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Study Examines Reliability of UPS Systems and Offers Approaches to Improving System Reliability

According to a new study by MTechnology, Inc., the Active Power flywheel-based CleanSource® 300 UPS (uninterruptible power supply) system is seven times less likely to fail versus a conventional, double conversion UPS system with batteries during a short power outage.

Commissioned by Active Power, the Reliability Study of Integrated Flywheel UPS System is a quantitative analysis of the in-service reliability of the company’s CleanSource 300 UPS system versus a double conversion UPS system with batteries. The study utilizes probabilistic risk assessment (PRA), a formal technique used by the aerospace and civilian nuclear power industry. Both systems were analyzed in conjunction with two classes of utility failure – short outages lasting less than 10 seconds and long outages exceeding 10 seconds. The Electric Power Research Institute (EPRI) reports 96 percent of all sags and outages last 10 seconds or less. In a short utility outage, the UPS energy storage is sufficient to support the load until the utility is restored. In a long utility outage, the AC source is transferred to a generator.

“We found that demand failures of the flywheel energy storage system are extremely unlikely,” said Steve Fairfax, president of MTechnology, Inc. “If the flywheel is operational at the moment an outage occurs, the flywheel will almost certainly function. Conversely, a system supported by an electrochemical battery has a much higher probability of suffering a demand failure in a utility outage. Even with good monitoring and testing practices, it is difficult to predict the performance of the battery during an interruption. The flywheel-based CleanSource UPS is a dynamic electromechanical system so conditions that could lead to failure are relatively easy to detect and repair. Changes in values that determine the system’s health can be observed before an outage occurs.”

Key findings from the independent study include:

  • CleanSource UPS is approximately seven times less likely to fail versus a double-conversion UPS with batteries during a short utility outage.
  • The most likely failure mode of a double conversion UPS system with batteries is due to undetected battery failures. Detecting battery cells that will fail on the next demand has proven to be difficult.
  • In outages lasting longer than 10 seconds where transfer to standby generator by means of an automatic transfer switch is necessary, CleanSource UPS again performed better than the conventional double-conversion UPS.

Mission critical operations such as healthcare, broadcast and data centers require system performance with no interruptions in service as downtime can prove costly. EPRI reports that system downtime is doubling in cost every year due to power sags, surges and outages across all business sectors. These types of customers require uptime 24 hours a day seven days a week and expect their backup power systems to be extremely reliable.

“We felt the CleanSource UPS system was a more economical solution with lower maintenance than conventional battery-based UPS systems,” said Matt Servis, director of Engineering for Vermont based WFFF-TV (FOX) and WVNY-TV (ABC). “Within the first six months of installing the system, it was evident we made the right choice. We had more than 200 power incidents in this timeframe with absolutely no interruption in service to our viewers. The system is just a workhorse. And now after two years of service at the top of Vermont’s highest peak, we have not been off the air for even a moment due to power related issues.”

”We are excited about the results of the study which further supports our inherently reliable system design,” said Jim Clishem, president and CEO of Active Power. “Customers are increasingly focused on reliability and cost savings and we are proud to deliver a system with superior reliability and lower lifecycle cost than any other UPS system available on the market.”

For access to the Reliability Assessment of Integrated Flywheel UPS versus Double Conversion UPS with Batteries whitepaper, please visit

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