Introduction to Waveform Analysis

Transcription

1 Introduction to Waveform Analysis Course: PQ-WF201 Presented by: PowerCET Corporation 3350 Scott Blvd., Bldg. 55 Unit 1 Santa Clara, CA USA 408/ FAX 408/ training@powercet.com consulting@powercet.com Web Page:

2 Introduction to Waveform Analysis Course: PQ-WF201 Presented by: PowerCET Corporation 3350 Scott Blvd. Bldg. 55 Unit 1 Santa Clara, CA / Fax 408/ training@powercet.com URL: Measurement Fundamentals 1

3 Electrical Fundamentals Voltage V (e) Difference in potential Current A (i) Electron flow Wattage W Energy dissipated 3 Direct Current Measurements Peak measurement Maximum value = 2 Average measurement Mean of a waveform = 20/10 = 2 RMS Root Mean Squared = 40 (40/10) 1/2 = 2 4 2

4 Alternating Current Measurements Peak Maximum value = 2 Average Half cycle = Full cycle = 0 RMS Half cycle (360/180) 1/2 =1.414 Full cycle (720/360) 1/2 = Half Cycle RMS Calculation 10 points per half cycle Squared sample values = 20 {(20/10) 1/2 = 1.414} 6 3

5 Sinusoidal Waveform - AVG vs RMS Average = Mean Area = / 180 = 6.36 RMS Squared area = 9000 (9000/180) 1/2 = 7.07 Form Factor RMS/Avg = Sinusoidal Properties Sinewave Peak = 10 RMS = 7.07 Avg = 6.36 Form Factor RMS/Avg = 1.11 Crest Factor Peak/RMS =

6 SWM Current - Avg vs RMS Average = Mean Area = / 180 = 2.07 RMS Squared area = 2460 (2460/180) 1/2 = SWM Properties Peak = 10 RMS = 3.69 AVG = 2.07 Form factor RMS/Avg = 1.78 Crest factor Peak/RMS =

7 Measurement Comparison Sinusoidal vs pulsed Peak: 10 vs 10 Average: 6.36 vs 2.07 RMS: 7.07 vs 3.69 Form factor (RMS/Avg) Sinusoidal waveforms = 1.11 Switch mode current = 1.78 Rectangular waveforms < 1.11 Pulsed waveform > 1.11 Crest factor (Peak/RMS) Sinusoidal = Nonsinusoidal = 2.71 (or higher) Pulsed waveforms >> Waveform Effect on Multimeters WAVEFORM DESCRIPTION 10 UNITS PEAK 60 HZ AVERAGE SENSE RMS CALIBRATED PEAK SENSE RMS CALIBRATED TRUE RMS SENSE SINUSOIDAL TRIANGULAR SQUARE RECTIFIED PULSED POLE RECTIFIED

8 Slew Rate Effect on Multimeters Measurement of Sinusoidal Waveforms Frequency 60 Hz 180 Hz 420 Hz 1 khz 5 khz 10 khz Averaging DMM RMS Calibrated Clamp-on Ammeter/DMM Wideband DMM True RMS Test Signal = 10 Volts Peak 13 Waveform Effect on Multimeters RMS is the dc equivalent Average Sense RMS Calibrated 3% low with triangular waveforms 12% high with square waveforms 41% low with switch mode 45% low with 6 pole Peak Sense RMS calibrated 19% high with triangular waveforms 12% high with square waveforms 16% low with switch mode 38% low with 6 pole 14 7

9 Data Acquisition Signal Acquisition Snapshot Manual or automatic acquisition of instantaneous values Waveform data may include momentary disturbances Triggered Threshold activated acquisition of waveform data Waveform data WILL include momentary disturbances Composite Waveform data made up of averaged data 64 cycles of data averaged into a single cycle Averaging reduces the effects of momentary disturbances and high frequency interference. 16 8

10 Signal Processing Low and high pass filters separate regular voltage and current waveforms from transient waveforms. Peak hold easier to provide than complete digitized signal. 17 Aliasing and Digitizing Devices Nyquist theorem Sample rate > 2x highest frequency Insufficient sampling rate = Aliasing 18 9

11 Harmonic Frequency Content 1 8K sample/second No aliasing Harmonic Order Fund = 50/60 Hz 3rd = 150/180 5th = 250/300 7th = 350/420 9th = 450/540 11th = 550/ Harmonic Frequency Content 2 1K Sample/sec Very aliased Harmonic Order Fund - OK 3rd - marginal 5th - sawtooth 7th - verge of aliasing 9th - aliased 11th - aliased 20 10

