Introduction – Frequency Response Requirements for BESS plants
Since Grid Code, Issue 6 Revision 17, new under-frequency response requirements were introduced for Energy Storage Modules, such as Battery Energy Storage Systems (BESS), as part of the frequency response requirements for BESS plants. These requirements were brought in following GC0148 – “Implementation of EU Emergency Restoration Code Phase II”.
The new requirements include the so called “De-Load Function” and the “Latching Function”. The De-Load Function requires a more aggressive frequency droop response during import mode compared to the standard export mode requirements, while the Latching Function requires that the frequence response is held until the frequency rises back above 49.5Hz.
Power Plant Controller (PPC) OEMs are still in the process of adapting to these new frequency response requirements for BESS plants and so we have seen many BESS project face delays in the compliance process as these new requirements have been either missed or misinterpreted. In this article we hope to shed some light on the new requirements.
The De-Load Function
The De-Load Function, described in ECC.6.3.7.2.3.1, has been introduced as an alternative to Low Frequency Demand Disconnected (LFDD) which would require Energy Storage sites to disconnect 12.5% of their assets in following an under-frequency event. The De-Load Function specifies that plants operating in import mode should operate with an under-frequency droop setting of 0.6% – 1.2%, which is considerably more aggressive than the requirement in export mode which is <10%, although practically most plants operate in the range of 3% – 5%.
The 0.6 – 1.2% droop requirements give rise to the acceptable response area for import mode, which is given by the blue shaded region in the figure below. Note that once frequency falls below 48.9Hz the plant must have ceased import of active power entirely. Below 48.9Hz the plant is expected to be exporting active power. However, there is a clause (ECC.6.3.7.2.3.1(e)) which permits maintaining operation at 0MW rather than transitioning to export.
In practice, this means having two different LFSM-U droop settings, one for export mode and another for import mode. The PPC must then be capable of automatically switching between these droop settings depending on the operating mode.
Latching Function
The Latching Function, described in ECC.6.3.7.2.3.2, requires that following an under-frequency event the active power is held constant even as the frequency recovers; only when the frequency rises above 49.5Hz is the plant permitted to reduce the active power level. The acceptable response is given by the blue shaded area in the figure below. As the frequency falls, the plant follows the solid black-line defined by the De-Load Function droop, but as the frequency starts to increase the plant response should follow the dashed black-line
Compliance Studies
For Energy Storage Modules the frequency response compliance must now be demonstrated for import mode in addition to the standard export mode scenarios. The list of Simulation Scenarios that are required to be performed are outlined in ECP.A.3.6.9 to ECP.A.3.6.12 of the Grid Code and are described in further detail in Guidance Document for Electricity Storage (EU Code Users Issue 4). In total there are 18 new scenarios required for import mode, a significant increase on the 5 scenarios required for export mode, including under frequency events of 49.0Hz, 48.8Hz and 48.3Hz.
Let’s look at a 48.8Hz under-frequency event as an example. Two example responses are shown in the figures below. Example A provides the correct response for a plant with a 1.2% De-Load Function droop, while Example B is a typical non-compliant response from a plant controller which has not been appropriately modified to reflect the new Grid Code Requirements.
Example A (Compliant)
Example B (Non-Compliant)
In Example A the active power can be seen to increase to +6.6MW, which corresponds to a 1.2% droop, while in Example B, the active power only increases to -21.3MW corresponding to a 3% droop and is therefore outside of the acceptable response range.
On frequency recovery, Example A is seen to hold the active power level until the frequency rises above 49.5Hz and therefore respects the Latching Function requirements, while in Example B the active power can be seen to decrease much earlier.
Our Experience
It seems PPC OEMs are still adjusting to these new requirements as we are seeing non-compliance issues in several models we have received. This is leading to delays for many projects as models need to be sent back for updates repeatedly to achieve the required performance. Therefore, we recommend that dynamic models are provided earlier to validate the performance against the Grid Code requirements and mitigate against project delays.
