DETECTION OF LOSS OF NEUTRAL

A method for detecting a break in connection of a neutral of a three-phase electricity network, implemented in a processing unit of an item of electrical equipment connected to the electricity network includes acquiring, at a time T, a first phase voltage (V1), a second phase voltage (V2) and a third phase voltage (V3) measured by voltage sensors of the item of electrical equipment; evaluating a first quantity representative of a ratio between a maximum voltage and a minimum voltage from the first, second and third phase voltages; if the first quantity is greater than a predetermined threshold: evaluating, based on the first, second and third phase voltages, a second quantity; detecting a break in the neutral at the time T when the second quantity satisfies a predetermined reference criterion.

The invention relates to the field of electrical power distribution networks and equipment connected to said networks.

BACKGROUND OF THE INVENTION

An electricity distribution network is used to transport electrical power from a power generation unit to one or more connected electrical installations. Electrical power is generally transported in three-phase form, the electricity network then being made up of three phase conductors and one neutral conductor. An electricity network is also typically equipped with a plurality of electricity meters used to measure the electrical power consumed by the connected electrical installations.

In a three-phase electricity network, a break in the neutral connection may occur upstream of one or more meters. Such a break is the responsibility of the energy supplier in charge of the network and can cause major problems at the connected electrical installations. Indeed, depending on the load impedances of the connected electrical installations (downstream of the break point of the neutral), a significant imbalance of the phase voltages carried by the three phase conductors may occur. It is therefore possible for high voltages to be present at connected electrical installations, which may possibly destroy said installations.

It is therefore important to be able to reliably, simply and quickly detect a sudden break in the neutral connection in a three-phase electricity distribution network. This allows preventive and/or protective measures to be taken quickly for connected electrical installations.

Conventionally, the detection of a break in the connection of a neutral is based on the detection of an abnormal imbalance between the phase voltages of the distribution network and/or an absence of current flowing in the neutral conductor. However, this method is generally unreliable as it does not correctly identify whether the observed imbalance is related to a break in the neutral or to a break in one or more phases. Furthermore, detecting the absence of current through the neutral conductor requires the use of current sensors positioned on said neutral conductor, which increases costs and gives rise to physical and electrical implementation problems.

Another known method for detecting a break in the connection of a neutral is to measure the downstream load impedances (of the connected electrical installations) in order to determine the expected imbalance of the phase voltages in the event of a break in the neutral connection (and therefore to detect it when said imbalance actually occurs). However, the effectiveness and reliability of this method are directly related to the reliability of the downstream load impedance measurements. This is a limiting factor, because determining the expected imbalance between the phase voltages typically uses a theoretical analysis for which the downstream load impedances are assumed to be linear and constant over time (which is not necessarily the case in reality). Moreover, in the event that the break in neutral affects several electricity meters, each having distinct (uncorrelated) and unbalanced downstream load impedances, the method described above can no longer detect the break in the neutral connection. Indeed, for a given meter, the measured downstream load impedance only considers the electrical installations downstream of this meter. However, the imbalance between the phase voltages is linked to the equivalent downstream load impedance resulting from the combination of all the downstream load impedances of all the meters affected by the break in the neutral connection. Since the equivalent load impedance is not measurable, the method described above is no longer applicable in this case.

OBJECT OF THE INVENTION

The object of the invention is a method for detecting, in an electricity distribution network, a break in the connection of a neutral upstream of one or more electricity meters in a quick, simple and reliable manner.

SUMMARY OF THE INVENTION

In order to achieve this object, a method for detecting a break in connection of a neutral of a three-phase electricity network is proposed, the detection method being implemented at least partially in a processing unit of an item of electrical equipment connected to the electricity network, and comprising the steps, repeated regularly, of:acquiring, at a time T, a first phase voltage measured between a first phase of the three-phase electricity network and the neutral, a second phase voltage measured between a second phase and the neutral, and a third phase voltage measured between a third phase and the neutral, the first, second and third phase voltages being measured by voltage sensors of the item of electrical equipment;evaluating a first quantity representative of a ratio between a maximum phase voltage and a minimum phase voltage from the first, second and third phase voltages;if the first quantity is greater than a predetermined threshold:evaluating, based on the first, second and third phase voltages, at least a second quantity representative of a current balance between said first, second and third phase voltages;detecting a break in connection of the neutral at the time T when the at least one second quantity satisfies a predetermined reference criterion.

The detection method according to the invention is therefore particularly advantageous because it makes it possible to detect a break in the connection of a neutral in a distribution network (upstream of an electricity meter) from the simple measurement of the phase voltages carried by the phase conductors of said network. The detection method is therefore simple to implement (because it only requires voltage sensors) and inexpensive.