12 Sample Rate vs Accurate Measurements 120 samples per cycle 60th harmonic 3kHz at 50Hz; 3.6kHz at 60Hz 128 samples per cycle 64th harmonic 3.2kHz at 50Hz; 3.8Khz at 60Hz 256 samples per cycle 128th harmonic 6.4kHz at 50Hz; 7.6Khz at 60Hz 21 Measurement Concerns Sampling Sampling >/= 2x maximum harmonic of interest 2 n samples for FFT Synchronous sampling Vertical resolution 8 bit = 256 discrete levels 10 bit = 1,024 discrete levels 12 bit = 4,096 discrete levels 14 bit = 16,384 discrete levels 16 bit = 65,536 discrete levels 22 11

13 Signal Modes There are 15 measurements for 3 phase power Line-to-line: A-to-B; B-to-C; C-to-A Line-to-neutral: A-to-N; B-to-N; C-to-N Neutral-to-ground Three phases of current plus neutral and ground current INTERFERENCE MODE MEASUREMENT METHOD NORMAL MODE V LINE NEUTRAL GROUND VOLTAGE DIFFERENTIAL LINE-TO-NEUTRAL PHASE-TO-PHASE CURRENT MEASUREMENT PHASE CURRENT NEUTRAL CURRENT COMMON MODE V LINE NEUTRAL GROUND VOLTAGE DIFFERENTIAL NEUTRAL-TO-GROUND CURRENT MEASUREMENT LINE & NEUTRAL SUMMATION GROUND CURRENT LONGITUDINAL MODE V LINE NEUTRAL GROUND VOLTAGE DIFFERENTIAL IMBALANCE BETWEEN CONDUCTORS NEUTRAL-TO-GROUND CURRENT MEASUREMENT MULTIPLE CONDUCTOR SUMMATION 23 Single Ended versus Differential Shielded cables may develop shield voltages Differential is best but not used much at high frequencies A DIFFERENTIAL SIGNAL = A-B B A SINGLE ENDED SIGNAL = A-G 24 12

14 Power Measurements DC vs AC current and voltage Power = Voltage (V) x Current (I) for DC Power = Voltage (e) x Current (i) for AC x Cosθ 25 Typical Measurement Accuracy Voltage & current channels: 1% Usually expressed as a percentage of full scale Power: 2% Combination of voltage and current channel accuracies Current probes Accuracy varies with signal amplitude versus range Less accuracy with light loading Accuracy varies with conductor position Accuracy rolls off with higher frequencies 26 13

15 BMI A-116 CT Accuracy 27 Measurement Recap Know your meter!!! Use only true RMS for AC measurements Higher bandwidth means better measurements Beware high frequency distortion effects Match current probes to the load current If available use highest bit structure 28 14

16 Adding RMS Values Waveform Displacement 30 15

17 Basic Measurements Voltage & current (RMS only please) VA (Volt-Ampere) = Vrms * Arms Watts -- Average of instantaneous power curve 31 Inductive/Resistive Loading Single phase measurements RMS current = 7.3 Amperes; Power = 540 W True PF(Watts/Voltampere) = 0.63 Displacement PF = 51 lagging (0.63) 32 16

18 Capacitive/Resistive Loading Single phase measurements RMS current = 6.4 Amperes; Power = 560 W True PF(Watts/Voltampere) = 0.75 Displacement PF = 41 leading (-0.75) 33 Inductive/Capacitive/Resistive Single phase measurements RMS current = 4.9 Amperes; Power = 570 W True PF(Watts/Voltampere) = 0.98 Displacement PF = 6 lagging (0.99) 34 17

19 Computer Power Supply (SWM) Single phase measurements RMS current = 13.9 Amperes; Power = 1.19kW True PF(Watts/Voltampere) = 0.76 Displacement PF = 8 lagging (0.990) 35 Drives (VFD, VSD, ASD) Single phase measurements RMS current = 11.2 Amperes; Power = 1.05kW True PF (Watts/Voltampere) = 0.80 Displacement PF = 3 lagging (0.998) 36 18

20 Combined SWM & VSD Single phase measurements RMS current = 22.3 Amperes; Power = 2.31kW True PF (Watts/Voltampere) = 0.92 Displacement PF = 7 lagging (0.993) 37 Harmonic & Phase Angle Effects Adding RMS numbers with phase angles oresultant higher than true value o = 13.7 in previous slides VS 4.9 Adding RMS numbers with harmonic oresultant higher than true value o = 25.1 in previous slides VS 22.3 RMS cannot take into account effects of adding harmonics or phase angle displacement 38 19

