M.G. Twitchett,
IEng, FIIE, MSOE, MIPlantE(Consultant, Electrical and Plant Engineer)
The purpose of a site distribution system is to provide a means of economically and reliably distributing power from one, or occasionally more than one, main location to a number of geographically dispersed load centres within a defined site bound- ary. The source of power is usually an intake from the local Distribution Network Operator (DNO) but may also be from on-site generating plant. The economics of the system will be largely dependent on the voltage at which it is decided to operate it. The ability of the system to reliably supply power to where it is needed and when it is needed will depend initially on the quality of the equipment and the installation workmanship. In the long term it will depend also on the flexibility avail- able to continue to supply that power to where it is needed, despite the need to maintain parts of the system and possibly despite the occurrence of faults on the system. While much that will be discussed may also apply to distribution systems within buildings (especially buildings with a large electrical demand), such installations are dealt with in Chapter 4.
If the site is other than very compact or has only low demand, it is likely to prove economic to distribute power at high voltage rather than at 400 V. (Throughout this chapter the term ‘high voltage’ is used to mean ‘exceeding 1000 V a.c.’ as is com- monly understood in UK practice. It should be noted that the term ‘medium voltage’
meaning ‘exceeding 1000 V but not exceeding 12 000 V’ is used in some other coun- tries and due to the globalisation of markets this meaning is increasingly being encountered in the UK.)
Most DNO local distribution networks operate at 11 kV or in some localities at 6.6 kV, and in most cases it is usually sensible to distribute power around the site at this ‘intake voltage’. When the supply is provided at high voltage the DNO does not have to provide the step-down transformers or pay for the losses in those trans- formers. The electricity can therefore usually be purchased at lower cost when taken at high voltage. The level of demand which makes this economically viable must be assessed in the early stages of design.
This book is confined to systems up to 11 kV but it should be borne in mind that on some very large, high demand sites it becomes economic to take a supply at 33 kV or even 132 kV. It is unusual, however, for the consumer to operate the site network at more than 11 kV.
The capital cost implications of operating at, say, 11 kV become clearer if one considers as an example the need to transmit 500 kVA over a distance of 500 m. At
67
400 V this would require two 4-core cables of 240 mm2 in parallel (assuming a requirement for a neutral). The cost of 1000 m of such cable would be in the region of £14 600. While in theory the same power could be conveyed at 11 kV by a 3-core, 6 mm2cable, in practice the smallest available cable for this voltage is likely to be 3-core, 25 mm2. Nevertheless the cost of 500 m of this cable would still only be in the region of £3000. A transformer would be needed at the remote end and a 500 kVA oil-filled transformer would cost approximately £4700. The 11 kV system would thus show an approximate saving of almost £7000 over the 400 V system for what is quite a modest power distribution requirement. (These calculations are based on prices at December 2001.)
INTAKE ARRANGEMENTS
While low voltage intake arrangements are generally simple single feeders there are many possible configurations for the interface between the consumer and the DNO on high voltage supplies. Some of the more common arrangements are described below.
The exact point of demarcation between the consumer and the DNO should be the subject of formal agreement. It becomes particularly important, and sometimes contentious, in the event of faults and the allocation of repair costs and may some- times be pin-pointed down to the cable lugs and the bolts securing them inside a cable box. The simplest arrangement is for a single transformer to be owned by the consumer with the h.v. switchgear supplying it owned and operated by the DNO, as shown in Fig. 3.1. This arrangement relieves the consumer of any high voltage switching duties but makes him completely dependent on the DNO for making live the transformer or isolating it for maintenance. (Facilities are usually provided to
Fig. 3.1 A single high voltage intake with h.v. switchgear under control of the DNO.
enable the consumer to trip the DNO’s switch or circuit-breaker in an emergency, but not to reclose it.)
