Category B Active Network Management is one of two systems being rolled out by National Grid Electricity Distribution to control the technical limits of ANM and allow more generation to connect to the power network.

Article by Pete Aston – acknowledged expert in ANM systems

Pete joined Roadnight Taylor from Western Power Distribution (WPD), the UK’s largest Distribution Network Operator (DNO) and world-leading pioneers of ANM. Heading up WPD’s system planning team, Pete was responsible for the design of, and all connections to, its extra-high voltage networks. He was also responsible, amongst other things, for overseeing the roll out of ANM across all four of WPD’s licence areas.

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7th December 2021

Active Network Management (ANM) systems have both technical and commercial limits. Accordingly, National Grid Electricity Distribution (NGED) has developed different categories of Active Network Management (ANM) systems, starting with Category Z ANM and Category B ANM.

NGED’s Category B ANM system has been developed to allow additional generation to be connected to a network, over and above what would be technically feasible under Category Z ANM. This will have better checks and processing built into it than the Category Z system, and will also be more resilient to faults such as a failure of the communications network.

The worst-case scenario for Category B is not as extreme as for Category Z. It considers:

• the ANM system failing; and
• all generators in the network then operating at full output; and
• no circuits and/or transformers switched out (an intact network)

In this worst-case scenario, they will allow up to 110% loading on any overhead line or 125% through any other item of plant (such as a transformer or underground cable).

The diagrams show how much generation could be connected to a Category B ANM system on a standard two-transformer 132/33kV substation.

The example below is a substation with two 90MVA transformers, some generation, and a minimum demand of 20MVA.

Ignoring ANM, a DNO will typically assess how much generation can connect by assuming one transformer is switched out (for maintenance or a fault). In the case below, the rating of the one remaining transformer is 90MVA, plus the minimum demand of 20MVA, means up to 90+20=110MVA of generation can connect without ANM.

If  NGED wants to use a Category B ANM system to connect more generation to this substation, it needs to consider the worst-case scenario outlined above. With no transformers switched out, the rating of both transformers is 180MVA. So the max allowed power flow through those transformers is 180MVA x 125% = 225MVA. This an additional 225-90 = 135MVA of generation that can be added by using a Category B ANM scheme, as shown below.

So in this example, the Category B ANM system has allowed an additional 122.7% of generation.

It’s worth pointing out that Category B and Category Z ANM don’t change the level of curtailment experienced by the generators. In two identical networks with the same generators, the first customer in the ANM queue would have the same curtailment in both Category Z and Category B. The only difference is that the Category B system would allow more generators to connect, so it could ultimately have a much longer ANM queue. One outcome of this is that Category B systems, which allow more generators to connect, will inevitably lead to customers at the bottom of the queue experiencing high levels of predicted curtailment. Category Z systems will typically fill up before curtailment levels get extremely high.

In many parts of its network, NGED has had to move to offering Category B ANM, because the level of accepted offers had already filled up to the Category Z limits. However, NGED does not yet have the Category B ANM system finalised, so it is likely to be a few more years until it is ready to be rolled out. If a connection offer says that it requires Category B ANM, it is possible that the connection of the site will need to wait until it is available on that part of the network.

In some scenarios, even the Category B ANM system will reach its allowed capacity (due to lots of accepted offers). When this happens, NGED will need to revert to issuing offers with conventional reinforcement, so increasing the capacity of the network by installing bigger overhead lines, underground cables and transformers. This, of course, comes with it a high price tag and long time delays.

Eventually ANM will run out of capacity in every location and DNOs will need to invest in the network. ANM is a great tool, but it is a short-term fix before a bigger and stronger grid is needed.