AU2013254917A1

AU2013254917A1 - Method and system for determining the primary voltage of a transformer, and transformer substation including such a determinaton system

Info

Publication number: AU2013254917A1
Application number: AU2013254917A
Authority: AU
Inventors: Denis Boss; Philippe Deschamps
Original assignee: Schneider Electric Industries SAS
Current assignee: Schneider Electric Industries SAS
Priority date: 2012-11-09
Filing date: 2013-11-07
Publication date: 2014-05-29
Anticipated expiration: 2033-11-07
Also published as: EP2730929B1; ES2537929T3; FR2998060B1; CN103809009A; AU2013254917B2; CN103809009B; FR2998060A1; EP2730929A1

Abstract

Method and system for determining the primary voltage of a transformer, and transformer substation including such a determination system The method according to the invention makes it possible to determine a primary voltage of an electric transformer (18). The transformer has a no-load transformation ratio and comprises a primary winding (52) capable of having the primary voltage between its ends and of being passed through by a primary current (11a, 11b, 11c), a secondary winding (54) capable of having a secondary voltage between its ends and of being passed through by a secondary current (12a, 12b , 12c). The determination method comprises measuring the secondary voltage (U2a, U2b, U2c) using a voltage sensor (59A, 59B, 59C), and measuring the primary current (11a, 1lb, 11c) using a first current sensor (56A, 56B, 56C). The method further comprises the following steps: a) calculating a secondary voltage drop between the ends of the secondary winding (54), as a function of specific characteristics of the transformer (18), of the load factor of the transformer (18) and of the power factor of a load connected to the secondary winding (54) at the output of the transformer (18); b) calculating the modulus (I Ulj ) of the primary voltage as a function of the measured secondary voltage, of the voltage drop calculated in step a) and of the no-load transformation ratio; and c) calculating the phase shift of the primary voltage (U1j) relative to the measured primary current (11j) as a function of the phase shift of the measured secondary voltage (U2j) relative to the measured primary current (11j). Figure 1 T Iw ---------- ---------- -- --- --- Wt - - - - - - - - S Measurement of the I secondary voltage U2 Measuremnent of the primary an~d 1 sec ondarl currents iI ano 12 SCaiculation of the voitage dro A 2 IDetermnination of the position of 1B the adjusting member ECalculation of the nnodus of the primary voltage U! SCaloulation of the phase shift of the primary' voltage U I with the primary Current .g