Furthermore, the detection method according to the invention is also highly reliable because, when there is a suspected break in the neutral (i.e., when the first quantity is greater than the predetermined threshold), the second quantity is evaluated and makes it possible to confirm without any possible doubt that a break in the neutral connection has indeed occurred.

Since the second quantity is evaluated directly on the basis of the phase voltages (the second quantity is evaluated only on the basis of the first, second and third phase voltages and therefore does not require any additional measurements), the detection method according to the invention can quickly detect a break in the neutral connection.

Moreover, a detection method as previously described is proposed, in which the at least one second quantity comprises a second quantity that is a function of a sum of pairwise products of root mean square values of the first, second and third phase voltages.

a detection method as previously described is proposed, in which said second quantity is equal to:

G⁢2G⁢2=13⁢(V1⁢eff⁢V2⁢eff+V2⁢eff⁢V3⁢eff+V3⁢eff⁢V1⁢eff),where V1eff, V2effand V3effare respectively a root mean square value of the first phase voltage, a root mean square value of the second phase voltage and a root mean square value of the third phase voltage,BorneInf≤G2≤BorneSup the predetermined reference criterion being that:BorneInf≤G2≤BorneSup.

Moreover, a detection method as previously described is proposed, further comprising the steps of:detecting whether:

V1eff=VnomandV2eff=VnomandV3eff=Vnom,where Vnomis a nominal root mean square value of the phase voltage of the electricity network;

if this condition is met, defining and as follows:

Moreover, a detection method as previously described is proposed, in which the at least one second quantity comprises a second quantity that is a function of an area of an actual triangle formed by the first, second and third phase voltages in the Fresnel diagram.

Moreover, a detection method as previously described is proposed, in which the area of the actual triangle is determined by using the formula: where A is the second quantity, V1eff, V2effand V3effare respectively the root mean square values of the first, second and third phase voltages, and φ12is a first phase shift between the first phase voltage and the second phase voltage, φ23is a second phase shift between the second phase voltage and the third phase voltage and φ31is a third phase shift between the third phase voltage and the first phase voltage, the reference criterion then being that the second quantity is such that:

A=12⁢(V1⁢eff⁢V2⁢eff⁢sin⁢φ1⁢2+V2⁢eff⁢V3⁢eff⁢sin⁢φ2⁢3+V3⁢eff⁢V1⁢eff⁢sin⁢φ3⁢1)where Arefis an area of a reference triangle and ε1is a first predetermined measurement uncertainty.

Moreover, a detection method as previously described is proposed, in which, if the first, second and third phase voltages are perfectly balanced, the area of the predetermined reference triangle is evaluated by using the formula:

where Arefis the area of the reference triangle and Vnomis the nominal root mean square value of the phase voltage of the electricity network.

U12U23U31Φ1−ε2≤U12≤Φ1+ε2−ε2≤U23≤Φ2+ε2Φ3−ε2≤U31≤Φ3+ε2Moreover, a detection method as previously described is proposed, in which the at least one second quantity comprises second quantities which comprise a first line-to-line voltage representative of a difference between the first phase voltage and the second phase voltage, a second line-to-line voltage representative of a difference between the second phase voltage and the third phase voltage and a third line-to-line voltage representative of a difference between the third phase voltage and the first phase voltage, the reference criterion then being that:

U12U23U31Φ1−ε2≤U12≤Φ1+ε2Φ2−ε2≤U23≤Φ2+ε2Φ3−ε2≤U31≤Φ3+ε2where Φ1, Φ2and Φ3are reference values of the first, second and third line-to-line voltages measured during operation at a reference time T0preceding the time T and ε2is a second predetermined measurement uncertainty.

Moreover, a detection method as previously described is proposed, in which the at least one second quantity comprises second quantities that comprise a first phase shift between the first phase voltage and the second phase voltage, a second phase shift between the second phase voltage and the third phase voltage and a third phase shift between the third phase voltage and the first phase voltage, the predetermined reference criterion then being that the first, second and third phase shifts are each non-zero and different to 120 degrees.

Moreover, a detection method as previously described is proposed, further comprising the step of detecting a break in the neutral connection when it has been detected that the second quantity satisfies the predetermined reference criterion a predetermined number of times, corresponding to consecutive instances it is satisfied, spaced apart two by two in time by a predetermined duration.

Moreover, a detection method as previously described is proposed, in which, when a break in the neutral connection has been detected, the method further comprises the step of generating an alarm signal that can be timestamped in a memory of the item of electrical equipment and/or that can be transmitted to an item of equipment external to said item of electrical equipment.

Also proposed is an item of electrical equipment comprising voltage sensors and a processing unit arranged to implement the detection method as previously described.

Also proposed is an item of electrical equipment as previously described, the item of electrical equipment being an electricity meter.

Also proposed is a computer program comprising instructions that cause the item of electrical equipment as previously described to perform the steps of the detection method as previously described.

