Monitoring circuit for control means and selective breakaway means in modular supply systems
BACKGROUND OF THE INVENTION 
     The present invention relates to a monitoring circuit for control means and selective breakaway means in modular power supply systems, particularly for telecommunications equipment. More particularly, the present application relates to a circuit designed to enable the right distribution of the load among the elements which compose the system as well as the location and the selective switching off of the units that are out of order. The supply systems of the type described above are usually comprised of a plurality of basic unit (such as rectifiers, or inverters, etc.) that are connected to each other in parallel. This type of connection is used, for example, when the power to be supplied by the system is not delivered by a single power unit and the power is therefore supplied by several power units that are connected in parallel to each other. In addition to the power units that are necessary to supply the operating power, usually at least one other power unit is added, in order to act as a hot reserve to be used in case of a failure of one or more of the operating power units. 
     In these types of systems, optimum conditions are obtained when the total current absorbed by the load is equally supplied by the n elementary units, that is when each power unit supplies a current I having a value of It/n. 
     Furthermore, it must be noticed that when the power units are comprised of inverters, besides the optimization problems as described above, further new technical problems occur in connection with the phase relationship of the oscillations that are supplied by each inverter. In fact, an inverter is constituted by a unit that is designed to receive a continuous voltage on its input and the inverter is adapted to deliver a sinusoidal voltage to the output so that the frequency of the inverter normally coincides with the frequency of the electrical power distribution line. 
     The inverters of the previous generation are characterized by the fact that they present an output impedance that does a not have an excessively low value. Consequently, the problems relating to an equal power distribution as described above can be solved through slow control adjustment systems. 
     On the other hand, modern generation inverters are configured to approximate an ideal voltage generator and are consequently characterized in that they have a particularly low output impedance. The low impedance values result from the use of means that are designed to act in such a manner that the oscillations generated by the inverters have the same level and phase at any time. In fact, if the power system is constituted by n elementary units, having one unit which supplies an oscillation that is shifted in phase with respect to the oscillations generated by the further n-1 units, the power delivered by the n-1 units is delivered to the unit that is out of phase. Consequently possible damage to the unit may be caused the power supplied to the load may be reduced. CL SUMMARY OF THE INVENTION 
     Therefore, the basic technical problem of the present invention is how to realize a supply system with a monitoring circuit that is particularly fast for allowing level and phase variations in the oscillations, generated by each inverter, in a time as short as possible. A short time will minimize the duration of the negative effects caused by a possible phase shift as described above and will consequently avoid the danger of any damage. The Purpose of the embodiments in the present application is to solve the aforementioned technical problem through the use of circuitry that is particularly simple and economical. 
     For this purpose and according to the present embodiments of the supply system is of the type comprising n elementary power units, that are connected in parallel to each other with each having control and selective breakaway means. 
     The monitoring circuit includes the presence of the following characteristic elements: 
     First means, designed to receive a first signal on their input, that is proportional to the current supplied by the relevant power unit. Also a second signal that is proportional to the average current value is supplied by the n power units and is adapted to calculate the entity of an error signal corresponding to the difference between the first signal and the second signal; 
     Second means, designed to algebraically add the error signal at the output of the first means to a third signal. The value of the third signal is proportional to the voltage that is supplied by the relevant power unit. Furthermore, the second means is designed to send the third signal, obtained as described above, to the control and selective breakaway means. 
     The control means modifies the level of the signal relevant power unit until the error signal is cancelled. On the other hand, the selective breakaway means switched off the relevant power unit from the parallel connection when the signal that is supplied by the second means deviates from a nominal value by a predetermined entity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Further characteristics of the application will be more evident in the following description only one embodiment of the invention is given as an example and is complemented by the attached drawings in which: 
     FIG. 1 shows a block diagram of a supply system that is referred to in one embodiment of the present application. 
