This month the cost and research departments of Mott Green & Wall and Davis Langdon & Everest examine the processes and options for the mains power supply connection from an electricity company's network.
Privatisation of the Electricity Board monopolies began in 1990 with the formation of Regional Electricity Companies (RECs). Competition for the supply of electricity has been phased-in and it is now traded like any other commodity. Competition has also been introduced for meter provision and operation.

In 2001 the RECs were required by the regulator to separate their operational business units into 'supply' and 'distribution', and some REC holding groups now own either supply or distribution companies in their rivals' geographical areas. Partly as a consequence of privatisation, a new breed of consultant has emerged, known as 'new connection managers' who will arrange new or upgraded connections anywhere in the country. Naturally, some of the RECs offer this type of service, which can cover temporary site power and the transition through to occupation.

Connection and metering options
Figure 1 illustrates the decision-making process associated with the mains supply connection. For loads below about 300 kVA an lv connection, simply jointed onto a local lv distribution main cable, is the simplest and most cost-effective option. The standard commercial lv supply is connected via a three-phase 400 A 'cut out' – the fused service head installed by the REC to protect the network from faults on the client's installation.

If the load is predicted to exceed the lv limit, then one or more transformers will be required, and the design team has to decide whether the client's point of connection and metering will be at lv or hv. The implications of this decision are summarised in table 1.

Procurement of the REC connection
Most major developments require an hv connection and the REC will need to be given details of:

  • the connection load in kVA;
  • any large potentially 'disturbing loads' such as uninterruptible power systems;
  • generating plant if paralleled with mains for standby change over or for export of power;
  • any supply security requirements above the normal standard.

The REC will then assess the available network capacity and provide an estimate of the connection charge, which may be broken down as:

  • normal costs – representing the installation of cables, the site intake switchgear and, if lv metered, any transformers, plus the cost of any necessary network reinforcement;
  • a fee to cover design and management;
  • a tariff support allowance – a rebate reflecting future capacity charges;
  • abnormal charge – for awkward jobs or customers;
  • repair and maintenance charge – an up-front sum, capitalised for the asset value and life.

For large or complex projects where more cost certainty is needed, RECs will, for a further fee, carry out a feasibility study including route surveys. However, the actual connection charge will not be confirmed until after the REC tenders its sub-contract works, and can sometimes exceed the estimate by a considerable margin.

The construction works for the new connection, ie the trenching, ductworks, cabling, reinstatement, supply and installation of the intake switchgear and any transformers, may be sub-contracted by the client, to a contractor approved by the REC. The REC reserves the right to design the construction works, or at least to approve the design of others, and it will insist on carrying out any necessary network reinforcement works. Finally, the REC will adopt the cables and substation equipment, and make the capitalised repair and maintenance charge as outlined above.

REC connection costs
The complexity of the electrical supply market, the variability of available network capacity and the various design options result in a wide range of connection costs, even for projects that appear broadly similar.

Tables 2 and 3 have been extrapolated from recent central London projects all having a required connection capacity of 1500 kVA. Table 2 (Click here, 23 Kb PDF) highlights the difference between lv and hv metering, and table 3 (Click here, 23 Kb PDF) compares projects with different types of hv connection and network security.

Supply security and restoration
Security of power supply is a business critical issue for many occupiers, and there are a range of multiple connection options available. The supply security benchmark used by RECs is the total number of customer service interruptions per 100 connected customers per annum (CI/100CC). A typical benchmark is 30 CI/100CC, a target which includes all customers, and as a result is difficult to interpret.

This benchmark equates, in very simple terms, to a mean time between failure of three years for a single connection. Once a failure has occurred, restoration times can vary widely, although on average at least 99·9% of all interruptions are restored within 24 hours.

With this performance in mind, many m&e consultants will design for more than one connection. However, the effectiveness of this approach depends on the network configuration and whether the client's operations can tolerate the predicted interruption frequency and restoration period.

REC tariffs
Analysis of the wide range of tariffs available from the RECs is a specialist task that is beyond the scope of this article. One element of the tariff, the availability charge, should be considered at the design and specification stage, because it is charged on the agreed capacity whether that capacity is used or not.

The agreed capacity is fixed by the REC connection construction contract; it may not be reduced for a period of five years, and it will include the additional capacity reserved in any standby connection. The current availability charge, at around £1.00/kVA/month, is equivalent to £12000 per annum for every 1 MVA of agreed connection capacity.

Experience shows that most sites run well below their agreed capacity, and designers should be careful not to overstate their power requirements, nor to include unnecessary contingency allowances. In those cases where power demand is expected to increase, the infrastructure should if possible be modular, or else designed for the ultimate requirement, with the initial agreed connection capacity as low as is practical, then reviewed as and when necessary.

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