7. pq sources
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Electric Power Quality
TutorialPart II: Sources and Mitigation
Schemes
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Sources of Power Quality
Problems
• Nonlinear Loads• Sources of Harmonics
• Sources of Flicker
• Sources of Sag• Different Converter Schemes
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Proliferation of Nonlinear Loads
• Impact customer loads, distribution feeders
and substation equipment.
• Impacts individual customers’ neighbors andultimately the source.
• Lack of National standards complicate the
issue.
• Utilities, customers and suppliers need to work
together.
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Sources of Harmonics
• Power Electronic Devices
– Phase-angle regulators in lighting/heating
controllers
– Rectifiers/Inverters
– Adjustable speed motor drives
– Interface of wind /solar power converterswith the utility
– HVDC systems
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Sources of Harmonics & Flicker
• Ferromagnetic devices
– Transformers (saturation non-linearity)
• Arcing Devices
– Fluorescent lamps
– Arc welders
– Arc furnaces
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• Starting of Heavy Loads – Large motors
• Brown Outs
– Large loads
• Fault clearing times on distribution
feeders: 5 to 15 cycles.
• Sags range from 20 to 50%.
Sources of Sags
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Effects of Sags
• Voltage sags not perceptible to human eye.
• Sensitive electronic equipment affected byvoltage sags.
• Sags interrupt service to loads such asautomated processes for many hours.
• Results in loss of revenue of millions ofdollars.
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Sags
• Need compensation devices to avoid interruption.
• Need to evaluate the role of protective devices.
– Reclosings after a fault
– Operating speed of circuit breakers, fuses and reclosers.
• Need to analyze causes for faults.
– e.g.: Tree falling.
– Multiple reclosings maybe questionable from powerquality perspective.
• Adopt optimum tree trimming policies.
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Phase-angle Regulators
• Example: Lamp dimmer
• Comments:
– Current distortion can be reduced by proper sizing of the choke.
– THD and radiated EMI is low for triggering angles close to 0 and 180 (i.e., full or zero brightness)
– THD and radiated EMI is highest for angles to 90 (half-bright).
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Lamp dimmer circuit waveforms
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Spectrum of current drawn by lamp dimmer circuit
• High frequency components which lead to EMI are reduced by the choke.
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Single-phase Rectifiers
• Examples: Computer power supplies, Battery chargers
• The rectifier conducts only when the line voltage magnitude exceedsthe capacitor voltage.
• The capacitor gets charged by drawing current at the peak of thevoltage cycle and gets discharges slowly into the switching regulatorbetween the voltage peaks.
• Thus the circuit draws short pulses of current during line voltagepeaks.
Typical computer power supply front-end
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Current Drawn by a Computer Power Supply
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Sequence Classification of Harmonics
• In AC systems, the current
and voltage waveforms have
rotational symmetry. – even harmonics will not be
present.
• Power system harmonics are
hence predominantly theodd, i.e 3rd, 5th, 7th, etc.
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Three-phase RectifiersSix-pulse Rectifier
• Used in motor drives, traction, electrochemical plants, etc.
• The high inductance in the dc side causes the dc current,
Id to be essentially constant.
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Three-phase Rectifiers (cont.)
Six-pulse Rectifier
• The Fourier series for the line current for a diode
rectifier is:
• For symmetrical ideal triggering, only harmonics of the
order 6n
1 are present in the AC side currents.
• The presence of source reactance and commutation
effects lead to smoother current waveforms.
...t13sin13
1
t11sin11
1
t7sin7
1
tsin55
1
tsin I
32
t)( dai
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Supply voltage and current waveforms for three-phase bridge
with highly inductive load
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Twelve-pulse Rectifier (cont.)
• Used in high power motor drives, traction, hvdc converters, etc.
• The Fourier series for the line current for a twelve-pulse dioderectifier is:
• For symmetrical ideal triggering, only harmonics of the order
12n 1 are present in the AC side currents.
...13sin
13
111sin
11
1tsin
32)( t t I t i
d a
Supply voltage and current waveforms for twelve pulse bridge with highly inductive load
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Cycloconverters
• Used in large mill drives in cement and mining industries.
• The characteristic harmonics generated are:
061 nf f pm f h
f o=output frequency of the cycloconverter;
m=1,2,3,… ; n=0,1,2,…
Typical input current harmonics of a six-pules cycloconverter with 5-Hz output frequency
• The harmonic spectrum
varies as the output
frequency is varied.
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Integral-cycle Controllers or Pulse Burst Modulation
(PBM)
• This technique is used in applications such as heating, ovens,furnaces, etc.