Description

1 Method and system for determining the primary voltage of a transformer, and transformer substation including such a determination system The present invention relates to a method for determining the primary voltage of an 5 electric transformer, the electric transformer having a no-load transformation ratio and comprising a primary winding capable of having the primary voltage between its ends and being passed through by a primary current, a secondary winding capable of having a secondary voltage between its ends and being passed through by a secondary current. The method comprises the following steps: 10 - measuring the secondary voltage using a voltage sensor, - measuring the primary current using a first current sensor. The invention also relates to a system for determining the primary voltage of an electric transformer intended to carry out such a method. The terminal distribution of public power grids for distributing a polyphase current to 15 energy consumers is done at a low voltage from medium-voltage / low-voltage sub-stations, also called MV/LV substations. Each MV/LV substation is in the form of a radial structure including multiple low-voltage lines, each serving a certain number of consumers. Each low voltage line includes a phase conductor for each phase of the delivered polyphase current and a neutral conductor. Each of these MV/LV substations is in turn connected on a medium 20 voltage line, the origin of which is on a high-voltage/medium-voltage substation, also called HV/MV substation. The low voltage may vary based on the loads connected to the low-voltage grid and characteristics of the low-voltage lines. The compliance of the delivered voltage is monitored using two methods. 25 A first method consists, during design of the grid, of calculating the dimensioning of the cables and the distribution of the loads, in order to guarantee a minimal voltage drop in the conductors compatible with the acceptable voltage limits of the loads. A second method, which can be done under operating conditions, consists of adjusting the voltage on the low-voltage lines to adapt to the conditions of the moment. The 30 second method also comprises adjusting the voltage upstream. On the one hand, the load adjusters of the HV/MV substations, controlled automatically and continuously, make it possible to adjust the voltage at the head of the grid to minimize online losses while preserving an acceptable voltage downstream. On the other hand, no-load adjusters of the MV/LV substations make it possible to adjust the voltage upstream. These no-load adjusters 35 include manual switches, also called adjusting members, positioned in the distribution 2 transformers. They make it possible to choose the most suitable winding of the transformer based on the structure of the grid to which it is connected. The no-load adjusters are maneuvered very occasionally, i.e., several times throughout the entire lifetime of the transformer, to follow the structural evolutions of the grid and consumers. 5 In this context, inserting decentralized production into the grid, in particular low voltage production, locally causes the maximum voltage to be exceeded. In fact, the dimensioning of the grid, calculated during its design, does not account for the energy flows coming from decentralized productions. Regular monitoring is then necessary of the current circulating in the primary winding of the transformer, also called primary current; the voltage 10 between the ends of the primary winding, also called primary voltage; the current circulating in the secondary winding of the transformer, also called secondary current; and the voltage between the ends of the secondary winding, also called secondary voltage. Simple and cost-effective solutions are known for measuring the primary current and/or secondary current, for example using opening toroids. Solutions are also known for 15 measuring the secondary voltage. To measure the primary voltage, the use is known of voltage measuring transformers, capacitor voltage dividers, resistive dividers, or solutions based on a simplified calculation that depends on the secondary voltage and the transformation ratio of the transformer. However, such measurements of the primary voltage are relatively imprecise or too 20 expensive, in particular in the case of the voltage measuring transformer. They are also intrusive, as they require accessing parts of the primary circuit of the transformer that are powered on or cables that are connected thereto. The aim of the invention is therefore to propose a more precise method for determining the primary voltage of a transformer, that method having a limited cost and being 25 compatible with the existing fleet of MV/LV substations. To that end, the invention relates to a method for determining the primary voltage of a transformer of the aforementioned type, characterized in that the method further comprises the following steps: a) calculating a secondary voltage drop between the ends of the secondary winding, 30 as a function of specific characteristics of the transformer, of the load factor of the transformer and of the power factor of a load connected to the secondary winding at the output of the transformer; b) calculating the modulus of the primary voltage as a function of the measured secondary voltage, of the voltage drop calculated in step a) and of the no-load transformation 35 ratio; and 3 c) calculating the phase shift of the primary voltage relative to the measured primary current as a function of the phase shift of the measured secondary voltage relative to the measured primary current. According to other advantageous aspects of the invention, the determination method 5 comprises one or more of the following features, considered alone or according to all technically possible combinations: - during step a), the secondary voltage drop is calculated using the following equation: AU =(Ur cosp+Ux sinq()*n + *(Ux cos( - Ur sing() *n 2 U 200 10 where Ur and Ux represent the resistive and reactive components specific to the transformer, n represents the load factor of the transformer, and cos <p represents the power factor of the load of the transformer; - the electric transformer further comprises an adjusting member capable of modifying the no-load transformation ratio of the transformer, and during step b), the modulus of the 15 primary voltage is calculated as a function of the measured secondary voltage, of the voltage drop calculated in step a), of the no-load transformation ratio and of the position of the adjusting member; - during step b), the modulus |U1j of the primary voltage, j being the index of the corresponding phase, is calculated using the following equation: 20 Ulj = Kpos * Uo* U2j U2o 1 AU U where Kpos is a coefficient that depends on the position of the adjusting member, is the U2o no-load transformation ratio of the transformer, U2j is the secondary voltage, and AU is the secondary voltage drop calculated during step a); - during step c), the phase shift ph(Ulj, l1j) between the primary voltage and the 25 primary current is calculated according to the following equation: ph(Ulj,llj) = Tr - ph(llj,U2j) where ph(llj,U2j) is the phase shift measured between the primary current and the secondary voltage ; - the method further comprises measuring the secondary current using a second 30 current sensor and in that, during step b), the position of the adjusting member of the transformer is calculated as a function of the primary and secondary currents; 4 - the position of the adjusting member is determined using the knowledge of the primary rated voltages for each of the positions of the adjusting member of the transformer, the secondary rated voltage, as well as using the following equation: 12j UlIj Ilj U2j 5 where l1j and 12j are the measured primary and secondary currents, U1j and U2j are the primary and secondary voltages. The position of the adjusting member is determined by identifying the position making it possible to obtain the ratio between the primary and secondary rated voltages, corresponding to the second term of the preceding equation, closest to the ratio between the measured secondary and primary currents corresponding to 10 the first term of the preceding equation. The invention also relates to a system for determining the primary voltage of an electric transformer, the electric transformer having a no-load transformation ratio comprising a primary winding capable of having the primary voltage between its ends and being passed through by a primary current, a secondary winding capable of having a secondary voltage 15 between its ends and being passed through by a secondary current. The system comprises a voltage sensor capable of measuring the secondary voltage, and a first current voltage capable of measuring the primary current. The system further comprises: - first means for calculating the secondary voltage drop between the ends of the 20 secondary winding, based on specific characteristics of the transformer, the load factor of the transformer and the power factor of a load connected to the secondary winding at the output of the transformer; - second means for calculating the modulus of the primary voltage as a function of the measured secondary voltage, the calculated voltage drop, and the no-load 25 transformation ratio; and - third means for calculating the phase shift of the primary voltage relative to the measured primary current as a function of the phase shift of the measured secondary voltage relative to the measured primary current. According to other advantageous aspects of the invention, the determination system 30 comprises one or more of the following features, considered alone or according to all technically possible combinations: - each current sensor includes an opening toroid; - each current sensor is connected to the third calculating means via wireless communication means.

