Planning principles for extended stability considerations in the European power grid

Project state

With the amendment to the German law for prioritising renewable energies (Renewable Energy Sources Act - EEG) from 2012, the expansion of renewable energy conversion units has been further promoted. The temporal and spatial inhomogeneity of the feed-in from renewables is increasing demand for power transport capacities and, consequently, grid capacities. In order to identify necessary grid expansion measures, stationary power flow analyses are being mainly used that focus on the power transport taking in account steady-state limits for current and voltage.

P-V curve for the (N-0), (N-1) and (N-2) events for a static P load characteristic
© RWTH Aachen University

The consideration of stability aspects is based here on existing planning principles concerning (n-1) and selected (n-2) investigations in "critical" grid usage events, whose identification is based on long-term operating experience. However, the suitability of such an approach needs to be questioned in view of the ongoing restructuring of the energy supply systems. The substitution of conventional power generators with converter-based, decentralised generation units as part of the restructuring, coupled with the increase in long-range power flows, suggests that stability aspects such as the voltage stability will take on greater significance in future both in the network planning and network operational management. The revision of existing planning principles is therefore of central importance if existing planning approaches are to prevent hardly foreseeable infrastructure misallocations at an early stage and thus not jeopardize the rapid progression of the energy transition.

Reactive power supplies for stable grids

Schematic representation of the stability limits for the (N-0), (N-1) and (N-2) events with a given operating point (OP) and the safety margins
© RWTH Aachen University

A key problem is the reactive power capacity of the networks. The increasing utilisation of existing transmission lines as well as large-scale power transitions are increasingly resulting in a high-current operation that requires an increasing dynamic reactive power supply. At the same time, little is known about the effects of such operating modes on the voltage stability or where exactly the operating limits of the existing transmission grids lie. This raises the question as to the future robustness of the transmission grid. For example, ecologically motivated measures such as infrastructure bundling are increasing the probability that large transport capacities will simultaneously fail.

The failure of several of the planned HVDC connections in Germany could lead to critical loads on the surrounding three-phase (AC) network, whose stable operation requires considerably more than the normal reactive power supply. Accordingly, it needs to be checked as to how much static and dynamic reactive power is required for the grid situations being controlled, while taking the necessary safety margins into consideration. This key aspect is currently not taken fully into account and requires a fundamental review of existing network planning principles.

The objectives of this study in detail:

  • An assessment of cascade effects in the creation of voltage collapses in hybrid networks, taking into account the relevant time ranges, was concluded in the first phase of the project in order to define the model requirements and relevant components. This resulted in two model approaches: firstly a stationary model for analysing the long-term voltage stability and a second model for simulating dynamic time-range processes for the short-term voltage stability.
  • Simulation and analysis of the voltage stability in hybrid network structures taking into account large power transients and (n-1) or selected (n-2) faults. The voltage stability is being investigated for the "post-fault" state of the system in stationary analyses, whereby it needs to be clarified as to which problems are to be expected in accordance with the current guidelines and grid connection conditions. To this end, methods have already been developed within the project in order to evaluate the system stability and quantify the gap between a given operating point and the nearest stability limit. In addition, a method has been developed to identify relevant (N-k) events that can have a major impact on the system stability.
  • In addition to the stationary tests, the dynamic requirements for the reactive power supply are being determined on the basis of selected dynamic simulations and defined fault events in the time range. The relevant fault events are being selected using the aforementioned methods for determining critical (N-k) events. Initial dynamic simulations have already shown the increased demands on the dynamic reactive power supply through the increased use of converter technologies in the form of HVDC, generators or loads.
  • Particularly in connection with the dynamic requirements in accordance with the different penetration rates and operating modes of the new energy conversion technologies, investigations are being conducted aimed at improving the reactive power supply and voltage stability. Initial sensitivity analyses have already been carried out as part of the project.
  • Based on the different penetrations, safety margins will be identified that must be adhered to in order to ensure safe operation.

Finally, there is a need to investigate and further develop existing concepts for system-stabilising measures ("defence plans") in the event of serious faults. Initial concepts for effective load shedding as a defence measure have been developed and are currently being tested.

Expanding planning principles

Schematic presentation of the iterative method for determining the closest stability limit
© RWTH Aachen University

Based on the results obtained, possible adjustments or extensions to existing planning principles will then be investigated. These include, for example, adjusting the safety margins or further network analyses of the network planning or operation management. An adjustment of the system services provided in accordance with the network situation as well as an adjustment of the technical connection conditions and defence plans is also possible. The latter is aimed at reducing the impact of major disturbances. This applies in particular to the evaluation of control options for preventing cascade effects and grid collapses.

As an associated partner, the transmission grid operator Amprion is supporting the entire research work and is advising on the later practical applicability of the methods and results.

Project duration

07/2015 - 12/2017


Dipl.-Ing. Moritz Mittelstaedt
Institute for High Voltage Technology, Head of Department Sustainable Transmission Systems
RWTH Aachen University
Schinkelstraße 2, Raum SG 105
52056 Aachen
+49 241 80-94781
+49 241 80-92135

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