The present invention relates to an emulator device for emulating a variable impedance, and in particular a variable reactance, for use in particular as a variable power reactor for various applications.

More specifically, the present invention relates to an impedance emulator device of the type defined in the preamble of Claim 1.

An impedance emulator of this type is described in US 5 699 219 A. This emulator device comprises an inductor having a plurality of intermediate terminals or taps which divide it into a plurality of segments, each of which is connected in parallel to a corresponding pair of SCRs or thyristors which are connected to each other in anti-parallel. This solution can be used to vary the reactance in rather coarse steps.

A first object of the present invention is to provide an improved emulator device for emulating variable impedance and in particular reactance, this device providing high- precision control, in modulus and in phase, of the emulated impedance or reactance.

This and other objects are achieved according to a first aspect of the invention by means of an emulator device whose principal features are defined in the attached Claim 1.

In . one embodiment, the control means associated with the array or matrix of electronic switches are, in particular, designed to apply to the control terminals of its electronic switches square-wave driving signals with a modulatable pulse width or duration, such that the modulus of the impedance emulated by means of said array or matrix of switches can vary, in a practically continuous way, as a function of the pulse width or duration of said driving signals.

Additionally, said control means are conveniently designed to apply to the control terminals of the electronic switches of said array or matrix driving signals whose pulse width or duration is modulatable, for instance in an essentially sinusoidal manner, such that the phase of the reactance emulated by means of the aforesaid array or matrix of switches varies as a function of the width or duration of said driving signals. Another object of the present invention is a system for the control and regulation of the state of the neutral conductor in a three-phase system for the distribution of alternating current power, comprising a variable impedance emulator device of the type defined above, connected between the neutral conductor of said power distribution system and earth, said control and regulation system having the features defined in the attached Claim 6.

Further characteristics and advantages of the invention will be made clear by the following detailed description, provided purely by way of non-limiting example, with reference to the appended drawings, in which:

Figure 1 is a circuit diagram illustrating a three-phase power distribution system with the neutral conductor earthed by means of a Petersen coil or inductor according to the prior art,

Figure 2 is a circuit diagram, partially in block form, showing a system according to the invention, for the control and regulation of the state of the neutral conductor in a three- phase system;

Figures 2a, 2b and 2c are circuit diagrams similar to those of Figure 2, showing variant embodiments,

Figure 3 is a block diagram of a matrix of groups of controlled electronic switches, used in the control and regulation system shown in Figure 2;

Figure 4 is a circuit diagram illustrating a possible structure and the procedures for interconnection of the groups of switches included in the matrix of Figure 3;

Figures 5 and 6 show two different architectures of driver transformer devices for controlling the matrix of switches of Figure 3; and

Figures 7 to 1 1 show different embodiments of an inductor which can be used in a system for the control and regulation of the state of the neutral conductor according to the invention. Figure 1 illustrates part of a three-phase alternating-current electric power distribution system, indicated by TDS, comprising a line with three conductors or phases A, B and C, and with a neutral conductor N. The symbols CAB, C _A C and CBC indicate the capacitances between the phases A, B and C. The symbols CA, CB and Cc indicate the capacitances between each of the phases A, B and C and earth, which is indicated by T.

In Figure 1 , a switch S W represents an earth fault T.

There are various known methods of earthing the neutral conductor N.

In systems where the neutral conductor is earthed (directly or through an impedance), earth faults become short circuits, which are far less current-limited, requiring the intervention of switches even in the case of transient faults.

In isolated neutral systems, however, the line can be kept operational during temporary earth faults, because the fault current is small. On the other hand, the excess voltages are high in such cases, and the isolated neutral system must be designed appropriately to ensure that excess voltages can be withstood without damage for a given time.

The system TDS of Figure 1 has its neutral conductor N earthed in a known way by means of an inductor L, also known as a Petersen coil. This neutral earthing method has the advantage of ensuring continuity of the power distribution service in case of temporary earth faults.

