ac power problems and measurement sreevidhya@students
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Power Analyser FundamentalsPower Analysis and Harmonics
106-004/1 ©Voltech 2003
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www.voltech.com 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.
106-004/1 ©Voltech 2003
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Historically, only simple measurements required:
AC Motor Fixed Speed
Lamp Light
Heater Heat
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www.voltech.comReal and apparent power
Instantaneous power is the product of instantaneous voltage and current.
Resistive load. – instantaneous W is always (+)ve– W = V x A
W
Voltage /Current
Power
-ve Power lost due to phase difference of current & voltage
+ve
-veTime
50W Average
100V RMS
1A RMS
100 2V peak
1 2A peak
100
60 o
W
Voltage /Current
Power Time
100V RMS
1A RMS
100 2V peak
1 2A peak
100 2 x 1 2=200W peak
100
Inductive load– current is phase shifted– instantaneous W sometimes (-)ve– W V x A
Apparent Power = V x A Power Factor = Cos
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www.voltech.comModern electronic loads
PWM Drive Variable speed
More controlGreater efficiency
HF Ballast Light
CompactGreater efficiency
Complex waveforms demand sophisticated analysis Drive to better efficiency - more accurate analysis
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Rectified and Capacitor Smoothed inputs
Input Voltage
Input Current
Load
(Charging when input voltage greater than capacitor volts)
Capacitor Voltage
Capacitor Current
Current only flows when AC supply voltage is greater than capacitor voltage.
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www.voltech.comPower factor and cos
The power factor is reduced . Not by phase displacement, but by shape distortion.
Why is this important?
Power factor of <1 means generation and transmission must be oversized:• Power lines and transformers sized to supply VA, not Watts.
Power factor <1 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.
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www.voltech.comPower factor and cos
Voltage
IEC / IEEE Definition Power Factor = Watts Volts x Amps
PF = 0.65
CosCurrent Harmonics
Fundamental (1st Harmonic)
3rd Harmonic
CosAngle between fundamental (1st harmonic) voltage and current.
Total Current
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www.voltech.comMulti-phase measurements
Single Phase.
100W
1W
1W
• 2W lost for every 100W delivered (example)• Induction motors require extra winding to start.
Three Phase – 120° apart.
100W
1W
1W
100W
1W100W
1W
A
B
Sum of voltage always = 0A can be joined to B.
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100W
1W
Multi-phase measurements
Three-phase “delta”
• 1W lost for every 100W delivered (1/2 the copper required for 3 x 2 wire)• Induction motors require extra winding to start.
1W
1W
• “Star” or “Wye” Connection.• Centre point, N is called star or Neutral point.
N
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www.voltech.comMulti-phase measurements
Power Station
3-phase and 1-phase loads415V phase to phase230V 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
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www.voltech.comTriplen harmonics
Phase One Current
-1.5
-1
-0.5
0
0.5
1
1.5
0 180 270 360Amps
FundamentalThird Harmonic
90
Phase Two Current
-1.5
-1
-0.5
0
0.5
1
1.5
0 180 270 360Amps
FundamentalThird Harmonic
90
Phase Three Current
-1.5
-1
-0.5
0
0.5
1
1.5
0 180 270 360
Phase angle in degrees
Am
ps
FundamentalThird Harmonic
90
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.
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www.voltech.comTriplen harmonics in
3 phase power systems
Burnout of neutral conductor or damage to related components Overheating of distribution transformers
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.
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www.voltech.comAC motors
Input 2 Wattmeter or3 Wattmeter method.
OutputTorque and SpeedMechanical Power
V I
Electrical to Mechanical Efficiency
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Distorted AC Input DC Bus PWM Output Torque & Speed
xxx NM&
Rev / Min
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www.voltech.comPWM voltage spectrum
Fundamental(E.g. 40 Hz)
Switching or carrier(E.g. 10, 000 Hz)
Total Power = Fundamental + Harmonics (f) + Carrier + Harmonics(c) + Noise
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www.voltech.comPWM 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
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www.voltech.comPWM Mode - Example
Fundamental(E.g. 40 Hz)
Switching or carrier(E.g. 10, 000 Hz)
Fundamental230 V
Filtered240V
Total255V
Total Power = Fundamental + Harmonics (f) + Carrier + Harmonics(c) + Noise
What’s required?• Total Power (1MHz bandwidth)• Fundamental Power
Motor efficiency ~ Fund Power x 100% Total Power
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Motor Start-Up and Transients
Waveform
• 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.
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Motor Start-Up and Transients
Waveform
A rms
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www.voltech.comIEC Standards
HarmonicsTo 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.
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www.voltech.comIEC 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 voltageacross 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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www.voltech.comHow 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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www.voltech.com 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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www.voltech.comVoltech Measurement
Solutions• Formed by power electronics engineers to solve everyday power electronics test and measurement problems.• World’s first commercial digital power analyser.• Patent on automatic transformer testing.
PM3000A
PM100 Single PhasePM300 Three PhaseLow-cost Analysers
AT3600 Automatic Transformer Tester
ATi LCR Meter / Automatic Tester