Industry Spotlight: Using VFDs with Reciprocating Compressors

Posted by Mackenzie Inman 20/03/2015 2 Comment(s)

A Variable Frequency Drive (VFD) provides the solution to large motor starting currents and voltage drops while simultaneously saving thousands

 

A large 2500HP Medium Voltage drive rated at 275 amps (4160V) controls the green energy reciprocating compressor at a Chicago area municipal landfill converting  landfill gas (LFG) into electric power.  

Problem:

In early December 2013, a large landfill energy developer faced a problem at one of their Chicago area plants with the local electrical utility company. Despite the use of a Reduced Voltage Soft Starter, their 2250HP compressor motor would draw so much current that the voltage dropped in neighboring residential areas. The voltage drops would shut off televisions, reset computers, cause dimming and flickering lights, as well as many other power disruptions. Calls of complaint from the neighboring community would flood into the utility company with each start of the compressor. Finally, with the tension high between the utility company and the local community, the utility company issued a cease and desist letter to the generation facility informing them that until a

solution was implemented for the voltage drop issues, the motor could no longer be started. The shutdown resulted in over $30,000 a day, or over $200,000 a week, in production losses. With such a high production loss, it was imperative that the generation facility not only found a solution, but found one that met their needs quickly.

(Picture) Two different views of the reciprocating compressor.

 

Solution:

If the landfill energy developer found a way to stop their compressor from drawing so much current on start-up, the voltage drops in the surrounding community would stop. To do that, they needed to find a way to eliminate the high starting currents when they started their large motor. Variable Frequency Drives (VFD) not only have the ability to eliminate the high currents associated with starting a large motor, but they also have a very high power factor, helping to reduce voltage drops.

Now that the solution had been identified, the difficulties of finding a manufacture that could deliver and install a VFD while meeting a quick timeline became a challenge. Since many Medium Voltage VFDs are used on applications that involve very specific requirements, it was unlikely that a manufacturer would stock an option ready for shipment immediately. Average lead times for manufacturing Medium Voltage VFDs can take anywhere from 16 to 20 weeks, which was time the landfill energy developer did not have with such high production losses.

In addition to the challenge of finding a manufacturer, the upcoming holiday season created additional concerns.  Slow shipping over the holidays, a question about whether an experienced engineer would be available for start-up and programming, and if he would be free to fly into Chicago on such short notice over the holidays, were all major concerns.

Fortunately, all of those concerns were addressed and met. The landfill energy developer placed their order with VFDs.com on December 18th 2013 and only two days later on December 20th, the VFD shipped to the facility. Knowing the difficulties associated with providing a Medium Voltage Drive so quickly, the turnaround and delivery was significant.

 

(Picture) Shown above is the 2500HP, 275 Amp Medium Voltage Drive after installation at the facility.

 

Result:

After the order was placed and shipped to the Chicago area, an engineer from VFDs.com traveled to the facility over the holiday weekend between Christmas and New Year’s to oversee start up and programming. Upon completion, a call to the utility company was made and the generation facility was given the green light to start their compressor again. The success of the VFD was evident after the compressor had been started and the electrical utility operator had to ask afterwards if the compressor had been turned on yet, as the voltage remained steady on his testing equipment.

While the VFD was the solution to the motor’s high starting current and voltage drops, VFDs can also be a leading cause of power quality problems on electrical grids. In order to ensure that the VFD implemented at the facility did not create yet another utility problem, a 24-pulse “clean power” VFD was supplied. By using this 24-pulse system, negligible harmonic distortion would be produced and any potential problems with harmonics and power quality would be averted. If no plan had been in place to control the high frequency noise from the VFD, it is likely additional power quality problems would have been created, eventually leading to the utility issuing another order of shut down. 

Knowing this, it was crucial that the facility worked with qualified engineers and VFD specialists so that they were able to avoid creating another potential problem which would have cost them additional thousands in production loss if they were forced to shut down again.

A beneficial byproduct of the facility shutdown was the significant energy savings generated from using the VFD on the large compressor. After the VFD was implemented at the facility, less energy was used to operate the compressor which meant more energy available for sale going back to the grid. The energy savings realized for this specific facility has produced a verified savings in excess of $100,000 per year. In addition, there have been no customer complaints received concerning voltage drops caused by the compressor since the installation of the VFD at the landfill energy developer’s facility.

