter Journal diameter Height
1395 mm 1200 mm 1850 mm 3 m * 1.835 m
CK 45 V, forged steel 325 mm 400 mm 2200 mm (approx.)
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Turbine Bearing Data: Journal diameter Bearing axial length L/D ratio No. of lobes Lobe arc length Offset factor Groove angle Set clearance Pre load Oil supply pressure Speed a) Normal b) Runaway Bearing load Specific load Bearing loss Minimum film thickness Life Material
400 mm 92.7 mm 0.2318 dim 6 dim 50 degree 0.5 dim 10 deg 469.9 micron 0 dim 0.1 N/sq. mm 428.7 rpm 820 rpm 30000 N 0.809 N/mm2 1.8 KW 29.97 micron 5000 times starting/stopping of machine Babbit
Shaft seal: Make Type Material
TITAN industry (AUS) Stuffing box TITAN 4431, braided from pure PTFE silky yarns with lubricant addative
Butterfly valve: Manufacturer Type Normal diameter Body material Disc material Type of seal Opening time Closing time Leakage rate (max) static head under fully closed condition Operating mechanism Operating pressure Closing mechanism
KLINGER/Austria butterfly 1700 mm st 52 st. 52.3 rubber 46 sec 50 sec as per DIN 3230 hydraulic 100 bar dead weight
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By pass valve: Type Size Type of operating
ND 200 150 mm Electrical motor operated
Power Transformers:
Two sets of 7800/8300 KVA capacity step-up transformers are provided for two units. The transformers are provided for two units. The transformer is of 3-Φ, two winding, oil immersed, forced air cooled/self cooling type . Temperature gauges are provided to monitor the gas pressure and pressure relief device is provided for safety of the transformer. Silica gel in a glass housing is provided to indicate the moisture content in the coil. Specification: Manufacturer Type Rated power Type of cooling Phases Frequency Vector group Standard Percentage impedance Ratio (HV/LV) Taps Temperature rise winding oil Type of insulating oil Exciting current at nominal tap and rated voltage (LV) Rated current (HV) Rated current (LV) BIL (HV/HVN/LV) Mass of insulating oil Total mass No load loss Load loss at 75˚ C (7.8 MVA base) Auxiliary loss taken by fan motor Fan motor rating Lightning Arrestor: Manufacturer Type Voltage Rating Continuous operating voltage Classification
Hyundai Heavy Ind. Ltd. Korea TL 0649 7.8/8.3 MVA ONAN/ONAF 3 50 Hz Ynd11 IEC-76 7.3% (at rated tap 7.8 MVA base) 132000 V/6600 V ± 2*2.5% 60˚/55˚ C IEC 296 class 10.8 A 34.1/36.3 A 682.3/726.1 A 550/150/60 KV 9900 kg 31800 kg 10.42 KW 42.2 KW 0.18 KW 0.4 KW
Bowthrope Emp Brighton UK MBA3 120 120 KV rms 96 KV rms Station type 19
Standard Pressure relief class Full wave impulse One min dry power frequency One min wet power frequency Total creepage Line discharge class Accessories provided Quantity
IEC 99-4 1991 20 KA rms (Class B) 545 KV crest 355 KV rms 320 KV rms 3320 mm 2 Polyfiber base insulator surge counter SC-12 3*3 (9 pcs.)
