Uninterruptible Power Supply

Diesel Progress North American Edition, May, 2001 by Henry C. Lengefeld

Making a paradigm shift in distributed power for critical loads

When a paradigm shift in the market is demanded, major changes in foundational technology must be in place to support the resultant new model. Critical power users are being faced with rolling utility brownouts, blackouts and distribution and generation confusion. This demands an immediate solution that can only be achieved with on-site distributed power. Success requires a higher level of engine generator frequency regulation and synchronization especially for sensitive critical loads. New integrated systems, that combine the latest in engine generator controls and uninterruptible power supply (UPS) technology, are proving more reliable than the traditional building block model of the past.

Uninterruptible power supply has generally been applied as a power conditioner between the utility and the critical load. When the UPS is running on engine generator, the UPS acts as a power conditioner and independent frequency converter compensating for the frequency swings of the engine generator. This frequency conversion feature had been required in the past when data centers used the 60 Hz sine wave as a synchronizing signal. However, today's data center and the Internet are equipped with power supplies that tolerate a wide range of frequency to enable application worldwide.

The problem is not at what input frequency the load must be powered, but at what frequency regulation for synchronization and phase matching for load transfers.

In the past, the UPS took on this task by converting frequency using double conversion UPS technology. This same double conversion UPS technology compounded the frequency regulation problem for the engine generator since the rectifier of the UPS represented a difficult harmonic load that often required harmonic filters just to run from the engine.

Although the UPS would run from the engine generator, the UPS rarely could phase lock synchronize to the engine generator and confirmed this with a constant "bypass not available" indication. Bypass not available indicates that the UPS' automatic system bypass cannot be used to bypass the UPS in the event of an internal failure or to provide critical overload current during events such as fault clearing. Overload and fault clearing current has become especially crucial now that the UPS industry has converted the output inverter to the much more limited IGBT power conversion device.

The most important aspect of bypass not available goes to the core of reliability studies that consider mean-time-before-failure or MTBF analysis. In these analysis, the probability of internal failure at the same time as a utility failure is a common assumption. It is easily seen that bypass not available contradicts this assumption and must be corrected.

Uninterruptible power supply or soft loading has been a necessity for many years, but this uses UPS battery run time. Selection of a UPS technology with a linear soft start for the engine generator reduces battery consumption and leads to faster stability.

Once up and running, bringing the engine generator into synchronism can be difficult to do and maintain with some UPS as the load. The UPS is usually the largest single load on the engine generator. Since both the engine generator and the UPS are regulated systems, their respective regulators tend to interact and can result in instability.

Earlier systems that have been the workhorse of batch mode processing operations for years, are no longer reliable enough for today's 7/24/365 operations--continuous run data centers connected worldwide, critical communications centers and Internet facilities. Hyper critical, these facilities strive for the highest availability now measured in 9s. Two 9s is 99.0% available or 3.7 days per year without power. Six sigma or six 9s is 99.9999%. That means 32 seconds without power per year. The ultimate is no 9s.

The drive toward zero downtime has brought us to distributed redundant systems. In the past, the UPS has been used to synchronize at the load to enable transfer from one side to the other for redundancy. This has meant the UPS, as a source with the least capacity for overloads and inrush, is called on to manage this critical function. At the same time, the engine generators are allowed to free wheel, further loosing the benefit of the generator subtransient reactance via the bypass. This can be corrected at minimum cost and is being done with great success, especially for distributed power applications today.

The foundational technology of engine generator tighter speed control -- and thereby more precise frequency regulation for phase lock synchronization -- is in place. More compatible Delta Conversion UPS technology for engine stability, combined with precise engine frequency regulation, makes bypass available.

According to Steve Wetter, program manager, Electric Power Group, at Caterpillar Inc., all engine generators manufactured today are tested to ISO Standard 8528 Part 5 that dictates specific frequency recovery and transient response to loads. Further investigations show that many dealers are applying engine generators with phase lock controls to enable them to continuously track a source and pickup a load with a static transfer switch.