1 power quality -- bhanu bhushan -- june, 2011. 2 how close is the supply voltage waveform to...
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• How close is the supply voltage waveform to sinusoidal, and how close are the supply voltage and frequency to the rated ?
• What Power Quality do we actually have ? What Power Quality do we really need ?
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POWER QUALITY
in a wider, Indian perspective
GRID - Level & CONSUMER - Level
1) SUPPLY CONTINUITY
2) FREQUENCY
3) VOLTAGE
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SUPPLY INTERRUPTIONS
a) LOAD - SHEDDING due to own or others’ over-drawal : Maximize generation, and
allow over-drawal, as long as grid can
sustain it, and it is paid for.
b) LOAD - SHEDDING to curtail over-loading or under-voltage : If too frequent, ask for system augmentation, additional capacitors.
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c) TRIPPING due to a fault or equipment failure : Minimize outage duration,
: Reliable protection, Auto-reclosing,
: Ask for building redundancies.
FREQUENCY : covered in another session.
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VOLTAGE PROBLEMS
• HIGH / LOW : Can be corrected by transformer tap-changing and reactive compensation : shunt / series capacitors, reactors, SVC, MVAR generation change.
• SWELLS & SAGS, SPIKES & DIPS, FLICKER : Caused by switching on / off of large loads, capacitor banks, electric furnaces, welding machines.
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• PHASE UNBALANCE : Caused by single - phase or unbalanced loads (e.g. railway traction), pole - discrepancy, break in a phase (conductor snapping), break in neutral, hanging faults, non-transposition.
• WAVE FORM DISTORTIONS (HARMONICS and DC offset) : caused by HVDC, SVC, FACTS, Converters, UPS, power / speed controllers, computers, TVs, chargers, printers, tube-lights, CFLs, fan regulators, electronic ballasts,
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communication equipment, arc furnaces, welding, railway traction, etc.
Circuit breaker and isolator operation (switching transients), L.A., transformer magnetizing current inrush, lightning,
Faults and their clearance, insulator flash- over, corona, faulty grounding.
Adverse effects : Over-heating & noise, resonance, telephone interference, hum, capacitor failure, mal-operation of control device and medical equipment.
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POSSIBLE SOLUTIONS : Circuit segregation, harmonic filters, U.P.S.
DAMPING by synchronous and induction machines.
A use of harmonics: Harmonic restraint in transformer differential relays.
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Simple examples around us :
• Ceiling fan regulators
• Tube lights
• Lap-tops & peripherals
• Domestic inverters
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Ceiling fans -- 1- ph Induction motors,
shaded - pole or split - winding, inverted.
Fan regulators -- 3 different types:
i) Choke : weighty, costly, low PF
ii) Resistor : energy loss, heating
iii) Electronic : voltage and current
distortions, harmful on both sides.
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Supply voltage and fan current, regulated by old resistance type regulator, at full-speed
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Supply voltage and fan current, regulated by old resistance type regulator, at low speed
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Fan current, regulated by electronic regulator, at full speed
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Fan current, regulated by electronic regulator, at low speed
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Harmonics in fan current, at low speed, with electronic regulator
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Voltage wave form, after electronic regulator
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Harmonics in fan voltage, at low speed
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Tube lights : non-linear discharge lamps.
Smoothening & PF improvement by choke
and capacitor. Not a serious problem.
Electronic ballasts : V & I distortions ?
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Tube light current waveform
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Lap-tops & peripherals : AC / DC adapters draw non-sinusoidal current.
Domestic inverters : Battery charging current is always non-sinusoidal.
INDUSTRIAL U.P.S.
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Voltage Wave form – UPS output
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Harmonics in UPS output voltage
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IEEE Standard 519 - 1992
Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems