CHAPTER 3 VOLTAGES AND FREQUENCIES

3.1       STANDARDISATION IN 1947

The Electricity Act, 1947, in setting up the Central Electricity Generating Board (and its Scottish equivalents) as well as the various Area Electricity Boards, continued the process of standardisations of voltages and frequency started in 1926.

The centralising of generation and the interconnection of power stations clearly required the standardising of frequency throughout the United Kingdom, and this was set at 50Hz.  In order that manufacturers could standardise on equipment, local distribution voltage was set at 415V, 3-phase, 4-wire, giving 240V single-phase line-to-neutral.

3.2       VOLTAGES

3.2.1    The National Grid and Supergrids

The National Grid originally interconnected all the CEB power stations and transmitted power at 132kV.  Later a ‘supergrid’ at 275kV was superimposed on the National Grid, and more recently still 400kV transmission with nationwide cover has come into use.  These supergrids in effect interconnect on a national level the 132kV systems which are relatively local to their generating stations.

The 400kV system is now the main trunk route for the transportation of bulk power. The 400kV, 275kV and 132kV networks can be likened respectively to motorways, dual carriage-ways and A-roads.

Figure 3.1 shows the 400kV (red) and the 275kV (blue) ‘supergrid’ networks as at present installed in England, Wales and Scotland. Many of the 275kV lines, when originally planned, were built with a view to possible conversion to a higher voltage later.  When the 400kV grid system was inaugurated, the majority of the longer 275kV lines were re-insulated for 400kV and are now operated at that voltage.  Those 275kV lines that remained were comparatively local to the main centres of population, except for Scotland where no 400kV is used, except a small amount in the Glasgow area.  This can be seen in Figure 3.1.

It should be noted that none of these networks is a simple ‘radial’ system emanating from a single power source, but each forms an interconnected system between several sources of power.  Moreover the links between the 400kV and 275kV systems are made at several different points through 400/275kV auto-transformers.

Initially the 132kV power transmitted by the national grid to all the Area Board regions was transformed on receipt to 33kV or 11kV for distribution by each Area Board.  This arrangement is now (1985) being modified.  Most of the 132kV systems are being turned over to the various Area Boards, where they then become ‘distribution’ rather than ‘transmission’.  The new system is shown typically in Figure 3.2.  The CEGB operates the 400kV and 275kV systems and only retains a 132kV unit if it includes a generating station.  The transfer process is continuing.  The actual grid transformers (400/132kV and 275/132kV) and the associated high-voltage switchgear at the boundary between the CEGB and Board Areas remain the property of the CEGB.

On the Continent 700kV transmission has been introduced because of the long distances involved, and even 1 000kV is being considered, although the technical problems in using such voltage levels are immense.

FIGURE 3.1

PRINCIPAL 400kV AND 275kV SYSTEMS AND CROSS CHANNEL LINK

3.2.2    The Cross-Channel DC Link

The UK network of the CEGB is interconnected with the Continental network of Electricité de France (EdF) by a cross-channel direct-current link, indicated in yellow in Figure 3.1.  The period of peak loading in the UK differs from that in France by about one hour, so that each country’s system can support the other during such peak periods by a two-way interchange of power.  The link is also a very valuable back-up in the event of either side having system faults which leave either one short of generating capacity.

The link is operated by direct current for the following main reasons:

(a)        Smaller and fewer cables are needed as compared with a.c. for a given transfer of power.

(b)        Direct current gives better control of power flow by reason of the thyristors.

(c)        It would be very difficult (though not impossible) to synchronise two such vast systems as the UK and French networks.

(d)        By the use of a d.c. link without synchronism each country retains its freedom to operate its own network by frequency control.

(e)        A fault on one side would not be fed by the other.  Large faults are mainly reactive, and the d.c. link cannot pass reactive power.

(f)        A long a.c. cable has considerable capacitance.  This would cause a large voltage rise and severe switching problems (see Chapter 4).  It would also give rise to capacitive currents which could thermally overload the cable and thereby limit the useful energy arriving at the far end.