The neutral earthing method using a Petersen coil also considerably reduces the risk of intermittent arcing to earth, and automatically suppresses the highly destructive effects of earth faults. This method allows directional wattmeter earth protection to be used, thus combining rapidity of fault selection and elimination with low fault currents in the earthing systems. In existing Petersen coil systems, the neutral conductor is earthed through a variable reactance, which is tuned, by means of an automatic measurement and computing system, to the capacitive reactances associated with the distribution lines. The reactance of the Petersen coil is typically varied by means of an electric motor which varies the air gap of the inductor by moving a metal core with suitable characteristics into or out of a winding. The control system controls this motor in accordance with the results of calculations carried out for the purpose of maintaining the tuning between the various reactances associated with the line.

The neutral earthing reactance is equipped with high-power resistors which are connected or disconnected in order to adapt the active part of the neutral current. High-power resistors in series are also provided, in order to adapt the time constant of the system. The use of these Petersen coil earthing devices is widespread, and is of strategic importance for the assurance of continuity in the power supply.

There are also known systems for the control and regulation of the state of the neutral conductor, based on the use of electric or electronic components of the active or passive type.

In active systems, current is injected into the neutral conductor to compensate for the unbalance and earth fault currents. The current injection is carried out by various methods, all based on the use of power originating outside the regulated system.

Passive systems use an earthing coil of the fixed type, provided with secondary windings, and these systems require a division of the phase on a secondary of this earthing coil.

According to one aspect of the present invention, an innovative system is proposed for the control and regulation of the state of the neutral conductor of a three-phase power distribution system. The system according to the invention is essentially of the passive type, and will now be described with reference to the attached drawings, and in particular to Figure 2 and the subsequent figures.

In Figure 2, a system for the control and regulation of the state of the neutral conductor of a three-phase power distribution system TDS is indicated as a whole by 1.

In the embodiment illustrated by way of example in this figure, an inductor L and a resistor R, in series with each other, are connected between the neutral N of the distribution system TDS (of the three-phase type, for example) and the earth T.

The inductor L has a terminal 2 connected to the neutral N, and a terminal 3 connected to the resistor R.

The control and regulation system 1 of Figure 2 comprises an ordered array, particularly a matrix, of solid state electronic switches, indicated as a whole by 5.

The array or matrix 5 of electronic switches has three terminals 10, 1 1 and 12, connected, respectively, to terminals 2 and 3 of the inductor L, and to earth T.

A possible structure or architecture of the ordered array or matrix 5 will be described subsequently with reference to Figures 3 and 4.

The array or matrix 5 of electronic switches is associated with a driver device 6 which in turn is controlled by a controller 8, constructed for example with the use of one or more microprocessors.

The controller 8 is connected to the driver device 6 and to further devices, which will be described subsequently, by means of a communications bus 7.

This bus is connected to a current measurement device 9 associated (for example) with the resistor R, and intended to supply signals or data indicating the strength of the current flowing between the neutral conductor N and earth T.

The bus 7 is also connected to an apparatus 13 which can supply signals or data indicating the currents in the phases A, B and C and an apparatus 14 which can supply signals or data indicating the voltages between these phases.

By means of the communications bus 7, the data supplied by the measurement devices or apparatus 9, 13 and 14 are delivered to equipment 15 designed to analyse the state of the neutral conductor N. This equipment can include a first section 15a essentially comprising a so-called neutral analyser device of a known type, and a second section 15b for measuring the strength of the leakage currents flowing to earth T. This second section is also of a known type.

The analyser equipment 15 is connected to, and interacts with, the controller 8.

In alternative embodiments (not shown), the controller 8 and the analyser equipment 15 could be integrated in a single device.

To summarize very briefly, the analyser equipment 15 collects the data relating to the voltages and currents in the controlled distribution system TDS, these data being required in order to determine the most suitable impedance to be created between the neutral conductor N and earth T, for the purpose of completely, or at least substantially, neutralizing the current generated by all the unbalances present in the three-phase line A, B, C.

Various types of neutral analyser equipment are known, and are often based on current injection using various methods.

In a first method, a voltage at power frequency is applied between the neutral N and earth T in order to perturb the balanced state of the line, and to obtain data for use in setting the correct reactance between the neutral and earth. In other methods, current is injected with special or non-periodic waveforms, such as Dirac pulses.

The system 1 for the control and regulation of the state of the neutral conductor according to the invention can use neutral analyser equipment using any of the known systems, and in particular those briefly outlined above.

The controller 8 interacts with the analyser equipment 15, and generates a system of control signals for the driver device 6, as a function of the detected values of the magnitudes or parameters monitored by this analyser equipment 15.

The driver device 6 applies driving signals to the electronic .switches of the array or matrix 5, these driving signals being capable of controlling the current conduction of the switches, in such a way that the array or matrix 5 as a whole (in conjunction with the inductor L) can emulate in a controlled way an impedance which is variable in modulus and in phase.

With reference to Figure 3, in the embodiment illustrated therein the array or matrix 5 comprises a plurality of groups 20 of electronic switches. As seen in Figure 4, each group 20 includes at least one pair of electronic switches Ql and Q2, such as transistors of the MOSFET type, interconnected in anti-series.

Alternatively, the electronic switches Ql and Q2 could be, for example, transistors of the IGBT type, or bipolar transistors.

In the specific example shown in Figure 4, the two transistors Ql and Q2 have their respective gates connected to each other and to a first line conductor 21 , while their sources are also connected to each other and to a second line conductor 22.

The drain of the transistor Ql of each group 20 is connected to a line conductor 23 and the drain of the transistor Q2 is connected to a line conductor 24. In general, the array or matrix 5 can comprise n rows of groups of switches and m columns, where n and m are integers greater than or equal to 1. In the example shown in Figure 3, the array 5 comprises m=3 columns and, for example, n=60 rows. As can be seen in Figure 3, in the matrix 5 the groups 20 of electronic switches in each row are connected in parallel with each other between a pair of line conductors 23 and 24, and the rows of groups 20 of switches are connected in series with each other.

The controller 8 is conveniently designed to supply to the driver device 6 control signals such that this device applies to the gates and sources of the switches Ql , Q2 of the array or matrix 5 square-wave driving signals with a modulatable pulse width or duration, in such a way that this array or matrix of switches as a whole emulates an impedance whose modulus varies in a manner corresponding to the pulse width or duration of these driving signals.

The controller 8 is also conveniently designed to supply to the driver device 6 control signals such that this device applies to the control terminals (gate and source) of the switches of the array or matrix 5 driving signals having a pulse width or duration which is modulated, in an essentially sinusoidal way for example, such that the array or matrix 5 of switches emulates an impedance whose phase varies as a function of the pulse width or duration of said driving signals.

In the embodiment shown in Figure 3, designed for the emulator device of Figure 2, the array or matrix 5 of switches comprises two sub-arrays or sub-matrices 5a, 5b which are essentially in series with each other, of which the first, 5a, is positioned between terminals 10 and 1 1 and is connected in parallel with the inductor L, while the second, 5b, is positioned between terminals 1 1 and 12 and is connected in parallel with the resistor R. In any case, each sub-array or sub-matrix 5a, 5b also takes the form of an array or matrix. The number of groups or modules 20 of the matrix 5, and the number of pairs of electronic switches in each group or module 20, depend on the intended capabilities in terms of operating voltage and the current strength. By way of example, if we assume that each module or group 20 can handle a voltage of 500 V a.c. and conduct a current of 250 A (rms), then in order to achieve a total operating voltage of 30 V and a total current of 750 A it will be necessary to use a matrix comprising a total of 180 groups or modules 20, that is to say 60 "rows" connected in series with each other and each comprising 3 groups or modules 20.

