Understanding DNO Network Reinforcement Requirements and Costs

DNO Network Reinforcement – A Rough Developers Guide

Developers adding generation to the UK distribution network will be aware of the schedule problems and costs of the DNO’s connections, and in many cases the DNO connection costs can make or break a project. What many developers do not realise is that the cost and magnitude of this reinforcement depends on their own system configuration and fault contribution and there are various design steps that can be taken in the conceptual stage, that can significantly reduce these costs and schedule issues. Interested? Then please read on!

Once a site has been selected, and the Developer (whether Solar/PV, Wind, Hydro, Thermal or Battery Storage) has carried out some preliminary design and cost analysis to decide the system configuration they must approach the DNO to get a connection cost. The DNO will evaluate their network to see if it has sufficient capacity to accommodate the connection, and if not what reinforcement works are necessary. DNO network reinforcement can be anything from minor protection changes, up to major upgrades of their network, and the DNO is obligated to provide a competitive price for carrying out these works. The costs associated with this work is known as the DNO Network Reinforcement Costs.

The cost of the connection depends on the amount of reinforcement work needed by the DNO. The DNO’s will provide a breakdown of what work is required, but it is very important to note that DNOs are not normally proactive on these issues, but it is equally important to understand that there is a good reason for the DNO attitude – they don’t know the scheme background, financing and context, so they cannot really suggest alternatives unless there is a very obvious solution and also it is not in their remit to provide a free consultancy service to the Developer. To put this into context let is consider a case where a developer has asked for a 30MW STOR connection at 33kV with standard sets, then the DNO will carry out an estimate on this basis and may quote, for example, £1.5m for a connection. What the developer might not realise is that by spending money on modifying their design (i.e. increasing a transformer impedance or specifying high sub-transient reactance on the generators), or even accepting a 25MW rating, these costs could be reduced significantly and the corresponding schedule impacts reduced. (More on this later!)

So what drives the costs the DNO quotes? As mentioned previously, it is the amount of reinforcement work that the DNO deems necessary to meet the requirements of your project. The more reinforcement work needed, the more expensive the connection cost. Electrically, these reinforcement costs can be loosely summarised below:

  • Local Demand and Conditions
  • Cable and overhead lines overloaded or substation transformer capacity overloaded
  • Fault levels exceeded on the DNO switchgear

DNO Network Reinforcement – Geographical Location / Local Power Demand

Firstly, it is always worth considering the geographical location of a plant, as this is often a key driver. In many ways, this is perhaps rather self-explanatory, and a lot of network problems will depend on the local geographical distribution of power consumers in the area. In simple terms, building a 45MW solar/PV farm in the remote Scottish highlands is likely to encounter a lot more problems, as the grid will be weaker and power has further to travel to be useful. Conversely a 45MW solar site in the South West near London, is likely to have less issues, as the electrical infrastructure will be reasonably good – although potentially overloaded.  Weak parts of the grid are far more susceptible to problems of compliance for voltage deviations (P28 studies), harmonic emissions (G5/4 studies), fault levels and in some cases also power system stability. Similarly connecting a site near to a major DNO substation is likely to mean a strong network, and hence less problems than trying to connect to a very small rural substation.

The key aspect of this issue is understanding that choosing a geographical area to develop should be done with a degree of caution, as whilst remote site may appear attractive they can often contain a host of hidden problems that do not appear until some of the preliminary studies are being carried out. More developed parts of the network are able to cope with bigger systems more easily. Good locations to look out for, are old industrial areas, that have been mothballed or closed as these typically have good electrical infrastructure already in place.

Many developers are aware that the DNOs are often very helpful in this areas, and provide network ‘heat’ maps that shown the general issues faced by key areas within their network that are constrained by various technical problems. What is less widely known is that the DNOs also publish their Long Term Development Statements (LTDS), which contain a whole host of useful data and can indicate how much spare capacity is available on the network. This data is presented in a very technical format however, and is not always easy to access and understand.

DNO Network Reinforcement – Equipment Overloaded

Secondly, another common issue is that the DNO network does not have enough steady state spare capacity in the area. For example, consider a project where a Developers wants to connect a 45MW generation project at 132kV to a nearby overhead line. If the line is rated to say 80MW, and there is already 40MW of generation on it, adding another 45MW will exceeds its rated capacity and trigger a major upgrade and result in a large connection cost. There is of course a simple technical approach to this issue – reduce the capacity of the generation site, this can be done by physically reducing the size of the plant, or alternatively by agreeing to an Export Limitation System (ELS), when a plant controller ensure the plant output does not exceed a set figure. Taking the above example, if the project design is amended to 39.9MW either through a design change or via an ELS, then the project would not reach the capacity limit of the line, and the connection cost would be reduced.

It is perhaps useful to understand the ease or difficulty of uprating existing overhead line, cables or transformers from the DNO point of view. Uprating overhead lines, is actually relatively simple (up to a point), as in the majority of cases the wooden poles, or lattice towers can be left in place and the conductor removed and replaced with larger rated conductors. This can be a bit of headache, but shouldn’t be that expensive. Uprating cables, however is a different matter. Cables are not easy to replace, and typically this means complete replacement and a lot of digging and trenching work – which means cost and time, particularly in urban areas. Uprating substation transformers, is also relatively easy in terms of engineering design – however it can be expensive and lengthy! Grid sized transformers, are very expensive and take a long time to manufacture and ship to site, getting ‘outages’ on transformer for replacement is much more difficult than on overhead and cable circuits, as they pose a bigger risk to the DNO network. This kind of upgrade is therefore typically only justifiable on very large schemes.

