ITMI20130899A1 - DEVICE AND METHOD FOR DETECTING AN OPERATING CONDITION IN THE ISLAND OF AN ELECTRIC SUBSYSTEM - Google Patents

Description

`` DEVICE AND METHOD FOR DETECTION OF AN ISLAND OPERATING CONDITION OF AN ELECTRICAL SUBSYSTEMâ €

DESCRIPTION

The present invention relates to a device and a method for detecting an island operating condition of an electrical subsystem.

As is well known, configurations of electricity distribution systems are becoming more and more widespread in which, to the energy usually supplied by the primary or secondary power stations, the energy produced by the so-called â € œdistributed generationâ € is added, that is, from one or more electrical energy production units, typically starting from renewable sources, and from other active components, such as electrical energy storage systems. The production units are generally controlled to maximize the extractable active power from renewable sources and keep the reactive power or the power factor of the apparent power fed into the grid constant. The active components are also generally controlled to ensure an assigned trend of the active power and to keep the reactive power or the power factor of the apparent power fed into the grid constant. In the following description, without loss of generality, we therefore refer only to the production units and all the considerations made below are also valid for the other active components.

A problem that is encountered in these configurations of electrical energy distribution systems is the so-called â € œislandingâ € or â € œislandingâ € phenomenon.

This phenomenon occurs when, for various reasons, the power supply of electricity supplied by the substation (s) to a certain sub-part of the electrical system is interrupted (for example, due to maneuvers, protection interventions or inefficiencies), while distributed generation production units continue to power this subpart.

The quantity of energy supplied by the generation distributed to this portion of the electrical system can reach values comparable to the energy required by the loads. In this circumstance, islanding or islanding occurs, which takes the form of a substantial balance between generation and load in the absence of connection to the main power supply.

The island operating condition is an undesirable situation for several reasons:

1) it can lead to the collapse of the network;

2) it can cause a lightning strike for operators intervening on lines that they believe are inactive;

3) it can cause damage to the equipment of users connected to that portion of the grid affected by the islanding phenomenon, due to the uncontrolled operation of the electrical subsystem, potentially outside the voltage and frequency limit conditions;

4) can cause damage to users' equipment during the transient restoration of the connection of the electrical subsystem operating in isolation with the main power supply, as synchronization is not guaranteed.

In order to achieve an adequate level of safety and reliability of the electrical system, it is necessary to install devices that are able to detect the island operating condition, in order to undertake all the necessary actions, in accordance with the technical rules of interconnection, able to prevent the consequences related to this phenomenon.

The state of the art provides various techniques for detecting island operation. These techniques are based on different methods that can be classified into three types: passive methods, active methods and methods based on communication systems.

The methods of the first type (passive) exploit the local measurement of some electrical quantities. They are based on the following principle: in the presence of island operation there is an imbalance between the power produced by the units of the distributed generation and that absorbed by the loads, with consequent variation of some electrical quantities, typically frequency and voltage.

The devices that use the passive method include the classic minimum / maximum voltage and minimum / maximum frequency relays. If the load is more prevalent than the distributed generation, the latter is not able to support the island operation. Under these conditions, voltage and frequency decrease and islanding is detected. If, on the other hand, the distributed generation is prevalent with respect to the load, then there will be overvoltages and frequency increases which are detected by the relays. It is evident that these relays are not able to detect islanding when the power produced by the units of the distributed generation is close to that absorbed by the loads.

Other devices using the passive method have been proposed in the following documents.

The document EP 2538517 describes a method which is based on the variation of the voltage phase at the terminals of a synchronous generator which appears with the passage from the operation in connection to the main power supply to the island operation; to evaluate the phase variation it is necessary to generate a phase reference signal that is synchronous with the network inside the detection device.

WO 2008/153419 describes a method that uses harmonic distortion. In the case of distributed generation production units equipped with inverters, the currents injected by these units are deformed. The method detects the voltage harmonics caused by the aforementioned current harmonics and, in particular, detects their increase in case of island operation.

The document EP 1743409, on the other hand, proposes a passive method based on the phase-shift or "phase-shift" of the voltage waveform that appears in the transition to island operation. The phase variation is calculated by comparing the variations in the time distances between two consecutive passages of the voltage for zero.

WO 2009/032876 describes a method in which a relationship is established between the current generated by the generating unit of the distributed generation and the voltage measured at its terminals. When the waveform of the voltage is not consistent with the relationship established, the condition of island operation is detected.

The methods of the second type (active) detect the effects caused by perturbations intentionally introduced through the production units of distributed generation. They are also known as "perturb and observe" methods. They are based on the following principle: if the operating condition is connected to the main power supply, the perturbations introduced cause negligible effects, while in isolated operating conditions, the effects of the perturbations are significant and detectable by appropriate devices.

Among the devices that use the active method, the best known is based on the detection of the so-called "external network impedance". An intentional variation of current injected by the production unit of distributed generation determines a variation of voltage at its terminals and the external impedance of the network is estimated from the ratio of these variations. The island operating condition is detected if the estimated value of the external impedance exceeds a certain threshold.

Some devices using the active method have been proposed in the following documents.

The document EP 1764894 proposes an active method based on the introduction of a reactive current (or power) reference in the form of a square wave and on the measurement of the consequent derivative of the voltage (or frequency).

The document EP 1926844 proposes an active method based on the introduction of a current or reactive power reference in the form of a ramp, square, sine wave with harmonic frequency, trapezoidal, and on the measurement of the consequent variation of the voltage. It is based on the assumption that, if there is a connection to the mains, the voltage does not substantially change.

Document EP 1780861 describes a method based on the detection, at the terminals of the distributed generation production unit, of a "mains presence" signal, superimposed on the mains voltage and generated at a point of the power supply mains.

The document EP 2501014 describes a method based on the introduction of a current variation, on the measurement of the consequent voltage variation and on the verification if the latter exceeds a fixed threshold. This threshold is established starting from a value attributed to the grid impedance in the conditions of connection to the main power supply.

The document EP 2532065 describes a method which is based on the introduction of a predefined "pattern" of variation of the phase of the current injected by an inverter of the production unit of the distributed generation. The effect of this disturbance on the voltage at the inverter terminals is detectable only in island operating conditions; in this case, the device amplifies the pattern until the inverter minimum / maximum frequency protections are triggered.

The document WO 03/038970 proposes an active method based on the injection of an impulsive disturbance of current generated by the inverter of the production unit of the distributed generation. The effect of this disturbance on the voltage at the inverter terminals is detectable only in island operating conditions.

The document EP 0810713 describes a method in which a distortion of the current injected by the inverter of the production unit of the distributed generation is introduced and the consequent variation of the frequency caused by this distortion is detected, measuring the variation of the zero crossing of the distorted voltage waveform.

Finally, the methods of the third type (based on communication systems) base their operating principle on the presence of a communication vector between the distributed generation and the primary or secondary cabin. In case of loss of this communication, island operation is detected.

Some devices using the communication systems-based method have been proposed in the following documents.

Document WO 2011/140559 describes a system which is based on the communication of "synchrophasors" from the cabin to the production units of the distributed generation. The received "synchrophasors" are compared with the "synchrophasors" derived from the voltage measurement at the production unit terminals; if the difference exceeds a certain threshold, the condition of island operation is detected.

The document WO 2012/032111 proposes a method based on the communication systems between the control systems of the cabin and those of the production units of the distributed generation. In case of loss of this communication, island operation is detected.

Document WO 2012/065359 instead presents a method based on the communication of a signal superimposed on the fundamental component of the mains voltage, sent from the cabin and detected by the distributed generation. In case of loss of this signal, island operation is detected.

None of the methods described above is free from operational limitations and / or drawbacks.