Figures 5 and 6 are qualitative illustrations of two possible architectures of the driver device 6.

In the exemplary embodiment of Figure 5, the driver device 6 comprises a pulse transformer with a primary winding Wl connected to a plurality of secondary windings W21-W2n, the number n of the secondary windings being equal to the number of rows of the array or matrix 5. In particular, each secondary winding is connected to a corresponding line formed by a pair of conductors 21 and 22 (Figures 3 and 4).

The secondary windings W21-W2n are insulated from each other in order to be able to withstand the total insulation voltage of the system. In the variant shown in Figure 6, the driver device 6 comprises n pulse transformers Tl -Tn, that is to say a transformer for each row of the array or matrix 5.

In other variants which are not illustrated, the driver device 6 can be connected to each group or module 20 of electronic switches by means of optical fibres.

In particular, a first optical fibre can supply these modules or groups of switches through a corresponding photoreceiver; this fibre is illuminated by a monomode or multimode optical power generator, and the fibre in turn illuminates the photoreceiver associated with a group or module of switches of the array or matrix 5, thus generating a sufficient supply voltage.

A second fibre serves to transfer the control pulses generated by the controller 8, to regulate the current conduction of the electronic switches of the array or matrix 5.

With reference to Figure 2, the inductor L can be produced by various methods of construction and with the use of various connecting systems, for example in the ways which will be described now with reference to Figures 7 to 1 1.

In the solution preferred at present, the inductor L is in air and has its core in air. However, it is possible to use other solutions, for example with an iron core immersed in oil or embedded in resin or of a dry type with air as insulation.

The winding of the inductor L can be produced in various ways, either by assembling different coils, or in the form of an autotransformer with additional residual inductance.

In the solution of Figure 7, the inductor L comprises an elongate O-shaped core indicated by 30, on one branch of which two coils 31 and 32 are wound, these coils being positioned, respectively, between terminals 2 and 3 and between terminal 3 and an intermediate tap 4.

In the embodiments shown in Figures 8 and 9, the inductor L comprises an O-shaped core 40 with an intermediate branch or cross-piece.

In the solution of Figure 8, the winding comprises two coils 41 and 42 wound, respectively, on an end branch and on the intermediate branch of the core 40, and connected in series between terminals 2 and 3. Terminal 4 represents an intermediate tap of the coil 42.

In the solution according to Figure 9, the winding of the inductor L comprises two coils 41 and 42, both wound on the intermediate branch of the core 40, and connected in series between terminals 3 and 4, terminal 2 being connected to the junction between these coils. Figure 10 shows a variant embodiment of the inductor L, whose core comprises two concentric toroids 50 and 51. The winding comprises a coil 52 wound on the outer toroid 51 , and a coil 53 wound on the inner toroid 50. The coils 52 and 53 are connected in series with each other, between terminals 2 and 3, and terminal 5 is connected to an intermediate tap of the coil 52.

In the variant shown in Figure 1 1 , the core of the inductor L comprises a single toroid 51 , on which is wound a coil 52, between terminals 2 and 4, terminal 3 being connected to an intermediate point of this coil.

In the control and regulation system 1 according to the invention, the controller 8 can advantageously be designed to run test algorithms such as those which will now be described.

As stated previously, the array or matrix 5 of electronic switches emulates, in use, an impedance (reactance) which is variable in amplitude and phase as a result of the modulation of the square wave signals applied to the control terminals of these switches.

If this modulation is synchronized, for example, with a phase voltage of the controlled three-phase system or network TDS, an intentional homopolar voltage is produced as a result, enabling the control and regulation system 1 to execute a measurement of the characteristic parameters of the network.

This type of test can be conducted in such a way as to create no operating problems in the controlled three-phase network TDS, and can be conducted at any moment by the controller 8.

