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Ac Power Problems And Measurement Sreevidhya@Students


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Ac Power Problems And Measurement Sreevidhya@Students

  1. 1. Power Analyser Fundamentals Power Analysis and Harmonics
  2. 2. AC power problems and measurement solutions <ul><li>True power and power factor. </li></ul><ul><li>Fourier transforms. </li></ul><ul><li>Three-phase systems and harmonic problems. </li></ul><ul><li>Pulse Width Modulated Motor Drives. </li></ul><ul><li>IEC Standards. Harmonics & Flicker. </li></ul><ul><li>Solutions and Measurements. </li></ul>
  3. 3. Historically, only simple measurements required: AC Motor Fixed Speed Lamp Light Heater Heat
  4. 4. Real and apparent power <ul><li>Instantaneous power is the product of instantaneous voltage and current. </li></ul><ul><li>Resistive load. </li></ul><ul><ul><li>instantaneous W is always (+)ve </li></ul></ul><ul><ul><li>W = V x A </li></ul></ul><ul><li>Inductive load </li></ul><ul><ul><li>current is phase shifted </li></ul></ul><ul><ul><li>instantaneous W sometimes (-)ve </li></ul></ul><ul><ul><li>W  V x A </li></ul></ul><ul><li>Apparent Power = V x A </li></ul><ul><li>Power Factor = Cos  </li></ul>W W
  5. 5. Modern electronic loads More control Greater efficiency Compact Greater efficiency <ul><li>Complex waveforms demand sophisticated analysis </li></ul><ul><li>Drive to better efficiency - more accurate analysis </li></ul>PWM Drive Variable speed HF Ballast Light
  6. 6. Rectified and Capacitor Smoothed inputs Current only flows when AC supply voltage is greater than capacitor voltage.
  7. 7. Power factor and cos  <ul><li>The power factor is reduced . Not by phase displacement, but by shape distortion. </li></ul><ul><li>Why is this important? </li></ul><ul><li>Power factor of <1 means generation and transmission must be oversized: </li></ul><ul><li>Power lines and transformers sized to supply VA, not Watts. </li></ul><ul><li>Power factor <1 due to harmonic distortion: </li></ul><ul><li>Cannot be corrected by capacitors. (Capacitors may be damaged). </li></ul><ul><li>Distorts Voltage which increases the losses in motors. </li></ul><ul><li>Creates electromagnetic interference. EMI. </li></ul><ul><li>Neutral burn-out in 3 phase 4 wire systems. </li></ul><ul><li>Distortion is measured and controlled in terms of harmonics. </li></ul>
  8. 8. Fourier transforms Any repetitive waveform may be described by a series of sinewaves….
  9. 9. DFT vs FFT <ul><li>Two methods of performing the calculation. DFT and FFT. D – Discrete F - Fast F ourier T ransform </li></ul><ul><li>Sampling Rate and Synchronisation </li></ul><ul><ul><li>FFT must be 2 n , E.g. 2 10 = 1024 samples per cycle Difficult when sampling rate fixed! </li></ul></ul><ul><ul><li>Problem corrected by the use of windows (e.g. Hanning) </li></ul></ul><ul><ul><li>DFT is synchronised by definition </li></ul></ul><ul><ul><li>No problem with DFT, no filters or windows (E.g. ‘Hanning’) to correct errors. </li></ul></ul><ul><li>FFT also accumulates errors - DFT error the same for each order. </li></ul>
  10. 10. Power factor and cos  IEC / IEEE Definition Power Factor = Watts Volts x Amps PF = 0.65 Cos  Cos  Angle between fundamental (1st harmonic) voltage and current. Voltage Current Harmonics Fundamental (1st Harmonic) 3rd Harmonic Total Current
  11. 11. True Watts computation
  12. 12. Functions derived by computation <ul><li>Watts </li></ul><ul><li>Volts RMS </li></ul><ul><li>Amps RMS </li></ul><ul><li>Volts DC </li></ul><ul><li>Amps DC </li></ul><ul><li>Volts Pk </li></ul><ul><li>Amps Pk </li></ul><ul><li>Frequency </li></ul><ul><li>Volt-Amperes </li></ul><ul><li>Volt-Amperes Reactive </li></ul><ul><li>Power Factor </li></ul><ul><li>Volts Crest Factor </li></ul><ul><li>Amps Crest Factor </li></ul><ul><li>Harmonics </li></ul>All primary power parameters can be computed from digital samples
