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Summary of network design policies

Policies for the design of the electricity network are set out in the following electricity policy documents (EPD).

EPD279 - Distribution system design - general requirements

All new work is to be designed in accordance with current policy and will comply with all relevant statutes and licence conditions, and with the distribution code. Connections to customers shall be made at the lowest practicable voltage. All new work shall use only approved equipment. The minimum standard of security is as set out in ER P2/6 and design policy will achieve at least that level of security. Designs shall take account of the nature of customers' load and ensure that it does not interfere with other users of the network. All protection systems shall comply with policy and particular care shall be taken over the protection of three-circuit groups. Maximum design fault levels and standard methods of system earthing are defined.

EPD280 - Distribution system design - 132kV network

Close liaison with National Grid must be maintained on all developments affecting the 132kV network or the final metering scheme. Extensions will generally be by 132/33kV feeder-transformers in substations designed to be equipped with two such transformers of up to 90MVA rating. Alternatively 132/11kV substations with up to two transformers of up to 30 MVA rating may be considered. The numbers of circuit addresses and isolation points are limited in accordance with ER P18. Interconnection at 33kV may be used to provide security to single transformer substations or to provide load transfer facilities.

EPD281 - Distribution system design - 33kV network

Approved circuit and substation configurations are defined. Extensions will generally be by 33/11 or 33/6.6kV feeder transformers in substations designed to be equipped with two such transformers of 23MVA rating. Transformer-feeder circuit ratings will normally match those of the transformers. For interconnectors or where further extension is anticipated, circuits of up to 38MVA rating may be considered. The numbers of circuit addresses and isolation points are limited. Where more than two underground cable circuits feed a common load group, they should not all be laid in the same trench. Closed interconnectors at 11 or 6.6kV may be used to provide security to single transformer substations.

EPD282 - Distribution system design - 11/6.6kV network

The network is to be developed on the basis of radial circuits forming open rings on 33/11 or 33/6.6kV substations or interconnectors between them. Switched alternative circuits must be sufficient to restore any part of the network after fault with a maximum of two load transfer stages. Opportunities to convert 6.6kV parts of the network to 11kV must be used whenever practicable. New designs should be such that no more than 200 customers or 500kVA of transformer capacity would remain disconnected for repair time after any one fault. The number of customers on a circuit between a 33/11 or 33/6.6kV substation and a normal open point should not exceed 2500. The maximum rating of distribution transformer to be used on the network is 1000kVA and all newly commissioned or recommissioned ground-mounted distribution transformers must have local means of HV isolation. All new cables for use on either 11 or 6.6kV networks must be rated for 11kV non-effectively earthed (i.e. impedance earthed) systems. All new cables shall have a cross-section of 300mm2 Aluminium except for specified situations where 95mm2 Aluminium cable may be used. Conditions for the use of teed circuits and pad-mounted transformers are defined. Overhead circuits must be provided with sufficient isolation points to facilitate fault sectionalising and restoration. Existing unit-protection schemes on 11 and 6.6kV networks should be maintained as long as is reasonably practicable.

EPD283 - Distribution system design - low voltage network

The LV network is operated at a design voltage of 230/400V and a maximum voltage drop between transformer terminals and customers' exit points of 7%. LV distributors operate radially with limited interconnection via link-boxes to provide sufficient security to substations fed from teed circuits or limited alternative feed for use during plant maintenance. All new extensions to the LV network shall use protective multiple earthing. Aerial bundled conductor is the preferred construction for LV overhead lines. Maximum values of prospective short circuit current are defined. Networks shall be designed such that all mains are fully protected by the substation fuses. No more than 200 customers may be connected to any one three-phase fuseway. Any new service shall have a minimum size of 35 mm2 aluminium single-phase, a maximum length of 30m, a maximum voltage drop of 2% and shall not be looped. As an exception, a new service to a street electrical fixture may have a minimum size of 4 mm2 copper single-phase up to a maximum length of 15m.

EPD259 - Generation connected to the Electricity North West distribution network

Customer protection will generally be allowed, including that required in compliance with ER G59/1, to trip the distribution metering circuit breaker at the connection interface. Synchronizing on this circuit breaker will not be allowed. If for some reason the nature of the customers' system or protection indicates a relatively arduous duty, we may insist on the customer installing his own circuit breaker to protect his system. Metering circuit breakers will be selected from their standard specifications for ground mounted circuit breakers: pole mounted circuit breakers will not be used for this duty.

A tripping supply may be provided to the customer's protection from the network operator's battery, subject to the drain imposed. However each case will be judged on its merits. In particular the use of the battery is not likely to be granted if the DC wiring energised from it is routed outside the curtilage of the substation.

The Electricity North West specification for the construction of single circuit 132kV lines on wood poles is the ENA Technical Specification 4350 (Specification for single circuit overhead lines on wood poles for use 132KV).

For information on how to get copies of these documents visit the documentation page.


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