5 The invention also relates to a substation for transforming a first electrical current having a first alternating voltage into a second electrical current having a second alternating voltage, comprising: - a first panel including at least one incoming electrical conductor that can be 5 connected to the electrical grid, the incoming conductor having a primary voltage, - a second panel including at least one outgoing electrical conductor, the outgoing conductor having a secondary voltage, - a transformer comprising a primary winding connected to the first panel, and a secondary winding connected to the second panel, and 10 - a system for determining the primary voltage of the transformer, wherein the determination system is as defined above. These features and advantages of the invention will appear upon reading the following description, provided solely as a non-limiting example, and done in reference to the appended drawings, in which: 15 - figure 1 is a diagrammatic illustration of a transformation substation comprising a first panel, a second panel connected to the first panel by means of a transformer, and a system for determining the primary voltage from measurements of primary and secondary currents and the secondary voltage, - figure 2 is a detailed view showing the communication between an information 20 processing unit of the determination system and the secondary current and secondary voltage sensors shown in figure 1, - figure 3 is a flowchart of a method for determining the primary voltage of the transformer, according to the invention. In figure 1, a transformation substation 10 connected to an alternating electrical grid 25 12, comprises a first panel 14, a second panel 16, an electric transformer 18 provided with an adjusting member 20 and connected between the first panel and the second panel, as well as a system 22 for determining a primary voltage U1j of the transformer 18. The transformation substation 10 is capable of transforming the electrical current 11j delivered by the grid 12 and having a first alternating voltage U1j, into an electrical current 12j 30 having a second alternating voltage U2j. The electrical grid 12 is an alternating grid comprising P phases, P being an integer greater than or equal to 1, such as a three-phase grid. In other words, in the embodiment described in figure 1, P is equal to 3. The voltages Ulj, U2j and currents 11j, 12j of each phase conductor are Ula, Ulb, Ulc, U2a, U2b, U2cand 11a, 1lb, 11c, 12a, 12b, 12c.

6 The electrical grid 12 is a medium-voltage grid, i.e., a grid whereof the voltage is above 1000 V and below 50,000 V. The first three-phase voltage U1j is then a medium voltage. The first panel 14 includes several incoming cables 24A, 24B, each incoming cable 5 24A, 24B having a first 26A, 26B, second 28A, 28B and third 30A, 30B incoming conductor. Each incoming conductor 26A, 26B, 28A, 28B, 30A, 30B is connected to the electrical grid 12 for example using a respective incoming circuit breaker 31. The three-phase current 11j circulating in the corresponding incoming conductors 26A, 26B, 28A, 28B, 30A, 30B has the first three-phase voltage U1j. 10 The first incoming conductors 26A, 26B, the second incoming conductors 28A, 28B, the third incoming conductors 30A, 30B are respectively connected to a first primary connecting conductor 32, a second primary connecting conductor 33, and a third primary connecting conductor 34. These primary connecting conductors connect the incoming cables 24 to the adjusting member 20 of the transformer 18. 15 The second panel 16 comprises a first 35, second 36, third 37 and fourth 38 secondary connecting conductor and a plurality N of outgoing cables 40A, 40B, ..., 40N, i.e., a first outgoing cable 40A, a second outgoing cable 40B, ..., an Nth outgoing cable 40N, each outgoing cable 40A, 40B, ..., 40N being capable of delivering a three-phase voltage U2j. The secondary connecting conductors 35, 36, 37, 38 connect the transformer 18 to the outgoing 20 cables 40A, ..., 40N. Each outgoing cable 40A, ... , 40N is a low-voltage outgoing cable U2j, i.e., an outgoing cable whereof the voltage is below 1000 V. The second three-phase voltage U2j is then a low voltage. The first outgoing cable 40A includes a first 42A, second 44A, third 46A and fourth 25 48A outgoing conductor and three outgoing circuit breakers 50. The first, second, third outgoing conductors 42A, 42B, 42C are respectively connected to the first, second and third secondary connecting conductors 35, 36, 37 by means of a corresponding outgoing circuit breaker 50. The fourth outgoing conductor 48A is directly connected to the fourth secondary connecting conductor 38. 30 The secondary connecting conductors 35, 36, 37 and the outgoing conductors of the corresponding outgoing cables 42A, 44A, 46A have the same voltage U2j, each corresponding to a respective phase U2a, U2b, U2c of the second three-phase voltage. The other outgoing cables 40B, ..., 40N are identical to the first outgoing cable 40A described above, and include the same elements, each time replacing the corresponding 35 letter A with the letter B, ..., N for the references of the elements.

