Ametek TR-2100, Operation Manual

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TR-2100 User Manual 77
Harmonic Measurement
A single cycle window is used to calculate the harmonic content of voltages and currents. A
standard Discrete Fourier Transform (DFT) technique is used to calculate the amplitude and
angle of each component. A minimum of 4 samples per cycle is required to extract a
harmonic component accurately. A number of samples are taken over the integrating interval
(10 mins for EN50160) and averaged.
The maximum, minimum and average amplitude is stored every 10 minutes. The phase
angles are also stored at the 10 minute boundary. Data for the fundamental and up to the 64
th
harmonic are recorded. THD, thd, TDD and K factor are computed on the host PC.
Flicker
Flicker is measured using the technique defined in IEC1000-4-15. This corrects some errors
that were in the original standard IEC60868. The sub-synchronous spectrum is extracted and
weighted using the defined curves. This instantaneous flicker value is recorded for 10
minutes and a statistical average is taken (as defined in the standard). This short term flicker
value - Pst, is recorded in a separate long term log every 10 minutes.
Real Power
True power is computed by multiplying individual current and voltage samples and then
averaging over one cycle. The actual number of samples per cycle will vary with the sample
rate.
P
n
n
n
V I
=
=
∑
1
96
1
96
*
Real power for a circuit with a delta wound VT is derived using the two wattmeter
method. This makes two power measurements using two of the line currents. It can
be proved that the sum of the two powers is same as the total circuit power:
I V I V I V
c c b b a a
P
+ + =
Total circuit power
0
= + +
I I I
c b a
Currents sum to 0
Substituting the second equation into the first and collecting terms yields:
( ) ( )
I V V I V V
c
b c
a
b a
P
− + − =
The total circuit power can be measured using two wattmeters on the phase-to-phase
voltages. Note that the phasing of the VTs is important.
Apparent Power
Apparent power is computed from the RMS voltage and current.
S
rms rms
V I
=
*