Two methods of performing the calculation. DFT and FFT. D – Discrete F - Fast F ourier T ransform
Sampling Rate and Synchronisation
FFT must be 2 n , E.g. 2 10 = 1024 samples per cycle Difficult 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.
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
1W lost for every 100W delivered (1/2 the copper required for 3 x 2 wire)
Induction motors require extra winding to start.
“ Star” or “Wye” Connection.
Centre point, N is called star or Neutral point.
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
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.
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.
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
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.
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.
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.
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.
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.
Windows TM PC software for presentation, storage, and analysis of results.