7 The electric transformer 18 is capable of transforming the current 11j from the electrical grid having the first alternating voltage U1j into the current 12j delivered to the second panel 16 and having the second alternating voltage U2j. The electric transformer 18 includes a primary winding 52 connected to the first panel 14 and a secondary winding 54 5 connected to the second panel 16, each winding 52, 54 including an electromagnetic coil, not shown, for each phase a, b, c of the corresponding alternating current. The current capable of circulating in the primary winding 52 of the transformer is called primary current and is denoted 11j, j being the index of the corresponding phase, equal to a, b or c. The voltage between the ends of the primary winding 52 is called primary voltage 10 and is denoted U1j for each phase j of the corresponding alternating current. The current capable of circulating in the secondary winding 54 of the transformer is called secondary current 12j, and the voltage between the ends of the secondary winding 54 is called secondary voltage U2j for each phase j of the corresponding alternating current, j being equal to a, b or c. 15 The determination system 22, shown in figure 1 and part of which is shown on a larger scale in figure 2, comprises sensors 56A, 56B, 56C for the primary current, sensors 58A, 58B, 58C for the secondary current, sensors 59A, 59B, 59C for the secondary voltage, and an information processing unit 60. The determination system 22 is capable of using the primary current sensors 56A, 20 56B, 56C to measure the primary currents 11a, 1lb, 1lc passing through each of the three primary connecting conductors 32, 33, 34 connected to the primary winding 52 of the transformer. The determination system 22 is capable of using the secondary current sensors 58A, 58B, 58C to measure the secondary currents 12a, 12b, 12c passing through each of the three 25 secondary connecting conductors 35, 36, 37 connected to the secondary winding 54 of the transformer. The determination system 22 is capable of using the voltage sensors 59A, 59B, 59C to measure the secondary voltages U2a, U2b, U2c, relative to the neutral conductor 38, of each of the three secondary connecting conductors 35, 36, 37 connected to the second 30 winding 54. The information processing unit 60 comprises a processor 62 and a memory 64 associated with the processor 62. The memory 64 is capable of storing first software 66 for calculating the voltage drop AU across the terminals of the transformer on the side of the T secondary winding 54, second software 68 for calculating the modulus of the primary voltage 8 Ula, U1b, Ulc of the transformer, and third software 70 for calculating the phase shift of the primary voltage Ula, U1b, Ulc with the primary current 11a, 1lb, 11c. The information processing unit 60 comprises communication software 72 capable of communicating with the current sensors 56A,..., 58C. 5 The processing unit comprises software 73 for acquiring values measured by the primary current sensors 56A, 56B, 56C, the secondary current sensors 58A, 58B, 58C and the secondary voltage sensors 59A, 59B, 59C. These five software applications 66, 68, 70, 72, 73 are able to communicate with each other. 10 The first calculation software 66 is capable of calculating the voltage drop using equation [1] AU 12 [1] = (Ur cosqo + Ux sinqg).n + - (Ux cosp - Ur sinqg) . n' U 200 where Ur and Ux represent the resistive and reactive components specific to the transformer 18, n represents the load factor of the transformer 18, and cosqp represents the 15 power factor of the load of the transformer 18. The second calculation software 68 is capable of calculating the modulus of the primary voltage from equation [2]: Ulo U2j [2] Ulj = Kpos. . U2o 1 AU U where Kpos is a coefficient depending on the position of the adjusting member 20, 20 Ulo is the no-load transformation ratio of the transformer 18, U2j is the measured U2o secondary voltage, and AU is the voltage drop previously calculated using equation [1]. C The second calculation software 68 for the modulus of the primary voltage is capable of determining the coefficient Kpos depending on the position of the adjusting member in a preliminary step before the calculation of equation [2]. 25 This preliminary step makes it possible to deduce the position of the adjusting member from the knowledge of the primary rated voltages for each of the positions of the adjusting member of the transformer, the secondary rated voltage, and equation [3]: [3] 12j U j Ilj U2j where 11j and 12j are the measured primary and secondary currents, U1j and U2j are 30 the primary and secondary voltages.