Fig. 3.2 depicts a more flexible and more common arrangement for a single feeder supply from a DNO. Both the DNO’s and the consumer’s circuit-breakers or switches are effectively connected to one set of busbars, but responsibility for the busbars changes at the declared point of demarcation. With this arrangement the consumer has control over his own network ‘downstream’ of his circuit-breakers or switches but there is mutual dependence for isolation of the busbars.
Where the security of duplicate DNO feeders is required the most common arrangement is as shown in Fig. 3.3. In this arrangement the DNO’s section of the busbars often forms a part of a ring main. It may be equipped with either circuit- breakers or fault making/load breaking switches. The consumer’s circuit-breaker (sometimes referred to as the metering circuit-breaker) is actually configured as a bus-section switch and is normally the responsibility of the DNO. There is mutual dependence for isolation of the consumer’s section of the busbars although the DNO is still able to keep its section of the busbars in service. In some cases the entire switchboard will be within one room but quite commonly the DNO section Fig. 3.2 Single feeder h.v. DNO supply with consumer’s h.v. switchgear.
Fig. 3.3 Typical duplicate feeder DNO h.v. intake arrangement.
will be partitioned off or in a separate room to the consumer’s gear. In such cases either a busbar extension chamber or a bus-zone cable is used to carry the busbars through the dividing wall. In critical cases where a greater degree of supply secu- rity and flexibility is required, an arrangement like that shown in Fig. 3.4 may be necessary. Here the DNO is usually responsible for the centre section of the switch- board and the bus-section circuit-breaker as shown.
The factors that must be carefully assessed before deciding on the intake con- figuration are as follows:
(1) Effects on the business of partial and total losses of supply (2) Required availability
(3) Likelihood of faults on either DNO’s or consumer’s equipment (4) Maintenance requirements
(5) Possibility of and economics of generator usage to cover supply outages (6) Cost
Consultation with the local DNO is required at an early stage in order to agree on a mutually acceptable design of intake substation and on the division of ownership and responsibilities. Accommodation for the DNO’s equipment will be required and they will require easy access to it at all times. All these aspects should be the subject of a formal agreement signed by both parties. Provision for metering must also be made and in this connection it is very probable that the consumer may wish to pur- chase electricity from a supplier other than the DNO and that provision should be made to accommodate sophisticated metering with remote monitoring.
SITE DISTRIBUTION NETWORKS
As with the configuration of the intake arrangements, so there are similar decisions to be made regarding the configuration of the distribution network and its cost.
Local supplies may be required at other than 400 V if large motor drives operating Fig. 3.4 Duplicate DNO feeders supplying sectioned busbars.
at high voltage (usually 3.3 kV or 6.6 kV but occasionally at 11 kV) are to be installed. The simplest arrangement is to supply single transformer substations via single radial feeders emanating from the intake substation, as shown in Fig. 3.5. The major disadvantage of this system is that supply to any particular load centre will be lost if any outage due to a fault or for maintenance occurs on any item on the single supply route. Increased security of supply can be achieved by providing dupli- cate radial feeders and transformers, as shown in Fig. 3.6. If the transformers are each rated to carry the total local load, and bus-section circuit-breakers are installed on the l.v. switchboard, greater flexibility is afforded to deal with maintenance or faults.
Probably the most cost effective and common means of providing duplicate feeders to all the load centre substations is to employ one or more ring mains. Figure 3.7 shows a typical ring main supplying a number of both single and two transformer substations of various configurations. Such a system enables all substations to be supplied in the event of an outage of any one leg of cable on the ring.
For economic reasons most ring mains incorporate substations equipped with fault making/load break switches rather than circuit-breakers on the ring main cable feeders. Such switches do not provide automatic means of opening in the event of faults. It is therefore good practice in normal circumstances to operate the ring-main with one of the switches (at some strategically suitable point) open, so that a fault anywhere on the system only causes a loss of supply to the substations on that side of the network.
