FR2738963A1 - Improvements to Petersen coil earthed HV distribution system - Google Patents

Abstract

The primary substation's arc-suppression or Petersen coil's admittance (YNG) is determined by the system capacitive current at any earth fault point. Any one phase (10, 12, 14) can be earthed through an eg. 8 ohm resistor (18) by a selector switch (16) operated by a central control unit (20), receiving input from feeder (Dl..Di) residual current (24l..24i) and coil voltage (22) measurements. Residual current measurements are corrected by comparing with pre-existing values related to phase admittance (eg. Y1G1, Y2G1, Y3G1, etc.) asymmetry. A phase/earth resistive fault causes the affected phase to be earthed (16, 18) at the substation, a delay allowing transient faults to clear. This results in reduced potentials at the fault point. The control unit monitors the fault current, tripping the affected feeder if it exceeds a permitted value.

Description

 The present invention relates to multi-phase electrical energy distribution networks and relates in particular to a method of maintaining the supply of electrical energy in the presence of a fault on a defective phase of a starting line of such networks.

 Single-phase non-fugitive faults, that is to say those which are not of the self-extinguishing type and which affect a phase of the network, are mainly due to faulty insulation between a phase and the earth, to the presence of transformers defective, or broken conductors.

 These faults generate a fault current and can cause a considerable rise in potential in the vicinity of the defective network portion and therefore present a significant danger for the environment (risk of fire), for network equipment or for persons. nearby (possibility of electric shock).

 In compensated networks, the network neutral is connected to earth by a compensation coil also called "Petersen" coil. This coil minimizes the fault current generated by weakly resistive faults of a transient nature.

Indeed, it is possible, by acting on the value of the inductance of the coil to modify the "reactive" current flowing in the coil, so as to compensate for the "capacitive" current of the network which circulates in the equivalent resistance of the fault. between the faulty phase and the network earth.

 This type of network neutral earthing therefore makes it possible to limit the fault current. It thus allows the operators of electrical energy distribution networks to maintain the supply of electrical energy on the portion of the network concerned during the detection and repair of the fault.

 This maintenance of the supply nevertheless presents risks during the appearance of more resistive faults which are accompanied by an increase in the potential in the vicinity of the relatively large defect.

 To overcome this drawback, the operators of the French electrical energy distribution network recommend the earthing of the defective phase by a "shunt" resistance of the order of a few tens of ohms in order to short-circuit the resistance of the network fault to reduce the fault current and limit the rise in potential near the fault.

 This solution is relatively easy to implement but is not very effective for certain weakly resistive faults due to the drop in line voltage due to the load current or in the case of highly resistive faults caused for example by a drop or a rupture of a conductor, and generating a potential rise in the vicinity of the fault very high.

 Consequently, maintaining the supply poses significant risks.

 The invention aims to overcome these drawbacks.

 It therefore relates to a method of maintaining the supply of electrical energy in a polyphase electrical energy distribution network, in the presence of a fault on a defective phase of a starting line of the network, comprising the steps of detection and earthing the defective phase so as to reduce the fault current propagating in the network under the effect of said fault, characterized in that it also comprises the steps of measuring the residual current of the line flow, calculating the difference between the measured residual current of the flow line and the residual current propagating in the flow line in the absence of a fault, under the same voltage conditions between the neutral and the earth of the network , said difference being representative of the value of the fault current, and of maintaining the supply of electrical energy as long as said difference is less than a maximum value e of threshold representative of the maximum value of the fault current authorized to circulate in the network.

 According to a characteristic of the invention, the overall admittance and the asymmetry of each starting line are further measured, before the appearance of the fault, the residual current of the starting line in the absence of a fault being calculated. from the measured values of the admittance and the asymmetry of the faulty feeder and the voltage present between the neutral and the network earth in the presence of the fault.

 According to another characteristic of the invention, the method further comprises a step of adjusting the voltage present between the neutral and the earth of the network, subsequent to the step of calculating the difference between the measured residual current and the residual current. propagating in the starting line in the absence of fault, in order to reduce the fault current propagating in the faulty phase.

 According to a first embodiment, the step of adjusting the voltage present between the neutral and the earth of the network is carried out by adjusting the value of the impedance of an element with adjustable impedance for earthing the defective phase. in view of the variation of said voltage present between the phase and the neutral of the network.

