View
269
Download
1
Category
Preview:
Citation preview
Power Quality Issues
Basic Requirements for Critical Loads
Continuous, No Break in Power
Voltage Regulation Sine wave Supply Isolation Constant Frequency
LoadVs
Is
Understanding Reactive Power and
Power Factor
Vs
Is
R
Is
Vs XL
Reactive P
ower(Q
) in V
AR
Appare
nt
Power(
S) in
VA
Imp. Phase
Angle
True Power(P)in Watts
XL
Vs
Is
R
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 volt-amperes. This includes fundamental and all harmonic components ”
Understanding Crest Factor
Non Linear Load
Vs
Is
Current drawn by single Phase diode rectifier
Crest Factor = --------------------------Peak Amplitude
RMS Value
Understanding Harmonics
Non Linear Load
Vs
Is
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 harm onics
square of amplitude of fundamental x 100
Mathematically, THD of a voltage wave form ca n be defined as,
THD = V
V
100 x h
h
h 2
122
Power Quality
Supply Voltage at load must
The Voltage at any point in the distribution system is uniquely described by
Where v,f are constants (declared Value) for all t
)2(2 V(t) ftVSin
have fundamental component only
be balanced
have declared magnitude under all conditions
PQ affected by Polluting Load
Polluting Load
A
B
PCC
Distorted voltage
Pure Sinusoidal
Line Impedance ZL
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 currentsOversize 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
Causes for Sag Motor Starting Transformer Energization Transmission Faults
Causes for Swell Single line to ground fault Removing a large load /
adding a large capacitor bank
0 1 2 3 4 5 6 7 8 9 10 11190
195
200
205
210
215
220
RM
S v
olta
ge in
V
Time in Cycles
0 5 10 15 20 251010.110.210.310.410.510.610.710.810.911
Time in Cycles
RM
S v
olta
ge in
kV
Voltage sag due to Motor Starting
Voltage sag due to Transformer Energization
Voltage UnbalanceDefinition 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
Causes for Unbalance Unequal system impedances Unequal distribution of
single-phase loads
Phase to Phase loads Unbalanced Three phase loads
Va
VbVc
1200
1200
1200
Va
VbVc
1200
1210
1190
Balanced System Unbalanced System
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
M
3Ø Diode Rectifier
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 FlickerDefinition
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
Effect lamp flicker Human eye is most sensitive to voltage waveform modulation around a frequency of 6-8Hz.
Causes Induction Motor drive
• Arc furnaces• Arc welders• Frequent motor starts
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 DisturbancesTransient disturbances are caused by the injection of energy by switching or by lightningCauses
Lightning Capacitor Switching Load switching
Oscillatory transient waveform caused by capacitor energizing
Need for Power QualityBusiness 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
h 11< 11 h 17 17 h 23 23 h 35 35 h TDD4.0 2.0 1.5 0.6 0.3 5.07.0 3.5 2.5 1.0 0.5 8.010.0 4.5 4.0 1.5 0.7 12.012.0 5.5 5.0 2.0 1.0 15.0
ISC / IL 20
20-5050-100100-1000 1000 15.0 7.0 6.0 2.5 1.4 20.0
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
Recommended