Another common issue that can arise, is that the DNO network can be running ‘hot’, and operating at higher than its nominal voltage, i.e. an 11kV network is operating at 11.5kV or 12kV instead of its usual 11kV. This is a headache for DNOs who need to operate within certain limits, and by adding additional generation to this network the problem will be made worse. Where this problem is encountered, there is no easy solution, other than the DNO changing the transformer tap changer, or providing compensation equipment – neither of which is cheap or easy.

DNO Network Reinforcement – Fault Level Problems

Thirdly, and by far the most common problem is that of ‘fault level’ on the DNO network. Fault levels can be a tricky subject for anyone who is not an electrical engineer – in simple terms they relate to the ability of the electrical system to withstand and clear a fault safely. Any kind of generation plant will contribute (increase) the DNO fault level – some types of plant such as Solar/PV and certain types of wind (full converters) will make a fairly modest contribution, other such as STOR generation plants and DFIG wind turbines can be much more significant. A reasonably good analogy to a fault level constraint is the Impact Force, or breaking strain on a safety rope. If the rope is rated at 50kN, and you breaking load is rated at 49.9kN you are safe, if however your breaking load is rated at 50.1kN the situation is not safe, as the rope could snap, and a larger rope is needed.

Fault levels are usually defined in terms of kA (kilo-amperes), and relate to both equipment rating and contribution from the generating plant and elsewhere on the DNO network. A switchboard fault level is expressed in terms of its fault rating in kA and its fault level in kA. Its fault rating is one of its design characteristics and a fixed value, whilst the fault level depends on what equipment is connected to the switchboard. The actual fault level on the switchboard must always be less than the fault rating, otherwise the switchgear is not safe. The amount of spare ‘kA’ on the switchboard is known as its headroom. For example an 11kV switchboard may be rated at 25kA, the existing fault level is 23kA and so the DNO has a headroom of 2kA.

Reducing Fault Level to Prevent Triggering a System Upgrade

So, in a similar manner to the steady state problem, let us consider a scenario with a 20MW STOR generation plant connected at 33kV to the DNO network. The DNO in the above example has advised that they have a headroom of 2kA. The system studies consultant and/or DNO carries out a fault study and find that their fault contribution is 2.5kA, this exceeds the DNO headroom so the DNO will need to replace the entire switchboard. However, if we can modify the design of the plant to reduce its fault level to say 1.8kA, then we would be under the headroom of the DNO’s fault level and thus not trigger an upgrade of the DNO system (in theory at least!).

Reducing Fault Level to Reduce the Cost to a Developer

A different scenario occurs, when we cannot quite get the fault level down enough to prevent the DNO from needing a system upgrade. What we can do instead is try and reduce the fault level contribution of the site, so that the it reduces the cost apportionment to the Developer.  The reason for this is partly due to the legislation brought in by Ofgem to increase competition in connections and support distributed generation. The key parameter that the DNOs to assign a switchboard upgrade cost to a developer is known as the Cost Allocation Factor  (CAF), this factor is determined by a specific formula the DNO must use and apply. The formula is very simple, and is shown below:

Fault Level CAF = (3 * Fault Level Contribution From Generation Site / New Fault Level) *100

To see how it is used let us consider a scenario where the DNO advised that a 33kV switchboard needs replacing, with a 25kA switchboard and the overall cost will be £500k.

If the new generation site contributes a fault level of 4kA to the DNO Network

Fault Level CAF = (3 *4 / 25) * 100 = 48%

So the developer would pay 48% of the £500k cost (i.e. £240k)

Suppose however, we modify our design to reduce the fault level contribution to the DNO by increasing the main transformer impedance of specifying generators with high sub-transient reactance. This design change results in our fault level contribution to the DNO reducing to say 2kA.

Fault Level CAF = (3 *2 / 25) * 100 = 24%

So the developer would pay 24% of the £500k cost (i.e. £120k), thus resulting in a saving of £120k!

In practice, this becomes an an optimisation exercise, as nothing in engineering is ‘free’. Reducing the fault contribution can be done relatively easily (up to a point), but it will affect the cost of the equipment purchased by the developer. Suppose in the above example, the Developer increases the transformer impedance from a standard value if say 10% up to a specialist high impedance value of 25%. The transformer manufacturer increases their manufacturing cost by £20k to account for the extra design and materials, but overall the design change would still have saved the Developer £100k.


In summary, it is therefore important that scheme developers understand what drives the DNO network reinforcement costs, and the DNO’s attitude to providing connection costs. If Developers engage with system studies consultants, such as Aurora Power Consulting, they can provide many practical ways to increase a projects viability and reduce cost and schedule issues. Options that can be looked at are optimising the size of the agreed connection, accepting Export Limitation Schemes and modifying the design to reduce the fault level contribution to the DNO. Please contact Aurora Power Consulting if you would like to know more details.