lecture8 electrical systems notes 2016

8/18/2019 Lecture8 Electrical Systems Notes 2016

1/17

Energy Management (EN 410/607) Notes 1

En erg y Man ag em en t (EN 607/ EN 410) 2016 San tan u Ban dyop ad hyay

1

Electrical Systems

Lecture 8

En erg y Man ag emen t (EN 607/ EN 410) 2016 San tan u Ban dyop ad hy ay

2

Outline

Topics Number of

lectures

(approx)

Electrical Systems: Demand control,

power factor correction, load

scheduling/shifting, Motor drives-

motor efficiency testing, energy

efficient motors, motor speed

control.

2 (17, 19

Feb)


Electrical Load


4

Electrical Energy Terms

• Direct Current

• Alternating Current

• Current• Voltage

• Resistance

• Ohm' Law

• Frequency

• Apparent power (kVA)

• Reactive power

• Active Power


2/17



5

Load curve of a typical day –MSEB(8/11/2000 source: WREB annual report-2001)

10260 MW9892 MW

6000

7000

8000

9000

10000

11000

1 2 3 4 5 6 7 8 9 1 0 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Time hours

D e m a n d , M W

morning

peak

Eveningpeak


6

Analysis of System Load Curve

• A load curve defines power vs time

• Load Factor = (Average Power)Peak Power

System Load Factor

• Capacity Factor (plant load factor)

= Energy generated by a plant

Energy generated if operating at max capacity


7

Loads and Demands• Connected Load - sum of the continuous (or

nameplate )ratings of equipment.

• Contract Demand: electric power that the

consumer agreed upon with the utility• Average Demand

• Load Factor - ratio of the average demand tothe maximum demand

• Demand factor- ratio of maximum demandto connected load

• Power Factor (PF) is the ratio of activepower (kW) to the apparent power (kVA)


8

Electricity Tariff

• Residential - Block - Energy charge

• Agricultural – Horsepower

• Industrial – Two part –Energy, Demand

• Commercial – Block

• Public Works


3/17



9

Electricity Tariff-Components

• Maximum demand Charges

• Energy Charges

• Power factor penalty or bonus rates

• Fuel cost adjustment charges

• Electricity duty charges levied w.r.t units consumed

• Meter rentals

• Time Of Day (TOD)

• Penalty for exceeding contract demand

• Surcharge if metering is at LT s ide in some of the utilities


10

Example (MSEB LT tariff-1/6/08)

• LT Domestic

Less than 30 kWh/m Rs 0.40/kWh +Rs 3 service0-100 kWh/month Rs 2.05/kWh

101-300 kWh/month Rs 3.90/kWh

301-500 kWh/month Rs 5.30/kWh

>500 kWh/month Rs 6.20/kWh

Service connection: Rs 30/single ph, Rs 100/3-ph

Additional Fixed charge of Rs. 100 per 10 kW load

or part thereof above 10 kW load shall be payable.


11

Example (MSEB LT tariff-1/6/08)

• LT Non-Domestic

Less than 20 kW Rs 3.40/kWh +Rs 150 service

20-50 kW Rs 5.50/kWh + Rs 150/kVA>50 kW Rs 7.50/kWh + Rs 150/kVA

• LT Public Works

20-40 kW Rs 1.75/kWh + Rs 50/kVA

• LT Agriculture

Non-metered: Rs 2.41/kW

Metered: Rs 1.10/kWh + Rs 20/kW


12

Example (MSEB HT Tariff-1/6/08)

• HT Industrial

Demand charges Rs 150/kVA/month

Energy charge Rs 4.00-5.00/kWh

TOD – Energy charge

• 2200 hrs – 0600 hrs (-0.85)

