CA1166310A

CA1166310A - A.c. electricity power supply system and methods of and apparatus for load control thereon

Info

Publication number: CA1166310A
Application number: CA000381599A
Authority: CA
Inventors: Robert A. Peddie; John S. Fielden
Original assignee: South Eastern Electricity Board
Current assignee: South Eastern Electricity Board
Priority date: 1980-07-14
Filing date: 1981-07-13
Publication date: 1984-04-24
Also published as: ATE11471T1; JPH0456535B2; NZ197667A; US4385241A; DE3168465D1; EP0044181A1; AU542286B2; EP0044181B1; GB2080640B; JPS5749332A; GB2080640A; AU7271881A

Abstract

ABSTRACT OF THE DISCLOSURE

In an a.c. electricity power supply system, to guard against a "blackout", that is to say a cumulative supply failure caused by a fault leading to generating stations being overloaded and having to cease supplying a network, consumers are each provided with means responsive to the supply frequency and operative to reduce or cut out the consumer's load when the supply frequency falls below a predetermined value, which may be different for different consumers.

Description

3 :~ ~

. ..... , ~ ~ . .... .

~A~C. EL~CTRICITY POWER SUPPLY SYSTE~ AN~ METI~ODS

~ACKGROU~D 0~ T~E ~~VENT~ON
.
S lo Fi~l~ of the Inv~ntion This invention relat s to ~.c. electriclty power supply ~y5tcm5 and to methods o~ and apparatus for controlling th~ load thereon.
In some parts of the world, there have in recent years been widcspread power ~allures lastlny many hours arising ~rom the power d~mand excaoding the supply. Such failures can occur partlcularly ln syst~ms which arc not ~trongly interconnected and where, for ex~lple, damage o~ a 5ingl~ supply llne can deprive a wide area o~ ~ subst~ntial ~ource of power. ~hen this happens, the oth~r generatlng stations, w'lich still r~main connected to 3upply the loa~, become over-loadcd~ The ovcrload~ns 1~
. accumulative a~d rap~d wlth thc con~equent necessity fox ~mmed~te opcniny o~ circu~t ~re~kers to disconncc~
~he load. ~n xec~nt ye~rs wid~spread loss of power o~ this n~ture, commonly r~crrcd to ~ a b.l~ekout, ,' ~.

3 ~ ~) ha~ occurxed ~n a numbe~ o ountr~e~
20 ~r~o~ ~rt V~rious lo~d manag~m~nt ~f~t~m~ hav~ been propo~ed to ena~le a utility to ~ect some control of power d~m~nd at con~umer's px~mi es by the tr~nsmlssion o control signal~ from th~ utility to e~fect s~lcctlve swltching out of loads, thereby enabling a rcductlon u demand tG be obtain~d by selectlve swltchlng of non-essenti~l or less essential loads. Such techni~ues however are not ~act enough to prevent a blackout such as ha~ bcen described above.

SUMM~RY OF T~{E I~VEN1~ION
It is one o~ the ob~ects o~ the present: lnventio to provide an ~nproved method of and apparatus or s~feguarding an alternating current elcctrlc power ~upply system against such a bl~ckout.
Th~ pre~ent invcntion makes use of the drop ~n th~ suppl~ ~requcnc~ whi~h occurs when ~n alternator ~t a generating sta~lon becomes o~erloaded.

,.

