Application Date: March 19, 1936. No. 8295/36.
Complete Specification Left: March 18, 1937.
Complete Specification Accepted: Nov. 19, 1937.
Improvements in or relating to Voltage Regulating Systems
I, ALAN DOWER BLUMLEIN, a British Subject of 32, Audley Road, Ealing, London, W.5, do hereby declare the nature of this invention to be as follows:-
The present invention relates to voltage regulating system.
It is an object of the present invention to provide a new or improved system for voltage regulation in alternating current networks.
According to the present invention there is provided a system comprising a generator and two or more loads or one or more loads and two or more generators, the generators and loads being inter-connected through tapped tightly coupled impedances, the arrangement, including the magnitudes of the impedance and the positions of the tapping points thereon, being such as to minimise interaction of one source or load on another source or load.
The tightly coupled impedances may comprise autotransformers having impedances bridged across a part or the whole thereof or the electrical equivalents of such circuit arrangements. Such an electrical equivalent may for example be the choke having a central or non-central intermediate tapping, the two parts of the choke being tightly coupled to one another.
If the generators are of the polyphase type comprising n phases, then n tightly coupled impedances must be employed for each interconnection of two loads and a generator or two generators and a load.
An embodiment of the invention will now be described by way of example and reference will be directed to the accompanying drawing which shows the inter-connection of a single phase of a system of polyphase generators and loads.
It will be assumed that it is required to eliminate or reduce the voltage fluctuations of a source such as three phase A.C. mains, so that a supply is available the voltage of which is substantially unaffected by voltage changes of the mains (assuming frequency to be constant) although of course the voltage will be affected by variations of the load. Suppose now a synchronous machine of internal reactance X per phase is bridged across the mains. The voltage fluctuations of the mains will be reduced by an amount EQUAT. HERE where Y is the impedance per phase looking back into the mains. If however the three terminals of the machine are connected to the centre points of three tightly coupled chokes of total impedance 4X each, the mains conductor being connected to one end of each choke and the load conductor to the other end of each choke, the variation of the mains voltage will be substantially eliminated.
An arrangement of the type just described comprising tightly coupled impedances is equivalent to a machine having an E.M.F. (E) but no reactance and having two sets of output terminals and each having reactances 2X in series. Hence any variation of mains voltage will cause current to flow through the reactance on the mains side, but will not alter the E.M.F., which alone is effective in driving current through the load and the reactances 2X in series. The open circuit E.M.F. presented to the load will be the E.M.F. generated by the machine. The volt-amperes which must be capable of being handled, depend upon the variation of the mains volts expected. If the mains volts fluctuate by an amount v, then the volt-amperes will be EQUAT. HERE E, and provided that the reactance of the machine is small compared with the load impedance, and that the load has a power factor of substantially unity, there will be negligible increase of machine loading due to the presence of the load. If however the load is reactive, the machine must provide the powerless volt-amperes of the load.
The above approximate machine ratings were based on the assumption that the machine has negligible losses when running light. It is possible to drive the machine in order to put power into the mains, or to use it as a motor in order to derive power from the mains. It can under both conditions still serve to smooth the voltage to the load. The rating of the machine must be sufficient to carry the additional current required by driving into or being driven from the mains.
The tight coupled impedances described above are of the equal ration (1:1) type, but unequal ratios may alternatively be used, the values being calculated as described in co-pending Application No. 19170/35. Using unequal ratios, it is possible to obtain a low impedance facing the load, but there is a greater risk of the machine pulling out of synchronism if excessive power loads are taken from the arrangement. By employing the high impedance towards the load, relatively poor regulation is obtained, but better smoothing, since there is less chance of the machine pulling out of synchronism if a regulating device is employed in this way.
It is possible to obtain a substantially exact balance for either impulsive voltage variations, or slow voltage variations, but it is impracticable to obtain an exact balance for both, since the impedance of the machine is composed of both the reactance of the windings, which operates immediately, and the reaction of the field, which operates slowly. If the value of the choke is based on the winding reactance, a balance is obtained for impulsive changes. If the value of the choke is calculated on the total impedance of the machine including armature reaction on the field, then a balance is obtained for slow variations of load. The smaller the effects of the armature reaction on the field (e.g., saturated excitation), the smaller will be the discrepancy between the impulsive and the slow balance conditions.
The method described above may be used with advantage to regulate the voltage at the end of a long transmission line where during peak load a separate generating station is used to supply some of the load. If the various machines in the station are each connected through suitable centre tapped chokes to the incoming and the outgoing bus bars, during light loads these machines can run light and serve solely to provide a voltage regulation, the power being obtained from the incoming feeder.