Also proposed is a computer-readable storage medium on which the previously described computer program is stored.

The invention shall be better understood in the light of the following description of specific and non-limiting embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In reference toFIG.1, an electricity distribution network1is shown. The distribution network1allows electrical power to be transported from a power generation unit2to one or more electrical installations, in this instance a first electrical installation3and a second electrical installation4.

The distribution network1is a three-phase electricity network that comprises a first phase conductor5A, a second phase conductor5B, a third phase conductor5C and a neutral conductor6. For the sake of simplicity, the term “conductor” shall be omitted hereinafter in the description, with reference being made simply to a first phase5A, a second phase5B, a third phase5C and a neutral6.

The distribution network1carries a first phase voltage V1between the first phase and the neutral6, a second phase voltage V2between the second phase5B and the neutral6and a third phase voltage V3between the third phase5C and the neutral6. The first, second and third phase voltages V1, V2, V3are alternating (sinusoidal) voltages with a frequency of 50 Hz.

FIG.2shows the first phase voltage V1, the second phase voltage V2and the third phase voltage V3in the Fresnel diagram when the distribution network1is operating in nominal conditions.

The root mean square value of the first phase voltage V1eff, the root mean square value of the second phase voltage V2effand the root mean square value of the third phase voltage V3effare all equal to the nominal root mean square value of the phase voltage, identified as Vnom, which is equal to 230V.

Moreover, the first, second and third phase voltages V1, V2, V3are each out of phase with one another. Therefore, a first phase shift φ12is present between the vector representation of the first phase voltage V1and that of the second phase voltage V2, a second phase shift φ23is present between the vector representation of the second phase voltage V2and that of the third phase voltage V3and a third phase shift φ31is present between the vector representation of the third phase voltage V3and that of the first phase voltage V1. The first phase shift φ12, the second phase shift φ23and the third phase shift φ31are all equal to 120°.

FIG.3also shows a first line-to-line voltage U12which corresponds to a difference between the first phase voltage V1and the second phase voltage V2, a second line-to-line voltage U23which corresponds to a difference between the second phase voltage V2and the third phase voltage V3and a third line-to-line voltage U31which corresponds to a difference between the third phase voltage V3and the first phase voltage V1.

In reference once more toFIG.1, a first meter7is connected to the distribution network1between the generation unit2and the first installation3. The first meter7is intended to measure the consumption of electrical power supplied to the first installation3via the distribution network1. The first installation3is in this instance represented by a first electrical impedance Z1connected between the first phase5A and the neutral6, by a second electrical impedance Z2connected between the second phase5B and the neutral6and by a third electrical impedance connected between the third phase5C and the neutral6. Therefore, during operation in nominal conditions, the first impedance Z1has the first phase voltage V1at its terminals, the second impedance Z2has the second phase voltage V2at its terminals and the third impedance Z3has the third phase voltage V3at its terminals.

A second meter8is also connected to the distribution network between the generation unit2and the second installation4. The second meter8is intended to measure the consumption of electrical power supplied to the second installation4via the distribution network1.

The first meter7and the second meter8are three-phase meters.

The architecture of the first meter7will now be described.

The first meter7comprises a processing unit7A which comprises at least one processing component which may be a DSP (Digital Signal Processor), a processor, a microcontroller, an FPGA (Field-Programmable Gate Array) or an ASIC (Application-Specific Integrated Circuit).

The first meter7further comprises a memory7B connected to the processing unit7A or integrated into the processing unit7A. The memory7B forms a computer-readable storage medium, on which at least one computer program is stored comprising instructions for at least partially implementing the detection method described below.

The first meter7further comprises a communication module7C connected to the processing unit7A and arranged to transmit data using PLC (Power Line Communication) technology. It should be noted that other communication standards may be used, cellular radio technology, for example.

The first meter7further comprises voltage sensors7D connected to the processing unit7A and arranged to measure the first phase voltage V1, the second phase voltage V2and the third phase voltage V3of the distribution network1.

The voltage sensors7D of the first meter7will now be described in greater detail in reference toFIG.3.

The voltage sensors7D comprise a first branch10, a second branch11and a third branch12.

The voltage sensors further comprise a first analogue-to-digital converter13, a second analogue-to-digital converter14and a third analogue-to-digital converter15. In order to simplify the description, they will be referred to hereinafter as the first ADC13, the second ADC14and the third ADC15.

The first branch10comprises two first resistors R1A, R1B. The first resistor R1Acomprises a first terminal10A connected to the first phase5A and a second terminal10B connected to an input13A of the first ADC13. The first resistor R1Bcomprises a first terminal10C connected to the input13A of the first ADC13and a second terminal10D connected to the neutral6. The two first resistors R1A, R1Bthus form a voltage divider bridge between the first phase5A and the neutral6. A first measurement voltage V1mis therefore present between the input13A of the first ADC13and the neutral6. The first measurement voltage V1mis representative of the first phase voltage V1and is consistent with the input voltage range of the first ADC13.