HV (132 KV) SF6 CB: One set of single pole/three pole SF6 CB for Pokhara feeder and two sets of 3 pole high speed for two power transformers are present. SF6 gas insulated type CBs are installed in the switchyard. The CBs for transmission line is capable of making single pole auto-reclosing operation. Specification: Manufacturer Type Rated voltage Rated normal current Rated frequency withstand Voltage (50 Hz, 1 min) a) to earth b) across open switching device Rated short circuit breaking current a) rms value of ac component of current b) percentage dc component Minimum opening time First pole to clear factor Rated transient recovery voltage a) Peak voltage b) Rate of rise Short time fault a) Surge impedance b) Peak factor Rated short circuit (peak) making current Rated out phase breaking current Rated duration of short circuit Rated operating frequency Rated line charging breaking current Rated cable charging breaking current Rated lightning impulse withstand voltage(peak) a) to earth
ALSTOM Energietechnik S1-145 F3 14031 145 KV 3150 A 50/60 Hz 275 KV 275 KV 40 KA 36% 35µs 1.5 249 KV 2 KV/ms 450 Ω 1.6 100 KA 10 KA 3 sec 0-0.35-CO-3 min-CO resp CO-15s-CO 50 A 160 A
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b) across open switches device
650 KV 650 KV
HV Switchyard (CT and PT)
CAPACITIVE VOLTAGE TRANSFORMER TYPE FREQUENCY RATED PRIMARY VOLTAGE RATED PRIMARY VOLTAGE RATED BURDEN THERMAL BURDEN INSULATION LEVEL (KV) MAXIMUM TEMP. RISE (*K) RATING KGT- 145 50 Hz 132/√3 KV 110/√3 V 200 VA 1000 VA 145/ 275 /650 65
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132KV CURRENT TRANSFORMER TYPE
RATING
CT 145/275/650 275KV/650KV 50 Hz 25 KA/1 SEC
INSULATION LEVEL FREQUENCY SHORT TIME CURRENT RATING RATED CONTINUOUS THERMAL CURRENT TOTAL CREEPAGE DISTANCE CORE RATIO CLASS
180 A
3625 mm
75/1,150/1 5P, 0.2, PS
Auxillaries:
1) Pressure Oil System: Pressure oil system for each unit consists of one pressure tank and two motor driven oil pump units, valves, controls and other accessories. The oil tank has sufficient volume of pressure oil for guide vane operaion, brakes and inlet valve opening. 2) Sump Pump: 22
The water from all the coolers is discharged to the tailrace. The seepage water inside the power house is collected to the sump pit and drained to the tailrace by means of two sets of motor driven drainage pumps. 3) Fire Protection System: Automatic CO2 release system is provided for the generator fire extinguishing. The complete system consists of nine nos. of CO2 cylinders, piping system, smoke and heat detectors, control cucible, solenoid valve, discharge nozzles, CO2 actuating cylinders,etc. 4) Station service transformer: One set of 250 KVA dry type transformer. 5) Battery and Battery Charger: Lead acid, splash proof, totally enclosed, maintenance free batteries are used and arranged in double tier rack. The charger is 3 phase full wave thyristor bridge rectifier with control and protection cucible. Type of DC voltage is 110 V which is used as auxillary dc source for the control, protection and annunciators for different equipments and emergency lightning in control room The batteries are installed in battery rooms and chargers in control room. 6) Elevator: An elevator with three stops with a capacity of 1000 kg is provided in power house. 7) Air conditioner: Two sets of air conditioners with exhaust system are provided in powerhouse. One set is provided in control room and other in office room. 8) Water level indicators: Water level indicators and measuring devices are provided in two locations at head work area.
Anciliaries
1. 2. 3. 4. 5. 6. 7. HP & LP compressor Pump Overhead Crane Ventillation System Air Conditioning Fire Alarm Emergency lighting Lighting and Power distribution 23
8. Telephone and Communication 9. Diesel Generator for backup Circuit Breakers: High voltage (132 KV) One set of single pole/ three pole for Pokhara feeder and two set of 3 pole high speed for two power transformers are used. SF6 gas insulated type CB’s are insulated in the switch yard and the CB’s for transmission line is capable of making single phase auto-reclosing operation. The ratings of CB’s are already mentioned in equipment section.
Generator Neutral Earthing
A neutral grounding resistor of 38.1 Ω (at 20°C) is used at generator neutral terminal. The specification is as follows: Neutral grounding resistor Type Standard Manufacturer Coil material Rated current Rated voltage Rated value Highest system voltage Time rating CT Disconnection switch
HKR-N IEEE 32 HANKOOK Industrial Resistor Co. SS4I 100A 6600/√V -38.1Ω at 20°C a6 KV/min IEEE32 60 sec 7.2 KV, 100/5A, 10P20, 50 UA 7.2KV, 400A, 1 pole
Design data for grounding system design including the details are given below: Average measured resistance at site 4.7 Ω Soil resistivity 147.7 Ωm Fault clearing time 1 sec Fault clearing time of CB 0.5 sec Depth of burial of conductor 0.5-0.7 m below earth Highest system voltage 145 KV Grounding conductor/ connection type bore copper conductor Ground fault current (max): 20KA 24
Grounding rsistance
less than 0.5 Ω
Grounding Grid The grouding grid system was employed both for power house as well as 145 KV switchyard. The data are as follows: Power house: Mesh: Cage type Totla conductor length: 400m 145 KV Switchyard Area: 40m×40m Mesh size: 5m×5m Total conductor length: 720 There is a common earthing system for high and low voltage equipments Lightning Arrester: There are three lightning arrestors used in transmission line feeder. 6 nos (two sets) of Lightning Arresters are used for two sets of power transformers. Specification: Manufacturer Type Voltage Rating Continuous Operating Voltage Classification Standard Pressure relief class Full wave impulse Min. dry power frequency Min. wet power frequency Total crrepage Line discharge class Accessory provided Quantity Bowthrope Emp Brighton UK MBA 3120 120 KV rms 96 KV rms Station type IEC 99-4, 1991 20 KA rms (Class B) 545 KV crest 355 KV rms 320 KV rms 3320 mm 2 Poly fiber base insulators surge counter SC-12 3*3 (9 pcs.)