16 3 ~ ~

According to one aspect of the present invention, a method of safeguarding an a.c. electric power system against the effects of the power demand on the system exceeding the generating capacity of the system comprises the steps of, for each of a plurality of consumers, sensing when the power supply frequency falls below a predetermined frequency by using a phase-lock loop arranged for synchron-ising an oscillator with the power supply frequency, the phase-lock loop being arranged to lose phase lock when the power supply frequency falls below a predetermined frequency, sensing the loss-oi- phase lock to provide a control signal, and interrupting the power supply to at least part of the load taken by each said consumer in response to said control signal indicating a fall of frequency to below said predetermined frequency. Preferably, for each consumer, said at least part oi- the load is reconnected to the supply when the supply frequency rises above said predetermined frequency or above a higher frequency.
Wi-th this method, automatic disconnection of loads occurs as soon as the frequency drops below some predetermined value. It is not necessary to use the same predetermined frequency for every consumer. In general it is preferable that the load shedding is not instantaneous but occurs over a short period of time so that the system comes to a stable condition where the generating capacity can meet the connected load demand. More particularly r it is desirable that any automatic reconnection is effected gradually to prevent over-loading of the system. Thus said ~663 ~ 4--prc~c~.'ned r~e~uency ls pref~r~bly ~ ~requency wl h~n ~ predetermlncd frequency band or b~nds~ wlth ~h~ ~xac~ ~requeno1es d~fering ~or difforen~
consumers. ~t ~s lnev~t~blc ln pxactice that the ~xact frequency ~ operatlon ~n any practlcal s~stem will ~y slig~tly for dif~erent consumer~. Wlth ~
50 Hz supply requency, ~ might b~ arranged that thc consumexs are eu~ out at a ~requency within the ra~ge ~f 46~5 to 48~5 Hz. Thus, a~ the frequency drops~
some consumers w~ll be d~sconnected when the frequency P~lls to 4~5 and gradually more wlll have their 8witch~s opened ~s the frequcncy ~alls further;
~11 or su~stant~all~ all would ~ave lost power by the tlme the frequency is down to 4G.S~
lS ~t ~s preferxed to ~ve automa~c re~toration o th~

power suoply ~l~en the ~re~uency rises ~nd, to ~llow a mar~n to pre~en~ switch~n~ on and o~, the supply ~hould be restored to a co~sumer when the frequency ~ sl~ghtly htgher th~n the fxequency at whlch it

2~ pr~ousl~ cu~ of~. -.
- Th~ load switch~ng ~or cons~mers may be e~fectcd tn two or mor~ ~tep~, s~ttch~ng of~ of p~rt of th~ lo~d occ~rrl~g when the ~-equ~nc~ ~as ~allen ~o a flrst ~lue ~nd further s~tch~ng bclng ~ccted when ~he ~5 frequency falls to a s~cDnd lower value.

.. ' ^`` '' ~

~ :L 6 6 3 ~

Xt wlll ~e r~dlly ap~x~n~ ~h~t dcslr~bly ~11 or ~ubs'cantl~lly ~ll coslswr er~ should h~Ye their pow~r controlled ln thi~ w~y althou~h ~t Is obviously po~sible to provide sa~eguard~ ~or c~rtain prlorlty lo~d~, ~uch as hospital , kldncy machln~s etc. ~o that they rcmain connectcd to the supply dcs~lte thc lc55 o ~requency~ Since the control me~ns ar~ now a~sociated with the indivIdual cons~mcrs and ~a~ 1~
~ecessary b~ ~s~ociatcd with Individual loads, ~uch prioxity XacLl~tie~ may be pro~Ided.
It has prevlously be~n proposed, see ~or example oux Canadian Patent Application Serial No.34355~ ~ilcd ~anuary ~th 1980, ~o c~ect measuremcnt o oner~y consumptinn by a consum~ supp11ed with altcrn~tlng curr~nt power by ma~ing u5e o data procosslng mcan~
havlng a clock, controllin~ the dat~ procc~ing, which clock 1~ synchronl~d wlth ~h~ incom~n~ malns frequency.
In ~uch a system, employlng a phase-lock loop ~o ~ynchronise the clock frequency witll ~he lncomlng malns, there is lnhcrently a slgnal av~ila~l~ rcpres~ntatlYe o~ the lncomin~ mains ~requency or of the devlatlon o the malns ~requency ~rom a datum~ This ~ignal may be U~Pd for controllin~ the a~ore~entlon~d ~w~tch ox ~witches for int~rrupting th~ 5Upply 0 th~ ~oad or load~ on the consumer'3 prem~se~.
phas~-lock loop will commonly operato only ov~r a ~imlted ~requency rang~ and it ~5 convenl~nt to makc th~ switch mcans operativo ~o disconn~ct tho load vr s . . ..