The second branch11comprises two second resistors R2A, R2B. The second resistor R2Acomprises a first terminal11A connected to the second phase5B and a second terminal11B connected to an input14A of the second ADC14. The second resistor R2B comprises a first terminal11C connected to the input14A of the second ADC14and a second terminal11D connected to the neutral6. The two second resistors R2A, R2Bthus form a voltage divider bridge between the second phase5B and the neutral6. A second measurement voltage V2mis therefore present between the input14A of the second ADC14and the neutral6. The second measurement voltage V2mis representative of the second phase voltage V2and is consistent with the input voltage range of the second ADC14.

The third branch12comprises two third resistors R3A, R3B. The third resistor R3Acomprises a first terminal12A connected to the third phase5C and a second terminal12B connected to an input15A of the third ADC15. The third resistor R3Bcomprises a first terminal12C connected to the input15A of the third ADC15and a second terminal12D connected to the neutral6. The two third resistors R3A, R3Bthus form a voltage divider bridge between the third phase5C and the neutral6. A third measurement voltage Vim is therefore present between the input15A of the third ADC15and the neutral6. The third measurement voltage V3mis representative of the third phase voltage V3and is consistent with the input voltage range of the third ADC15.

The voltage divider bridges of each of the first, second and third branches10,11,12make it possible respectively to reduce the amplitudes of the first, second and third phase voltages V1, V2, V3in order to make them compatible with the measurement range of the first ADC13, the second ADC14and the third ADC15.

The first ADC13comprises an output13B connected to the processing unit7A. The first ADC13therefore produces first measurement samples representative of the first phase voltage V1and provides them to the processing unit7A.

The second ADC14comprises an output14B connected to the processing unit7A. The second ADC14will therefore produce second measurement samples representative of the second phase voltage V2and provide them to the processing unit7A.

The third ADC15comprises an output15B connected to the processing unit7A. The third ADC15will therefore produce third measurement samples representative of the third phase voltage V3and provide them to the processing unit7A.

It is assumed that the first ADC13, the second ADC14and the third ADC15all have suitable characteristics (number of bits, sampling frequency) to correctly convert the first measurement voltage V1m, the second measurement voltage V2mand the third measurement voltage V1m, respectively. In this instance, the first ADC13, the second ADC14and the third ADC15each have a number of bits greater than or equal to 12 bits and a sampling frequency of at least 2 ksps (kilo samples per second).

The influence of a break in the connection of the neutral6in the distribution network1will now be described.

In reference toFIG.4, it is assumed that a break in the connection of the neutral6occurs in the distribution network1at a break point16situated upstream of the first meter7. The term “upstream” should be understood to mean on the generation unit2side, and the term “downstream” should be understood to mean on the first installation3side.

When the break in the connection of the neutral6occurs, a node17becomes a floating node.

The first phase voltage V1downstream of the break point16then balances spontaneously according to the first impedance Z1of the first installation3. The first impedance Z1therefore has the first phase voltage V1at its terminals, which is the voltage carried by the first phase5A downstream of the break point16. The first phase voltage V1is expressed as function of a first nominal phase voltage V10(which is the voltage carried by the first phase5A upstream of the break point16) and a difference in potential Vneinduced by the break in the connection of the neutral6:

Similarly, the second phase voltage V2downstream of the break point16balances spontaneously according to the second impedance Z2of the first installation3. The second impedance Z2therefore has the second phase voltage V2at its terminals, which is the voltage carried by the second phase5B downstream of the break point16. The first phase voltage V2is expressed as function of a second nominal phase voltage V20(which is the voltage carried by the second phase5B upstream of the break point16) and the difference in potential Vne:

Similarly, the third phase voltage V3downstream of the break point16balances spontaneously according to the third impedance Z3of the first installation3. The third impedance Z3therefore has the third phase voltage V3at its terminals, which is the voltage carried by the third phase5C downstream of the break point16. The third phase voltage V3is expressed as function of a third nominal phase voltage V30(which is the voltage carried by the third phase5C upstream of the break point16) and the difference in potential Vne:

It should be noted that, when the neutral6is connected correctly, V1=V10, V2=V20and V3=V30.

In reference toFIG.5, the Fresnel diagram shows the influence of the break in connection of the neutral6.

It is interesting to note that the break in the connection of the neutral6causes the first phase voltage V1, the second phase voltage V2and the third phase voltage V3to rebalance with each other. Except in the specific scenario in which the first, second and third electrical impedances Z1, Z2, Z3are perfectly balanced in the first, second and third phases5A,5B,5C, the new balance will be different from the initial balance (when the neutral6was connected correctly).