 As a variant, the step of adjusting the voltage present between the neutral and the earth is carried out by adjusting the value of the inductance of a compensation coil for earthing the neutral of the network in order to adjust said voltage. present between neutral and network earth.

 Said step of adjusting the voltage present between the neutral and the earth of the network is carried out by connecting to the earth, each phase of the faulty starting line by an element with adjustable grounding admittance and by adjusting the value of their admittance for adjusting asymmetry.

 It follows that when a resistive single-phase fault occurs on one of the phases of the network, the various adjustable elements of the network are adjusted so as to reduce the fault current generated by the fault and consequently to reduce the voltage present in the vicinity of the fault, and the supply of electrical energy is maintained as long as the fault current is less than a maximum authorized value.

 The invention also relates to a device for maintaining the supply of electrical energy in a polyphase electrical energy distribution network, in the presence of a fault on a defective phase of a starting line of the network, comprising means for detecting the fault and the starting line and means for selective connection of the defective phase to earth so as to reduce the fault current propagating in the network under the effect of said fault, characterized in that it comprises in addition a device for measuring the residual current of the starting line, a device for measuring the voltage between the neutral and the earth of the network and a central processing unit adapted to calculate the residual current propagating in the starting line in the absence of a fault, under the same voltage conditions between the neutral and the earth of the network, and of the difference between said residual currents measured and calculated, in view e to authorize the maintenance of the supply of electrical energy as long as said difference is less than a maximum threshold value representative of the maximum value of the fault current authorized to circulate in the network.

 According to another characteristic of the invention, the selective connection means comprise a switch arranged in series on an adjustable element for earthing the defective phase and controlled by the central processing unit with a view to adjusting the voltage present. between neutral and network ground.

 According to one embodiment of the invention, the adjustable element for earthing the defective phase consists of an element with adjustable impedance for varying the voltage present between the neutral and the earth of the network.

 According to another embodiment, the adjustable element for earthing the defective phase is constituted by an element with adjustable admittance for adjusting the asymmetry of the network, the other phases of this starting line being each connected to the earth by a second element with adjustable admittance for adjusting the asymmetry of the network.

 As a variant, the device further comprises a compensation coil for earthing the network neutral, said coil being constituted by a coil with adjustable inductance controlled by the central processing unit to adjust the phase of the voltage present between the neutral and network ground.

The invention will be better understood on reading the description which follows, given solely by way of example and made with reference to the appended drawings in which
- Figure 1 shows the equivalent circuit of a three-phase electrical energy distribution network;
- Figure 2 shows the network of Figure 1, equipped with a device for maintaining the supply of electrical energy
- Figure 3 is a flowchart showing the different phases of the method according to the invention;
- Figure 4 is a flowchart showing the different phases of a second method according to the invention; and
- Figure 5 shows the network of line 1, equipped with a supply maintenance device according to another embodiment.

 Figure 1 shows the equivalent circuit of a three-phase electrical energy distribution network.

 The network supply station consists of a three-phase voltage source supplying a busbar, 10, 12 and 14, to which the network start lines are connected.

 In this figure, V1, V2 and V3 denote the voltages delivered by the power station, I1, I2 and I3 denote the currents delivered by this power station, I ".t denotes the total residual current at the output of the station d 'supply, Di denotes the starting line ni, and Ire denotes the residual current of the starting line n i.

 Furthermore, Yogi, Y2Gi and Y3Gi denote the global phase-earth admittances of each of the three phases of the start i.

 The network neutral N is connected to earth by a reactive X admittance compensation coil.

 If we consider that the resistive part of the phase-earth admittances is negligible compared to the capacitive part, when a single-phase fault appears on one of the phases, the reactive current IS passing through this compensation coil compensates for the capacitive current of the network, which limits the fault current generated by the fault.

 As mentioned above, with this neutral earthing system, it is possible to maintain the supply of electrical energy in the presence of a single-phase fault due to the relatively low value of the fault current.

 Maintaining the supply can nevertheless present a danger in the case of weak resistive faults, due to the drop in line voltage due to the load current, or in the case of highly resistive faults, caused for example by breaking or fall of a driver from a starting line.

 FIG. 2 shows the equivalent circuit of a distribution network equipped with a device making it possible to overcome this drawback and to manage the maintenance of the supply of electrical energy during the appearance of a single-phase fault.