The system is also applicable to the coupling up of a number of power supplies so as to reduce the voltage variation and in particular to reduce the surges on short circuit. For example, a station may have two or more sets of bus bars which are connected to various loads of incoming transmission lines or both loads and transmission lines, and each machine of the station (or the machines of the station as a whole) may be coupled to these bus bars through suitably tapped chokes. An arrangement for connecting a generator to two loads is shown in Figs. 1 and 2 of co-pending Application No. 19170/35 and to three loads in Fig. 3 of that application, which method may be extended to any number of loads or bus bars. The various bus bars will now be isolated from each other as regards variation of load affecting voltage, so that any surge arriving on one bus bar will not affect the voltage on the other bars, except inasmuch as power will be drawn from the machines which may regulate their speed. A very severe short circuit on one set of bus bars will take power out of the other bus bars by way of the machines and as the speed of the machine will not be immediately affected, there will be time for overload devices to operate to isolate the short circuit. Arrangements may be made to alter the tapping points on the reactor so as to divide the reactance to the various bus bars in accordance with the power requirements.
The accompanying drawing shows one phase only of five power stations connected up according to the present invention. Power station A has three bus bars 1, 2, 3 coupled to two groups of machines 4, 5 through separate choke arrangements 6, 7 and 8, 9. In the other stations only one machine is shown at each point, although more machines or groups of machines may be connected. The lines bearing arrows and the reference L represent feeders for connection to loads, and in the case of power station A coupled chokes 10, 11 are provided for isolating two loads L1 and L2 from one another. Stations B and C have two loads tapped directly off the incoming station interconnecting lines. It will be seen that as regards the station interconnection, no surge can pass from one feeder to the other, since if the chokes are properly proportioned, there is substantially no interaction as regards voltage or current apart from the interaction for power supply. The arrangement shown is intended only as a diagram of how the system may be extended and many similar diagrams of possible arrangements of station interconnection can be drawn. The feeder 12 between C and E might be a long transmission line as described above. The machines 13 at E under conditions of light load serve to regulate the voltage on the bus bars but can under conditions of heavy load be used to supply extra power. The remaining phases of a multiphase system are similarly connected.
In the arrangement shown in the attached figure, the position of the tapping points (and the value of the chokes as explained in co-pending Application No. 19170/35) can be designed so that under conditions of maximum load the currents in the two parts of each of the chokes cause no magnetisation, and hence no increased reactance. Under conditions of full load the total generated volt-amperes will not be increased due to the introduction of the chokes.
Dated this 19th day of March, 1936.
REDDIE & GROSE,
Agents for the Applicant,
6, Bream’s Buildings, London, E.C.4.
Improvements in or relating to Voltage Regulating Systems
I, Alan Dower Blumlein, a British Subject of 32, Audley Road, Ealing, London, W.5, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:
The present invention relates to voltage regulating systems, and has for its object to provide a new or improved system of voltage regulation in alternating current networks.
In a system comprising a load fed with alternating current from a source such for example as the A.C. mains, variations of load elsewhere in the system give rise to fluctuations in mains voltage, and consequent variations of voltage across the load in question. It has already been proposed, in order to reduce these variations of load voltage, to feed the load from a generator which is driven mechanically by a synchronous motor fed with driving current from a source of alternating current power. In such an arrangement, provided that the frequency of the voltage of the source remains constant, the synchronous motor is driven at uniform speed independently of fluctuations in the magnitude of this voltage, and a constant voltage is accordingly supplied to the load by the generator.
Patent Specification No. 461,004 describes and claims apparatus for the supply of electrical energy to two loads from a common alternating current source, and provides means for substantially reducing or eliminating the effect on the voltage at one load due to variations in the current in the other; this result is achieved by the use of two coupled impedance elements, connected respectively between one load and the source and between the other load and the source.
The present invention also makes use of coupled impedance elements, and one of its objects is to reduce or eliminate the effects of variations in the voltage of a source of alternating current power upon the voltage at a point in an electrical network associated with that source.
The present invention accordingly provides an alternating current electrical network comprising a synchronous electric machine connected through a first inductive impedance element to a first source of alternating current power, and through a second inductive impedance element to a supply point, the supply point being connected either to a feeder adapted to supply a load or to a second source of alternating current power associated with a feeder adapted to supply a load, wherein said impedance elements are so constituted, and are inductively coupled to one another in such a manner, that fluctuations in the voltage at said supply point due to variations in the voltage of said first source of alternating current power are substantially reduced or eliminated, and wherein the ratio of the impedances of said impedance elements is substantially equal to the inverse of the ration of the squares of the normal mean of full-load currents which flow, in operation, in said impedance elements.
