CIRCUIT TO INHIBIT VOLTAGE AND CURRENT RETURNS TO THE ELECTRIC POWER DISTRIBUTION LINE IN AN INVERTER-TYPE POWER PLANT

Abstract: The circuit is suitable for application to inverter-type power plants where the electric distribution line (RD) is used as stand-by unit. The circuit is composed by a ballast impedance (I) and by an electric mesh comprising: the energizing circuit of first sectioning means (S1) suitable to insulate power plant from the distribution line (RD); the contacts of second sectioning means (S2) which are monitored by the supervisory means (SM) of the power plant; an electric probe (SE) suitable to make available a voltage or a current when current flows through said distribution line (RD).

Description: Circuit to inhibit voltage and current returns to the electric power distribution line in an inverter-type power plant DESCRIPTION The present invention refers to a circuit suitable to inhibit voltage and current returns to the distribution line in an inverter-type power plant 5 which uses said distribution line as stand-by unit to which the load is switched in presence of faults in one or more inverters or in presence of an overload condition caused for example by a fault in the utilizing units. 10 As known, the inverters are suitable to receive a direct voltage at their input and to make available at their output an alternating voltage which feeds a load. The power plants of said type comprise one or more 15 inverters, connected in parallel, as well as one or more stand-by inverters or the electric distribution line - as stand-by unit. • Said power plants are then used to feed systems which require an high degree of reliability and it is 20 therefore necessary to provide working and stand-by devices, as well as supervisory means. The present invention can be applied when the electric distribution line is used as stand-by unit. In particular, when the supervisory means detect the 25 presence of faults in the working devices, they provide the disengagement of a first set of static switches which, by assuming the condition of "open switches", cause the sectioning of the connection between the load and the working devices. Said static 30 switches may be centralized (located on the multiple unit which connects the parallel of the inverters to the utilizing units) or remotely located (i.e. in cascade to each inverter). At the same time the supervisory means release a second set of static 35 switches which, by assuming the condition of "closed switches" connect the load to the distribution line which is the stand-by unit, as specified above. During the normal operation (load feeded by inverters) and due to a fault in the system, it is possible that the second set of static switches unduly assume the configuration of "closed switches" and connect in this way the distribution line to the inverters. Said fault causes both accident prevention- and handling problems. The problems relating to the accident prevention arise when the faults mentioned above occur and determine voltage and current ' returns to the electric distribution line. When said faults occur during a maintenance operation (subsequent for example to a sectioning operation of a network section which is therefore insulated from the power generators located upstream from the network section interested), problems relative to the accident prevention may arise, as the personnel works in the convinction that the mains is not connected to any power source. The handling problems arise in presence of conditions similar to the above (load feeded by inverters and with a fault in the second set of static switches) and they are caused by a power transfer to the distribution line in the absence of any contractual regulations suitable to control said transfer. The units which arrange the electric energy distribution do not usually allow the direct parallel between the network and a set of inverters, except when a specific contractual regulation is provided and adequate technical means for the execution of said operation are arranged. In the known power plants, due to faults in the static switches and/or in the supervisory means, the voltage and current returns to the electric distribution line -J is inhibited by a current return device connected in series to the electric distribution line. As known, said devices are adequate to carry out a sectioning on said line when they detect an energy transfer in the direction opposite to the normal working direction. Said devices are too expensive and have an operating time too high. They are therefore suitable to solve the handling problems cited above, but are not adequate to solve the problems connected to the accident prevention, as the personnel usually intervenes when the sectioning has been already carried out: no current therefore flows and said current return device may not operate. In fact, as the operating time of these devices are too high, the sectioning of the connection may be not effected (if the current intensity of the human body is inferior to the operating current) or may be effected when the accident has already occured. The known devices are not fit therefore to solve problems of accident prevention nature and caused by voltage returns to the electric distribution line. Purpose of the present invention is a circuit, particularly simple, reliable and cheap, suitable to solve the problems connected both with the accident prevention and the handling, as stated above. Object of the present invention is therefore a circuit for the inhibition of voltage and current returns to the electric distribution line, particularly suitable for the application in inverter-type power plants comprising: one or more inverters connected in parallel which feed a load over a first set of static switches; first sectioning means for the connection of the load to the electric distribution line through a second set of static switches; supervisory means suitable to release the first set of static switches » respectively the second set of static switches in relation to the inverters condition, i.e. to the feeded