An electric power system is considered to be stable when the system returns to a stable balanced state after a disturbance.
The extensive build-up of renewable energies (especially wind and solar energy) can cause transmission grids to exceed their limits, control zones be subject to higher utilization rate, and the grid be forced to transport the power over long-distance lines.
This is especially the case when there are certain areas in an state or country which are subject to higher solar radiations or higher amount of wind energy in a specific geographical area.
The outcome of such circumstances would be power transmission networks operate closer to their operational and stability limits.
The extensive build-up of renewable energies (especially wind and solar energy) can cause transmission grids to exceed their limits, control zones be subject to higher utilization rate, and the grid be forced to transport the power over long-distance lines.
At the same time, the rise of inverter-based renewable energies, which are replacing large power plants, leads to reduction of the rotating mass (instant reserve). This fundamentally changes the dynamic behavior of the power system.
From grid management point of view, changes undergoing in the generation mix, has changed the dynamic behavior of the system, and the increasing volatility results in higher complexity and a variety of operational situations which have to be carefully observed and controlled, in a predictive and proactive fashion.
Voltage and Rotor Angle Stability
A power system is considered unstable from voltage point of view when the physical transmission capacity of the networks has exceeded due to large amount of power transports. The electric system is then no longer able to keep the voltage within the acceptable limits. This can lead to voltage collapse which makes the system unstable.
The rotor angle stabilitydefines the ability of the synchronous machines in the network to remain stable and synchronous after a large fault such as a three-phase short circuit happens.
The instability triggered by a sudden imbalance of the angular momentum happens within a few seconds; leading to the situation of transient stability problem.
What does this mean for network management?
Normally, the dynamic network stability is assessed by offline simulation tools, based on worst case scenarios, which were performed once or twice a year as part of planning studies. Subsequently the stability limits including “security buffers“ for the later use in grid management are derived.
The previously determined stability limits can lead to inefficient network operations since the worst case consideration is too pessimistic for most of the network situations. On the other hand, increasing the thermal limits (for example by dynamic overhead power line limit rating or use of high temperature conductors) may mean that the stability limits of the network operations become relevant by limiting the power to be transmitted.
The previously determined stability limits can lead to inefficient network operations since the worst case consideration is too pessimistic for most of the network situations.
For the rotor stability analysis, in medium term, steady state network security calculations are in specific circumstances insufficient to provide reliable analysis of the system security in critical situations.
This means that new tools for assessing network stability as part of operation planning (day-ahead and intra-day) are needed, to improve situational awareness and provide support for determining and evaluating counter measures in critical situations.
How does the dynamic stability analysis work?
The setup of the data model for the dynamic stability analysis is based on extending the already existing data model for the steady state data analysis, by additional dynamic data such as controller data, detailed models of the generators, high voltage DC transmission, FACTS elements, etc.
The analysis of the transient stability behavior primarily uses the trajectory of the rotor and voltage angles over time under various defined fault scenarios.
The critical fault clearing time is a key variable for quantifying the transient rotor angle stability. The fault clearing time shall define the maximum permitted duration of a three-phase short circuit for which the system remains stable.
The fault clearing time shall define the maximum permitted duration of a three-phase short circuit for which the system remains stable.
The visualization concepts
The purpose of the visualization should be to enable fast detection and assessment of the stability state and to create situational awareness by holistic, clear and concise overviews.
In order to fulfill this requirement, a hierarchical visualization concept with multiple presentation layer seems plausible.
On the highest presentation level, the global stability states are displayed in a so-called stability monitor. The global stability state aggregates the results across all fault scenarios for a time slot.
The purpose of the visualization should be to enable fast detection and assessment of the stability state and to create situational awareness by holistic, clear and concise overviews.
If critical state exist, additional information must be provided to the network operator. This shall be done through a second presentation level. Here the stability state related to individual fault scenarios including all relevant information shall be displayed.
The third level enables detailed analysis of the simulated events, on the basis of the relevant elements or their variables such as the change over time of the rotor angles or generator voltages in form of curves or tables with the respective values.
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