The Redirection of Flows over Interconnectors (RFI) is an operational tool that Parties (in this document meaning ELIA, NGET, RTE, TenneT B.V., CORESO, BritNed and NGIC) can use during real-time operations. When available, it constitutes a very efficient method for recovering and maintaining AC system security when the grid is under strain. The method can also help to lower balancing costs for HVDC link owners in the event of an HVDC link failure and to reduce TSO’s operating costs after gate closure. Flows redirected over interconnectors by the Parties have no impact on the power market or on market-based flows.
Redirecting flows over interconnectors is a way of relieving network constraints, irrespective of their origin, during the operational phase in one or more of at least two neighbouring AC synchronous areas connected by at least two HVDC links. Overloads may or may not be predicted, and can have various causes, such as deviations from planned AC load flows, i.e. excess of wind power, and/or AC component failures. Changing the set-point schedule for parallel HVDC links for the same quantity of power (but in opposite directions) sets up a loop flow that relieves the congested AC area. This may offer an alternative to real-time redispatching, which may very often be extremely expensive and sometimes even be impossible to execute.
Flows may only be redirected over interconnectors if sufficient spare capacity in the relevant direction is available during the respective operational period along all the AC and DC corridors comprising the loop. RFIs may only be initiated post market closure so they are transparent and do not affect commercial positions. By definition, RFIs yield no power balance and have no frequency- or price-related consequences in the AC synchronous areas involved. The net changes in the HVDC links involved, i.e. rescheduling, always adds up to 0 when viewed from either of the two synchronous areas.
The Parties have looked into the benefits of implementing RFIs and reached the following conclusions:
- For NGET, the key benefit is to enable small flow changes to take place that allow filter banks to be switched and so ease high voltages at night. It also regards RFIs as a useful tool for pre- and post-fault control that can be used to decrease overloads on elements in South East England.
- For TenneT, the key benefit is to allow greater control of filter-bank switching, particularly to alleviate high-voltage events. But RFIs is also a useful tool for pre- and post-fault control, enhancing PST coordination between Germany, Belgium, the Netherlands and France.
- For Elia, the key benefit concerns pre- and post-fault control, with RFIs enhancing PST coordination between Germany, Belgium, the Netherlands and France.
- For RTE, NGIC and BritNed, the key benefit of redirecting flows over interconnectors is smaller imbalances once an interconnector has been tripped.
Any TSO can initiate RFIs (RTE, ELIA, TenneT, NGET – also on behalf of NGIC and BritNed) in either a clockwise or an anti-clockwise direction. These RFIs are then coordinated by CORESO and enacted by the Parties.
On Thursday, 19 November 2015 (between 11 a.m. and noon GMT), Coreso coordinated the first redirection of flow (RFI) on the IFA and BritNed interconnectors after market closure. The schedules of both IFA and BritNed were simultaneously altered by 50 MW, but in opposite directions, to set up a loop power flow. At the same time, 50 MW trades were executed on all continental borders in the HVDC loop to ensure the commercial balance of each Party. BritNed’s emergency rating was used during the test, since BritNed was already operating at full commercial capacity during the trial hour.
The purpose of the RFI trial was to test:
- RFI concept;
- feasibility of RFIs using BritNed and IFA;
- operating protocol;
- communications between the Parties;
- commercial settlement processes.
The technical results of the trial show that RFI is highly efficient on the AC networks between the HVDC links (up to 74% effective in some cases). In addition, no frequency deviation was observed as a result of flow redirection, since ramping by IFA and BritNed was accurately synchronised at the beginning and end of the trial. Operational communications between all the Parties involved went as planned, and their individual execution of the RFI took less than 10 minutes. The trial was successful for all Parties and proved the technical and operational feasibility of the RFI concept.
Given the success of the trial, the TSOs involved in this project hope that an enduring service (limited in the first instance to emergency situations) can be established. Private interconnector companies need to assess the commercial viability of RFIs as a product before committing further resources. Coreso will publish theoretical conclusions and the results of experimental trials and analysis.