9 The primary rated voltages and the secondary rated voltage are also called respectively primary nominal voltages and the secondary nominal voltage. Each position of the adjusting member determines a primary voltage, therefore a ratio between the primary and secondary voltages. This ratio between the primary and secondary 5 voltages is equal to the ratio between the measured secondary and primary currents. The determination of the position of the measuring member during this preliminary step therefore consists of identifying the ratio between the primary and secondary voltages, via the measured currents, and therefore deducing the position of the adjusting member therefrom. The third calculation software 70 is capable of calculating the phase shift of the 10 primary voltage U1j with the primary current 11j for a configuration D yn1 1 of the transformer 18 from equation [4]: [4] ph(U1j,11j) = -rr - ph(11j,U2j) where 11j is the primary current measured for each phase j, U2j is the secondary voltage measured for each phase j, U1j is the primary voltage for each phase j, and 15 ph(11j,U2j) is the phase shift between the primary current 11j and the secondary voltage U2j for each phase j. The configuration D yn11 of the transformer 18 corresponds to a three phase system with the coils of the primary winding 52 connected in a triangle and the coils of the secondary winding 54 connected in a star with the neutral brought out. In the example embodiment of figures 1 and 2, the communication between the 20 current sensors 56A, ..., 58C and the information processing unit 60 occurs wirelessly. The information processing unit 60 then comprises a wireless transceiver 74 and a wireless antenna 75. The determination system 22 is capable of determining the primary voltage U1j of the electric transformer 18 for measurements of the primary 11j and secondary 12j currents, as 25 well as the secondary voltage U2j. In figure 2, each secondary current sensor 58A, 58B, 58C includes a first toroid 76 positioned around each of the secondary connecting conductors 35, 36, 37 and a first winding 78 arranged around the first toroid 76, as shown in figure 2. The circulation of the current 12a, 12b, 12c through the corresponding secondary connecting conductor 35, 36, 37 is 30 capable of creating an induced current proportional to the intensity of the current in the first winding 78. The first toroid 76 is for example a Rogowski coil. The first toroid 76 is preferably an opening toroid, so as to facilitate its arrangement around the corresponding conductors 35, 36, 37.

10 For each current sensor 58A, 58B, 58C, the circulation of the current 12a, 12b, 12c through the corresponding secondary connecting conductor 35, 36, 37 is capable of creating a signal proportional to the intensity of the current in the first winding 78. The primary current sensors 56A, 56B, 56C are identical to the secondary current 5 sensors 58A, 58B, 58C described in figure 2. A power supply member 90, shown in figure 2, is capable of supplying power to a communication unit 92 capable of shaping the measured secondary currents 12j and communicating them to the information processing unit 60 via a wireless antenna 93. The power supply member 90 includes, for each of the first, second and third secondary 10 connecting conductors 35, 36, 37, a second toroid 94 positioned around the corresponding secondary connecting conductor 35, 36, 37 and a second winding 96 arranged around the second toroid 94. The circulation of the current in the corresponding secondary connecting conductor 35, 36, 37 is capable of creating an induced current in the second winding 96. The power supply member 90 includes a converter 98 connected to each of the 15 second windings 96 and capable of delivering a predetermined voltage to the communication unit 92. Each second toroid 94 is an iron toroid. Each second toroid 94 is preferably an opening toroid so as to facilitate its arrangement around the corresponding conductors 35, 36, 37. In other words, the transmission of the measurements of the secondary current 12j is 20 self-powered by means of the power supply member 90 including the second toroids 94 adapted to recover the magnetic energy coming from the circulation of the current in the corresponding secondary connecting conductors 35, 36, 37. The information processing unit 60 is capable of communicating with the primary current sensors 56A, 56B, 56C, the secondary voltage sensors 59A, 59B, 59C, and the 25 secondary current sensors 58A, 58B, 58C via the communication unit 92 and the antenna 93. In the example embodiment of figure 2, the current measurements 12a, 12b, 12c are transmitted to the information processing unit 60 using a transceiver 95 included in the communication unit, and the wireless antenna 93 via a wireless data link 99. 30 The operation of the system for determining the primary voltage U1j of the transformer 18 will now be explained using figure 3. Figure 3 shows a flowchart of the steps of a method for determining the primary voltage U1j of the transformer 18 implemented by the determination system 22, in particular by the primary 56A, 56B, 56C and secondary 58A, 58B, 58C current sensors, the secondary voltage sensors 59A, 59B, 59C, and the 35 information processing unit 60.

11 In figure 3, during a first step 100, the secondary voltage sensors 59A, 59B, 59C measure the secondary voltages U2j, j for example being equal to a, b, c, of each of the first, second and third secondary connecting conductors 35, 36, 37 relative to the fourth secondary connecting conductor 38, i.e., the neutral conductor. These voltages U2a, U2b, 5 U2c are communicated to the information processing unit 60. During a second step 110, the primary current sensors 56A, 56B, 56C and the secondary current sensors 58A, 58B, 58C measure the primary currents 11a, 1lb, 1lc and secondary currents 12a, 12b, 2c on either side of the transformer 18 using the determination system 22 described in figure 2. During this step 110, the primary and secondary current 10 sensors 56A, 56B, 56C, 58A, 58B, 58C send the current measurements via the data link 99, such as a wireless link, to the information processing unit 60. The current measurements are received by the communication software 72 and taken into account by the acquisition software 73, so as next to be delivered to the calculation software 66, 68, 70. Once the primary current measurements 11a, 11b, 11c, secondary current 15 measurements 12a, 12b, 12c and secondary voltages U2a, U2b, U2c are centralized in the information processing unit 60, the latter may, using the calculation software 66, 68, 70, carry out the calculation steps making it possible to determine the corresponding primary voltages Ula, U1b, U1c of the transformer 18. In a first calculation step 120, the first calculation software 66 calculate the voltage 20 drop across the terminals of the transformer 18, on the side of the secondary winding 54. The calculation of this voltage drop is done from data of the resistive Ur and reactive Ux components of the short-circuit voltage measured in tests on the transformer 18, the load factor of the transformer 18 and the power factor of the load of the transformer 18. This voltage drop is calculated using equation [1]. It is expressed as a percentage of the no-load 25 rated voltage, the latter being indicated on a signal plate, not shown, associated with the transformer 18. The additional measurements of the load factor of the transformer, i.e., the measurement of the secondary current 12j, and those of the power factor cos(p of the load will also be taken into account by the information processing unit 60 to determine the voltage 30 drop for the considered load rating. After this step 120 and an intermediate step 125 for determining the position of the adjusting member Kpos, the second calculation software 68 calculates, during a second calculation step 130, the modulus |U1jl of the primary voltage Ulj from equation [2]. During the intermediate step 125, the position of the adjusting member Kpos is 35 determined as a function of the number of turns of the primary winding 52 taken into account.

12 In general, an adjusting member 20 comprises multiple adjusting positions, such as first P1, second P2 and third P3 positions when the electric transformer 18 is used in France. The second position P2 is the position where a rated voltage is obtained. Selecting the first position P1 makes it possible to take a higher number of turns into account, therefore to 5 establish a greater primary voltage U1j. Conversely, selecting the third position P3 causes a lower number of turns to be taken into account and therefore imposes a lower primary voltage U1j. When the electric transformer 18 is used in Switzerland, the adjusting member 20 comprises five or six adjusting positions. The value Kpos is equal to 1 for the second position P2 of the adjusting member 20, 10 i.e., to establish the rated value of the primary voltage. In the example of a primary voltage U1j greater than 2.5% of the rated voltage, the adjusting member is in the first position P1 and the value of Kpos is for example equal to 1.025. Kpos is for example equal to 0.975 for an adjusting member in the third position P3. The value of Kpos is for example determined using the knowledge of the primary 15 rated voltages corresponding to each position of the adjusting member, the secondary rated voltage, equation [3], and the measurement of the primary 11j and secondary 12j currents. Calculating the first term of equation [3] makes it possible to identify the position of the adjusting member Kpos verifying that the ratio between the primary U1j and secondary U2j voltages is equal to the first term of equation [3] resulting from the measurements of the 20 primary and secondary currents. Once the position of the adjusting member Kpos is known, the modulus of each primary voltage U1j is calculated during step 130 using the second calculation software 68. This module depends on the position of the adjusting member Kpos, the ratio of the no-load transformation of the primary and secondary rated voltages, the measurement of each 25 secondary voltage U2j, and the voltage drop calculated during step 120. Lastly, during a step 140, the phase shift of each primary voltage U1j is calculated with the corresponding primary current Ij1. To that end, the starting principle is used that the phase shift on the side of the primary winding 52 only significantly depends on the phase shift of the corresponding primary current 11j relative to the corresponding measured 30 secondary voltage U2j. In configuration D yn11 of the transformer 18, the relationship is given by equation [4]. The modulus and the phase of each primary voltage U1j are thus known. The determination system 22 according to the invention makes it possible to determine each primary voltage U1j in a usage context and to improve the existing grid.

13 Furthermore, it is possible to establish the determination system 22 without cutting the voltage, for example owing to the use of opening toroids as current sensors. It furthermore does not require physical access to the parts of the primary side of the transformer 18 that are powered on. 5 The placement of the determination system 22 does not require any adjustment or particular parameterization throughout its entire operating lifetime. In particular, the position of the adjusting member 20 of the transformer is automatically taken into account. None of the current and voltage sensors 56A,..., 59C require medium voltage insulation. 10 Furthermore, this determination system 22 makes it possible to calculate the position Kpos of the adjusting member 20 of the transformer 18 and to make that information remotely accessible and digitally available. Historically, the no-load chargers of distribution transformers are positioned during the installation of the substation according to the rules specific to each distributor, and the recording of their position in a database is only rarely 15 guaranteed. Currently, the only way to know the actual position of a wall charger consists of visiting the substation to perform a visual verification. This functionality allowing remote reading of the position Kpos provides a solution to the problem of a voltage swing of the grid inasmuch as it allows the operator to remotely assess the adjustment capacity at its disposal locally, in the location of the problem. 20 One skilled in the art will understand that the system and method previously described in case P=3, P being the number of phase conductors, applies for any value of P, and also P =1, i.e., in the case of a monophasic grid. One can see that the determination system 22 according to the invention is more precise and less expensive, while being easy to install, easy to use and compatible with the 25 existing grid.

Claims

1.- A method for determining the primary voltage (U1j) of an electric transformer (18), the electric transformer having a no-load transformation ratio and comprising a primary 5 winding (52) capable of having the primary voltage (U1j) between its ends and of being passed through by a primary current (11), a secondary winding (54) capable of having a secondary voltage (U2j) between its ends and of being passed through by a secondary current (12j), the method comprising the following steps: 10 - measuring the secondary voltage (U2j) using a voltage sensor (59A, 59B, 59C), - measuring the primary current (11j) using a first current sensor (56A, 56B, 56C), the method being characterized in that it further comprises the following steps: a)- calculating a secondary voltage drop between the ends of the secondary winding (54), as a function of specific characteristics of the transformer (18), of the load factor of the 15 transformer (18) and of the power factor of a load connected to the secondary winding (54) at the output of the transformer (18); b)- calculating the modulus (I U1jl) of the primary voltage as a function of the measured secondary voltage, of the voltage drop calculated in step a) and of the no-load transformation ratio; and 20 c)- calculating the phase shift of the primary voltage (U1j) relative to the measured primary current (11j) as a function of the phase shift of the measured secondary voltage (U2j) relative to the measured primary current (11j).

