ELECTRICAL COMPONENTS OF MHP

Electrical Component Design Standard Electrical Components of MHPs

Generator Control Equipment Protection Equipment Earthing System Transformers Primary Distribution System Secondary Distribution System Power Limiting Device Metering Equipment

Generators Types

Synchronous Generator(SG) Asynchronous Generator(Induction Motor as

Generators or IMAG)

Synchronous Generator(SG)Asynchronous Generator(Induction Motor as Generators or IMAG)

Principle of Synchronous Generator

Exploded View-Synchronous Generator

Schematic diagram of an Induction Generator

Principle of an Asynchronous Generator

Exploded View of an Squirrel Cage Induction Machine

Comparison Between SG and IMAGSynchronous Generator(SG)

Asynchronous Generator(IMAG)

Generally competitive in high power range >100kVA

Not readily available in developing countries

Complicated & liable to breakdown; High maintenance cost

Complicated Needs to be ordered

Generally competitive in small power range <30kVA

Readily available in small power range

Rugged & simple; Reliable and low maintenance cost

Simple Possible with normal design

(4P or more)

Comparison Between SG and IMAG contd…Synchronous Generator(SG)

Asynchronous Generator(IMAG)

Self exciting Can supply reactive power Higher full load efficiency; good

part load efficiency High Rotor Inertia Good Voltage regulation Generally good technical support

available Good over load and motor start

capacity Can be made to sustain SC and

clear fault Fairly simple Selection,

calculation

External source needed for excitation External source needed for supplying

reactive power Lower full load efficiency; poor part

load efficiency Low Rotor Inertia Poor Voltage regulation Technical support generally does not

exist May de-excite under such condition De-excites under SC and might need

re-magnetization Needs calculation and re-adjustment

Types based on relative position of Armature and Field Winding Rotating Armature Type(Brushed Type)

Generally not preferred in MHPs due to high maintenance cost(Slipring and Brush)

Rotating Field Type(Brushless Type) Exciter Armature is mounted in the same shaft as that of

the main generator and AC voltage generated in the rotating exciter armature is converted to DC voltage though the use of 6 Nos of rotating diodes to feed to the main generator rotating field winding

Synchronous Generators(bit more) Generally Cylindrical Rotor Type and Salient Pole

Type Cylindrical Rotor Types(4/2Pole machine) are high

RPM(mostly 1500/3000RPM for a 50Hz Generation and 1800/3600RPM for 60Hz Generation) and invariably used with Steam Turbine/Gas Turbine

Salient Pole machines are generally low RPM type and generally manufactured in RPM range starting from 142.8 RPM and upto 1000RPM for a 50Hz system.

Practical Approach As standard Diesel Engines (as prime mover for DG

sets) manufactured in Indian Subcontinent have RPM as 1500, off the shelf Salient Pole Generators are also designed for an RPM of 1500 to suit direct coupling.

In MHPs of Nepal,1500RPM Synchronous Generator becomes natural choice.

Few renowned manufacturers of Synchronous Generators(in Mini/Micro Hydro range) of India origin are KEL, Kirloskar Electric company,Cummins,Crompton,Greaves,Jyoti etc.

Practical Approach KEL make Generators have Compounding

Transformer based Excitation System(to maintain voltage within +-5%) and no need of electronic AVR for size upto 125kVA,Starting Current Capability upto 2-3 times rated current.

Compounding Transformer based

Excitation System

Synchronous Generator Sizing Various Factors to Consider

Temperature factor(A) Altitude factor(B) ELC Correction Factor(C) Power Factor(D)Generator kVA=Installed Capacity in kW/(A*B*C*D)

Generator rating FactorsMaximum ambient temperature (oC)

20 25 30 35 40 45 50 55

A Temperature factor 1.10 1.08 1.06 1.03 1.00 0.96 0.92 0.88

Altitudes (m) 1000 1250 1500 1750 2000 2250 2500 2750

B1 Altitude factor 1.00 0.98 0.96 0.945 0.93 0.915 0.90 0.88

Altitudes (m) 3000 3250 3500 3750 4000 4250 4500

B2 Altitude factor 0.86 0.845 0.83 0.815 0.8 0.785 0.77

C ELC correction factor 0.83D Power factor When load is only light bulbs 1.0

When load includes tube lights and other inductiveloads

0.8

Synchronous Generator Sizing contd…Generator size as calculated above is multiplied by 1.3 to

allow for the following conditions The output of the turbine could be higher than expected due

to better design(efficient design) If large motors (> 10% of generator size) are supplied by the

generator, the generator should be able to handle the starting current.