By means of a second test algorithm, the modulus and phase of the impedance emulated by the array or matrix 5 of electronic switches can be varied continuously, in steps or in bursts. It is considered that this variation, using the phase voltages of the controlled TDS network as a generator, enables the profile of the actual impedance of the network to be identified. This second type of test can also be conducted in such a way as to create no operating problems in the controlled three-phase network TDS, and can be conducted at any moment by the controller 8. As stated in the preceding description, the control and regulation system 1 can vary the reaction time of the array or matrix 5 of switches as a function of the variation of the values of the monitored magnitudes or parameters of the controlled network TDS. This can conveniently be executed by using an algorithm of the PID (proportional integral derivative) type. This type of algorithm can optimize the rate of variation of the reactance emulated with the array or matrix 5 of switches, with minimum overshoot.

The use of a PID algorithm also makes it possible to benefit from other known advantages of such an algorithm.

In the preceding description and in the illustration of the control and regulation system 1 of Figure 2, the array or matrix 5 of electronic switches is connected to an actual inductor L interposed between the neutral N and earth T, but, according to the invention, the effective control and regulation of the state of the neutral conductor N can also be achieved, if necessary, without using the inductor L, in other words by interposing between the neutral N and earth T, if necessary, only the array or matrix 5 of electronic switches, with or without the resistor R. Furthermore, although the array or matrix 5 of electronic switches has been illustrated in the preceding description in relation to its use in a system for the control and regulation of the state of the neutral conductor N of a three-phase system or network, this array or matrix of electronic switches, if suitably driven, can find general application as an emulator device emulating impedance, and in particular reactance, which is variable in a controlled way in modulus and phase.

Without prejudice to the above description, some variant embodiments will now be outlined with reference to Figures 2a, 2b and 2c. In these figures, parts and elements described previously have been given the same alphanumeric references as those used previously, particularly in Figure 2 and in the description thereof.

In the variant of Figure 2a, the inductor L is connected in parallel to the sub-array or sub- matrix 5b, between terminals 1 1 and 12, in other words between terminal 3 and earth T. The sub-array or sub-matrix 5a is connected between terminals 2 and 3, in other words between terminals 10 and 1 1 , in series with the sub-array or sub-matrix 5b. In the diagram of Figure 2a the resistor R is not present, although it could be provided, between terminals 2 and 3 for example.

In the variant shown in Figure 2b, the array or matrix 5 is divided into three sub-arrays or sub-matrices 5a, 5b and 5c, connected in series with each other, between terminal 2 and earth T. The inductor L is connected in parallel with the sub-array 5a, between terminals 10 and 1 1 , and the resistor R is connected in parallel with the sub-array 5b, between a terminal 1 la and terminal 12, the latter of which is connected to earth T. The intermediate sub-array 5c is connected between terminals 1 1 and 1 la. The inductor L and the resistor R are connected in series with each other, between terminal 2 and earth T, via the intermediate sub-array 5c.

In the variant shown in Figure 2b, the sub-array of switches 5b can be used to modulate the time constant R/L of the emulated impedance between terminal 2 and earth T, by a similar procedure to that used in the solution shown in Figure 2.

The variant shown in Figure 2c includes an array or matrix 5 of switches divided into three sub-arrays or sub-matrices in series, again indicated by 5a, 5b and 5c. In this variant, the resistor R is positioned between the neutral conductor N and terminal 2, in parallel between terminals 10a and 10 of the sub-array 5a which can be used to modulate the time constant R/L. On the other hand, the inductor L is connected between terminals 2 and 3, in other words between terminals 10 and 1 1 of the intermediate sub-array 5b. The sub-array 5c is connected between terminal 3 and earth T.

The mode of operation of the variants shown in Figures 2a, 2b and 2c is fairly similar to that of the solution shown in Figure 2, with the appropriate changes made, as will be evident to persons skilled in the art. Naturally, the principle of the invention remaining the same, the forms of embodiment and the details of construction may be varied widely with respect to those described and illustrated, which have been given purely by way of non-limiting example, without thereby departing from the scope of the invention as defined by the attached claims.