Off-grid detection techniques based on passive methods can give incorrect results. For example, if a balance is achieved between distributed generation and loads, island operation can persist and, theoretically, be permanent. In practice, the balance between generation and load translates into the presence of a “non-intervention zone” or “non-detection zone” of devices based on minimum / maximum frequency and minimum / maximum voltage relays. Another example in which passive methods lead to incorrect results occurs in the presence of disturbances on the main power supply such as to generate undesirable variations in voltage and / or frequency. Under these conditions, passive methods are not able to discriminate the occurrence of isolated operating conditions from other events, in the sense that they are not able to establish whether the cause of such variations is attributable to isolated operation or to disturbance.

Active methods, by injecting perturbations, alter the waveform of the voltage and therefore cause a deterioration in the power quality of the supply. Further limitations on the use of active methods are due to the intervention time of the device, which can be quite high, and in some cases to the cost, as measuring devices and high performance processing units are required. Active methods always have a non-intervention zone, even if much less wide than that present in passive methods.

Methods based on communication systems overcome the limitations linked to the use of passive and active methods, as they do not present areas of non-intervention. On the other hand, they are not exempt from possible untimely signals, that is, from false signals of island operation. Furthermore, the methods of the third type, being based on the presence of a communication system, are feasible for new developments of distribution networks, as they involve costs for the construction of new infrastructures that are difficult to sustain for the existing networks. To avoid introducing a new physical means of communication, the technologies used to exchange information can be based on "Power Line Carrier" or "wireless" networks. In any case, the complexity of the system requires high investments.

The object of the present invention relates to the detection of the transitions of an electrical subsystem from an operating condition in connection to the grid / power station to an island operating condition; in this second condition, the subsystem loads are powered by the distributed generation units connected to the subsystem itself, in the absence of connection with the power supply station. The invention also makes it possible to detect inverse transitions, ie the transitions from an island operating condition to a condition of connection to the grid / power station.

The invention preferably provides for the execution of three steps which are carried out in sequence:

1. acquisition of the voltage and current measurements at the point where the device according to the invention is inserted;

2. elaboration of the measurements for the estimation of the parameters of the Thevenin equivalent circuit, representative of the electrical system seen from the point where the device is inserted;

3. determination of the operating condition of the electrical subsystem (connected to the power supply network or in isolation) as a function of the estimated values of the parameters of the Thevenin equivalent circuit.

Preferably, the sequence of the three steps is performed cyclically at predetermined time intervals.

From a conceptual point of view, the invention provides for the application of the model represented by the equivalent Thevenin circuit to distinguish, on the basis of the values assumed by the parameters of this model, the isolated operation from that in connection to the network / station. Power supply. The parameters that characterize the model are two complex numbers: the no-load voltage and the equivalent impedance. The adoption of this type of model represents an advance compared to the models used by the techniques made available by the state of the art, in particular by those that use the passive method of the so-called external network impedance. In fact, according to these techniques, the external network impedance is estimated starting from a different model, i.e. a difference model that excludes the no-load voltage and represents the external impedance as a ratio between the voltage variations obtained in response changes in current, and the changes in current themselves.

A further aspect of the invention concerns the modalities with which the estimated values of the parameters of the model are used to determine the operating condition of the electrical subsystem in question.

The purpose of the present invention is therefore to provide a device and a method that allow to reliably detect the occurrence of island operation.

Another object of the invention is to provide a device and a method which can be easily applied both to existing electrical networks and to electrical networks under construction.

Another purpose of the invention is to make available a device and a method that do not require complex and expensive structures to be made and put into operation.

A further object of the invention is to provide a device and a method which use a technique based on the passive method, and which therefore do not perturb the system during the detection of the electrical quantities of interest.

Another object of the invention is to make available a device and a method that detect the island operating condition of the electrical subsystem without the need for any communication system with the power supply station or with other devices.

A further object of the invention is to provide a device and a method that can detect an island operating condition regardless of the configuration of the subsystem in question, as long as the point to which the device is connected is part of it, regardless of the power and location. of loads, and the number, power, type of electricity production units that are part of distributed generation.

Another object of the invention is to make available a device and a method that implement a technique for detecting isolated operating conditions which, at least from a theoretical point of view, does not have areas of non-intervention.

A further object of the invention is to provide a device and a method that can operate correctly even if the instrumentation used is connected to a point of the electrical subsystem which is different from the connection point of the production units of the distributed generation.