2.- The method according to claim 1, characterized in that during step a), the 25 secondary voltage drop is calculated using the following equation: AU =(Ur cosp+ Ux sinq()*n + - * (Ux cos( - Ur sing() * n 2 U 200 where Ur and Ux represent the resistive and reactive components specific to the transformer (18), n represents the load factor of the transformer (18), and cos (p represents the power factor of the load of the transformer (18). 30

3.- The method according to claim 1 or 2, characterized in that the electric transformer (18) further comprises an adjusting member (20) capable of modifying the no load transformation ratio of the transformer (18), and in that during step b), the modulus of the primary voltage (U1j) is calculated as a function of the measured secondary voltage 15 (U2j), of the voltage drop calculated in step a), of the no-load transformation ratio and of the position of the adjusting member (20).

4.- The method according to claim 3, characterized in that during step b), the modulus 5 (1 U1j ) of the primary voltage (U1j), is calculated using the following equation: Ulj = Kpos * Uo* U2j U2o 1 _AU U where Kpos is a coefficient that depends on the position of the adjusting member (20), U10 U2o is the no-load transformation ratio of the transformer (18), U2j is the secondary voltage, and AU is the secondary voltage drop calculated during step a). 10

5.- The method according to one of the preceding claims, characterized in that during step c), the phase shift ph(Ulj, l1j) between the primary voltage (U1j) and the primary current (11j) is calculated according to the following equation: ph(Ulj,llj) = -rr - ph(llj,U2j) 15 where ph(llj,U2j) is the phase shift measured between the primary current (11j) and the secondary voltage (U2j).

6.- The method according to one of the preceding claims, characterized in that it further comprises measuring the secondary current (12j) using a second current sensor (58A, 20 58B, 58C) and in that, during step b), the position of the adjusting member (20) of the transformer (18) is calculated as a function of the primary (11j) and secondary (12j) currents.

7.- The method according to claim 6, characterized in that the position of the adjusting member (Kpos) is determined using the knowledge of the primary rated voltages for each of 25 the positions of the adjusting member of the transformer, the secondary rated voltage, as well as using the following equation: 12j UlIj Ilj U2j where 11j and 12j are the measured primary and secondary currents, U1j and U2j are the primary and secondary voltages, the position of the adjusting member (20) is determined 30 by identifying the position (Kpos) making it possible to obtain the ratio between the primary 16 and secondary rated voltages, corresponding to the second term of the preceding equation, closest to the ratio between the measured secondary (12j) and primary (11) currents.

8.- A system (22) for determining the primary voltage (U1j) of an electric transformer 5 (18), the electric transformer (18) having a no-load transformation ratio and comprising a primary winding (52) capable of having the primary voltage (U1j) between its ends and of being passed through by a primary current (11j), a secondary winding capable of having a secondary voltage (U2j) between its ends and of being passed through by a secondary current (12j), 10 the system comprising: a voltage sensor (59A, 59B, 59C) capable of measuring the secondary voltage (U2j), and a first current voltage (56A, 56B, 56C) capable of measuring the primary current (11j), the system being characterized in that it further comprises: 15 first means for calculating the secondary voltage drop between the ends of the secondary winding (54), based on specific characteristics of the transformer (18), on the load factor of the transformer and on the power factor of a load connected to the secondary winding (54) at the output of the transformer (18); second means for calculating the modulus (I U1j ) of the primary voltage (U1j) as a 20 function of the measured secondary voltage (U2j), the calculated voltage drop and the no load transformation ratio; and third means for calculating the phase shift of the primary voltage (U1j) relative to the measured primary current (11j) as a function of the phase shift of the measured secondary voltage (U2j) relative to the measured primary current (11j). 25

9.- The determination system according to claim 8, characterized in that each current sensor (56A, 56B, 56C, 58A, 56B, 56C) includes an opening toroid.

10.- The determination system according to claim 8 or 9, characterized in that each 30 current sensor (56A, 56B, 56C, 58A, 56B, 56C) is connected to the third calculating means via wireless communication means.

11.- A substation (18) for transforming a first electrical current (11j) having a first alternating voltage (U1j) into a second electrical current (12j) having a second alternating 35 voltage (U2j), comprising: 17 a first panel (14) including at least one incoming electrical conductor (26, 28, 30) adapted to be connected to the electrical grid (12), the incoming conductor having a primary voltage (U1j), a second panel (16) including at least one outgoing electrical conductor (42, 44, 46), 5 the outgoing conductor having a secondary voltage (U2j), a transformer (18) comprising a primary winding (52) linked to the first panel (14), and a secondary winding (54) linked to the second panel (16), and a determination system for determining the primary voltage (U1j) of the transformer (18), 10 characterized in that the determination system is according to any one of claims 8 to 10.