Ac Supply System

Abstract

The voltage stability of an AC supply system having a plurality of AC current supply assemblies connected in parallel to a common load is improved when a malfunction is detected in one of the current supply assemblies by first connecting the load to a standby AC main with which the current supply assemblies are synchronized, disconnecting the malfunctioning current supply assembly from the load, and, after adjusting the properly functioning current supply assemblies to supply the total power output prior to the malfunction, disconnecting the load from the standby AC main.

Description

BACKGROUND OF THE INVENTION

This invention relates to an improved AC supply system of the type including a plurality of individual current supply assemblies connected in parallel to a common load. More particularly, this invention relates to a method and apparatus for improving voltage stability of such a system when one of the current supply assemblies is malfunctioning.

In AC supply systems it is quite common to provide a standby AC main with which the individual current supply assemblies or sets of the system are synchronized and to which the load is switched or connected in case of a malfunction in the system in order that the power supplied to the load continue without interruption. Various schemes for connecting the current supply assemblies to the load and for switching or connecting them to the standby AC main are known.

In one known type of AC supply system providing for the switching of a load from a current supply assembly to a standby AC main or to a substitute current supply assembly without interruption in the event of a malfunction in the current supply assembly, at least two current supply assemblies are provided which, when the system is operating normally, individually feed identical partial loads which are separated from one another but which together form the total load. The two current supply assemblies are also designed to be able to individually supply the current for the total load. Whenever a malfunction is detected in one of the current supply assemblies, the respective partial load is automatically switched without interruptions in the current supply, by means of an electronic switching device, directly to the output of the properly functioning current supply assembly. If the latter current supply assembly should then also malfunction, so that both assemblies are then inoperative, the entire load is switched automatically and without interruption to the standby AC main.

In another known type of AC supply system, a single current supply assembly feeds the load under normal conditions and a substitute assembly and a standby AC main are available for emergencies. When the current supply assembly malfunctions, the substitute assembly is automatically put into operation and the load is switched by means of electronic switching devices, and in particular an electronic switch common to both assemblies and a switch connected to the standby AC main which are switched with overlapping closing times, first temporarily to the standby AC main and then to the substitute assembly.

It is also known to provide an AC supply system having a plurality of current supply assemblies, e.g. thyristor inverters, which operate in parallel for a single load in order to increase the output or dependability of the system. In such a system, if a malfunction occurs in one of the current supply assemblies, the load voltage may deviate greatly from the rated or desired value due to the influence exerted on the parallelly operating current supply assemblies, depending on the type of malfunction, until the malfunction assembly is separated or disconnected from the load. This large voltage deviation is produced due to the fact that the malfunctioning assembly represents a short circuit type load for the properly operating assembly or assemblies. For some loads, for example data processing systems, a continuously constant supply voltage is an absolute must and accordingly, this type of supply system can not normally be used for such loads.

SUMMARY OF THE INVENTION

It is accordingly the object of this invention to provide a solution for the above-mentioned problem with AC supply systems having a plurality of current supply assemblies connected in parallel to a single load and requiring a minimum number of switching devices. According to the invention, this object is achieved in that whenever a malfunction of one of the current supply assemblies is detected, the common output of the current supply assemblies and hence the load is switched to the standby AC main, with which the assemblies are synchronized, through the aid of an electronic switch and, after separation of the malfunctioning assembly from the system by opening the associated switching device connected in the respective output line of the assembly, and after stabilization of the remaining assemblies not affected by the malfunction to the total power output provided by the system prior to the malfunction, the remaining properly functioning current supply assemblies are again disconnected from the standby AC main.

According to a feature of the invention, energy storage devices, e.g. chokes and capacitors of filter circuits, are disposed in the outputs of the current supply assemblies. These energy storage devices ensure that the load voltage will not suddenly change when an internal malfunction of one of the current supply assemblies occurs, but rather remains almost constant until the malfunction is detected and the electronic switching triggered. Due to the synchronism between the AC supply assemblies and the standby AC main, switching can occur at any moment.

According to a further feature of the present invention, the current supply assemblies are monitored for the occurrence of internal malfunctions as well as for undue deviations in the load voltage so that switching to the standby AC main without interruptions is performed upon the malfunctioning of one of the current supply assemblies as well as when there is an undue deviation in the voltage, caused, e.g., by an external short circuit.

In order to simplify the monitoring of malfunctions in a current supply system, according to a further embodiment of the present invention, a plurality of electronic switches which are individually associated with the current supply assemblies and which are disposed between the standby AC main and the common output of the system are utilized. In this embodiment, the individual current supply assemblies can each be separately monitored by a monitoring member and the monitoring thus becomes simpler than with central monitoring of the entire system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of an AC supply system according to the invention utilizing a central monitoring and control device.

