ABSTRACT 
Industrial processes are often some of the most challenging environments for application of power quality equipment. In addition to being extremely sensitive to power disturbances, the critical loads involved in manufacturing process operations can be very difficult to support due to high harmonics and load transients. This paper presents the application of sophisticated flywheel-based UPS technology in an industrial environment to resolve utility voltage sags primarily due to inclement weather. Integration issues with variable speed drives and other process machinery found in industrial environments are addressed. In addition, the strategic approach in UPS distribution and implementation as well as financial benefits are discussed.
Introduction
A plastic extrusion plant utilizes flywheel-based, battery-free, CleanSource ® UPS technology systems to increase efficiency by protecting numerous extrusion processes from outages due to utility sags. The plant is located in Oklahoma, which suffers from utility power events due to weather effects and other disturbances on the grid. The plastic extrusion processes contain both AC and DC variable-speed motor drives.
The plant operates 16 plastic extruders, which often shut down because of the power events. These power events can last up to 5 seconds and occur up to four times per week during the winter months, causing excessive downtime. Because an extruder requires three operators approximately four hours to reestablish normal operations after an unexpected shutdown, the cost of downtime is significant. The frequency of occurrence and the number of downtime hours represented substantial financial and efficiency impacts to the plant operations.
The company wanted to find the most economical solution, with a long service life, to allow for investment payback. Battery-based solutions were quickly ruled out due to cycle rate of outages and the elevated temperatures (up to 105ºF) found on the factory floor. The plant did not have existing backup generator sets, and the customer did not want to introduce a generator-based solution. The company decided that a flywheel-based solution was best suited for the application.
They considered two flywheel UPS options for the application. One of the solutions considered was four 1.6-MVA UPS systems. However, this solution was rejected due to multiple issues associated with its size and installation. Installation would have required added expense due to the physical size and logistics of the UPS. In addition, the large amount of rewiring for power distribution in the building would have been expensive as well, and the interruptions to the facility would have resulted in lost productivity and revenue.
The second and ultimate solution was to install sixteen 16 CleanSource 300-kVA and one 900-kVA flywheel UPS systems directly on the factory floor. Since the factory is quite large and each extruder line has its own power feed, the UPS systems were installed adjacent to the equipment to be protected. This solution presented minimum logistical problems since each 300-kVA UPS requires only 14-ft 2 of floor space. In addition, the harsh environment on the factory floor would not present a problem since the flywheel systems operate in ambient temperatures up to 105ºF with minimum performance issues. Furthermore, the close proximity to each extruder allowed for the least amount of rewiring. The UPS systems were installed between the existing power breaker and the extruder equipment, as shown in the figure below.
The CleanSource UPS system was less expensive to install and allowed for minimum impact to power distribution during installation, thus keeping the overall cost and impact to plant operations as low as possible. Furthermore, the distributed approach allows for continued operations if a single UPS requires maintenance.
APPLICATION
Plant Power Overview
The utility company provides power to the plant at 480-VAC through several step-down transformers and distribution circuits. These circuits vary in capacity in relation to the quantity of equipment being supplied. The figure below shows the protected electrical circuits and the way the extruder and bag-making equipment were configured on the step-down transformers.
Equipment
Each 300-kVA UPS protects a single extruder, which takes plastic pellets and makes 48-inch wide, 2-ply thick, rolls of plastic. The plastic is then fed into bag-making machines. The extruders are designed to run for weeks without stopping, as the rolls of plastic are removed without interrupting the process. The extruder and associated equipment represent a number of varied loads on the UPS. Each extruder has the following primary loads where the combined load is approximately 200-kVA on the UPS at a 0.6 lagging power factor:
• 125-HP DC drive;
• 60-HP DC drive;
• 15-HP AC drive;
• 2 x10-HP AC drives; and
• 50-KW of heaters.
Drive Basics
A variable speed motor drive is a power-electronics product placed between the utility and a motor. The motor drive allows a DC motor to be controlled over a wide speed range by varying the DC voltage that is applied to the motor. Since the utility supplies its customers with AC voltage, a DC motor is generally installed with a motor drive to provide the variable DC voltage.
The main power circuit in a DC drive is a 6-pulse phase-controlled rectifier, which provides DC voltage to the armature of the motor for speed control. Phase-controlled rectifiers monitor their AC input to determine when to gate the SCRs to obtain the proper DC voltage. A phase locked loop (PLL) is typically involved in this monitoring circuit for noise immunity.