21 Phase Angle & Measurements Generating Electricity Polyphase Generator Sinusoidal Voltage 40 20

22 Cosine of Angular Displacement 1.0 = cos0 ; 0.98 = cos10 ; 0.93 = cos20 ; 0.86 = cos30 ; 0.76 = cos40 ; 0.64 = cos50 ; 0.50 = cos60 ; 0.34 = cos70 ; 0.17 = cos80 ; 0.0 = cos90 41 Wye Vs Delta Measurement of VFD 42 21

23 Wye Measurement of VFD 43 Delta Measurement of VFD 44 22

24 Wye Vs Delta Measurement Wye measurement mode VTHD = 5.06%THD; 5th is 4.184% Fundamental displacement is 6 Delta measurement mode VTHD = 4.79%; 5th is 4.059% Fundamental displacement is 35 Measurement considerations Some monitors automatically perform wye measurements in delta measurement mode It is best to monitor in the mode that the load draws power Some loads have so much phase shift the wye is the only choice 45 Phase/Neutral & Phase/Phase 46 23

25 Loads & Services Single Phase SWM 48 24

26 Single Phase DC Drive 49 Single Phase Motor 50 25

27 240/120 Volt Service & SWM Load /120 With Computer Loading 52 26

28 240/120 Measurements 53 3 Phase Wye & SWM Loads 54 27

29 3 Phase Wye & ASD Loads 55 3 Phase Wye & Motor Load 56 28

30 High Leg Delta (Sheet Fed Press) Phase A Phase B Phase C 57 Hi-Leg Measurements 58 29

31 Monitor Measurements VAR (volt-ampere reactive) Measurement left over from days of sinusoidal loads Indication of reactive elements Cosine of angle = resistive element Sine of angle = reactive element VA (Volt-Ampere) = Vrms * Arms Watts -- Average of instantaneous power curve 60 30

32 Phase Angle Displacement Angular displacement between voltage and current fundamentals Also called displacement power factor Does not include harmonics effects if based upon fundamentals 61 PF (power factor) = watts / volt-ampere True power factor includes both displacement & harmonics Displacement PF = Cosine of E&I angle -- provides +/- sign Min = 0.5, Max = 0.99, Median = 0.85, 62 31

33 Three Phase Wye SWM PF 63 Vthd & Ithd Total harmonic distortion of voltage and current E=IZ -- can't have one without the other 64 32

34 Comparative Plot - Irms & Ithd When sinusoidal loads are turned off at night then I THD will rise because THD is usually based upon a percentage of the fundamental. 65 Voltage unbalance Percentage expressing deviation in phase voltages Voltage unbalance affects motors and polyphase power supplies Voltage unbalance commonly caused by single phase loads 66 33

35 Icf (current crest factor) Ratio of waveform peak vs rms level Used for UPS inverter sizing 67 KF (K factor) Numeric indication of harmonic waveform content Used for transformer rating Summation of all odd order harmonics -- H# 2 *pu

36 Flicker PST (short term) & PLT (long term) RPM data older standard 69 Current Sources 35

37 Resistive Loads Light bulbs & heaters VA = W; PF = 1 71 Single Phase Angle Control Loads Light dimmers and heater controls Odd order harmonics dominant Power factor and THD depend upon phase angle Even order harmonics present when firing angle varies 72 36

38 Full Wave Power Supply Computer loads Harmonics Odd orders dominant 3rd, 5th, 7th, 9th, 11th... Zero sequence harmonics Triplens - 3rd, 9th, 15, 21st... THD can exceed 100% Referenced to fundamental Power factor typically Ferroresonant Lighting Ballast Power factor = 0.4; Current distortion = 19%THD This is the type of load that led to changes in lighting ballasts

39 Electronic Ballast CBM (Certified ballasts manufacturers) Odd harmonics present out to 50th harmonic 17% THD -- low distortion but high frequency effects 75 Electronic Discharge Lighting Dominant harmonics to 50th harmonic Largest harmonics -- 25th, 31st, 37th 49% THD 76 38

40 Compact Fluorescent Electronic Ballast Prounounced harmonics beyond the 50th 128% THD 77 Half Wave Power Supply Half cycle pulsed current Asymmetrical current DC bias Power Factor 0.5 true power factor Displacement power factor if load is inductive Harmonics Odds - 3rd, 5th, 7th, 9th... Evens - 2nd, 4th, 6th, 8th... Source of zero sequence harmonics (triplens) - 3rd, 9th, 15, 21st

