A system approach using tuned filters solves PF and harmonic distortion problems with DC drives at ski areas.
Holy Cross Electric Association, the local utility for the Vail, Colo. area, began experiencing problems on its distribution system. Vail Mountain's 25 chair lifts, utilizing nearly 8000 hp, were suspected as the cause. To verify this, preliminary harmonic measurements were recorded at key sites by Holy Cross.
The total harmonic distortion (THD) of the voltage in and around the town of Vail ranged between 6% and 10%. The 25kV feeder that supplies the mountain had current and voltage distortions of 13% and 8% respectively. On the mountain, the numbers were even higher: The 1000 hp Chair Lift No. 16 had a THD of 12.9% and a total demand distortion (TDD) in excess of 27%. As a gauge of the severity of this distortion, IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Power Systems, recommends that maximum voltage distortion be 5% (for voltages below 69kV).
Obviously, there was a problem; actually there were several problems. Due to the levels of harmonic distortion created by Vail's chair lifts, the utility's other customers were beginning to experience their own problems. This condition had to be corrected immediately.
Harmonics occur anywhere non-linear loads are present; at Vail, the non-linear loads are in the form of DC variable frequency drives (VFDs). These drives operate at a very poor displacement power factor (PF) due to the phase control of the SCRs. In fact, measurement shows that a typical DC drive PF ranges between 0.60 to 0.80, depending upon its speed and load. Typically, AC VFDs operate at PFs ranging between 0.90 and 0.98. Fig. 1, on page 86, shows a typical waveform and spectrum for a 700 hp DC drive. Regardless of the drive type, these magnitudes of harmonic currents result in a utility voltage distortion level of 7% to 8%. The harmonic currents injected by the lifts onto the utility system were well in excess of the recommended limits of IEEE 519-1992.
To counter the harmonic problems, Vail and Holy Cross began researching possible solutions. The initial proposed solutions included increasing the size of conductors and transformers, and even installing isolation transformers. These were considered to be temporary fixes, doing nothing toward solving the problems.
Since Holy Cross treats each lift as a separate; stand-alone entity (each lift at Vail has its own dedicated transformer and utility meter), a preliminary proposal was presented a year later to Vail and Holy Cross, one that recommended placing individual harmonic filters at virtually every drive. From a purely technical standpoint, this approach would significantly reduce the amount of harmonic currents at each drive, thereby bringing Holy Cross' distribution system into compliance with IEEE 519-1992.
There were, however, some major concerns with this approach.
* Cost. The equipment and installation costs of between 20 and 25 filters would be exorbitant.
* Installation time. Not all of the filters could be installed in time for the next season. Therefore, the project would need to be spread over two seasons.
* Filter overload. During the first season, with only half the filters on-line, there would be a potential for premature filter failure as each installed filter would try to absorb harmonic currents from non filtered drives as well as those from its own drive.
Thus, installing individual filters was not deemed a viable solution.
The above plan was modified in April of that year on the basis of a change of viewpoint: Holy Cross and Vail decided that the Vail and Beaver Creek Mountains be viewed as single circuits, instead of a combination of many industrial circuits. By doing so, the harmonic content of each mountain could be reduced or managed by installing fewer filters, strategically sized and placed. This approach afforded Vail the flexibility of not always having to add filters for every future lift or drive upgrade. Now that Holy Cross was willing to treat the two mountains as single circuits, a systems approach could be implemented in the design of the filtering equipment.
During the peak ski season of the following year, the filter manufacturer recorded the harmonic spectra of all the lifts over 100 hp at both mountains. At the utility end, similar data were recorded at Holy Cross' substations and at other key locations along its distribution system.
With this information, the manufacturer, with some assistance, built computer models of the utility and mountain distribution systems. Once these models were constructed, simulations were run using different tuning frequencies, quantities, sizes, and locations of filters. These simulations, presented in a final proposal to Vail and Holy Cross in April of that year, showed that placing five filters at Vail and two at Beaver Creek would bring the utility's distribution system and main substation into compliance with IEEE 519-1992. Since Vail was planning on adding another quad detachable lift on Vail Mountain the next year, it was decided to install a sixth filter here to further reduce the harmonics on the mountain. Vail Associates and Holy Cross approved this approach, and the installation of the eight filters was begun.
These filters, each sized at 464kvar, are tuned to the fifth and seventh harmonic currents typically generated by VFDs. Seven of the units are housed in free standing NEMA 3R enclosures, which are mounted on concrete slabs next to each Holy Cross transformer. The eighth unit is configured to mate up to existing drive cabinets and low voltage switchgear, and is installed in the lift house of a new detachable chair lift.
Since these filters are required to absorb the harmonic currents of their respective drives as well as those from other nonfiltered lifts, 600V capacitors derated to 480V and single-phase iron core reactors were specified.
In any passive filter design (as shown in Fig. 2, on page 88) the capacitors and reactors are connected in series. As harmonic currents flow through the reactor, a voltage, current, and kvar rise across the capacitor occurs; this rise may exceed the capacitor's designed duty limits. Failure to recognize these capacitor limits may result in premature capacitor failure. The combination of 600V capacitors with properly sized single-phase reactors helps avoid this problem.
For a project of this magnitude, funding is a very important factor. The utility was concerned with the quality of its power, since it was not in compliance with IEEE 519-1992; Vail was the main contributor to Holy Cross' poor power problem. In addition, Holy Cross' rate structure penalizes its customers whenever PF falls below 0.95. Vail's average PF ranged between 0.77 and 0.80, which translates into an energy penalty of nearly 15%. Thus, both parties had to work together to solve each other's problems.
The funding solution was unique: Vail would pay for the project through the savings it realized by improving its PF. Thus, a six-year note, payable to Holy Cross in monthly installments, was signed by Vail Associates.
There are many benefits to Vail with this agreement.
* Its operating costs will not increase due to the purchase and installation of the filters.
* It will retain ownership of the equipment and thereby gain tax advantages.
* It will realize the savings on its energy bill through improved PF after six years.
Startup and field testing
After completion of installation the following year, the manufacturer, in attendance with Holy Cross and Vail personnel, energized the systems and conducted preliminary testing under no load conditions. The initial data indicated that the filters were performing as designed. The real testing was conducted over a Thanksgiving weekend, with harmonic measurements recorded by the manufacturer at all filter locations. The filtered lifts were loaded, but the entire mountain was not fully operational. These measurements showed that harmonic distortion levels at each lift were reduced to below IEEE 519-1992 thresholds. The filter performance measured at the 1000 hp Chair No. 16 transformer is shown in Fig. 3.
Prior testing (done before filter installation) by Starboard Electric, an authorized Colorado sales and service representative for one make of drive, revealed that the harmonic distortion and "notching" of the waveform at the drive could cause problems in several areas. The VFD's armature current feedback circuit is of a very small signal level (50mV for 100% current) and is susceptible to any harmonic noise in the drive cabinet; this noise causes motor overcurrent trips and erratic acceleration and deceleration. With the filters on-line, Starboard found that these problems all but disappeared.
Harmonic verification measurements under full load conditions
In December, Holy Cross, Public Service Company of Colorado, Starboard Electric, Vail Associates, and the filter system manufacturer set up test equipment at the Vail-Beaver Creek substations to monitor the results of three year's worth of planning. With both mountains fully loaded, Holy Cross' substations were brought into compliance with IEEE 519-1992.
As a side note, Holy Cross, because of this project, is considering changing its PF billing practices from reading and charging a penalty for each lift to a totalizing billing system on Vail and Beaver Creek mountains. This will further reduce Vail Associates' PF penalties.
There are certain key points in harmonic filtration projects at ski areas.
* They require a coordinated effort between the customer, utility, and equipment vendor.
* They are very complicated and should not be hurried along to meet some arbitrary deadlines.
* To ensure success, a system approach should be used, one that uses computer modeling to aid in the design of the filtering equipment.
* Once a mountain is brought into compliance with IEEE 519-1992 or with the serving utility's requirements, the future installation of non-linear loads and/or capacitors should not be done without first analyzing the impact the devices will have on the harmonic profile of the mountain.
Finally, the equipment vendor selection should be based on the vendor's ability to do onsite harmonic and power data acquisition; computer modeling of the distribution system, hardware design and manufacturing; system startup; and harmonic verification measurements under full load conditions.
Steve Hyland is Manager, Lift Operations, Vail Associates, Inc., Vail, Colo.; William McConnell, III is Sales Manager, Var+Controls, Inc., Whitmore Lake, Mich.