 It can be seen in this FIG. 2 that the network, identical to that of FIG. 1, is provided with a three-stage switch 16 making it possible to selectively connect each of the bars 10, 12 and 14 to an element 18 for connection to earth.

 The switch 16 is controlled by a central processing unit 20 connected to a member 22 for measuring the voltage M; present between neutral N and earth of the network and to a set of measuring devices 24 1 ... 24i of the residual current flowing in each starting line Dî, ... Di.

 The central processing unit 20 is of known type and will therefore not be described in detail below.

 When a resistive single-phase fault appears on a phase of a starting line, for example on phase 1 of line D1, this fault is detected by the central processing unit 20 which controls the switch 16 to connect phase 1 defective at the earth connection element 18.

 Advantageously, the switching of the switch 16 is carried out after a sufficient time to wait for the possible self-extinction of the fault.

 According to a first embodiment, the ground connection element 18 consists of a resistor, for example having a value close to 8 ohms.

 The short-circuit current flowing in the earth connection element 18 generates a voltage between the defective phase and the earth at the supply station which compensates for part of the voltage drop due to the load current in line .

 The rise in potential at the location of the fault, being dependent on the voltage drop in the faulty phase, the connection to ground by the element 18 makes it possible to reduce the rise in potential generated by the presence of the fault and therefore allows maintaining the supply of electrical energy in the presence of this fault.

 The central processing unit also provides continuous monitoring of the fault current.

 Thus, as soon as the value of the fault current exceeds a maximum admissible value, it causes the supply station to be disconnected.

 The method of maintaining the supply of electrical energy in the network of FIG. 2 will now be described with reference to FIG. 3.

 During this process, the distribution network is continuously monitored, during step 30, with a view to detecting a fault on a phase of a starting line of the network.

 The detection of a fault can be carried out by various known techniques, for example by using wattmetric protections on each starting line of the network, or by using known methods for detecting resistive faults.

 During this step 30, the voltage maza present between the neutral and the earth of the network is also measured, as well as the overall admittance X; i and the asymmetry K; i of each phase, in order to be able to calculate, as will be described in detail below, the residual current Ir, .i which would flow in the defective starting line, in the absence of a fault.

 During the following step 32, the defective starting line and the defective phase are also determined, using known techniques, in order to cause the switching of the switch 16 to connect the phase concerned to earth, during the step 34.

 During the following two steps 36 and 38, it is monitored that the fault current flowing in the network is less than a predetermined threshold value representative of the maximum threshold value authorized to flow in the network.

 This maximum fault current threshold value is determined by the standards in force in each country, which requires limiting the rise in potential across the fault to a given value. It is for example 4 amps.

 If the fault current is less than this maximum threshold value, the supply of electrical energy is maintained during step 40.

 If this is not the case, the resistor 18, not being sufficient to limit the rise in potential in the vicinity of the fault, the supply of electrical energy is then interrupted during step 42. The interruption of the supply in this case eliminates all risk of incidents for people or the environment.

 Monitoring of the fault current is carried out by the central processing unit 20, by calculating the difference between the residual current IrDi flowing in the faulty starting line n "i and the residual current Iresi which would flow in this starting line nor in absence of fault, this difference being representative of the value of the fault current.

Thus, as soon as a fault is detected, the central processing unit measures, during step 36, the residual current IrDi of the defective line, by means of the measuring member 24i, and calculates, also at during this step 36, the residual current -1re.i which would flow in the starting line concerned according to the following relation = = YtDi x yNG + KDij (1) Ctref Vnom (1) in which
Vnom is the nominal simple network voltage, i.e. the voltage between a phase and the network neutral,
Ctref is a constant capacity value whose value is, in a known manner, freely fixed,
2 ;; G is the voltage between neutral and earth measured during step 36,
w is the pulsation of the electric energy flowing in the starting line, j # designating the associated complex number, and in which the asymmetry KDi is defined by the relation
X1Gi + a2 x x2ai + a x3Gi (2) j # Ctref in which a and a2 are complex operators equal to ej Z / 3 and e1 4n / 3 respectively.

