Industrial and Commercial Power SystemsTopic 9

POWER QUALITY

The University of New South Wales

School of Electrical Engineeringand Telecommunications

Power quality (PQ) problem = any problem that causes voltage, current, or frequency deviations in the supply and may result in mal-operation or failure of end-user equipment.

In majority of cases, power quality actually refers to quality of the voltage. Therefore, PQ standards mostly specify requirements on supply voltage.

Some effects

C/B trips when it shouldn’t.

Computers lose data or crash

Neutral conductors overheat

Digital clocks show erratic time

Adjustable speed drives trip or suffer damage

Power factor correction capacitors blow up

Lights flicker

Power quality definitions [Ref: R.C. Dugan]

Factors that cause power quality problems:

Increased use of power quality-sensitive equipment.

Increased use of equipment that generates power quality problems

Increased inter-connectedness of power system

Deregulation of power industry

Tolerance envelope for IT equipment.Developed by Information Technology Industry Council (ITIC), formerly

Computer and Business Equipment Manufacturers Association (CBEMA).

Summary of power quality problems[Ref: R.C. Dugan]

Reliability measures for distribution systems:

Supply availability: actual time supply is available to customers, expressed as percentage.

utilities aim to deliver 4-nine or higher availability (total outage of 52 minutes in one year )

Reliability Index or SAIDI (System Average Interruption Duration Index): average duration lost per customer per year

SAIFI (System Average Interruption Frequency Index)

CAIDI (Customer Average Interruption Duration Index)

MAIFI (Momentary Average Interruption Frequency Index)

[Ref. Electricity Network Operation Standards, July 2004]

NSW Electricity Information (Independent Pricing and Regulatory Tribunal)

Customer Service Standards (EnergyAustralia)

Customers can claim a payment if experience:

(i) too many interruptions in one year or(ii) an interruption that lasts too long

Amount: $80 per claim, max $320 per year per premise

Ref. EACustomer Service Standards brochure

Customer Service Standards (EnergyAustralia)

Ref. EA Customer Service Standards brochure

Principal phenomena causing electromagneticdisturbances as classified by the IEC.

Categories and characteristics of power systemelectromagnetic phenomena [Ref: R.C. Dugan]

Simplified classification system by EnergyAustralia

ENOS Electricity Network Operation Standards, June 2006

Transients:

2 categories: impulsive and oscillatory. These terms reflect waveshape of transient.

Sometimes, also categorized according to their mode: common mode or normal mode.

Transients: Impulsive transients

Uni-directional impulse. Characterized by rise and decay times.

Most common cause is lightning: Voltage impulse wave of 1.2/50µs Current impulse wave of 8/20µs

Due to high frequencies involved, shape of impulsive transients can be changed quickly by network components.

Transients: Impulsive transients

Lightning stroke current impulse transient .

Transients: Oscillatory transients

High-frequency oscillatory transient: primary frequency component >500kHz, typical duration in µs.

Medium-frequency oscillatory transient: primary frequency between 5kHz and 500kHz with duration of tens of µs.

Low-frequency oscillatory transient: primary frequency <5kHz, duration from 0.3 to 50ms. Frequently encountered on sub-transmission and distribution systems. Caused by many types of events, most frequent is capacitor bank energisation.

Oscillatory transient by back-to-back capacitor switching

Low-frequency oscillatory transient caused by capacitor-bank energisation

Low-frequency oscillatory transient caused by ferro-resonance of an unloaded transformer

Long-duration voltage variation:

Over-voltage: increase in rms ac voltage >110% at 50Hz for duration >1 minute

Under-voltage: decrease in rms ac voltage <90% at 50Hz for duration >1 minute

Sustained interruption: voltage is <0.1pu for duration >1 minute

Short-duration voltage variation:

3 types: interruption, sag, swell. Each can be designated as instantaneous (0.01-0.5s), momentary (0.5–3s) or temporary (3-60s).

Interruption: voltage decreases to <0.1 pu for duration <1 minute

Sag (dip): decrease in rms 50Hz ac voltage to between 0.1 and 0.9pu for duration <1 minute

Swell: increase in rms 50Hz ac voltage to between 1.1 and 1.8pu for duration <1 minute.

Short-duration voltage variation:

Momentary interruption due to a fault and reclosing.

Short-duration voltage variation:

Voltage sag caused by single line to ground (SLG) fault.

Short-duration voltage variation:

Temporary voltage sag caused by motor starting.

Short-duration voltage variation:

Instantaneous voltage swell on un-faulted phase during a single-line to ground (SLG) fault.

Voltage sag performance of EnergyAustralia distribution networks.

Note: ITIC Information Technology Industry Council of America voltage tolerance envelope

Voltage imbalance (unbalance):

Result from unbalanced network impedances or unequal distribution of single-phase loads.

can cause overheating and mal-operation of certain types of 3φ motors.

EnergyAustralia’s objective is not to exceed 6% difference between highest and lowest phase or line steady state voltage (5 minute average) for LV network.

Voltage imbalance (or unbalance)

Waveform distortion:

Steady-state deviation from ideal 50Hz sine wave.

5 different types of waveform distortion. DC offset Harmonics Inter-harmonics Notching Noise

Waveform distortion: DC offset

Presence of DC voltage or current

Caused by equipment with different operating characteristics in each half cycle (e.g. half-wave rectification), geomagnetic disturbance

According to NSW Electrolysis Committee, DC voltage component of neutral conductor w.r.t. earth can reach +/-10V.