If the fault is on a cable (which is the most likely event), once it has been located switching can be carried out to isolate the defective leg and restore supplies to all the substations. Of course, if a fault occurs on the busbars of one of the substations Fig. 3.5 Single radical feeders supplying load centres.
(a rare but not unknown occurrence), then that substation (or section of busbars if provided with a bus-section switch) must be repaired before its supply can be restored. The location of faults is greatly facilitated by the provision of earth fault indicators at each cable terminating box around the system. These make unneces- sary the practice of repeatedly closing onto a fault in order to locate its whereabouts, or the time consuming technique of isolating each section in turn in order to conduct insulation tests. A visual inspection of the earth fault indicators will determine which section of the ring main is faulty. Earth fault indicators with remote monitoring are also available at increased cost which must be balanced against the cost of delay in locating the fault and carrying out the necessary switching to restore supplies.
Where continuity of supply is crucial and even short unplanned outages due to cable faults cannot be tolerated, it is necessary to provide circuit-breakers through- out the ring main and to operate the system as a closed ring in normal circumstances.
In this mode of operation conventional current and/or time graded protection will not provide proper discrimination since the direction of power flow around the ring main is likely to vary under different load conditions. In order to provide continu- ity of supply to all substations in the event of a cable fault, it is therefore necessary to employ a differential protection scheme for each leg of the ring main or to install a suitably graded scheme utilising directional overcurrent and earth fault relays.
ON-SITE GENERATION
Generators may be installed on site for one or more of the following reasons:
Fig. 3.6 Duplicate feeders and duplicate transformers.
(1) Standby in the event of loss of DNO supply
(2) Total stand alone system, possibly incorporating combined heat and power (CHP)
(3) Peak lopping
(4) Utilisation of available process steam through a pass-out turbine.
Site generating plant may be operated in a number of different modes:
(1) ‘Island mode’, i.e. disconnected from any DNO supply (2) In parallel with the DNO under normal conditions
(3) In parallel with the DNO supply only briefly to permit continuity during changeovers.
Consultation and formal agreement with the DNO are required if paralleling with their supply, even briefly, is envisaged. Where generating plant is intended to have a ‘standby’ role it is essential that the risks that it is intended to cover are fully assessed and that it is decided in exactly what circumstances its deployment will be required. For example, in Fig. 3.8 the 11 kV generator provides an alternative in the Fig. 3.7 A typical h.v. ring-main system.
event of a loss of the DNO supplies but cannot be deployed if the intake substa- tion busbars have to be isolated for maintenance.
Generators may be provided at strategic low voltage switchboards in order to maintain supplies to essential loads in the event of either loss of the high voltage DNO supply or outages on parts of the site high voltage network. In some cases such generators are used to ‘backfeed’ the site h.v. network by using the local trans- former in a ‘step-up’ mode in order to provide emergency supplies to the entire site, as shown in Fig. 3.9. (Precautions must of course be taken to avoid backfeeding into the DNO system.) However, in the majority of cases all the transformers on the h.v.
network will have delta connected h.v. windings, usually vector group Dyn11. It would be more appropriate to connect a low voltage generator to the high voltage network via a transformer with star connected h.v. windings, (e.g. vector group YNd11). If the h.v. network is supplied from delta connected transformer windings when disconnected from the DNO supply, then the site h.v. network is operating without any earthed reference point, and normal earth fault protection schemes will be ineffective. If an earth fault were to occur on the unearthed system it would there- fore go undetected. Beside the obvious undesirability of the system continuing to operate with an earth fault, there is the possibility of damage to any voltage-graded insulation in machine windings. In order to overcome this problem either a star- connected earthing transformer should be connected to the system or, perhaps less expensively, the system could be protected by a neutral point displacement relay supplied from a special voltage transformer and arranged either to disconnect the system or to initiate an alarm.
SWITCHGEAR
While switching devices with a wide range of capabilities is possible, the devices that generally have practical application on h.v. site distribution networks are:
Fig. 3.8 Generator connected to h.v. intake substation.
