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Finding a power quality needle in a 30-mile haystack

Major power problems at ski resort may not be the result of an on-site equipment failure. Crystal Mountain, WA 2005

At the end of April 2005, Crystal Mountain Resort in Washington suffered catastrophic failure of two-power filters associate with their chair lifts. The ski resort had just closed down for the season. Maintenance personnel were doing the end of season maintenance and cleanup when they smelled the overheated reactors on the power filters. Crystal Mountain has four Var+Controls harmonic power filters located in the lift houses of their highest horsepower detachable chair lifts. Two were installed in 1997, one in 1998, and one in 2001. All of the filter units are programmed with two fixed steps to absorb the distortion of the utility grid and the rest of the steps automatically turn on to obtain a unity power factor for the greatest electrical efficiency when the lifts are in operation.

Crystal Mountain electrical personnel measured the harmonic current in the power filters to determine the damage to the units. With the help of Bill McConnell of Var, the team found that only the C-phase reactors in two of the lift power filters were damaged. They replaced those reactors and continued investigating the cause of the failure throughout the summer. The filters were noisier than usual but seemed to work with their associated lift.

At the start of the 2005-2006 ski season, the electrical maintenance personnel noted that, as more chair lifts were brought on line, the more overloaded the power filters became. They turned the filters off in an attempt to save the equipment and decrease the noise and smell in the lift houses. With the power filters off line, the lift dc drives were noisy, pumping systems on the mountain malfunctioned, variable frequency drives on carpet conveyors tripped off line, and data capture systems failed. At that point, the team called Bill McConnell of Var and Vaughn DeCrausaz of Starboard Electric to help troubleshoot the ski area's overall power system. Given the approaching Thanksgiving and Christmas holidays, finding a solution was of paramount 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 community on the main power line feed from Enumclaw to Crystal Mountain. The voltage distortion was huge at 14.04% as shown below.

Greenwater convenience store outside 120vac receptacle. All lifts at Crystal Mountain running Filters off

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. 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 lifts running and NO filters on line.

Lift 11, Filter off. All other filters off

With all of the lifts running Saturday, DeCrausaz used a Fluke 199 Scopemeter© to check the Lift 11 dc drive armature SCR (Silicone Controlled Rectifier) pulses. The dc pulses to the motor were very erratic, varying in time duration and amplitude. This was caused by the varying distorted voltage waveform to the drive. The drive had difficulty finding the voltage zero crossing for the armature SCR firing. The distorted voltage waveform was also affecting the dc motor field magnetism. With the collapse of the voltage waveform due to the armature SCR firing, voltage and current were decreased to the fields. In effect, the DC motor tried to run faster, with less torque (higher armature 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 could turn a power filter on without self-destructing. They took the following current measurements with a Fluke 3 phase RPM (Reliable Power Meter) at 5 PM at the power filter of Lift 11:

Utility 5th Harmonic Current

The 5th harmonic current rating of each filter step is 75 amps. The 5th harmonic is the whole number multiple of the fundamental frequency.
Harmonic = whole number x fundamental frequency
5th Harmonic = 5 x 60 cycles per second = 300 cps = 300Hz

Now the challenge was to locate the cause of the 53% 5th harmonic current imbalance and the C-phase harmonic current magnitude.

At night with no lifts running, the voltage distortion on the mountain was 6.1% Total Harmonic Distortion (THD). The Institute of Electrical and Electronic Engineers (IEEE) 519-1992 Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems lists 5% THD. This "no load" distortion on the utility grid indicated a resonance either on the utility side or at Crystal Mountain. This resonance or amplification of the 5th harmonic could be caused by high voltage capacity banks lowering the resonant frequency of the power system or the high voltage underground leaking current to ground at some point.

DeCrausaz and McConnell made several measurements along the power line coming from Enumclaw: The first at the supply end of the utility grid, the second through the town of Greenwater approximately 20 miles toward Crystal, and the third at Crystal. There was a decommissioned Weyerhaeurser sawmill five miles outside of Enumclaw on the 57kv (57,000 volt) utility line that fed Greenwater and Crystal. The distortion at the Weyerhaeurser plant was only 1.9% THD, indicating that the utility feed to that point was good. On the side of the Greenwater store, the distortion was 14.04% with the lifts running and 6.01% with the lifts off. Greenwater is fed from the 57kv line transformed down to 12kv for the local area. Crystal is further fed from this 12kv line in Greenwater through a transformer to 34kv. The 34kv line has approximately five miles of relatively new underground with the remaining six miles run overhead almost to Crystal. The utility has a backup 2000kw generator just off Crystal's property at 4160 volts transformed to 12kv to match the 34kv delivery voltage that is then transformed to 12kv for distribution around the ski area. Crystal is primary metered on this 12kv line.

The utility felt that the problem was caused by some of the old 12kv underground line located on the ski area. So, the team tested this theory by starting the generator at night with no lifts running and monitoring the voltage and current at one of the power filters. With the generator on in parallel (co-generating) with the utility (Crystal being fed by both the generator and utility) the 5th harmonic current to the power filter reduced to 60 amps, 72 amps, and 88 amps, respectively. With the utility removed from the circuit, the 5th harmonic current was further reduced and balanced to 25 amps, 22 amps, and 24 amps. This test proved the problem was on the utility system from Greenwater to the generator and not on Crystal's property.

After further investigation, a 900kvar-capacitor bank was located on a pole near the Greenwater substation on the 12kv line servicing the town. The fuses had been thrown on at the end of April for a still undetermined reason. During the summer the C-phase fuse had blown several times and been replaced. The C-phase fuse was out and hanging down when the capacitor bank was located. At that point, the utility removed the remaining two fuses and said it planned to remove the capacitor bank from the pole so that it couldn't be inadvertently energized.

Later at the ski area, the following voltage waveform was recorded at Lift 11 with the mountain shutdown for the evening. Note the amount of distortion still on the utility system.

Lift 11 off, filter off. All other lifts off, Utility caps off

Lift 11 on, filter-3 steps
Lift 10 on, filter-3 steps
Lift 9 on, filter-3 steps
Lift 3 on, filter-2 steps
All other lifts running

That evening at the ski area, all the power filters were put on-line and measurements taken to prove their health. The voltage distortion at the area was reduced from 18%THD to 3.1%THD. The next day technicians turned on all of the lifts with all four filters and all of the pumping stations and carpets online. The voltage waveform below is at Lift 11 with all the lifts and filters on-line.

Lessons learned from this adventure:

  • Have a good working relationship with your utility company. Without their help this problem would not have been isolated, located, and solved.
  • Have good electrical drawings of your facility and surrounding area as far back toward the source as possible. Visually check and update the documentation periodically.
  • Pay particular attention to the utility's equipment as far back as 50 miles from your facility. Capacitor banks and voltage regulators are sure signs of past anomalies. Note step down and buck up transformer combinations.
  • Monitor your utility voltage and current real time for any problems and consult the supplier as soon as any abnormality occurs.
  • Special electrical equipment is needed to pinpoint this type of power problem. Taking voltage and current measurements do not tell the whole story. Even looking at the waveforms with an oscilloscope may not be good enough. Each harmonic in the voltage and current waveforms must be quantified.
  • Running all lift equipment and snowmaking together in the fall is a good test of the power system, lift communication, and equipment reliability.
  • Test the ground resistance not only at the dc drive motor and wiring but also at the power wiring from the utility transformer through the service to the drive.