Power Analyser Fundamentals

1

• Power Analyser Fundamentals
• Power Analysis and Harmonics

2
AC power problems and measurement solutions

• True power and power factor.
• Fourier transforms.
• Three-phase systems and harmonic problems.
• Pulse Width Modulated Motor Drives.
• IEC Standards. Harmonics Flicker.
• Solutions and Measurements.

3

• Historically, only simple measurements
  required

4
Real and apparent power

• Instantaneous power is the product of
  instantaneous voltage and current.
• Resistive load.
• instantaneous W is always ()ve
• W V x A

• Inductive load
• current is phase shifted
• instantaneous W sometimes (-)ve
• W ? V x A
• Apparent Power V x A
• Power Factor Cos??

5
Modern electronic loads
More control Greater efficiency
Compact Greater efficiency

• Complex waveforms demand sophisticated analysis
• Drive to better efficiency - more accurate
  analysis

6
Rectified and Capacitor Smoothed inputs
Current only flows when AC supply voltage is
greater than capacitor voltage.
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Power factor and cos?

• The power factor is reduced . Not by phase
  displacement, but by shape distortion.
• Why is this important?
• Power factor of lt1 means generation and
  transmission must be oversized
• Power lines and transformers sized to supply
  VA, not Watts.
• Power factor lt1 due to harmonic distortion
• Cannot be corrected by capacitors. (Capacitors
  may be damaged).
• Distorts Voltage which increases the losses in
  motors.
• Creates electromagnetic interference. EMI.
• Neutral burn-out in 3 phase 4 wire systems.
• Distortion is measured and controlled in terms of
  harmonics.

8
Fourier transforms
Any repetitive waveform may be described by a
series of sinewaves.
9
DFT vs FFT

• Two methods of performing the calculation. DFT
  and FFT.D Discrete F- Fast Fourier
  Transform

• Sampling Rate and Synchronisation
• FFT must be 2n, E.g. 210 1024 samples per
  cycleDifficult when sampling rate fixed!
• Problem corrected by the use of windows (e.g.
  Hanning)
• DFT is synchronised by definition
• No problem with DFT, no filters or windows (E.g.
  Hanning) to correct errors.

• FFT also accumulates errors - DFT error the same
  for each order.

10
Power factor and cos?
Cos?1
PF 0.65
IEC / IEEE Definition Power Factor
Watts Volts x
Amps
Cos?? Angle between fundamental (1st harmonic)
voltage and current.
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True Watts computation
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Functions derived by computation
All primary power parameters can be computed from
digital samples

• Watts
• Volts RMS
• Amps RMS
• Volts DC
• Amps DC
• Volts Pk
• Amps Pk
• Frequency
• Volt-Amperes
• Volt-Amperes Reactive
• Power Factor
• Volts Crest Factor
• Amps Crest Factor
• Harmonics

13
Multi-phase measurements
Single Phase.

• 2W lost for every 100W delivered (example)
• Induction motors require extra winding to start.

Three Phase 120 apart.
B
Sum of voltage always 0 A can be joined to B.
A
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Multi-phase measurements
Three-phase delta
1W

• 1W lost for every 100W delivered (1/2 the
  copper required for 3 x 2 wire)
• Induction motors require extra winding to start.

1W
100W
1W

• Star or Wye Connection.
• Centre point, N is called star or Neutral point.

15
Multi-phase measurements
Distribution Transformer
Industrial, Office and Domestic Loads
Power Station
3-phase and 1-phase loads 415V phase to
phase 230V phase to neutral
N
For balanced loads (same current in each phase),
neutral current 0
Small neutral conductor
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Triplen harmonics
Third harmonics of each phase IN PHASE with each
other.
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?
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Triplen harmonics in 3 phase power systems
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.

• Burnout of neutral conductor or damage to related
  components
• Overheating of distribution transformers

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.
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Multi-phase measurements
Neutral Current Measurements. Kirchoffs 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.

• All data returned to central processor at ground.
• Vector computation of inter-phase quantities.
• Harmonics of neutral. Can also be used to
  calculate ground current for three-wire
  motor-drive connection.

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AC motors

• Input 2 Wattmeter or 3 Wattmeter method.
  

Output Torque and Speed Mechanical Power
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PWM motor drives
Distorted AC Input DC Bus PWM Output
Torque Speed
xxx NM Rev / Min
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PWM voltage spectrum
Fundamental (E.g. 40 Hz)
Switching or carrier (E.g. 10, 000 Hz)
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PWM mode
Fundamental Data
Filters 5Hz to 1kHz
Low Frequency Measurements
Frequency
Frequency
Sync
High Frequency Measurements
Total W, V, A...
Measurements to full 1MHz bandwidth
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PWM Mode - Example
Fundamental 230 V
Filtered 240V
Total 255V
Fundamental (E.g. 40 Hz)
Switching or carrier (E.g. 10, 000 Hz)

• Whats required?
• Total Power (1MHz bandwidth)
• Fundamental Power

Motor efficiency Fund Power x 100
Total Power
25
Motor Start-Up and Transients

• PWM Waveform difficult to synchronize with
  steady-state.
• Very difficult when fundamental frequency
  changing.
• Unique cycle-by-cycle mode of the PM3000A
  avoids the need for time-consuming calculations
  upon raw data points.
• Gives what is of interest,
• Total power, PF, V, A etc for every cycle.

26
Motor Start-Up and Transients
A rms
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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

• DFT measurement of 16 cycles of waveform.
• Anti-aliasing filter as specified.
• Rectangular window - No gaps
• Harmonic accuracy better than 0.2.
• AC source voltage, frequency and harmonics
  measured during the test.
• Single low value shunt ensures compliance
  throughout dynamic range.
• Certified, traceable accuracy.

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.
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How the PM3000A meets the requirements IEC
Flicker Testing

• Complete IEC61000-4-15 flicker meter for
  IEC61000-3-3.
• dc, dt and dmax testing.
• Flicker accuracy proved by using both sine and
  square modulations.
• Impedance network available for full compliance
  measurements.
• WindowsTM PC software for presentation, storage,
  and analysis of results.

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Solutions

• Drives with harmonic correction Passive or
  Active.
• External Filters.
• Zig-zag transformers.
• Controlling inrush and transients to meet flicker
  standards.

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PM3000A Measurement Summary

• All measurements made digitally - No analogue
  error Stable over time Quick to
  calibrate
• Accuracy maintained with wide range of input
  signal level and frequency 0.05 Basic - 600V
  rms, 30A rms, DC to 1MHz
• Single input shunt technology, accurate with high
  reliability.
• Specification maintained with distorted power
  waveform.
• DFT (not FFT) harmonic analysis system
• Sample by sample calculation across 3 phases
  allows neutral calculation.
• Useful additional functions like Inrush Current,
  Distortion and a.c. Impedance
• Easy to use, all main power parameters available
  at push of button.
• Non volatile memory to save any complicated front
  panel setup configuration.
• Single phase, three phase and independent channel
  operation.
• Fast computation for real time feel
• Many special modes PWM Motor drive, Ballast,
  IEC testing etc.
• Application specific software available, VPAS,
  (PM3000A) VPAS Lite (PM100), IEC1000-3-2/3 for
  Windows (PM3000A)

32
Voltech Measurement Solutions

• Formed by power electronics engineers to solve
  everyday power electronics test and measurement
  problems.
• Worlds first commercial digital power
  analyser.
• Patent on automatic transformer testing.

PM3000A
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