RTE, as a transmission system operator, is carrying out interaction studies to assess the risks associated with the presence of power electronics-based equipment in the proximity of installations already connected to the transmission grid. A collaboration contract signed between RTE and EDF is opening up research to establish a method for studying the risk of sub-synchronous torsional interactions. A survey of interests is open to other market participants interested in this matter.

Use of power electronics in the proximity of installations connected to the network

The energy transition in France and Europe has brought about profound changes in electricity transmission systems which take many different forms.

The development of renewable energy leads to a change in transmission system active power flows, thus requiring the construction of new linear infrastructures, potentially using direct current technologies. It also introduces new technologies within our customers' installations that interact with the transmission system.

Increasingly, particularly in wind and solar fields, our customers are using power electronics components, potentially in the vicinity of other installations already connected to the grid.

Interaction studies carried out by RTE

The role of RTE, as a transmission system operator, is therefore to ensure that the new installations connecting to its network operate in a way that is compatible with other operating components, and in particular with installations of other customers nearby.

For this reason, RTE is conducting studies to ensure that there is no harmful interaction for transmission system equipment and for the equipment of customer installations, in particular in the case of the insertion of network elements containing power electronics equipment.

Interaction risks may arise when a power electronics-based installation (a customer connection or a DC conversion station) is built near a power plant with synchronous machines.

Among the known risks of interaction are “sub-synchronous oscillations (SSO)”, and more particularly the so-called “Sub-Synchronous Torsional Interactions (SSTI)”, which can cause vibrations of generator shafts.  These are generated by power exchanges between generators and other equipment connected to the grid at frequencies below the system frequency (50 Hz).

Figure 1 : Examples of Mechanical Torque Magnitude variations on the generator shaft after a step on the HVDC active power reference (extracted from EMT simulations)

These power exchanges are potentially harmful, on the one hand, to the integrity of the equipment of the generation units and, on the other, to the safety of the power system. Accordingly, in the context of interaction studies, RTE is looking to establish a methodology to define and characterise the situations leading to the presence of such a risk.

In simple terms, the presence of this risk is mainly due to:

• The structure of the generator shafts, creating a mechanical system oscillating at certain frequencies, known as torsional frequencies,
• The presence of power electronics-based equipment controlled by various command systems, generating different dynamic behaviour depending on the nature of the electrical variations at the connection point,
• The grid between these generators and Power Electronics-based equipment, acting as the vector transporting a disruption from one of these items of equipment to the other.

An “unfortunate” combination of these 3 conditions can cause significant vibrations in the generation units’ shafts, thus potentially resulting in them breaking, as well as divergent behaviour in power electronics-based equipment. All of these consequences can impact the safety of the operation of the power system.

A collaboration agreement between RTE and EDF

Noting that there is a physical (and electrical) proximity on the French grid of a nuclear power generation site (the Gravelines CNPE belonging to EDF) and a link based on power electronics equipment (the HVDC IFA2000 connection between France and England belonging in part to RTE), a collaboration agreement was concluded between EDF and RTE to develop of a method for studying the risk of sub-synchronous resonance on torsional interactions, thus taking as an example the specific case of the IFA2000 HVDC link/Gravelines power plants.

In this way, each of the parties can contribute its own essential skills to the establishment of this methodology. The principle of this collaboration will initially focus on conducting off-line EMT studies (see article "EMTP software, RTE simulates the power grid") to constitute the theoretical and practical principles of the methodology. These principles will then be confirmed and improved through real-time simulations, using a replica of the real control of the IFA2000 HVDC link.

Survey of interests

The methodology aims to be applicable to all types of situations, whether it concerns equipment based on Power Electronics (HVDC connection, wind farms, etc.), or generation units for which torsional frequencies of under 50 Hz have been identified.

In this context, RTE invites any party interested in contributing their specific expertise to the development of the methodology to contact the engineers working on this theme.

Find out more

See the article "EMTP software, RTE simulates the power grid"

Contact

If you are interested in this process, you can contact Bertrand Clerc at the following address: bertrand.clerc@rte-france.com