41 Half Wave Rectified Loads Transformer Primary Transformer Secondary kva UPS UPS trips off-line Occurs during outages Voltage B-N = 120Vrms THD = 4% Odd harmonics = 3.2% Even harmonics = 2.5% Current THD = 43% Balance = 23, 31, 18 (Arms) Odd harmonics = 39% Even harmonics = 17.7% 2nd harmonic = 17.4% 3rd harmonic = 35.5% 4th harmonic = 2.2% 80 40

42 6 Pulse Voltage Fed Load Each half cycle Double pulsed current Symmetrical current Power Factor 0.8 true power factor - typical No displacement power factor Harmonics Odd harmonics Frequency doublets (6n +/- 1) 5th, 7th; 11th, 13th Pulse Current Fed Load Pulsed current each half cycle Asymmetrical current due to commutation differences Power Factor 0.6 to 0.8 true power factor Displacement power factor due to motor load Harmonics Odd harmonics Doublets (6n +/- 1) 5th, 7th; 11th, 13th... Even harmonics - limited amounts 82 41

43 6 Pole Combination SCR/Diode Current duty cycle In-phase pulsed current for one half cycle Quadrature current for alternate half cycle Asymmetrical current Power Factor 0.5 to 0.8 true power factor Displacement power factor due to load and inductance Harmonics - ITHD = 46.23% Odd harmonics - 5th, 7th, 11th, 13th % THD Even harmonics - 2nd, 4th, 6th, 8th % THD Source of zero sequence harmonics (triple Pulse Voltage Fed Load 84 42

44 12 Pulse Current 85 Harmonic Order Harmonic # Harmonic Sequence 3 Pulse (L-N) 6 Pulse 12 Pulse 18 Pulse 24 Pulse 3 0 x 5 - x x 7 + x x 9 0 x 11 - x x x 13 + x x x 15 0 x 17 - x x x 19 + x x x 21 0 x 23 - x x x x 25 + x x x x 27 0 x 29 - x x 31 + x x 33 0 x 35 - x x x x 37 + x x x x 39 0 x 41 - x x 43 + x x 45 0 x 47 - x x x x 49 + x x x x 86 43

45 Voltage Distortion-- IEEE 519 IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems Voltage distortion not to exceed 5%THD at PCC and no single harmonic to exceed 3%. These numbers are frequently used in equipment guidelines. V harmonic PCC I harmonic 87 Current Distortion -- IEEE519 There are no equipment specific limits for current harmonics. Current harmonics normally cause neutral current problems. Current harmonics effects depend upon system impedance Maximum Harmonic Current Distortion in Percent of IL Individual Harmonic Order (Odd Harmonics) ISC / IL <11 11 h < h < h < h TDD <20* < < < > Even harmonics are limited to 25% of the odd harmonic limits above. Current distortions that result in a dc offset, e.g., half-wave converters, are not allowed. *All power generation equipment is limited to these values of current distortion, regardless of actual Isc / IL. where Isc = maximum short-circuit current at PCC IL = maximum demand load current (fundamental frequency component) at PCC

46 Delta-Wye Current Flow Current in each phase of secondary develop as line currents on two phases of the primary. H1 supplies current to phase A and returns current from phase C. 89 Delta-Wye: SWM Primary Secondary 90 45

47 Delta-Wye: 6 Pulse AC Drive Primary Secondary 91 Facility Load Combinations 92 46

48 Interference Susceptibility Voltage Waveform Notching 94 47

49 Zero Cross Voltage Error Extra zero voltage crossings can cause process errors for equipment that rely upon voltage waveforms for frequency reference. This can affect SCRs, Triacs, clocks. 95 Oscillatory Transients DC notch & overshoot 96 48

50 Repetitive Transients Continual presence increases potential for adverse effects (simply a timing issue) Transients present on one phase of a residential 240/120 volt service 97 ANSI/IEEE C IEEE Recommended practice on surge voltages in low-voltage ac power circuits. Category A (receptacles) Low exposure inside building: 2kV Medium exposure closer to service entrance: 4kV High exposure at service entrance: 6kV Electrical fast transient Test severity level 1: 1kV Test severity level 2: 2kV Test severity level 3: 4kV 98 49

51 CBEMA Computer & Business Equipment Manufacturers Association 99 ITIC Information Technology Industry Council

52 Full Range ITIC Plot Acceptable limit for transient voltage 875% of nominal = 3500 for 1uS!!! 101 ANSI Amercian National Standards Institute 350% is 1400 volts for 1mS!!!

