Aging beauty: Integrated approaches to plant automation upgrades

The operating life of the Bahraini Rifaa II power plant was increased by 15+ years following an integrated upgrade
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It is all too easy to forget that some of the Middle East region’s power plants are not as young as they used to be. This is a particular problem for many of the original auxiliary components. Sascha Zarmutek of Alstom Power highlights these problems and offers some integrated solutions.

Sascha Zarmutek, Alstom Power, Switzerland

The Middle East region has many power plants that have now been in service for over 15 years. Although the mechanical lifetime of the engine itself has still not expired à‚— most machines still have another 15 to 25 years of service with the correct inspection and maintenance à‚— the same is unfortunately not true for many or most of the original auxiliary components.

These include such items as the hydraulic/mechanical components and the electrical and control systems. As a direct consequence of this, an increasing number of owners are facing difficulties with undesired trips, reduced availability of the machines and even increasing maintenance, sourcing and breakdown costs.

These older units are still important for this region. New capacities are planned or under construction, but not yet available to cover the faster growth of electricity demand in the Middle East region à‚— at least double compared with the global average. In addition, Saudi Arabia as the second largest power market with an installed capacity of more than 37 GW faces problem with forced outages and grid blackouts due to peak load deficits.

By modernizing and upgrading long running power plants, additional peak load capacities (e.g. fast start-up units) could be available, new operation requirements could be implemented (e.g. frequency response) and the time delay until the new power plants are in operation could be bridged; all aspects bringing economic benefit to the plant owner, the consumer and the region.

Rather than considering the replacement of individual components or even sets of components from various suppliers, which could create performance issues because of the inter-relationship of the system with the existing plant, what would be beneficial is a single supplier with the turbine know-how and modern automation solutions that could provide an integrated plant-wide technology migration. Alstom’s ‘Integral Automation Upgrade’ is one such package.

Alstom’s Integral Automation Upgrade concept is designed to be executed during a major inspection and to extend the plant’s lifetime for another two decades. The scope of work is the upgrade of the protection and control systems of the gas turbine, steam turbine and generator by using the latest and proven technologies.

Turbine protection and control

Certain hydraulic/mechanical parts, especially the hydraulic, closed-loop control components like the temperature controller, speed governor and the fuel changeover valve, as well as the operating devices, are of an obsolete design. The maintenance and adjustment, as well as spare parts procurement for these components are time-consuming, difficult and expensive.

Alstom’s service turbine control system is available for gas turbines of the types GT9, GT11 and GT13, and also for steam turbines applications. It replaces the existing hydraulic/mechanical control relay logic and hardwired system.

The turbine protection and control system uses the latest and proven technology. This ensures that no additional capital investment in this area is required for the remaining lifetime of the turbine.

To secure the turbine control and safety functionality, the field instrumentation for the following measurements needs to be replaced or modified: turbine outlet temperature measurement; compressor end pressure measurement; flame monitoring measurement system; and speed measurement and electronic over speed protection. The upgraded turbine safety system will fulfill the functional safety requirements according IEC 61508 for critical protection criteria.

This Alstom turbine control system is a microprocessor-based, digital control system and consists of the process control system, a critical protection system and the necessary infrastructure for data communication (Figure 1).

Figure 1: Architecture and network structure
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Closed- and open-loop control

The process control system is divided into an open-loop control, closed-loop control and engine protection. Two interconnected and independent controllers are dedicated for process control, supervision of limitations and act over protection circuitries to the trip solenoids. The open-loop control task is to control, protect and supervise all actuators that have an on/off or open/close control function. The system is hierarchically structured, starting with the lowest ‘drive level’, ‘function group level’ and ending with the ‘process automation level’.

For example, the main function groups for the turbine process include: lube oil, fuel gas, fuel oil, starting device and turning gear, synchronization. Process automation is achieved through the turbine sequencer that controls the different function groups.

The closed-loop control controls continuous process variables based on set-point values. The following main control modes secure reliable gas turbine operation:

  • Start-up Control à‚— mainly to accelerate the gas turbine from ignition to minimum control speed.
  • Speed/Load Control à‚—the speed controller takes over from the start-up controller and accelerates the unit to synchronizing speed and is activating in idle operation. The load controller is active whenever the gas turbine is connected to the grid.
  • Frequency response operation à‚— automatic variation of the gas turbine load maintaining the balance between generation and consumption of electrical energy in the grid. The load change is proportional to the measured deviation from nominal frequency.
  • Temperature control ensures that specific temperatures in the turbine do not exceed allowed limits.

The protection system maintains the integrity of the turbine and the safety of people in case a parameter exceeds the allowed limits. The protection is divided into an engine module and critical protection module. The engine protection includes protection criteria, which are processed within the process control system. The voting circuitries are designed according to process importance from ‘1 out of 1’, up to ‘2 out of 3’ logics. Process redundancies are always reflected in the control system configuration.

The critical protection consists mainly of an overspeed protection and a burner protection. Both are fully independent from the closed- and open-loop controls. There are various pre-alarms that inform the operators of an unacceptable operating condition that will cause a trip or fast load shedding, unless some corrective action is taken.

Protective load shedding (PLS) as a new feature serves to minimize the stresses in the machine by preventing unnecessary trips. It is initiated automatically and reduces the load at a fixed preset rate. If the cause of malfunction disappears during the unloading, the procedure is stopped automatically and the gas turbine can be loaded again after resetting the PLS command.

A trip is defined as the immediate closing of the trip shut-off valves and leads to an instantaneous decrease of the load, temperature and mass flow. The gas turbine trip is typically a two-channel system, where each trip solenoid is connected to a separated trip channel. Engineering, maintenance and updating of the controller application software is performed by the engineering station. It provides tools for efficient entry of engineering data, online debugging and forcing based on graphical function block application programmes according to IEC 61131.

