Method for calculating the electric field in a dielectric material layer of a high-voltage direct current electrical cable and system therefor

Description

[0001] The present invention relates to the sector of the transmission of electrical energy in direct electric current and in particular it concerns a method for calculating the electric field within a dielectric material layer of a high voltage electric cable for direct electric current.

[0002] More specifically, said method is designed to automatically and substantially calculate in real time the electric field distribution over time within a dielectric material layer included in the electric cable itself.

[0003] The present invention also relates to a system for calculating said electric field by means of said method.Prior art

[0004] An electric cable is an electrical component that includes from the inside to the outside in succession: a conductor made of metallic material, at least a first layer of semiconductive material, arranged around the conductor, a dielectric material layer, arranged around said first semiconductive material layer, at least a second semiconductive material layer, arranged around said dielectric material layer, at least one metallic material layer called screen.

[0005] Further layers may be present in the electrical cable depending on the type of application of the electrical cable.

[0006] In general, the main function of the dielectric material layer is to withstand the electrical stress to which it is subjected during the operation of the electrical cable, ensuring that the conductor and screen are electrically insulated from each other.

[0007] The thickness and type of dielectric material are selected and sized on the basis of the electrical stress and also the thermal and mechanical stress to be withstand, to prevent the properties of said dielectric material from being degraded.

[0008] Currently, a method is known to monitor in real time the maximum temperature inside the conductor and the maximum temperature on the external surface of a high voltage electric cable for direct electric current by measuring the electric current flowing in the conductor in the axial direction. (i.e. along the longitudinal axis of the electric cable) with the aim of preventing the temperature associated with the conductor from being greater than a predetermined temperature value or temperature threshold value.

[0009] Furthermore, this method allows the following values of electric current to be calculated in real time: i) a first maximum value associated with an admissible permanent electric current which can flow continuously for an indefinite time in a conductor under the conditions of use without exceeding a temperature threshold under nominal conditions; ii) at least a second value associated with an admissible permanent electric current which can flow continuously for a predetermined time in a conductor in the conditions of use without exceeding a further temperature threshold in emergency conditions.

[0010] This method is the same method used to monitor high voltage electrical cables for alternating current.

[0011] However, the use of a method designed to monitor high voltage electrical cables for alternating current in order to monitor high voltage electrical cables for direct current has some disadvantages.

[0012] A disadvantage is that a method designed to monitor high voltage electric cables for alternating current does not allow real time monitoring of the electric field inside the dielectric material layer of a high voltage electric cable for direct current.

[0013] Therefore, monitoring a high voltage electric cable for direct electric current using the same method with which a high voltage electric cable for alternating current is monitored does not allow to evaluate the risks deriving from the accumulation of spatial electric charge inside the dielectric material layer.

[0014] If on the one hand these phenomena are not relevant in a high voltage electric cable for alternating electric current, on the other hand, in a high voltage electric cable for direct electric current, said phenomena cause premature aging and a loss of dielectric properties of the dielectric material layer of the electric cable.

[0015] In fact, the electrical stress is conditioned not only by the voltage applied between the conductor and the screen, but also by accumulation phenomena of spatial electric charge that occur over time, when the electric cable is in use, in which said phenomena are related to the electric field variations over time.

[0016] Therefore, if the time-varying electric field is not monitored and consequently not known, it is not possible to evaluate the entity of said phenomena and the dielectric material layer of the electric cable can be subjected over time to the risks mentioned above (i.e. aging and loss of dielectric properties).

[0017] Relevant prior art is disclosed in CN 110 427 637 A which discloses a simulation method for the space charge distribution of DC cables taking into account the influence of temperature and electric field gradient.Aim of the invention

[0018] The aim of the present invention is to overcome said disadvantage by providing a method for calculating automatically and substantially in real time the electric field within a dielectric material layer of a high voltage electric cable for direct electric current.

[0019] Advantageously, by means of said method, the possibilities that said dielectric material layer is subject to premature aging and / or a loss of its dielectric properties and / or runs the risk of failure are limited.

[0020] A further object of the present invention is to provide a system for carrying out this method.Object of the invention

[0021] Object of the invention is a method for calculating the electric field within a dielectric material layer of a high voltage electric cable for direct electric current according to claim 1.

[0022] Further preferred embodiments are described in the method dependent claims.

[0023] A further object of the invention is a system for calculating the electric field within a dielectric material layer of a high voltage electric cable for direct electric current according to claim 15.

[0024] Further preferred embodiments are described in the system dependent claims.Figure list

[0025] The present invention will be now described, for illustrative, but not limitative purposes, according to its embodiment, making particular reference to the enclosed figures, wherein: Figure 1 is a schematic view of an electric cable comprising from inside to outside a conductor, a first semiconductive material layer, a dielectric material layer, a second semiconductive material layer and a metallic material layer called shield and an outer layer which is a sheath, in which a plurality of planes transverse to the longitudinal axis of the electric cable, temperature measuring means, an electric current measuring device, a voltage measuring device between said conductor and said shield, and a computer, connected to said temperature measuring means, to said electric current measuring device and to said voltage measuring device, are visible; Figures 2A, 2B, 2C show a respective computing domain associated with a respective area that includes a respective section of the electric cable, in which said area is arranged on a respective transverse plane to the longitudinal axis of the electric cable; Figures 3A, 3B, 3C show a respective further computing domain associated with a respective further area, in which said further area is arranged inside said electric cable, between the first semiconductive material layer and the second semiconductive material layer of the electric cable (in Figures 3A, 3B 3C, the conductor, the shield and the outer layer of the electric cable are not shown); Figure 4 is a schematic view of a grid of the computation domain of Figure 2A comprising a plurality of finite elements to be used for the computation of a temperature distribution by means of a finite element method; Figure 5 shows an enlargement of a detail of Figure 4 to show some finite elements of said computing domain, arranged along a radial direction of the electric cable; Figure 6 shows an enlargement of figure 5 of a detail of Figure 4 to show some finite elements belonging to the further calculation domain, arranged along a radial direction of the electric cable, to be used for the calculation of an electric field distribution by means of a further finite element method. Detailed description of the invention

[0026] With particular reference to Figure 1, a method for calculating an electric field inside a dielectric material layer 13 of a high voltage electric cable 1 for direct electric current.

[0027] In particular, said electric cable 1 comprises a plurality of layers and more particularly from the inside towards the outside: a conductor 11 having a longitudinal axis A, a first semiconductive material layer 12, said dielectric material layer 13, a second semiconductive material layer 14, a metallic material layer 15 called shield,

[0028] Moreover, said electric cable 1 further comprises an outer layer 16 which is a sheath.

[0029] In general, additional layers may be present inside the electric cable 1 according to the type of application of the electric cable itself.

[0030] For example, the electric cable can comprise further first semiconductive material layers and / or further second semiconductive material layers and / or further external layers.

[0031] However, in the embodiment being described, said electric cable 1 is constituted by said conductor 11, by said first semiconductive material layer 12, by said dielectric material layer 13, by said second semiconductive material layer 14, by said metallic material layer 15 called screen and from said outer layer 16.

