Download PDF: Case Study: More Snow With Power Quality ?
Tools: Fluke 41B Power Quality/Harmonics Analyzer
Measurements: Voltage, THD, Current TDD, KVA
Team: Colorado Ski Resort, Starboard Electrical Contractors, Var+Technologies, PQ specialists, Denver Water Board
Note: The Fluke 41B Power Quality Harmonics Analyzer is no longer available. In its place, Fluke recommends the 434 Power Quality Analyzer.
Scenario
A ski resort near Silverthorn, CO., depends on its snowmaking capacity to augment nature's snowfall, especially during drought years. Pressurized water and compressed air are forced through special spray nozzles to atomize the water in freezing temperatures and create snow.
Background
Air is readily available; however, water availability depends on rainfall. To solve this problem, the resort arranged to divert water from the nearby Snake River, in cooperation with the Colorado Conservation Board and the Denver Water Board. To ensure the river maintained minimum flow levels, the resort would pump water out of the Roberts Tunnel below, via the 900-foot deep Montezuma Shaft, and into the Snake River, above.
In 1996 the resort installed a pumping system in the tunnel comprised of a 350 horsepower SCR (silicone controlled rectifier) VFD (variable frequency drive) operating at 480 volts ac. Because the pump and motor were 900 feet down the shaft, the 480-volt power from the VFD was transformed to 4160 volts at ground level and wired down the shaft to the motor. The shaft pumps could deliver 7.8-cfs (about 3,500-gpm) to the river when both ran full capacity. That allowed the resort to divert about 9.4-cfs (about 4,200-gpm) for snowmaking.
The power source for the pumping system was a 1000 KVA transformer fed by a 25,000 volt, 3 phase #2 overhead power line five miles from the resort going on past the pumping system to the community of Montezuma. This pumping system worked well for several years with only the 350 horsepower pump.
Evolution
As the resort expanded its snowmaking system, its water demands increased. The project installed an additional 750 horsepower VFD in 2002 with a new 12-inch pipe from the tunnel, up the shaft to the Snake River. However, the system was never run to capacity because the power line and 1000 KVA transformer could not support the total 1100 horsepower of VFD load.
If both drives were run at capacity, they would drop offline because of their undervoltage protection. When the system was on-line, local residences complained of flickering lights and the noisy transformer and VFDs. Meanwhile, new management assumed ownership of the resort.
Problems
During the 2002-2003 ski season, the new management was disappointed with the snowmaking coverage. The new lift manager worked together with local engineering and electrical contractors and (re)discovered that the problem was not a lack of water but a lack of power to pump the water. They contacted Var+Technologies to see if harmonic power filters could solve the lack of power at the pumping station. Var+ had already supplied six filters that had cured several power issues on the mountain (harmonic related problems with failed battery chargers, night lighting transformers, drive field supplies, and chair lift speed instability).
Investigation
Using a Fluke 41B Power Quality Analyzer, the team measured power at the pumping station in February 2003. The team could only run the 750 hp pump at approximately 50 % load because of the power line and transformer capacity. The team then calculated horsepower and power (Table 1) and compared that to calculations from the actual measurements (Table 2).
Findings
The 350 horsepower pump was running 120 % of the expected load. If the 750 horsepower pump were assumed to run at 120 % also, the KVA required for both pumps would be 1065 with 313 amps of 5th harmonic current. 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 = 300 Hz
With these numbers and 10.24 % THD (Total Harmonic Distortion) voltage distortion plus 30.78 % TDD (Total Demand Distortion) current distortion, it was no wonder the power line and transformer couldn't handle the load. The Institute of Electrical and Electronic Engineers (IEEE) 519-1992 Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems lists 5 % THD and 8 % TDD as guideline maximums.
Solution
The team thought a Var+ harmonic filter - 900 kVAR at 600 volt (594 kVAR at 480 volt) in 9 steps - should be able to manage the reactive power (KVAR) and the 5th harmonic current as well as improve system efficiency, allowing the pumps to run at maximum capacity. (See sidebar on reactive power.) The filter should also reduce the voltage and current distortions to the IEEE guidelines, by filtering out the 300 Hz current with a reactor (inductor)/capacitor network.
The resort purchased and installed the harmonic filter and activated it in October 2003. Several of the pump manufacture service personnel were on site and they were skeptical about pump drives operating with a power filter and the capacity of the 1000 KVA transformer. The utility also had several technicians there to measure the load, power factor, and distortion on their transformer. Including resort snowmaking personnel and management, there were approximately 15 people anticipating a "fire and light show."
A snowmaker tech started the 350 hp pump and slowly dialed up the capacity. The harmonic filter banged in several 66 KVAR steps to hold the power factor at unity. When the 350 hp pump was at full capacity, the 750 hp pump was started and slowly increased in speed and capacity. With both pumps at full capacity, the harmonic filter had seven of the nine steps energized and the system was pumping approximately 4500 gallons/minute.
The snowmakers walked over to the pumping system outlet pipe and were amazed at the amount of water discharging into the Snake River. The team experimented with different combinations of pump settings and start/stop sequences to check the operation of the drives and harmonic filter. After a half hour, the crowd dissipated because there was nothing to see except water pouring into the river.
To verify the harmonic filter's operation, the team took one last set of measurements in December 2003 with the Fluke 41B. These measurements were taken at the point of common connection (PCC) of the drives and harmonic filter.
The KVA had been reduced from a calculated value of 1065 to 964, the voltage increased from 474 to 492, and the 5th harmonic (300 HZ) current reduced from a calculated value of 313 to 59-amps. The harmonic filter injected 424 KVAR of reactive power.
The pumping system has been running for the last three snowmaking seasons without problems.
Lessons Learned:
- Having the proper test equipment to measure voltage, current harmonics and power factor is paramount to identifying the problem and specifying a solution.
- Reactive power (VARS) should be added to the "power systems" to achieve a unity power factor.
- Do not believe everything you hear about a project without proper documentation.
Examples:- 1100 horsepower of motor load can't operate on a 1000 KVA transformer.
- Two large motor drives can't be connected on one large transformer.
- Electronic drives can't operate with harmonic power filters.