Control and supervision of the turbine takes place from the local control room or main control room and is accomplished by the Human Machine Interface (HMI) with the following operation functions: operation by means of displays, using the mouse and keyboard as pointing and activating devices; monitoring by means of process displays, navigation on object displays, curve displays for trends along with status lists; and supervision by means of alarm and events displays, the system self-diagnostics and system status displays. Additionally, an emergency back-up panel located in the control cubicles allows operation in case of a HMI or network failure.

For the older electrical equipment, spare parts procurement is becoming more expensive and difficult day-by-day, the maintenance on the generator protection or excitation systems often requires time-consuming recalibration and the availability of specialists for these systems is not always guaranteed, hence reducing availability or increasing downtime might occur. The modular concept offers a reliable and cost-effective solution for upgrading electrical systems and has been sized to fit to the service turbine control systems.

Generator protection and control

To secure the stability of the generating unit on the grid, a reliable and fast acting excitation system is of great importance. In case of severe disturbances on the grid, the power train is protected by the generator and transformer protection system, allowing the unit to stay in island operation until the dispatcher gives grid clearance and initiates the automatic re-synchronizing to the grid.

A microprocessor-based protection system is used for the generator and transformer protection. It detects electrical malfunctions in the power train systems such as the generator, power transformers, bus ducts and/or generator circuit breaker and so prevents or at least minimizes major failures.

The excitation system serves to regulate the generator voltage of synchronous machines. The equipment receives the AC power supplied either via an excitation transformer or power voltage transformer connected to the generator leads. For smaller machines, permanent magnet generators are used to feed the field winding of an exciter.

The generator excitation control is termed Automatic Voltage Regulator (AVR). It is a closed-loop sensing of the generator terminal voltage, comparing it with a set value and controlling the excitation current to counteract any deviation in voltage.

An important additional feature is the Power System Stabilizer (PSS), which dynamically modulates the excitation current to counteract low-frequency oscillations between the grid and the turbine generator. This functionality is a requirement for power plants on weak grids, where the energy consumption is higher than the capacity.

The synchronizing system is switched on by the turbine control system. The device then automatically adjusts the turbine speed and phase (frequency controller) and the generator voltage (AVR) to the grid.

The synchronizing system is a microprocessor-based single- or dual-channel synchronizer with automatic synchronizing and a manual synchrocheck feature. An optional metering system with statistical counting of the unit energy production and own consumption of active and reactive power can also be applied.

Gas turbine starting system

The availability of a power plant depends highly on a reliable starting system; especially power plants with more than one gas turbine require flexible starting concepts. While another focusing on high market prices during peak periods will rely on its short starting and loading features after being released for dispatch. Therefore improved start-up time is essential.

The gas turbine starting system is initiated by a starting motor or a static starting device (SSD). The starting system accelerates the unit until the turbine generates enough power to attain synchronous speed.

The major components of a SSD are a medium-voltage circuit breaker, static frequency converter including control and monitoring unit, starting excitation and thyristors.

To secure starting reliability and plant availability, one SSD with a change-over facility is installed for each pair of gas turbines. To increase redundancy in the event of a failure, each gas turbine can be started at any time via one of the adjacent systems.

To exploit the benefits of the above different packages, a secure and stable auxiliary power supply is recommended.

In normal operation the battery charger supplies all DC consumers and keeps the battery bank charged. The inverter supplies the 230 VAC1 essential distribution board, from where the turbine control system, flame monitors, printer(s) and HMI monitor(s), among others, are supplied.

All essential DC loads necessary for a safe shutdown of the turbine unit in case of an AC failure are supplied from the integrated DC distribution board. The low voltage AC motor control center supplies the auxiliary drives and consumers for the relevant unit and secures the low voltage AC distribution.

Makkah 2 power plant, Saudi Arabia

Commissioned in 1980, the single-cycle Makkah 2 power plant is powered by three GT11D5 gas turbines. After a fire in the control module in 2003 the plant owner had two alternatives: either to clean and repair each affected device or to replace the control module completely. To save costs and time, the owner looked for an integrated concept to be provided by a single partner that was able to offer a plant-specific solution.

Based on the customer’s requirements, the unit rehabilitation had to be performed without disturbing the operation of the other two units. To minimize the on-site integration work scope, interfaces to the other units and balance of plant, the turbine control and electrical systems were fully assembled and tested in an air-conditioned container. This led to a considerable reduction of the time needed on-site for erection and commissioning. The life extension upgrade took six weeks as originally scheduled.

Rifaa II power plant, Bahrain

The single-cycle Rifaa II power plant consists of six type GT13D2 gas turbines operating on fuel gas. The plant was built in 1983, the gas turbines are 22 years old, and the plant required significant investment to ensure low emission levels and its long-term performance and competitiveness in a growing power market.

In 2000, the Ministry of Electricity and Water (now the Electricity and Water Authority) began to compare the benefits of various rehabilitation options with the alternative of building a new plant. Alstom proposed an integrated GT13DM gas turbine upgrade and power plant modernization package including new components, control systems and auxiliaries that would increase the life of the plant by 10-15 years.

The scope of the project, besides the turbine upgrade, included the replacement of the electrical system, the installation of six fuel filter skids, overhauls of the pre-warming unit and gas reducing station, as well as the adjustment of the control system for a proper plant-wide integration of the entire scope.

The last upgraded unit was handed to the customer on May 2009. The integrated upgrade has lengthened the plant life by at least 15 years, including an increase in power and efficiency. MEE

1 This value is country specifc and be adjusted.

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