[0032] Said method comprises the following steps: A) identifying one or more areas S j , with j=1,2...N where N is a positive integer, each of which is arranged along said electric cable 1, on a respective plane P j transverse to the longitudinal axis A of said electric cable 1 and has a geometric shape defined by a closed reference line; B) associating a respective computational domain DT J with each area S j , as well as associating a respective further computational domain DE J with a further area which is a portion of said area S j and is arranged inside said electric cable 1, between said first semiconductive material layer 12 and said second semiconductive material layer 14, wherein said computational domain DT J is discretized through a set of finite elements e J1 ,e J2 ...e JM , wherein J indicates the area and M is a positive integer and said further computational domain DE J is discretized through a further set of finite elements e JX ,e JY ...e JL , wherein J indicates the area and X, Y and L are positive integers used to indicate any finite element, and wherein said further area is included between a first closed line 120 and a second closed line 140 and in transversal section said first closed line 120 is a first contact line between the conductor 11 and said first semiconductive material layer 12 and said second closed line 140 is a second contact line between said second semiconductive material layer 14 and said shield 15; C) acquiring: a value of temperature TE J at each area S j through temperature measuring means TS J ; a value of voltage V between said conductor 11 and said shield 15, wherein said value of voltage V is obtained through a voltage measuring device 18 or is a predetermined value of voltage; a value of electric current I flowing along the longitudinal axis A of the conductor 11, wherein said value of electric current I is obtained through an electric current measuring device 19 or is a predetermined value of electric current; D) calculating, by means of a numerical finite element method, a respective temperature distribution Te J1 ,Te J2 ...Te JM for each computational domain DT J , wherein each value of temperature of a temperature distribution is associated with a respective finite element e J1 ,e J2 ...e JM of a respective computational domain DT J , each temperature distribution being calculated by solving: o Fourier's law for conducting heat in transient conditions, starting from said temperature values TE J , from said measured electric current values I, and from a predetermined temperature distribution T 0 ; E) calculating, by means of a further numerical finite element method, a respective electric field distribution Ee JX , Ee JY ...E JL for each further computational domain DE J , wherein each value of electric field of an electric field distribution is associated with a respective finite element e JX ,e JY ...e JL of said further computational domain DE J , each electric field distribution being calculated by solving: o Gauss's law for an electric field E in transient conditions, applied to each finite element e JY falling into the dielectric material layer 13, o the law of continuity of electric current in stationary conditions applied to each finite element e JL which falls in the first semiconductive material layer 12 or in the second semiconductive material layer 14 and which does not contact one or more finite elements e JL which fall into the dielectric material layer 13, ∘ Schottky's law referred to an injection of electric charges in correspondence of a contact line between different materials, applied to each finite element e JX which falls into the first semiconductive material layer 12 or in the second semiconductive material layer 14 and contacts one or more finite elements e JY which fall into the dielectric material layer 13 (the expression "contact line" or "interface" means the portion of a contact surface between different materials on the section of the electric cable 1 inside the respective area S j ), o the relationship between electric field E and said voltage V, starting from boundary conditions applied to said first closed line 120 and to said second closed line 140 of the respective further computational domain D EJ , in which said boundary conditions depend on said voltage value V and on a predetermined electric charge density distribution ρ 0 .

[0033] With particular reference to step A, one or more areas S j are identified along said electric cable 1.

[0034] Each area S j is arranged on a respective plane P j which is transverse to the longitudinal axis A of the electric cable 1 and has a geometric shape delimited by a closed reference line.

[0035] In the embodiment being disclosed, as shown in Figure 1, a plurality of areas S j are identified along said electric cable 1 and each area S j is external to electric cable 1 and arranged around the electric cable itself, on a respective plane P j .

[0036] In particular, Figure 1 shows a plurality of areas S 1 ,S 2 ,...S N , each of which is arranged on a respective plane P 1 ,P 2 ,...P N .

[0037] However, each area S j can be within said electric cable 1, without departing from the scope of the invention.

[0038] The step A can be performed by a processing unit (which can comprise or be constituted of a microcontroller), not shown.

[0039] In the embodiment being disclosed, said processing unit is arranged inside a computer C.

[0040] With particular reference to step B, reference is made to two computational domains: a respective computational domain DT J associated with to each area S j , and a respective further computational domain DE J associated to a further area which is a portion of said area S j and is arranged inside said electric cable 1, between said first semiconductive material layer 12 and said second semiconductive material layer 14.

[0041] Figures 2A, 2B and 2C show a respective computational domain DT 1, DT 2,... DT N associated with a respective area S 1 ,S 2 ,...S N .

[0042] Figures 3A, 3B and 3C show a respective further computational domain DE 1, DE 2,... DE N associated with a respective further area which is a portion of a respective area S 1 ,S 2 ,...S N .

[0043] Each further computational domain DE 1, DE 2,... DE N is included in a respective computational domain DT 1, DT 2,... DT N .

[0044] As said, said further area is included between a first closed line 120, which is a first contact line 120 between the conductor 11 and said first semiconductive material layer 12, and a second closed line 140 which is a second contact line between said second semiconductive material layer 14 and said shield 15.

[0045] Figures 3A, 3B and 3C show for each further computational domain DE 1, DE 2,... DE N the respective first closed line 120 and the second closed line 140.

[0046] In particular, each computational domain DT J is discretized through a respective set of finite elements e J1 ,e J2 ...e JM and each set of finite elements forms a respective grid GT J .

[0047] Furthermore, the finite elements e J1 ,e J2 ...e JM are arranged in such a way as to divide the computational domain DT J in a plurality of slices, each of which is arranged along a predetermined radial direction of the section of the electric cable 1 in said respective area S j .

[0048] Figure 4 shows a grid GT J for a general computational domain DT J associated with a general area S J .

[0049] Figure 5 shows an enlargement of a detail of said grid GT J .

[0050] Furthermore, each further computational domain DE J is discretized through a respective further set of finite elements and each further set of finite elements forms a respective further grid GE J .

[0051] Figure 6 shows an enlargement of a detail of said further grid GE J .

[0052] In the embodiment being disclosed, each further set of finite elements is a sub-set of finite elements e JX ,e JY ...e JL of a respective set of finite elements e J1 ,e J2 ...e JM .

[0053] Therefore, the further grid GE J is a portion of the grid GT J .

[0054] Step B can be performed by said processing unit.

[0055] With particular reference to step C, in the embodiment being disclosed, a respective value of temperature TE J at each area S j is measured through said temperature measuring means TS J , a value of voltage V between said conductor 11 and said shield 15 is measured through said voltage measuring device 18, and a value of electrical current I flowing along the longitudinal axis A of the conductor 11 is measured through said electric current measuring device 19.

[0056] Said temperature measuring means TS J comprise a plurality of temperature sensors for measuring a temperature at a respective area S j .

[0057] In the embodiment being disclosed, said temperature sensors are indicated with the reference TS 1 ,TS 2 ,...TS N and are arranged on a respective portion of said outer layer 16 of said electric cable 1.

[0058] However, said temperature sensors can be arranged inside said electrical cable 1 or at a predetermined distance from said outer layer 16, without departing from the scope of the invention.

[0059] In an alternative, not shown in Figures, said temperature measuring means can be comprise a distributed temperature sensor with a longitudinal resolution to measure a temperature at each area S j .

[0060] Said distributed temperature sensor can be arranged inside said electric cable 1 or on said outer layer 16 of said electric cable 1 or at a predetermined distance from said outer layer 16.

[0061] With reference to the voltage measuring device 18, in the embodiment being disclosed, said voltage measuring means is a voltmeter.

[0062] With reference to the electric current measuring device 19, in the embodiment being disclosed, said electric current measuring device 19 is an ammeter

[0063] In an alternative, the value of voltage can be a predetermined voltage value (i.e. a constant value) which is not measured by any voltage measuring device, but is acquired by a processing unit, described below, configured to acquire said predetermined value that can be established by a user.

[0064] Also the value of electric current value can be a predetermined electric current value which is not measured by any electric current measuring device, but is acquired by said processing unit configured to acquire said predetermined electric current value which can be established by a user.

[0065] However, the measurement of the voltage value and the electric current value is advantageous to correctly calculate the electric field when there are variations in voltage or variations in electric current.

[0066] With reference to step D, a respective temperature distribution Te J1 ,Te J2 ...Te JM for each computational domain DT J is calculated by means of a numerical finite element method.

