GB1584396A

GB1584396A - Standby electric power supplies

Info

Publication number: GB1584396A
Application number: GB33655/77A
Authority: GB
Original assignee: Westinghouse Brake and Signal Co Ltd
Current assignee: Siemens Mobility Ltd
Priority date: 1978-05-30
Filing date: 1978-05-30
Publication date: 1981-02-11

Description

(54) IMPROVEMENTS IN OR RELATING TO STANDBY ELECTRIC POWER SUPPLIES (71) We, WESTINGHOUSE BRAKE AND SIGNAL COMPANY LIMITED, a Company incorporated under the Laws of Great Britain, of 3, John Street, London, WC1N 2ES, England, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement This invention relates to standby electric power supplies, and particularly to electric supplies which provide a source of alternating current in the event of loss of a normal a.c.

supply.

It is known to provide standby electric power supplies which are normally inactive but are called into operation in the event of a mains supply failure. Standby electric power supplies may be divided into two classes, namely the temporary interruption type and the "ho-break" type. In the temporary interruption type, upon the failure of the main electricity supply, the standby supply is not available until, for example, a diesel engine is started and run up to appropriate speed. The "no-break" type is available more-or-less instanteously with the loss of the normal supply, although there may be a small voltage transient upon changeover to the standby supply.

The present invention is concerned with the "no-break" type of standby power supply in which electrical energy is stored in batteries.

This type of supply can be divided into two further sub-types: those which turn direct current from the batteries into alternating current by means of rotating machines and those which embody static inverters. The static inverter being a solid-state device requires no periodic maintanance and does not wear out mechanically. Rotating machines on the other hand may be considered basically simpler and preferred by maintainance technicians without specialised electronics experience. Also the flywheel effect of the rotating components can be useful for short-term energy storage to provide continuity in the event of-mains supply failure.

It is an object of the present invention to provide a type of hybrid standby power supply including a rotating machine to provide a flywheel effect and a minimum of sophisticated electronics.

According to one feature of the invention standby a.c. power supply means comprises battery power storage means, normally operative battery charging means for connection to an a.c. mains supply, normally inoperative d.c. to a.c. inverting means connected to receive d.c. current from the battery means and provide a.c. current to standby a.c. supply terminals, a rotary a.c. machine the armature winding of which is connected to the standby a.c. supply terminals and the field winding of which is connected to a source of excitation current, and control means responsive to failure of the a.c. mains supply to render the d.c. to a.c. inverting means operative to provide standby a.c.

power from power stored in the battery means the arrangement being such that the rotary a.c. machine normally rotates when the a.c.

mains supply is present and acts to maintain the supply voltage when the a.c. mains supply ceases so that the angular momentum of the machine provides for smooth changeover to the standby a.c. power supply upon cessation of the a.c. mains supply.

The inverter and/or the a.c. machine may be directly connected to said a.c. line, or said inverter and/or said a.c. machine may be coupled to said a.c. line through one or more transformers. Alternatively, the inverter may be coupled to said a.c. line by being connected or coupled to tappings on the a.c. windings of said a.c. machine, or connected or coupled to further a.c. windings of said a.c. machine.

Usually the armature winding will comprise the stator winding of an alternator and the field winding the rotor windings.

Referring first to Fig. 1, the reference 10 designates a three-phase three-wire a.c. consuming load whose supply must not be interrupted. The load terminals 12 are connected to a three-wire a.c. supply line 14 connected via a dis-connecting switch means 16 to a normal a.c. supply 18, shown by way of example as the star connected secondary winding of a distribution transformer powered by the national electricity supply. (Suitable forms of the switch means 16 will subsequently be detailed with reference to Figs.

3 and 4.). Three-phase four-wire supplies and loads may be accomodated by providing the supply line 14 with a fourth wire (not shown) forming a neutral line and connected to the star point of the supply 18.