The new balance involves a variation in the root mean square value of the first phase voltage V1eff, the root mean square value of the second phase voltage V2effand the root mean square value of the third phase voltage V3effwhich are then no longer necessarily equal to the nominal root mean square value of the phase voltage Vnom.

The new balance also involves a variation in the first phase shift φ12, the second phase shift φ23and the third phase shift φ31which are then no longer all necessarily equal to 120°.

However, the new balance retains the first line-to-line voltage U12, the second line-to-line voltage U23and the third line-to-line voltage U31.

In reference toFIG.6, the steps of the method for detecting a break in the connection of a neutral according to the invention is now described.

The detection method according to the invention is implemented in the processing unit7A of the first meter7. More particularly, the detection method according to the invention is implemented continuously by the processing unit7A of the first meter. The steps of said detection method are therefore repeated regularly over time.

The method starts at a time T by acquiring the first phase voltage V1between the first phase5A and the neutral6, the second phase voltage V2between the second phase and the neutral6and the third phase voltage V3between the third phase5C and the neutral6(step E1).

More specifically, the processing unit7A acquires the first measurement samples representative of the first phase voltage V1, the second measurement samples representative of the second phase voltage V2and the third measurement samples representative of the third phase voltage V3. The first measurement samples, the second measurement samples and the third measurement samples are transmitted to the processing unit7A by the voltage sensors7D.

Using the first measurement samples, the second measurement samples and the third measurement samples, respectively, the processing unit7A determines (possibly by applying calibration parameters) a root mean square value of the first phase voltage V1eff, a root mean square value of the second phase voltage V2effand a root mean square value of the third phase voltage V3eff. The root mean square values V1eff, V2effand V3effare in this instance determined respectively by the processing unit7A each second based on the first, second and third measurement samples acquired the previous second. The root mean square values V1eff, V2effand V3effcould also be determined over a sliding second or over any other appropriate time window.

The processing unit7A next defines and/or updates a predetermined reference criterion (step E2). The predetermined reference criterion will be described below.

It should be noted that step E2is optional. Indeed, the predetermined reference criterion may be determined a single time during the installation of the first meter7in the distribution network1(the predetermined reference criterion being stored in the memory7B). The predetermined reference criterion could also be defined and/or updated during maintenance of the first meter7.

The processing unit7A then evaluates a first quantity representative of a ratio between a maximum phase voltage and a minimum phase voltage from the first, second and third phase voltages V1, V2, V3(step E3). More specifically, the first quantity is in this instance a number M that is determined by using the following formula:

Evaluating the first quantity M makes it possible to check whether or not there is an imbalance between the first, second and third phase voltages V1, V2, V3.

To this end, the first quantity M is compared to a predetermined threshold (step E4). The predetermined threshold is in this instance equal to 1.2 in order to take into account, in particular, normal dispersions of the first, second and third phase voltages V1, V2, V3(intrinsic in the distribution network1). If the first quantity M is greater than the predetermined threshold, there is a significant imbalance between the first, second and third phase voltages V1, V2, V3, and this constitutes a first indicator of a suspected break in the connection of the neutral6in the distribution network1upstream of the first meter7.

However, the simple fact that the first quantity M is greater than the predetermined threshold is not sufficient to be certain that a break in connection of the neutral6has actually occurred. In particular, a simple break in the connection of one phase (from the first, second and third phase5A,5B,5C) could also lead to the same observation (the first quantity M is greater than the predetermined threshold). It is therefore necessary to confirm the suspected break in the connection of the neutral6by determining an indicator that is characteristic of a break in the connection of a neutral in a three-phase electricity network.

It should be noted that, if the first quantity M is less than the predetermined threshold, the detection method according to the invention determines that there is no break in the connection of the neutral6, and therefore returns to step E1.

If the first quantity is greater than the predetermined threshold, the processing unit7A evaluates, based on the first, second and third phase voltages V1, V2, V3, at least one second quantity representative of a current balance between said first, second and third phase voltages (step E5). Indeed, as shown inFIG.5, the break in the connection of a neutral gives rise to a new balance between the first, second and third phase voltages V1, V2, V3.

The processing unit7A then detects whether the at least one second quantity satisfies the predetermined reference criterion.

Several methods are now possible and, in particular, several different second quantities as well as several predetermined reference criteria can be defined in order to effectively confirm that a break in the connection of the neutral6has occurred.

According to a first embodiment, the at least one second quantity comprises a second quantity G2 that is a function of a sum of the pairwise products of the first, second and third phase voltages V1, V2, V3. More specifically, the second quantity G2 is a function of a sum of pairwise products of root mean square values of the first, second and third phase voltages V1eff, V2eff, V3effThe approach proposed here is based on the fact that, when there is a suspected break in the neutral connection, the first, second and third phase voltages V1, V2, V3become balanced according to the new balance, which is not completely random.