Another purpose of the invention is to make available a technique for detecting operating conditions on an island that allows to install several devices in the same subsystem, without interactions or couplings between them that could cause interference and / or malfunctions. .

These and other purposes are substantially achieved by a device and a method according to what is described in the attached claims.

Further features and advantages will become more apparent from the detailed description of a preferred and non-exclusive embodiment of the invention.

This description is provided below with reference to the accompanying figures, also having purely illustrative and therefore non-limiting purposes, in which:

- figure 1 shows a block diagram representative of the invention;

Figure 2 shows a detailed block diagram of a part of the diagram of Figure 1;

Figure 3 shows a flow chart representative of a possible implementation of the method in accordance with the present invention.

With reference to the accompanying figures, 100 indicates a device in accordance with the present invention as a whole.

The device 100 (fig. 1) is inserted in a context in which an electrical subsystem 10 is connected to at least one power supply station 20 forming part of an electrical power supply network 30.

The electrical subsystem 10 can be constituted, in practice, by a portion of the electrical network, from which some loads are derived, which is normally connected to the power supply network 30.

The power supply station 20 can in practice be constituted, for example, by a primary or secondary substation, from which the electrical subsystem 10 draws electrical energy.

The power supply network 30 can be, by way of example only, a 400 V distribution network, at a frequency of 50 Hz.

In addition, one or more electricity production units 40 are connected to the electrical subsystem 10 and are part of a distributed generation 50.

40 production units can use renewable energies, such as solar radiation.

During the normal operation of subsystem 10, the loads of the same subsystem absorb the electricity they need from the power supply network 30 and from the production units 40 of distributed generation 50.

As mentioned, the device 100 has the task of recognizing when an island operating condition of the subsystem 10 occurs, in which the loads of the same subsystem absorb the electrical energy they need exclusively from the production units 40 of the distributed generation 50, in the absence of connection to the mains 30.

The device 100 in particular is able to detect the transitions to / from island operating conditions of the subsystem 10.

In greater detail, the device 100 is connected to a determined point P of the electrical subsystem 10, so as to be able to detect the electrical quantities of interest and perform the required analyzes.

The device 100 comprises a first operating module 110 which detects the electrical operating quantities of the electrical subsystem 10 in said determined point P.

Preferably, the operating module 110 carries out the measurements of the voltage in said determined point P and of the current in a branch afferent to said determined point P and determines the values of the corresponding phasor electrical quantities.

From a practical point of view, the first operating module 110 can operate in such a way as to acquire, with any suitable method, the voltage and current measurements (single-phase or three-phase) in the said determined point P, obtaining the amplitude and phase values representative of the voltage and current phasors at the fundamental operating frequency of the electrical system and tracking their variations over time.

The device 100 comprises a second operating module 120 configured to determine a first parameter P1 representative of a no-load voltage at said certain point P.

The device 100 also comprises a third operating module 130 configured to determine a second parameter P2 representative of an equivalent impedance in said determined point P.

In practice, the second and third operating modules 120, 130 determine, respectively, the Thevenin no-load voltage `` seen '' by the device 100 in the determined point P, and the equivalent impedance of Thevenin `` seen '' by the device 100 in the determined point P.

Preferably, the second operating module 120 is configured to determine the first parameter P1 as a function of a plurality of determinations of the voltage and current phasors produced by the first operating module 110.

Preferably, the third operating module 130 is configured to determine the second parameter P2 as a function of a plurality of determinations of the voltage and current phasors produced by the first operating module 110.

The second and third operating modules 120, 130 can then adopt any suitable method to estimate the parameters of the Thevenin equivalent circuit (no-load voltage, represented by the first parameter P1 and equivalent impedance, represented by the second parameter P2) and to track the variations, on the basis of the acquisition over time of the pairs of values of voltage and current phasors and starting from the model derived from the representative equations of the equivalent electric circuit.

By way of example, the following input-output model can be used:

y (tj) = Ï • (tj) Î ̃

where the vectors of output variables y (tj) and input variables Ï • (tj) and the matrix of parameters to be estimated Î ̃ are defined as follows:

y (tj) = [Re {V (tj)} Im {V (tj)}]

Ï † (tj) = <[> Re <{> I <(> tj <)}> Im <{> I <(> tj <)}> cv <]>

ï £ ® Req, th Xeq, th ï £ ¹

Î ̃ = ï £ ̄

ï £ ̄ ∠’X ï £ º

eq, th Req, th ï £ º

ï £ ̄ï £ ° Re {V0, th} cv Im {V0, th} cv <ï £ º> ï £ »

being <V (tj), I (tj)> the pairs of values of voltage and current phasors available at instant tj, with a generic fixed quantity, Zeq, th = Req, th jX eq, th the equivalent impedance of Thevenin and <V> 0, th the no-load voltage, and having indicated with Re and Im the real and imaginary part of a phasor.

To this model it is possible to apply a technique of recursive estimation of the parameters of the matrix Î ̃.

It is also possible to adopt different models and, in particular, higher order models in which the quantities measured in the previous cycles are kept in mind.

The techniques adopted for the recursive estimation of the parameters of each model can be different. For example, one technique is based on the Constrained Recursive Least-Squares method.

The device 100 further comprises a processing unit 140 associated with said second and third operating modules 120, 130.

Processing unit 140 is configured to compare the first parameter P1 with a first reference value Ref1, and to compare the second parameter P2 with a second reference value Ref2.

The first and second reference values Ref1, Ref2 can be determined in different ways.

Preferably, the first reference value Ref1 is determined as a function of a nominal value of an operating voltage of the electrical subsystem 10.

By way of example, Ref1 can be substantially set equal to h∠™ Vn, where h is a suitable coefficient (h <1) which takes into account the possible variations of the voltage regime under normal operating conditions of the electrical system, and Vn the nominal value of the operating voltage of the electrical system.

Preferably, the first reference value Ref1 is determined as a function of a voltage measured at the determined point P during the operation of the electrical system.

By way of example, Ref1 can be set substantially equal to h∠™ Vmeasured, in which h is a suitable coefficient (h <1) which takes into account the possible variations of the voltage regime under normal operating conditions of the electrical system, and Vmeasured the value effective of the voltage measured at the given point P.

Preferably, the first reference value Ref1 is determined as a function of an estimated no-load voltage at the determined point P during an initial calibration phase of the device 100.

By way of example, Ref1 can be set substantially equal to h∠™ <V> 0, th, tar, in which h is a suitable coefficient (h <1) which takes into account the possible variations of the voltage regime under normal operating conditions of the electrical system, and <V> 0, th, will calibrate the Thevenin no-load voltage module estimated by the second sensing module 120 during a calibration phase carried out during an initial period, after the device 100 has been installed but before it is considered operational in all respects, and in a situation where the operation is not in isolation.

Preferably, the second reference value Ref2 can be determined as a function of the impedances of the lines and transformers that make up the electrical subsystem 10 and the power supply network 30.

By way of example, Ref2 can be set substantially equal to k∠™ <Z> connections <+ Z> transformat ore, in which k is a suitable coefficient (k> 1) which takes into account the possible variations of the network configuration, Zconnection ̈ the resultant of the impedances of all the components that connect the determined point P to the power supply station 20 and Ztransformat ore is the impedance of the transformer T of the power supply station 20.

Preferably, the second reference value Ref2 is determined as a function of an equivalent impedance estimated at the determined point P during an initial calibration phase of the device 100.

By way of example, Ref2 can be set substantially equal to k∠™ <Z> eq, th, tar, in which k is a suitable coefficient (k> 1) that takes into account the possible variations of the network configuration, and <Z> eq, th, calibrates the Thevenin equivalent impedance module estimated by the third operating module 130 during a calibration phase carried out during an initial period, after the device 100 has been installed but before it is considered to all intents and purposes operational, and in a situation where the operation is not in isolation.

As said, the processing unit 140 compares the first and second parameters P1, P2 with the respective reference values Ref1, Ref2.

