Category B Active Network Management is the system currently being rolled out by National Grid Electricity Distribution to control power flow 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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First written: 07 December, 2021. Last update: 31 July, 2025

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 (which is no longer being rolled out) 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 the older Category Z ANM. It has 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 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.

Roadnight Taylor can help you understand ANM connection for your projects.