If one of the root mean square values of a phase voltage from the root mean square values of the first, second and third phase voltages V1eff, V2eff, V3effdecreases sharply, while the other two root mean square values remain constant, this does not indicate a break in the connection of the neutral6. For example, if the root mean square value V1effdecreases sharply while the root mean square values V2effand V3effremain constant, it is rather a question of a voltage dip in the first phase5A or a break in the connection of said first phase5A. However, if one of the root mean square values of a phase voltage from the root mean square values of the first, second and third phase voltages V1eff, V2eff, V3effdecreases sharply and, at the same time, one of the two other root mean square values increases (or indeed the two root mean square values increase) it is highly probable that this is linked to a break in the connection of the neutral6. For example, if the root mean square value V1effdecreases sharply and the root mean square values V2effand V3effsimultaneously increase, it is highly likely that this is linked to a break in the connection of the neutral6.

Similarly, if one of the root mean square values of a phase voltage from the root mean square values of the first, second and third phase voltages V1eff, V2eff, V3effincreases sharply, while the other two root mean square values remain constant, this does not indicate a break in the connection of the neutral6.

In order to confirm the suspected break in the connection of the neutral6, the processing unit7A calculates the second quantity G2 by using the following formula:

where V1eff, V2effand V3effare respectively the root mean square value of the first phase voltage, the root mean square value of the second phase voltage and the root mean square value of the third phase voltage.

The second quantity G2 is proposed because it is limited when a break in the connection of the neutral6occurs. In the first embodiment, the predetermined reference criterion which is defined by the processing unit7A is therefore to check that the second quantity G2 is limited between a lower limit BornInf and an upper limit BornSup: BorneInf≤G2≤BorneSup.

In order to correctly define BornInf and BornSup, the detection method further comprises the steps of:detecting whether the first, second and third phase voltages V1, V2, V3are perfectly balanced, i.e., whether or not the root mean square value of the first phase voltage V1eff, the root mean square value of the second phase voltage V2effand the root mean square value of the third phase voltage V3effare all equal to the nominal root mean square value of the phase voltage Vnomof the distribution network1:

V1eff=VnomandV2eff=VnomandV3eff=Vnom,if this condition is met, defining BorneInf and BorneSup as follows:

Annex 1 proposes a mathematical demonstration for determining the expression of BornInf and BornSup.

If the second quantity G2 is limited between the lower limit BornInf and the upper limit BornSup (i.e., if the second quantity G2 satisfies the predetermined reference criterion), the detection method according to the invention detects a break in the neutral connection at the time T upstream of the first meter7.

In the first embodiment, it is also possible to detect a break in the connection of the neutral6when one phase from the first, second and third phases5A,5B,5C is disconnected. In this instance, the third phase5C is considered to be disconnected. If a break occurs in the connection of the neutral6, there is a characteristic relation between the root mean square value of the first phase voltage V1effand the root mean square value of the second phase voltage V2eff.

If the first, second and third phase voltages V1, V2, V3are perfectly balanced, i.e., if: V1eff=Vnom, and V2eff=Vnomand V3eff=Vnom, the characteristic relation is the following:V1eff+V2eff=√{square root over (3)}Vnom, where Vnomis the nominal root mean square value of the phase voltage.

If the first, second and third phase voltages V1, V2, V3are not perfectly balanced, i.e., if: V1eff=α1·Vnomand V2eff=α2·Vnomand V3eff=α3·Vnom, (α1≠α2≠α3) the characteristic relation is the following:V1eff+V2eff=Vnom√{square root over (α12+α22+α1α2)}, where Vnomis the nominal root mean square value of the phase voltage.

A more complete and detailed algorithm for implementing the detection method according to the first embodiment of the invention is presented in Annex 2.

According to a second embodiment, the detection method comprises the step of evaluating the at least one second quantity which comprises a second quantity A that is a function of an area of an actual triangle19formed by the first, second and third phase voltages V1, V2, V3in the Fresnel diagram.

The actual triangle19has a first edge constituted by the first line-to-line voltage U12, a second edge constituted by the second line-to-line voltage U23and a third edge constituted by the third line-to-line voltage U31.

The second quantity A is determined by the processing unit7A by using the formula:

where A is the second quantity (which is the area of the actual triangle19), V1eff, V2effand V3effare respectively the root mean square values of the first, second and third phase voltages, and φ12is the first phase shift, φ23is the second phase shift and φ31is the third phase shift. The first phase shift φ12, the second phase shift φ23and the third phase shift φ31are determined by the processing unit7A based on the first, second and third phase voltages V1, V2, V3via a zero-crossing method and appropriate filtering.