Connected in parallel to the supply line 14 is a rotary synchronous three-phase a.c.

machine 20 having a stator 22 provided with a.c. windings (not shown) which are connected to the line 14, and a controllably excitable d.c. rotor 24. In the context of this invention, the machine 20 may be of any suitable form capable of being selectively operated either as a rotary synchronous capacitor or as a rotary synchronous alternator. A flywheel (not shown) may be coupled to the rotor 24, either directly or through gearing or a belt drive, to increase the stored kinetic energy of the rotating parts when the machine 20 is running, thereby to increase the energy that may be extracted from the machine 20 when running as an alternator, for a given frequency droop.

Coupled in parallel to the line 14, and hence also in parallel to the machine 20, is a threephase inverter 26 comprising six gate-controlled thrysitors 28 coupled in a three-phase bridge configuration between positive and negative lines 30 and 32 respectively. The gate control circuits and connections for the thyristors 28 are omitted for the sake of clarity. In normal operation of the embodiment of Fig. 1, the thyristors 28 are all in a blocking (totally non-conducting) state while the switch means 16 is closed to connect the supply 18 to the line 14 and thence to the load 10.

To power the inverter 26 when it is required to produce an a.c. output, as will subsequently be described in detail, there is provided a rechargable multi-cell battery 34 of a suitable nominal voltage when charged. The battery 34 may be of any suitable form, for example, a lead-acid battery or a nickelalkali battery. The negative terminal 35 of the battery 34 is directly connected to the common negative line 32, and the positive terminal 36 of the battery 34 is connected via a suitable inductance 38 to the positive line 30 of the inverter 26. The purpose of the inductance 38 is to enable a reasonably smooth current to flow from the battery 34 to the inverter 26 despite a possibly large ripple voltage at the inverter feed lines 30 and 32.

To charge the battery 34 when it requires charging, and to maintain the battery 34 fully charged once charged, a controllable converter bridge 40 is provided, the negative side of the bridge 40 being connected to the common negative line 32 and the positive line 42 of the bridge 40 being connected via a suitable d.c. smoothing choke 44 to the positive terminal 36 of the battery 34. As shown in Fig. 1, the bridge 40 may optionally be a single-phase full-wave bridge, or a threephase full-wave bridge, a.c. to be converted to d.c. by the bridge 40 being derived by, as appropriate, single-phase or three-phase connections to the line 14. The thyristors 46 of the bridge 40 are controlled in known manner to a delay angle depending on the output voltage and current required by the d.c. circuit.The gate control circuits and connections for the thyristors 46 are omitted for the sake of clarity, and are of any suitable conventional form, except that provision is made to render all the thyristors 46 blocking (totally nonconducting) in the event of the switch means 16 being opened to disconnect the supply 18 from the line 14 and the load 10, thereby to prevent an unwanted drain of a.c. from the line 14 when the embodiment of Fig. 1 is operating in emergency standby conditions as will shortly be explained. In respect of the bridge circuits 40 and 26 other known arrangements may be used with equal effect, the a.c.

to d.c. converter 40 may be replaced by a known arrangement of battery charger. The battery charging means may be connected to the side of the switch means 16 remote from the line 14 whereby the charging circuit for the battery 34 automatically becomes ineffective and does not provide an unwanted a.c. load in emergency conditions, i.e. when the supply 18 has failed.

Since the rotor 24 of the machine 20 requires direct current for its excitation, the battery 34 most conveniently provides the source of excitation current. Therefore connections 47 are made from the positive battery terminal 36 and the negative common line 32 to an excitation current control circuit 48, and from the circuit 48 to the rotor 24 via d.c. leads 50.

The circuit may take any suitable form converting the relatively constant voltage of the battery 34 to the appropriate level of rotor excitation current; for example, the circuit 48 may be a known form of transistor orthrysitor chopper such as may be found in electrically propelled battery powered vehicle traction current controllers.

Operation of the embodiment of Fig. 1 will now be described. Let us first assume the national electricity supply is operative, i.e.

a.c. is available from the supply 18. With the switch means 16 closed, the line 14 will be energised and a.c. will be provided to the load 10 from the supply 18. The machine 20, having been started and synchronised by any of the well known methods for starting and synchronising synchronous rotary machines, is running synchronously with the supply 18 and hence stores a certain amount of kinetic energy. The circuit 48 may be operated such that the excitation of the machine 20 improves the power factor of the load on the supply 18 (e.g. if the load 10 were inductive such as by reason of being one or more asynchronous cage motors, the rotor 24 would be excited highly enough that the windings 22 took a leading current).However, power factor connection by controlled excitation of the machine 20 is not an essential feature of the present invention, whereas it is an essential feature that the machine 20 possesses kinetic energy and may be operated as an alternator. While the supply 18 is maintained, the inverter 26 is kept inactive and the converter 40 is operated as required to keep the battery 34 as fully charged as is feasible.

