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
Power system state estimation is the core activity of on-line security analysis. It acts as a filter between the raw measurements received from the power system and all the application functions which require the most reliable data base to estimate the current state of the system. In almost all state estimation implementation, measurements obtained by a Supervisory Control And Data Acquisition (SCADA) system at approximately the same time over the whole supervised network are centrally processed periodically by a static state estimator, or at the operator request. State estimation is based on the mathematical relations between system state variables (i.e. bus voltage magnitudes and phase angles) and the measurements used to compute the real-time system state (e.g. real and reactive power measurements and bus voltage measurements).
The use of Phasor Measurement Units (PMUs) appears therefore to be very promising for improving the robustness and accuracy of the state estimation process for any large power system. An overall phasor measurements network architecture has been defined. Taking into account that it has to comply with the other Two-Step State Estimation algorithm developed within the PEGASE project, a hierarchical structure has been adopted with Phasor Data Concentrators both at the TSO and Pan European Transmission System levels. A set of minimum requirements specific to the State Estimation application has been defined and a PDC/TSO has been developed according to these requirements.
Tests on real data illustrate the improvements brought to state estimation performance by the introduction of phasor measurements. The reported tests have been performed on the 400/220 kV Spanish system including PMU at ten 400 kV substations. The influence of the weights associated to the phasor measurements has also been analyzed. The following main conclusions can be drawn:
The TSO state estimation prototype including phasor measurements was proved to perform properly on real data coming on one hand from SCADA snapshots for conventional measurements and on the other hand from the Phasor Data Concentrator output streams for phaser;
As expected, the introduction of phasor measurements leads to higher estimates accuracy, more particularly in the direct neighbourhood of the substations provided with a PMU;
Increasing the weights associated to phasor measurements corresponds to a slight global enhancement of accuracy but at the price of more suspected measurements. The choice of too high weights is thus not recommended. A compromise has to be reached between accuracy and number of false alarms in bad data detection;
The reported tests considered a rather limited number of phasor measurements. It is expected that increasing the proportion of phasor measurements would confirm the above conclusions and provide even higher benefits.
According to the PEGASE Consortium Agreement, the prototype software is owned 100% by Nucleo. The IPR holder (Nucleo) will include the Phasor Data Concentrator within its product’s portfolio, leading the Phasor Data Concentrator commercialization, and providing technical support and developing improvements to the existing prototypes. The Phasor Data Concentrator provides features that are not available in the PDC currently in the market, viz. new functions linked to the needs identified during the PEGASE project. The novelty introduced for calculating the status of the network, relies on the way that the measured values (PMU) are gathered and processed (Phasor Data Concentrator). The prototype has enabled REE to assess its accuracy and the robustness of results compared to conventional state estimators. Thus REE intends in the coming years to integrate large amounts of PMUs within the State Estimator deployed within the energy management system (EMS).