53 Neutral-to-Ground Voltage NEC fine print note (a) FPN No.4 Branch circuits sized for maximum 3% drop with total 5% drop for feeder and branch circuit. 120 Vac 5% drop = 6 volts 3 volts lost in supply and 3 volts lost in return per phase Polyphase systems N/G can reach 5 volts (3*1.73 = 5.2) 103 Floated Services 52

54 CNC Case Study Floating Power Source 105 Phase-to-Phase Summaries

55 Phase-to-Ground Summary 107 Power Monitor - Event Triggered

56 Floated Delta-Delta Service Ground referenced voltages vary with loading Ground referenced voltages vary with fault conditions -- external and internal 109 Floated Delta Service With Faults Event data falls above and below ANSI curve Some short duration events reach 350% of nominal

57 Delta-Delta Ground Referenced Voltage Ground referenced voltage reaches almost 900 volts Equipment failures at site included drives and MOVs 111 Delta-Delta Waveforms Ground referenced voltage rise is so large that it exceeds input range of the power monitor

58 UPS Output Waveforms Output Waveforms Sinusoidal increasingly dominant Pseudo-sine still popular

59 Pseudo-Sine Peak voltage roughly equivalent to sinusoidal value Root-mean-square value equivalent to sinusoidal waveform Load powered normally Vrms = 118; Vthd = 4% Irms = 1.5; Ithd = 111% Watts = 118 Load powered via inverter Vrms = 114; Vthd = 35% Irms = 1.1; Ithd = 41% Watts = Slow Transfers To/From Inverter Transfer time about 30 milliseconds Load continues to operate because internally stored energy of power supply maintains output

60 Frequency Fluctuations Initial frequency following transfer is about 240Hz. Steep slope of waveforms might affect PFC circuitry of load 117 Momentary Output Voltage Loss

61 Sinusoidal Output Waveform quality affected by output filter network Inverter stability affected by stored energy and dc transfer 119 Load Demand Effects Current pulls output filter out of tune Current magnitude limited by inverter/filter

62 Load Harmonic Effects 121 UPS Bypass Phase Angles Project: Install manual bypass for installed UPS without shutting down the UPS or loads Check input and output phase angles and select the correct stepdown/bypass transformer with the correct phase angle

63 Same Input & Output Use a wye-to-wye transformer in bypass 123 Shifted Input & Output Use a delta-wye transformer to match shift

64 Case Studies Rapid Light Bulb Failures

65 Voltage At The Site 7 voltage sags over a 35 day period 127 Voltage Sag Event Voltage sags accompanied by change in current at start and end of sag

66 Site Voltage Distortion Voltage distortion reaches 9% 129 Single Voltage Distortion Worst voltage distortion occurs late at night Store closes at 9:00PM Lights remain on all evening

67 Voltage Distortion & Lamp Current For every transition in voltage distortion there is a change in current 131 Bulb Failure Conclusions Lighting fixtures very susceptible to momentary voltage transitions Problem referred to manufacturer Solutions for site: UPS on the lighting circuits to stabilize voltage and remove voltage distortion

68 Transformer Current Imbalance Dry-type transformer kva with 3.9%Z Current imbalance discovered during routine preventive maintenance 133 Transformer Measurements Transformer input power and current varies much more than output Transformer is lightly loaded with 2.2 kw internal loss Input: 12.2 kw & 39.5 kva Output: 10 kw & 13.1 kva Phase Amperes kw kva PF DPF A B C A B C

69 Transformer Waveforms Input voltage/current Output voltage/current 135 Chiller Dropout Problem Chillers dropout frequently One severe storm caused several dropouts

70 Chiller Voltage Waveforms Continual voltage transient activity 137 Voltage & Transient Data - Chillers

71 Mains Voltage Waveforms No significant voltage distortion 139 Voltage & Transient Data - Mains

72 Package Unit - Chiller Investigation Start time End time Duration 05 : 37 : : 42 : : 05 : : 00 : : 13 : : 13 : : 57 : : 03 : : 06 : : 14 : : 17 : : 02 : : 52 : : 52 : : 00 : : 53 : : 53 : : 00 : : 54 : : 55 : : 00 : : 56 : : 56 : : 00 : : 08 : : 13 : : 04 : : 17 : : 18 : : 00 : : 18 : : 18 : : 00 : : 21 : : 21 : : 00 : : 32 : : 33 : : 00 : Lab Equipment Investigation

73 Concluding Comments It is always easier to evaluate monitor data if equipment logs of problems are available. Equipment specifications, if available, also help data evaluation. If the data does not make sense...maybe it isn't a power problem. Beware of events that do not appear natural...busy fingers can make a lot of problems