[0067] In particular, the distribution temperature Te J1 ,Te J2 ...Te JM was calculated assuming that the temperature values vary along a radial direction of the electric cable 1 and along a tangential direction.

[0068] In this way, it is possible to take into account the environment in which said electric cable 1 is installed, for example underground, in which the heat exchange between the electric cable 1 and the surrounding environment is different for different portions of each section of the electric cable 1 within a respective area S j .

[0069] The calculation of each temperature distribution is performed by said processing unit.

[0070] Said processing unit is connected to each of said temperature TS 1 ,TS 2 ,...TS N and configured to acquire a respective temperature value TE 1 ,TE 2 ,...TE N measured by each said temperature sensors.

[0071] Furthermore, said processing unit is connected to said electric current measuring device 19 and configured for acquiring an electric current value I from said electric current measuring device 19.

[0072] Each temperature value of a respective temperature distribution Te J1 ,Te J2 ...Te JM is associated with a respective finite element e J1 ,e J2 ...e JM of a respective computational domain DT J .

[0073] Each temperature distribution is calculated by solving the Fourier's law for conducting heat in transient conditions, starting from said temperature values TE J (measured through said temperature measuring means TS J ), said measured electric current value I, a predetermined temperature distribution T 0 .

[0074] Said predetermined temperature distribution T 0 can be stored by the processing unit in storage means (such as a memory), not shown, and said processing unit can be connected to said storage means.

[0075] In the embodiment being disclosed, said processing unit comprises said storage means.

[0076] In particular, the Fourier's law is expressed in discrete form and is solved with a spatial discretization which is represented by the finite elements of the computational domain DT J , mentioned above, and a temporal discretization.

[0077] This temporal discretization has a predetermined time step Δt.

[0078] This predetermined time step Δt can be of the order of seconds.

[0079] In the embodiment being disclosed, said predetermined time step Δt is constant and equal to 10s.

[0080] Furthermore, in an initial time instant t 0 , when a quantity of direct electric current begins to flow along the longitudinal axis A of the conductor 11, it is assumed that the predetermined temperature distribution T 0 is constant and equal to the ambient temperature.

[0081] With particular reference to step E, a respective electric field distribution Ee JX ,Ee JY ...E JL for each further computational domain DE J is calculated by means a further numerical finite element method.

[0082] Each electric field value of an electric field distribution is associated with a respective finite element e JX ,e JY ...e JL of said further computational domain DE J .

[0083] Each electric field distribution is calculated by solving: o Gauss's law for an electric field E in transient conditions, applied to each finite element e JY falling into the dielectric material layer 13, o the law of continuity of electric current in stationary conditions applied to each finite element e JL which falls in the first semiconductive material layer 12 or in the second semiconductive material layer 14 and which does not contact one or more finite elements e JL which fall into the dielectric material layer 13, o Schottky's law referred to an injection of electric charges in correspondence of a contact line between different materials, applied to each finite element e JX which falls into the first semiconductive material layer 12 or in the second semiconductive material layer 14 and contacts one or more finite elements e JY which fall into the dielectric material layer 13, o the relationship between electric field E and said voltage V.

[0084] The boundary conditions are applied to said first closed line 120 and to said second closed line 140 of the respective further computational domain D EJ and depend on said voltage value V measured between said conductor 11 and said shield 15 and on a predetermined electric charge density distribution ρ 0 .

[0085] In particular, in said initial time instant t 0 , when a quantity of direct electric current begins to flow along the longitudinal axis A of the conductor 11, it is assumed that the predetermined electric charge density distribution ρ 0 is constant and equal to zero.

[0086] Gauss's law for an electric field E in transient conditions is expressed in discrete form and is solved with a spatial discretization that is represented by the finite elements of the further computation domain DE J , mentioned above, and a temporal discretization.

[0087] Said predetermined time step Δt was also used for this time discretization.

[0088] In particular, this temporal discretization can be the same as that used to solve said Fourier's law in transitory conditions.

[0089] The law of continuity of electric current in stationary conditions is expressed in discrete form and is solved with a spatial discretization that is represented by the finite elements of the further computation domain DE J , mentioned above.

[0090] Schottky's law referred to an injection of electric charges depends on the temperature value, the electric field value, the materials and factors relating to the contact lines or interfaces between materials.

[0091] Since the temperature and the electric field associated with the elements belonging to the first semiconductive material layer 12 or to the second semiconductive material layer 14 vary over time and are calculated in real time using the aforementioned method, the injection of electrical charges also varies over time.

[0092] Schottky's law can be expressed by the following formula: J Schottky = A el T 2 ⋅ exp − q ⋅ φ el k b ⋅ T ⋅ exp − q k b ⋅ T − q ⋅ E m 4 πε + A lac T 2 ⋅ exp − q ⋅ φ lac k b ⋅ T ⋅ exp − q k b ⋅ T − q ⋅ E m 4 πε where: A el e A lac are respective constants, each of which depends on the materials and the injected electrical charges, in particular: A el is a first constant dependent on the materials and on the fact that the injected electric charges are electrons, i.e. negative electrical charges; A lac è una costante dipendente dai materiali e dal fatto che le cariche elettriche iniettate sono lacune, i.e. cariche elettriche positive; is a constant dependent on the materials and on the fact that the injected electrical charges are electron holes, i.e. positive electric charges; T is the temperature of the element in which Schottky's law is applied; q is the elementary electric charge (which is a known constant); φ el is the potential barrier for the injection of electrons; φ lac is the potential barrier for the injection of electron holes; k b is Boltzman's constant (which is a known constant); ε is the permittivity of the material of the element in which Schottky's law is applied; E m èis the electric field of the element where Schottky's law is applied.

[0093] The parameters φ el and φ lac are parameters that depend on the injected electrical charges (i.e. if the injected electrical charges are electrons or electron holes), on the materials and on one or more factors concerning one or more contact lines or interfaces between different materials, such as material roughness, protrusions of materials, presence of impurities, presence of gaps between materials due to a non-perfect adhesion between the materials (for example due to a detachment of the materials), etc..

[0094] The transient conditions for Gauss's law for an electric field E and the stationary conditions for the law of continuity of electric current allow to calculate the electric field distribution Ee JX ,Ee JY ...E JL with significantly reduced calculation times, substantially in real time, compared to known finite element methods used for the calculation of the electric field.

[0095] Furthermore, the electric field distribution Ee JX ,Ee JY ...E JL can be calculated assuming that the electric field values vary along a radial direction of the electric cable 1 and along a tangential direction.

[0096] Alternatively, the electric field distribution Ee JX ,Ee JY ...E JL can be calculated can be calculated only along a radial direction, assuming that the electric field values vary only along the radial direction of the electric cable 1 and not along the tangential direction, as explained below.

[0097] The calculation of each electric field distribution Ee JX ,Ee JY ...E JL is performed by said processing unit.

[0098] Said processing unit is connected to said voltage measuring device 18 and configured to acquire a voltage value V from said voltage measuring device 18.

[0099] Said predetermined electric charge density distribution ρ 0 can be stored by said processing unit in said storage means.

[0100] The method may further comprise the following steps which are performed for each area S j periodically at predetermined time intervals Δt (in which each predetermined time interval coincides with the predetermined time step mentioned above): F) calculating an electrical conductivity distribution σe JX ,σe JY ...σe JL for each finite element e JX ,e JY ...e JL of each further computational domain DE J , starting from said calculated temperature distribution Te J1 ,Te J2 ...Te JM and from said calculated electric field distribution Ee JX ,Ee JY ...E JL ; G) calculating an electric current density distribution Je JX ,Je JY ...J JL for each finite element e JX ,e JY ...e JL of each further computational domain DE J through the Ohm's law; H) calculating a respective value of electric charge density ρe JX ,ρe JY ...ρe JL for each finite element e JX ,e JY ...e JL of each further computational domain DE J by solving: o the law of continuity of electric current in transient conditions, wherein said law of continuity of electric is applied to each finite element e JY of each further computational domain D EJ which falls within the dielectric material layer 13, and o Gauss's law for an electric field E in stationary conditions, wherein said Gauss's law is applied to each finite element e JX , e JL of each further computational domain DE J which falls in the first semiconductive material layer 12 or in the second semiconductive material layer 14; I) updating said predetermined temperature distribution T 0 with the temperature values of said calculated temperature distribution Te J1 ,Te J2 ...Te JM and updating said predetermined electric charge density distribution ρ 0 with the calculated electric charge density values ρe JX ,ρe JY ...ρe JL ; J) repeating the steps from C to E.

