Reactive power component
Expenditure is incurred for the provision of reactive power. Alternating current systems have to receive excitation current continuously. The effect is that there are large reactive currents being transmitted across the network causing real energy losses and voltage drop (regulation). The result produces additional capital and running costs. These costs are reflected in tariffs by use of appropriate metering which records the various quantities and time of day and date.
At peak load times, efforts are made to reduce the kW and kVAr maximum demand. This maximum demand charge can be onerous and difficult to reduce.
For many years there have been recommendations to install equipment to improve the power factor and to manage the energy demand. The long term finan- cial recovery from such outlay has not often been sufficiently attractive to those managements who are anxious to reduce forced outages.
New technology
The introduction of digital, data handling systems with programmable devices and transmission capability has provided a powerful new tool for control and monitor- ing of the supply. Protection and interlocking have been improved with the intro- duction of information technology equipment in substations.
A typical, classical substation secondary system would perhaps have 40 cores per unit marshalled at the switchgear for transmission, via the alarm and station control cubicle, to the network control centre.
The equivalent digital control system has single parallel connections between each of the switchgear units and their protection panels. There is a single serial con- nection from each unit to the local control panel, plus a further serial connection to each of the computer input/output modules and the network control centre.
Procedural planning and design
Remote monitoring and control, together with a degree of automation, can be intro- duced into existing substations at a reasonable cost and with suitable checking facil- ities. This ensures that wrong procedures leading to unsafe conditions are not possible. Illustrations of modern equipment are shown in Figs 2.1 and 2.2.
The downside is that operational sequences based on logic diagrams must be pro- duced with the co-operation and approval of the client’s engineer. Comprehensive checks are needed, preferably physically on a mock-up on the supplier’s premises, to ensure that the procedures are safe. It should not be possible to defeat interlocks either deliberately or accidentally.
It is important that the installation contractor understands the purpose of sequences and is able to test the installed hardware connected to the various items of plant. A thorough knowledge of the software and the required settings is essen- tial. The designer must be able to supply this information well in advance of the final commissioning procedures.
Electricity distributors provide a single point of supply to a site which can be at different levels of security depending on the importance of the electrical loads
Fig. 2.1 11 kV switchboard, Jersey (Siemens plc).
Fig. 2.2 Digital control panel, Jersey (Siemens plc).
served. If the load demand is of sufficient magnitude, the consumer will find it prac- tical and economic to take the supply at high voltage (h.v.). For the purpose of this handbook the upper limit is 11 kV, although for very large complexes the intake might be at 33 kV or even higher. It is the consumer’s responsibility to control and distribute the electricity around his site or premises to suit his own requirements.
The main substation serving a network incorporates the supply intake point, but there may be other substations located at strategic areas in a large complex and connected to the main substation by a cable network. The manner in which this interlinking occurs depends on a number of factors and is fully discussed in Chapter 3.
If the supply is taken at 11 kV the main substation will include 11 kV switchgear which may serve other substations at 11 kV throughout the site. Stepdown trans- formers at the site substations provide power at the consumer voltage. For very small installations the intake may be a single cable providing a three-phase 400/
230 V supply to a switch and fuse board. In between are a variety of arrangements.
Depending on the importance of the load served, there may be a standby gene- rator(s) linked to the system and arranged to supply essential loads. Some manufacturing complexes may generate their own power, particularly where the generation of process steam is related to the electrical requirements. Generally speaking it is uneconomic to rely on self-generation totally because of the cost of standby generators. Such sites would normally have a standby supply from the mains network. In that case synchronising is required in the main substation to allow paralleling of the two systems. Alternatively, the two systems are interlocked to prevent accidental paralleling. Particular attention would be required to limit the fault power, the effects of reverse power and for discrimination under fault condi- tions. There would also be a problem in sharing responsibility. The magnitude and the importance of the load dictate the nature of the supply and distribution network and hence the design of the substations and control facilities.
Two points should be noted in respect of the demarcation between the supply authority and the consumer’s equipment. The first is that there may be annual rates levied on any transformer between the supply point and the consumer’s distribu- tion system. Secondly, while the supply authority may provide, install, operate and own all of the equipment up to the point of supply, it may require the consumer to make a contribution to the capital cost of making this supply available and will in many cases ask for a site on the consumer’s premises to house it. It is common but not invariable for the supply authority to insist on a separate room or locked-off area for its apparatus.
There is therefore a wide variety of possible arrangements. The client’s contrac- tor is normally responsible for ensuring that the whole scheme is installed in a safe and secure manner with due regard to all health and safety regulations in accor- dance with the Health and Safety at Work etc. Act and in compliance with the client’s specifications. The site engineer therefore has a heavy responsibility. If a design drawing shows an unsafe arrangement or one which conflicts with good prac- tice, he must draw this to the attention of his employer so that it may be remedied.
Electrical power systems can cause death and injury if not properly installed. The local authority and the fire authority inspect any new properties to check that they are safe. If they are not, the remedial work could be expensive, especially if it
involves civil works. The supply authority will normally require information about any new installation but apart from checking the busbars immediately after their own protection the supply authority does not have a responsibility for the client’s installation.
