The covering of peak loads : note by the Secretariat

UNITED NATIONS

ECONOMIC AND

SOCIAL COUNCIL

Distr.

LIMITED

E/CN.14/EP/23

2 September 1963 ENGLISH

Original: FRENCH

■ficoEroMic;coiyiMissrbN

Ajfjfican-Bl^otriO'Power Meeting.

Addis Ababa,. 21-3*1 October I963

THE COVERING OF PEa'-Ji LU/J)S

(Note by the secretariat)

63-3^

... . v .■.TID3 COVERING OF P2.AK LOADS ■-...,.. ; : Intr odiicti on

Since electric po-^er has to be generated at the moment at which it is consumed, the load that has to bo met at any given time by electric power plants is determined by consumer demand alono. It is obvious that this load varies within rather broad Hsiis? depending or. the habits of consumers, and is characterized by daily, weekly

and annual cyclee superimposed upon p. pattern of steady growth*

The most inpor-iairi factor ^ororning oho preparation both of equipment programmes and of annual programmes for the operation of such equipment; is the trend of the maximum annual load. By means of its investment programme, the electricity under taking has ±o encore that the equipment available is adequate to cover successive annual peak loads* It also has to organize the maintenance and overhauling of

equipment ir_ such a way -that sufficient capacity is available (allowing for breakdown probability of only r fev years? per century), to cover the peak load each year.

However, the coveriEg oi daily peak loads also raises certain technical and

economic proclecic vMch have a bearing >oth on the operation of the generators and

transmission oqniv-roir; ■■ ?7 v-te no-Work and <:a the choice of the investments necessary for its gredua?. oxtc:ini:ru, xt is w:th all these problems and the methods used in their solution !>hnt t'.io prc;;on" docament is concerned.

The var-j.ri+ior.s of Vr.o da,i.'.y load curve of an electricity network have a

characteristic pattorn -.vith onj? wo or e'en tliree peaks (morning, afternoon and evening) as well an £ glcch period at night.

A charpx-beristic load variation also appears during the week, and different load

curves can be dic.ti.r.gv.-ishQd Pars

- the workii-,rr dayr, c^ -i h-^ ^ot-k (Tuesday, ViTednesday? Thursday, Friday) - the days bofo:;o v.ztf after holidcys (Saturday, Monday)? and

- holidays (Sunday).

Finally- i;> cddiVicn to weekly variations? there are seasonal load variations, which are reflected in changes in the values and form of the daily load curve during

the various seasons of 'oho year*

Variations in the load of an electricity network are caused by a great variety of

factors, which can? howeverr be classified as follows:

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page 3

(a) The number* nature, and characteristics of the consumers;

(b) Naiural and atmospheric conditions in the region supplied by the; network i

in question; ■■- .'■ : . • ■ ■ , ■ ,, ,

(c) Working hours* public holidays and vacations; - . ' ■ (d)- The daily activities of the population and its level of living;

(e) Measures adopted by electricity suppliers*

With respect to the nature and characteristics of the consumers, electricity:

consumers axe generally classified by broad categories? such as industry* transport^

agriculture, handicrafts, trade and households- The gradual changes which take place in the structure of these groups of consumers as a whole lead to the modification of

load curves.

The use of electricity for heating, lighting and other purposes depends on the

natural and atmospheric conditions prevailing in the region supplied by an electricity

network.

For this reason, the hours of sunrise and sunset, brightness and temperature, :

govern the shape' of the load curve- -

The load curve is also influenced by the relation of the working hours of the population to daylight hours as well as by its daily activities. .

The introduction of sunnnertime, which brings working hours more into line with daylight hours,; leads to a reduction in peak load in certain cases* •

■- The introduction of the five-day working yreek modifies the Saturday load curve^

because industrial consumers no longer take pov/er from the network....

Lastly, electricity supply undertakings also influence the load curve more or less directly by Deans of tariffs? agreements with customers or statutory regulations*

In this connexion; the usual practice is to encourage consumption during, off- • peak periods and to reduce it during peak periods-

In addition to periodical load variations, a steady increase in consumption is taking place, even in the most developed countries* This results to some extent from population growth, but to a much greater extent from the increased use of

electric power in industry, rail transport and agriculture and for domestic purposes, partly in substitution for other types of energy. This annual increase in electric power consumption may amount to between 15 and 20 per cent in countries where

electrification is still in its initial stages, although it tends to decline as

electrification proceeds iand"leve'ig~(5ff at about 7 per cent, a figure which appro5criiStTs"t"6' tfKe" "do^Tlfig^l-n'coliLStimption e^ery ten-years in industrialized ■

countries* "

The maximum animal load is aiso; showing a gradual upward trend,: However, in countries where^e^ec^rifriSa^ the increase in the maximum

annual load is generally less pronounced than -that in total consumption, because,'

as electrification'proceeds, the uses of this form of energy multiply and consumption gradually increases. In other words, in countries where electrification is in

progress, -theioad" factor tends to increa'se, although this tendency soon becomes less : marked, and in the developed bountries the "annual increase in the maximum load1 ■

corresponds in practice to the increase in total electric power consumption;

It will be noted that the regional interconnexion of small local networks tends to'result" in a substantial increase in the load factor of the interconnected network : as a whole, owing to both systematic and random variations in individual consumption;

The benefit derived from these variations as a result of the interconnexion of regional networks into large interconnected grids is, however very small, because

"diversity doesn't pay'Wice". Another variation relates to the exact time of the peak load. This is influenced by the habits of the consumers and depends on the time '

of twilight* which varies with both longitude and latitude. This variationy however, is slight and could not, of itself, justify the construction of interconnexion line;sV

2• Technical aspects of the covering of peak loads

The regulation of generating equipment in accordance with load presents a number ' of technical problems* Although hydraulic turbines can tolerate substantial

variations, this is not true of steam turbines or hif^ capacity boilers* It often takes more than ten hours to start up large thermal sets with a capacity of 100-150 MW and the load of ohese machines cannot be maintained for long at below 10 per cent of the rated load without risk of damage, I/tore over? high-capccity generators may also be damaged by too rapid" or too freauent load variations, The load variatioh;i6f nuclear

power stations will also probably be Ignited*

' Nevertheless, all these problems have been? or gradually will be j solved as a1 result of improvements in the design of■ equipment and in operating techniques. The problem of seasonal variations is dealt with by means: of an adequate inspection

programme for thermal povrer plants in order to1 ensure that almost, all thermal 'capacity

is available during the winter months when the load is heaviest TBiere day-to-day operations are concerned, load curves can be estimated with a high degree of accuracy.

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page 5

Thus, electric power plants are operated on the basis of pre-established programmes which make it possible to distribute load variations among the largest number of units in service and to reduce the extent of variations in any one unit.

Use is now also to be made of high-capacity thermal sets to regulate load variations?

either by shutting them down and starting them up rapidly, or by allowing them to run at a very reduced load.

The interconnexion of thermal and hydro-electric plants has also led to the more rational utilization of the power produced by making it possible to employ the

latter plants as load regulators* Where hydro-electric plants have been unable to

fulfil this function owing to insufficient capacity? plants with pumped storage have been constructed,, Other methods of covering peak loads include:

- steam plants specially designed for adaptation to load variations at a good average efficiency;

- steam plants specially adapted for overloading during peak hours;

- obsolete steam plants specially adapted for operation during peak hours;

- gas-turbine plants* either equipped with free-piston gas generators or aircraft-type jet engines, or with a tank of compressed air, replenished by electro-compressors during off-peak periods;

- hydro-electric plants with reserve equipment, etc.

The technical problems which arise in this field can thus be solved in a wide variety of ways* The choice of the solution to be adopted to meet load variations

in any given case is therefore essentially a matter of economics.

^• Economic aspects of the covering of peak loads

Equipment for the production, transmission and distribution of electric power involves substantial investments, the total amount being determined by the maximum demand. The annual servicing charges on these investments, plus salaries, wages and other administrative end operational costs, represent the fixed operating costs of an

electricity undertaking-

Account must also be taken of certain costs which are proportional to the amount of electric power produced. These costs depend mainly upon the price of fuel in the case of thermal power plants and do not arise in the case of hydro-electric plants.

Every increase in the load factor is reflected in a reduction in the average cost

of the power produced.

The first economic problem which arises in this connexion is therefore that of

■■■:.■ ■-. .■■ -\t ^r . . . : ■ ' ■ -■ r-. - " •- . ■ '■ . ■- ■ :- ■:■"■". -" . improving the shape of the load curve and reducing the scale of variations, in short, of obtaining the highest possible load factor. This can be achieved by a suitable tariff structures

- the two-part tariff, comprising a fixed charge, depending on the maximum demand, and a fixed price per kWh, so that the consumer is encouraged to increase his consumption without increasing the maximum load;

- the dual tariff, under which the price per kWh used during the peak period is higher than the price per ktfh used during off-peak periods;

- the night-hour tariff, under which electric power can be used at a reduced price during the nightj

- tariffs based on the marginal cost of the electric power and consisting of fixed charges depending on the power contracted for in each of several clearly defined periods? and a fixed price per kti/h used? also depending on the period during which it is used0

The last-mentioned tariffs are based on the principle that the value of electricity varies according to the tine at which It is used. The instantaneous value is equal to the partial cost per kli/h produced in the least economic plant which has to be operated in order to cover the load*

In addition to the above list, there are the special agreements concluded with certain industries* stipulating, for example, that the consumer shall reduce the load during peak periods or giving the electricity undertaking the right to interrupt supplies, if necessary; in order tc reduce the peak load.

The interconnexion of local networks has a two-fold advantage. In the first place, owing to the diversity of local loads? it improves the load curve of the

i . . . i . . . --.■-. - ,■',■■..■

interconnected network as a whole, and, in the second place, it tiakes it possible to use a larger number of generating sets in the economic distribution of the load. However, interconnecting feeders onta.il capital investment and maintenance costs, the amount of which depends on the length, voltago and transmission capacity of the network.

