By Teresa Hansen, Associate Editor
Power supply is critical to any paper mill. When a major paper manufacturer found that its early 1980s vintage turbine-generator’s unplanned downtime, mostly due to obsolete spare parts and maintenances issues, was jeopardizing one of its mill’s production capacity, the manufacturer decided it was time to overhaul the aging turbine-generator.
Taking advantage of a planned outage, plant personnel decided to replace the outdated electronic and hydraulic systems, and contacted Alstom Power for assistance. Using technology from GE Fanuc Automation, Alstom successfully retrofitted the generator and installed faster controls. The upgrade led to increased reliability and efficiency by optimizing turbine start-ups and performance, and by constantly monitoring and adjusting for thermal design constraints. Additionally, the plant has reduced maintenance and spare parts costs, improved diagnostics for less downtime and faster return to service, improved regulatory reporting, and provided nearly 100 percent uptime to power its mill.
Power to the Paper
Alstom Power’s Controls and Commissioning (CONCOM) group, based in Midlothian, Va., developed the plan for the turbine control system (TCS) upgrade. The scope of the work included upgrading the TCS with a redundant turbine controller, auxiliary controller, protection controllers, automatic voltage regulator (AVR), and a new human machine interface (HMI) solution. The control system solutions are part of Alstom’s P400 Turbine and Generator Control System family, which uses GE Fanuc controllers and HMI software.
The plant’s turbine-generator is an early 1980s vintage single casing, three-pressure section machine.
The plant’s turbine-generator is a single casing, three-pressure section machine, which at full load is capable of running with steam inlet conditions of 1,800 psi at 1,000 F and 800 million lbs/hr steam mass flow. The turbine-generator system includes a high-pressure steam turbine, intermediate-pressure turbine, low-pressure turbine, condenser, generator, two controlled extractions, live steam piping and low-pressure exhaust.
The new TCS is divided into two systems, the closed-loop turbine controller and the open-loop turbine safety system and auxiliary controllers. The safety system provides a second line of protection against potentially dangerous service conditions such as overspeed, loss of vacuum, and loss of bearing oil supply.
The new TCS also incorporates speed control, load and pressure control, automatic turbine run-up, thermal stress calculations, and protection (or “limiter”) functions into a single controller platform. To achieve the highest reliability and uptime possible, Alstom incorporated a GE Fanuc hot standby redundant controller, featuring the PACSystems RX7i Programmable Automation Controllers. Alstom migrated existing code from its previous TCS designs using GE Fanuc Series 90-70 PLCs to the PACSystems RX7i and PACSystems Control Memory Xchange (CMX) – resulting in the plant’s execution time dropping from 40 msec to 14 to 16 msec and helping it realize target process improvements through faster CPU performance.
“By migrating our design to the PACSystems RX7i Hot Standby CPU Redundancy solution, we achieved a tremendous improvement in system performance,” explains Stephen Altman, control system product engineer with Alstom Power’s CONCOM group. “The improved processing speed translates to tighter regulation of the turbine and greater efficiency. Synchronized data flows much, much faster, significantly reducing our cycle times. For the plant, this means a bumpless system – operations will stay up, and the machine won’t trip.”
Tools, such as Proficy Machine Edition, helped speed the controller migration and allowed Alstom to design the system within the scheduled timeframe. According to Altman, Proficy Machine Edition offers a set of development and management features such as multiple target support per project, a tool chest, data monitor and watch windows, all combining to make project creation and execution an efficient process.
The Alstom P400 Turbine and Generator Control System uses GE Fanuc controllers and HMI software.
“The GE Fanuc development tools provided a means to design the custom functions used in the power industry, such as Integrator and PI functions,” Altman explains. “We were able to utilize the C programmer’s toolkit and the parameterized subroutine block feature to design a custom function library using industry standard control algorithms – to develop a much more capable system in less time.”
No Power Trips
To further ensure uptime and reliability, Alstom implemented GE Fanuc Proficy HMI/SCADA – iFIX in a redundant operator interface configuration. The HMI system makes it easy to access the controls through graphical screens in a familiar Microsoft Windows environment. Rapid supervisory control and monitoring permit faster response to any process issues. The system’s flexibility allows for unlimited alarm areas, exception-based alarms, priorities and remote management of alarms, reducing troubleshooting time and increasing uptime. Additionally, iFIX collects and manages data, stores historical data and exports data to common databases – with the ability to quickly connect to a plant-wide historian, as database needs and analysis change. The system allows automatic and long-term sampling, storing, reporting and displaying the turbine process data, all in an environment that reduces training time and speeds response.
