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POWER QUALITY

-- Bhanu Bhushan --

<a3bhanu@gmail.com>

June, 2011

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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