The first undersea cables were laid in 1961 between Lydd in Kent and Echinghen on the French side.  The system operated with two cables at 200kV d.c. and transferred 160MW of power. At each end were ‘Converter Stations’ which used grid-controlled mercury arc rectifiers.  They rectified the a.c. when transmitting d.c. power and acted as inverters when receiving d.c. power and converting it to a.c. I n time the cables, which were laid on the seabed, became increasingly damaged by trawls and ships’ anchors, and they were difficult to repair in mid-Channel.  They were finally abandoned in 1982.

A new and much larger system was started in 1982, further to the east of the old one, between Sellindge in Kent and Les Mandarins near Calais.  It transfers 2 000MW of power at 540kV d.c., using four parallel pairs of cables at +270kV and -270kV. Each cable is 45km long, and they are spaced 1 km between each pair.  They are buried in pairs in four deep seabed trenches to prevent damage and to avoid magnetic disturbance to ships’ compasses.  The converter stations at Sellindge and Les Mandarins use solid-state thyristors for rectifying and inverting.  They employ filter equipment to reduce the harmonics caused by the rectifiers and inverters.  This, together with the transformers and switchgear, covers some 34 acres at each end.

The d.c. link is connected at Sellindge into the 400kV supergrid transmission line from Dungeness to Canterbury.

The cost of the whole 2 000MW project was only half that of a new power station of the same output, and the UK paid only half of that cost.  Moreover the project has brought benefits to both countries.

 

FIGURE 3.2

TYPICAL GENERATION, TRANSMISSION AND DISTRIBUTION

3.2.2    Choice of Voltage

The choice of transmission voltage depends mainly on the length of the line.  The passing of currents along a line gives rise to I²R power losses, and the higher the voltage (and hence the lower the current for a given power) the smaller the losses.  This saving on losses has to be set against the extra cost of very high voltage lines, transformers and switchgear, and therefore a compromise has to be reached.  A useful ‘rule of thumb’ for choice of transmission voltage is 1.2kV per mile, or 0.75kV per kilometre.

The voltage of actual generation can be chosen as desired, since it is not used outside the power station.  It was originally between 11kV and 22kV but is now more usually at 25kV.  The generated voltage is immediately transformed at the station up to the transmission voltage (132kV, 275kV or 400kV).  Often the generator and step-up transformer form a single electrical unit without any switchgear between them, the transformer LV terminals being connected direct to the generator terminals, the so-called ‘unit construction’.  They stand or fall together.

In the UK domestic and small power users’ voltage has been set at 415/240V, and this is used in all onshore oil installations.  On the Continent, however, 380/220V is not uncommon, still at 50Hz. In North and South America 440V, 3-phase at 60Hz is standard, but for lighting and small power the 440V is transformed to 117V single-phase instead of using the phase-to-neutral single-phase voltage of 254V as used elsewhere.

Most offshore platforms have followed the American practice of using 440V, 60Hz, 3-phase for distribution, but, instead of transforming to 117V for lighting and small power, a 4-wire system is used to give 254V single-phase line-to-neutral.

3.3       CONTROL OF FREQUENCY AND VOLTAGE

It is the statutory duty of the CEGB to hold their frequency to 50Hz within specified limits (±1%), and their system of Regional Control Centres organises this.  Whenever the loading on the whole national network exceeds the total power being put into it by all generating stations, the national frequency will slowly begin to fall.  This is detected at power stations by observing the loss of actual cycles compared with a standard.  The National Control Centre orders selected running stations to take on more load, or it starts up additional stations, until a balance is achieved, the loss of frequency arrested and lost cycles recovered.  It should be noted that Area Boards have no control over the frequency of the power which they deliver.

Similarly, CEGB Regional Control Centres monitor the voltage at various parts of their systems and take steps to see that it is kept within limits.  There is no statutory limit for transmission voltages, but there is for voltage at the consumer’s terminals (0 to +12% for high voltage and ±6% for low voltage), and it is for the Area Boards to see that this is met.

Within a consumer’s premises there is little that can be done by the consumer himself to regulate frequency, or to regulate voltage unless he has a supply transformer with an on-load tap changer.  The frequency is that of the national grid, and the voltage should be within the statutory limits.  Some voltage adjustments within the installation could be made at the local transformer if it had an off-load tap changer, but such a change could be made only by interrupting the supplies and could not be carried out at will whenever needed.