The reference criterion is then that the second quantity A is such that: Aref−ε1≤A≤Arefε1, where Arefis a predetermined area of a reference triangle18and ε1is a first predetermined measurement uncertainty (typically, +/−1% or +/−2%).

In reference toFIG.5, the area of the reference triangle18Arefis actually the area of the triangle formed by the first, second and third phase voltages V1, V2, V3in the Fresnel diagram in nominal conditions (i.e., when there is no break in the connection of the neutral6).

Indeed, it is interesting to note that the area of the reference triangle18and the area of the actual triangle19are similar in the event of a break in the connection of the neutral6. Therefore, by comparing the second quantity A (the area of the actual triangle19) with the predetermined area of the reference triangle18Aref, it is possible to effectively determine whether a break in the connection of the neutral6has occurred.

In order to correctly define the area of the reference triangle18Aref, the detection method further comprises the steps of:detecting whether the first, second and third phase voltages V1, V2, V3are perfectly balanced, i.e., whether or not the root mean square value of the first phase voltage V left-, the root mean square value of the second phase voltage V2effand the root mean square value of the third phase voltage V3effare all equal to the nominal root mean square value of the phase voltage Vnom, of the distribution network1:

V1eff=VnomandV2eff=VnomandV3eff=Vnom,if this condition is met, the area of the reference triangle18is evaluated by using the formula:

where Vnomis the nominal root mean square value of the phase voltage.

Otherwise, if said condition is not met, and therefore if:V1eff=α1·Vnomand V2eff=α2·Vnom, and V3eff=α3·Vnom, where a1, a2and a3are real coefficients such that α1≠α2≠α3and Vnomis said nominal root mean square value, the area of the reference triangle18is evaluated by using the formula:

where Vnomis said nominal root mean square value of the phase voltage.

If the second quantity A satisfies the predetermined reference criterion, the detection method according to the invention detects a break in the neutral connection at the time T upstream of the first meter7.

According to a third embodiment of the invention, the at least one second quantity comprises second quantities which comprise the first line-to-line voltage U12, the second line-to-line voltage U23and the third line-to-line voltage U31.

In reference toFIG.5, in the event of a break in the connection of the neutral6, the first line-to-line voltage U12, the second line-to-line voltage U23and the third line-to-line voltage U31remain constant.

The reference criterion checked by the processing unit7A is therefore that of checking whether:

where Φ1, Φ2and Φ3are respectively reference values of the first, second and third line-to-line voltages U12, U23, U31measured during operation at a reference time T0preceding the time T and ε2is a second predetermined measurement uncertainty (typically, +/−1% or +/−2%).

If the second quantities U12, U23and U31satisfy the predetermined reference criterion, the detection method according to the invention detects a break in the neutral connection at the time T upstream of the first meter7.

According to a fourth embodiment of the invention, the detection method comprises the step of evaluating the at least one second quantity which comprises second quantities which comprise the first phase shift Φ12, the second phase shift Φ23and the third phase shift Φ31.

In reference toFIG.5, it is interesting to note that the first phase shift Φ12, the second phase shift Φ23and the third phase shift Φ31are no longer equal to 120° when a break in the connection of the neutral6occurs.

The reference criterion defined by the processing unit7A is then satisfied if the first, second and third phase shifts Φ12, φ23, Φ31are each non-zero and different to 120 degrees.

If the first, second and third phase shifts Φ12, Φ23, Φ31satisfy the predetermined reference criterion, the detection method according to the invention detects a break in the neutral connection at the time T upstream of the first meter7.

In the four embodiments of the invention disclosed above, if the second quantity or quantities do not satisfy the predetermined reference criterion, the detection method moves on to step E1′, which is similar to step E1, and possibly includes a step E2′ which is similar to step E2.

In the four embodiments of the invention disclosed above, a suitable margin of uncertainty, relating in particular to the accuracy of the voltage sensors7D of the first meter7, is to be taken into consideration. Typically, the margin of uncertainty is in the region of +/−1% or +/−2%.

In the four embodiments of the invention disclosed above, the expressions used for the definition of the predetermined reference criterion (BornInf, BornSup, Aref, Φ1, Φ2and Φ3) could be determined by the processing unit7A by means of several successive calculations in order to mitigate parasitic phenomena (such as a transient overvoltage of the shockwave or micro-cut-off type) that could decrease the reliability of the detection method according to the invention. The use of an averaging technique is quite feasible assuming that the distribution network1is stable for a given time period.

Irrespective of the embodiment, the detection method according to the invention is implemented continuously by the processing unit7A of the first meter. The steps of said detection method are therefore repeated regularly over time.