[0101] With reference to the time interval Δt, said time interval is greater than of the computational time for solving calculations menzioned in steps from C to I.

[0102] Preferably, said time interval Δt is three orders of magnitude greater than said computational time.

[0103] Furthermore, once the computational time is known to calculate the temperature distribution Te J1 ,Te J2 ...Te JM , the electric field distribution Ee JX ,Ee JY ...E JL , the electrical conductivity distribution σe JX ,σe JY ...σe JL , the electrical current density distribution Je JX ,Je JY ...Je JL , and the electric charge density ρe JX ,ρe JY ...ρe JL , at a given time instant, steps F to J can be performed after a further time interval Δt' equal to the difference between said time interval Δt and said computational time, so as to obtain a result in real time every time interval Δt.

[0104] With particular reference to step F, an electrical conductivity distribution σe JX ,σe JY ...σe JL was calculated by the processing unit of computer C for each further computational domain DE J , starting from the calculated temperature distribution Te J1 ,Te J2 ...Te JM and the calculated electric field distribution Ee JX ,Ee JY ...Ee JL .

[0105] With particular reference to step G, an electrical current density distribution Je JX ,Je JY ...Je JL was calculated by the processing unit of computer C for each further computational domain DE J using Ohm's law.

[0106] In particular, the electrical conductivity distribution σe JX ,σe JY ...σe JL , the electrical current density distribution Je JX ,Je JY ...Je JL , and the electric charge density ρe JX ,ρe JY ...ρe JL can be calculated assuming that the values vary along a radial direction of the electric cable 1 and along a tangential direction.

[0107] Alternatively, when said electric field distribution Ee JX ,Ee JY ...E JL is calculated only along said radial direction, also the electric conductivity distribution σe JX ,σe JY ...σe JL , the electric current density distribution Je JX ,Je JY ...Je JL , and the electric charge density ρe JX ,ρe JY ...ρe JL can only be calculated along said radial direction, assuming that they do not vary along the tangential directions. In this way, the computation time is further reduced.

[0108] With particular reference to step H, by means of the processing unit of computer C, a respective electric charge density value ρe JX ,ρe JY ...ρe JL has been calculated for each finite element e JX ,e JY ...e JL of each further computational domain DE J , solving the law of continuity of electric current in transient conditions and Gauss's law for an electric field E in stationary conditions.

[0109] The transient conditions for the law of continuity of electric current and the stationary conditions for Gauss's law for an electric field E allow to calculate the electric charge density ρe JX ,ρe JY ...ρe JL with significantly reduced computation times, substantially in real time, compared to known finite element methods used for the calculation of the electric charge density.

[0110] The law of continuity of electric current in transient conditions and Gauss's law for an electric field E in stationary conditions are expressed in discrete form.

[0111] With particular reference to step I, the predetermined temperature distribution T 0 is updated with the temperature values of said calculated temperature distribution Te J1 ,Te J2 ...Te JM and the predetermined electric charge density distribution ρ 0 is updated with the calculated electric charge density ρe JX ,ρe JY ...ρe JL .

[0112] The updated values of the predetermined temperature distribution T 0 and the updated values of the predetermined electric charge density distribution ρ 0 can be stored in said storage means.

[0113] In light of the above, for each time instant t k with k = 1.2 ... Z where Z is a positive integer, excluding the initial time instant t 0 mentioned above, the values of the predetermined temperature distribution T 0 and the values of the predetermined electric charge density distribution ρ 0 are equal respectively to the temperature values Te J1 ,Te J2 ...Te JM and to the electric charge density values ρe JX ,ρe JY ...ρe JL calculated in the previous time instant of time t k-1 (t k - t k-1 = Δt).

[0114] With particular to step J, the steps C, D and E are repeated at periodic time intervals Δt.

[0115] The method steps described above can be used in addition to other steps to monitor the electrical stress to which the electrical cable 1 is subjected with respect to an electrical stress for which the electrical cable was designed or a predetermined electrical stress.

[0116] To this end, said method can further comprise the following steps in addition to the steps A to E or steps A to J: K) identifying a respective maximum value of electric field E MAXJ in each further computational domain DE J ; L) comparing each maximum value of electric field E MAXJ with a predetermined value of electric field E REF ; M) if said maximum value of electric field E MAXJ is greater than said predetermined value of electric field E REF , establishing that at least one section of electrical cable 1 at each said area S j is subjected to an electrical stress greater than an electrical stress for which said electrical cable 1 was designed or to a predetermined electrical stress.

[0117] In other words, after having identified for each further computational domain DE J a respective maximum value of electric field E MAXJ , each value of electric field E MAXJ is compared with a predetermined value of electric field E REF which is a threshold value and if one or more maximum values of electric field E MAXJ are greater than said predetermined value of electric field E REF , it means that the section of the electric cable 1 at the respective area S j is subjected to an electric stress higher than an electric stress for which said electric cable 1 has been designed or to a predetermined electrical stress.

[0118] With reference to the finite element method, said finite element method mentioned in step D can solve a respective system of linear equations for each computational domain DT J : A 1 j * T j = B 1 j dove A1 j is a first square matrix with a number of rows and a number of columns equal to the number of finite elements e J1 ,e J2 ...e JM of said computational domain DT J , and each element of said matrix is a constant numeric coefficient depending on the geometric properties of said computational domain DT J and from the thermophysical properties of at least one material associated with a finite element corresponding to a portion of said electric cable 1, T j is a first column vector with a number of elements equal to the number of finite elements e J1 ,e J2 ...e JM of said computational domain DT J and each element of said first column vector is a value of temperature which is unknown, B1 j is a further first column vector with a number of elements equal to the number of finite elements e J1 ,e J2 ...e JM of said computational domain DT J and each element is a numeric value variable over time and depending on the boundary conditions of heat exchange between electric cable 1 and external environment, on the temperature of said predetermined temperature distribution T 0 associated with each finite element e J1 ,e J2 ...e JM of said computational domain DT J , on a thermal power value calculated on the basis of the measured electric current values I flowing in the conductor 11 and the thermophysical properties of at least one material associated with a finite element corresponding to said portion of said electric cable 1.

[0119] Furthermore, step D can comprise the following sub-steps: D1) calculating a respective numeric value associated with each element of each first vector B1 j on the basis of a value of temperature Te J1 ,Te J2 ...Te JM associated with each finite element e J1 ,e J2 ...e JM of said computational domain DT J and with the values of thermal power due to the passage of electric current I in the conductor 11 and losses in the dielectric material layer 13; D2) multiplying said first vector B1 j by a further first matrix A1 j ', which is the inverse matrix of said first matrix A1, to obtain a respective value of temperature Te J1 ,Te J2 ...Te JM associated with each finite element e J1 ,e J2 ...e JM of said computational domain DT J ; D3) for each computational domain DT J calculating a difference between a respective value of temperature TE J measured at each area S j and a respective value of temperature between the calculated values of temperature Te J1 ,Te J2 ...Te JM , wherein said value of temperature is associated with a respective finite element between the finite elements e J1 ,e J2 ...e JM of said computational domain DT J at which said temperature value has been measured, to obtain a respective temperature variation value ΔT J ; D4) for each finite element e J1 ,e J2 ...e JM of said computational domain DT J calculating a respective updated value of temperature Te J1 ,Te J2 ...Te JM by adding a respective value of temperature obtained at sub-step D2 and a respective value of temperature variation ΔT J obtained at sub-step D3.