• Subharmonics are predominant. DC component can also bepresent.
• High frequency harmonics above 200 Hz are practically
absent.
Pulse-burst-modulation power conditioning .Current wave: n=6; g=4/6
Harmonic spectrum for g6/8.
Currents generated by a typical PBM system.
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A Demonstration That a Balanced 3-Phase Load Can
Result In Neutral Current
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Types of Filters
• Passive filters
– provide low impedance path to ground at resonance
frequency,
– use tuned RLC components,
– economical.
• Active filters
– inject harmonic currents (or voltages) out of phase
with the ambient harmonics, – use components such as switches and amplifiers,
– expensive.
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Passive Filters
• Series tuned circuit offers very low impedance atresonance frequency
• Parallel tuned circuit offers very high impedance
at resonance frequency
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Capacitor as a Filter
• A shunt capacitor is the simplest form ofpassive filter – economical,
– also provides reactive power (Q) compensation.
• Guidelines for sizing capacitive filters – resonance between capacitor and circuit inductive reactance
should not occur exactly at an integer multiple of fundamental
frequency.
– sensitivity of resonant point to drift in capacitor value shouldbe investigated,
– voltage and var support provided should not be excessive,
– IEEE Standard 18 should be consulted for sizing and
placement of capacitor.
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Active Filters - Voltage and Current Type
• Voltage type active filter
– capacitor (dc source),
– voltage source inverter (VSI).
• Current type active filter
– inductor (current source),
– current source inverter (CSI).
Voltage type (left) and current-type active filters.
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Active Filters - Technology Overview
• Active filters are also referred to as active power
line conditioners.
• Reference [3] gives an extensive literature survey of
existing and proposed line conditioning
methodologies.• Active filters may also be classified depending on
their correction method.
– correction in time-domain,
– correction in frequency domain.
• Table 1 in [3] gives a summary of the various
publications in the area of active filters - a few
entries of which are shown here in Table 1.
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Active Filters - Technology Overview
• In Table 1,
– ‘Type’ indicates voltage type (V) or current type (I),
– ‘P.E. Device’ indicates the power electronic switch used.
Table 1: Summary of publications on active power filters [3].
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Static Var Compensator
• Consists of electronically switched capacitor and/or
inductor.
• Some SVC technologies
– Thyristor Controlled Reactor (TCR) with fixed capacitor (FC) – TCR with thyristor switched capacitor (TSC).
• The Adaptive Var Compensator (AVC), developed
at the University of Washington, is essentially a
bank of TSCs.
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Static Condenser (STATCOM)
• FACTS and Custom Power Device
– reactive power compensation,
– voltage regulation (by reactive power compensation), – harmonic current compensation.
• Behaves as a voltage source connected in shunt to
the power system through an inductor.
Figure 22: Functional
block diagram of a
STATCON.
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Dynamic Voltage Restorer (DVR)
• FACTS and Custom Power Device
– voltage regulation (by series compensation),
– harmonic line voltage compensation.
• Behaves as a voltage source connected with the power
line.
Figure 23: Functional
block diagram of a DVR.
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Solid State Breaker (SSB)
•Custom Power Device – for instantaneous fault clearing, – subcycle reclosing,
– zero current/voltage closing.
Figure 24: Functional schematic of a solid state breaker.
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Adaptive Power Quality Compensator
• Decouple reactive and harmonic
compensation
• Arrange Converters so that – slow switching reactive converter bears most of
the stress
– fast switching harmonic converter handleslower voltages and currents.
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ShuntShunt CompensatorCompensator ImplementationImplementation
Sensing&
Control
HPFilter
PWM inverter - phase A
Stepped-wave inverter -reactive compensator
Gate s ignals
AB
C
CH
LH
LQ
IH
IQ
LoadSource
ILI S
CQ
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Shunt Compensator Simulation Results
Compensation of Rectifier Load
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Operation of reactive converter
Shunt Compensator Simulation Results
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Current sharing between reactive and PWM converters
Shunt Compensator Simulation Results
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The Adaptive Var Compensator (AVC)
• No harmonics or transients are introduced by the AVC.
• Compensates on cycle to cycle basis
– power factor control,
– voltage control.
– Voltage flicker control
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The Adaptive Var Compensator (AVC)
• Essentially a bank of switched capacitors.
• Capacitors are in a binary ratio (1:2:4).
• SCRs switched at zero current and zero voltagecrossing.
AVC is a bank of electronically switched capacitors in a binary ratio.
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