• 0600 hrs – 0900 hrs 0

• 0900 hrs – 1200 hrs 0.80

• 1200 hrs – 1800 hrs 0

• 1800 hrs – 2200 hrs 1.10


4/17



13

Load curve of a typical day –MSEB

TOD Tariff

10260 MW9892 MW

6000

7000

8000

9000

10000

11000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Time hours

D e m a n d , M W

morning

peak

Eveningpeak

-0.85 0.80 1.10


14

Impact of Load Factor on Price

Base tariff

0

2

4

6

8

0 0.2 0.4 0.6 0.8 1

Load factor


15

Maximum Demand Control

• Typically, demand charges constitute a

considerable portion of the electricity bill

• Integrated load management to effectively

control the maximum demand

– Generate load curve

– Analyse load curve for various demands

– Identification of critical and re-schedulable

loads for maximum demand and TOD tariff


16

Maximum Demand Control

• Rescheduling of large electric loads and

equipment operations

– prepare an operation flow chart and a process chart

• Reduce the maximum demand by building up

storage capacity

• Shedding of Non-Essential Loads

• Operation of Captive Generation

• Maintain the desired Power factor of system


5/17



Power Factor


18

Representation in Phasor diagram

Real Axis

I m a g i n a r y o r j a x i s

V V

I I

sincos je j


19

Basics of Power

)()()( t it vt P

)cos(2)cos(2 t I t V

)cos()cos(2 t t VI

)cos()cos(coscos2 B A B A B A

)2cos()cos()( t VI VI t P

Constant w.r.t time Sinusoidally varyingw.r.t time


20

Basics of Power

T

dt t pT

P0

)(1

)cos( VI

)cos( Power factor

Phase angle difference between V and I

power factor angle


6/17



21

Power terms

VI S

cosVI P

sinVI Q

S

P

Q

kVA

kW

kVAr

22 QPS S – Apparent Power

P – Active Power

Q – Reactive Power


22

Power factor correction

))tan()(tan( 21 P

1

S1

P

Q1kVar

2 Q2

S2

Capacitor rating =Q1-Q2

Maximum Demand Saving

= S1-S2 kVA


23

Power Factor Correction

• Static Capacitors (Fixed/ Switchable)

Automatic PF Correction

• Reduced line current ( and I2R losses)

• Improved Voltage

• Reduced Maximum Demand

• Capacity for expansion

• Reduction in tariff


24

Cost Benefits

• Reduced kVA (Maximum demand) charges in

utility bill

• Reduced distribution losses (kWh) within theplant network

• Better voltage at motor terminals and improved

performance of motors

• A high power factor eliminates penalty charges

imposed and may be reduction in utility bill

• Capacity deferred costs


7/17Energy Management (EN 410/607) Notes 7


25

Location of Capacitors

• Maximum benefit of capacitors is derived bylocating them as close as possible to the load

• The rating of the capacitor should not be greaterthan the no-load magnetizing kVAr of the motor,if connected directly (over voltage protection)

• motor manufacturers specify maximum capacitorratings

• A circuit breaker or switch will be required if acapacitor is installed for many appliances

• With high voltage breaker, capacitor bank tofeeder


26

b

c

Ia

Ib

Ic

Iab

Ibc

Ica

a

Delta connected load


27

Ic

Ia

Ib

a

bc

n

Y connected load


28

)cos(3 L L I V P

Delta connected

L I P I 3 LV PV

Y connected

PV 3 LV P I L I

Power in 3-phase circuits




Load Management


30

Common LM Options (SEB)

• Staggering of working hours of largeconsumers

• Staggering of holidays of large consumers

• Specified energy and power quotas for major

consumers

• Rostering of agricultural loads

• Curtailment of demand - service interruptions

(load shedding)


31

Load Management Options

• Direct Load Control (DLC) – Utility hascontrol of directly switching off customer

loads• Interruptible Load Control (ILC)- Utility

provides advance notice to customers toswitch off loads

• Time of Use (TOU) Tariffs – price signalprovided – customer decides response


32

Sample Industrial Load Profile (Mumbai)




33

ILM Research Objective

• Determine optimal response of industry for

a specified time varying tariff – develop ageneral model applicable for different

industries

– Process Scheduling- Continuous/ Batch

– Cool Storage

– Cogeneration


34

Process Scheduling

• Variable electricity cost normally not included

• Flexibility in scheduling• Optimisation problem – Min Annual operating

costs

• Constraints – Demand, Storage and equipment

• Models developed for continuous and batchprocesses (Illustrated for flour mill and mini steelplant)

• Viable for Industry


35

Process Scheduling

• Batch processes- batch time, quantity,

charging, discharging, power demand

variation (load cycles)

• Raw material constraints, Allocation

constraints, Storage constraints,

Sequential Constraints, maintenance

downtime


36

30 T MeltingArc furnace

Bar mill

Wiremill

40 T Melting Arc

furnace

St. steelScrap mix or

Alloy steelscrap mix

Alloy steel

scrap mix

Convertor (only for

St Steel)