[0120] Furthermore, in addition to or alternatively to the sub-steps from D1 to D4, said step D can comprise the following sub-steps: D5) calculating a temperature gradient of said temperature distribution Te J1 ,Te J2 ...Te JM inside a respective section of said electric cable 1 in each area S j along a plurality of radial directions, wherein each value of temperature gradient is associated with a respective finite element e J1 ,e J2 ...e JM of a respective computational domain DT J , wherein said finite element is arranged inside said electric cable 1; and D6) identifying a radial direction of said plurality of radial directions associated with a maximum value of said calculated temperature gradient.

[0121] The partition of the computational domain DT J into segments along said predetermined radial directions advantageously allows to calculate said temperature gradient along all the radial directions associated with each segment in a simple manner, without the need for any interpolations.

[0122] The radial direction associated with a calculated maximum temperature gradient value can be advantageously used to speed up the calculation of the electric field Ee JX ,Ee JY ...E JL in step E, as already mentioned.

[0123] In fact, the electric field distribution Ee JX ,Ee JY ...E JL can only be calculated along said radial direction identified in sub-step D6.

[0124] In this way, it is possible to reduce the calculation times, since the electric field Ee JX ,Ee JY ...E JL is calculated only along the radial direction subject to the maximum temperature gradient used as an index to identify the worst situation.

[0125] Furthermore, also the electrical conductivity distribution σe JX ,σe JY ...σe JL , the electrical current density distribution Je JX ,Je JY ...Je JL , and the electric charge density ρe JX ,ρe JY ...ρe JL can only be calculated along said radial direction identified in sub-step D6 .

[0126] This further reduces the computation time for calculating the electric field.

[0127] With reference to the further numerical finite element method mentioned in step E, said further numerical finite element method at step E can solve a respective linear equation system for each further computational domain DE J A 2 j * E j = B 2 j where A2 j is a second square matrix with a number of rows and a number of columns equal to a number of finite elements of said further computational domain DE J and each element of said matrix is a constant numeric coefficient depending on the geometric properties of said further computational domain DE J and on the dielectric properties of at least one of a material associated with said portion of said electric cable 1 which falls inside said further computational domain DE J , E j is a second column vector with a number of elements equal to the number of finite elements e JX ,e JY ...e JL of said further computational domain DE J and each element is a numeric value of electric field Ee JX ,Ee JY ...E JL associated with each finite element e JX ,e JY ...e JL of said further computational domain DE J , B2 j is a further second column vector with a number of elements equal to the number of finite elements e JX ,e JY ...e JL of said further computational domain DE J , wherein each element is a numeric value depending on the boundary conditions of electric field E, on said value of voltage V and on said predetermined electric charge density distribution ρ 0 associated with each finite element e JX ,e JY ...e JL of said further computational domain DE J .

[0128] Furthermore, the step E can comprise the following sub-steps: E1) calculating a respective numerical value associated with each element of each second vector B2 j on the basis of an electric charge density value ρ 0 associated with each finite element e JX ,e JY ...e JL of said further computational domain DE J and said measured voltage value V; E2) multiply said second vector B2 j by a further second matrix A2 j ' which is the inverse matrix of said second matrix A2 j to obtain a respective electric field value Ee JX ,Ee JY ...E JL associated with each finite element e JX ,e JY ...e JL of said further computational domain DE J .

[0129] Said method further comprises the following steps for taking into account a thermal conduction along the longitudinal axis A of the conductor 11: N1) calculating for each area S j the mean value of temperature T cj in the conductor 11; N2) calculating: a first axial temperature gradient GR1 J as a difference between the mean value of temperature T cj in the conductor 11 at an area S j and the mean value of temperature T cj-1 in the conductor 11 at a previous area S j-1 by dividing for a value equal to a first distance d 1 between said area S j and said previous area S j-1 , and a second axial temperature gradient GR2 J as a difference between the mean value of temperature T cj+1 in the conductor 11 at an area S j+1 and the mean value of temperature T cj in the conductor 11 at a previous area S j by dividing for a value equal to a second distance d 2 between said area S j+1 and said previous area S j ; N3) multiplying the result due to the difference between said first axial temperature gradient GR1 J and said second axial temperature gradient GR2 J for a mean value of thermal conductivity of the conductor 11 for obtaining a respective thermal power flow value Q j for each area S j ; N4) for each area S j calculating, according to a method to discretize differential equations, each element of said further first vector B1 j taking into account said thermal power flow value Q j .

[0130] In the embodiment being disclosed, said first distance d 1 is equal to said second distance d 2 .

[0131] Furthermore, the method can further comprise further steps in addition to steps A to M for obtaining a value of waiting time t RP which is the time to wait before the polarity of said voltage V between said conductor 11 and said shield 15 is inverted after that the value of said voltage V has been set equal to zero, without said maximum value of electric field E MAXJ is greater than said predetermined value of electric field E REF : When a high voltage electric cable for direct electric current is connected to a converter, such as a current source converter, the polarity inversion of said voltage V allows to reverse the direction of the electric current I flowing along the longitudinal axis of the conductor 11.

[0132] To this end, said method can comprise the following steps: O1) associating a value equal to zero with said waiting time t RP ; O2) repeating the steps from C to M or from C to N4 for a predetermined number iterations NI considering a predetermined further time interval Δt RP , wherein said value of voltage V and said value of electric current I have been set equal to zero; O3) for each iteration, adding the value of said predetermined further time interval Δt RP to a respective value of waiting time t RP to obtain a respective updated value of waiting time t RP ; 04) repeating the steps from C to M or from C to N4 for a predetermined further number of iterations MI considering said predetermined further time interval Δt RP , wherein said value of voltage V is in absolute value equal to a nominal voltage value and has a sign opposite to the sign of said value of voltage V before the polarity of said voltage V is inverted, and said value of electric current I is equal to zero; O5) identifying for each area S j the maximum value of electric field E MAXJ and identify for each iteration of said predetermined further number of iterations MI a respective maximum value of electric field E MAXRP between said maximum values of electric field, wherein said maximum value of electric field E MAXRP is the maximum value of electric field reached after an inversion of polarity of the voltage V between said conductor 11 and said shield 15 with a waiting time equal to the waiting time t RP ; 06) repeating the steps from O1 to O5 by progressively increasing said predetermined number of iterations NI until the maximum value of electric field E MAXRP is less than or equal to said predetermined value of electric field E REF and storing the value of waiting time t RP corresponding to said maximum value of electric field E MAXRP .

[0133] In general, in use, the conductor 11 is connected to a converter and the shield 15 is connected to ground.

[0134] In particular, when the conductor 11 is connected to a current source converter, to prevent the dielectric material layer from being subjected to an electrical stress greater than a predetermined electrical stress (which can be a desired or required stress) or stress for which the electric cable 1 has been designed, an operator can invert the polarity of said voltage V through said converter after having waited a time greater than or equal to t RP after the value of said voltage V has been set equal to zero.

[0135] Consequently, the method comprise the step of connecting said conductor 11 to a converter and said shield 15 to ground.

[0136] The present invention relates also to system for calculating a electric field inside a dielectric material layer 13 of a high voltage electric cable 1 for direct electric current, disclosed above.