Let us now assume a loss or interruption of the national electricity supply, i.e. a.c. no longer available from the supply 18. By the use of supply voltage monitoring means (not shown), or a self-sustaining relay contact within the switch means 16, the switch means 16 is opened to disconnect the supply 18 from the line 14. Because the machine 20 is rotating and possesses kinetic energy, and with suitable excitation of the machine rotor 24 from the circuit 48 (which receives an uninterrupted d.c. supply from the battery 34), the machine 20 functions as an alternator, and thus maintains the line 14 energised and continues a supply of a.c. to the load 10 without interruption. With suitable operation of the circuit 48, voltage transients at changeover to standby supply may be minimised.The frequency of the alternator 20 will droop as its kinetic energy is converted to electrical energy, but before the frequency of the standby supply has fallen unacceptably, the inverter 26 is started up to convert d.c. from the lines 30 and 32 to three-phase a.c. both to supply the load 10 and maintain the speed of the machine 20 with energy from the battery 34, this now being the sole source of standby power until the supply 18 is restored. Upon starting the inverter 26, the excitation of the machine 20 may be increased and maintained such that the machine 20 acts as a synchronous rotary capacitor in known manner, and draws a leading current from the inverter 26 such that the thyristors 28 commutate naturally.This obviates the need for complex and expensive commutation circuits for the inverter 26 which may be simpler and cheaper thereby as compared to prior art "no-break" inverter supplies.

Control means (not shown) for the inverter 26 desirably but not essentially causes a phase advance in the sequence of firing of the thyristors 28 gradually to increase the frequency of the system until it is back at the pre-interruption frequency (thereby making up the lost kinetic energy of the machine 20), or some other predetermined frequency.

Increasing the kinetic energy of the rotor 24, for example by making the rotor 24 more massive than normal and/or coupling a flywheel thereto, as aforesaid, will reduce the frequency droop in the maintained supply to the load 10 from the alternator 20 unitl the inverter 26 starts to generate a.c.

During the standby power supply mode of operation of the Fig. 1 arrangement (which may be brought about during the presence of the supply 18 in order to test the functioning of the standby supply), the battery charging converter 40 will of course be rendered inoperative by its control means (not shown) since the only a.c. available for rectification is the a.c. provided by inversion of the battery d.c. (i.e. a self-powered d.c.-to-a.c.-to-d.c.

circular conversion which produces no useful effect and disipates power).

Referring now to Fig. 2, this illustrates the second embodiment of the invention and parts therein which correspond in function to like parts in Fig. 1 are given the same reference numerals. (Two parts, namely 26/40 and 38/44, are given double reference numerals to indicate that they serve a dual function, as will subseqeuntly be explained in detail).

The second embodiment (Fig. 2) is in general principle identical to the first embodiment (Fig. 1), but differs in that the short but significant time during which all power comes from running the machine 20 as an alternator without undue frequency droop (which might necessitate the use of a flywheel as aforesaid) is used to allow a simple electromechanical changeover switch to operate, enabling, as will be described, certain parts of the second embodiment to be used for a dual purpose, according to whether the power supply is running on a mains or standby mode, and thereby offer potential cost reduction.

In Fig. 2, a double-pole, double-throw switch 52 which is linked to the switch means 16 is shown in the position it occupies when the switch means 16 connects the supply 18 to the line 14 and the load 10. In this position of the switch 52, the three-phase full-wave thyristor bridge 26/40 functions as a controlled rectifier equivalent to the converter 40 of Fig.

1, drawing power from the supply 18, charging and maintaining the charge of the battery 34 through the smoothing choke 38/44 (equivalent in this mode to the smoothing choke 44 in Fig. 1). The machine 20 is running as in Fig. 1, i.e. idling or functioning as a power factor improver.

