Here are highlights from some articles recently posted at Besides presenting Control Engineering's entire print editions, Control Engineering Online also delivers daily news, Web Exclusives, and Online Extra articles that add value to in-print features.


Here are highlights from some articles recently posted at . Besides presenting Control Engineering's entire print editions, Control Engineering Online also delivers daily news, Web Exclusives, and Online Extra articles that add value to in-print features.

Modular VFD with cell bypass eliminates interruptions

To get closer to uninterrupted operation, or at least achieve five-year run time without a failure, Marathon-Ashland Petroleum (Rockford, Ill.) recently installed an advanced variable-frequency drive (VFD) as part of a multi-year, $85-million renovation of its Fluidized Bed Catalytic Cracking Unit (FBCCU) refinery system.

The company evaluated three VFD technologies, including load commutating inverter (LCI), neutral point clamp (NPC) with integrated gate commutated thyristor (IGCT), before selecting an insulated gate bipolar transistor (IGBT) cell-based VFD. Marathon-Ashland chose Perfect Harmony medium-voltage VFD with Advanced Cell Bypass from ASI Robicon (New Kensington, Pa.) because of its redundant capabilities, which increased reliability because the drive can bypass disabled IGBT cells without drive speed or voltage reductions.

For more information, visit .

Hydrogen purity ensures safe steam-turbine operation

Hydrogen's high thermal conductivity makes its ideal for cooling and optimizing performance of steam turbine generators, but its purity must be precisely controlled and maintained at a level of 85% in generators' cooling jackets to ensure its efficiency and prevent the potential for explosions.

To reduce maintenance costs and increase safety, Southwestern Electric Power recently replaced the hydrogen gas analyzer on the 650-MW Westinghouse generator at its H.W. Pirkey plant (Longview, Tex.) with ABB's (Wickliffe, O.) 6540 203K hydrogen analyzer system with two sensors. One sensor is for hydrogen and the other is for carbon dioxide, which is used to purge the hydrogen during jacket cleanings. Plant engineers report that ABB's true gas purity analyzer was easier to read; more accurate than others proposed, which measured purity as a function of density; and required at least 50% less maintenance than the generator's previous unit.

For more information, visit .

Linux-based PLC: high performance, community- driven, and free

To try to move the control and automation industry toward public-domain, community-driven, free automation software based on Linux, Curt Wuollet recently launched the Linux PLC project, also known as Puffin PLC. Mr. Wuollet's project aims to create an open-source PLC alternative that runs on a PC with tools for flowcharts, ladder logic, and HMI; drivers for I/O points, serial ports, fieldbuses and Ethernet; and popular database support. This will allow engineers to build control systems with a free, growing knowledge base, and receive support and contribute improvement to an open-source community.

Linux PLC is also approaching its first software release, which will automate tasks with Linux, memory mapping, demo I/O, and utility code. Web servers, TCP/IP and Modbus are available, and drivers for Ethernet, Modbus TCP/IP, and other fieldbuses are being tested.

For more information, visit .

Back-to-Basics: power conditioning with a CVT

To prevent potentially damaging voltage distortions, engineers often condition power by applying a ferro-resistant, voltage regulator, or Contant Voltage Transformer (CVT), to clean up ac line problems and protect downstream electronics, according to Jill Normandin, marketing director, Sola-Hevi Duty (Skokie, Ill.).

A CVT includes a saturating transformer that uses a resonant or tank circuit, which is formed by the transformer's reactance and a capacitor. This provides a "pool" of power that supplies constant, clean output voltage free of most power disturbances, and has no worrisome response times. The transformer can be more than 90% efficient at full loads because its saturating section allows it to be less efficient at lighter loads.

Because they have no mechanical or active components, CVTs can be used in systems for decades. They can regulate input voltage swings from 20-40%, and more if a system isn't operating at maximum load. CVTs will also provide hold up of 3 milliseconds, as well as surge suppression and noise reduction. Likewise, undervoltage, overvoltage, swells, and even some sags can be isolated from sensitive loads. CVTs act as overload protectors, and limit their output to between 150-200% of overcurrent.

For more information, visit .

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