It should be noted that in the case where the overall admittances YtDi and the asymmetry P; i of the defective starting line are not directly measurable, these can be obtained in a known manner from the value of the voltage VNG present between the neutral and the earth of the network, of the value of the residual current Iresi circulating in the line neither corresponding, in the absence of fault, from two measurements a and b independent according to the following relationships:
yNGb - yNGo XtDi (3)
Iresib - Iresia and
Iresia YtGi X yNGa
Mi i --------------------- (4)
jo) X Ctref x
After having measured and calculated the residual currents in the presence and in the absence of a fault, the central processing unit 20 calculates the difference between the corresponding values and then compares the result of this difference with the maximum threshold value to consider, if necessary. if necessary, the disconnection of the defective starting line, the other healthy starting lines being thus always supplied.

 It is understood that this method of maintaining the supply of electrical energy in a distribution network equipped with a switch and a grounding resistor for the defective phase allows the limitation of the fault current generated by a fault present on a phase, limiting the rise in potential across the fault and therefore maintaining the distribution of electrical energy as long as the fault is present. This method also makes it possible to interrupt the supply on the defective line when this limitation is no longer sufficient, while maintaining the supply on the other healthy starting lines.

 According to another embodiment of the invention, the earth connection element 18 is constituted by a variable impedance controlled by the central processing unit 20. This variable impedance can be a continuously adjustable impedance, this is that is to say made up of a single adjustable ohmic conductor, or with discontinuous adjustment, that is to say made up of a set of ohmic conductors arranged in parallel and selectively interlockable.

 In this case, it is possible, by acting on the value of the ohmic conductor to influence the voltage present between the defective phase and the earth so as to improve compensation for the voltage drop in the defective phase.

 The different steps of the method of maintaining the supply by means of such a variable resistance for connection to the earth of the defective phase are represented in FIG. 4. In this method, steps 44, 46, 48, 50 and 51 are identical to steps 30 to 40 of FIG. 3.

 When the fault current propagating in the network is higher than the maximum threshold value mentioned previously, the value of the inductance of the compensation coil is modified so as to minimize the fault current by acting on the voltage! present between neutral and network earth.

 To do this, it is first verified, during step 52, that the maximum number of adjustment of the compensation coil is not reached, this maximum number being fixed beforehand.

 If this is the case, it is decided, during step 56, to interrupt the supply of electrical energy on the defective starting line.

 If this is not the case, the value of the inductance of the compensation coil is adjusted in step 54, for example by using a coil with a plunger core of known type, until the difference between the measured residual current of the defective starting line and the residual current of the starting line in the absence of a fault is minimum, so that the fault current is minimized.

 After having adjusted the phase of the voltage present between the neutral and the earth of the network, it is checked, in the following step 58, that the fault current is less than the maximum threshold value.

 If this is the case, the supply of electrical energy in the defective line is maintained at step 62.

 If this is not the case, the value of the impedance of the ohmic conductor 18 is adjusted to adjust the modulus of the voltage between neutral and earth to a value making it possible to compensate for the drop in line voltage.

 We then return to step 50 during which we check that the fault current is always less than the maximum authorized threshold value.

 In the process which has just been described, it has been considered that the distribution network is provided with a compensation coil connecting the neutral of the network to earth.

 Of course, if the network does not have a compensation coil, steps 54, 56 and 58 are eliminated.

 In the embodiments which have just been described, the ground connection element 18 is constituted by a purely resistive element.

 It is however possible, with reference to FIG. 5, to consider providing the distribution network with variable admittances for earthing the phases of the faulty starting line and, in particular, to provide each phase with one or more variable admittance earth connection elements controlled by the central processing unit and capable of being selectively switched.

 In this FIG. 5, which differs from FIG. 2 only in the nature of the phase earth connection elements, these variable admittance elements are designated by the reference numerals 64a, 64b, 64c.

 It is therefore conceivable, with reference to equation (2), that a variation in the admittance of the earth connection element generates a modification of the total asymmetry of the network.

In a compensated network, the voltage between neutral and earth is determined by the relationship
K
---------------- (5)
M + jD in which M is the network mismatch, zero when the network is in the tuned state and equal to -1 when the neutral N of the network is isolated, K is the total asymmetry of the network and D is the damping , determined by the network configuration.

 It is therefore possible, by acting on the value of the grounding admittance of the defective phase, to act on the voltage LG.

 The method of maintaining the supply of the network equipped with an adjustable admittance for earthing the defective phase differs from the method of FIG. 4 only by the nature of steps 54 and 60 during which, respectively, the artificial asymmetry phase by retaining its module so as to minimize the fault current, and the artificial asymmetry module by retaining its phase until a minimum fault current is obtained.

