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.


ASI Robicon's medium-voltage, variable frequency drive with Advanced Cell Bypass helps conserve energy at Marathon-Ashland Petroleum's cracking unit.

U ninterrupted operation, or total process reliability, is an essential goal at processing companies, especially those in environments where even a momentary interruption can cost millions of dollars in lost production. Marathon-Ashland Petroleum (Robinson, Ill.) was on the same quest when it installed an advanced variable frequency drive (VFD) that it expects to operate continuously for five years without interruption. This new drive technology is an integral part of the company's critical Fluidized Bed Catalytic Cracking Unit (FCCU) refinery system. All of the unit's components were picked to meet the same five-year runtime criteria.

Marathon-Ashland began seeking to optimize and update its 50,000 bbl/day FCCU in 1997. As the largest producer of high-octane gasoline blend stock, the FCCU is one of the most important units in a refinery. The scope of Marathon-Ashland's $85-million project included a complete replacement of the FCCU reactor, combustion air blower and wet gas compressor, as well as the addition of a flue gas scrubber to reduce particulate emissions. The project was scheduled for completion during a planned refinery maintenance turnaround in November 2000.

Gaining efficiencies

Marathon-Ashland expected its renovation project to conserve substantial energy, as well as improve product yield, reduce maintenance costs, improve reliability, and enhance environmental compliance. Efficiencies generating energy savings would come largely from the new motor-driven combustion air and wet gas compressors, which would deliver a design efficiency of more than 80%, compared with existing steam turbine-driven centrifugal compressors, which usually operate at 65% efficiency.

Installing the new compressors made additional efficiencies possible by allowing installation of a variable-frequency drive (VFD), also known as an adjustable speed drive. This is because speed control is the most efficient method of controlling the capacity of centrifugal machines. Other methods include suction throttling and spillback control. Movable stator vanes may be used to control axial machines. A feasibility study showed that the wet gas compressor would be a good candidate for a VFD, due to improved operating efficiency and a quick projected payback on initial cost.

Evaluating VFDs

Once Marathon-Ashland decided to install a VFD on its wet gas compressor, the company held a technology evaluation session to examine three different VFD technologies for reliability and efficiency. The three VFD technologies evaluated were load commutating inverter (LCI), neutral point clamp (NPC) with integrated gate commutating thyristor (IGCT), and a Perfect Harmony design with insulated gate bipolar transistor (IGBT).

While the LCI technology was proven and was the lowest cost alternative, Marathon-Ashland ruled it out because it was not fault tolerant and was the least efficient. The newer, higher-efficiency NPC-IGCT and Perfect Harmony technologies were further evaluated, using lifecycle cost analyses. Reliability analysis indicated that the fault tolerant Perfect Harmony drive had redundant technologies for maximum reliability, as well as the lowest lifecycle costs. As a result, it was selected to drive the wet gas compressor.

Matching the mission

The project team's next task was to evaluate and select a VFD configuration that could meet the company's five-year continuous operations goal. This five-year reliability plan also checked the reliability of other system components. Reliability of machine trains has increased over the years, so five-year runs without trips or overhauls can be expected. This also meant the same reliability would be expected from the VFD, since it would be an integral part of the compressor system. Consequently, a modular VFD with redundancy was required, and Marathon-Ashland evaluated several options. The lifecycle cost advantages of a base VFD system were compared against systems with redundant power modules and bypass contactors.

Based on these comparisons, Marathon-Ashland selected a medium-voltage VFD from ASI Robicon (New Kensington, Pa.) that was equipped with its Advanced Cell Bypass technology. In the event of a power module failure, for example, this feature bypasses the disabled module without major drive speed or voltage reductions. These redundancy and bypass features enhance the drive's overall reliability, providing a real opportunity to operate without interruption for the planned five-year period.

Checking, picking components

Besides picking the VFD, Marathon-Ashland's design review process included identifying and eliminating possible single-point failures that could lead to a drive system fault or trip. All cooling system pumps and fans are redundant, and designed so that a failed circuit can be isolated and replaced while the drive is operating. The power supply for the control system is suitable for conventional cooling, and it was fitted with a blower to provide additional cooling. A second fan was later added for redundancy.

The system's only non-redundant components are the input isolation transformer, which has very high reliability, and the central control processor. While the modular nature of the drive system's design distributes some control functions to each module, such that a failure at that point can be bypassed, the remaining functions in the central control are not duplicated.

The design review process also identified places where upgraded components or methods could enhance reliability. Hence, cooling system pumps were specified to be API pumps with IEEE 841 motors. All auxiliary components, such as pumps, motors, switchgear, terminations, were chosen to be common with other equipment in the refinery, which would minimize the cost of spares, tools and training.

Maintaining process availability

Throughout the renovation of its FCCU, compressors and other equipment, maintaining process availability was paramount for Marathon-Ashland. Consequently, the new system communicates status data to the refinery's process control for decision-making. All sensor data and diagnostic messages are communicated via a combination of Modbus data link, digital I/O and analog signals. All signals from process control to the VFD are via discrete analog or digital inputs. Back-up manual controls and default conditions are defined in case any signals are lost.

Marathon-Ashland reports that combining module bypass with neutral-shift enhances the reliability of modular medium-voltage drives. These advanced features were fundamental in creating a carefully designed process control system that is now in the midst of running continuously for five years.

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