Disconnecting switch: Four sets of disconnecting switches of three pole, single throw, central relating type, electromagnetic locking type are provided in the switchyard. Two sets are used for two power transformers, one in feeder side of 132 KV bus and one to the tr. Line side with earth switch. All the disconnecting switches are locally/remote operated type with the provision of manual operation also.
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Practice of testing: As per our information, there is no any testing practice or method to check if the soil resistivity is as the designed value or has changed. The grounding system has worked fine till date. Generator Protection Relays: Numerical relay is employed for generator protection. The main features of this relays are fast acting high response time, robust in construction, high reliability, programmable and longer life. 110/24 V dc/dc chopper is provided for relay auxiliary power source. Relay type: LG PG 11 Manufacturer: ALSTOM, UK Protection employed: i) Differential protection (87 G) ii) Voltage restraint overcurrent iii) Negative phase sequence iv) Reverse power v) Loss of field/excitation vi) Sensitive earth fault (stator) vii) Restricted earth fault viii) Over voltage ix) Under voltage x) Over frequency xi) Under frequency xii) Rotor earth fault relay
Generator Transformer Scheme: Unit generator transformer scheme is used. Each generator generates at 6.6 KV and 6.6/132 KV transformer is used for each generator and then only high voltage 132 KV busbar is constructed.
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CB B Y Δ Y Δ
G
G Fig: Unit Generator Transformer Scheme
HV busbar:
The single bus bar scheme is used in the hydropower. It has many disadvantages but as per our inquiry, we were informed that since bus bar maintenance is a rare phenomenon, so this scheme was considered right by the working personnel.
Excitation System:
The excitation system provides the necessary field current to the rotor winding of the synchronous machine. The amount of excitation depends on the power factor, speed of the machine and load current. For larger currents, lower speed and lagging power factor, the excitation requirement is more. The main requirement of an excitation system are reliability under all conditions of service, simplicity of control, ease of maintenance, stability and high transient response. In Modi Khola HP, the excitation system used is brushless type and consists of ac exciter, rotatory rectifiers, field circuit breaker and voltage regulating control cubicle. Brrshless excitation system: Fig. below shows the schematic of a brushless excitation system with rotating noncontrolled diode rectifier excitation system. It consists of an ac exciter and a rotating diode bridge mounted on generator shaft. A small PMG provides excitation current to the stator ac exciter field. The excitation current supplied to stator of ac exciter field is controlled by stationary AVR by automatic control. Brushless excitation system has no brushes/slip rings/commutators. The ac exciters-rotors and rotating diode rectifier bridge are mounted on the generator shaft without the need of brushes. Alternator field winding is connected to two terminal plated of Rotating Diode 27
Rectifier Bridge. The rotating rectifier bridge receives three phase input from ac exciter rotor and gives dc output to alternator field. The ac exciter stator has dc winding which receives dc power from PMG through AVR control.
The flow of excitation power is as follows: Field current for stator of ac exciter from PMG through AVR.
3-Φ rotor winding of AC exciter on generator shaft.
Diode bridge on rotor shaft of generator
Main field of generator.