[0137] Said system comprises: temperature measuring means TS J ; a processing unit connected to said temperature measuring means TS J and configured to: o identifying one or more areas S j , with j=1,2...N where N is a positive integer, each of which is arranged along said electric cable 1, on a respective plane P j transverse to the longitudinal axis A of said electric cable 1 and has a geometric shape defined by a closed reference line; o associating a respective computational domain DT J with each area S j , as well as associating a respective further computational domain DE J with a further area which is a portion of said area S j and is arranged inside said electric cable 1, between said first semiconductive material layer 12 and said second semiconductive material layer 14, wherein said further area is included between a first closed line 120 and a second closed line 140, said computational domain DT J is. discretized through a set of finite elements e J1 ,e J2 ...e JM , said further computational domain DE J is discretized through a further set of finite elements e JX ,e JY ...e JL (in transversal section said first closed line 120 is a first contact line between the conductor 11 and said first semiconductive material layer 120 and said second closed line 140 is a second contact line between said second semiconductive layer 14 and said shield 15; ∘ acquiring: a value of temperature TE J at each area S j through temperature measuring means TS J ; a value of voltage V between said conductor 11 and said shield 15, a value of electric current I flowing along the longitudinal axis A of the conductor 11; o calculating, by means of a numerical finite element method, a respective temperature distribution Te J1 ,Te J2 ...Te JM for each computational domain DT J , wherein each value of temperature of a temperature distribution is associated with a respective finite element e J1 ,e J2 ...e JM of a respective computational domain DT J , each temperature distribution being calculated by solving: Fourier's law for conducting heat in transient conditions, starting from said temperature values TE J , from said measured electric current values I, and from a predetermined temperature distribution T 0 ; o calculating, by means of a further numerical finite element method, a respective electric field distribution Ee JX ,Ee JY ...E JL for each further computational domain DE J , wherein each value of electric field of an electric field distribution is associated with a respective finite element e JX ,e JY ...e JL of said further computational domain DE J , each electric field distribution being calculated by solving: Gauss's law for an electric field E in transient conditions, applied to each finite element e JY falling into the dielectric material layer 13, the law of continuity of electric current in stationary conditions applied to each finite element e JL which falls in the first semiconductive material layer 12 or in the second semiconductive material layer 14 and which does not contact one or more finite elements e JL which fall into the dielectric material layer 13, Schottky's law referred to an injection of electric charges in correspondence of a contact line between different materials, applied to each finite element e JX which falls into the first semiconductive material layer 12 or in the second semiconductive material layer 14 and contacts one or more finite elements e JY which fall into the dielectric material layer 13, the relationship between electric field E and said voltage V, starting from boundary conditions applied to said first closed line 120 and to said second closed line 140 of the respective further computational domain DE J , in which said boundary conditions depend on said voltage value V and on a predetermined electric charge density distribution ρ 0 .

[0138] In other words, said processing unit is configured to perform the steps from A to E of the method disclosed above.

[0139] In particular, in the embodiment being disclosed, said system comprises- a voltage measuring voltage 18 for measuring said voltage value V between said conductor 11 and said shield 15 and said processing unit is connected to said voltage measuring device 18 for acquiring said voltage value V.

[0140] However, said voltage value V can be a predetermined voltage value set by a user and said processing unit can be configured to acquire said predetermined voltage value, according to step C of the method described above.

[0141] Furthermore, in the embodiment being disclosed, said system comprise an electric current measuring device 19 for measuring said electric current value I flowing along the longitudinal axis of the conductor 11 and said processing unit is connected to said electric current measuring device 19 for acquiring said electric current value I, according to step C of the method described above.

[0142] However, said electric current value I can be a predetermined electric current value set by a user and said processing unit can be configured for acquiring said predetermined electric current value.

[0143] In particular, said system can comprise storage means (such as a memory) and said processing unit can be connected to said storage means for storing in said storage means said predetermined voltage value and / or said predetermined electric current value, when the voltage value V and the electric current value I are not measured.

[0144] With reference to step D of the method, said processing unit can be configured to perform the sub-steps from D1 to D4 and / or from D5 to D6 of said step D.

[0145] Furthermore, said processing unit can be configured to perform the steps from F to J of the method disclosed above or to perform in addition or as alternative to said steps from F to J, the steps from I to M of said method.

[0146] In particular, said processing unit can be configured to perform the steps from N1 to N4 of the method.

[0147] More particularly, said processing unit can be configured to perform the steps from O1 to O6 of the method disclosed above, for obtaining a value of waiting time t RP which is the time to wait before the polarity of said voltage V between said conductor 11 and said shield 15 is inverted after that the value of said voltage V has been set equal to zero, without said maximum value of electric field E MAXJ is greater than said predetermined value of electric field E REF : Furthermore, said processing unit can be configured to store said predetermined temperature distribution T 0 and said predetermined electric charge density distribution ρ 0 , independently of the storage of said predetermined voltage value and / or said predetermined electric current value.

[0148] Said processing unit can also be configured to store in said storage means said predetermined electric field value E REF (which is a threshold value) and / or said waiting time t RP .

[0149] Said system can comprise displaying means, such a display D of the computer C, for displaying the values of one or more quantities calculated using the method described above and / or the values of one or more quantities derived from said calculated quantities.

[0150] By way of example, it is possible to display in numerical form or by charts: one or more temperature maximum values in the conductor 11 of a respective temperature distribution Te J1 ,Te J2 ...Te JM calculated for a respective area S j ;and / or ore or more temperature maximum values on the outer layer 16 of a respective temperature distribution Te J1 ,Te J2 ...Te JM calculated for a respective area S j ; and / or one or more difference values indicating the temperature difference between the temperature associated with the first closed line 120 and the temperature associated with the second closed line 140 of a respective temperature distribution Te J1 ,Te J2 ...Te JM calculated for a respective area S j ; and / or said electric field distribution Ee JX ,Ee JY ...E JL and said predetermined electric field E REF ; and / or said electric charge density distribution ρe JX ,ρe JY ...ρe JL ; and / or one or more temperature maximum values in the conductor 11 of a respective temperature distribution Te J1 ,Te J2 ...Te JM calculated along the longitudinal axis A of the electric cable 1 ; and / or one or more difference values indicating the temperature difference between the temperature associated with the first closed line 120 and the temperature associated with the second closed line 140 of a respective temperature distribution Te J1 ,Te J2 ...Te JM calculated along the longitudinal axis A of the electrical cable 1; and / or one or more electric field maximum values of a respective electric field distribution Ee JX ,Ee JY ...E JL calculated for each area S j along the longitudinal axis A of the electric cable 1.

[0151] Said displaying means can also be used to display: the position of the area S j associated with the temperature maximum value in the conductor 11 obtained by said temperature distribution Te J1 ,Te J2 ...Te JM calculated for each area S j ; and / or the position of the area S j associated with the electric field maximum value of said electric field distribution Ee JX ,Ee JY ...E JL calculated for each area S j .

[0152] Furthermore, when the polarity of the voltage V between the conductor 11 and the shield 15 of the electric cable 1 is inverted, said displaying means can display: the electric field value calculated during step O2; and / or the chart of the electric charge density distribution ρe JX ,ρe JY ...ρe JL referred to an area S j which is the area in which the maximum value of the electric field is the maximum between the maximum values of electric field; and / or the chart of the electric field distribution Ee JX ,Ee JY ...E JL referred to an area S j which is the area in which the maximum value of electric field is the maximum between the maximum values of electric field and / or said predetermined electric field value E REF . Advantages

[0153] Advantageously, the method object of the invention allows to calculate, substantially in real time, the electric field within a dielectric material layer of a high voltage electric cable for direct electric current.

[0154] A further advantage is given by the fact that this calculation can be performed on any computer, as it requires reduced computational resources.

[0155] Furthermore, through the information regarding the electric field and the comparison with a predetermined electric field value, it is possible to monitor the electric stress of said electric cable with respect to the stress for which the electric cable was designed or to a desired stress.

[0156] Consequently, it is possible to improve the reliability of the electrical cable and increase the life span of the electrical cable.