The construction of interconnexion networks and their gradual extension therefore raises complex economic problems in which the advantages they offer as a means of covering peak loads represent an important factor,

* ■*

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Once the interconnexion has been effected? another series of economic problems arise in respect of the optimum economic distribution of the load for any given load

curve •

A programme for the operation of all available generating sets must be drawn up on the basis of load forecasts* For this purpose, accurate methods which take the influence of weather conditions into account? have been developed for making detailed forecasts of the load curve.

Generally speaking, programmes for the operation of plants are then drawn up by placing at the base of the load diagram the production of run-of-stream hydro electric plants* geothermic plants and plants for the combined production of heat and electricity whose production of heat for the requirements of the heating system

generally governs the production of electric power.

In &scend:;_n2 orrder oJ' tho 3p&c:.:?i2 ccjt oC fual consumed per leWh, these are followed by thermal and hydro-electric storage plants- In estimating the production of these latter plants* account is taken both of the energy stored in their water reserves and of short and long-term forecasts of the rate at which these reservoirs will be filled. The problem of load distribution is further complicated if allowance must be made for the limiting capacity of the transmission lines comprising the

interconnexion network^, transmission losses due to load transfers, additional fuel consumption due to the shutting-down £,nd re--star-;ing of certain thermal plants, the use of gas turbines and plants with pumped storage for covering peak loads and the

efficient filling in of troughs in the load curve by consumption for pumping purposes.

A final, series of economic problems concerns the choice of investments in

additional generating arjl transmission equipment to meet the steadily growing demand.

For this purpose, the number of plants of each type to be constructed must be determined, bearing in mind the role of oach such plant in the operation of the network as a whole D :

' From the economic point of view, the various types of generating units, such as steam plants; diesel plants, gas turbine plants? run-bf-stream hydro-electric plants, hydro-electric storage plants? plants with pulped storage etc3, differ with respect to the specific cost of construction per kTF and the partial cost of each k$h produced*

Generally speaking, plants whose construction costs ore relatively high? such as hydro-electric plants with seasonal reservoirs and thermal plants with high steam pressures and temperatures produce electric power at a relatively low partial cost per kffih.

They must therefore be operated at a high load factor. On the other hand, plants whose construction costs are relatively low, such as diesel plants and gas turbine plants, produce power at a relatively high partial cost per kfilh and are

economical only if they are operated at a relatively low load factor, in other words, . if they are used to cover peak loads and to provide make-up supplies in emergencies..

: It is also necessary to determine the economic size of these different plants, since this factor has a bearing on their efficiency, operating costs, etc. and on the reserve capacity that must be provided in order to maintain breakdown probability within acceptable limits.

Furthermore, the expansion of the interconnexion network must be planned in such a way as to take full advantage of the different characteristics of each category of generating plants* In other words, many extremely complex problems are involved, the solution of which is farther complicated by uncertainty surrounding certain basic data, such as fuel price trends1.

Rule of thumb methods are frequently used in their solution., For example, the first step taken is to determine the number of plants of the same type needed to meet foreseeable demand and the economic size of their generating units. The next step is to determine how this set of plants can be made more efficient by replacing some of the plants by peak-load plants.*

Operational research methods have also been applied to this problem, linear programming techniques being used to draw up a rough construction programme, which is then elaborated in detail by means of marginal studies. The use of digital

computers makes it possible to compare a large number of solutions by simulation studies? and to select the most advantageous. In the case of networks in the

developing countries, however} the problem is often more straight forward and'rough selection methods generally produce satisfactory results.

4» Results of the study of problems relating to the covering of peak loads If is clear from vrhat has been said above that the covering of peak loads in tlectric power networks.raises a number of extremely complex.technical and economic

..pr oblens. ■ ■ .. : ■ ■ ■ - ■■-.•■ ■; . "

. ■■;. Since it was felt that all these problems could usefully be studied, the Committee on Electric Power'of the Economic Commission for Europe set up an Ad Hoc Group of Experts to Study the Covering of Peak Loads. This Group of: Experts

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prepared a document, published under the symbol ST/ECE/EP/l9, dealing with all the

problems in this field, as well as the methods used in their solution.

In addition, a Symposium on Beak Load Coverage was held at Venice in May 1963, . to which seventy-six reports were submitted* The main theme of this symposium was A subdivided into the four following sections:

- General aspects of the problems posed by the load fluctuations;

- Solutions to the technical problems of load fluctuations in connexion with the construction and operation of thermal power stations;

- Solutions for technical problems of load fluctuations in connexion with the construction and operation of hydro-electric power stations; and

- Solutions to the economic problems posed by load fluctuations*

Document ST/ECE/EP/22 contains detailed information on the organization of this

Symposium, a list of the seventy-six reports submitted, and a summary of the

discussions which followed the consideration of these reports-