LadleArc

furnace

VDor VOD

station

Bloom caster

Billet caster

Bloom mill

ooo

ooo

Reheat furnace

Reheat furnace

Reheat

furnace

Wireproducts

for finalfinish

Rods, Bars for final

finish

Open store

Open store

Open store

Open store

STEEL PLANT FLOW DIAGRAM




37

0

10

20

30

40

50

60

Time hours

L o a d M W

Optimal with TOU tariff

Optimal with flat tariff

2 4 6 8 10 12 14 16 18 20 22 24

Steel Plant Optimal Response to TOU tariff


38

Process Scheduling Summary

Example Structure Results Saving

Flour Mill

Continuous

Linear, IP

120 variables

46 constraints

Flat- 2 shift

- 25%store

TOU-3 shift

1%

6.4%

75%peak

reduction

Mini Steel

Plant

Batch

Linear, IP

432 variables

630

constraints

Flat

TOU

Diff loading

8%

10%

50% peak

reduction


39

LM Options

• Cool Storage – Chilled water , Ice,

Phase change material storage- operatecompressor during off-peak

• Water pumping systems

• Cogeneration – Operating strategy

• Power pooling with other industries

• Evaluate Process Storage possibilities


40

Cool Storage

• Cool Storage – Chilled water operate compressorduring off-peak

• Commercial case study (BSES MDC), Industrial casestudy (German Remedies)

• Part load characteristics compressor,pumps

• Non- linear problem – 96 variables, Quasi NewtonMethod

• MD reduces from 208 kVA to 129 kVA, 10%reduction in peak co-incident demand, 6% bill saving




41

Cool Storage of Commercial Complex

-under TOU tariff

129 kVA

208 kVA

0

50

100

150

200

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24Time hours

k V A

with optimal cool storageLoad following (without storage)


42

Analysis of Utility’s Load Profiles

• Utility :Taloja EHV (100kV/22kV) substation

• Analysis of load profiles (before and after theintroduction of TOD tariff) Total substation (system)

Industrial feeders

Express feeders

• 191 log sheets are available

• Hourly average demand (pf = 0.98)


Motors


44

End Use (HT Industry)

• Motor/Pump/Fan 52 %

• Air Compressor 9 %

• Air Conditioning/Refrig. 5 %

• Melting 16 %

• Electrical Heating 11 %

• Lighting 4 %

• Others 4 %


12/17




49

DC and Synchronous Motors

• DC motors are used in special applicationswhere high starting torque or where smoothacceleration over a broad speed range isrequired

• AC power is fed to the stator of the synchronousmotor. The rotor is fed by DC from a separatesource.

• The rotor magnetic field locks onto the statorrotating magnetic field and rotates at the samespeed.


50

Motor Characteristics

• Synchronous Speed (SS) = 120 f/p

• Slip (s) = 1 - (Rated speed/SS)• Power Factor: lagging due to induction

– At part load, the active current reduces.

– However, no reduction in the magnetizing

current (proportional to supply voltage)

– Reduction in power factor


51

Motor Effic iency

• Ratio of mechanical output to electrical input

• May be determined directly or indirectly through

intrinsic losses• Efficiency is a function of operating temperature,

type of motor, speed, rating, etc.

• Squirrel cage motors are normally more efficient

than slip-ring motors

• Higher-speed motors are normally more efficient

than lower-speed motors


52

Rotor copper

loss

Losses

Constant Variable

Mechanical lossCore/Iron loss Copper loss Stray

load loss

Eddy current

loss

HysteresisStator copper

loss

Friction

lossWindage

loss




Problem• Motor Specifications

• Rated power: 34 kW

• Voltage: 415 Volt

• Current: 57 Amps

• Speed: 1475 rpm

• Connection: Delta

53

• No load test Data

• Voltage: 415 V

• Current: 16.1 A

• Frequency: 50 Hz

• Rstator (30°C): 0.264 Ω/ph

• No-load power: 1063.74 W

• Stator copper loss at no-load:

• Iron plus friction and windage loss:

• Assume operating temperature of 120°C

• Stator resistance at 120°C:


Problem• Motor Specifications

• Rated power: 34 kW

• Voltage: 415 Volt

• Current: 57 Amps

• Speed: 1475 rpm

• Connection: Delta

54

• No load test Data

• Voltage: 415 V

• Current: 16.1 A

• Frequency: 50 Hz

• Rstator (30°C): 0.264 Ω/ph

• No-load power: 1063.74 W

• Stator copper loss at full-load:

• Full load slip:

• Rotor power input:

• Assume stray loss of 0.5% of rated output

• Motor input:

• Efficiency: and Power factor:


