GE PQM II, Instruction Manual

Page: 160 / 216

6–22 PQMII POWER QUALITY METER – INSTRUCTION MANUAL
CHAPTER 6: MONITORING
6.4 A3 Power Analysis
6.4.1 Power Quality
PATH: ACTUAL VALUES
Ö
A3 POWER ANALYSIS
ÖØ
POWER QUALITY VALUES
• Ia/Ib/Ic CREST FACTOR : The crest factor describes how much the load current can
vary from a pure sine wave while maintaining the system’s full rating. A completely
linear load (pure sine wave) has a crest factor of
(1 /0.707), which is the ratio of the
peak value of sine wave to its RMS value. Typically, the crest factor can range from
to 2.5.
• Ia/Ib/Ic THDF : The Transformer Harmonic Derating Factor (THDF), also known as
CBEMA factor, is defined as the crest factor of a pure sine wave ( ) divided by the
measured crest factor. This method is useful in cases where lower order harmonics are
dominant. In a case where higher order harmonics are present, it may be necessary
to use a more precise method (K-factor) of calculating the derating factor. This method
also does not take into consideration the losses associated with rated eddy current in
the transformer. The EnerVista PQMII Setup Software provides the K-factor method of
calculating the derating factor, which is defined on a per unit basis as follows:
(EQ 6.3)
where: I
h
= RMS current at harmonic h , in per unit of rated RMS load current
6.4.2 THD
PATH: ACTUAL VALUES
Ö
A3 POWER ANALYSIS
ÖØ
TOTAL HARMONIC DISTORTION
„
POWER QUALITY [
Z
]
VALUES
Ia CREST FACTOR =
1.233
MESSAGE
Ib CREST FACTOR =
1.008
MESSAGE
Ic CREST FACTOR =
1.000
MESSAGE
Ia THDF = 0.944
MESSAGE
Ib THDF = 0.999
MESSAGE
Ic THDF = 0.988
2
2
2
K I
h
2
h
2
×
h 1
=
h
max
∑
=
„
TOTAL HARMONIC [
Z
]
DISTORTION
PHASE A CURRENT THD=
5.3%
MESSAGE
PHASE B CURRENT THD=
7.8%