State-of-the-art energy management system to improve Venezuela`s hydro-electric plant operations

Venezuela`s concern for protecting the environment, and the country`s need for more efficient power generation, were major factors in EDELCA installing a state-of-the art energy management system.

Ing Francisco Castro

EDELCA

Walter Wissmeier and Ing Herbert Goldmann

Siemens AG

The plans of the Venezuelan electrical utility CVG Electrification del Caroni CA (EDELCA) to develop the considerable hydro-electric potential of the Caroni River has been taking shape for a number of years. This ambitious long-term project calls for the building of three dams down stream from the Guri Dam, which was completed in 1986. The three dams, Macagua, Caruachi and Tocoma, are being built in several stages. The first stage of the Macagua scheme, Macagua I, was completed during 1961 and the second stage, Macagua II, will be completed towards the end of 1994. The second stage, which will include a new control center, will incorporate Siemens` energy management system (EMS).

The increasing need for electric power by the growing industrial region around the town of Ciudad Guayana, coupled with the requirements of better energy management and growing environmental concern, caused EDELCA to consider the advantages offered by a state-of-the-art computer-based energy management system. Subsequent to careful economic and technical appraisal, the decision was made during 1992 to build a control center for both stages of the Macagua project and install a modern instrumentation and control system.

One of the far reaching aspects is the system`s open distributed architecture, a system which brings not only a degree of operational flexibility not previously attainable, but also increased security and the capacity for the system to be readily updated.

The plant (Figure 1) is approximately 10 kilometers upstream from where the Caroni River flows into the Orinoco. Macagua I has an installed capacity of 420 MW. Macagua II will have a capacity of 3,208 MW derived from 12 vertical-shaft Francis turbines coupled to 250-MW generators and two Kaplan turbines coupled to 104-MW generators. The plant will be fed from a 50 square kilometer reservoir with a nominal head of 54.5 meters and a maximum capacity of 363,000,000 cubic meters, making it one of the largest hydro-electric schemes in Venezuela.

Progress in EMS

Since Macagua I was commissioned there have been considerable advances in EMS technology. At the end of the sixties the first supervisory control and data acquisition (SCADA) systems were introduced to provide monitoring and remote control of sub-stations, load monitoring and data logging. Subsequent systems were characterized by more powerful data processing brought about by the use of real-time mini-computers.

Close on the heels of these came power systems application programs configured to meet the requirements of control room staff. This software improved general energy management involving such key requirements as security of supply, efficiency of operation and management analysis. The adoption of standard systems with distributed architecture and user-friendly features, such as graphical user interfaces, has added a further dimension.

Integrating these systems with SCADA and energy management concepts has led to what are called EMSs. It is this synergy that forms the basis of the Macagua instrumentation and control system. These types of systems cover a range of applications from relatively small distributed networks for industrial complexes to large-scale applications such as the Macagua scheme.

System overview

The new center will control Macagua I and II hydro-electric plants. It will include a master station and a first line data acquisition system both supplied by Siemens. The plant control can be grouped with the master station and the computer system, which essentially interrogates the 28 remote terminal units (RTUs). The RTUs are located at the generators and other key positions around the Macagua facility.

The essential SCADA/EMS facilities afforded by the master station include generation control and load scheduling, spillway gate control, dispatch simulation for training, an expert system for disturbance analysis and computer-managed plant maintenance. In common with the latest energy management thinking, the system supplied at Macagua is based around distributed hardware using fully graphical man-to-machine interfaces comprising UNIX-based work stations and servers with reduced instruction set computing (RISC) architectures making use of high-speed parallel processing.

Each server will have a particular function: SCADA, power system management, load dispatch and simulation training (Figure 2). A standard feature of the work stations is the use of accelerated graphics to provide features such as smooth zooming and panning, decluttering and windowing for easier multi-tasking.

As in any computer-controlled system, the heart of the master station is data acquisition (DAS). This vital sub-system has a great deal of built-in redundancy to ensure both high reliability and high availability. Other functions provides the master station/telecontrol interfaces for communicating with the plant via the RTUs. This sub-system is also used for passing on pre-processed power data from machinery (protocol emulation, old/new comparison and dimensional arithmetic conversion). It also acts as a gateway terminal server for communicating with other control centers.

An important part of the master station is the flexible redundancy concept that allows any critical function to be assigned priority status by switching to servers carrying out time-critical duties and relegating less urgent task to non-critical servers.

All the sub-systems communicate over a standardized Ethernet local area network (LAN) working up to the transport layer of the ISO-7 model. This data bus, termed the process LAN, is used for open communication with administration departments. The separation of the two LANs in this way eliminates any chance of spurious communication with external sources.

Remote terminals

The 28 remote terminal units are used for the telecontrol of generators, switch yards and water spillway gates. Communications between the master station and the various RTUs is over fiber-optic cable. The duties of the RTUs are:

– Data collection and processing

– Supervisory control such as connect/disconnect, raise/lower

– Communication with master station and PC

– Generation start-up and shut-down

– Loop control of automatic generation control facility

– Loop control of automatic voltage control facility

– Water spillway gate control

All of the RTUs are based on 32-bit microprocessors and are in effect the input/output interfaces for the system. The RTUs can be controlled either from the control center or locally by an operator, via portable personal computer (PC). For this reason all RTUs can execute data acquisition and control functions independently of the master station. The master station software resides jointly in the redundant data acquisition and communications servers. These redundant servers detect status changes in order to update the system database and to filter data and command outputs.

To increase operational flexibility, the personal computer acts in place of the master station computer for local data acquisition and control.

The microprocessors in the RTUs are continuously scanning the analog and digital inputs and transmitting the resultant data to the master station`s computer in the control center whenever a change of state occurs. An RTU will also accept control commands from the master station`s computer as well as local control commands, automatic stop/start sequencing and open/close control. For security reasons, an uninterruptible power supply backs up the main supply to both the master station in the control center and the RTUs.

Basic operations

The procedures for starting and stopping generation are conducted through one of 14 RTUs. The start-up/shut-down sequence program controls the respective turbine-generator set through a series of steady-state transitions, starting from the set in a stationary position and finishing at no-load speed. Automatic generation control works in conjunction with control loop software within the generator RTUs. This control function will be performed from either the existing Despacho control center or from the new Macagua center. However, normal operation will be under Despacho control where power generation requirements will be received every few seconds.

Each of the generator RTUs will be provided with firmware to facilitate unit (MW) power control according to setpoint levels sent from the master station in the control center. Here closed-loop proportional/integral control regulates each megawatt of output. In an independent mode of operation the power set-point value can be set via the portable PC. The same method may be used for setting automatic voltage control.

The spillway gate control is set to satisfy optimum flow requirements by coordinating the timing and volume flow of the 12 spillway gates. Here the operating setpoints will be transmitted from the master station to the redundant spillway RTU. When both RTUs are in operation, the command signals will be sent to both RTUs from the master station to ensure identical operation. Every minute gateway positions will be interrogated and the positions transmitted to the master station. The actual operation of the gates will be effected from the control systems.

Software

With the Macagua-II project the total system software comprises some one million lines of high-level language code. It is generally organized at three levels: data, programs and inter-task communication. The data level is realized by a real-time relational database and the necessary access routines. For rapid access and fast data updating, the database contents is distributed among the various servers and work stations.

Responsibility for supporting the work station displays is shared by the respective servers, which hold the stored data. The real-time topology-oriented power plant systems data is held in the communication servers. The related archives and working schedules are located in the administration/historical and future data (ADM/HFD) server.

The static display data and the continuously refreshed graphical displays are in the seven man-to-machine interface workstations. To facilitate administration of the system, the complete master database and its system parameters are located in the ADM/HFD server.

The program layer consists of some 200 programs distributed among the hardware components of the system. All of the programs, whose structures are identical, include unified database access calls and softbus commands for inter-task communications.

The inter-task communications layer, realized by a proprietary program called “Softbus,” is an I/O-interface that ensures efficient communication between all programs. These programs can be on the same or different servers or workstations. The Softbus program is the de facto equivalent of the three upper layers of the ISO/OSI 4.4 according to the seven-layer model.

Considerable effort was put into developing a powerful but user friendly data management system. Here the computer database has been designed to be easily modified by operational staff at any time to accommodate changes in the power system configuration without disturbing the functioning of the main system. However, there is no need for the plant operators to be computer specialists.

The functionality of the Macagua-II EMS comprises the full range of functions including data processing, supervisory control, operational report recording, historical data recording plus on-line and off-line applications software and a operator training simulator.

On-line application functions are:

– Network status analysis

– System alarm analysis

– Automatic generation control

– Automatic voltage control

– Spillway gate control

– Work permits

Off-line application software includes plant maintenance that provides inter-related routines for maintenance scheduling, work permit servicing, maintenance and repair reviews, equipment records, component failure histories, spare part inventories and maintenance statistics. These functions will be provided by means of the relational database management system. An important feature of this facility is that it can be used interactively at the maintenance workstation in the control center and for data consultation from administration office workstations.

In configuring the Macagua EMS system, Siemens put particular emphasis on reliability, availability and economy. In addition, the Macagua system is sufficiently flexible to allow it to be readily adapted and up-graded to meet most future requirements.

Reliability and availability are largely the result of redundant system configurations built around fault-tolerant sub-systems and resilient computer software. Servicing and maintenance schedules were designed with maximum economy in mind without reducing operational safety standards.

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Figure 1. Aerial view of the Macagua hydro-electric power project in Venezuela

AUTHORS

Ing Francisco Castro is manager of electric and mechanical projects for C.V.G. Edelca, Venezuela. He has a degree in electrical engineering from Carabobo University in Venezuela and a diploma in engineering from Aston University in England.

Walter Wissmeier is a sales manager with Siemens AG in Germany and was awarded a technical degree by Siemens in 1964.

Ing Herbert Goldmann has a diploma in engineering and is employed as a project engineer/manager with Siemens AG in Germany.