3 :~ ~

loads when the phase~lock loop fails to maintain phase synchronisation due to a fall in frequency. If the phase-lock loop fails to maintain synchronisation, the phase error increases rapidly and this signal may be used for effecting switch control.
According to another aspect of the invention, in an a.c. electric power supply system ha~ing generating means arranged for supplying power to a plurality of consumers, at least some of said consumers are each provided with means for sensing when the power supply frequency falls below a predetermined frequency, said means for sensing a fall in frequency comprising an oscillator, a phase-lock loop arranged for synchronising the oscillator with the power supply frequency, the phase-lock loop being arranged to lose phase lock when the supply frequency falls below the predetermined frequency, and means operatively responsive to loss of phase lock of the loop to provide a control signal, and switch means operatively responsive to the control signal and arranged for interrupting the power supply to at least part of that consumer's load in response to a sensed fall of frequency to below said predetermined frequèncy.
Each of said least some consumers may have means operative to reconnect said at least part of that consumer's load to the supply in response to a sensed rise of the supply frequency above said predetermined frequency at which the switch means for that consumer was operated to interrupt the supply or above a higher i3~

frequency.
The aforementioned switch means may control supply to only part of a consumer's load and further switch means may be provided operatively responsive to sensed fall of frequency to a still lower value below that at which the first-mentioned switch means is operated.
The control switches are preferably solid state switches and may be switches provided for some other purpose, e.g. for automatic control to limit maximum demand. It will thus be seen that the protection of the system against a blackout due to demand exceeding the generating capacity can readily be provided on consumers' premises with little additional equipment beyond that which is installed Eor other purposes.
According to a further aspect of the present invention there is provided apparatus comprising means responsive to the frequency and alternating current electric power supply being fed to a load and switch means controlling the supply to the load which switch means are operative to cut off the supply when the frequency sensed by the frequency responsive means falls below a predetermined frequency, said means for sensing when the power supply frequency falls below a predetermined frequency comprising a phase-lock loop for maintaining the output of an oscillator in phase synchronisation with the alternating current electric power supply, said phase-lock loop being operative only to maintain synchronisation when the power supply frequency is above said predetermined frequency, and means for sensing loss of phase lock. Preferably means are ~, . . .

3 1~3~

provided for automatically restoring the supply when the frequency rises above said predetermined value or above a high frequency.
The switch means conveniently comprises a solid state switch or switches. Said phase lock loop may be operative to l~aintain phase lock over a frequency range around the nominal frequency of the supply system but to lose lock when the frequency falls below a predetermined value; in this case the switch means may be made responsive to the loss of phase-lock in the loop.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a block diagram illustrating part of an electricity supply system supplying alternating power to consumers and diagrammatically illustrating one embodiment of the invention;
Figure 2 is a graphical diagram for explaining the operation of part of the apparatus of Figure 1; and Figure 3 illustrates a modification of part of the apparatus of Figure 1.
DESCRIPTION OF THE PREFERRED ~MBODIMENTS
Referring to Figure 1 th~re is shown diagrammatically an electric power generating system ha~ing a number of generating stations 10, 11, 12 interconnected by a power distribution network indicated diagrammatically . , - . .