Prior to on-site implementation, Alstom conducted a complete and aggressive factory acceptance test (FAT) with the plant. The team tested all systems against Alstom’s proprietary turbine simulator. With the FAT completed, implementation was quick and there were minimal changes during commissioning.
“Since the upgrade, the plant has recognized benefits such as increased availability and reliability,” Altman says. “The team minimized spare parts issues and reduced training requirements. The team also expects a decrease in maintenance costs.”
Turbine Control System Design and Functionality at a Glance
Redundant Turbine Controller (Alstom P400-Turbine Control)
PACSystems RX7i in the hot standby CPU redundancy configuration communicates with the protection controllers via the Genius network. Communication to the auxiliary controller is over a fiber-optic ring, utilizing PACSystems CMX modules. Features include:
- Turbine primary speed governing. This includes proportional speed control, speed limitation and acceleration limitation.
- Turbine run-up speed control (auto and manual) from turning speed to synchronous speed.
- Turbine speed control (auto and manual) during no load operation to permit synchronization with the grid system (by external synchronizer).
- Valve position manual control.
- Closed loop MW load control. The operator sets a target load setpoint. When the load control is engaged with the unit synchronized, the load control function will act to maintain the generated load at this setpoint.
- Live steam pressure control. The turbine is unloaded in proportion to decreasing inlet steam pressure until a preset minimum load is achieved. This includes using minimum live steam pressure limiter and live steam pressure gradient limiter.
- Exhaust steam pressure control. The turbine is unloaded in proportion to rising exhaust steam pressure until a preset minimum load is achieved.
- Coordinated boiler and turbine load control.
- Hydraulic system reset.
- Stop valve control.
- Frequency adjustment (60Hz ±0.05Hz).
Auxiliary (Open Loop) Controller (Alstom P400-AC)
PACSystems RX7i in the standalone controller configuration with expansion I/O rack. Features include:
- Auxiliary systems control (lube oil, jacking oil, turning gear, etc.)
- Turbine start-up program
- Field measurement conditioning
- Automatic system testing (lube oil, safety, control fluid, etc.)
- Synchronizing function group
- Turbine generator auxiliary control
Three-channel Turbine Protection System (Alstom P400-TP)
Three Series 90-30 PLCs with the CPU364 processor, processing the same field inputs and executing the same core logic. Communicates with turbine controllers over the Genius network and provides two-out-of-three voting for signals that may trip the unit. Features include:
- Three channel overspeed protection
- Remote turbine trip according to I/O specification
- Protection system tests as required by process, typically:
- Digital overspeed protection tests
- Stop valve movement tests
- Stop and control valves tightness tests
Binary Signal SOE Recording (Alstom P400-SOE)
Series 90-30 PLC with sequence of events recorder CPU from Horner Electric, connected to another Series 90-30 PLC with a data concentrator module also from Horner. Features include:
- Binary signal COS recordings with timestamps
- Data to HMI communication
- On-board clock synchronization
Series 90-30 PLC communicates with the protection, auxiliary and turbine controllers over Ethernet, as well as over Modbus to a legacy DCS. Features include:
- Setpoint and reading process data from all TCS controllers
- Data collection from Bentley Nevada vibration monitoring equipment
Redundant Automatic Voltage Regulator (Alstom P400-AVR)
Two Series 90-30 PLCs processing the same field inputs and executing the same core logic. Hardwired signals are used for redundant process synchronization. GE Fanuc Datapanel operator interface is mounted on the front door of the excitation equipment for maintenance and local information. Communicates with the HMI over Ethernet.
- Automatic voltage regulator
- Manual regulator
- Automatic voltage built up with progressive voltage raise (soft start)
- Over excitation limitation
- Under excitation limitation
- Volts/Hertz limitation
- Supervision of generator measurement voltage for the automatic voltage regulator
- Positive droop: compensation of voltage drops induced by active and reactive currents
- Negative droop: reactive balance for two generators working in parallel
- Field flashing sequence in case of self excited generator
- Local information through a digital control panel connected to the regulator
- Excitation logic sequences (field breaker control, start, stop)
- Generator stator current limitation
- Reactive power or power factor superimposed control
- Communication with another control system, like plant HMI, through a TCP/IP link
Redundant Operator Interface
GE Fanuc Proficy HMI/SCADA – iFIX provides process visualization, data acquisition and supervisory control over manufacturing and production processes. Features include:
- Supervisory control: rapid process monitoring allowing rapid response to process issues
- Advanced alarming: reliable, flexible and easy-to-use system, with unlimited alarm areas, exception-based alarms, alarm priorities and remote management of alarms
- Data management: ability to collect and manage data, store historical data and export data to common databases
- Historical data: automatic and long-term means of sampling, storing, reporting and displaying process data