If the first parameter P1 is less than the first reference value Ref1 and the second parameter P2 is greater than the second reference value Ref2, then the processing unit 140 generates an output signal OS representative of the fact that the electrical subsystem 10 operates in island operating conditions.

The OS output signal can be a notification signal, for example of an acoustic and / or visual type, transmitted by means of suitable signaling tools and / or by other technologies, so that the operators in charge can be suitably informed. The output signal OS can also be sent to suitable devices of the electrical subsystem 10, for example to switching devices, and / or to the electrical energy production units 40 of the distributed generation 50, so as to activate any automatic actions provided for in island operating conditions.

Otherwise, if the first parameter P1 is greater than or equal to the first reference value Ref1 and the second parameter P2 is less than or equal to the second reference value Ref2, then the processing unit 140 o does not generate the output signal OS o generates an output signal OS representative of the fact that the electrical subsystem 10 operates in connection with the power supply network 30. This signal can be sent to suitable devices of the electrical subsystem 10, for example to switching devices, and / or to the electricity production unit 40 of distributed generation 50, and activate any automatic actions envisaged in operating conditions in connection with the power grid 30.

During the operation of the processing unit 140, two other cases may arise. In the first case, the first parameter P1 is greater than or equal to the first reference value Ref1 and the second parameter P2 is greater than the second reference value Ref2. In a second case, the first parameter P1 is less than the first reference value Ref1 and the second parameter P2 is less than or equal to the second reference value Ref2. These cases can arise when the subsystem 10 has changed the operating condition and the device 100 is estimating the new parameters P1 and P2; during this phase it is possible that the two parameters do not cross simultaneously the thresholds represented by the respective reference values Ref1 and Ref2. In both cases the processing unit 140 will be able to generate an OS output signal according to various criteria.

Preferably in both cases the processing unit 140 does not modify the output signal OS, continuing to indicate the same operating condition previously detected, whether it is in isolation or connected to the power supply network, in accordance with a conservative criterion.

Preferably in both cases the processing unit 140 generates an output signal OS representative of the island operating condition, in accordance with the criterion for which it is sufficient that the comparison of one of the two parameters P1 or P2 with the respective value reference Ref1 or Ref2 check the island condition to signal this condition.

Preferably in both cases the processing unit 140 generates an output signal OS representative of the operating condition in connection with the power supply network, in accordance with the criterion for which it is sufficient that the comparison of one of the two parameters P1 or P2 with the respective reference value Ref1 or Ref2 do not check the island condition to signal the operating condition in connection to the power supply network.

Preferably in both cases the processing unit 140 generates a "pre-alarm" or "warning" output signal OS, different from the OS output signals representative of the operating conditions on the island and in connection to the power supply network.

The detection / control technique described above is based on the following considerations: in operating conditions of the electrical subsystem 10 in connection with the power supply station 20, the parameters of the equivalent Thevenin circuit seen from the determined point P where the device 100 is connected they will be characterized by the following values: - Thevenin no-load voltage value close to the value of the rated operating voltage of subsystem 10;

- Thevenin equivalent impedance value of the same order of magnitude as the value obtained by adding the resultant of the impedances of all the components that connect the determined point P to the power supply station 20 and the impedance of the transformer T of the same station.

On the other hand, in isolated operating conditions the same parameters of the equivalent Thevenin circuit seen from the determined point P are characterized by the following values:

- Thevenin no-load voltage value close to zero;

- Thevenin equivalent impedance value of an order of magnitude greater than the corresponding value assumed in conditions of connection to the power supply station 20.

In particular, it should be noted that significant variations in the values of the parameters of the Thevenin equivalent circuit also occur in the presence of several electricity production units, even of different types and connected in different points of the subsystem. In fact the production units, since they have not been specifically designed to allow their operation in isolation, cannot be modeled, from an electrotechnical point of view, as ideal current or voltage generators. They follow an active and / or reactive power reference and, consequently, are modeled as an electromotive force whose value depends on the injected currents, behaving like a negative load. For this reason, with reference to the Thevenin equivalent circuit seen from the determined point P where the device 100 is connected, the production units do not contribute to the value of the no-load voltage but alter the equivalent impedance. As a result, the no-load voltage of the Thevenin equivalent circuit assumes zero values in isolated operating conditions.