VFDs.com and EMC

Based out of Salt Lake City, Utah, VFDs.com is a division of Energy Management Corporation (EMC). For over 30 years, EMC has been a leading provider of high performance Medium Voltage and Low Voltage Motor and Drive Systems. Services offered include: a wide variety of VFDs, custom VFD panels, engineering services, and drive repairs.

Landfill Energy Developer

Landfill gas-to-energy projects have been designed around the world to generate renewable energy from discarded and harmful waste. Landfill gas-to-energy projects are not only a significant source of renewable energy, but they also use readily available resources present in local communities to offset the use of non-renewable resources.

 

Leave a Comment

2 Comment(s)

admin:
03/09/2015, 09:07:51 AM
Reply

Hello William, we have consulted our VP of engineering. Here is his response.
1. We did not test the RVSS prior to arrival on site for startup. Issues with regard to RVSS centered around voltage drop. RVSS starting currents are typically 250% to 500% at very low power factor. In addition to the large currents, low power factor exacerbates voltage drop. VFD current starts at very low values (f.e. 10%) and gradually rises to the operating current of the system.
2. I do not recall the specifics of the compressor controls.
3. We were not involved in the original installation.
4. We suspect that one of the problems entailed the building of homes near the landfill over a period of years. This is my assumption only and may or may not be true. In any case, it was homeowner complaints that eventually led the utility to require the VFD.
5. We have no motor test data. No special drive settings were required. The VFD was tuned to the motor and operates in open loop flux vector mode.
6. My experience in this and many other medium voltage applications is that there is a place for both soft starts and VFDs in medium voltage applications. We have no amp draw data with the RVSS since we arrived on site after the installation of the VFD. However, there is a HUGE difference in both amperes and power factor with the two technologies as noted in #1 above. The voltage drop problem was solved by reducing the amount of current and increasing the power factor of that current. Power savings was obtained be reducing the operating speed of the compressor.
7. The customer did not indicate any problems with motor windings prior to installation of the VFD and there have been no problems since.
8. This was a short case study written by an intern at our company for purposes of highlighting one of our many medium voltage VFD applications. We did not, and probably will not, spend the time necessary to produce a detailed white paper.

Hope this helps! Let me know if you have other questions.

William Babbitt:
27/07/2015, 05:04:48 AM, www.jciind.com
Reply

Are the specific values and differences between the existing RVSS and the drive available?

What issues were found with the existing RVSS? Also were unloaders and other control devices used in the system, and of what value if any?

How was the problem of voltage drops missed prior to the original installation? Surely the total capacity was known, one would think such a large power use would be identified as it related to the entire system capacity, or not?

Was the motor tested and is any data available prior to or after the installation? Both original and prior to the drive.

Is data available as to the drive settings on start? Any data as to amp draw between the two starting methods?

We are finding soft starts to be inadequate for medium voltage, not quite this much power but significant, approximately 1/3 of this example. We are curious as to why the soft start failed to perform with enough drop to avoid the power losses outside the unit itself.

Any damage found to the motor windings, were any special winding data or connections used in anticipation of the soft start?

Is there a white paper available for this solution?

admin:
10/09/2015, 04:49:23 PM

Hello William, we have consulted our VP of engineering. Here is his response.

1. We did not test the RVSS prior to arrival on site for startup. Issues with regard to RVSS centered around voltage drop. RVSS starting currents are typically 250% to 500% at very low power factor. In addition to the large currents, low power factor exacerbates voltage drop. VFD current starts at very low values (f.e. 10%) and gradually rises to the operating current of the system.

2. I do not recall the specifics of the compressor controls.

3. We were not involved in the original installation.

4. We suspect that one of the problems entailed the building of homes near the landfill over a period of years. This is my assumption only and may or may not be true. In any case, it was homeowner complaints that eventually led the utility to require the VFD.

5. We have no motor test data. No special drive settings were required. The VFD was tuned to the motor and operates in open loop flux vector mode.

6. My experience in this and many other medium voltage applications is that there is a place for both soft starts and VFDs in medium voltage applications. We have no amp draw data with the RVSS since we arrived on site after the installation of the VFD. However, there is a HUGE difference in both amperes and power factor with the two technologies as noted in #1 above. The voltage drop problem was solved by reducing the amount of current and increasing the power factor of that current. Power savings was obtained be reducing the operating speed of the compressor.

7. The customer did not indicate any problems with motor windings prior to installation of the VFD and there have been no problems since.

8. This was a short case study written by an intern at our company for purposes of highlighting one of our many medium voltage VFD applications. We did not, and probably will not, spend the time necessary to produce a detailed white paper.

Hope this helps! Let me know if you have other questions.