As stated, the arrangement of supply to consumers’ premises depends primarily on the nature of the load. For loads from say 100 to 300 kVA, a 400/230 V three- phase supply from an Electricity Distributor’s substation is normally satisfactory. A load of between, say, 200 and 500 kVA may have its own Electricity Distributor’s transformer. Above 500 kVA one might expect an 11 kV supply and above 1000 kVA it would normally be split into two or more 11 kV circuits. Where a duplicate supply is provided the incomers are generally arranged to have manual changeover facili- ties or to be fully automatic with feeder protection depending on whether or not a firm supply is required. Firm supplies are normally provided for all loads above 5 MVA.
Internal to the factory or other industrial complexes single radial feeders are gen- erally acceptable provided that there is no safety hazard or process which is unduly sensitive to the loss of supply. A smelter or furnace could be badly damaged by the solidification of molten metal caused by a supply failure. In such circumstances a ring main may be installed or duplication of connections to the sensitive load. It is also important to remember that electrical equipment including busbars, trans- formers and circuit-breakers needs maintenance, and therefore bus-section switches, duplicate transformers and interconnecting cabling may be inescapable for a continuous process.
An alternative supply may be better obtained from an in-house standby genera- tor than a duplicate feed. This is particularly so in remote areas where it is recog- nised that there is only a single overhead line, or in overseas territories with unreliable power supply systems.
It is simpler and there is less risk of damage to equipment if the standby supply is not synchronised with the public supply. This is generally satisfactory since a small diesel generator can pick up full load in about a minute. Where there are installa- tions such as computers for which a continuous supply is essential, a battery- operated inverter may be used. Emergency lighting is traditionally energised by direct current (d.c.) and it is wise to consider whether circuit-breaker closing and tripping supplies should be d.c. together with supplies to contactors and electrically operated valves.
SUBSTATIONS
Early consultation with the local Electricity Distributor is essential for agreement on a mutually approved substation to act as the intake point for a particular site.
This consultation is usually before detailed knowledge of the plant or project is known but it is essential that a fairly accurate load requirement be determined. Plant manufacturers must be approached to provide information relative to the equip- ment they are supplying and this, together with experience, enables a reasonably reliable load demand to be ascertained. Having this knowledge enables the rating of the power transformers and of associated switchgear to be decided.
All substations ought to be designed to be capable of extension unless it is obvious that such a facility will not be needed.The extent of such provisions must be weighed against the monetary outlay and be agreed as a viable proposition. Depending on the nature of the system the substations may be required to accommodate h.v.
and/or l.v. switchgear, transformer(s) and protective and control facilities. They can be wholly or partially outdoor or indoor and can take many forms.
Standard equipment should be selected as far as possible to keep cost and deliv- ery to reasonable levels. This equipment should comply with an appropriate stan- dard; in the UK this would be a British standard.
One can conveniently divide the subject into two sections: the various network supply arrangements possible which determine the nature of the equipment utilised, and how this equipment is laid out and protected, i.e. the type of enclosure provided.
High voltage substations
There are three common ways that an 11 kV supply may be provided to a site: by a ring main, by duplicate feeders, or by a radial feeder or a single spur from a radial feeder.
The duplicate supply may be provided with either automatic or manual change- over facilities.
Ring-main unit
A ring-main unit consists of two manually operated incoming isolators and a tee- off circuit to an 11 kV/400 V/230 V transformer. The tee-off may be controlled by a circuit-breaker or a fuse-switch, both providing a measure of protection to the trans- former and acting as a back-up to the l.v. consumer’s network. This is a very common arrangement.
If the site contains three-phase motors at various points remote from the substa- tion, the designer may provide a motor control centre at the substation either inte- gral with the main distribution board or adjacent to it. Alternatively, a number of circuits may be established to supply motor starters at sites closer to the motors.
The main distribution board may also supply local general services such as lighting and small power directly in addition to circuits for more remote sub-distribution boards.
It is preferable to use motor control centres for process plant such as food or chemical manufacture where the production is automatically controlled. The cross connections between the monitoring equipment and the drive controller may be more readily effected. Duplicate supplies may also be provided economically to the motor control panel.
Where the supplies are only for small power and lighting the main l.v. switch- board may be less elaborate with fuse-switches, mccbs and fuse boards. Where split- ter boards are used a 400 V isolating switch is needed to enable fuses to be changed in safety. This should be so interlocked that access to the fuses is not possible until the switch is open. A typical ring-main unit is shown in Fig. 2.3 where the tee-off is controlled by a circuit-breaker and Fig. 2.4 where a fuse-switch is employed for this purpose.
Duplicate supply substation
A more elaborate and expensive design of substation is shown in Fig. 2.5. This would provide firm power to a site where security of supply is important. Generally the bus-section switch is kept open and the two halves of the board are supplied by their respective cable feeds. If there is a failure of one of the incoming cables the bus-section switch is closed and the faulty cable is isolated by its circuit-breaker.