FIG. 2 is a block diagram of a further embodiment of an AC supply system according to the invention utilizing separate monitoring and control devices for each of the current supply assemblies.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown a block circuit diagram of an AC supply system having two AC current supply or inverter assemblies 1, 2 whose outputs are connected in parallel to a common load 3. In a manner well known in the art, the inverters 1 and 2 which may, for example, be static single phase or three phase inverter circuits containing thyristors, bipolar thyristors or transistors, are controlled by a common control unit 4 which is synchronized with a standby AC main N by means of a synchronizing device 5, thus also synchronizing the inverter assemblies 1 and 2 with the standby AC main N. Connected in the output line of each of the current supply assemblies 1 and 2 in a known manner, is an energy store El or E2, respectively, which may, for example, be the capacitors or inductances of a filter, and a switch S1 or S2, respectively. Although the switches S1 and S2 are indicated as mechanical switches, it is to be understood that they can also be electronic switches. In order to monitor the operation of the system, a central monitoring and control member 6 is provided having, for this purpose, signalling lines a1, a2 leading from the current supply assemblies 1 and 2, respectively, and a further signalling line leading from the control unit 4. It should be noted that the control lines a1 and a2 are schematic in nature and may in fact contain a plurality of separate lines which monitor not only the input and output voltages of the current supply assemblies, but also internal operations of the assemblies, for malfunctions.

A thyristor-type converter having an energy store (filter) is disclosed in an article by J. Schmidt, entitled "Gesicherte unterbrechungsfreie Stromversorgungsanlagen mit Thyristorwechselrichtern" ETZ-B, vol. 29 (1969), No. 16, pages 387-392.

In addition to the switches S1 and S2, according to the invention, the system is further provided with a switch 8, preferably electronic, which is connected between the common output line 9 of the system and the standby AC main N. Closure of this switch in the event of a malfunction will thus have the effect of connecting the common output line 9 and hence the load to the AC main N. To control the operation of the various switches and adjust the system upon the occurrence of a malfunction, the monitoring and control member 6 is provided with control lines d1 and d2 for the switches S1 and S2, respectively, c for the switch 8, and e1 and e2 for the inverters 1 and 2, respectively. These latter control lines e1 and e2 are utilized to adjust the output of the functioning inverter when a malfunction in the system is discovered.

Turning now to the operation of the system, it should first be noted that all of the switches are shown in their positions for normal operation of the system. If now during operation of the supply system a malfunction occurs, for example, at inverter 1 so that it ceases to function and as a consequence the power fed to the load decreases, whereby, due to the increased load on the still functioning inverter 2 from inverter 1, the supply voltage may decrease accordingly, this malfunction of inverter 1 is signalled through the signalling line a1 to the monitoring and control circuit 6. Circuit 6 then immediately transmits an actuation or control signal to the switch 8, via signalling line c, which sets this switch into its closed position. The common inverter output line 9 and the load 3 are thus connected to the standby AC main N which replaces the missing power for the load 3. The switch 8 remains temporarily closed until the malfunctioning inverter 1 has been separated or disconnected from the load 3 by opening its switch S1 and the inverter 2 has been adjusted to produce the total supply power. These latter functions are carried out by means of two actuating signals which are produced by the circuit 6 almost simultaneously with the actuation signal to the switch 8, and are transmitted via signalling lines d1 and e2 to the switch S1 and the inverter 2, respectively. After opening contactor S1 and adjusting the inverter 2, the monitoring and control circuit 6 transmits a further actuating signal via signalling lines c to the switch 8 which causes it to return to the open position, thus disconnecting the load 3 from the AC main N.

If inverter 2 now also ceases to function properly due to a malfunction, this malfunction is signalled to the monitoring and control circuit 6 via signalling line a2 and a new closing signal is then transmitted through signalling line c to the switch 8 and an opening signal is transmitted through line d2 to the switch S2. Since in this condition the standby AC main N feeds the load 3 by itself, the switch 8 remains closed and the load 3 remains connected to the standby AC main N. After repair of one or more of the inverters, the load 3 can be switched back to the current supply assemblies according to known methods which need not be discussed here in detail. Of course, in the event of a malfunction in the control circuit 4, indicating that the entire system is malfunctioning, the signal transmitted to the circuit 6 via line b will also cause the switch 8 to be closed and connect the load 3 to the AC main N.

Referring now to FIG. 2, there is shown another embodiment of the invention wherein in place of a single malfunction control switch 8 and a central monitoring and control circuit 6, separate malfunction switches and monitoring and control circuits are provided for each current supply assembly or inverter in the system. As shown in the figure, in addition to the switch 8, a second switch 8' is provided which is disposed, as is the switch 8, between the standby AC main N and the common assembly output line 9 having the load 3 connected thereto. With two available switches 8, 8', one of them, for example switch 8, as indicated, is associated as to its function with inverter 2 and the other, switch 8', is associated with inverter 1. This is intended to mean that when inverter 2 experiences a malfunction, the load 3 is temporarily connected to the standby AC main N by means of switch 8 and when inverter 1 is malfunctioning such a switching is accomplished by means of switch 8'.