55

Losses

• Core losses vary with the core material, coregeometry, and input voltage

• Friction and windage losses are caused byfriction in the bearings of the motor,aerodynamic losses associated with ventilationfan, and other rotating parts

• Copper losses are I2R losses

• Stray losses arise from a variety of sources.Typically, proportional to the square of the rotorcurrent


56

Typical Load Vs Loss curve for design B, 50-HP, 1800 RPM

induction motor




57

Typical performance curves for design B, 10-HP, 1800RPM, 220-V, Three Phase, 60 HZ induction motor


58Source: Larson and Subbiah : ESD


59

ENERGY EFFICIENT MOTORS


60

Technology Characteristics of Motors

Range Typical

Rating hp

STD Motor

Efficiency

(%)

EEM

Efficiency

(%)

Cost of

Standard

Motor

(Rs)

Cost of

EEM

(Rs)

1-5 3.0 79.7 86.8 7,500 9,750

5-10 7.5 84.4 88.6 13,300 17,290

10-15 12.5 87.3 91.0 24,100 31,330

15-20 17.5 88.4 92.0 28,500 37,050

20-50 35.0 90.6 92.0 56,200 73,060

>50 100.0 93.0 94.5 187,100 243,230




61

Efficiency Testing Methods

• Load Test – No load test and six loadtests

• Equivalent Circuit Test – No load test,

Locked Rotor test , Variable voltage

(IEEE Std 112-1984, JEC 37, IEC –34-2,

ISI –4889)


62

No Load Test

• The motor is run at rated voltage and frequency withoutany shaft load

• Input power, current, frequency and voltage are noted

• The no load P.F. is quite low and hence low PF watt-meters are required

• From the input power, stator I2R losses under no loadare subtracted to give the sum of Friction and Windage(F&W) and core losses

• plot no-load input kW versus Voltage; the intercept isFriction & Windage kW loss component

• F&W and core losses = No load power (watts) - (No loadcurrent)2 × Stator resistance


63

Stator and Rotor I2R Losses

• The stator winding resistance is directly

measured

• The resistance must be corrected to theoperating temperature

• The rotor I2R losses are calculated

• Rotor I2R losses = Slip × (Stator Input –

Stator I2R Losses – Core Loss)

• Stray Load Losses fixed at 0.5%

Cin:235

235 1

0

1

t

t

t

R

R

o


64

Example

• Motor Specifications:

– Rated power, Voltage, Current, Speed,

Connection

• No load test Data:

– Voltage V, Current I, Frequency F, Stator

phase resistance at 30°C, No load power Pnl

• Calculate:

– Stator cu loss at 30°, Iron and fw loss, stator

loss at 120°, FL slip, rotor i/p [= Pr/(1-s)], total

i/p, FL efficiency, FL pf [=P/√3 VI]




65

Motor Loading

• Part load = measured i/p to nameplate i/p

• Part load = i/p load current to i/p ratedcurrent

– Current varies approximately linearly withload up 75% of full load.

– Below the 75% load, pf degrades and therelation is non-linear

• Part load = actual slip to rated slip

• With voltage correction: 2

load part

r

op

r

op

V

V

s

s


66

Source: BEE Manual


67

ECOs for motors

• Replace with std motor of lower size

• Replace with EE motor of lower size

• Add capacitors to improve pf

• Replace V belt drive by flat belt

• Put timer/controller to switch off during

idling

• Two-speed motor/ variable speed

(application specific)


68

References/Further Reading• MSEB Tariffs, MSEB, Mumbai

• S.Ashok, R.Banerjee,IEEE Trans on Power Systems, Nov 2001,p879-884

• S.Ashok, R.Banerjee,IEEE Trans on Power Systems, Vol 18, May2003, p931-937

• S.Ashok, R.Banerjee,Energy, 2003• Witte, Schmidt, Brown, Industrial Energy Management and

Utilisation, Hemisphere Publ,Washington,1988

• WREB, Annual Report, 2001

• O. I.Elgerd Electric Energy Systems Theory,TMH, 2001

• Larson and Subbiah, Energy for Sustainable Development, Vol 1,1994, p 36-38

• J.C.Andreas, Energy Efficient Motors, Marcel Dekker, 1992, NewYork

• H.E.Jordan, Energy Efficient Motors & their application, 1983, VanNostrand

• Nagrath, Kothari, Electric Machines, Tata Mc Graw Hill, 1996

• BEE Guide Book (www.em-ea.org/gbook1.asp)

lecture8 electrical systems notes 2016

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