  13. 13. Multi-phase measurements Single Phase. <ul><li>2W lost for every 100W delivered (example) </li></ul><ul><li>Induction motors require extra winding to start. </li></ul>Three Phase – 120 ° apart. A B Sum of voltage always = 0 A can be joined to B. 100W 1W 1W 100W 1W 1W 100W 1W 100W 1W
  14. 14. 100W 1W Multi-phase measurements Three-phase “delta” <ul><li>1W lost for every 100W delivered (1/2 the copper required for 3 x 2 wire) </li></ul><ul><li>Induction motors require extra winding to start. </li></ul>1W 1W <ul><li>“ Star” or “Wye” Connection. </li></ul><ul><li>Centre point, N is called star or Neutral point. </li></ul>N
  15. 15. Multi-phase measurements Power Station 3-phase and 1-phase loads 415V phase to phase 230V phase to neutral Distribution Transformer Industrial, Office and Domestic Loads N For balanced loads (same current in each phase), neutral current = 0 Small neutral conductor
  16. 16. Triplen harmonics Definition: ‘Odd Harmonics divisible by three’. E.g. 3, 9, 15, 21, 27, 33… Why are they so important to the three phase power industry? Third harmonics of each phase IN PHASE with each other.
  17. 17. Triplen harmonics in 3 phase power systems <ul><li>Burnout of neutral conductor or damage to related components </li></ul><ul><li>Overheating of distribution transformers </li></ul>All harmonics reduce the efficiency of a power system and loads connected to it. Triplen harmonics are significant because the harmonics in each conductor are in Phase. Triplen harmonics can therefore be much more damaging.
  18. 18. Multi-phase measurements 2 Wattmeter vs. 3 Wattmeter connections #1 3 Wattmeter #2 2 Wattmeter 2 wattmeter connection provides all the information required for 3-wire measurements. V1 V2 V3 V1 ph-ph V2 ph-ph
  19. 19. Multi-phase measurements Neutral Current Measurements. Kirchoff’s Law: I1 + I2 + I3 = 0 When the sum is not 0, this current must be flowing in the Neutral. This calculation performed on a SAMPLE by SAMPLE basis. <ul><li>All data returned to central processor at ground. </li></ul><ul><ul><li>Vector computation of inter-phase quantities. </li></ul></ul><ul><ul><li>Harmonics of ‘neutral”. – Can also be used to calculate ground current for three-wire motor-drive connection. </li></ul></ul>V1 A1 V2 A2 V3 A3 Central Processor Samples Isolation Display
  20. 20. AC motors Input 2 Wattmeter or 3 Wattmeter method. Output Torque and Speed Mechanical Power V I Electrical to Mechanical Efficiency
  21. 21. PWM motor drives Distorted AC Input DC Bus PWM Output Torque & Speed xxx NM & Rev / Min
  22. 22. PWM voltage spectrum Fundamental (E.g. 40 Hz) Switching or carrier (E.g. 10, 000 Hz) Total Power = Fundamental + Harmonics (f) + Carrier + Harmonics(c) + Noise
  23. 23. PWM mode Filters 5Hz to 1kHz Low Frequency Measurements Frequency High Frequency Measurements Sync Fundamental Data Frequency Total W, V, A... Measurements to full 1MHz bandwidth
  24. 24. PWM Mode - Example Fundamental (E.g. 40 Hz) Switching or carrier (E.g. 10, 000 Hz) Fundamental 230 V Filtered 240V Total 255V <ul><li>What’s required? </li></ul><ul><li>Total Power (1MHz bandwidth) </li></ul><ul><li>Fundamental Power </li></ul>Motor efficiency ~ Fund Power x 100% Total Power Total Power = Fundamental + Harmonics (f) + Carrier + Harmonics(c) + Noise