The generator is always generating rated power when using an ELC

Practical Calculation Suppose we need to find Correct Generator Size for a 15kW

installed capacity scheme. Assume ambient temp of 40 deg Celcius,A=1 Altitude above mean sea level as 1250m,B=0.98 ELC correction Factor,C=0.83 Load as a mix of predominantly Incandescent lighting(PF:1)

plus some inductive load like FT(PF:0.5) so as to give an average power factor,D = 0.8

Generator Size in KVA considering safety factor of 1.3=1.3*15/(1*0.98*0.83*0.8)=29.966=30kVAHence we see that for all practical purposes the Generator Size

in kVA to be chosen is 2 times the installed capacity in kW

IMAG IMAG is Squirrel Cage Induction Motor run as

Induction Generator Induction motor when rotated above its rated

synchronous speed by some prime mover(hydraulic turbine in case of MHP)and fed required excitation though capacitors or through import of reactive power from Grid, Generates Electrical Power. Slip in such case is negative.

Very Rugged Construction due to general ruggedness of squirrel cage rotor. Highly common to use in Pico/Peltric Sets.

Practical Approach Though IMAG is cheaper in range up to 30kVA size, SG

is preferred over IMAG in sizes above 10kW due to practical considerations

IMAG is however used almost invariably in Pico/Peltric (Pletric: Pelton+Electric Generator) sets upto 5kW

MHP in operation: Synchronous Generator in Focus

MHP in operation: Double Jet Pelton Turbine in Focus

Control Equipment Voltage Control Equipment

Compounding Transformer for voltage regulation in the range of +-5% for generators upto 125kVA

Additionally fitted electronic AVR for sizes >125kVA• Frequency Control Equipment

Electronic Load Controller(ELC) upto installed capacity 100kW

Hydromechanical/Electronic/Digital Governor for installed capacity>100kW

Electronic Load Controller(ELC) An equipment which allows constant power

generation( no flow control) and maintains constant system frequency by dumping additional generated power to electric immersion heaters used as ballasts.

ELCs are considered to be cheaper alternatives of costly flow control governors in case of MHPs.

Since ELC’s use Thyristors as switching device, a lot many harmonics are generated requiring derating of generators(ELC correction Factor: C)

ELC Panel Board

Inside View of an ELC Panel Board

ELC Continued Sizing of ELC and associated Ballast for installed

capacity upto 50kW Size of ELC=Installed Capacity in kW Size of Ballast=1.2*Installed Capacity in kW(to take care

of possible additional power generated due to better system efficiency)

Sizing of ELC and associated Ballast for installed capacity > 50kW and upto 100kW Size of ELC=0.6*Installed Capacity in kW Size of Main Ballast=1.2*0.6*Installed Capacity in kW Size of Extension=0.4*Installed Capacity in kW

Protection Equipment Plant protection Equipment Outdoor Protection Equipment

Plant protection Equipment

Overcurrent Protection Thermal Adjustable Type Moulded Case Circuit Breaker

for Protecting generator from normal overload/short circuit in the distribution line(Rating:25% oversized)

HRC fast fuse for Protecting ELC power components(Thyristors etc) from

short circuit/earth fault occurring in ballast circuit

Plant protection Equipment contd…

Overvoltage/Undervoltage Protection(set at 110% and 85%) Under voltage Protection optional for plants <30kW

Over Frequency/Under Frequency Protection(set at 51.5Hz and 48.5Hz) Under frequency Protection optional for plants <30kW

Varistor (to protect ELC board components from transient over voltages)

Outdoor Protection Equipment Lightening Arrestors to protect lines from Lightning

0.5kV Metal Oxide(Gapless)Arrestors for 400Volt Line 9kV,5kA Metal Oxide(Gapless) Arrestors for 11kV Line• Drop Out(DO) Fuse to Protect Transformers from Over

Current

Earthing System All non current carrying metal parts to be earthed to

protect equipment/human being from safety hazard An earth Continuity Conductor(ECC) to run though

the perimeter of the powerhouse Generator body, Turbine Enclosure, Belt Guard, ELC

Panel board, Ballast Tank, Metallic Door, Window Frame etc (if used) all to be connected to ECC

Earthing System Continued Generator(Star Connected) Neutral to be earthed

separate from the earthing of non current carrying metal parts

Lightning Arrestors fitted at first pole should have separate earthing from that of neutral GND.

600mm*600mm*3.15 mm copper plate is used for earthing purposes(weighs around 10kg)

Earth Resistance value <5 Ohm is desirable but in no case should exceed 10 Ohm.