DC drives typically use an outer control loop (speed) that feeds an inner control loop (current). The tuning of these control loops is critical in order to maintain the accuracy of the motor's speed. In the extrusion process, poor motor speed control results in plastic that is not useable due to its non-uniform thickness. Since DC drive manufacturers have no control over what type of DC motor will be connected, the gains of these loops have to be field-adjustable. Some DC drives will have a "Self-Tune" feature that will attempt to tune both loops automatically. While it is understood this Self-Tune feature can achieve acceptable results in some applications, it does have flaws. Placing a UPS in front of a DC drive may amplify these flaws, resulting in the requirement to 'field tune' these control loops.
DC drives can generate at least 30 percent current THD and will notch the AC input voltage. This line-notching and current THD will result in a 2-5 percent distortion of the output voltage of the UPS. It is good practice to install AC line reactors in front of each DC drive to reduce the line notch depth.
System Adjustments
The following problem arose during installation of the flywheel UPS in the facility. It was observed that the DC drive's AC input current would oscillate when the UPS was in Bypass and oscillate twice as much when the UPS was Online. At certain times, these oscillations were severe enough to make the UPS discharge excessively. The current feedback loop gain was increased to bring the oscillations under control, thus reducing the oscillations under all conditions. The tuning parameters were set for all the drives throughout the factory, which allowed the system to remain stable and prevented extruder line shutdowns during power events.
Another adjustment made was in the UPS slew rate and the DC drive PLL error parameter. After discharge following utility power failure, the input returns and is qualified by the UPS. The UPS must then synchronize its output to the input before the load can be transferred from flywheel to utility. This synchronization is done at a slew rate that is defined by a UPS operating parameter. If the slew rate is too fast, the DC drive's PLL circuit may generate an error and turn the drive off. For this application, the slew rate was reduced to 0.6Hz/sec and the drives PLL error window was widened to 10 m sec.
Connecting DC drives to the output of a flywheel UPS should not be considered 'plug & play' and certain guidelines should be followed to ensure success. DC drive manufacturers do not recommend connecting more than 5 DC drives (2.5 extruders) to the same source without some isolation (smaller transformers or AC line reactors). Using the smaller kVA UPS systems on each extruder provides this isolation.
Additional points to consider are listed below:
The DC drive load should not exceed 50 percent of the UPS capacity.
- Some DC drives have the ability to 'regenerate' power back onto the AC line. UPS systems cannot handle reverse power flow and should never be placed in front of DC drives that can "regen."
- Placing a UPS in front of a DC drive WILL accentuate any DC drive tuning weaknesses that may not be as obvious when the drive is connected to the utility, which is a much 'stiffer' source. This may cause the need for DC drive adjustment to ensure stable operation.
Additional Benefit
A DC drive can have extremely poor, lagging power factor, depending on the DC motor's speed and load. Generally the faster the motor is turning, the better the power factor. Under many conditions, the flywheel UPS will provide a dramatic improvement in the power factor that the utility will see. This adds to the economical benefits of the flywheel UPS installation. The input power factor of the UPS at this site is 0.95 to 1.0 even though the load power factor is approximately 0.6 lagging.
Monitoring & System Communications
The factory in which the UPS systems are installed is quite large. The company wanted a way to monitor operations within the factory, as well as remotely. A single phone line to each UPS would have solved the problem, but increased installation cost. However, the flywheel UPS has communications options that allow for both modem and Ethernet monitoring. In this application, the Ethernet connection option on each UPS was connected to a simple hub. A local PC runs the monitoring software, " UPS View," and collects data when required. The software used has the capability to allow for connection from any computer around the world. Thus, company and service personnel monitor the system from time to time, ensuring continuous operations with no downtime. Additionally, the monitoring software will page a service technician if any of the systems generate an alarm. The figure below shows the communication configuration used in this application.
Financial
In a factory setting, the distributed approach has many benefits to the overall efficiency of plant operation. The utilization of 17 flywheel-based UPS systems saved enormous cost at the time of installation. The ability of the UPS to function in the high heat environment of the factory floor allowed for a distributed approach that minimized factory downtime and maximized plant efficiency. Furthermore, the small footprint of the 300-kVA flywheel UPS aided in placement in an already densely populated factory floor.
After 14 months of operations, the flywheel UPS systems work at keeping the factory running. The factory downtime for all lines was reduced 700 hours for the year 2002. Each hour saved represents not only the revenue of product manufactured in that time, but also the avoidance of lost revenue due to lost productivity. The flywheels are performing beyond the customer's expectations, and the factory reports that the payback of the capital investment will be one half the original time estimate.
Terry Ault is Product Support Manager and Scott Richey is Product Manager for Activ Power at 2128 W. Braker Lane, BK12, Austin Texas 78758