Corrosion to earthing system.

AS3100 Approval and Test Specification – Definitions and general requirements for electrical materials and equipment.

Current waveform and harmonics of an adjustable-speed-drive.

Waveform distortion: Harmonics

Waveform distortion: Harmonics Sinusoidal voltage/current components at multiple

frequencies of 50Hz (e.g. 150Hz is third harmonic)

Caused by non-linear devices and loads on system.

AS/NZS 61000.3.6.2001 (Electromagnetic compatibility - Assessment of emission limits for distorting loads in MV and HV power systems). Utilities seek to restrict their contribution at the pcc to 30-50% of these limits.

Waveform distortion: Inter-harmonics

Caused by waveforms that have frequency components that are not integral multiples of fundamental (50Hz)

Main causes are static frequency converters, cyclo-converters, induction motors, arcing devices.

Can cause light flicker, audible noise in audio equipment, vibration in induction machines.

Allowable limits specified in IEC Std 1000.3.9

Mains signaling voltages, injected onto network by the utility, e.g. 492, 750 and 1050 Hz.

Waveform distortion: Notching

Periodic voltage disturbance caused by switching operation of power electronic devices (current commutation)

Extent of distortion determined by depth and width of notch

Frequency components due to notching can be quite high

Present Australian standards limit notch depth to 20% of peak supply voltage at point of common coupling.

Waveform distortion – Notching

Voltage notching caused by three-phase converter

Waveform distortion: Noise

Any unwanted signals that cannot be classified as harmonic distortion or transients and have spectral content < 200 kHz

Caused by power electronic devices, control circuits, arcing equipment, switching power supplies.

Improper grounding exacerbates problem.

In Australia, acceptable limits set by Spectrum Management Agency.

Voltage fluctuation:

Rapid changes in voltage within allowable limits of nominal voltage

AS60038-2000 specifies new standard nominal voltage to be 230V (reduced from 240V). Tolerance is +10% to -6% which means actual supply voltage range will not necessarily be lowered.

EnergyAustralia aim to maintain steady-state voltage (ten-minute average) within +/-10% of nominal voltage of 240V (i.e. between 264V and 216V), at consumer’s terminals. If possible, it will aim for 226-253V range.

Voltage fluctuation:

Voltage flicker caused by arc furnace operation

Note: Flicker can cause lamps to blink rapidly, and is visible to human eyes at flickering frequencies of 6-8Hz.

Power frequency variation:

Deviation of power system fundamental frequency from nominal value (50Hz or 60Hz)

Power system frequency directly related to generator rotating speed. Slight variations in frequency as dynamic balance between load and generation changes. .

Standards set by NEMMCO (National Electricity Market Management Co Pty Ltd).

Affect clock accuracy. Policy of maintaining “electrical time” in NSW to within 3s of Australian Eastern Standard time.

Power frequency variation:

Symmetrical components

Harmonic distortion

Very little distortion in sine wave voltage generated at power stationsCaused by non-linear loads at the end-usersHarmonic distortion = periodic distortion in every AC cycle

Current distortion caused by non-linear resistance

Harmonic current flowing through system impedanceresults in harmonic voltage at the load.

Fourier Analysis

Any periodic waveform can be expressed as a Fourier series.Advantage of using Fourier series analysis

Total harmonic distortion (THD):

For voltage:

(IEEE definition)

Note: IEC defines THD as % of total rms

THDV is a meaningful parameter but THDI may not be.

Total demand distortion (TDD) refers to fundamental of peak demand current.

Crest factor (CF):

Measure of effective value of harmonic components

Effects of harmonics on power factor

Harmonicresonance

TroubleshootingPower Quality Problems

What are the problems ?

Where are the problems occurring ?

When do they occur ?

What else is happening at the same time ?

PQ problem solvingflowchart

Equipment for PQ investigations

Voltmeters, ammeters, multi-meters

Oscilloscopes

Harmonic analyzers

Power quality analyzers

Other devices, e.g. infrared detectors

Fluke 43 power qualityAnalyzer (single phase)

Fluke 430 power qualityAnalyzer (three phase)

DRANETZwww.dranetz-bmi.com

Protection measures

Protection measuresEnergyAustralia Electricity Network Operation Standard (June 2006)

Test next week covers:TransformersFault current calculationsProtection

Thank you

The Electrical SystemAs

A Tandem Bicycle

Ref: Stefan Fassbinder, Sept. 2005www.leonardo-energy.org

Tandem bicycle moving at constant speed

loads

Transmissionnetwork

generators

Constant velocity = constant frequencyConstant tension = constant voltage

Gear = transformer

Inductive power and its compensation

Inductive load compensation

Harmonic distortion

Hyperactive blue rider:

Constantly bending forward and backwards

Not at rhythm of bike but few times faster

Causing bike to jerk forward and backwards.

Saddle mounted on castors = harmonic filter

Keeping constant voltage and frequency

Voltage dip

Voltage rise

50Hzin-phase

Connect to system

Disconnect from system

Three different types of power stations

Traditional power stations(coal-fired turbo-generators)

Slow-speed hydro-generators

Wind turbinesvariable speed drives

Three different types of loads

Resistive loads

Reactive loads

Electrical motors

Electrical energy heat