A - Background and objectives
The European Transmission Network (ETN) has been evolving significantly in the ways that network operators work and cooperate with each other, in particularly by setting up common operating procedures and tools such as network simulation tools . In the domain of power system state estimation and transmission network time simulation, very few innovations have emerged over the past decade (2000-2010) in Europe. Breakthroughs are therefore expected in power system simulation along four directions to serve the ETN: state estimation, optimization algorithms, time domain simulation and power system component modelling.
The PEGASE project aimed to remove several algorithmic barriers related to the monitoring, simulation and optimization of very large power systems, therefore paving the way for improved close to real-time operations and system security .
B - Description of the result
A power system is a complex dynamic process displaying a series of possibly unstable phenomena encompassing, for instance, the loss of synchronism of generators, unstable growing power oscillations, grid stability issues (voltage stability, frequency stability), cascade trippings. Dynamic simulation of the power system requires the use of an extended electrochemical model (EEM), which includes a detailed representation of the generating units and their controllers. The mathematical features of the EEM are very specific and difficult to handle: it is large (typically 5 times the static model), non-linear, stiff (mixing fast and slow variables), oscillating, poorly damped and full of discontinuities. Numerically solving the EEM requires a very robust integration algorithm: implicit, simultaneous, A-stable and using a variable step size. The development targets of time domain simulation software prototype are ambitious, viz. simulating the whole pan European Transmission System (the size of the EEM reaches about 125.000 state variables), and an off-line simulation with no compromise on accuracy in less than 15 minutes.
A full accuracy time-domain simulation prototype has been developed. It includes a new fine grain parallelization, the best up-to-date linear algebra and the new step-size control. Generic models for wind turbines, wind farms, and Voltage Source Converter (VSC) HVDC have been developed. The models can be adapted to some FACTS (Flexible AC Transmission System) devices as well. The models are simplified to the extent that they are capable of reproducing only the phenomena affecting the power system stability. They may not necessarily represent specific control strategies or rely on parameters of a particular wind turbine. As a result the same model is capable of simulating wind turbines of different manufacturers or even different turbine concepts, or entire wind farms consisting of any realistic number of wind turbines. Tests have proved that the full accuracy prototype is able to achieve an unprecedented ratio accuracy/performance for large systems, on local events simulation, but more importantly on full-scale scenarios involving large parts of the system. The scenarios include network splitting with cascade tripping of overloaded lines, inter area oscillations, or whole areas losing synchronism. Each of these tests was performed in less than 10 minutes, viz. a voltage collapse scenario activating more than 10 LTC and 2 generator field current limiter (4.4 minutes), a simple generator set point change (0.3 minute), changing the AC grid impedance in parallel with an embedded VSC HVDC (1.4 minute), an extreme case of voltage collapse with system splitting (9.8 minutes)
According to the PEGASE Consortium Agreement, the Accurate Time-Domain simulation prototype of Transmission Systems is owned jointly by RTE and TRACTEBEL, which will share jointly the prototype commercialization.