Under control

Daniel Thiery* explains the merits of a distributed control system and its application at the Sanchong pumped storage plant in South Korea

Today’s power markets are ever more demanding in terms of the quality of service users expect from a plant’s digital control system. The Alspa 8000-P320 distributed control system, from French company Cegelec, encompasses all areas of control and data processing required by hydro power plants and substations. The aim of the system is to provide a total solution to the problems of site automation and management.

Alspa 8000-P320 is the fruit of extensive experience accumulated since the installation of Cegelec’s first auto-mated plant control system in France’s Rhone valley in 1971.

The new system has been designed to meet the requirements of both operators and investors, even in the most complex installations such as the Sanchong pumped storage power plant now under construction in South Korea.

Marking an advance in the design of distributed control systems (DCSs), the new system’s open and flexible nature incorporates distributed intelligence. It is designed as a series of electronic blocks communicating with each other via communication networks, dispensing with the need for rigid, centralised controller cubicles. The system is based on three main elements.

Controbloc

The control functions (Controbloc) are performed by cells composed of a cell controller responsible for sequential control functions and control loops, and field controllers handling input/output functions distributed over an open F8000 (WorldFip) fieldbus.

This fieldbus provides open connections to electrical protection facilities, smart instrumentation devices and LV/MV drawers (Alsthom Gemstart). The fieldbus also allows open Modbus connections to other functions such as machine or temperature control. A full range of controllers is available, enabling the system to be adapted to the technical and economic restrictions inherent in each individual power project.

The cell controllers are linked by an S8000 unit communications network available either with WorldFip technology or as an Ethernet link.

Described as reliable, efficient and resistant to electromagnetic interference, the Controbloc automation cells are designed to operate in all industrial environments. Fast, secure communi-cation links mean that the different control system elements can be located nearer the processes they are controlling.

Centralog

The Centralog system is the essential element in the control room and incorporates screen-based plant control and supervision functions. It enables the operator to increase the productivity of units and is integrated with open system technologies (Unix, Windows NT etc). Stations exchange data over an Ethernet network which is also open to Internet/Intranet communications used for remote process maintenance.

The system has been ergonomically designed for power plant control to ensure operator comfort and help limit machine downtimes. The Centralog control room incorporates the following supporting software which comprises the automation suite:

•Automatic site control (co-ordinated unit operation).

•Operator help (procedure and alarm files).

•Advanced control loops.

•Process simulation.

•Tuning and maintenance assistance tools.

•Energy saving.

•Vibration monitoring and smart instrumentation.

The special hydro power automation suite optimises operational capacities and power management, providing better service for site operators and owners.

Controcad

Configuration studies are supported by a software engineering tool called Controcad which is utilised in the design office during the design phase and on site during implementation of the system. Controcad meets iec standard 1131.3 and uses diagrams to produce control function codes and documentation to ISO9001/ISO9002 quality standards.

Other features of Alspa 8000-P320 include varying levels of ruggedness and operational dependability. The ruggedness of the system makes it resistant to external disturbances such as vibrations and temperature changes, and, in particular, immune to electromagnetic interference (to IEC 1000 standards). The system’s reliability is achieved by rigorous monitoring, which includes strict software management procedures.

The fieldbuses and unit networks are particularly rugged and generally do not require the use of fibre optic cable, which is reserved for outside links between buildings.

Sanchong pumped storage plant

The Sanchong pumped storage power plant is located in Chiri Mountain approximately 260km south of Seoul in South Korea. Despite strong international competition, a French consortium of GEC Alsthom and Cegelec signed a contract with Korea Electric Power Company for work at the plant in September 1994. Cegelec’s work consists of engineering, supply, supervision and commissioning of the electrical equipment and the Alspa 8000-P320 control system. The equipment was delivered in May 1998.

The plant will be fitted with two reversible units rated 350MW each and will be connected to the 345kV bus of the Jinyand substation through two transmission lines. A static frequency converter supplied by Cegelec will be used to start the units in pumping mode. It will also be possible to start unit one as a pump with the back-to-back method, using unit two as a launcher.

The Alspa 8000-P320 system was chosen to integrate sophisticated processes in the pumped storage plant. These include the automatic control of units, dam gates, switchyards and static frequency converters. To allow the exchange and transmission of information between the different areas of the plant such as the upper reservoir, powerhouse, lower reservoir, high voltage switchyard and the control building, two networks are supplied:

•An Ethernet network inside the control room.

•An S8000 network in other areas of the plant with an optical fibre interconnection between the switchyard and the powerhouse.

There are five unit operational modes, two launching modes, six stable status modes and three intermediate stable status modes at Sanchong (see table right). The Alspa 8000-P320 system is able to implement these automatically, switching between the various different modes if necessary. Automatic site control is carried out in the control room through Centralog. The operational modes of the units can be selected through man-machine dialogue at the Centralog video stations. In addition, different control modes are available from the Centralog for active and reactive power management according to the operational modes of the units.

For active power management, the units may be controlled individually or jointly (from a global power setpoint). For reactive power management, the possible control modes are:

•An individual voltage setpoint or an individual power factor setpoint entered by the Centralog operator if the unit is in a generating mode.

•An individual voltage setpoint or an individual reactive power setpoint entered by the Centralog operator if the unit is in condenser (generator or pump) operational mode.

The reactive power management control modes apply to each unit separately. Transitions between the different control modes may be initiated by the operator through man-machine dialogue and are automatically performed by the Centralog. Hydraulic limits at Sanchong are controlled by the software system, which essentially acts as a safety control. For example the units are shut down if threshold levels in the reservoirs are reached. When in a generating mode, a gradient of load increase and load decrease is provided (predefined in the database) to avoid serious oscillations in the surge tank.

Other calculations which are undertaken by the Alspa system include an assessment of the plant’s capabilities, where the following items are calculated: minimum available active power, maximum available active power, active power of the coupled units and the maximum active power of the coupled units. Special calculations which are logged by the system include performance calculations, which are carried out for pumping operation, generating operation or a combination of the two operations. Operation of the dam gates is also recorded. The setpoints are transmitted to the dam controller station which sends open/close orders accordingly. Gate number and gate opening values are automatically printed out on the event log in the control room Centralog.

The economic management of energy can also be achieved at Sanchong through the joint control of the power units using the remote Electrical Management System. This optimises the energy production of the power plant by taking into account the hydraulic resources, integrated diagnostic functions and the redundancy of main components, which reduce unit stoppages.