In addition to the monitoring and control circuit 6, a second control circuit 6' is provided so that the circuits 6 and 6' are associated with the inverters 2 and 1, respectively. In this embodiment, however, each of the monitoring and control circuits 6'. 6 need only monitor one of the inverters 1 or 2, respectively, and accordingly, they are connected to their associated inverters by means of monitoring or input signal lines a'1 and a2, respectively. The circuits 6 and 6' are also provided with monitoring signal lines b and b', respectively, for monitoring the control circuit 4.

In order to provide the required control functions according to the invention, each of the circuits 6, 6' is provided with a respective control line c or c' for controlling the switches 8 and 8', respectively, a control line d2 or d'1, respectively, for controlling the respective switches S2 and S1 associated with the associated inverter being monitored, and a control line e1 or e'2 for adjusting the power output of the other inverter in the circuit. The operation of this embodiment in the event of a malfunction of one or more of the inverters is the same as that for the embodiment of FIG. 1.

Independent of whether one operates with one or two switches 8, 8', i.e., the embodiments of FIGS. 1 or 2, the respective switches 8 and 8' can be bridged in a known manner by means of an emergency contactor S8 or S8', respectively, and the synchronizing device 5 monitored by a separate monitoring device 5'. When the synchronizing device 5 is malfunctioning, the monitoring member 5' initiates a signal via a signalling line f, f' which causes the switches 8, 8' to be blocked and a relay SN in the standby AC main to be opened. Switches 8 and 8' can then no longer be closed when one of the inverters 1 or 2 malfunctions. Instead, in this case the monitoring member 6 or 6' initiates a signal over lines d2 or d'1, respectively, and sends it to the associated switch S8 or S8' causing it to close, so that the load 3 can be connected to the standby AC main N at least with an interruption in its supply voltage.

Several possibilities for controlling the current supply assemblies are known. As shown in the above embodiments, all of the current supply assemblies may be controlled by a common control unit 4 which may be synchronized by means of a synchronizing oscillator 5 with the standby AC main N. Alternatively, each assembly can be controlled by a separate control unit which can be synchronized with the standby AC main by means of a common synchronizing oscillator 5. The separate control units should be so designed that they have a limited influence on each other as concerns their frequencies. It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

Claims

I claim:

1. A method of improving the voltage stability of an AC supply system including a plurality of current supply assemblies which are synchronized with an AC main and which are connected in parallel via individual energy storage devices to a common load upon the malfunction of one of said current supply assemblies comprising the steps of:

connecting said load to said standby AC main upon the detection of a malfunction of one of said current supply assemblies;

disconnecting said malfunctioning current supply assembly from said load;

adjusting the properly functioning current supply assemblies to provide the total power output supplied by said system prior to the malfunction; and

disconnecting said load from said standby AC main.

2. An AC current supply system comprising, in combination:

a plurality of current supply assemblies connected in parallel via individual energy storage devices to a common load;

means for synchronizing said current supply assemblies with a standby AC main;

first normally closed switch means for individually connecting said current supply assemblies to said load;

second normally open switch means for connecting said load to said standby AC main; and

control means responsive to the malfunctionings of one of said current supply assemblies for: load to said standby AC main;

b. disconnecting said malfunctioning current supply assembly from said load by opening the respective one of said first switch means;

c. adjusting the properly functioning current supply assemblies to provide the total power output supplied by said system prior to the malfunction; and then

d. disconnecting said load from said standby AC main by opening said second switch means, whereby said AC supply system then again supplies the entire power required by said load and the voltage stability of the system is maintained during the time required to remove the malfunctioning assembly from the system.

3. The AC supply system as defined in claim 2 wherein said second switch means comprises a plurality of individual switches, one for each of said current supply means, each of which is connected between the common output to said current supply assemblies and said standby AC main.

4. The AC supply system as defined in claim 2 wherein said control means responds to internal malfunctions of said current supply assemblies.

5. The AC supply system as defined in claim 2 wherein said control means responds to voltage drops and/or excess voltages at the inputs of said current supply assemblies.

6. The AC supply system as defined in claim 2 wherein said current supply assemblies are single phase static inverters containing thyristors, bipolar thyristors or transistors.

7. The AC supply system as defined in claim 2 wherein said current supply assemblies are three phase static inverters containing thyristors, bipolar thyristors or transistors.

8. The AC supply system as defined in claim 2 wherein said current supply assemblies are static inverters, and wherein said synchronizing means synchronizes said current supply assemblies with the said AC main via the control device for said static inverters.

9. The AC supply system as defined in claim 8 wherein all of said static inverters are controlled by a central control device.