The synchronous machine may be a single machine or a group of machines, and the machine or group of machines may be capable of supplying power to the associated network, or may comprise one or more motors which take power from the network. The synchronous machine or group of machines may be arranged on occasions to supply power to and at other times to draw power from the network.
When two inductance coils are coupled to one another by mutual inductance, the two coils being connected in series-aiding sense, it is permissible to represent the mutual inductance between them by a third coil connected to the junction point of the two first-mentioned coils, the three coils together forming a three-terminal electrical network. This method of representation may be extended to cover the case in which the two inductively-coupled coils are wound on an iron core, and in which substantial iron losses take place in the core; in this case, the third coil is replaced by a complex impedance comprising, in addition to an inductance, a resistance which is dependent in magnitude on the magnitude of the iron losses; similarly, if the two coils are shunted by a resistance, the third coil is replaced by a complex impedance comprising inductance and resistance, the magnitude of the resistance being dependent on the magnitude of the resistance shunting the coils. For the purposes of this specification, the term "mutual impedance" will be used to describe this complex impedance.
According to a feature of the invention, there is provided an alternating current electrical network comprising a synchronous electric motor connected through a first source of alternating current power, and through a second inductive impedance element to a supply point, the supply point being connected either to a feeder adapted to supply a load or to a second source of alternating current power associated with a feeder adapted to supply a load, wherein said impedance elements are so constituted, and are inductively coupled to one another in such a manner, that fluctuations in the voltage at said supply point due to variations in the voltage of said first source of alternating current power are substantially reduced or eliminated.
The impedance elements may be connected in series between two terminals a and c to form the whole or part of a three terminal coupling network, the intermediate terminal b of this coupling network being connected to the synchronous machine or motor. Fluctuations in the voltage at the supply point due to variations in the voltage of the first source can then be substantially eliminated or much reduced by arranging that the mutual impedance between the parts of the coupling network between terminals a and b and between terminals b and c respectively has an inductive component substantially equal to the armature reactance, or to the armature reactance together with the armature reaction, of the synchronous machine or motor, or equal to some value intermediate these two values.
The present invention still provides advantage if the mutual impedance be given a value somewhat less than any of those mentioned above; thus in certain cases it may be found satisfactory if the mutual impedance has a value which is a substantial part of the armature reactance of the machine or motor, or of the armature reactance together with the armature reaction.
Systems to which the present invention can most usefully be applied are generally polyphase systems and each phase of the system is then arranged to constitute a network in accordance with the invention.
The present invention also provides a system of connection between an electric alternating current generating unit and two feeders of an alternating current network comprising one or more other generating units, wherein each phase conductor of each of said feeders is connected to the first-named generating unit through an inductive impedance element and wherein the two impedance elements associated with each phase are inductively coupled together so as to neutralise the whole or at least a substantial fraction of the impedance of the first-named generating unit as regards its effects in producing direct electrical coupling between the two feeders, the arrangement being characterised in that the ratio of the impedances of said impedance elements is substantially equal to the inverse of the ratio of the squares of the normal mean of full-load currents which flow, in operation, in said impedance elements. As will be more fully explained later the impedance of the first-named generating unit may be taken as the armature reactance alone or together with the whole or a part of the armature reaction. The impedance elements may be substantially wholly inductive, or they may comprise also a resistive component of desired magnitude.
Because the machine impedance is largely inductive, the two coupled impedance elements are conveniently constituted by a tapped choke, and the coupling is preferably such that the mutual impedance between the two parts of the choke (between terminals a and b on the one hand and b and c on the other hand of the three terminal network above mentioned) is at least 70% of the square root of the product obtained by multiplying the impedances of the two parts by one another. For optimum working, however, the mutual impedance is preferably made greater than 95% of the product referred to and the impedances may then be referred to as tight-coupled. It must, however, be understood that tight coupling is not an essential feature of the invention and many of the advantages of the invention are obtainable when couplings less tight than those discussed are employed. From the point of view of economy of apparatus, however, tight coupling is a desirable feature.
Various embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which:-
Figs. 1 to 3 are explanatory drawings of one embodiment of the invention.
Figs. 4 and 5 illustrate two further embodiments of the invention.
Fig. 6 shows by way of example an application of the invention to power ***tion and distribution technique.
Referring to Fig. 1, there is illustrated the application of the invention to the production or elimination of the effect at a supply point o, and hence at a load L, of voltage fluctuations of a source of power constituted by one phase M of three-phase A.C. mains. The neutral conductor N is shown earthed. The object of the arrangement of Fig. 1 is to ensure that there is available to the load L a supply of power the voltage of which is substantially unaffected by voltage changes of the mains M, it being assumed that the frequency of the current in the mains remains constant.