Specifications of exciter used in Modi HP: Rated voltage 64 V DC 28
Rated current Capacity
792 A DC 50.7 KW
Rotating diode: Rotatory rectifier No. of diodes Rated voltage Rated current Excitation PT: Phases Frequency Voltage ratio Capacity IGBT Transistor Rectifier: Rated voltage Rated current Excitation control cubicle: Manufacturer Model Control Power Source Minimum Voltage Ceiling Voltage Ceiling Current Nominal Current Maximum current Mode of operation
Silicon diode type 6 1800 V DC 860 A AC
3 50 Hz 6600/140 V 1500 VA
600 V ac 150 A ac
ACEO, France R 630 230 V ac/ 24 V dc 0.3 Vn 1.6 Vn 1.6 Vn 20 A 35 A for 10 sec. Automatic/ Manual
Governor: The function of the governor is to keep the speed constant when the load on the turbine increases or decreases. To maintain the frequency of electric supply constant, the speed of alternator driven by the hydraulic turbine must remain constant. As the load changes, the governor changes the rate of flow of water to bring the speed back to normal speed and maintains a balance between the power input and output. A good speed regulating governor should be quite sensitive to the changes in shaft speed and should be rapid in action. However, it must not close the pipe so quickly as to cause water hammer in the penstock. The principal elements of a speed regulating system for hydraulic turbines are: 29
a) Speed responsive element- generally flyball mechanism. b) Relay valve to supply oil pressure to either side of servo- motor piston. c) Servo- motor along with oil pressure operated piston to move turbine control mechanism. d) Restoring mechanism to hold servo-motor in fixed position when input and output are equalized. e) Oil pressure supply required for action of servo-motor. The governor system in MHP is digital electro-hydraulic, cabinet actuator type with electrical speed sensing device (PID), gate opening control device each completed with pressure oil system, speed sensing device and nitrogen air filled air bladder system. The main functions of electronic governor are: Speed controller for starting the turbine from 0 to 100% speed. Load controller to adjust the power output by controlling the guide vane opening. Frequency controller for isolation operation.
Fig: Pressure oil system of Governor Operation modes: Manual operation mode: a) Service mode (automatic) b) Shut off mode. Three different shut off modes are possible. Normal stop Quick stop Emergency stop Interface module. a) Guide vane position limit switches Guide vane closed position Guide vane no load position Guide vane full open position b) Turbine speed points Turbine standstill: less than 1% of nominal speed Generator brakes ON: less than 30% of nominal speed Turbine under speed: less than 80% of nominal speed Excitation system ON: greater than 90% of nominal speed. Synchronization ON: 95%-105% of nominal speed. Turbine Over Speed: 120% (warning) of nominal speed Turbine Runaway Speed: 140% (trip) of nominal speed. Pressure oil system of Governor: 30
When the load on the generator decreases, the speed of the generator turbine coupled set increases. Since the shaft of the centrifugal rotor is coupled with generator turbine shaft by some means, the rotating fly-ball moves away due to centrifugal force. This will cause upward movement of the sleeve. As the sleeve moves up, the lever turns about the fulcrum and the piston rod of the control valve moves downward. Subsequently, the valve V2 opens and valve V1 closes as shown in the figure. Hence the relay cylinder gets more oil in L-space and the guide vane closes thus by reducing the water inlet into the turbines so that speed reduces again to the pre-set value. On the other hand, when load on the generator increases, the speed of the generator turbine set decreases. This will cause the rotating flyball to move downward. Then the sleeve moves down and the lever rotates about the fulcrum and the piston rod of the central valve moves upward. Subsequently, the valve V1 opens and V2 closes. Now, the oil under pressure will move through V1 and exert pressure on M-space of the relay cylinder results opening of guide vanes thus by increasing amount of water flow into the turbine so that speed increases again to a pre-set value. In Francis turbine, a relief valve called as pressure regulator, is provided in the penstock. When load decreased, the relief valve opens, diverting the water directly to the tailrace and thus avoiding water hammer in penstock.
Maintenance The main problems in the electrical equipments were related to PT, circuit breakers, battery chargers, penstock leakage, need of epoxy painting at penstock, cleaning of drainage in power house and power canal, and transformer. As a coincidence, as soon as we reached there, the trashrack was cleaned with the help of crane. As far as we could trace out, there was no maintenance schedule. According to the employees, there was no maintenance schedule, however practice was to supervise, monitor and go for maintenance work if any fault in the system arises. The management of maintenance was unambiguously weak obviously reflected from the lack of the proper maintenance schedule. The staffs for maintenance were divided into three teams for maintenance: a. Civil Structure Maintenance Team b. Mechanical Components Maintenance Team c. Electrical Components Maintenance Team Each of the staffs in their related field was the technical expertise in their own field. Each team had its own supervisor, overseer, technicians and an engineer. The main problem of the power plant were the turbine erosion, generator cooling system, transformer, battery back-up system being not in proper condition, the power transformer cooling system, the erosion of guide vanes etc.
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