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 network operation.
B - Description of the result
Dynamic security assessment (DSA) of a power system consists in simulating this system when facing a series of incidents like short-circuit, line switching or generator unit tripping. DSA is today most of the time run off-line by TSOs during the reliability assessment process (operational planning at transmission level). It is used to understand the dynamic behavior of the system, to tune the parameters of the controllers, to check the protection settings or to assess the stability of an operating point for the day-ahead operations. In case of well-known typical behaviors for the network of interest, some simplifications of the extended extended electrochemical model (EEM) can apply, for instance to measure the distance to instability resulting from a known phenomenon (such as voltage collapse). The resulting reduction in calculation times has allowed implementing on-line, phenomenon-oriented DSA.
Yet, the evolutions of the Pan European Transmission Network require reconsidering classical approaches for DSA. Indeed, implementing to-day the so-called N-1 criterion security rule consists in checking the existence of an acceptable steady state point after the tripping of each line, one by one, without considering the resulting system dynamics. This is no longer satisfactory since, when operating the system close to its stability limits, the trajectory to the new steady state equilibrium point could be unstable. But this kind of application requires performing simulations very fast. 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.
Simplified system simulations are usually performed by replacing a detailed model by a simplified one. The drawback of this approach is that two sets of models must be maintained. The proposed innovative approach consists in using only one model for both simulations and to introduce the simplification in the numerical scheme. Tests have proved that this algorithm (viz. the simplified numerical scheme) is able to achieve unprecedented performances for dynamic simulations on large systems while addressing slow dynamic phenomena: it thus makes online dynamic security analysis at the Pan European level possible. The following figures were obtained during tests performed on the “UCTE+TEIAS” system: trip a line below 400 kV (26 seconds) and tripping of two parallel 400 kV lines (44 seconds).
Both contingencies lead to significant voltage deviations over a large zone with the triggering of many automata. A whole online dynamic security assessment at the Pan European scale would require the consideration of 2000 N-1 contingencies in less than 5 minutes. With an average of 26 seconds per N-1 contingency, this target can be reached through the simultaneous use of 192 cores. This could be performed using less than the computation power available in a single modern blade enclosure.
According to the PEGASE Consortium Agreement, the Simplified Simulation Prototype for Dynamic Security Assessment is owned jointly by RTE (75 %) and TRACTEBEL (25%), which will share the prototype commercialization. In case TRACTEBEL and RTE needs external support beyond the end of the development project regarding future development of the simplified simulation prototype, they will request a proposal from ULg to ensure this external support.