Power Quality and Generators - Part 6: Generator Sizing and UPS
This is the sixth installment in an ongoing series that will cover basic engineering and code issues for standby generators. In a subsequent column, the author will address commissioning and training.
When a power event occurs, the uninterruptible power supply provides the temporary ride through until the generator kicks in. In this installment of the series, I summarize some of the rotary and flywheel UPS technology available today. These technologies will be evaluated in conjunction with standby generators for critical environments.
A majority of UPS backup systems I've designed are provided with battery arrangements. These systems are typically provided with 5 to 15 minutes of battery backup at full load. The majority of these systems in a critical environment are backed up with a standby generator. If the generator is also serving life safety loads, as specified in the National Electrical Code Article 700, it must be running and providing alternating current power within 10 seconds of a power outage. Even if the generator is not designed for life safety, it can typically be up and running within 10 to 15 seconds after a power failure.
Most of the flywheel UPS systems will supply 15 seconds of full-load power and can actually serve up to several minutes at less than full load. In addition, most of the utility disturbances will last for 5 seconds or less. For these reasons, a rotary or flywheel UPS can be a viable option over conventional battery backup systems.
DC flywheel systems
A DC flywheel system can be utilized by itself or in combination with batteries. The DC flywheel would feed the inverter of a UPS system. This combination approach with a DC flywheel and batteries would represent higher first cost, but would provide for a longer life and potentially simpler maintenance. In addition, a system that only implements a DC flywheel requires a smaller footprint than a battery system.
A conventional battery-powered UPS system that is augmented by a DC flywheel could serve the majority of utility outages - typically lasting for five seconds or less-through the flywheel and not the batteries. This would greatly increase the life of the batteries.
Other advantages of a flywheel (kinetic energy) system in lieu of a battery system include the following:
- Flywheel reliability is typically greater than a single battery string.
- A flywheel requires less maintenance than battery systems.
- Flywheel UPS systems do not require the controlled environment that a UPS battery system requires. A battery system must be kept close to 77 degree Fahrenheit for good performance and life expectancy. In addition, a battery system can also represent a risk of explosion or acid spill.
- A flywheel will take up less space than a UPS battery system.
- A battery system can require spill control and neutralization and hydrogen sensors. Eye wash stations can also be required with battery installations. In addition, the battery system should be continuously monitored to ensure top performance and life expectancy.
The rotational speed of the flywheel (kinetic energy) has a dramatic effect on the amount of stored energy available for critical loads. Doubling the rotations per minute of the flywheel will quadruple the available stored energy. High-speed flywheel systems require a different design approach than low-speed systems. The former are typically made from carbon or carbon and fiberglass composite materials. These materials can withstand the higher stresses associated with the high-speed systems. In addition, these systems utilize magnetic bearings and vacuum enclosures to reduce rotational friction and system losses.
Hybrid rotary UPS
In a hybrid rotary UPS approach, a motor generator incorporates two fully rated power paths. The first path is through a conventional rectifier and inverter and the second path is around the rectifier and inverter, through a static switch to the motor generator. (See diagram below).
In this situation, the rectifier continually provides power to the batteries, but the power is normally regulated and conditioned through the motor generator. The rotating inertia of the motor generator will provide a power ride through for brief utility power interruptions. The motor generator provides for true power isolation. No utility power disturbances pass through to the critical load. When the main power fails, the DC batteries provide power through the inverter and to the load through the motor generator. The inverter is normally off in this situation.
Benefits to this design scenario include the following:
- Provides for two conditioned paths of power
- Provides for two independent power sources; the batteries are only utilized if both paths of power fail
- Can have a greater overload capability
- Uninterruptible power supply system approach can offer high fault clearing ability without going into bypass
This diagram (to the left) represents a hybrid approach. The motor generator provided continuous conditioned power to the load. The rotating inertia of the motor generator will provide a power ride through for brief utility power interruptions. The inverter will only operate when the power fails for more than a brief interruption and the generator is in the process of starting.
Kinetic power cell technology
Another UPS system approach is called kinetic power cell technology. The system offers a line-interactive UPS system. Voltage transients are suppressed through the transient voltage surge suppressors. When the system senses voltage sag, the system takes energy from the flywheel to maintain the output voltage.
- Are matched with the generator to provide for seamless power
- Claim a UPS system efficiency of 96% to 97%
- Indicate there is no restriction on the number of energy discharges
In contrast, a large number of energy discharges on a battery system can reduce life expectancy.
Moreover, flywheel UPS are "generator friendly". By reducing harmonic distortion and by eliminating block loading, the generator systems may not have to be oversized when feeding a flywheel UPS. This is typically not the case with battery UPS.
Due to uninterruptible power supply system efficiency losses, the harmonic distortion created by the UPS system as well as the potential simultaneous charging of the battery supply after discharge and serving the critical loads, a generator feeding a battery UPS system may have to be oversized. Voltage distortion can be the result of a heavily loaded generator feeding a large UPS system that creates harmonic distortion.
Some of the flywheel UPS manufacturers indicate that a short ride through of 15 seconds can be adequate in lieu of the traditional 15 minutes typically provided in a battery UPS if the system is designed with higher reliability. The intent of the argument is that a highly reliable data center is counting on the generator to start the first time, every time. Strict maintenance and redundant starting circuits can ensure that optimum reliability is attained. In addition, redundant generators are typically designed into a high end data center.
This approach may not be appropriate in a typical Tier I data center or facility installation with a single standby generator. In addition, these installations will typically not have a 24-hour maintenance and operation staff that follow a rigorous fuel and battery system maintenance protocol. Generator starting reliability in these types of facilities is not going to be as high as in a Tier IV data center.
These manufacturers further indicate that the conventional wisdom is that a 15 minute backup system provided with a battery UPS system will allow time for an orderly shutdown and additional cranking of the generator. The argument against this conventional wisdom is that a 15 minute orderly shutdown will still result in an unacceptable loss of load.
Furthermore, if the generator does not start within the first few seconds, the chances are that it will not start within 15 minutes. In addition, flywheel UPS vendor literature indicates that the reliability of VRLA batteries drops off considerable two to three years into the life of the UPS system. This in and of itself can reduce reliability of the battery system over the flywheel option. One study indicates that batteries cause 70% of the failures in a conventional battery operated uninterruptible power supply system.
Design power supply for specific use
Generator starting reliability can be improved when a flywheel system provides 24 volts DC to the generator starting system. The power supply from the flywheel system to the generator starting system can be monitored by the UPS system to add even greater starting reliability.
The design engineer should understand the owner's objectives, maintenance and operations procedures and comfort level concerning back up times. With this information at hand, the rotary or flywheel UPS system options do represent a viable solution and should be considered on future projects.
Articles in This Series
- Part 1: Sizing and Code Issues
- Part 2: The Effects of Harmonics
- Part 3: Complying with the Codes and Controlling Noise
- Part 4: Fuel Configurations for Standby Gensets
- Part 5: Paralleling Generators in Critical Applications
- Part 6: Generator Sizing and UPS
- Part 7: Commissioning, Training and Long-Term O&M Programs
- Part 8: Basic Calculations for Sizing Generators and the Impacts of Certain Loads
- Part 9: Design Criteria for Grounding
- Part 10: Generators and the 2005 NEC