For what has been said, the adoption of the complete Thevenin model, consisting of two parameters, respectively, the no-load voltage and the equivalent impedance, is certainly more adequate than the methods based on the external network impedance, estimated as the ratio between voltage variations and current variations. These methods are based on difference models (variations in voltages and currents) and, consequently, do not consider the no-load voltage parameter, which instead is the parameter that has the largest and most reliable variation for detecting the transition from a condition of connection to the power supply network to an island operating condition. In fact, in the case of no-load voltage, the orders of magnitude of the two values it assumes in the two operating conditions are known with great reliability and these orders of magnitude are very different from each other. Conversely, in the case of the equivalent impedance, the values it assumes, in particular in isolated operating conditions, are not known a priori and depend on other factors, such as the extension and topology of the subsystem 10, the power of the loads, the number and type of production units.

Example of an implementation scheme

An example of one of the possible schemes that implement the functions of the device 100 is shown in fig. 2 where the blocks represent the following hardware devices:

1. phase voltage measuring device;

2. phase current measuring apparatus;

3. A / D converter for voltage measurement;

4. A / D converter for current measurement;

5. microprocessor computing system;

6. output signal generator which indicates the operating condition of the electrical subsystem 10 in isolation or in connection to the power supply network 30.

The microprocessor computing system 5 implements the functions of the device proposed and previously described.

The aforementioned first operating module 110 can advantageously comprise the measuring apparatus 1, the measuring apparatus 2, the A / D converter 3 and the A / D converter 4 and that portion (hardware and / or software) of the microprocessor 5 dedicated to the determination of voltage and current phasors.

The aforementioned second operating module 120 can advantageously comprise that portion (hardware and / or software) of microprocessor 5 dedicated to calculating the no-load voltage.

The aforementioned third operating module 130 can advantageously comprise that portion (hardware and / or software) of microprocessor 5 dedicated to the calculation of the equivalent impedance.

The processing unit 140 can conveniently comprise the hardware and / or software portion of the microprocessor 5 dedicated to the comparison of the no-load voltage and equivalent impedance with the respective reference values, and the output signal generator 6.

An example of a possible flow chart describing the software implemented on the calculation system is shown in fig. 3 where the blocks represent the following pieces of code:

1000. acquisition of the voltage value supplied by the A / D converter;

1010. calculation of the modulus and phase of the fundamental component of the voltage by means of a Kalman filter;

1020. acquisition of the current value supplied by the A / D converter;

1030. calculation of the modulus and phase of the fundamental component of the current by means of a Kalman filter;

1040. Estimation of the parameters of the Thevenin equivalent circuit by means of a recursive algorithm based on constrained least squares;

1050. determination of the operating condition of the electrical subsystem 10 (connected to the grid or isolated).

The seven steps are repeated with a temporal cyclicality equal to Ts (in fig. 3 the variable "t" represents time), while M is a suitable integer that determines the cyclicality M · Ts with which only the steps 1040 are performed and 1050 of the software.

The invention achieves important advantages.

First of all, the device and the method according to the invention allow to reliably detect the occurrence of island operation.

Furthermore, the device and the method according to the invention can be easily applied both to existing electrical networks and to electrical networks under construction.

Another advantage is found by observing that the device and the method in accordance with the invention do not require complex and expensive structures in order to be manufactured and installed.

A further advantage of the invention relates to the fact that a technique based on a passive method is used, and which therefore does not disturb the system during the detection of the electrical quantities of interest.

Another advantage emerges considering that the device and the method according to the invention detect the island operating conditions of an electrical subsystem without the need for any communication system with the power supply station or with other devices.

A further advantage consists in the fact that the device and the method in accordance with the invention can detect an island operating condition regardless of the configuration of the subsystem in question, as long as the point to which the device is connected is part of it, regardless of the power and location of loads, and the number, power, type of electricity production units that are part of distributed generation.