     FIG. 2 shows in detail one of the monitoring circuits CP from FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, UA 1 , UA 2 , . . . , UA n  refer to a plurality of power units that are connected in parallel to each other by means of the switches T 1 , T 2 , . . . , T n . A monitoring circuit CP, voltage control means RV and selective breakaway means DS are associated to each power unit UA. 
     Each monitoring circuit CP is connected by means of a contact K of the relevant switch T to a common bus B. There is a signal I med  on the common bus B that is proportional to the average value of the current supplied by each power unit UA. 
     Furthermore, the circuits CP receive a signal Iu i  on their inputs that is proportional to the current value supplied by the relevant power unit UA i  that is and a signal Vu i  proportional to the voltage supplied by the relevant power unit UA i . Each signal IU i  is respectively detected by means of a voltage transducer TV i . 
     The task of each monitoring circuit is to generate a signal VC in response to the presence of a voltage error on the relevant power unit UA of the same value. 
     In fact, the signal Vc of reaches the input of the respective voltage control means RV which modifies the level of the signal generated by the relevant power unit UA. The signal Vc is modified until a nominal value is reached and consequently the signal generated by the relevant power unit has a value that is within the predetermined tolerance limits. 
     Also, the signal Vc reaches the input of the respective selective breakaway means DS which activates on output when the difference between the signal Vc and nominal value of the respective selective breakaway means evidences the presence of a failure in the relevant power unit UA. Considering that a generical power unit can be considered to be out of order when the output voltage differs by more than 5% from a nominal value, the means DS has the task of detecting the presence of this condition and activating an output in order to determine the opening of the relevant switch T i . Thus, is switched off the unit UA that has failed. 
     FIG. 2 illustrates a generical monitoring circuit CP which comprises by first means PM adapted to calculate the entity of the error signal E on the relevant power unit UA. Furthermore, second means SM algebraically sums the error signal E to a signal that is proportional to the voltage supplied by the relevant power unit UA. 
     The first means PM comprises by an operational amplifier AO 1 , to the inputs of which the signal Iu i  (which is proportional to the current supplied by the relevant power unit) is applied by means of first and a second resistors R 1  and R 2  of equal value. Between the output of the amplifier A0 1  and the inverting input, a first feedback resistor R 3  is connected. While between the non-inverting input and ground, a second feedback resistor R 4  is connected having a value equal to that of the resistor R 3 . When a generical power unit UA i  is inserted into the parallel connection, the switch T i  and the relevant contact K i  are closed. As a consequence, the signal I med , which is proportional to the average value of the current supplied by the power units, reaches the non-inverting input of the unit AO 1 . 
     If the resistors described above are so dimensioned as to satisfy the relationship R 1  =R 2  and R 3  =R 4 , an error signal E, expressing the voltage error on the relevant power unit corresponds to the output of the unit AO 1 . In particular, the error signal E represents a signal that is proportional to the entity of a shifting between the voltage supplied by the power unit actually under examination and the average value of the voltage supplied by the remaining (n-1) units. 
     On the other hand, the means SM comprises by a second operational amplifier AO 2  which has the non-inverting input connected to a reference potential and the inverting input connected to the output of the unit AO 1  by means of a resistor R 5 . The inverting input also receives the signal Vu i  by means of a resistor R 6  and a feedback signal by means of a resistor R 7 . Under normal operating conditions the output voltages of the various power units are perfectly pulled in phase. The output voltage VU that is applied to the load Z is equal to the average value among the voltages at the outputs of the varius power units. Each power unit compares its own current output with the average current and adjusts its own output voltage until a current equal to the average value is supplied. 
     In the event that one of the power units delivers a higher current than the average current, the error signal E i  will appear to be in phase to the output voltage Vu i . Consequently, the regulation means RV i  receives a signal VC i  on the input with a higher value than the nominal one. As a consequence, the means RV i  arranges a reduction in the current supplied by the relevant power unit UA i  until the current value is approximately equal to the average current. On the contrary, if one of the power units supplies a lower current than the average current the error signal E i  will appear to be in phase opposition to the signal Vu i  and consequently the regulation means RV i  receives a signal Vc i  on the input having a lower value than the nominal signal. As a consequence, the means RV i  determines an increase in the current supplied by the relevant power unit UA i  until the current value is approximated equal to the average current. 
