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Transcript
Power Quality Issues
Basic Requirements for Critical Loads
Is
Vs
Load
Continuous, No Break in Power
Voltage Regulation
Sine wave Supply
Isolation
Constant Frequency
Understanding
Reactive Power and Power Factor
Is
Is
R
Vs
Vs
XL
XL
Vs
R
Imp. Phase
Angle
True Power(P)
in Watts
Reactive Power(Q)
in VAR
Is
Displacement and True Power Factor
Displacement Power Factor:
“Ratio of the active power of the fundamental, in
watts, to the apparent Power of the fundamental
wave, in volt-amperes”
True Power Factor:
“Ratio of the total power, in watts, to the total voltamperes. This includes fundamental and all harmonic
components ”
Understanding Crest Factor
Is
Vs
Non
Linear
Load
Current drawn by single Phase diode rectifier
Peak Amplitude
Crest Factor = -------------------------RMS Value
Understanding Harmonics
Is
Vs
Non
Linear
Load
Deviation from a perfect sine wave can be represented by Harmonics.
Sinusoidal Component having a frequency that is an integral
multiple of the Fundamental frequency.
Total Harmonic Distortion
 Defines the total harmonic content of current or voltage
 Ratio of the RMS of the harmonic content to the RMS of the
Fundamental, as % of Fundamental
THD =
sum of squares of amplitudes of all harmonics x 100
square of amplitude of fundamental
Mathematically,THD of a voltage wave form can be defined as,
2
h   Vh

THD =
x 100
2
h2 V
1
Power Quality
Supply Voltage at load must
have fundamental component only
be balanced
have declared magnitude under all conditions
The Voltage at any point in the distribution system is uniquely
described by

V(t)ft

2
VSin
(
2
)
Where
v,f are constants (declared Value) for all t
PQ affected by Polluting Load
PCC
A
Polluting
Load
Pure Sinusoidal
Line Impedance ZL
B
Distorted
voltage
Common PQ Disturbances
 Reactive Power Demand
 Harmonic Distortion
 Voltage sags and swells
 Undervoltages and overvoltages
 Voltage Unbalance
 Voltage Flicker
 Voltage Notching
 Voltage Interruption
 Transient Disturbances
 Frequency variations
Harmonics-Polluting Loads
 Rectifiers
 Arc furnaces
 Adjustable Speed drives
 Power Electronic converters
Harmonics-FFT Analysis
Current drawn by PC
Harmonic Spectrum of PC
Current Waveform
6 Pulse Converter/Battery Charger
Harmonic Spectrum of 6 Pulse
Converter/Battery Charger
Implication of
Reactive and Harmonics currents
Oversize of all installation equipments to transmit Reactive and
Harmonic currents namely
Transformer
Cables
Circuit breakers & distribution switch boards
Neutral overloads
Increase in Transmission & Distribution loss
Reduction in voltage stability margin
Overheating and loss of life & equipments
Major Increase in cost
Voltage sags and swells
•
Reduction in the ac voltage, at the power frequency, for durations
from a half-cycle to a few seconds.
•
Voltage Sag is Characterized by two parameters – Magnitude and
Duration
•
Power Electronics Loads are Sensitive to Voltage Sags
Causes of Voltage sags and swells
Voltage sag due to Motor Starting
220
Causes for Sag
 Transmission Faults
RMS voltage in V
 Motor Starting
 Transformer Energization
215
210
205
200
195
190 0
1
2
3
4 5 6 7
Time in Cycles
8
9
10 11
Causes for Swell
 Single line to ground fault
 Removing a large load /
adding a large capacitor bank
RMS voltage in kV
Voltage sag due to Transformer
Energization
11
10.9
10.8
10.7
10.6
10.5
10.4
10.3
10.2
10.1
10 0
5
10
15
20
Time in Cycles
25
Voltage Unbalance
Definition
In a balanced sinusoidal supply system the three line-neutral voltages
are equal in magnitude and are phase displaced from each other by 120
degrees V
V
a
1200
Vc
a
Balanced System
1200
1200
1200
Vb
Unbalanced System
1190
1210
Vc
Causes for Unbalance
 Unequal system impedances
 Unequal distribution of
single-phase loads
Phase to Phase loads
Unbalanced Three phase loads
Vb
Effect of Voltage Unbalance
 Induction Motor drive
•
•
•
•
Overheating and loss of Insulation life
Reduced Motor Efficiency
Noisy in their operation due Torque and speed pulsation
Motor derating factor
NEMA Induction motor derating curve
Effect of Voltage Unbalance
 AC Variable Speed drive
3Ø Diode Rectifier
M
Line Current of 3Ø diode
Rectifier for Balanced Input
Line Current of 3Ø diode
Rectifier for 5%unbalanced Input
Line Current of 3Ø diode Rectifier
for 15%unbalanced Input
• Draws uncharacteristic triplen harmonics
• Triplen harmonic current can lead to undesirable
harmonic problems
• Excessive thermal stress on diodes
Voltage Flicker
Definition
Repetitive or random variations of the voltage envelope modulated at
frequencies less than 25 Hz, which the human eye can detect as a
variation in the lamp intensity of a standard bulb due to sudden changes
in the real and reactive Power drawn by a load
Voltage waveform showing flicker created by an arc furnace
Voltage Flicker
Causes
 Induction Motor drive
• Arc furnaces
• Arc welders
• Frequent motor starts
Effect
 lamp flicker
Human eye is most sensitive to voltage waveform
modulation around a frequency of 6-8Hz.
Voltage Notching
Causes
 Adjustable Speed Drives
 Solid State rectifiers
Voltage Interruption
Complete loss of electrical supply
Cause
 Transmission Fault clearing time
• Opening / Recloser of circuit breaker
Transient Disturbances
Transient disturbances are caused by the injection of
energy by switching or by lightning
Causes
 Lightning
 Capacitor Switching
 Load switching
Oscillatory transient waveform caused by capacitor energizing
Need for Power Quality
Business Problems:
• Momentary disturbance can cause scrambled data, interrupted
communications, system crashes and equipment failure
• Lost productivity and idle people and equipment
• Overtime required to make up for lost work time
• Revenue and accounting problems such as invoices not
prepared, payments held up etc
According to Electric Light and Power Magazine, 30 to 40 percent
of all business downtime is related to Power Quality Problems
IEEE 519 Harmonic Standard
 IEEE 519 “Recommended Practices and Requirements for
Harmonic Control in Electric Power Systems”
 Specifies load current harmonic limits at PCC
 Specifies supply voltage harmonic limits at PCC
HARMONIC CURRENT DISTORTION LIMITS IN % OF IL
V 69 kV
I SC/ I L
 20
20-50
50-100
100-1000
 1000
h <11 11 h 17 17h 23 23 h 35 35h
TDD
4.0
7.0
10.0
12.0
15.0
5.0
8.0
12.0
15.0
20.0
2.0
3.5
4.5
5.5
7.0
1.5
2.5
4.0
5.0
6.0
0.6
1.0
1.5
2.0
2.5
0.3
0.5
0.7
1.0
1.4
PQ Mitigation
Reactive and Harmonic Demand
 Active Filter
 Passive Filter
Voltage sag and Swell
 Dynamic Voltage Restorer
 Tap changing transformer
PQ Mitigation
Voltage Unbalance
 Static Power balancer
 Redistribution of single-phase loads equally
to all phases (Utility level).
 Load Balancing (Plant level)
Voltage Flicker
 Distribution static VAR compensator
PQ Mitigation
Transient Disturbances
 Surge Arrester
 Isolation transformer
 Active/Passive Filter