The Tanesco-owned 200MW Kidatu hydro power station in Tanzania was built in two stages. Two units were commissioned in 1974 and two others followed in 1978. Over the years, the control system has undergone various rehabilitation programmes and extensions, resulting in a conglomerate of live systems that made the servicing and maintenance of the plant extremely difficult. voith-siemens Hydro of Norway recently completed the total replacement of the automation, communication and protection systems.
The Kidatu rehabilitation project was funded by Norwegian and Swedish development aid, and the contract with Voith Siemens Hydro for the electrical works was signed in 1998. The condition of the previous automation and protection systems was poor, and the communication systems to the dam and to the switchyard broke down years ago. Updating of the plant documentation was never completed as parts of the plant were replaced or extended.
Automation system
All parts of the old control system have been replaced by a new automation system, based on Simatic programmable controllers and a PC-based man-machine interface (SCADA). The control system is divided into three main parts, which work independently. Together they form the concept for the Voith Siemens Hydro power control system.
The basic package for the SCADA system is the IGSS from 7-Technologies, extended by Voith Siemens Hydro with real-time functions to fulfill the requirements for use in power control systems. The SCADA system software is run on standard PCs with Microsoft Windows.
Common overall automation functions and system management are taken care of by the station controller (STC), materialised by a Simatic Controller.
The generating units, auxiliary systems, switchyard and dam all have autonomous object controllers (OBC), on which all the unit specific software runs. Associated backup systems enable the generating units to also be operated without the SCADA, STC and OBCs in service.
All communication between the different parts of the control system is carried out by standard and open international data communication protocols. The specific configuration of the data communication ensures the necessary level of reliability, high resolution and exact time setting of events.
During the early stages of the project, it was questioned if it was advisable to implement a computer-based automation system in a country where, at that time, computers were not very well known. During the last few years, however, Tanzania has undergone rapid development so that PCs and the internet are now parts of daily life in certain areas of the country.
As an early part of the contract, the power plant operators were given basic and advanced computer and system training by Voith Siemens Hydro and, during the implementation period on site, the staff got a good grasp of the new technology. Advanced training and proven technology together made it possible to operate the power plant without any disturbances that cannot be rectified directly on site.
Water household system
Part of the automation system is the advanced water household system (WHS). The RunAid system from Norconsult together with special SCADA functions are used for these tasks. The WHS is an online special software system, implemented on standard automation hardware and consists of:
• Online joint control (optimal unit load distribution)
• Online optimal operation (full scale power generation optimisation including automatic start and stop for units)
• Power generation planning (including consideration of start/stop costs)
• Online calculation of water flow / consumption
• Comparison of different operation scenarios (advisory)
• Automatic grid voltage regulation
• Spinning reserve setting and monitoring
• Offline power generation simulator
Based on the available knowledge of the present water volume in the basin, the characteristics of the waterways, turbines and generators, the WHS has resulted in a significant increase in the average energy output of the power plant, and the extra investment has already been recouped.
Protection system
The status of the original electrical protection system was rather poor. Because of out-dated technology, lack of spare parts and unreliable trip signal communication, the generators had until recently suffered a rather ‘unprotected’ existence.
For the new protection system, the SIPROTEC family from Siemens was chosen. Each unit has been equipped with a full range duplicated electrical / mechanical protection system. The two systems work completely independently (mutual reserve) and are based on state-of-the-art and self-monitoring standard equipment. The reliability of the trip signal communication to the switchyard circuit breakers has been improved significantly by two separate and self-monitored circuits, established for this purpose.
Before commissioning of the plant, Voith Siemens Hydro prepared a comprehensive relay coordination study for the whole scheme on which all relay settings were based. As the Kidatu power plant represents a heavy power load into a relatively weak grid, all protection relay settings had to be accurate. The experience has also proven that with Kidatu’s new protection scheme, grid stability in Tanzania has improved noticeably.
Communication system
Due to technology failures, poor maintenance, landslides and bush fires, all communication between the power house, dam and switchyard had recently been out of action. The power house-switchyard communication was easily restored by fibre cable.
Several methods of communication to the dam site 10km upstream were evaluated. Satellite communication remains underdeveloped in the area and the operational costs could not have been justified. The possibility of a radio link was investigated by Norwegian experts, but the geographical conditions and poor security at possible antenna tower sites made this option too risky.
As one of the 230kV power transmission lines from the switchyard happened to pass close by the dam site, and one of the phases was unoccupied, a power line carrier (PLC) system was identified as the best solution. The necessary work on the transmission line was planned in detail and completed during a very short shut down period.
Furthermore, a modern remote control communication to Tanesco’s load dispatch centre has been implemented. The required protocols and modems were integrated into the new automation system, according to Voith Siemens Hydro standards.
Documentation
The last part of the rehabilitation project was the improvement of the plant documentation system. Voith Siemens Hydro performed a thorough survey and fact-finding study of the plant. All the knowledge gained by this study has been used in the new plant documentation. Operation and maintenance staff can now get a clear overview of all details in minimal time from a single comprehensive system and perform necessary service or fault rectifying.
Conclusion
Reliability of power supply is very important for developing countries through both new generation and also the rehabilitation of existing plants. Through the Kidatu rehabilitation project, Scandinavian developing aid organisations (Sida and NORAD), together with Voith Siemens Hydro of Norway, have shown the importance of cooperation between funds and technology from industrialised countries to improve the power generation reliability and general progress in developing nations.
