SCADA systems ensure availability in third-world transmission networks
The UEB installed a SCC for the republic`s entire transmission and distribution network, resulting in less than one hour of unavailability per year
Dipl. Ing. Klaus Schultz-Fademrecht
In the late 1980s, Uganda annually produced about 655 million kWh of electricity. Almost all power was generated in the Owen Falls hydroelectric power plant on the Victoria Nile near Jinja, the second-largest town in Uganda. In this time, the Uganda Electricity Board (UEB) installed a new supervisory control center (SCC) for the centralized operation of the interconnected transmission network (lines and substations) of the entire republic. The UEB erected the SCC in Lugogo at the outskirts of Kampala.
Under the republic`s “Rehabilitation and Upgrading of the Uganda Electricity System” program, the new SCC comprised all necessary equipment, software and services for the remote stations, including the complete system for voice and data communication.
For the realization of this highly sophisticated program, the UEB commissioned Lahmeyer International (Frankfurt, Germany) for the design, tender evaluation, system supervision, construction and factory, as well as site acceptance tests of the new SCC. The manufacturer of the SCADA systems was ABB Network Control, Sweden, with the aim to control and supervise the nationwide 132-kV grid, using 14 remote terminal units (RTU), and the 132/33/11 kV distribution system from the capital Kampala.
The UEB supplies electrical energy to nearly 20 million inhabitants of Uganda, as well as to the industrial and business enterprises. In addition, part of the electricity generated by the Owen Falls hydroelectric power plant is exported to the neighboring countries of Kenya and Tanzania.
The SCADA system
Computerized supervisory control and data acquisition (SCADA) systems have now been in operation worldwide for 25 to 30 years. During recent years, a trend has been established to develop a standardized system for SCADA systems` functions so that it can be adapted to the diverse operations of various utilities and authorities. The general hardware and software concepts for SCADA systems are to provide a flexible set of functions and equipment. The actual use of a new system is specified by the parameters defined in the SCC database.
The new SCC in Lugogo is installed with the standardized workstation-based, on-line and hot-standby computer system (Figure 1), connected via local-area network. The visualization of the process to control and supervise the power network happens with two operator consoles equipped with two fully graphic color video display units (VDU). These control consoles have SCADA software for signal alarm processing, historical data storage, commands, monitoring and metering, as well as peripheral systems and front-end computers. The requirements for a modern and secure communications system for protection signaling, speech, and data transmission with double routing and automatic switch-over are fulfilled via optical fiber systems, power-line carriers, radio systems, and microwave and telephone systems.
Data acquisition and processing
The basic information with regard to the power system is collected by ABB RTU 400s, installed in the various substations and the power plant. Process variables collected by the RTU400s include analog input, single digital input, digital values, double digital inputs, command outputs and SCADA alarms. The communication protocol used between the front-end computer and RTUs is RP570. The front-end computers poll the RTUs with this protocol, and all device control commands are initiated from the operator consoles. The data processing comprises:
– measured value processing,
– calculated value processing,
– indication processing,
– calculated indication processing,
– energy counter processing,
– event processing and
– alarm processing.
The time-tagged data function is also in use, collecting, storing and accessing time-related data. The main features of these functions are: future planning data; plan value period extension; validity marking; and initiate user calculation. In case of disturbance in system data collection, the time-tagged data function creates a list called sequence of events (SOE). The data acquired from the different RTUs are merged chronologically in the SOE list. The post-mortem review function is also part of the system.
Figure 1 briefly outlines the hardware configuration. The computer system is a redundant VAX server configuration (Digital Equipment Corp.). Each of the two Micro VAX 3100 model 10e servers is equipped with 20 MB primary memory, an RZ56 Winchester disk (665 MB), a TZ50 cartridge magnetic tape unit (95 MB) and a VT420 console terminal. The printers for the event and report lists are managed by a DEC-Server 200. Two stall alarm units are mounted in VDU display generator cubicles. The man-machine subsystems contain two workstations in the SCC in Lugogo and one in Amber House, the main administration office in Kampala. ABB developed the three workstations (WS200). The man-machine subsystems also comprise the following:
– three display generators,
– three keyboard controllers,
– three printers,
– one hard copy unit,
– five display monitors,
– six various keyboards,
– three trackballs and
– different modems.
Information about the process is presented in a number of pictures defined by ABB with UEB. Table 1 shows the scope of pictures in the system at delivery.
One of the most important aspects for computer systems installed for essential functions in the third world is the availability of the system. Availability calculations are based on the assumption of constant component failure rate with respect to time and preventive maintenance. The total system availability is the combination of all the subsystems` availability, calculated individually after structuring the SCADA system into subsystems. A parallel redundant structure increases the availability by having redundant components so that the system continues to function even though some components have failed. The environmental conditions also affect system availability. For example, a temperature rise of 10 C to 15 C doubles the failure risk in systems without redundancy.
Figure 2 shows the structure of the UEB SCADA system. The system is operational if two of the following three systems are available: man-machine subsystem, main computer system or front-end system.
In calculating the system availability, assume
l (total system) = 7.048e10-6,
where l is the constant failure rate. Values expressed here are in failures per hour. Then, if the mean time between failure = 1/l = 141,884 h, then the total system availability is
A = 141884 / (141884+12) = 0.9999.
This means the unavailability of the system is less than one hour per year. This result is achieved by utilizing the highly reliable components in the system. END
Dipl. Ing. Klaus Schultz-Fademrecht has done the site acceptance tests for the SCADA and communication part of the Uganda Power Network Rehabilitation Project. He has a degree in control and automation engineering from Darmstadt Technical University in Germany and has been working in the SCADA and communication field since 1991.