  25. 25. Motor Start-Up and Transients <ul><li>PWM Waveform difficult to synchronize with steady-state. </li></ul><ul><li>Very difficult when fundamental frequency changing. </li></ul><ul><li>Unique cycle-by-cycle mode of the PM3000A avoids the need for time-consuming calculations upon raw data points. </li></ul><ul><li>Gives what is of interest, </li></ul><ul><li>Total power, PF, V, A etc for every cycle. </li></ul>Waveform
  26. 26. Motor Start-Up and Transients A rms Waveform
  27. 27. IEC Standards Harmonics To overcome the problems described with harmonics, the IEC lays down limits that equipment must meet. Standards cover equipment up to 75A. The PM3000A: <ul><li>DFT measurement of 16 cycles of waveform. </li></ul><ul><li>Anti-aliasing filter as specified. </li></ul><ul><li>Rectangular window - No gaps </li></ul><ul><li>Harmonic accuracy better than 0.2%. </li></ul><ul><li>AC source voltage, frequency and harmonics measured during the test. </li></ul><ul><li>Single low value shunt ensures compliance throughout dynamic range. </li></ul><ul><li>Certified, traceable accuracy. </li></ul>
  28. 28. IEC Standards. Flicker. The IEC aim to limit level of voltage fluctuations that equipment may cause to AC power lines. Standards exist for up to 75A. Change in the load current will cause change in voltage across the load. That change in voltage causes a squared change in lamp output intensity. The perception of ‘flicker’ is dependant on the response of the lamp, our eyes and our brain.
  29. 29. How the PM3000A meets the requirements IEC Flicker Testing <ul><li>Complete IEC61000-4-15 flicker meter for IEC61000-3-3. </li></ul><ul><li>dc, dt and dmax testing. </li></ul><ul><li>Flicker accuracy proved by using both sine and square modulations. </li></ul><ul><li>Impedance network available for full compliance measurements. </li></ul><ul><li>Windows TM PC software for presentation, storage, and analysis of results. </li></ul>
  30. 30. Solutions <ul><li>Drives with harmonic correction – Passive or Active. </li></ul><ul><li>External Filters. </li></ul><ul><li>Zig-zag transformers. </li></ul><ul><li>Controlling inrush and transients to meet flicker standards. </li></ul>
  31. 31. PM3000A Measurement Summary <ul><li>All measurements made digitally - No analogue error Stable over time Quick to calibrate </li></ul><ul><li>Accuracy maintained with wide range of input signal level and frequency: 0.05% Basic - 600V rms, 30A rms, DC to 1MHz </li></ul><ul><li>Single input shunt technology, accurate with high reliability. </li></ul><ul><li>Specification maintained with distorted power waveform. </li></ul><ul><li>DFT (not FFT) harmonic analysis system </li></ul><ul><li>Sample by sample calculation across 3 phases allows neutral calculation. </li></ul><ul><li>Useful additional functions like Inrush Current, Distortion and a.c. Impedance </li></ul><ul><li>Easy to use, all main power parameters available at push of button. </li></ul><ul><li>Non volatile memory to save any complicated front panel setup configuration. </li></ul><ul><li>Single phase, three phase and independent channel operation. </li></ul><ul><li>Fast computation for ‘real time’ feel </li></ul><ul><li>Many special modes: PWM Motor drive, Ballast, IEC testing etc. </li></ul><ul><li>Application specific software available, VPAS, (PM3000A) VPAS Lite (PM100), IEC1000-3-2/3 for Windows (PM3000A) </li></ul>
  32. 32. Voltech Measurement Solutions <ul><ul><ul><li>Formed by power electronics engineers to solve everyday power electronics test and measurement problems. </li></ul></ul></ul><ul><ul><ul><li>World’s first commercial digital power analyser. </li></ul></ul></ul><ul><ul><ul><li>Patent on automatic transformer testing. </li></ul></ul></ul>PM3000A PM100 Single Phase PM300 Three Phase Low-cost Analysers AT3600 Automatic Transformer Tester ATi LCR Meter / Automatic Tester