 The invention which has just been described therefore makes it possible on the one hand to maintain the distribution of electrical energy in a network in the presence of a fault, and this, as long as the fault presents no danger and to cut the distribution in the event danger, and, on the other hand, to minimize the influence of the fault on the network.

Claims (11)

 1. A method of maintaining the supply of electrical energy in a polyphase electrical energy distribution network, in the presence of a fault on a defective phase of a starting line (Di) of the network, comprising the steps of detection and for earthing the defective phase so as to reduce the fault current propagating in the network under the effect of said fault, characterized in that it also comprises the steps of measuring the residual current (ire) of said starting line, calculating the difference between the measured residual current (IrDini) of the starting line (Di) and the residual current (1 ,,,) propagating in the starting line in the absence of fault, in the same voltage conditions (VNG) between the neutral earth of the network, said difference being representative of the value of the fault current, and of maintaining the supply of electrical energy as long as said difference is less than a va their maximum of threshold representative of the maximum value of the fault current authorized to circulate in the network.  2. Method according to claim 1, characterized in that, in addition, before the appearance of the fault, the overall admittance (YDi) and the asymmetry (KDA) of each starting line, the residual current ( lresi) of the starting line in the absence of a fault being calculated from the measured values of the admittance (YDi) and the asymmetry (ksi) of the faulty start and of the voltage (Y; G) present between the neutral and network earth in the presence of the fault.  3. Method according to one of claims 1 and 2, characterized in that it further comprises a step of adjusting the voltage (VNG) present between the neutral (N) and the earth of the network, after the step for calculating the difference between the measured residual current (irai) and the residual current (Ir ,, i) propagating in the starting line in the absence of a fault, with a view to reducing the fault current propagating in the phase defective.  4. Method according to claim 3, characterized in that said step of adjusting the voltage (2; G) present between the neutral (N) and the network earth is carried out by adjusting the value of the impedance of an element (18) with adjustable impedance for earthing the defective phase with a view to varying said voltage (y) present between the phase and the network neutral.  5. Method according to one of claims 3 and 4, characterized in that the step of adjusting the voltage () present between the neutral and the earth is carried out by adjusting the value of the inductance (YNG) of a network neutral compensation coil for adjusting said voltage (9G) between the neutral and the network earth.  6. Method according to any one of claims 3 to 5, characterized in that said step of adjusting the voltage (2; G) present between the neutral and the network earth is carried out by connecting each phase of the faulty starting line by an adjustable admittance element (64a, 64b, 64c) for earthing and adjusting the value of their admittance for the purpose of adjusting the asymmetry.  7. Device for maintaining the supply of electrical energy in a polyphase electrical energy distribution network, in the presence of a fault on a defective phase of a starting line (Di) of the network, comprising means for detecting the fault and the defective phase and means for selective connection (16,18) of the defective phase to earth so as to reduce the fault current propagating in the network under the effect of said fault, characterized in that further comprises a device for measuring the residual current (irai) of the starting line (24i), a device for measuring (22) the voltage between the neutral and the earth of the network and a suitable central processing unit (20) to calculate the residual current (I ,,. i) propagating in the starting line in the absence of a fault and under the same voltage conditions (fic) between neutral and earth and the difference between said residual current measured ( IrDi) and calculated lé (irai), in order to authorize the maintenance of the supply of electrical energy as long as said difference is less than a maximum threshold value representative of the maximum value of the fault current authorized to circulate in the network.  8. Device according to claim 7, characterized in that the selective connection means comprise a switch (16) arranged in series on an adjustable element (18) for earthing the defective phase controlled by the central processing unit (20) for the adjustment of the voltage (yAG) present between the neutral (N) and the network earth.  9. Device according to claim 8, characterized in that the adjustable element (18) for earthing the defective phase is constituted by an element with adjustable impedance for varying the voltage present between the neutral and the earth of the network. .  10. Device according to one of claims 8 and 9, characterized in that the adjustable element (18) for earthing the defective phase is constituted by an adjustable admittance element for adjusting the asymmetry of the network, the other phases of this starting line being each connected to earth by a second element with adjustable admittance for adjusting the asymmetry of the network.  11. Device according to any one of claims 7 to 10, characterized in that it further comprises a compensation coil for earthing the network neutral, said coil being constituted by an adjustable inductance coil controlled by the central processing unit (20) for adjusting the voltage present between the neutral and the earth of the network.