Advantageously, and irrespective of the embodiment, the detection method according to the invention may further comprise the step of detecting a break in connection of the neutral6when it has been detected that the second quantity satisfies the predetermined reference criterion a predetermined number of times (for example, 10 times), corresponding to consecutive instances it is satisfied (i.e., consecutive implementations of the detection method) spaced apart two by two in time by a predetermined duration (for example, 1 second). This increases the reliability of the detection method according to the invention.

Optionally, when a break in the neutral connection has effectively been detected, the detection method may further comprise the step of generating an alarm signal that can be timestamped in the memory7B of the first meter7and/or that can be transmitted (via the communication module7C) to an item of equipment external to said first meter7, for example an information system of the distribution network1.

Optionally, the alarm signal may be the displaying of a specific message on a local display of the first meter7and/or the alarm signal may be the illumination of an indicator light located on said first meter7and/or the issuing of a sound signal by a loudspeaker of the first meter7.

Ideally, all of the electricity meters of the distribution network1are similar and are arranged to implement the detection method according to the invention. Therefore, the whole of the distribution network1is monitored correctly.

Optionally, when a break in the neutral connection has effectively been detected and the imbalance between the first, second and third phase voltages is considered to be too great, the detection method may further comprise the step, if said detection method is implemented in a specific meter comprising a breaker, of opening said breaker in order to protect one or more electrical installations downstream of said specific meter.

Naturally, the invention is not limited to the described embodiments, but covers any variant that falls within the scope of the invention as defined by the claims.

It is quite possible to freely combine the different embodiments disclosed above. Therefore, the processing unit7A of the first meter7may determine several second quantities, for example the second quantity G2 (disclosed in the first embodiment) and the second quantity A (disclosed in the second embodiment). This could increase the reliability of the detection method according to the invention.

The voltage sensors7D of the first meter7comprising a first ADC13, a second ADC14and a third ADC15have been disclosed, but the voltage sensors7D may quite possibly comprise a single ADC comprising three distinct inputs each respectively acquiring the first phase voltage, the second phase voltage and the third phase voltage.

Annex 1: Formal Demonstration of the Relations Proposed for the First Embodiment

The assumption for the present demonstration is that the first, second and third phase voltages V1, V2, V3are perfectly balanced, i.e., that the root mean square values V1eff, V2eff, V3effare all equal to the nominal root mean square value of the phase voltage Vnom. This is not necessarily accurate in practice, but can be used as a first approach.

It should be noted that voltages in the demonstration are in vector form and that, throughout the demonstration, they are complex root mean square values.

In reference toFIG.4, once a break in the neutral connection has occurred, it is possible to write:

{right arrow over (V10)}−{right arrow over (V1)}={right arrow over (Vne)}

{right arrow over (V20)}−{right arrow over (V2)}={right arrow over (Vne)}

{right arrow over (V30)}−{right arrow over (V3)}={right arrow over (Vne)}

{right arrow over (I1)}+{right arrow over (I2)}+{right arrow over (I3)}=0

I1, I2, I=are the electric currents flowing respectively through the first phase5A, the second phase5B and the third phase5C.

Using a first admittance Y1, a second admittance Y2and a third admittance Y3(Y1=1/Z1, Y2=1/Z2, Y3=1/Z3), the following applies:

{right arrow over (V10)}Y1+{right arrow over (V20)}Y2+{right arrow over (V30)}Y3={right arrow over (Vne)}(Y1+Y2+Y3)

Substitution {right arrow over (Vne)}: (1) is carried out

{right arrow over (V10)}Y1+{right arrow over (V20)}Y2+{right arrow over (V30)}Y3=({right arrow over (V10)}−{right arrow over (V1)})(Y1+Y2+Y3)

Calculation of V1

Based on the relation (1), the following applies:

Applying the modulus gives: (2)

Calculation of V2

Based on the relation (1), the following applies:

It follows that:

Applying the modulus gives: (3)

Calculation of V3

Based on the relation (1), the following applies:

It follows that:

Applying the modulus gives: (4)

Using the relation (2), (3) and (4) and positing V10=Vnomgives:

Studying the Limit Cases:

It follows that (the limit case studied here constituting the lower limit)—Viand Vjbeing respectively the phase voltages of the ith phase and jth phase:

It follows that (the limit case studied here constituting the upper limit):

We have shown, by studying limit cases, that the sum of the ViVjremained very precisely limited between Vnomand 1.056Vnomin the event of a break in the neutral connection:

The proposed formula can then be generalized, by the same calculation process, to take into account initial conditions for which the distribution network does not have perfectly balanced phase voltages, by positing:

The fact is that, considering the effect of the initial imbalances between the phase voltages coming from the distribution network, the asymptotic study cases to be considered (which make it possible to obtain a bounded limit of the result expected in the event of a break in the neutral connection) increase from 2 to 6, due to loss of symmetry.

The 6 limits are given by:

And Finally: