Case Studies

2 Fluke Corporation Power quality accountability
conveyors tripped off-line and data capture
credit card systems failed. At that point, the team
called Bill McConnell of Var+Technologies and
Vaughn DeCrausaz of Starboard Electric to help
troubleshoot the ski area’s overall power system.
Given the approaching Thanksgiving and Christ-
mas holidays, finding a solution was of para-
mount importance.
On the way up to the ski area, DeCrausaz took
a measurement with a Fluke 41B power meter at
an outside receptacle of a convenience store in
Greenwater. Greenwater is a bedroom commu-
nity on the main power line feed from Enumclaw
to Crystal Mountain. The voltage distortion was
huge at 14 % shown below.
When first on-site, DeCrausaz and McConnell
made an attempt to measure the voltage and
current of the problem power filters with the
lifts running, it turned out this was impossible
due to the possibility of damaging the filter. The
filter would have to absorb the entire harmonic
current of the utility system plus the harmonic
current generated by the dc drive of the lift. Note
below the voltage waveform at Lift 11 with all
the lifts running and NO filters on line.
With all of the lifts running Saturday,
DeCrausaz used a Fluke 199 Scopemeter® to
check the Lift 11 dc drive armature SCR (Silicon
Controlled Rectifier) pulses. The dc pulses to
the motor were very erratic, varying in duration
and amplitude. This was caused by the distorted
voltage waveform affecting the dc motor field
magnetism. With the collapse of the voltage
waveform due to the armature SCR firing, volt-
age and current were decreased to the fields.
The erratic firing of the SCR’s caused the motor
to try to run faster, lowered its torque, increased
its current draw and also pulled power away
from its field windings. In effect, the dc motor
tried to run faster, with less torque (higher arma-
ture current) and with decreased fields.
The team consulted the local utility company
to verify whether any changes in their system
had occurred at the end of April. Were any new
underground lines installed? Had any capacitor
bank been installed or modified? Had any large
loads been installed or removed?
Finally, after the ski area had been shut
down for the evening, the techs were able to
turn a power filter on without damaging it.
They measured harmonics currents with a Fluke
3 phase RPM Power Recorder at 5 pm at the
power filter of Lifts 10 and 11. The table below
shows the current at the 5th harmonic (300 Hz).
Not only was the distortion abnormally high,
but it was also very different between phases.
The imbalance of 53 % pointed to a significant
physical difference between the phases.
At night with no lifts running, the voltage
distortion on the mountain was 6.1 % Total
Harmonic Distortion (THD). The Institute of Elec-
trical and Electronic Engineers (IEEE) 519-1992
Recommended Practices and Requirements for
Harmonic Control in Electrical Power Systems
lists 5 % THD as an upper limit. This “no load”
distortion on the utility grid indicated a reso-
nance either on the utility side at Crystal
Mountain. The resonance or amplification at
the 5th harmonic could be caused by high volt-
age capacitor banks lowering the resonant
frequency of the power system or high voltage
underground distribution lines coupling current
to ground.
Greenwater convenience store 120 vac receptacle.
All lifts at Crystal Mountain running. Filters off.
Lift 11 with all lifts running and no filters on line.
Lift Phase A
(amps)
Phase B
(amps)
Phase C
(amps)
Imbalance
Ph A-C
10 98.2 118.7 150.4 53.2 %
11 109.6 122.6 160.1 46.1 %
Average 103.9 120.65 155.25 49.4 %
Utility 5th (300) Hz Harmonic Current