[0157] The present invention has been described for illustrative, but not limitative purposes, according to its preferred embodiment, but it is to be understood that variations and / or modifications can be carried out by a skilled in the art, without departing from the scope thereof, as defined according to enclosed claims.

Claims

1. Method for calculating an electric field inside a dielectric material layer (13) of a high voltage electric cable (1) for direct electric current, wherein said electric cable (1) is of the type comprising from the inside towards the outside a conductor (11) having a longitudinal axis (A), a first semiconductive material layer (12), said dielectric material layer (13), a second semiconductive material layer (14), a metallic material layer (15) called shield, said method comprising the following steps: A) identifying one or more areas Sj, with j=1,2...N where N is a positive integer, each of which is arranged along said electric cable (1), on a respective plane Pj transverse to the longitudinal axis (A) of said electric cable (1) and has a geometric shape defined by a closed reference line; B) associating a respective computational domain DTJ with each area Sj, as well as associating a respective further computational domain DEJ with a further area which is a portion of said area Sj and is arranged inside said electric cable (1), between said first semiconductive material layer (12) and said second semiconductive material layer (14), wherein said further area is included between a first closed line (120) and a second closed line (140), said computational domain DTJ being discretized through a set of finite elements eJ1,eJ2...eJM, said further computational domain DEJ being discretized through a further set of finite elements eJX,eJY...eJL, in transversal section said first closed line (120) being a first contact line between the conductor (11) and said first semiconductive material layer (120) and said second closed line (140) being a second contact line between said second semiconductive layer (14) and said shield (15); C) acquiring: - a value of temperature TEJ at each area Sj through temperature measuring means (TSJ); - a value of voltage V between said conductor (11) and said shield (15), wherein said value of voltage V is obtained through a voltage measuring device (18) or is a predetermined value of voltage; - a value of electric current I flowing along the longitudinal axis (A) of the conductor (11), wherein said value of electric current I is obtained through an electric current measuring device (19) or is a predetermined value of electric current; D) calculating, by means of a numerical finite element method, a respective temperature distribution TeJ1,TeJ2...TeJM for each computational domain DTJ, wherein each value of temperature of a temperature distribution is associated with a respective finite element eJ1,eJ2...eJM of a respective computational domain DTJ, each temperature distribution being calculated by solving: o Fourier's law for conducting heat in transient conditions, starting from said temperature values TEJ, from said measured electric current values I, and from a predetermined temperature distribution T0; E) calculating, by means of a further numerical finite element method, a respective electric field distribution EeJX,EeJY...EJL for each further computational domain DEJ, wherein each value of electric field of an electric field distribution is associated with a respective finite element eJX,eJY...eJL of said further computational domain DEJ, each electric field distribution being calculated by solving: o Gauss's law for an electric field E in transient conditions, applied to each finite element eJY falling into the dielectric material layer (13), o the law of continuity of electric current in stationary conditions applied to each finite element eJL which falls in the first semiconductive material layer (12) or in the second semiconductive material layer (14) and which does not contact one or more finite elements eJL which fall into the dielectric material layer (13), o Schottky's law referred to an injection of electric charges in correspondence of a contact line between different materials, applied to each finite element eJX which falls into the first semiconductive material layer (12) or in the second semiconductive material layer (14) and contacts one or more finite elements eJY which fall into the dielectric material layer (13), o the relationship between electric field E and said voltage V, starting from boundary conditions applied to said first closed line (120) and to said second closed line (140) of the respective further computational domain DEJ, in which said boundary conditions depend on said voltage value V and on a predetermined electric charge density distribution ρ0.

2. Method according to claim 1, wherein said method comprises the following steps which are performed periodically at predetermined time intervals Δt for each area Sj: F) calculating an electrical conductivity distribution σeJX,σeJY...σeJL for each finite element eJX,eJY...eJL of each further computational domain DEJ, starting from said calculated temperature distribution TeJ1,TeJ2...TeJM and from said calculated electric field distribution EeJX,EeJY...EJL; G) calculating an electric current density distribution JeJX,JeJY...JJL for each finite element eJX,eJY...eJL of each further computational domain DEJ through the Ohm's law; H) calculating a respective value of electric charge density ρeJX,ρeJY...ρeJL for each finite element eJX,eJY...eJL of each further computational domain DEJ by solving: o the law of continuity of electric current in transient conditions, applied to each finite element eJY of each further computational domain DEJ which falls within the dielectric material layer (13), and o Gauss's law for an electric field E in stationary conditions, applied to each finite element eJX, eJL of each further computational domain DEJ which falls in the first semiconductive material layer (12) or in the second semiconductive material layer (14); I) updating said predetermined temperature distribution T0 with the temperature values of said calculated temperature distribution TeJ1,TeJ2...TeJM and updating said predetermined electric charge density distribution ρ0 with the calculated electric charge density values ρeJX,ρeJY...ρeJL; J) repeating the steps from C to E.

3. Method according to any one of the previous claim, wherein said method comprises the following steps: K) identifying a respective maximum value of electric field EMAXJ in each further computational domain DEJ; L) comparing each maximum value of electric field EMAXJ with a predetermined value of electric field EREF; M) if said maximum value of electric field EMAXJ is greater than said predetermined value of electric field EREF, establishing that at least one section of electrical cable (1) at each said area Sj is subjected to an electrical stress greater than an electrical stress for which said electrical cable (1) was designed or to a predetermined electrical stress.

4. Method according to any one of the previous claims, wherein said finite element method at step D solves a respective system of linear equations for each computational domain DTJ: A 1 j * T j = B 1 j where A1j is a first square matrix with a number of rows and a number of columns equal to the number of finite elements eJ1,eJ2...eJM of said computational domain DTJ, and each element of said matrix is a constant numeric coefficient depending on the geometric properties of said computational domain DTJ and from the thermophysical properties of at least one material associated with a finite element corresponding to a portion of said electric cable (1), Tj is a first column vector with a number of elements equal to the number of finite elements eJ1,eJ2...eJM of said computational domain DTJ and each element of said first column vector is a value of temperature which is unknown, B1j is a further first column vector with a number of elements equal to the number of finite elements eJ1,eJ2...eJM of said computational domain DTJ and each element is a numeric value variable over time and depending on the boundary conditions of heat exchange between electric cable (1) and external environment, on the temperature of said predetermined temperature distribution T0 associated with each finite element eJ1,eJ2...eJM of said computational domain DTJ, on a thermal power value calculated on the basis of the measured electric current values I flowing in the conductor (11) and the thermophysical properties of at least one material associated with a finite element corresponding to said portion of said electric cable (1), wherein said step D comprises the following sub-steps: D1) calculating a respective numeric value associated with each element of each first vector B1j on the basis of a value of temperature TeJ1,TeJ2...TeJM associated with each finite element eJ1,eJ2...eJM of said computational domain DTJ and with the values of thermal power due to the passage of electric current I in the conductor (11) and losses in the dielectric material layer (13); D2) multiplying said first vector B1j by a further first matrix A1j', which is the inverse matrix of said first matrix A1, to obtain a respective value of temperature TeJ1,TeJ2...TeJM associated with each finite element eJ1,eJ2...eJM of said computational domain DTJ; D3) for each computational domain DTJ calculating a difference between a respective value of temperature TEJ measured at each area Sj and a respective value of temperature between the calculated values of temperature TeJ1,TeJ2...TeJM, wherein said value of temperature is associated with a respective finite element between the finite elements eJ1,eJ2...eJM of said computational domain DTJ at which said temperature value has been measured, to obtain a respective temperature variation value ΔTJ; D4) for each finite element eJ1,eJ2...eJM of said computational domain DTJ calculating a respective updated value of temperature TeJ1,TeJ2...TeJM by adding a respective value of temperature obtained at sub-step D2 and a respective value of temperature variation ΔTJ obtained at sub-step D3.