The mains are connected to one end a of a tightly-coupled choke C, the other end c of which is connected to the load L, and a tapping b in the choke is connected to a synchronous electric machine A1, the internal impedance of which is indicated by z. The choke C and any other impedances which may be connected between the terminals a, b and c form a three terminal network such as that already referred to, whilst the parts of the choke between terminals a and b on the one hand and b and c on the other hand constitute the impedance elements. The mutual impedance between the two impedance elements a, b and b, c is made substantially equal to the impedance z of the machine.
It is known that two impedance elements of magnitudes Z1, Z2 having between them a mutual impedance equal to z, and joined end to end in "series-aiding" sense, can be represented by a three-terminal network comprising two uncoupled impedances (Z1 + z) and (Z2 + z), joined together end to end and having an impedance –z in an arm of the network branched off from their junction point. Such an equivalent circuit is shown within the dotted rectangle in Fig. 2, which is the equivalent of Fig. 1, the impedances Z1 and Z2 being the impedance of parts a, b or b, c of choke C of Fig. 1. The point p within the rectangle has no physical reality, but the coupled impedances Z1 and Z2 operate, as far as can be detected, from any connections made at a, b or b, c as though the circuit was that shown within the rectangle. The machine is represented in Fig. 2 by a perfect source of E.M.F., A2, and the machine impedance z, has no physical reality. The E.M.F. of the source A2, when a motor is employed, is the back E.M.F. The internal impedance z of the machine is equal and opposite to the negative impedance –z in the branched arm of the circuit within the rectangle, since the mutual impedance between the two halves of the coupled choke C in Fig. 1 is made equal to z, and these two impedances cancel one another; we therefore have in effect a source of E.M.F. A2 with no internal impedance feeding current to the point p, and from that point through the impedances (Z1 + z) and (Z2 + z) respectively to the mains M, N and the load L. this arrangement is exactly equivalent to a circuit comprising two machines of similar internal E.M.F. on the same shaft, one having an internal impedance (Z1 + z) and the other an internal impedance (Z2 + z), and one machine being electrically connected to the mains and the other to the load; in such an arrangement, clearly, there can be no direct electrical transmission between the mains and the load. It should be borne in mind, however, that power transfer can take place between points a and c, Fig. 2, when the E.M.F. of the machine A2 lags or leads on the voltages at points a and c.
The equivalent circuit of Fig. 2 is shown somewhat modified in Fig. 3. In this case it is assumed that the two halves of the choke are tight coupled, in which case the impedance (Z1 + z) may be written z EQUAT. HERE and the impedance (Z2 + z) may be rewritten z EQUAT. HERE where s1 and s2 are respectively the numbers of turns in the parts a, b and b, c of choke C. As before, A2 represents a source of E.M.F. having no internal impedance, and it is clear that there can be no direct electrical transmission of electrical disturbances from the mains MN to the load L, power transfer taking place only on account of changes in the relative phase of the E.M.F. in A2 with respect to the voltage on the mains MN and the load.
In the arrangement described with reference to Figs. 1 to 3, therefore, variations in mains voltage are prevented from producing variations in the voltage across the load.
Thus, for example, referring to Fig. 2, if the mains voltage increases or decreases there will be produced between the mains and the machine A2 a voltage difference which is in phase with the mains voltage. Since the impedance (Z1 + z) is mainly reactive this voltage difference will produce in the impedance (Z1 + z) a current in quadrature, lagging on the voltage, and hence there will be no change in the power and the voltage at point p will remain unchanged.
Considering now the effect of a changed in loading upon the machine A2, it will be convenient to discuss first the case where a mechanical load is suddenly applied to the machine. The effect of such a mechanical load is suddenly applied to the machine. The effect of such a mechanical load will be that the machine will begin to lag in phase and its E.M.F. will lag with respect to the mains voltage. The voltage difference so produced will be substantially in quadrature and, since impedance (Z1 + z) is mainly reactive, will produce in this impedance a current which is substantially in phase with the mains voltage. The necessary additional current is thus supplied to the machine A2 to enable it to deal with the additional load and its lag will not then further increase. Meanwhile, the load L has been receiving E.M.F. of the machine A2 unaffected excepting in phase by the application of the mechanical load and by the consequent changes in power conditions between the mains and the machine A2.
A similar effect takes pace when the loading upon the machine is electrical instead of mechanical; for example, and increase in the mean current in the load L may cause the machine to being to lag in phase, and in these circumstances, additional current is supplied by the mains to prevent a further increase in the phase lag of the machine.
It should be pointed out, however, that the present invention does not enable constancy of the voltage across the load to be maintained if the load current fluctuates, since the voltage at the point p, Fig. 2, is applied to the load through an effective regulation impedance equal to EQUAT. HERE together with any other impedance, such as may be introduced by a feeder, tending to cause regulation.
If the machine has substantial mechanical inertia, the change in phase thereof introduced by a change in power conditions will be effected more slowly, and if changes in power conditions are only of short duration the consequent changes in phase of the machine may only be small.
For these reasons it will be seen that when the mutual impedance between the impedance elements is exactly equal to the impedance of the synchronous machine there is no direct electrical coupling between the load and the mains. Any direct electrical coupling which in practice may remain is due to imperfections in matching the impedances.
When determining the value of mutual impedance to be given to the impedance elements it should be borne in mind that alternating current machines of most usual kinds have, in effect, two possible values of internal impedance. One value represents the reactance of the armature inductance and is referred to as the armature reactance, and the other, which is referred to as the regulation impedance, includes the effect of magnetisation of the field system; it is the sum of the armature reactance an the armature reaction. The regulation due to the regulation impedance is relatively slow in manifesting itself compared to that due to the armature reactance. For this reason, although it is possible by suitable choice of the position of the tapping point b, to obtain a substantially exact balance between the mutual impedance of the tightly-coupled choke and the machine impedance of either impulsive voltage variations or slow voltage variations, it is impracticable to obtain an exact balance for both.
If the design of the choke C is based on the armature reactance, a balance is obtained for impulsive voltage changes. If the design of the choke is calculated on the basis of the regulation impedance then a balance is obtained for slow variations. The smaller the effects of the armature reaction on the field (e.g. under conditions of saturated excitation), the smaller will be the discrepancy between the impulsive and the slow balance conditions.
In practice, it may be found convenient to design the choke C on the basis of the armature reactance only, as this is the determining factor controlling surge conditions under short-circuit. The mutual impedance between the impedance elements may, however, be given a value intermediate the two values of machine impedance and in that case a compromise will be obtained.
The mutual impedance between the two parts of the tightly-coupled choke C constituting the impedance elements is mainly reactive, and this is usually satisfactory since in normal efficient alternating current machines, the regulation is due mainly to reactance. If the choke C is shunted by a resistance, the inherent resistance term of the mutual impedance is increased; this may be desirable in some cases, but has the disadvantage that the power losses of the system are increased. A convenient form of tightly-coupled windings on a common core, one end of each of the windings being connected together so that the coils are connected in series-aiding relationship. The construction of a choke of this kind is discussed in Patent Specification No. 461,004. The necessary inductance value is obtained by providing an air gap in the transformer core, the air gap being preferably within the limb on which the windings are placed. In chokes of large size, the air gap preferably consists of a number of small gaps in series, since a large leakage flux at a big air gap may cause serious stresses in the windings. Similarly, the two windings of the choke are preferably disposed one winding over the other, both windings surrounding equal parts of the core.
If a transformer or other transmission link is included between the machine A1 of Fig. 1 and the point b, the extra impedance so introduced must be allowed for in determining the correct value for the mutual impedance between the impedance elements a, b and b, c. In general, any extra impedance added between the machine and the coupled impedances, as for example due to a transformer or the impedance of the connection leads, can be considered as being part of the armature reactance; and it should be understood that armature reactance in this specification includes the inductance of any connections between the point of application of the coupled impedances and the actual terminals of the machine.
Now in the arrangement of Fig. 1, it will be noted that when the currents in the parts a, b and b, c of the choke C are in the ratio s2 to s1, it can be arranged that if the currents are in substantially the same phase, there will be substantially no magnetisation of the choke C; in these circumstances, as will be explained later, power losses can be kept small, and economies can be effected in the design of the choke C. Preferably, therefore, the arrangement is made such that the ratio of the impedance elements (that is to say, if Fig. 1, the ratio s12 to s22) is made substantially equal to the inverse of the squares of the normal mean or full load currents which flow, in operation, in the impedance elements.
Reference is now directed to Fig. 4, which illustrates the application of the invention to a three-phase system.
In Fig. 4 the reference 2 indicates a three-phase synchronous machine together with any auxiliary equipment, such as transformers, associated therewith; any symmetrical arrangement of a machine and transformers etc. whether connected in star or delta can be represented, as in Fig. 4, by the equivalent star arrangement, the three star-connected reactances 6, 7, 8 representing the equivalent reactances of the machine and its associated apparatus.
The three terminals of the machine 2 are connected respectively to the tapping points in three tightly-coupled chokes 3, 4, 5, one end of each of the three chokes being connected respectively to one phase of the mains (or other source of alternating current power) 1. The other ends of the chokes 3, 4, 5 are connected respectively to conductors 9 which pass to a load (not shown).
The mutual impedance between the two parts of each of chokes 3, 4, 5 (situated upon either side of the tapping points) is made substantially equal to the reactance of the limb (6, 7 or 8) which is connected to it. Where the reactances of the three limbs 6, 7, 8 are not all the same, the mutual impedances of the parts of the chokes 3, 4, 5 are consequently not the same for all three chokes.
For simplicity, let it first be assumed that chokes 3, 4 and 5 are centre-tapped. Then an arrangement of the kind described with reference to Fig. 4, assuming the limbs 6, 7, 8 to have the same reactance and each of chokes 3, 4, 5 to have a reactance from end to end equal to 4X, is equivalent to a machine having an E.M.F. (E) but no reactance, and having two sets of output terminals (one set connected to the mains and the other to the load) each terminal having a reactance of magnitude 2X between it and the machine. In other words, by the introduction of the tightly-coupled chokes 3, 4, 5 the machine 2 has effectively been divided into two machines, as already described with reference to Figs. 1 to 3. One of these two machines, for example that which may be regarded as associated with the load - may be effectively divided by the use of further tightly-coupled chokes into two further machines, and so on.
It will be clear that in Fig. 4 a variation of the mains voltage causes substantially wattless current to flow through the reactances (of magnitude 2X) on the mains side, but will not alter the E.M.F., which alone is effective in driving current through the load and the reactances on the load side. The open circuit E.M.F. presented to the load will be the E.M.F. generated within the machine, that is, the E.M.F. at point p in Fig. 2. The apparatus is arranged to be capable of handling volt-amperes dependent upon the variation of the mains voltage which is expected. If the mains voltage fluctuates by an amount v volts, then the volt-amperes to be catered for will be EQUAT. HERE, E, and provided that the reactance of the machine is small compared with the load impedance, and that the load has a power factor substantially equal to unity, there will be negligible increase of machine loading due to the presence of the load. If however the load is reactive, the machine must provide the powerless volt-amperes of the load.
The machine ratings given above are approximate, and are based on the assumption that the machine 2 had negligible losses when running light. As has already been explained, it is possible to drive the machine 2 so that power is passed into the mains from the machine, or the machine may be used as a motor deriving power from the mains; under both conditions, clearly, the machine can serve to maintain the voltage facing the load substantially constant as described above. The rating of the machine must be sufficient to permit the machine to carry the additional current required by driving into or being driven from the mains.
In the discussion of machine ratings for the arrangement of Fig. 4, it was assumed that the chokes 3, 4, 5 were tapped at their centres, so that each choke constituted two impedances of the same magnitude tightly coupled to one another; tightly-coupled chokes of unequal magnitudes may, however, be employed with advantage, and their design may be calculated as described in Patent Specification No. 461,004 so that the mutual impedance between their two parts is equal to the reactance of the three-phase limb with which they are associated: in order to apply this earlier Specification No. 461,004 to this calculation, the machine 2 is regarded as a source, and the mains 1 is treated as one load, the load connected at 9 being the other. Each tightly-coupled choke may in addition be so designed – as also described in Patent Specification No. 461,004 – that the ratio of the turns of the two parts thereof is substantially inversely equal to the ratio of the normal mean of full load currents in those two parts taken in the same order.
When tightly-coupled chokes tapped elsewhere that at their centres are employed, it is possible to obtain a low impedance facing the load connected at 9, but there is a greater risk of the machine 2 pulling out of synchronism if excessive power loads are taken from the arrangement. By arranging that there is high impedance facing the load, relatively poor regulation may be obtained, but more stable operation is ensured since there is less chance of the machine pulling out of synchronism on heavy overloads.
Reference is now directed to Fig. 5, which illustrates the use of tightly-coupled chokes in a delta-connected arrangement. The machine 24 is represented by three windings 25, 26 and 27 in delta, the arrangement being one into which any three-phase group of reactances may be resolved. If the resultant system is required to be symmetrical and balanced, six tightly-coupled chokes 28, 29, 30, 31, 32 and 33 are introduced. As in the arrangement of Fig. 4, 1 represents the mains or other source of alternating current power, and the conductors 9 are assumed to pass to a load, not shown.
Considering by way of example chokes 28 and 29, the mutual impedance between the two parts of choke 28, together with the mutual impedance between the two parts of choke 29, is made approximately equal to the impedance of the limb 25 of the machine 24. In practice, the pairs of chokes 28 and 29, 30 and 31, 32 and 33 may be wound on common cores, since the chokes of each pair operate in the same phase; in this way, it can be arranged that substantially the same amounts of copper and iron are required for the arrangement of Fig. 5 as for the arrangement of Fig. 4 provided negligible space is involved in insulating the turns of the windings.
