Section 14 HV Network Design
Section 14 HV Network Design
Section 14 HV Network Design
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normally be radial, unless planning provisions for the future development of the network require otherwise. The Distribution Planning Officer must be consulted in this regard. Where the design capacity requirement of a development or the combined design capacity with adjoining developments is greater than 630 kVA the supply to the development should be designed so that it will ultimately be incorporated into the interconnected backbone of the network. Developments comprised of multiple stages and/or which are bounded by undeveloped land will require a strategic network development plan for the general area involved. This will be developed in conjunction with the Distribution Planning Officer (DPO) and the Senior Design Officer who will also be jointly responsible for the maintenance and updating of the plan. Designers need to source information from developers, local government and government planning agencies to determine: the proposed sequence of staging and development timeframes. the proposed road structure and hierarchy. the likely future use of surrounding area.
The following provides some guidance in making design decisions and this, together with, consultation and the support of network planning and reliability personnel will enable the achievement of this objective. 1.2 Residential Developments (URD) Residential developments can range from small isolated developments through to the extensive developments covering large tracts of land. The network should be designed to achieve the minimum number of padmounted substations necessary to meet the calculated design demand and voltage regulation requirements. For conventional subdivisions a 500 kVA transformer is the largest padmount capacity that can practically be distributed, but in most cases a 315 kVA transformer capacity will be sufficient, (refer LV Network Design). As a design rule the transformer capacity should be chosen to achieve 80% utilisation, as a minimum, based on the estimated demand at the time of commissioning, given the limitations imposed by the range of transformer sizes available. For smaller isolated developments (up to 630 kVA, ultimate design demand) the incoming supply cable to the development should
Information will often be sketchy and incomplete and the strategy plan may need to cater for a number of contingencies, but nevertheless the plan will enable an orderly development of the network. Designers must also consult with the DPO and the SDO to establish whether the development they are designing is part of an established network development strategy plan. Guidelines for the selection of switchgear and cable sizes for these network arrangements are provided in Sections 2 and 5.
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For larger customers the substation should be situated as close as practical to the customers load centre, regardless of whether this makes the provision of an external low voltage interconnection impractical. This cannot however be hard and fast rule because of the different situations that will be encountered, and good engineering practice will need to be applied. Padmounts are preferred for supply requirements up to 1000kVA, provided a site satisfactory to Ergon and the customer can be established. Multiple padmounts may be used under certain conditions in consultation with Line Standards. Otherwise an indoor/chamber type substation will be required. When assessing the customers supply requirements, future growth must be taken into account as, it may be necessary to establish an indoor/chamber substation initially to cater for the longer term needs. Line Standards will, in the future, establish an Ergon Manual for indoor/chamber type substations and in the interim will provide support to designers and regions on design standards. Some industrial customers may, because of the nature of their business, seek to have an alternative HV supply. In such cases commercial considerations will apply. As a design rule Ergon will not normally provide a second supply cable and the associated switchgear in these situations. Designers must consult with the DPO before a ring feed is considered for network purposes in these circumstances. 1.4 Rural For rural developments the likely demand will not generally warrant an interconnected HV network, unless this is required for other reasons. The selection of the substation transformer size must be a practical balance between the cost of padmounts, HV cables, LV cables and reliability considerations.
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pedestrian use. Projects must benefit the local, if not the wider, community and generally require local government support.
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2.2 Interconnected Backbone Feeders Interconnected underground systems used by Ergon are normally a ring main arrangement with incoming and outgoing feeder cables being connected to load break / fault make switching at padmounts. The transformer is connected to the bus linking the feeder switches by a switch fuse combination as generally shown in Figure 2.1 below.
Pad Sub1 RMU Pad Sub2 RMU Pad Sub3 RMU
Figure 2.1 Interconnected backbone feeders. Pole top arrangements for the connection of the underground network to the overhead system are set out in Section 4. Different arrangements have been used by other utilities to reduce the amount of switchgear required; ie. looping in and out of a padmount unswitched. This, however, requires additional padmount / transformer inventory and the current extent and rate of growth of these networks in Ergon does not support the additional expense of holding the plant required. A watching brief will be maintained on this matter.
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In some circumstances it may be possible to provide a limited alternative supply from low voltage interconnections but this will generally be inadequate for the time required to repair a cable fault. While every case must be assessed individually, the general design rule is that demands < 630 kVA are acceptable for a radial connection subject to the availability of portable generation. Many commercial and industrial customers will desire an alternative high voltage supply. Where the design rule requirements for a radial supply apply and no other, planning, operational or reliability requirements affect this, commercial conditions may apply to the provision of the second cable. Designers should seek further advice if in doubt. 2.3.2 Connecting to an Underground Backbone For 22kV systems a ring main switch with an additional switch fuse unit is available. This enables a radial supply to be taken off an underground backbone to a padmount with a direct connection to the transformer (refer Underground Construction Manual PADMOUNTED SUBSTATIONS drawing Nos 5082/2 & /4). Because the cable is 2 fused at the supply end, 35mm cable with limited screen fault rating can be used.