(1) The circuit-breaker – a device capable of making and interrupting short-circuit current as well as operating on load current
(2) The fault making/load break switch – capable of operating on load current and of making short-circuit current but cannot interrupt short-circuit current (3) The switch-fuse – a fault making switch with series fuses. (Such units are often
provided with ‘striker pin’ fuses arranged to open the switch in the event of any fuse operating.)
All three of these devices have in the past been used in both fixed and withdraw- able extensible patterns but current common practice for site distribution sub- stations is to employ ring main units (RMU). Modern ring main units are often gas insulated with a single pressure vessel containing sulphur-hexafloride (SF6).
Two ring main switches and either a small SF6or a vacuum circuit-breaker to supply the local transformer, are contained within this pressure vessel. Extensible versions of such switchgear are also available for use where flexibility of the substation configuration is required. Remote operation of such switchgear can be provided by compressed gas or low voltage electric actuators controlled via a supervisory control and data acquisition (SCADA) system. Oil-insulated RMUs and individual switches and switchfuses have been extensively employed in the past and are still available.
Fig. 3.9 Generator connected to l.v. substation.
While circuit-breakers are more expensive, they provide the advantage of being capable of automatic and remote operation and are generally employed where rapid action to restore supplies following a fault is necessary, or where it is desirable to operate a ring main in a closed mode. Both fixed and withdrawable pattern circuit- breakers are available. In the case of the former, off-load isolators are usually pro- vided on either side of the circuit-breaker, whereas in the latter the circuit-breaker is truck mounted and may be bodily withdrawn from the switchboard. Until recent years the vast majority of circuit-breakers used in h.v. site distribution systems were oil-filled, with a small proportion of the air-break type. The current choice will prob- ably lay between vacuum and gas (SF6) types.
When specifying h.v. switchgear it is important to ensure that provision is made for proper means of isolating and earthing busbars, cables, transformers and any other connected apparatus on which work will at some time need to be carried out.
Low voltage switchgear is generally of the air-break type and in the rare case of it forming part of a site distribution system, it is unlikely to need to be essentially different from its normal application at a l.v. load centre.
CABLES
For h.v. distribution services, cables having polymeric insulation, usually cross-linked polyethylene (XLPE), are finding favour over PILC types due mainly to the sim- plicity with which they may be terminated and jointed. The most usual construction is XLPE/SWA/PVC. For l.v. systems the ubiquitous PVC/SWA/PVC is generally satisfactory. Where cables are routed through buildings or in cable tunnels, safety requirements may in some cases dictate the use of low smoke/low fume cable types.
The choice between copper or aluminium conductors and, in the case of alu- minium, solid or stranded construction, is influenced by cost and the practicalities of installation. Aluminium cables, where they are known to be such, are much less likely to be the subject of theft while awaiting, or during, installation.
As in any installation, conductor sizes must be chosen to meet the requirements of both current-carrying capacity and acceptable voltage drop. With regard to current-carrying capacity, decisions should be based on the usual factors and need not be discussed here. The question of cable size in relation to voltage drop is less clear. For low voltage systems the requirements of BS 7671 (IEE 16th edition) are deemed to be satisfied if the voltage drop between the origin of the installation and the fixed current-using equipment does not exceed 4% of the nominal voltage of the supply. This can be construed as being applicable to a ‘site distribution system’
operating at low voltage but there is at present no equivalent standard for high voltage systems. Where a h.v. system is supplying power to be utilised entirely at l.v., the voltage drop on the h.v. system may not be as important and can usually be com- pensated for by tap selection on the transformers at the load centre substations.
(Overvoltage under light load conditions must not then exceed a value that might be damaging to the connected equipment.) Where loads such as large motors are supplied directly at the h.v. system voltage, cable sizes should be selected to limit the voltage drop to a value acceptable to the manufacturers of the equipment in question.