loads and to the electric distribution line. The circuit is composed by a ballast impedance and by an electric mesh including: the energizing circuit of said first sectioning means; - an electric probe having a voltage or a current available at its ends when the current flows through said electric distribution line; the contacts of said second sectioning means, which are monitored by the supervisory means and are closed when the load is feeded by the inverters and open when the load is feeded by the mains. When the load is feeded by said set of inverters and in presence of a fault in the second set of static switches, even when the distribution line section joined to the power plant is sectioned and therefore without any load, the same current which flows through said ballast impedance (preferably capacitive or inductive type) flows through the electric branch downstream from the first sectioning means. Said current flow is detected by the electric probe, at the ends of which a voltage or a current is available and the current flows therefore through the energizing circuit of the first sectioning means. These latter disengage the power plant from the distribution line, avoiding in this way the accident prevention and handling problems mentioned above. Further characteristics of the circuit can be taken from the following description which refers to a non-limiting example shown in the following drawings, whereas: Figure 1 shows a single-phase power plant, including a first embodiment of the circuit object of the invention; Figure 2 shows the same power plant under 1, 5 including a second embodiment of the circuit object of the invention; Figure 3 shows a three phase power plant including said first embodiment of the circuit. The power plant of Figure 1 is so shaped as to 10 include: a plurality of inverters IN,, IN^ , IN , connected in parallel ; a first set of static switches, composed by two monitored diodes DC, and DC2 J which are jointly 15 released by supervisory means and are opposite connected so as an alternating current may flow through them; a second set of static switches composed by two monitored diodes DC, and DC,, which are jointly 20 released by the supervisory means so as an alternating current may flow; first sectioning means S, suitable to connect the electric distribution line RD - at wires R and N - to the second set of static switches (DC, and 25 DC 4 ); supervisory means SM suitable to verify predetermined parameters (for example, amplitude, phase, etc.) of the voltage available at the output both of the inverters N and of the line RD 30 and further suitable to provide the supply of the load through inverters IN, i.e. through the distribution line RD and further fit to monitor the closing and opening of the second sectioning means S . The circuit according to the invention is composed by a ballast impedance I and by an electric mesh including: the energizing circuit E of said first sectioning means S,; the contacts of said second sectioning means S 2 , which are monitored by the supervisory means S ; an electric probe SE, composed by a transformer TA, whose primary winding is connected in series to the network and whose secondary winding is connected to the ends of a resistance R, . When the load is feeded by the inverters IN the supervisory SM shall: release the diodes DC, and DC 2 ; - disengage the diodes DC, and DC.; cause the closing of the contacts of the second sectioning means S 2 . When no faults in the diodes DC 3 and DC, are found, they assume the condition of "open circuit" and inhibit voltage or current returns to the electric distribution line RD. Under these conditions no current flows through the electric mesh and the contacts of the sectioning means S, are therefore closed. When a fault in the diodes DC, and DC, (or their control circuits) occurs, they assume the condition of "closed circuit", causing voltage and current returns to the distribution line RD as well as problems connected to the accident prevention in the absence of the circuit object of the present invention. In this case a voltage is available at the ends of the electric probe SE, which causes the flow of the current through the energizing circuit E of the sectioning means S,; an opening of the relative contacts is therefore registered, by inhibiting in this way any voltage or current returns. .- According to a preferential embodiment of the invention, the ballast impedance I is of reactive type (preferably capacitive or inductive) in order not to disspate active power. 5 It is further possible to use a resistive impedance or a combination of the above resistances. It is useful to point out that said impedance I may result as non essential when a separator transformer is provided (here not illustrated) between the 10 distribution line and the load or other impedance, concentrated or distributed. In particular said concentrated impedance is not essential when the impedance absorbed by the distributed impedance, i.e. by the magnetizing current of the separator 15 transformer, is sufficient to cause the energizing of the means S, and the opening of the relative contacts. The circuit according to the invention is suitable to cause the opening of the contacts of the means S, even when the network section joined to the power plant has 20 been sectioned in order to allow the maintenance operations. In fact, when a fault in the diodes DC, and DC, is found, a reactive current is registered, caused by said ballast impedance I, which - if of reactive type - does not involve active energy 25 dissipation and causes the opening of the means S, independently from the condition of the knife switches arranged upstream from the network section under maintenance. It is further possible to use an energizing circuit of 30 the first sectioning means monitored by a current instead of by a voltage as illustrated in Fig. 1. In this case the resistance R, provided in the electric probe SE is not essential. It is necessary to point out that said electric mesh 35 may assume configurations which differs from what described