Figure 2.2 Interfeeder tie for 22kV / 11kV 2.3 Radial Feeders 2.3.1 General Radial supplies will generally supply only one or two padmounts and with this arrangement there is no alternative high voltage supply in the event of damage to the cable or a failure. The time to fix a high voltage cable fault can run into days and therefore the choice of a radial connection must take account of the consequential affect this will have on the customer and the community (essential services).
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For 2 substations on a radial supply the first will need to incorporate a RMU. Feeder cables may also be used for radial connections. This may be done to utilise accumulated short cable lengths from other projects or to allow future conversion to a feeder cable. In situations where there is a significant possibility of the LV load being disconnected for extended periods, padmount selection may require a switch fuse arrangement for transformer protection. This is to prevent ferroresonance overvoltages occurring following a single phase fuse operation at the cable source end. Feeder rated cable should be used for these applications. For 11kV systems a single switch fuse unit is available for a single padmount arrangement and may be used in conjunction with ring main units for a multiple padmount arrangement (refer Underground Construction Manual PADMOUNTED SUBSTATIONS drawing Nos 5096/1 & /2).
Figure 2.4 Backbone connection for 11kV / 22kV. This option should be used for 11kV application or 22kV application where a feeder rated cable for possible future use is a requirement. 2.3.3 Connecting to an Overhead Line The choice of padmount/s for a radial connection to an overhead system should be the lowest cost arrangement available. Generally, radial cables will need to be protected by a switch fuse combination at the pole top connection to protect the cable and transformer and avoid the risk of ferroresonance (see Section 4). For a single padmount arrangement Underground Construction Manual PADMOUNTED SUBSTATIONS drawing Nos 5094 for 11kV and 5104 for 22kV will apply.
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Pole top arrangements for the connection of the underground network to the overhead system are set out in Section 4.
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3.2 Common Earth Sites For 11kV Refer Underground Construction Manual PADMOUNTED SUBSTATIONS drawing Nos 5001/1 to /3, 5010, 5174 and accompanying drawings. For 22kV Refer Underground Construction Manual PADMOUNTED SUBSTATIONS drawing Nos 5114/1 to /3, 5116, 5176 and accompanying drawings. 3.3 Separate Earth Sites For URD installations a CMEN system is generally required. A separate earth arrangement is considered not practical due to the necessity of a considerably larger site, needed to provide clearance between the padmount site (HV) earth system and nearby conductive structures. Note also the required separation of the padmount site (HV) earth system from communications assets. Site requirement needs to be identified in the initial stages of design & negotiation with the Developer as an increase in size is almost certainly not practical at the time of construction. An exception to the foregoing may be where a padmount is located in parkland & the separation, site size, & other requirements are met. For 11kV Refer Underground Construction Manual PADMOUNTED SUBSTATIONS drawing No 5001/1 to /3 & 5175. For 22kV Refer Underground Construction Manual PADMOUNTED SUBSTATIONS drawing No 5114/1 to /3 & 5177.
It may not be possible to fully meet all these criteria and the local government authority may have preferences for these sites, which need to be taken into account. Site selection must also take into account the effect of Electro Magnetic Fields (EMF), in particular, on surrounding dwellings. In relation to padmounts the effective means of reducing EMF levels at surrounding buildings is to limit the transformer kVA rating and also provide reasonable separation between its LV bushing and the buildings. For the padmount locations as detailed in the Underground Construction Manual PADMOUNTED SUBSTATIONS drawings to apply, the LV cabinet must face the street and the maximum transformer rating for 11kV should not exceed 315kVA. Guidance on EMF issues may be obtained from Line Standards.
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5.2 Standard Cables The standard Ergon range of cables is set out in CABLES drawing 3351/1 and /2 together with their electrical characteristics and ratings. Other cables should not be used in the Ergon network without the agreement of Line Standards. Note: Ergon has adopted a rationalised range of 11kV, 22kV and 33kV cables and in some instances insect protected cables are the only option for use in areas where insect protection is not required. Refer to the following table.
11kV Not Insect Protected Application Station Exit Cable Feeder Cable / Non Fuse protected Radial Fuse protected Radial
# Note: Insect Protected cable to be used.
Cable #Triplex 400mm2 Al XLPE Triplex 185mm2 Al XLPE Triplex 35mm2 Al XLPE
Insect Protected Application Station Exit Cable Feeder Cable / Non Fuse protected Radial Fuse protected Radial Special Applications (Including Feeder Exits where
significant grouping factors apply)
Cable Triplex 400mm2 Al XLPE Triplex 185mm2 Al XLPE Triplex 35mm2 Al XLPE 1C 400mm2 Cu XLPE
22kV Not Insect Protected Application Station Exit Cable Feeder Cable / Non Fuse protected Radial Insect Protected Application Fuse protected Radial 33kV Insect Protected Application Feeder Cable / Non Fuse and Fuse protected Radial / Station Exit Cable (non sub-transmission) Feeder Cable / Non Fuse and Fuse protected Radial
APPROVED DATE PASSED C. Noel 10/04/03 A. Bletchly J. Hockings
Cable 1C 630mm2 Al XLPE Triplex 185mm2 Al XLPE Cable Triplex 35mm2 Al XLPE
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