university of...power factor and reactive power
TRANSCRIPT
Page 1
PSERC
Power Factorand
Reactive Power
Ward JewellWichita State University
Power Systems Engineering Research Center (pserc.org)
PSERC
Energy to lift a 5 pound weight2 feet high:
2 ft x 5 lb = 10 ft-lb= 0.0000038 kWh
= 0.0033 “calories” (which are actually kcal)
Value at 10.3 cents per kWh:(average residential US price, summer 2006)
0.000039 cents
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PSERC
As dragline bucket lowers, motors generate, return electricity to source
PSERC
Induction motor with no load
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018600
400
200
0
200
400
600
800735.249
465.196−
p t( )
0.0170 t
time (seconds)
pow
er (w
atts
)
energyto motor
energyfrom motor
0
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PSERC
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018600
400
200
0
200
400
600
800735.249
465.196−
p t( )
0.0170 t
time (seconds)
pow
er (w
atts
) averagepower:
130 watts
Induction motor
PSERC
Incandescent lights
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.0180
50
100
150
200
250
300
350306.8
0
p t( )
0.0170 t
0time (seconds)
pow
er (w
atts
)
averagepower:
150 watts
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PSERC
Incandescent Lights
PSERC
Induction motor with no load
Page 5
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Lights and Motor
5.1 A
1.3 A
Current
117.7 V0.13 kWInduction motor with
no load
118.0 V0.15 kWIncandescent lights
VoltagePower
PSERC
Why do the Volts and Amps matter?
Page 6
PSERC
Motors and Resistance Heat:100 MW
2.3 MW11.7 kVMotors
1.0 MW12.3 kVResistance Heat
Power lost in wiresCustomer voltage
PSERC
Incandescent Lights
Page 7
PSERC
Incandescent lights power:
Power = 118 V x 1.3 A= 153 W
= 0.15 kW= power measured by meter
PSERC
Incandescent Lights
Page 8
PSERC
Induction motor with no load
PSERC
Induction motor power:
117.7 V x 5.1 A= 600 W?= 0.6 kW?
NOT the power measured by meter
Page 9
PSERC
Induction motor with no load
PSERC
Define some new values:
Apparent power = volts x ampsFor the motor:
117.7 V x 5.1 A= 600 VA= 0.6 kVA
VA: volt-ampere
Page 10
PSERC
Define some new values:
Power Factor =
Average (“real”) (kW) powerApparent (kVA) power
For the motor:pf = 0.13 kW / 0.60 kVA
pf = 0.22
PSERC
Define some new values:the power triangle
for the motor:
real power = 0.13 kW
reactive power =
reac
tive
pow
er =
0.5
8 kV
AR
0.60kVA( )2 0.13kW( )2− 0.59kVAR=
VAR: volt-ampere reactive0.58 kVAR
VI2 – average power2
Appa
rent
pow
er =
0.6
0 kV
A
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PSERC
Induction motor with no load
PSERC
Lights and Motor
0.22
1.0
Power factor
0.60 kVA
0.15 kVA
Apparent power
0.58 kVAR
0 kVAR
Reactive power
5.1 A
1.3 A
Current
117.7 V0.13 kW
Induction motor with
no load
118.0 V0.15 kW
Incan-descent
lights
VoltageReal Power
Note: the motor’s reactive power will stay near its no-load value of 0.58 kVAR as its load and real
power (and thus apparent power and power factor) vary from no load to full load.
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PSERC
Power factor and reactive power are indicators of
power losses in wiresvoltage drop between supply and load
PSERC
Typical Power Factors
0.5-1.0Variable Speed Motor Drives
0.5-1.0Computers
0.6-1.0Battery Chargers
0.6-1.0Fluorescent lights
1.0Incandescent lights
1.0Resistance heat
0.7-0.8Induction motor
Page 13
PSERC
Power factor:lagging or leading?
Most loads with lower power factorare inductive.
Current lags voltage.Power factor is “lagging.”