3 ~6S3~t~
9~

at 13 whlch networ~ ~e~ds ~ l~rg~ number o~ consumers o~ whlch three are sh~wn ~t 209 21 and 22. The co~u~er~ 2~ 22 ~re inc~cat~d only dl~gr~m~ lc~lly whlle consumer 20 13 shown in slightly further detail as having a slnglc p~aSQ supply on two l~ds 30j 31 lead~ng to a plurallty of lo~ds shown at 32, 33 ~nd 34.. Xn t~L~S part~cular ~xample the loads 32 and 33 - cons~itute the ma~n loads o~ the cons~mer 20 whll~
the load 34 is a small load x~quired for certain essential purposes and which lt is requlrecl not to intexrupt i~ possibl~ when pxotectin~ th~ system ~g~lnst ~ "blackout" due for example ~o lnt~rruption ~r d~mage to a pc~er line 15 connecting one o~ the generators 10 ~o the power dic;trlbution network 13.
lS ~ solid state switch 36 is provid~d ~ n the supply to th@ loads 32; 33, th~ s swltch being controll~d by a æw:Ltch control sign~l on a lead 37.
~he present lnvention .~ concexncd more particularly with the operat:lon o~ such a switch ~o ~nte~xupt t~le power supply to consumers ' loads in the ~Y~nt of a gross failure resu~ting in ~he power dem~nd .

63~

~xceed~n~ the gener~t~n~ capAc~ty ~ tho 5y5te~. ~S
pxcviously explained, ~hen th~s oCCUr5~ thc frcquenoy of thc supply ~mmedlately be~lns to drop. It is ~onv~nient in ~e following dcscrlption t~ rcfer mor~
~pecif~c~lly to a supply at a nomlnal requency of 50 ~lzo It will be readtly apparent however that ~im~lar arr~ngements can be employcd for supplle~
at other ~requcncies~ c.g. 60 t~z~
The apparatus ~n tAe consumer~s prem~ses lncludes a clock ~ox d~tal d~ta process~ng of ln~ormation, wh~ch clock ~s normally sync~ronised with the incominq mains suppl~. Th~s clock comprises essentially a voltase controlled osclllator 40 operating ~t a ~requenc~ ~h~ch ~s a multiple of the mains supply lS ~re~uency, t~pically several ~housand times thc ma~ns æu~ply ~xequen~y. The output of the oscillator 40, ln dig~tal ~rml i~ dlvided down ln ~requency by a digital d~ider 41 to prov~de~for ex~mp1~,short durat~on pulses to a sampler 42 for sa~pl~ the ~ns ~ ~eous ~0 magnitudes o~ the voltage and curren~ wave~orm~ and thcr~y t~ proYldP d~ta for metering mean~ 4~ d~ter~ning the energy consu~pt~on o~ the consumer. ~h~ divider ~l provlde~
an output signal at ~h~ mains supply ~requency ln the ' ~orm o~ ~hort dur~tlon pulses th~ timing of whlch is . ~ .

3 1 t~
-~1 corP.p~,red ~n ~ m.~sX~tox 4~ w~t~ tl;nc o short àurat~on pul~ies ~t t~e ze~o ~olta~e crossover point o tho lncom~ng volta~e w~ve~Eorms on llnes 300 31.
For this purpose there iS ~ vol~nge tran5~0rmer 45 a,CrO55 the llne~ 30, 31 havlny an output windlng pro~riding ~o ~voltage ~ave~orm to the comparator ~mlt 430 The time di~erence detcnnlned by thc comp~r~tor 43 is measured digitally utilis~ng clock pulses ~rom ~e divlder ~1 on a 1~3ad 46 and the ~0 digit~l output is ~ measure c: f the phase error betwe~n the clock ~requenc~ and the tncvming s~avefor;n frc~uency~
This p~ase error ~s intcgrated dlgitally in an ~ntegrator 47 to provide on a lead 48 thc control slgnal ~ox ~he volt~ge controllcd oscillator 40. Such a phclsc lS control s~stem operates to control the requency o~ th~
oscilla~or ~0 so that tha output thereo~ is synchronlsed in phase with the incoming waveform. The phase error on the output ~rom the compa~a~or 43 is thu5 nonnally small so long as ~ynchronis~tion 1~ maintained. If synchxon~sation i~ lost howev~r th~ ph~se error 13 lncreased. ~os~ o~ phasa synchronisatlon can he axr~nged to occur as the ~re~uency decrea3es ~ ~ome predeten~in~difr~qucn~yt ~or example by limlting the x~ng~ of th~ output 5ign~1 ~rom the ~nt~yrator 47.
Th~ sudd~n incrcas~ ln phase ~rror is applied ~o 3 :~ ~

