The University of New South Wales. School of Electrical Engineering and Telecommunications. Industrial and Commercial Power Systems Topic 9

Transcription

1 The University of New South Wales School of Electrical Engineering and Telecommunications Industrial and Commercial Power Systems Topic 9 POWER QUALITY

2 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.

3 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

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

5 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

6 Tolerance envelope for IT equipment. Developed by Information Technology Industry Council (ITIC), formerly Computer and Business Equipment Manufacturers Association (CBEMA).

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

8 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)

9 [Ref. Electricity Network Operation Standards, July 2004]

10 NSW Electricity Information (Independent Pricing and Regulatory Tribunal)

11 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

12 Customer Service Standards (EnergyAustralia) Ref. EA Customer Service Standards brochure

13 Principal phenomena causing electromagnetic disturbances as classified by the IEC.

14 Categories and characteristics of power system electromagnetic phenomena [Ref: R.C. Dugan]

15 Simplified classification system by EnergyAustralia ENOS Electricity Network Operation Standards, June 2006

16 Transients: 2 categories: impulsive and oscillatory. These terms reflect waveshape of transient. Sometimes, also categorized according to their mode: common mode or normal mode.

17 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.

18 Transients: Impulsive transients Lightning stroke current impulse transient.

19 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.

20 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

21 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

22 Short-duration voltage variation: 3 types: interruption, sag, swell. Each can be designated as instantaneous ( s), 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.

23 Short-duration voltage variation: Momentary interruption due to a fault and reclosing.

24 Short-duration voltage variation: Voltage sag caused by single line to ground (SLG) fault.

25 Short-duration voltage variation: Temporary voltage sag caused by motor starting.

26 Short-duration voltage variation: Instantaneous voltage swell on un-faulted phase during a single-line to ground (SLG) fault.

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28 Voltage sag performance of EnergyAustralia distribution networks. Note: ITIC Information Technology Industry Council of America voltage tolerance envelope

29 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.

30 Voltage imbalance (or unbalance)

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32 Waveform distortion: Steady-state deviation from ideal 50Hz sine wave. 5 different types of waveform distortion. DC offset Harmonics Inter-harmonics Notching Noise

33 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.

34 Waveform distortion: Harmonics Current waveform and harmonics of an adjustable-speed-drive.

35 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 (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.

36 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, cycloconverters, induction motors, arcing devices. Can cause light flicker, audible noise in audio equipment, vibration in induction machines. Allowable limits specified in IEC Std Mains signaling voltages, injected onto network by the utility, e.g. 492, 750 and 1050 Hz.

37 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.

38 Waveform distortion Notching Voltage notching caused by three-phase converter

39 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.

40 Voltage fluctuation: Rapid changes in voltage within allowable limits of nominal voltage AS 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 V range.

41 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.

42 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.

43 Power frequency variation:

44 Symmetrical components

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50 Harmonic distortion

51 Very little distortion in sine wave voltage generated at power stations Caused by non-linear loads at the end-users Harmonic distortion = periodic distortion in every AC cycle Current distortion caused by non-linear resistance

52 Harmonic current flowing through system impedance results in harmonic voltage at the load.

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

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55 Total harmonic distortion (THD): Measure of effective value of harmonic components For voltage: (IEEE definition) Note: IEC defines THD as % of total rms THD V is a meaningful parameter but THD I may not be. Total demand distortion (TDD) refers to fundamental of peak demand current. Crest factor (CF):

56 Effects of harmonics on power factor

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59 Harmonic resonance

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62 Troubleshooting Power Quality Problems

63 What are the problems? Where are the problems occurring? When do they occur? What else is happening at the same time?

64 PQ problem solving flowchart

65 Equipment for PQ investigations Voltmeters, ammeters, multi-meters Oscilloscopes Harmonic analyzers Power quality analyzers Other devices, e.g. infrared detectors

66 Fluke 43 power quality Analyzer (single phase) Fluke 430 power quality Analyzer (three phase)

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71 DRANETZ

72 Protection measures

73 Protection measures EnergyAustralia Electricity Network Operation Standard (June 2006)

74 Test next week covers: Transformers Fault current calculations Protection

75 Thank you

76 The Electrical System As A Tandem Bicycle Ref: Stefan Fassbinder, Sept

77 loads Transmission network Constant velocity = constant frequency Constant tension = constant voltage generators Gear = transformer Tandem bicycle moving at constant speed

78 Inductive load compensation Inductive power and its compensation

79 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 Harmonic distortion

80 Voltage dip Voltage rise 50Hz in-phase Connect to system Disconnect from system Keeping constant voltage and frequency

81 Traditional power stations (coal-fired turbo-generators) Wind turbines variable speed drives Slow-speed hydro-generators Three different types of power stations

82 Resistive loads Electrical energy heat Electrical motors Reactive loads Three different types of loads

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