It has already been explained that, in any of the arrangements of Figs. 1 to 5, power may be flowing from the synchronous machine through both parts of the tightly-coupled chokes to the remainder of the system, in which case, the whole of the remainder of the system may be regarded as a load on the machine. With this fact in mind, certain details of a preferred design for tightly-coupled chokes for use in arrangements according to the invention will now be considered with reference to Fig. 1, and it will be assumed that the machine A1 is a generator from which both the mains M and load L are drawing power. Further, it will be assumed that under normal full load conditions, the load connected to terminal a is n times the total load, the load connected to terminal c being (1 – n) times the total load. The number of turns between a and b is made equal to EQUAT. HERE times the number of turns between b and c. Suppose the total number of turns is x, then the winding a, b will have (1 – n)x turns and the winding b, c will have nx turns. The mutual inductance between a, b and b, c is made equal to the reactance of the machine A1 to which terminal b is connected, so that if a short circuit to earth is introduced at the terminal a, the voltage across the whole winding will be equal to the open circuit voltage (V) of the machine, since the potential of the point a is substantially unaltered. The voltage per turn is then *** a sufficient number of turns and a sufficient core area must be provided to enable this voltage per turn to exist without complete saturation of the iron core occurring. As, however, short circuit conditions are usually temporary, heating due to iron losses is not of importance, so that on short circuit the core flux density may rise to more than, say, 15000 ***/sq. cm.
Suppose that the normal full load current of the machine under conditions of unity power factor is I, then under normal full load conditions the part-winding a, b carries a current nI. Furthermore, this part-winding must be capable of standing voltage across it of (1 – n) V volts, that EQUAT. HERE volts per turn. This part-winding and the core within it must therefore be rated to carry a power equal to n(1 – n). VI. Similarly, the part-winding b, c will have the same power rating. The choke, in that it has two coupled part-windings, therefore resembles a transformer with a power rating of the order of n(1 – n) times the power rating of the associated synchronous machine; the choke can, however, be less generously rated if desired, for two reasons. Firstly, under normal operating conditions, the iron losses of the core will be negligible, so that heat will only be produced by the upper losses, and thus a smaller cooling area, or greater power loss in the copper for a given cooling area, is permissible. Secondly, on short circuit, the flux density can bee allowed to rise to very high values so that an economical choke can be designed.
Once the necessary core size, number of turns and copper area have been fixed in the manner well known in transformer designing, a sufficient air gap is provided to obtain the desired reactance. For machines with reactances of magnitudes usually encountered in normal 50-cycle practice, the air gap will be so large that the variation of the inductance with current due to varying degrees of magnetisation of the iron, will be very small. For a machine of given size and a given distribution of load, an increase of the reactance of the machine will involve only reduction of air gap in the transformer, unless the reactance of the machine is increased so much that the gap becomes negligibly small, and the reactance too variable with current.
On the other hand, for normal practice, the choke may be designed with reference to the power output, and to the desired turns ratio a, b:b, c, and may then be adjusted by adjusting the air gap or gaps to suit the particular machine. It may prove necessary to pack the air gap or gaps to prevent closure due to magnetic pull in the manner well know in the constructions of chokes for smoothing rectified current. When a tightly-coupled choke is to be followed by further tightly-coupled chokes, which limit the short circuit currents to be expected, reduction in the size of the first-mentioned choke may be obtained, so that economical use is made of the iron. The chokes may be provided with tapped windings so arranged that the ratio of turns in the two windings may be altered, at the same time maintaining the product of the turns in the two windings constant, so as to allow for varying load conditions. Furthermore, the tappings may be altered so as to alter the mutual inductance, in order to allow for changes of the circuit from which they are fed.
In polyphase working, the chokes for the various phases may be wound on separate limbs of a common core as in standard practice for three phase transformers. Alternatively, a separate core may be used for each phase.
The present invention may be used with advantage to regulate the voltage at the end of a long transmission line in a system in which, under peak load conditions, a separate generating station issued to supply some of the load. If the various machines in this station are each connected through suitable tightly-coupled chokes to the incoming and the outgoing bus-bars in the manner outlined above, these machines can run light during light loads and serve solely to provide a voltage regulation, the power being obtained from the incoming feeder. Such a system is an example of the arrangement of Fig. 1.