above (for example, the second sectioning means are arranged in parallel to the energizing circuit of the first sectioning means), that are however based on the same principle according which the second sectioning means S 2 influence the activation of the first sectioning means S, through the mesh opening, i.e. by causing a short circuit of some of the mesh elements. It is further to note that, as illustrated in Fig. 1 with a dotted line, the electric probe SE may indirectly detect a current flow through the electric distribution line RD. It is possibile in particular to connect to the distribution line RD the primary winding of a step-down transformer TR, to the secondary winding of which the above transformer TA is connected. Fig. 2 shows a second embodiment of the invention, where the electric probe SE replaces the transformer TA mentioned above and provides the presence of a differential transformer TD, having: - a first winding through which the current produced by the inverters flows; a second winding through which the current for the load flows; a third winding through which the current flows when (in presence of a fault in the diodes DC, and/or DC,) the value of the current emitted by the inverters differs from the value of the current which comes to the load. - During the normal operation (load feeded by the inverters), in the absence of a fault in the diodes DC 3 and DC, on the third winding of TD, no current flows as the magnetic streams originated by the currents which flow through the first and the second winding are equal and opposite at the same time and therefore they annul each other. 3 When a fault in said diodes DC 3 and DC. is registered, part of the current produced by the inverters comes to the electric distribution line RD as well as to the ballast impedance I and the value of the current which flows through the first winding therefore differs from the value of the current which flows through the second winding. The magnetic streams originated by said currents do no annul! therefore each other completely, and on the third winding a current is induced, which causes the activation of the sectioning means S, according to procedures similar to those illustrated for the first embodiment of the invention. Fig. 3 shows a three-phase type power plant including said first embodiment of the circuit according to the invention. In addition to the neutral 'wire N and to the phase R - present in the single-phase power plants - there are the wires S and T. The first sectioning means S, , the first set of static switches DC, and DC 2 , the second set of static switches DC 3 and DC,, the primary winding of transformer TS and the ballast impedance I are constituted by three functional units instead of one unit as illustrated in Fig. 1. The second embodiment of the circuit arrangement according to the invention is not illustrated making reference to the three-phase power plant, as the relative diagram may be easily composed if the principles taken from Fig. 2 are applied to the diagram of Fig. 3. On the base of what above stated and according to the figures mentioned, the circuit meets the requirements stated in the invention and it is simple, reliable, cheap and may be applied both to the single-phase and to three-phase power plants.

Claims: CLAIMS 1) Circuit for the inhibition of voltage and current returns to the electric distribution line, particulary suitable for application in inverters-type power plants, including: one or more inverters connected in parallel and feeded by a load through a first set of static switches; first sectioning means suitable to connect the load to the electric distribution line over a second set of static switches; supervisory control means suitable to activate the first set of static switches, respectively the second set of static switches, relating to the inverters condition, i.e. to the feed load and to said electric distribution line. characterized in that it is composed by a ballast impedance I and an electric mesh including: - the energizing circuit (E) of said first sectioning means (S,); the contacts of second sectioning means (S 2 ) which are monitored by supervisory control means (SM); - an electric probe (SE) at the ends of which a voltage or a current is available when the current flows through said electric distribution line (RD). 2) Circuit claimed as under claim 1, characterized in that said electric probe (SE) is suitable to monitor said energizing circuit (E) through current and it is composed by a transformer (TA) having its primary winding connected in series to the electric distribution line (RD). 3. Circuit claimed as under claim 1, characterized in that said electric probe (SE) is suitable to monitor said energizing circuit (E) through voltage and it is constituted by a transformer (TA) having its secondary winding connected to the ends of a resistance (R,). 4. Circuit claimed as under claims 2 and 3, characterized in that said transformer (TA) is connected to the electric distribution line (RD) over an additional transformer (TR). 5. Circuit claimed as under claim 1, characterized in that said electric probe (SE) is constituted by a transformer having: a first winding crossed by the current produced by said inverters (IN); a second winding crossed by the current which feeds the load (Z); a third winding crossed by a current having a value equal to the difference among the currents which flow through the first and second winding. 6. Circuit claimed as under claim 5, characterized in that a resistance (R, ) is connected to the ends of the third winding. 7. Circuit claimed as under claim 1, characterized in that said ballast impedance (I) is of reactive type. 8. Circuit claimed as under claim 1 characterized in that said ballast impedance (I) is composed by the impedance of the secondary winding in a separator transformer.

Inventors: LENTINI SALVATORE, PATTI GIUSEPPE, LENTINI, SALVATORE, PATTI, GIUSEPPE
Applicants: ITALTEL SPA, ITALTEL SOCIETA ITALIANA TELECOMUNICAZIONI S.P.A.
Application year: 1988
Main classification: H02J7
IPC codes: H02J9