5. Method according to any one of the previous claims, wherein said step D comprises the following sub-steps: D5) calculating a temperature gradient of said temperature distribution TeJ1,TeJ2...TeJM inside a respective section of said electric cable (1) in each area Sj along a plurality of radial directions, wherein each value of temperature gradient is associated with a respective finite element eJ1,eJ2...eJM of a respective computational domain DTJ, wherein said finite element is arranged inside said electric cable (1); and D6) identifying a radial direction of said plurality of radial directions associated with a maximum value of said calculated temperature gradient, and wherein said electric field distribution EeJX,EeJY...EJL at said step E is calculated only along said radial direction identified at sub-step D6, assuming that the respective values of electric field vary only along said radial direction of said electric cable (1).

6. Method according the claims 2 and 5, wherein the electrical conductivity distribution σeJX,σeJY...σeJL at step F, the electric current density distribution JeJX,JeJY...JJL at step G and each value of electric charge density ρeJX,ρeJY...ρeJL at step H are calculated along said radial direction of said electric cable (1) identified at the sub-step D6.

7. Method according the previous claim, wherein said method further comprises the following steps for taking into account a thermal conduction along the longitudinal axis (A) of the conductor (11): N1) calculating for each area Sj the mean value of temperature Tcj in the conductor (11); N2) calculating: a first axial temperature gradient GR1J as a difference between the mean value of temperature Tcj in the conductor (11) at an area Sj and the mean value of temperature Tcj-1 in the conductor (11) at a previous area Sj-1 by dividing for a value equal to a first distance d1 between said area Sj and said previous area Sj-1, and a second axial temperature gradient GR2J as a difference between the mean value of temperature Tcj+1 in the conductor (11) at an area Sj+1 and the mean value of temperature Tcj in the conductor (11) at a previous area Sj by dividing for a value equal to a second distance d2 between said area Sj+1 and said previous area Sj; N3) multiplying the result due to the difference between said first axial temperature gradient GR1J and said second axial temperature gradient GR2J for a mean value of thermal conductivity of the conductor (11) for obtaining a respective thermal power flow value Qj for each area Sj; N4) for each area Sj calculating, according to a method to discretize differential equations, each element of said further first vector B1j taking into account said thermal power flow value Qj.

8. Method according to the previous claim, wherein said first distance d1 is equal to said second distance d2.

9. Method according to any one of the previous claims, wherein said further numerical finite element method at step E solves a respective linear equation system for each further computational domain DEJ: A 2 j * E j = B 2 j where A2j is a second square matrix with a number of rows and a number of columns equal to a number of finite elements of said further computational domain DEJ and each element of said matrix is a constant numeric coefficient and depends on the geometric properties of said further computational domain DEJ and on the dielectric properties of at least one of a material associated with said portion of said electric cable (1) which falls inside said further computational domain DEJ, Ej is a second column vector with a number of elements equal to the number of finite elements eJX,eJY...eJL of said further computational domain DEJ and each element is a numeric value of electric field EeJX,EeJY...EJL associated with each finite element eJX,eJY...eJL of said further computational domain DEJ, B2j is a further second column vector with a number of elements equal to the number of finite elements eJX,eJY...eJL of said further computational domain DEJ, wherein each element is a numeric value depending on the boundary conditions of electric field E, on said value of voltage V and on said predetermined electric charge density distribution ρ0 associated with each finite element eJX,eJY...eJL of said further computational domain DEJ.

10. Method according to claims 2 and 3 or according to claims 2 and 3 and any one of claims 4-9, wherein said method further comprises the following steps to obtain a value of waiting time tRP which is the time to wait before the polarity of said voltage V between said conductor (11) and said shield (15) is inverted after that the value of said voltage V has been set equal to zero, without said maximum value of electric field EMAXJ is greater than said predetermined value of electric field EREF: O1) associating a value equal to zero with said waiting time tRP; O2) repeating the steps from C to M or from C to N4 for a predetermined number iterations NI considering a predetermined further time interval ΔtRP, wherein said value of voltage V and said value of electric current I have been set equal to zero; O3) for each iteration, adding the value of said predetermined further time interval ΔtRP to a respective value of waiting time tRP to obtain a respective updated value of waiting time tRP; 04) repeating the steps from C to M or from C to N4 for a predetermined further number of iterations MI considering said predetermined further time interval ΔtRP, wherein said value of voltage V is in absolute value equal to a nominal voltage value and has a sign opposite to the sign of said value of voltage V before the polarity of said voltage V is inverted, and said value of electric current I is equal to zero; O5) identifying for each area Sj the maximum value of electric field EMAXJ and identify for each iteration of said predetermined further number of iterations MI a respective maximum value of electric field EMAXRP between said maximum values of electric field, wherein said maximum value of electric field EMAXRP is the maximum value of electric field reached after an inversion of polarity of the voltage V between said conductor (11) and said shield (15) with a waiting time equal to the waiting time tRP; 06) repeating the steps from O1 to O5 by progressively increasing said predetermined number of iterations NI until the maximum value of electric field EMAXRP is less than or equal to said predetermined value of electric field EREF and storing the value of waiting time tRP corresponding to said maximum value of electric field EMAXRP.

11. Method according to any one of the claims 1-10, wherein said area Sj is outside said electric cable (1).

12. Method according to any one of the claims 1-10, wherein said area Sj is inside said electric cable (1).

13. Method according to any one of claims 1-12, wherein said electric cable (1) comprises an external layer (16), and wherein said temperature measuring means (TSJ) comprise a plurality of temperature sensors (TS1J) for measuring a temperature at a respective area Sj and said temperature sensors are arranged inside said electric cable (1) or a respective portion of said external layer (16) or at a predetermined distance from said external layer (16).

14. Method according to any one of claims 1-12, wherein said electric cable (1) comprises an external layer (16), and wherein said temperature measuring means comprise a temperature distributed sensor with a longitudinal resolution for measuring a temperature at each area Sj and said temperature distributed sensor is arranged inside said electric cable (1) or on said external layer (16) or at a predetermined distance from said external layer (16).

15. System for calculating an electric field inside a dielectric material layer (13) of a high voltage electric cable (1) for direct electric current, wherein said electric cable (1) is of the type comprising from the inside towards the external a conductor (11) having a longitudinal axis (A), a first semiconductive material layer (12), said dielectric material layer (13), a second semiconductive material layer (14), a metallic material layer (15) called shield, said system comprising: - temperature measuring means (TSJ) for measuring a value of temperature at one or more areas Sj, with j=1,2...N where N is a positive integer, each of which is arranged along said electric cable (1) on a respective plane Pj transverse to the longitudinal axis (A) of said electric cable (1) and has a geometric shape defined from a closed reference line; - a processing unit connected to said temperature measuring means (TSJ) and configured to perform the steps of the method according to any one of the previous claims.

16. System according to the previous claim, wherein said system comprises: - a voltage measuring device (18) for measuring a value of voltage V between said conductor (11) and said shield (15); - an electric current measuring device (19) for measuring a value of electric current I flowing along the longitudinal axis (A) of the conductor (11); wherein said processing unit is connected to said voltage measuring device (18) and to said electric current measuring device (19) and configured to acquire a value of voltage V through said voltage measuring device (18) and said value of electric current I through said electric current measuring device (19).

17. System according to claim 15, wherein, said system comprises storage means and said processing is connected to said storage means is configured to store a predetermined value of voltage and / or a predetermined value of electric current and to acquire said predetermined value of voltage and / or said predetermined value of electric current from said storage means.

18. System according the claim 15 or 16, wherein, said system comprises storage means and said processing unity is connected to said storage means and configured to store said predetermined temperature distribution T0 and said predetermined electric charge density distribution ρ0.