Another advantage is found considering that a technique for detecting operating conditions in isolation is implemented which, at least from the theoretical point of view, does not have areas of non-intervention.

A further advantage concerns the fact that the device and the method according to the invention can operate correctly even if the instrumentation used is connected to a point of the electrical subsystem other than the connection point of the production units of distributed generation.

Another advantage consists in the fact that several devices can be installed in the same subsystem, without interactions or couplings between them that can cause interference and / or malfunctions.

Claims (10)

CLAIMS 1. Device for detecting an island operating condition of an electrical subsystem (10) connected to a power supply station (20) forming part of a power supply network (30), and to one or more production units of the electrical energy (40) forming part of a distributed generation (50), said device (100) being connected to a determined point (P) of said electrical subsystem (10), said device (100) comprising: a first operating module (110) for detecting the electrical operating quantities of said electrical subsystem (10) at the determined point (P); a second operating module (120) associated with said first operating module (110) to determine, as a function of said detected electrical quantities, a first parameter (P1) representative of a no-load voltage at said determined point (P); a third operating module (130) associated with said first operating module (110) to determine, as a function of said detected electrical quantities, a second parameter (P2) representative of an equivalent impedance in said determined point (P); a processing unit (140) associated with said second and third operating modules (120, 130) and configured for: comparing said first parameter (P1) with a first reference value (Ref1); comparing said second parameter (P2) with a second reference value (Ref2); if said first parameter (P1) is less than said first reference value (Ref1), and said second parameter (P2) is greater than said second reference value (Ref2), generate an output signal (OS) representative of the the fact that said electrical subsystem (10) operates in an island operating condition. 2. Device according to claim 1 in which said second and / or third operating module (120, 130) is configured to determine said first and / or second parameter (P1, P2) as a function of a plurality of measurements of electrical quantities of operation of said electrical subsystem (10) in said determined point (P). 3. Device according to claim 1 or 2 wherein said second operating module (120) is configured to determine said first parameter (P1) as a function of one or more detections of a voltage in said determined point (P) and as a function of one or more detections of a current flowing in a branch afferent to said determined point (P). 4. Device according to any one of the preceding claims, in which said third operating module (130) is configured to determine said second parameter (P2) as a function of one or more detections of a voltage in said determined point (P) and as a function of one or more detections of a current flowing in a branch afferent to said determined point (P). 5. Device according to any one of the preceding claims in which said first reference value (Ref1) is determined as a function of a nominal value of an operating voltage of said electrical subsystem (10). 6. Device according to any one of claims 1 to 4 wherein said first reference value (Ref1) is determined as a function of a voltage measured at said determined point (P) during the operation of the electrical system. 7. Device according to any one of claims 1 to 4 wherein said first reference value (Ref1) is determined as a function of an estimated no-load voltage at said determined point (P) by the second operating module (120) during a phase initial calibration of the device (100). 8. Device according to any one of the preceding claims in which said second reference value (Ref2) is determined as a function of the impedances of the lines and transformers that make up the electrical subsystem (10) and the power supply network (30). 9. Device according to any one of claims 1 to 7 wherein said second reference value (Ref2) is determined as a function of an equivalent impedance estimated at said given point (P) by the third operating module (130) during an initial phase device calibration (100). 10. Method for detecting an island operating condition of an electrical subsystem (10) connected to a power supply station (20) forming part of a power supply network (30), and to one or more energy production units electricity (40) forming part of a distributed generation (50), said method comprising the following steps: detecting electrical operating quantities of said electrical subsystem (10) at a determined point (P); determining a first parameter (P1) representative of a no-load voltage in said determined point (P) to which said electrical subsystem (10) is connected; determine a second parameter (P2) representative of an equivalent impedance in said determined point (P); comparing said first parameter (P1) with a first reference value (Ref1); comparing said second parameter (P2) with a second reference value (Ref2); if said first parameter (P1) is less than said first reference value (Ref1), and said second parameter (P2) is greater than said second reference value (Ref2), generate an output signal (OS) representative of the the fact that said electrical subsystem (10) operates in an island operating condition.