Typical Values for Soil ResistivityType of soil Moderate climate,

high rainfall (>500mm/year)Desert climate

(<250 mm/year)

Typical value(Ωm)

Range ofmeasured values

(Ωm)

Range of measured values (Ωm)

Clay 10 5-20 10-1000

Porous limestone 50 30-100 50-300

Sandstone 100 30-300 >1000

Quartz, marble, carbonaceous limestone

300 100-1000 >1000

Granite 1000

Slate, petrification, gneiss, rock of volcanic origin

2000

Transformers Few very simple practical formulas as below

VHV / VLV = THV / TLV , where the terms have their usual meanings.(Note that the voltages in the formula are to be phase voltages and not line to line voltages and the formula equally applies for both single phase and three phase transformers)

IHV =kVA/(√3*kV HV) and ILV =kVA/(√3*kV LV) for a 3 Phase Transformer and

IHV =kVA/kV HV and ILV =kVA/kV LV for a 1 Phase Transformer

Transformers contd…. Type based on purposes

Step Up Transformer(Used outside MHP P/H to step up voltage level form generation voltage ie 400V to primary distribution voltage ie 11000V)

Step Down Transformer(Used at loadcentres to reduce primary distribution voltage of 11000V to utilization voltage of 400V)

Oil Immersed Transformer

Plan and Legend of a Transformer

TYPICAL RATINGS AND FEATURE FOR 100kVA STEP UP TRANSFORMERTABLE 1

Type Three-phase, 11/0.4 kVRated capacity 100kVARated voltage - Primary 400V

- Secondary 11000 VHighest system voltage

- Primary 440 V- Secondary 12kV

Rated Frequency 50 HzConnection

- Primary Gnd. Wye - Secondary Delta

Cooling System ONAN Vector group YNd 11 Rated impedance voltage 3.5 - 4.5% BIL for windings andbushings for secondary side 75 kV Withstand voltage, 50 Hz, 60 Sec. - Primary 28 kV - Secondary 3 kV No load tap changer ± 2.5% to ± 5% on HV side

Mounting PlatformInsulation levels (IEC) 76 LI 75 AC 3/AC 28Insulation temperature class (IEC 76) A Maximum allowable noise level at 3 metre hemispherical radius <44 dBApplicable standard IEC

TYPICAL RATINGS AND FEATURE FOR 50kVA STEP DOWN TRANSFORMERTABLE 2

Type Three-phase, 11/0.4 kVRated capacity 50kVARated voltage - Primary 11 kV

- Secondary 400/230 VHighest system voltage

- Primary 12 kV- Secondary 440 V

Rated Frequency 50 HzConnection

- Primary Delta - Secondary Gnd. Wye

Cooling System ONAN Vector group Dyn 11 Rated impedance voltage 3.5 - 4.5% BIL for windings andbushings for primary side 75 kV Withstand voltage, 50 Hz, 60 Sec. - Primary 3 kV - Secondary 28 kV No load tap changer ± 2.5% to ± 5% on HV side

Mounting PlatformInsulation levels (IEC) 76 LI 75 AC 28/AC 3Insulation temperature class (IEC 76) A Maximum allowable noise level at 3 metre hemispherical radius <44 dBApplicable standard IEC

Sizing of Transformers Size of Step Up Transformer is based on following factors

Installed plant capacity Average Power Factor of Load

Calculating size for 30kW Installed Plant size with average power factor of load as 0.8 kVA Rating of Transformer required=Installed Plant

Size/Power factor=30/0.8=37.5 Now since 37.5kVA not being an standard transformer rating,

we should chose nearest higher rating ie 50kVA Necessary provisions may have to be incorporated for the

possibility of future load growth(in case it is planned to replace turbine generator set at a later date with more efficient turbine and suitable size generator)

Transmission and Distribution(T& D) Types

Suspended Overhead on Poles(Overhead Line) Buried Cables(Underground Cable)

OH Line(Features) Less Expensive Frequent Maintenance Required Less cost per maintenance Does not look aesthetically good in dense locality Not preferred in areas which experience heavy snowfall

T& D contd… UG Cable

Costly Installation(if done per standard 6 times costly compared to OH)

Less Maintenance Higher cost per maintenance Highly desirable in densely located areas Obvious choice for areas which experience heavy snowfall

Design Criteria of OH distribution line(as MHPs can not afford the luxury of UG cable) Maximum allowable voltage drop at the farthest end shall not

exceed 10% Maximum TL line to line voltage in case of isolated MHPs is 11kV ACSR(Aluminium Conductor Steel Reinforced) conductors are the

norms of the day(as against hard drawn copper which were used in earlier days)

Transmission and Distribution(T& D) contd…

Typical 3 Phase 4 Wire LT distribution

Primary Distribution Line Hardware and Fittings

Voltage Drop Calculation L-L Voltage Drop in case of 3-Phase balanced

distribution=√3 * I* (R*Cos φ +X*Sin φ)VoltWhere I is the load current in ampere, R is the

Resistance/Phase in ohm, X is the reactance/Phase in ohm ,and φ is the power factor angle.