     Besides allowing the adjustment of the current that is supplied by the relevant power unit, the signal Vc also enables indications about the relevant power unit operation. 
     In fact, the weight of the signal E i  has been determined in such a way that the power units which operate at nominal voltage, receive a signal Vc having a nominal value on the inputs. In particular, the weight of the error signal E i  has been determined so that the error that is picked up on the signal Vu i , due to the failure of a power unit, is the same vaue but opposte to the contribution brought by the signal E i . Consequently the signal Vc, which refers to the units operating at nominal voltage, presents a nominal value. 
     In contrast the faulty unit the error present on the signal Vu i  in consequence to the failure of the power unit, is added to the error E i . Consequently, the selective breakaway means DS i  of the faulty unit UA i  receives a signal Vc on the input that differs from the nominal value for the same error percentage that is present on the output voltage. 
     Let us suppose that the power system is composed of two elementary power units, the first of which is operating at nominal voltage, while the second presents an overvoltage error equal, for example, to 5%. If the output voltage is supplied by inverters having a low output impedance, then the voltge VU that is applied to the load will be equal to VU n  (1+2.5%), where VU n  represents the nominal output voltage. 
     On the basis of the material specified above, the voltage regulation means RV of the inverter that is not faulty, receives a signal Vc on the input having the same value as the nominal signal Vc=Vu n , where Vu n  represents a signal that is proportional to the nominal output voltage VU n . In fact: 
     
         (Vu+2.5% VU.sub.n)+(-2.5% Vu.sub.n)=Vu.sub.n =Vc. 
    
     This is due to the fact that the signal E i  appears to be in an opposite phase to the signal Vu i  and the contribution brought by E i  appears to be the same value but opposite to the variation present on the signal VU i . 
     The voltage control circuit RV of the inverter affected by the failure receives a signal Vc on the input that differs from the nominal value Vc n  for the same error percentage that is present on the relevant power unit. In fact: 
     
         (Vu.sub.n +2.5% Vu.sub.n)+(+2.5% Vu.sub.n)=Vu.sub.n +5%Vu.sub.n =Vc. 
    
     This is due to the fact that the signal E i  is in phase with the signal Vu i . 
     The selective breakaway means DS will not be described in detail herein as they are of a well known type that comprises threshold circuits designed to operate their outputs whenever signals are recieved on their input which differs from their own nominal value by a predetermined entity (for example 5%). 
     The activation of the output of the means DS i  opens the switch T i  which switches off the power unit UA i  affected by the failure. 
     The voltage control means RV will be not described in detail as they are well known to a person skilled in the art. 
     The monitoring circuits CP as claimed in the embodiment satisfy the requisites cited in the invention because they deliver the signal Vc, which allows the selective breakaway operations, to be carried out even when the output voltage deviates from the nominal value by a limited entity (for example ±5%). This has the advantage that the oversizing in current of the supply system, due the to current distribution, can be contained within low values (5% of the nominal current). In the control systems, already known, an oversizing of current to the order of 10% is required consequently, when the power system has to supply a higher power (for example 100 KW), a 5% reduction of the oversizing power constitutes a value that is ignored. 
     Furthermore, the monitoring circuit according to the embodiment, is characterized by the fact that an adjustment of the output voltage is allowed in a very short time. Thus, the monitoring circuit is suitable for use in combination with all types of power units, such as rectifiers, positive/negative voltage boosters or inverters. 
     Although the present invention has been described in connection with the preferred embodiment thereof, many variations and modifications may be made without departing from the scope or spirit of the invention. It is preferred, therefore, the present invention is not limited by the disclosure herein, but by the claims.