In the event of interruption of the supply 18, the switch means 16 opens and brings the switch 52 into the changed-over state relative to that shown. The battery then feeds the bridge 26/40 which is now operated as an inverter (analogous to the inverter 26 in Fig.

1), through the inductance 38/44 which is now functioning analogously to the inductance 38 of Fig. 1. Thus standby a.c. power is produced from the battery 34, the a.c. supply to the load 10 being maintained, in the interval between the opening of the switch means 16 and the commencement of operation of the inverter 26/40, by means of operation of the machine 20 as an alternator. Thus there is no fundamental difference between the operation of the Figs. 1 and 2 embodiments, the differences lying principally in making the thyristor bridge serve alternately as a d.c.-producing battery charger, and an a.c.-producing d.c.powered inverter. The machine 20 (and/or the load 10) still serves to draw the leading curtent that commutates the inverter in Fig. 2.

For most purposes, the switch means 16 may take the form of a simple three-pole contactor as shown in Fig. 3, including known provision for automatically opening (and remaining open until reset) in the event of loss of voltage on one or more incoming phases.

However, if it is important to avoid the disturbance of a mains voltage depression of sufficient duration that lasts as long as the electromechanical contactor of Fig. 3 takes to open, then a fast-acting semiconductor switch of the type shown in Fig. 4 may be employed instead. The switch of Fig. 4 comprises a respective pair of inverse-parallel connected thyristors in series between each incoming and outgoing connector of the switch means.

Other forms of switch means, including other forms of semiconductor switches (including but not restricted to triacs) may be employed within the scope of the invention.

The rotation of the machine 20 may be put to practical use in driving a fan or impeller or pump to move cooling air or liquid across the heat-producing or dissipating electrical parts of the embodiment of the invention: for example, the thyristor heatsinks. If necessary or desired, the machine 20 may be up-rated beyond its capacitor/alternator rating to cope with its motoring duties. Such use of the machine 20 as a motor obviates the need of a separate forced-cooling drive motor, and reliably serves this purpose since it does not stop rotating even when the supply 18 is interrupted.

Referring now to Fig. 5, this schematically illustrates a control system for controlling the operation of the Fig. 1 or Fig. 2 embodiments.

In Fig. 5, the lines between the blocks represent electric signal routes (in the case of the line 14, the supply voltage route).

As shown in Fig. 5, a two-wire tapping 54, which may be connections to two of the phases of the supply line 14 to to one such phase and to the neutral line if the latter is supplied, leads to a first comparator 56 as one input thereof. The other input 58 to the comparator 56 comes from a frequency standard 60 which provides a -signal denoting the desired nominal frequency of the system, e.g. 50Hz or 60Hz or 400Hz or any other desired frequency. The output 62 of the comparator 56 passes to a current reference circuit 64. The combined frequency-deviation/ current-reference output 66 from the circuit 64 forms one input of a second comparator 68.The other input 70 of the comparator 68 is a current signal derived by a suitable current sensing means 72, such as a current transformer sensing the output of the inverter 26 (Fig. 1 arrangement) or the a.c. output of the converter/inverter 26/40 (Fig. 2 arrangement) when the latter is functioning as an inverter.

The inverter 26 or 26/40 receives as an input, as shown in Fig. 5, positive and negative supplies along the lines 30 and 32 respectively from the battery 34. The inverter 26 or 26/40 delivers, as an output, three-phase a.c. to the polyphase a.c. windings 22 of the machine 20 and in parallel therewith also to the a.c. supply line 14. The voltage of the last-mentioned output is taken by a phase-phase or phaseneutral tapping 74 to form one input of a third comparator 76. Forming the other input 78 of the comparator 76 is a suitable signal from a voltage reference source 80. The output 82 from the comparator 76 controls the excitation regulator 48 whose controlled d.c. output passes via the leads 50 to cause a controlled excitation of the rotor 24 of the machine 20 in known manner.