PSERC
Induction motor with no load
Current lags voltage by about 3.6 milliseconds
3.6 ms
currentvoltage
Page 14
PSERC
Another way to calculate power factor
One 60 Hz cycle = 1/60 seconds = 16.7 ms
3.6 ms
16.7 ms
PSERC
Another way to calculate power factor:“displacement” power factor
(3.6 ms / 16.7 ms) x 360 degrees = 77 degreescurrent lags voltage by 77 degrees
cosine (77 degrees) = 0.22power factor is 0.22 lagging
pf = cos θθ = angle between voltage and current
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PSERC
Incandescent lights
Current and voltage are “in phase.”
PSERC
Incandescent lights:
displacement power factor:angle between voltage and current
= 0 degreespf = cos(0 degrees) = 1.0
true power factor:pf = 0.15 kW / 0.15 kVA
pf = 1.0
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If voltage and current are sinusoidaldisplacement pf (DPF) = true pf (PF)
motor lights
PSERC
Correcting (increasing)power factor
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Capacitors to improve power factor:capacitors release energy when inductors consume
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018
1
0.5
0
0.5
11.2
1.2−
iL t( )
ic t( )
0.0170 t
Inductorcurrent
Capacitorcurrent
PSERC
Induction motor with power factor correction capacitor
Page 18
PSERC
118.4 V
1.5 A0.960.18 kVA
0.11 kVAR
0.13 kW
Induction motor with capacitors
117.7 V
5.1 A0.220.60 kVA
0.58 kVAR
0.13 kW
Induction motor
VoltageCurrentPower factor
Apparent power
Reactive power
Real power
Induction motor with power factor correction capacitor
PSERC
Wire losses:motors with capacitors
1.0 MW12.3 kVMotors with power factor correction capacitor
2.3 MW11.7 kVMotors
Power lost in wiresCustomer voltage
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PSERC
Incandescent lights with power factor correction capacitor
PSERC
Incandescent lights with power factor correction capacitor
118.0 V1.3 A1.00.15 kVA0 kVAR0.15 kW
Incan-descent
lights0.23
leading
Power factor
0.66 kVA
Apparent power
0.64 kVAR
Reactive power
5.5 A
Current
119.9 V0.15 kW
Lights with capacitors
VoltageReal power
Page 20
PSERC
Wire losses:lights with capacitors
2.0 MW13.0 kVResistance heat with power factor
correction capacitors
1.0 MW12.3 kVResistance heat
Power lost in wiresCustomer voltage
PSERC
Leading power factorCurrent leads voltage in a capacitor.
Too much capacitance causes low leading power factor.
(just as bad as low lagging power factor)
Leading power factor causes high voltage and increased wire losses.
Use the correct amount of capacitance.(more is not better)
Switch capacitors off when motors are off(just put capacitor on same switch as motor)
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PSERC
If voltage and current are sinusoidaldisplacement pf = true pf
motor lights
PSERC
If waveform is not sinusoidal:PC voltage and current
Page 22
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If waveform is not sinusoidal:PC voltage and current
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Harmonic distortion
Page 23
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Low power factor caused by harmonic distortion cannot be
corrected by capacitors
Harmonic currents are not accompanied by harmonic voltage, so average (real) power
in harmonics is almost zero.pf = average power / apparent power
decreases
PSERC
Common harmonic loadscomputersmotor drivesbattery chargersrectifiersinduction heatersarc furnaces
To correct low power factor caused by distorted current waveforms, the
harmonic currents must be filtered.
Page 24
PSERC
Capacitors can make harmonic distortion worse:
Lights with power factor correction capacitor
This is rare, but should be considered in the presence of harmonic loads
PSERC
SummaryInduction motors and other inductive equipment load the electricpower system differently than incandescent lights and resistive heatersPower Factor and Reactive Power are indicators of power lost in wires and reduced customer voltageLow displacement power factor caused by induction motors (and other inductive loads) can be corrected with power factor correction capacitorsPower factor correction capacitors must be sized properlyPower factor correction capacitors cost much less than utility power factor charges and will eliminate those chargesPower factor correction capacitors should be disconnected when motors are disconnectedLow harmonic power factor is corrected with filters, not capacitors. Capacitors may make it worse.