Triplett Mitigator Loop Coil – 3232 - Manual (Page 17)

11.1.1: Over-Excited Transformer
When a transformer is over-excited by excessive primary voltage, it can create a harmonic current flow
if there is sufficient capacitance on the line to act as a sink. A typical signature of this condition usually
consists of the odd harmonics up to about the 13th harmonic (780Hz)[650Hz]. Specifically: 180Hz,
300Hz, 420Hz, 540Hz, 660Hz, and 780Hz. [150Hz, 250Hz, 350Hz, 450Hz, 550Hz, 650Hz]
11.1.2: Bad Capacitors
Actually, this signature really indicates that there is a reactive unbalance on 3 phase power lines. The
unbalance may be either capacitive or inductive. An inductive unbalance could be due to an inordinate
number of customer inductive loads improperly distributed across the phases. A capacitive unbalance
could be the result of a bad capacitor in a bank of capacitors, or a wrong size capacitor in a bank.
Usually, this problem causes a harmonic current at 300Hz [250Hz], but sometimes it is also seen at
420Hz or 660Hz [350Hz or 550Hz]. These are the 5th, 7th, and 11th harmonics and, notably, they are not
odd triple harmonics but are centered around 540Hz [450Hz], which is an odd triple harmonic.
11.1.3: Resonance
This condition will go unnoticed unless the resonance occurs at a frequency where there is sufficient
current flow to “excite” the condition. It usually occurs at a harmonic frequency (of 60Hz [50Hz]) be-
cause these are the most likely currents to be available in the power line. The actual source of the
harmonic currents may be an over-excited transformer or industrial machinery. The signature will tend
to follow the signatures of these phenomenon, but there will be a distinct peak in level at a “resonant”
frequency. The power line is considered to be in resonance if there is a predominant frequency that is at
least 6dB greater in level than the adjacent harmonics.
Conditions in the power system make resonance likely to occur at odd harmonics below 1000Hz. 540Hz
[450Hz] is a common resonant frequency. Do not forget, however, that resonance can occur at any
frequency if there is sufficient energy to excite it.
11.1.4: Rectifiers: DC Harmonics
Rectifiers are often used in industrial electronics. They might be found in commercial electroplating
facilities, any industry that uses large electronically controlled motors, commercial lighting systems, etc.
The harmonics produced on the DC side of the rectifiers is immediately obvious because they are at
“even” frequencies. If the rectifier assembly is not well balanced or has a defective rectifier, large even
harmonics will appear on the power line. In general, the more rectifiers involved in the assembly, the
fewer low frequency harmonics are produced.
6 Pulse Rectifier: 360Hz, 720Hz, 1080Hz, 1440Hz, 1800Hz, 2160Hz, 2520Hz
[300Hz, 600Hz, 900Hz, 1200Hz, 1500Hz, 1800Hz, 2100Hz]
12 Pulse Rectifier: 720Hz, 1440Hz, 2160Hz
[600Hz, 1200Hz, 1800Hz]
11.1.5: Rectifiers: AC Harmonics
Rectifiers, in addition to producing “even” harmonics, also produce odd harmonic currents on the AC
side (power line side) of the rectifier assembly. The odd triple harmonics, however, “cancel out” in 6
pulse rectifiers, and additional odd harmonics cancel out in 12 pulse rectifiers.
6 Pulse: 300Hz, 420Hz, 660Hz, 780Hz, 1020Hz, 1140Hz, 1380Hz, 1500Hz
1740Hz, 1860Hz, 2100Hz, 2220Hz, 2460Hz, 2580Hz, 2820Hz
[250Hz, 350Hz, 550Hz, 650Hz, 850Hz, 950Hz, 1150Hz, 1250Hz
1450Hz, 1550Hz, 1750Hz, 1850Hz, 2050Hz, 2150Hz, 2350Hz]
16