John Downing, Turbine Controls & Excitation Group (TC&E), celebrated the company’s 10th anniversary with a presentation last Wednesday at the 7F Users Group’s 2020 Digital Conference. He reminded the 300 or so owner/operators in attendance that many gas turbines are starting and stopping far more frequently than originally designed for. At the same time, independent system operators (ISOs) are imposing stiff fines for failure to meet startup and performance obligations.
Most plants have adapted to these more aggressive operating tempos and performance challenges, but one component that may be getting overlooked is the starting subsystem, especially for older machines equipped with a load commutated inverter (LCI).
Downing says that the Innovation Series™ and LS2100 LCIs, introduced in the late 1990s and early 2000s, consist of three subsystems, or functions: control section, silicon-controlled-rectifier (SCR) bridge section, and cooling section. For units not originally designed for lots of starts, it was common to have one LCI for multiple GTs, often in a one-to-two ratio.
For nearly 25 years, the LCI has provided reliable service. However, with an expected life of 20 years, many have failed, and a substantial number are at, or past, end of life. Failures usually are experienced in the controls and/or the cooling system.
Last production of the Innovation Series and LS2100 LCI controls supplied with GE machines was in 2013. Today, replacement parts are difficult to locate; acquisition times of four to six weeks are common. Also, qualified service and field engineering personnel are becoming harder to find as they retire out of the workforce.
Options to address this risk include the following:
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- Complete replacement of the LCI.
- Locate, purchase, and inventory critical spare parts, especially those relating to the control and cooling sections, and identify the engineers and technicians qualified to make the replacements when the time comes.
- Install a “digital front end” (DFE) control section replacement. In this case, the controls are replaced with a modern alternative that extends LCI life by 20 years.
TC&E partners with TMEIC, one of the world’s largest manufacturers of LCIs and drives, to provide a turnkey DFE solution that Downing says is far less expensive than the other two options.
Guts of the partnership’s offering are as follows: The obsolete programmable logic controller (PLC), standard VME (Versa Module Europa) rack, power supplies, and input/output (I/O) boards are replaced with new processor-based control circuit cards and a new PLC. The new digital controls fit in the existing panels and reuse most of the existing fiber optic cables and connectors.
The upgrade includes a local color touch-screen control panel and HMI interface.
In the control center, the associated software suite expands the programming, control, optimization, troubleshooting, and data logging of onsite operations, engineering, and maintenance personnel. The new LCI DFE utilizes appropriate communications protocol to talk directly to the GE Mark VI and Mark VIe control systems.
The typical upgrade project can be accomplished in about five days; a turbine shutdown is not necessarily required, although, of course, the LCI will be unavailable for that period.