Interlocking ensures that only two out of the three circuit-breakers (i.e. controlling the supply cables and the bus-section switch) are closed at any one time. With this system each intake must be capable of providing the full load of the network.
Single-supply substation
Where a consumer is able to accept an interruption in supply there is no necessity to go to the expense of a duplicate feed, either from a ring-main unit or two Fig. 2.3 A typical ring-main unit incorporating a circuit-breaker in the tee-off circuit.
Fig. 2.4 A typical ring-main unit incorporating a fuse-switch in the tee-off circuit.
separate cables. A single cable supplying an 11 kV switchboard can be used as shown in Fig. 2.6, and this could well be the responsibility of the Electricity Distributor.
Low voltage substations
For loads up to about 300 kVA the power is usually provided from the local supply authority’s network at 400 V. As for h.v. substations, either single or duplicate feeds may be provided to the consumer’s main switchboard. Quite often these boards are divided into sections, one supplying non-essential plant and the other essential equipment. In the event of a mains failure, the essential supplies can either be pro- vided by a duplicate mains cable or more likely from a standby generator set.
Fig. 2.5 Duplicate 11 kV supply substation.
Fig. 2.6 Single-cable supply to an 11 kV substation.
Generally, the main switchboard is a factory-built assembly often of composite design, incorporating circuit-breakers, fuse-switches and circuit-breaker panels.
Motor starters may also be included or may form a separate mcc board.
The rating of circuits is discussed in detail elsewhere, in Chapter 3, Site distribu- tion systems, but there are a number of general points which are often overlooked in respect of substation design, which make it easier to understand the ratings com- monly utilised.
A widespread site such as a dockyard or a large petrochemical plant may have a total load measured in megawatts. It is necessary therefore to have a number of substations.
Circuits supplying current-using equipment should not have a voltage drop exceeding 4.0% of the nominal voltage at the design current. However, it may be necessary to use a conductor larger than that required for the voltage drop to satisfy the motor starting conditions. In addition the cables and protective gear must be designed to match the prospective fault current.
To size a cable therefore requires consideration of:
(1) Full load continuous rating
(2) Voltage drop under full load conditions (3) Motor starting voltage drop
(4) Prospective fault current short time rating.
Each of these conditions is subject to additional constraints. For example, the full load rating must take into account the effect of a low voltage and a low power factor.
These conditions will also apply to the voltage drop. In addition, continuous full load rating must be available despite proximity to other cables and high ambient temperature. The motor starting situation will also be made more difficult by low mains voltage and power factor. The fault current is obviously a function of the supply characteristics. It is wise to allow for some strengthening of the supply system as time passes.
Fault clearance
The circuit-breakers and fusegear must be able to clear faults before cables are over- heated. They must also themselves be capable of accepting the mechanical, thermal and electrical stresses imposed by faults. Transformers, busbars, cable boxes and insulators must also be suitable for the fault level.
To assist in the correct selection of fuses, manufacturers offer a variety of fuse characteristics. These cover variations in current, voltage, time/current, Joule inte- gral, cut-offs, power dissipation and frequency including direct current.
The contractor must ensure that he is installing fuses which are appropriate to the duty. A fuse failure may result in an explosion or the emission of flame. Com- prehensive guides are available from such organisations as ERA (Report No.
87–0186R) and the manufacturers on the selection of fuses. It is particularly impor- tant to ensure that fuses in substations (which are subject to the highest fault levels on the system) have adequate short-circuit capacity and are fitted with the correct fuse link to protect the outgoing cables.
ENCLOSURES
Substations may be either outdoor or indoor types or a combination of both. Site substations generally are no different from main substations, having the same equip- ment and layout but usually on a smaller scale. All the same precautions have to be taken with respect to safety, access arrangements, protection, etc., as with main substations.
Outdoor substations
Where all the equipment is mounted in the open the enclosures must be of weather- proof design (i.e. suitable for all the relevant external influences); this generally relates to h.v. substations. Transformers are automatically suitable for outdoor mounting but if liquid-filled designs are employed they need to be provided with drainage facilities, as discussed later.
Ring-main units are often used outdoors and designs are available that are suit- able for this. Distribution cabinets are needed for the l.v. distribution feeder cables.
Railings or anti-vandal fencing are provided to enclose the equipment to form the substation (Fig. 2.7).
There are also packaged h.v./l.v. substations that utilise standard indoor equip- ment mounted inside a weatherproof enclosure. The transformer may be outside the housing which contains the h.v. and l.v. switchboards, separated by a corridor.
Room for rear access to the switchboards may have to be provided. A claimed advantage for this type of substation is that it allows the foundation to be pre- pared well in advance and is ready to accept the assembled equipment direct from the manufacturer. It is not so popular today because of economic considera- tions but has the advantage that maintenance on the switchgear is possible in all weathers.
An economic arrangement of outdoor substations is the so-called integrated design which has switchgear mounted on the transformer cable boxes, permitting a naturally cooled transformer to be employed (Fig. 2.8).
Fig. 2.7 Typical outdoor substation layout.