12~

control unl~ 50 ~o genarate a control ~iynal on the l~ad 37 ~or operat~ng ~he ~w~tch 36 to ~nterr~pt he supply to ~hc loads 32, 33.
It will be apprec~ated th~t tl~er~ are m~y other possible ways ln ~hi~h the reductlon ln ~re~uency of ~he mains ~upply ca~ be determined and utilised to operate a 5wl tch o Flgure 2 is a graphlcal diagram ln which curve 60 shows th~ relattons~ip be ween phase error (plo~ted as ordinates) and ~requency tplotted a3 abscissa) ~or one part~cular ~orm of phase-locked oscillator. As ~he frequency ~lls~ the output of the oscill~tor g~adu~ lags behind t~e incoming voltage waveform o~
the mains suppl~O At a re~uency of 49 H~., thcre l.s lS ~ phase error of 8. Th~ phase error i5 indicatlve of ~he control signal appl~ed to the aforementloned voltage control oscillator 40~ When some p~rt~cular phase error ha~ been reached, depending on the design o~ ~he equtpment, the pha~e-lock loop will no lon~er ~o b~ a~le to hold the ~re~uency to glve ph~se-lock ~th ~e incomlng waveform. When th~s occurs, ~he phase ~ror detenmined by the comparator 43 wlll ~n~rease and ~he swi~ch 36 wlll be oper~ted~

.
~.. :. .

.

Xt ~ envls~ged that, ~n ~,~ener~1,, all c:onswne~s ~hould ~lave ~ contro1 ~or s~utt~rlg vf thc loa~ whcn the frequency drops, slmllar to that ~hown for consumt~r 20 vf ~igure lo It 18 1nevltab1e that tht3 tolcra~ces S o ~ ~en~ ~ the phæloc3c 5yStan5 ~02' ~ercnt ~nsur~rs will ql~ san~ spre~ to the fra~n~ at whic~ the various ~wlt~cs 36 w~11 operate. In a ~ypical system, the spread rnight be over 2 Hz~ ~ng. from 46.5 Hz to 48.5 }l~. with ha1f the consumers 1~ng power when the frequency has droppe o some intermediate value, typica11y 47.25 Elz.
In ~gure 2 a curve 65 is showr. which is a dis~ribution curve ind~:cating the proportions o~ consumers whicn will lc: se lock at any p~rticul~r frcquency in thc ~xequency r~nge. It wilI be seen tllat ~11 cons~ners remain in lock at frequenciec~ above 48.5 Hæ but, when 1the ~requency ls dot~m to 46 ~læ on~y a small proportio~, . ~bout S~, wlll b~ left on. This curve makes allowa3lcc!
!eor the ~ac~t th~t certain consumers or certain ir~Aivl~lual loads w~.ll no~ be swltched of by the drop in rcquency, : 20 ~helr supplies belng maintained for sa~ety or other xeasons_ I~ m~y be that some consumers w111 have ~ir phase-10ck loops s~ to maint~in phase down to sl~nific3nt1y lower ~requenci~s than other~.