In operation of the control arrangement shown in Fig. 5, assuming it to be connected as indicated to the arrangement of Fig. 1, or to the arrangement of Fig. 2, the output of the comparator 76 tends to maintain stable the system voltage when operating in the standby mode, i.e. with the switch means 16 opened and the a.c. power for the load 10 being derived either from the kinetic energy of the machine 20, or the battery 34. The output of the comparator 68 controls the inverter 26 or the inverter 26/40 to produce a suitable level of current and to bring the system frequency during the standby mode up to the predetermined value (making up the afore-mentioned frequency droop of the machine 20) and to tend to maintain the frequency at that value despite variations in the power demand of the load 10 and the state of discharge of the battery 34.

While the invention has been described in relation to three-phase systems, it is applicable, with modifications obvious to those skilled in the art, to other numbers of phases, including but not restricted to single-phase systems.

Commutation notches may be observed in the a.c. waveform appearing at the standby supply terminals as a result of overlap caused by reactance in the rotary machine when the load is being supplied from the standby supply. Any-suitable filter or suppression network may be used to reduce the effect of the notches on the load voltage waveform, as -an example, a preferred circuit is shown in Fig. 6.

In Fig. 6, where like parts are given like references, the load 10 is decoupled from the converter 26 by a circuit comprisingtrans -formers 84, 86 and 88 and impedances 90, 92 and 94 connected in each of the three phase outputs as shown. The impedances 90, 92 and 94 are models of the subtransient impedance of the machine, or of whatever impedance is operative with respect to overlap in the converter. Thus, if the transformers have unity ratio the values of the impedances Z are substantially equal to the impedance of each phase of the machine. Where the transformer ratio is other than unity the impedance value is the appropriate proportion of the phase impedance.Assuming now that the transformers have unity ratio a transient current i flowing between the converter 26 and the machine 20 will induce a transient voltage iZ at the standby supply terminals 12, but also an additional votage -iZ in series with the load due to the external impedances 90, 92 and 94 and the phasing of the transformers.

The nett effect on the load 10 voltage is thus zero so that the load is effectively decoupled from the converter.

This arrangement does increase the impedance of the standby supply but in most applications this would not be expected to be an embarrassment. However, if required Z may be reduced to a fraction of the machine impedance and the transformer ratio chosen in corresponding proportion. This reduces the additional impednace in series with the load at the expense of an increase in the impedance seen by the converter.

In an alternative connection the impedances 90, 92 and 94 are connected in parallel with the other windings of the transformers 84, 86 and 88 respectively.

The impedance in question is predominantly reactive it can be combined conveniently with the transformer by using a gapped core in the construction of the latter. The electrical circuit of the arrangement is shown in Fig. 7 which shows a gapped core transformer 95 in one phase output line from the machine 20 which would replace, say, the impedance 90 and transfromer 84 in one of the phase outputs from machine 2 in Fig. 6.

In another embodiment the standby supply is modified by the addition of a prime mover which may be coupled to the a.c. machine in order to extend the useful period of the supply beyond that given by a single battery charge.

An internal combustion engine is a typical example of a suitable prime mover. Other modifications and variations may be made within the scope of the invention.

WHAT WE CLAIM IS: 1. Standby A.C. power supply means comprising battery power storage means, normally operative battery charging means for connection to an a.c. mains supply, normally inoperative d.c. to a.c. inverting means connected to receive d.c. current from the battery means and provide a.c. current to standby a.c. supply terminals, a rotary a.c.

machine the armature winding of which is connected to the standby a.c. supply terminals and the field winding of which is connected to a source of excitation current, and control means responsive to failure of the a.c. mains supply to render the d.c. to a.c. inverting means operative to provide standby a.c.

2. Standby power supply means according to Claim 1 including switch means responsive to cessation of the a.c. mains supply to disconnect the a.c. mains supply from the standby supply and the load and to activate the control means.

3. Standby power supply means according to Claim 1 or 2 wherein the rotary a.c. machine comprises an alternator.

4. Standby power supply means according to any preceding claim wherein the source of excitation current comprises a chopper circuit connected to receive current from the battery means and provide excitation current to the field winding of the a.c. machine.

5. Standby power supply means according to any preceding claim wherein the rotor of the a.c. machine is coupled to flywheel means to increase its moment of inertia.

