Dranetz Guide Guia do Utilizador descarregar pdf (Página 210)

210

Dran-View 6 User Guide

is the peak amplitude of the nth harmonic sine/cosine. In Dran-View you may

read rms amplitudes from the DFT Chart, a Text Label containing either the

$DFTDETAILSFULL or $DFTDETAILSBASIC control codes, or a Phasor. These

values may be converted to peak form by multiplying them by the square root of

two. Because the energy generation potential of a voltage or amperage harmonic

signal is proportional to the Root Mean Square (RMS) of the peak value, Dran-

View displays these values in RMS format for convenience. Using the rms values

you can easily compute the rms energy in any subset of harmonics by taking the

square root of the sum of their rms amplitudes squared. This becomes useful if you

are interested in the energy contribution of a small subset of harmonics rather than

the total harmonic energy which Dran-View provides directly. For example, to find

the rms contribution of the third and fifth, sum their squared values and take the

square root. To convert to percent of the total, divide by the Total RMS value.

ω is the angular frequency. Conventionally this is 2πf

where f

is the fundamental

frequency in reciprocal seconds (one over the period, T

) and 2π is in radians.

Dran-View expresses all angles in degrees so this value resolves to 360°/T

where

is the fundamental period in seconds. Note that in special situations the period T

may be an integral fraction of the time range used for the transform. This rule

applies primarily when more than one fundamental wave is highlighted, and

Min/Max/Avg mode is selected.

t is in seconds.

δ Is the positive modulo 360 degree phase angle offset at t equals zero.

Note that the phase offset, δ

, is subtracted in the cosine expansion and added in the

sine expansion. The cosine expansion is included as an option primarily because it

is the form that many texts on the subject of Fourier Analysis prefer to use. It is not

the preferred form for power calculations. Agreement with math texts is also the

reason that the minus sign was preserved in the cosine expansion above. Negating

the un-normalized cosine expansion δ

before presentation in the Dran-View would

have allowed the sine and cosine expansion forms to be analogous (it would be

much easier to remember that way). Because of the preponderance of cosine

transforms in texts, you are more likely to get agreement using the cosine transform

option if you are using a typical textbook application to do your own transforms on

the same data. In the power industry, the sine transform makes the most sense and

is the preferred form. For example, the phase relationship of a positive sequence

three phase system is usually expressed as 0, 240 and 120 degrees for phases A, B

and C, respectively.

Continued on the next page

1 2 ... 205 206 207 208 209 210 211 212 213 214 215 216 217 218

Comentários a estes Manuais

Sem comentários

Dranetz Guide Guia do Utilizador Página 210

Comentários a estes Manuais