6 3 ~

It w~ c ~srsnedlatel~ appal-ent that, in th~
event o~ 501~e gro3~ ~055 o:E p~3wer ~n thc ~en~rat~n~
system" the r~sult~nt drop ~n fx~ou~ncy ~ttill i~unediately c21use swltches 36 at th~ varlous consumers to be opened ~o as to reducc loadO In general ~ha xeductlon ln load will readily be made sufficient to malntain ~;tability of t}lC system provided some signif1cant - generat~ng capacity st~ 11 remains.
1~ the generating capacity is restored or i~ there ~s d~sconnect:ion of loads st~ll being fed wlt:h powcr, the frequency of the system will tcnd to rise. With ~he form o~ ph~sc~ock loop descrlbed above for the consumer 20~ when the frequency becomes suficicntly hlg}~, ph2se synchron~sat:ic)n will be re-ost~lishod ~nd .5 hence the slgnal to the control unit 50 will be removcd.
~n genera~, au~com~tlc restor~t:lon of the supply to the loads by closing th~ switch 36 would bc el~ectod by the contxol u~it 50. Pxo~ ion may be made ho~ever ~ ~or exampleg for au~oma~lc rests~rat~on o~ 5upply to certain loads bu~- ~or restora~ion o~ supply to other loads on ly c~ manu~lly resetting swltches . Because o thQ inheren t ~y~teresi~ effect in . h~ control loop, the frequency ~t which the restoration o~ power occurs will be hlghcr ~an thc frequ~ncy ~ whlch ~hc power was swltched off.
me dashed line curvc! ~6 lllustr~tes, for the particul~r , .: .
.

3 l ~) ~n~.bod~cnt undcr consi;dcr~at~on, the nunlb~r~; of con~ rs wh~cl~ w;lll ha.ve power res~orcd at the var~ous di~Qr~nt fxe~uencl~ a~ ~:he ~r~qu~ncy rises. It wlll be notcd ...... th~t ~he frequcncy must Xi5C to ~k lea~t 49.5 }iz b~fore 1:he l~st load ls rcstored. Howe~,er restor~t:ion Q~
loads cosnmence~ as soon as the ~requ~ncy bcglns to ri~e.
~n ~ha abovc-de~3cribed embodlment of th~ invcntion, the ~ailur~ a phase-lock loop to malnt~in pha~;e-loc~;
between a clock signal genexator and the lncoming lC) ~r~veform h~s been utilised t:o cont:rol the swltchlng, Such a phase-lock loop controlling a clock slgnal genexatox may ~o~n p~x~ o~ equipment for meter~ng tha l?ower consumption oi~ th~ indivldu~l consumer, ~s ~or !`' ~x~nple is de~cribcd in the speclfic~tion o~ Applicatlon No.343558. Thc swltch 36 ma~ be a switch u~lll3ed also fo~ load man~gement, e~ g~ for the automatic regul~tior o ~ha maxim~n d~m~nd by ~he cons-uner, utill~lnq nutoma~lc maximum dcmand rc~ul~ting apparatus. It will thus ~e ~een that the protection for tho sy~tem n~ins~ a large~scale blackout due to for ex~mple sudden loss of g~nerating capac~t:y, can b~ obtained wlth vcry li~
ndd~tion~l ~qulpment on the con~umer ' ~ p~esnises beyond that whlch ma~ be lnstallcd for other purpos s. T~
pro~ection for t:he sy~tem is ob'cained by 3witc~l1ng a tha .

} ~3~(~

ol6 cons~e~ pxe~ s. No ~nall~ng ovcx th~ tran~ssion lines ox ~ ~ther ~ean~ ~s re~u~red and thcre ls no need to pro~de ~ny special equlpment at tS2c generatin~
~t~ions or on the network 3part from the cq~lipmen~ on ~he consumer~s premlse~O
~n Flgur~ 3, there is shown a modl1cd orm of part of the apparatus of Figure l ~n which the sw~tchlng off o~ p~rts of the load at a cons~ex's pr~mlses occurs in two stage~. In Figure 3, the same reference c~aracters are used as ln ~i~ure l to denote corresponding components and mention wlll be made only o ~he distinctive feature~ of Figure 3. The consum~r ~s 5hown a~ havlng a ~urther load 70 which is o~ low pr~orityO A triac swltch 71. ~or cuttin~ off the supply to this load 70 is controlled ~ a compar~tor 7~
comparin~ the voltage output ~rom the compar~tor 44 wi~h a reference voltage from a x~erence source 73. This xe~erence ~olta~ ~s ohosen so ~h~t the s~itch 71 is ~perat~d to cut the ~uppl~ to load 70 when the ~requency has fallen to some predetermln~ value interm~di~te betwecn the normal suppl~ requency and the frequ~c~
at ~htch ~h~ 5wltch 36 operate~. P~ti~l loa~ sh~d~ln~
~n ~his wa~ by a lar~e nu~er o consumers m~y o~ten b~
su~c~ent to preven~ any fur~her ~requency drop bu~
2~ t~e ~wltche~ 36 woul~ be op~ratlve in tho e~ent of rlous overlo~dl~g of the supply network.