6. Standby power supply means accordidg to any preceding claim wherein the rotor of the a.c. machine is couplable to a prime mover.

7. Standby power supply means according to any preceding claim arranged and constructed for three-phase operation.

8. Standby power supply means according to any preceding claim wherein the d.c.

to a.c. inverting means comprises a gatecontrolled thyristor bridge and the thyristor gates are connected to gate control means operative to synchronously control the thyristor gates.

9. Standby power supply means according to any preceding claim wherein the battery charging means comprises a gate controlled thyristor bridge and the thyristor gates are connected to gate control means operative to synchronously control the thyristor gates.

10. Standby power supply means according to Claim 8 and 9 wherein the battery charging means and the d.c. to a.c. inverting means comprises a common thyristor bridge and gate control means operable in alternative charging and inverting modes according to the continuation of cessation respectively of the a.c. mains supply.

11. Standby power supply means accord

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. -formers 84, 86 and 88 and impedances 90, 92 and 94 connected in each of the three phase outputs as shown. The impedances 90, 92 and 94 are models of the subtransient impedance of the machine, or of whatever impedance is operative with respect to overlap in the converter. Thus, if the transformers have unity ratio the values of the impedances Z are substantially equal to the impedance of each phase of the machine. Where the transformer ratio is other than unity the impedance value is the appropriate proportion of the phase impedance.Assuming now that the transformers have unity ratio a transient current i flowing between the converter 26 and the machine 20 will induce a transient voltage iZ at the standby supply terminals 12, but also an additional votage -iZ in series with the load due to the external impedances 90, 92 and 94 and the phasing of the transformers. The nett effect on the load 10 voltage is thus zero so that the load is effectively decoupled from the converter. This arrangement does increase the impedance of the standby supply but in most applications this would not be expected to be an embarrassment. However, if required Z may be reduced to a fraction of the machine impedance and the transformer ratio chosen in corresponding proportion. This reduces the additional impednace in series with the load at the expense of an increase in the impedance seen by the converter. In an alternative connection the impedances 90, 92 and 94 are connected in parallel with the other windings of the transformers 84, 86 and 88 respectively. The impedance in question is predominantly reactive it can be combined conveniently with the transformer by using a gapped core in the construction of the latter. The electrical circuit of the arrangement is shown in Fig. 7 which shows a gapped core transformer 95 in one phase output line from the machine 20 which would replace, say, the impedance 90 and transfromer 84 in one of the phase outputs from machine 2 in Fig. 6. In another embodiment the standby supply is modified by the addition of a prime mover which may be coupled to the a.c. machine in order to extend the useful period of the supply beyond that given by a single battery charge. An internal combustion engine is a typical example of a suitable prime mover. Other modifications and variations may be made within the scope of the invention. WHAT WE CLAIM IS:

1. Standby A.C. power supply means comprising battery power storage means, normally operative battery charging means for connection to an a.c. mains supply, normally inoperative d.c. to a.c. inverting means connected to receive d.c. current from the battery means and provide a.c. current to standby a.c. supply terminals, a rotary a.c.

8. Standby power supply means according to any preceding claim wherein the d.c.

11. Standby power supply means accord

ing to any preceding claim wherein the d.c. to a.c. inverting means and/or the rotary machine is connected to the standby supply terminals via voltage transforming means.

12. Standby power supply means according to Claim 11 wherein the transforming means comprises the stator winding of the a.c.

machine in the manner of an autotransformer.

13. Standby power supply means according to Claim 11 wherein the transforming means comprises separate windings on the armature of the a.c. machine.

14. Standby power supply means according to any preceding claim further including filter means connected in the, or each, phase output of the supply so as to effectively decouple the load and the inverting means.

15. Standby power supply means according to Claim 13 wherein the filter means comprises a transformer the primary winding of which is connected in series with the output of the inverting means, the secondary winding of which is connected in series with the supply output terminals and an impedance connected in parallel with either transformer winding.

16. Standby power supply means according to Claim 13 wherein the filter means comprises a gapped core transformer in which the primary is connected between the inverting means and the a.c. machine and the secondary is connected in series with the standby supply terminals.

17. Standby power supply means substantially as described herein with reference to the accompanying drawings.