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Frequency event 22/07: Comparison of DC and FFR response from batteries

Bhanu Duggal's picture
Product Research Analyst EDF Energy

I am an Electrical Engineer with a Masters in Future Power Networks from Imperial College London. I currently work in PowerShift team as a Product Research Analyst with a key focus on...

  • Member since 2021
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  • Aug 6, 2021
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On July 22nd 2021 at 14:57, there was a frequency event as a result of sudden disconnection of the nuclear units at Heysham caused by a substation fire. The charts below show the response of all battery assets on EDF’s PowerShift platform to this frequency event as a result of their Dynamic Containment (DC) and Firm Frequency Response (FFR) contracts:


Fig.1 Frequency response of all assets managed by EDF’s PowerShift platform


 

Dynamic Containment Response


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Fig.2 DC Asset Response


We can see in Fig.2:

  1. The first delivery range of DC Low, kicking in as the frequency drops below 49.985 Hz. The second delivery range kicks in once the frequency hits 49.8 Hz.
  2. The frequency continues to drop rapidly as a result of the trip. As it moves through the operational limit of 49.8 Hz, the output of the battery increases by ~1.5 MW for each 0.01 Hz drop in system frequency.
  3. Thanks to the extremely rapid response of this battery and all the others providing dynamic containment (currently 891 MW and increasing each week), the rapid drop of frequency is curtailed, levelling off 49.65 Hz.
  4. As system frequency returns to the operational range, DC output drops to the point where the battery is outputting below 5% of the contracted quantity, providing a small amount of support within the operational frequency range.

It’s also worth noting that the battery went from zero response up to 25 MW in just 15 seconds and that the entire excursion from the operational frequency range lasted just 2 minutes. With National Grid procuring 798 MW of DC Low for delivery on the 22nd July, there would have been a cumulative response of ~407 MW as a result of this frequency event.

 

How does DC and FFR response differ? 


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Fig.3 DC vs FFR Response Curves


The graph in Fig.3 shows the response specifications of FFR and DC:

FFR

  • 49.985 Hz - 49.5 Hz: linear response from 0% to 100% if contracted MW

DC

  • 49.985 Hz - 49.8 Hz: linear response delivering 5% of the contracted MW
  • 49.8 Hz - 49.5 Hz: linear response delivering the remaining 95% of the contracted MW

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Fig.4 DC vs FFR Response Comparison


Fig.4 shows the relative response to the frequency event of an FFR asset and a DC Low asset. Both assets increase their output extremely quickly, with the FFR asset illustrating linear response as soon as the frequency dropped below 49.985 Hz whilst the DC asset kicked in once the frequency exited the operation frequency range.

Note! From Fig.3, it can be seen that FFR and DC response converge at the boundary of statutory frequency limits (49.5 Hz). This is evident in Fig.4 as the magnitude of normalised response is different for FFR and DC at nadir frequency of 49.65 Hz.

Once the frequency event had been contained, DC response dropped off very rapidly whilst FFR response was more gradual as it continued to support frequency within the operational range.

It is interesting to note that, whilst DC is the post-fault service and the response rate of DC is much steeper once the frequency drops below 49.8 Hz, an FFR asset still provides a greater level of response (per MW contracted) than an asset delivering DC.

 

Why is FFR being phased out?

Frequency response requirements are inversely correlated to system inertia. The transition to a net-zero system (high renewables penetration, falling ‘traditional’ generation (large coal/gas/nuclear generation assets)) has reduced inertia and made it harder to manage system frequency.

FFR is a wide frequency product, catering for any deviation within statutory limits. It provides a linear response to frequency deviations both pre-fault and post-fault.

Rather than having a single dynamic frequency response product, National Grid is in the process of introducing three new services:

  • Dynamic Containment – a fast-acting post-fault service to contain frequency within the statutory range of +/-0.5 Hz in the event of a sudden demand or generation loss (with the DC-Low service live since Oct-20).
  • Dynamic Moderation – a fast-acting service designed to manage sudden frequency imbalances in intermittent generation.
  • Dynamic Regulation – a pre-fault service designed to slowly correct continuous but small deviations in frequency.

With the introduction of these three new services, the separation of Low/High frequency products and the move to EFA block contracting, National Grid will be able to far better manage system frequency than through FFR alone. This will be ever more important as we transition to a net-zero system.

 

What’s next?

  • The DC-Low soft launch began in October 2020. Balancing Mechanism stacking was introduced in January 2021, with EFA block tendering (August 2021) and the DC-High service (October 2021) set to go live shortly
  • The two remaining services, Dynamic Moderation and Dynamic Regulation, are due for soft launch in January 2022

 

In my opinion, the new services are crucial to the development of a net-zero electricity system. They provide National Grid with the ability to manage frequency more dynamically than ever before which should promote competition in the market and bring down costs for end users. 

 

Note! FFR is a bi-directional service whereas DC has a separate market for each direction with DC HF expected to be launched in October 2021. In the above analysis FFR and DC have been compared for the low frequency region (less than 50 Hz).

 

(Originally posted on LinkedIn: https://www.linkedin.com/pulse/frequency-event-2207-comparison-dc-ffr-response-bhanu-duggal-/?trackingId=gdE3smzTS7e%2F5DDb8O3gag%3D%3D)

Bhanu Duggal's picture
Thank Bhanu for the Post!
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Matt Chester's picture
Matt Chester on Aug 6, 2021

With the introduction of these three new services, the separation of Low/High frequency products and the move to EFA block contracting, National Grid will be able to far better manage system frequency than through FFR alone. This will be ever more important as we transition to a net-zero system.

What were some of the major hurdles to getting these services online? Will other utilities readily be able to replicate National Grid's success or is this something with a steep learning curve? 

Bhanu Duggal's picture
Bhanu Duggal on Aug 13, 2021

I would say the requirement of upgrading to 20Hz performance metering was or is still even a major hurdle. With that also comes the challenges of validating the asset can respond within the acceptable range. The asset has to go through extensive testing before they can be certified as ready to deliver the DC service.

Not a drawback or hurdle but something to be aware of - if you are continuously out of the acceptable range for 4-5 50ms periods, then you risk the non-delivery of DC for the whole duration of your DC tender which is whole EFA day at the moment. This has put a real focus on the performance because you risk losing significant chunk of revenue for a service which is priced at 17£/MWh.

Could you please clarify what other utilities you have in mind as it may differ on region to region?

Jim Stack's picture
Jim Stack on Aug 6, 2021

Thank you for providing data on this great feature and it's fast response. An idea of the money saved and customers kept in service would be great. I have read article about the money saved that made it pay for itself. The GRID has needed battery assistance for 10p years and now it is here 

 

QUOTE= Thanks to the extremely rapid response of this battery and all the others providing dynamic containment (currently 891 MW and increasing each week), the rapid drop of frequency is curtailed, levelling off 49.65 Hz.

Bhanu Duggal's picture
Bhanu Duggal on Aug 13, 2021

Jim do you mean what would the cost have been if the demand/customers were disconnected had there not been a frequency response service? Its difficult to quantify that the situation was handled well before (although it got quite close) National Grid started to disconnect the demand. Having said that, a major frequency event happened on Aug 9, 2019 which caused a disruption in train, airport and hospital services. It could give you an idea of the money saved and customers kept in service this time around.

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