Voltage Drop in case of 1-Phase 2 wire system= 2 * I* (R*Cos φ +X*Sin φ)VoltWhere symbols have their usual meaning

Sizes and designations of ACSR conductors used in micro-hydropower schemes

Name Current rating in still air (amp) Resistance (Ω/km)

Inductive reactance at 50 Hz and 50 cm spacing (Ω/km)

Approximate weight(kg/km)

Squirrel (20 SQ.MM AL) 76 1.374

0.355 80

Gopher (25 SQ.MM AL)85 1.098

0.349 106

Weasel(30 SQ.MM AL)95 0.9116

0.345 128

Rabbit(50 SQ.MM AL)135 0.5449

0.335 214

Otter(80 SQ.MM AL)185 0.3434

0.328 339

Dog(100 SQ.MM AL)205 0.2745

0.315 394

Place Low tension High tension (1-11 kV)

Off road 4.5 m 5 m

Beside motorable road 5 m 5.5 m

Across from motorable road 5.5 m 6 m

Minimum vertical clearance for overheadconductors

A horizontal clearance of at least 1.2 m shall be observed in all cases.

Span (m) 20 30 40 50 60 80

Minimum sag (mm) 13 30 54 84 121 210

•The minimum sag for conductors up to 11 kV can becalculated by the following empirical formula:

2

8172

=

.Ld

where, d = sag in mL= length of span in m

Sag for spans of overhead cablesThe value of sag should be included when determining the ground clearance of atransmission line.

Poles used in MHPs Due to cost consideration , LT(Low Tension) poles are

entirely made out of hard wood. Seasoned Pole would be better option. Other possibilities could be PSC(Pre Stressed

Concrete) pole or steel tubular pole(where there is no cost constraint) or steel telescopic poles

Steel Tubular(either swaged type or folding type) poles are the norms for 11kV Distribution Line and also for the double pole structure which supports transformers.

Minimum length (m) 6.0 7.0 8.0 8.5 9.0

Maximum span (m) 35 35 35 35 35

Buried length (m) 1.0 1.2 1.5 1.7 2.0

Minimum top diameter (mm) 125 140 150 175 175

Minimum ground clearance (m) 4.0 4.6 5.5 5.8 6.1

Pole specification for hardwood poles

Insulators and Hardware Shackle Insulators of suitable voltage ratings (suitable

creepage path)---Up to 1000V OH Line Shackle Insulators --- Supported(Clamped)by D-Iron

set which is a clamp made out of galvanized steel Pin Insulators of suitable creepage path are used for

straight runs of 11kV Line Disc Insulators along with suitable tension Set are

used for deadening a line. Suitable Stay sets are provided at first pole, at line

end, at all poles set at an angle and at every fifth pole even if the poles are in a straight line

S. N Size Dimensions Weight Corresponding conductor

1 Small 55 mm x 55 mm

200 gm Squirrel, service wire

2 Medium

75 mm x 90 mm

600 gm Gopher, weasel and rabbit

3 Large 100 mm x 110 mm

1300 gm

Dog

Shackle Insulator specifications

Spacing between Conductors Conductor spacing

300mm upto 400V system 400mm upto 1000V System 600mm in case of 11kV line

Horizontal or Triangular alignment(ACSR conductor) Spacing between conductors=√(d+V/150)Where Spacing in m, V is the L-L voltage in kV and d is

the sag in metres. In general,70% should be added as safety factor on the value calculated above.

Service Wire Should be double insulated Additional voltage drop in service wire not to exceed

2% Service wire shall be of the same material as that of

line conductor to avoid electrolytic corrosion. Minimum size of 4-6 sq.mm. to be used depending on

the span

Power Limiting Device MCB of suitable rating is generally used 0.5A(Rated Current),5kA(Breaking Current) MCBs are

used for prescribed power of 100W The purpose is to disconnect the line against sustained

overload(control) and also to provide protection against short circuit in the household wiring.

Metering Types available

Electromechanical Type(Cheap but less accurate) Static Type(Costlier than EM type but far more accurate) Digital Type(Multifunctional Meter):Far more costlier

than Static Type but has tremendous advantages like far superior accuracy, Possibility of data logging, data communication, Multifunction Capability, Possibility of field adjustment