Successful Control Project at Bulgarian Nuclear Plant
"Operation of the steam generators within the prescribed limits is important for plant operation, so the triple modular redundancy of Tricon v9 and the 1E certified basic software and hardware very much suited the importance of this mission critical process control," explained Vladimir Urutchev, Chief Engineer at Kozloduy.
Invensys Nuclear has successfully completed a major plant improvement program at the Kozloduy nuclear power station in Bulgaria.
The plant operates a Russian-designed WER440 pressurized water reactor with six primary coolant loops providing multiple paths for cooling the reactor. This design allows plant personnel to isolate one or more of these loops for maintenance without the need for a complete plant shutdown.
However, while providing a degree of fault tolerance, such flexibility demands a complex control regime. Feedwater control in particular is critical. If the water level, or inventory, decreases significantly, reactor cooling is jeopardised. If the level increases too much, water passes through the steam header causing damage to the reactor's turbines.
In other words, there is no acceptable failure mode. Any control solution has to offer high and proven levels of reliability, availability and fault tolerance.
Although Triconex's Tricons are used in thousands of critical applications worldwide, this is the first time the technology has been involved with nuclear feedwater control.
Control at Kozloduy now involves a classic three-element system configured for feedforward-feedback performance to control feedwater flow. Dual and triple redundant sensors and transmitters are installed to improve reliability, matching fault tolerance in the field components.
"Operation of the steam generators within the prescribed limits is important for plant operation, so the triple modular redundancy of Tricon 9 and the 1E certified basic software and hardware very much suited the importance of this mission critical process control," added Urutchev.
The new system dynamically balances steam generator inventory against steam flow to the main turbine, feedwater flow and level. Tricon pressure and temperature control compensates for all three elements and uses an adaptive gain control algorithm to maintain water level during steady state and transient operation.
This control algorithm models the static and dynamic components of the process. During low power operations such as startup and shutdown, when flow measurement is inaccurate and unreliable, the program uses diverse measurements to estimate flow. Three element control is used over the full range of power operation, from 0% to 100% and back.
In addition, the development of a plant model and simulation computer during the course of the project allowed installation and start-up of the steam generator water level control system without the need for post start-up tuning.
The project also includes Wonderware InTouch-based operator interfaces. To review plant status, an operator simply uses the touch sensitive screen to select and view grouped parameters. Meanwhile, multiple security levels prevent unauthorised access to program code and/or stored data.
Triconex's contract, which was won with the help of DICS Intertrade Limited, also included all equipment such as transmitters, valves and wiring, together with programming, simulation testing and project implementation services.
"One of the means of achieving this involved developing and installing protection systems for cold overpressurisation of the reactor vessel," explained Vladimir Urutchev, Chief Engineer at Kozloduy. "We eventually selected the Tricon 9 platform from Triconex because it has a very good architecture and met our price requirements," he added.
So far the system has operated flawlessly and Triconex is confident that it will win future business at the Kozloduy plant.
"We are now considering using Tricon systems in other safety-related aspects such as reactor control, reactor protection systems, turbo generation control and total unit control," confirmed Urutchev.