The invention finds an important application in the coupling up of a number of power supplies so as to reduce voltage variations and in particular to reduce the surges on short circuit. For example, a generating station may have two or more sets of bus-bars which are connected to various loads of incoming transmission lines or both loads and transmission lines, and each machine of the station (or the machines of the station as a whole) may be coupled to these bus-bars through suitable tightly-coupled chokes. The design of the chokes for connection of a machine to its associated incoming transmission lines and its associated loads may be effected as herein described and as described in Patent Specification No. 461,004, such arrangements being capable of extension to any number of loads of bus-bars. In such an arrangement, the various bus-bars are isolated from each other as regards the effect of variations of load upon the voltage on the bus-bars, so that any surge arriving on one bus-bar will not affect the magnitude of the voltage on the other bars. When power is drawn from the machines there may be a change in their phase and hence in the phase of the voltage on the bus-bars. A very severe short circuit on one set of bus-bars will take power out of the other bus-bars by way of the machines, but as the phase of the machines will not be immediately affected, there will be time for overload devices to operate to isolate the short circuit. Arrangements may be made to alter the tapping points on the tightly-coupled chokes so as to divide the reactance to the various bus-bars in accordance with the power requirements.
Reference is not directed to Fig. 6, which shows the application of the invention to a system comprising five power stations, only one phase being shown for convenience. Power station A has three-bus-bars 11, 12, 13, bus-bar 13 is connected to the machines, or groups of machines, 14 and 15 through one part of each of the chokes 17 and 19, and the bus-bars 11 and 12 may be connected directly to the machines 14 and 15 through the other parts of chokes 17 and 19; preferably, however, the chokes 16 and 18 are provided as shown to prevent or reduce direct electrical transmission between bus-bars 11 and 12. In the other stations, B, C, D, E, only one machine is shown at each point, although similar design considerations apply when more machines or groups of machines are employed at these stations. The lines bearing arrows and the reference L represent feeders for connection to loads, and in the case of power station A, a tight-coupled choke 20 is provided for isolating two loads L1 and L2 from one another. Choke 20 may be constructed and arranged in the manner set forth in Patent Specification No. 461,004. Stations B and C have two loads tapped directly off the incoming station interconnecting lines. It will be appreciated from what has already been said that, as regards the interconnection of the various stations, if the tightly-coupled chokes are correctly designed, it can be arranged that there is substantially no interaction of voltages and currents in the various parts of the system (except insofar as the stations co-operate to supply the associated loads) and surges can therefore be prevented from passing from one feeder to another. The arrangement shown in Fig. 6 is intended only as a diagram of how one system of sources of power may be arranged according to this invention, and many diagrams of other possible arrangements of station interconnection can be drawn.
The feeder 22 between stations C and E might be along transmission line. The machine or group of machines 23 at station E, under conditions of light load, serve to regulate the voltage across its associated load, but under conditions of heavy load can be used to supply extra power to the remainder of the system.
Thus, under normal working conditions, the loads on the system of Fig. 6 are supplied from the generating stations A, B, C, D. Under extreme conditions of load, however, power flows from station E towards the rest of the system so that the rest of the system is a load on station E. The rest of the system is, however, very different from a dead load, since a short circuit on the feeder 22 will immediately divert power both from station E and from the rest of the system. Similarly, if the governor setting of the turbines, for example, at station E is altered so as to reduce the power generated at E, any excess power required by the load L, associated with station E will be derived from the rest of the system over feeder 22. The rest of the system therefore operates in relationship to station E as a source of alternating current power, since such power is available, although under particular conditions of load, power may be flowing from E towards the rest of the system.
In the arrangement shown in Fig. 6, the positions of the tapping points in the chokes associated with the five generating stations are conveniently so chose that under conditions of maximum load, or, if desired, under normal mean load conditions, the currents in the two parts of each of the chokes cause no magnetisation, and hence no increase of reactance looking towards the bus-bars. In these circumstances, the total volt-amperes generated will not be substantially increased due to the introduction of the chokes.
The invention has been described with particularly reference to the introduction of coupled impedances the mutual impedance between which is substantially equal to the impedance of the synchronous machine. If the mutual impedance were to exceed the impedance of the machine, a short circuit on one end of the coupled impedances would cause a serious voltage rise above normal voltage at the other end, so that values of coupling such that the mutual impedance is substantially greater than the machine impedance are not desirable. As has already been stated, the useful voltage-regulating and short-circuit protecting effects of this invention may, however, be obtained in part by coupling the two impedances so that the mutual impedance between them is less than the impedance of the machine. By employing impedances having between them a mutual impedance as low as half the impedance of the machine feeding them, helpful protection is still obtained and it may in certain cases be found that the reduction of electrical interaction so obtained is sufficient for the system under consideration. With such an arrangement the reactive volt-amperes under unbalanced load conditions will be less than if the mutual impedance between the two halves of the choke were closer to the machine impedance.
Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:-
Dated this 18th day of March, 1937.
REDDIE & GROSE,
Agents for the Applicant,
6, Bream’s Buildings, London, E.C.4.
Leamington Spa: Printed for His Majesty’s Stationery Office, by the Courier Press – 1937.