,

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-1. A method of safeguarding an a.c. electric power supply system against the effects of power demand on the system exceeding the generating capacity of the system comprising the steps of, for each of a plurality of consumers, sensing when the power supply frequency falls below a predetermined frequency by using a phase-lock loop arranged for synchronising an oscillator with the power supply frequency, the phase-lock loop being arranged to lose phase lock when the power supply frequency falls below the predetermined frequency, sensing the loss of phase lock to provide a control signal, and interrupting the power supply to at least part of the load taken by each said consumer in response to said control signal indicating a sensed fall of frequency to below said predetermined frequency.
2. A method as claimed in claim 1 wherein for each consumer, said at least part of the load is reconnected to the supply when the frequency of the supply frequency rises above said predetermined frequency or above a higher frequency.
3. A method as claimed in either claim 1 or claim 2 wherein said predetermined frequency is a frequency 6. An a.c. electric power supply system as claimed in claim 5 wherein each of at said least some consumers has means operative to reconnect said at least part of that consumer's load to the supply in response to a sensed rise of the supply frequency above said predetermined frequency at which the switch means for that consumer was operated to interrupt the supply or above a higher frequency.

7. An a.c. electric power supply system as claimed in either claim 5 or claim 6 wherein the switch means are solid state switches.

8. Apparatus for use by a consumer using power from an a.c. electric power supply system comprising means responsive to the frequency of the alternating current electric power supply being fed to a load and switch means controlling the supply to the load which switch means are operative to cut the supply when the frequency sensed by the frequency responsive means falls below a predetermined frequency, said means for sensing when the power supply frequency falls below a predetermined frequency comrpising a phase-lock loop for maintaining the output of an oscillator in phase synchronisation with the alternating current electric power supply, said phase-lock loop being operative only to maintain synchronisation when the power supply frequency is above said predetermined frequency, and means for sensing loss of phase lock.

within a predetermined frequency band or bands, with the exact frequency differing for different consumers.

4. A method as claimed in either claim 1 or claim 2 wherein the load switching for consumers is effected in two or more steps, switching off of part of the load being effected when the frequency has fallen to a first value and further switching being effected when the frequency has fallen to a second lower value.

5. An a.c. electric power supply system having generating means arranged for supplying power to a plurality of consumers wherein at least some of said consumers are each provided with means for sensing when the power supply frequency falls below a predetermined frequency, said means for sensing a fall in frequency comprising an oscillator, a phase-lock loop arranged for synchronising the oscillator with the power supply frequency, the phase-lock loop being arranged to lose phase lock when the supply frequency falls below the predetermined frequency, and means operatively responsive to loss of phase lock of the loop to provide a control signal, and switch means operatively responsive to the control signal and arranged for interrupting the power supply to at least part of that consumer's load in response to a sensed fall of frequency to below said predetermined frequency.

9. Apparatus as claimed in claim 8 and having means for automatically restoring the supply to said load when the frequency of the supply rises to a value above said predetermined frequency.

10. Apparatus as claimed in claim 9 wherein said switch means controls the power supply to part of a consumer's load and having further switch means operative at a different frequency for controlling the power supply to another part of the consumer's load.