FIELD OF INVENTION:
This invention relates to the field of a post average electric conversion voltage / low voltage (MV / LV).
BACKGROUND OF THE PRESENT INVENTION
A power distribution network average AC voltage such as is found in most European countries, usually consists of the following: a post source 1 which supplies the network, in which there are a high voltage transformer / medium-voltage 2, a neutral point impedance 3, and different line departures 4, equipped with means of protection against short circuits, including protections homopolar 5 which in particular occur in case of a fault between a phase line 6 and the earth,
MT 6 different lines, which can be either air-type bare conductor, either underground or overhead type insulated conductor with earth screen
Medium voltage transformer stations / low voltage 7, intended to provide the electricity needs of the clients served by the network.
CN 101017974 A
Abstract
The disclosed neutral-point indirect-grounding power network single-phase grounding fault location method comprises: all monitor control terminals are assembled on the distribution line switch or raing main unit, simultaneous collects and records the instant line zero-sequence current message before and after the fault to send to the host for analysis and find out the fault segment to isolation. This invention can fast diagnose fault segment to ensure normal power supplying.

US 6369996 Bl

BRIEF DESCRIPTION OF THE DRAWING
Further objects and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawings and wherein:
Figure 1 is a circuit diagram of an electrical distribution network average phase voltage of the prior art;
The Figure 2 illustrates the currents in the network of figure 1 ;
The Figure 3 is a diagram of a network portion MV including a transformer station according to the invention;
The Figure 4 illustrates the currents in the network of Figure 3 ;
The Figure 5 is a diagram of a transformer comprising the means of the invention;
The figures 6 and 7 are respectively a diagram and a perspective view of a transformer according to the invention
Various embodiments of the invention are described herein with reference to accompanying drawings, in which some, but not all embodiments of the invention are shown in figures. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that disclosure will satisfy applicable legal requirements.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
Forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that disclosure will satisfy applicable legal requirementsDescription translated from French
The invention relates to a post average electric conversion voltage / low voltage (MV/LV).

As shown figure 1 A power distribution network average AC voltage such as is found in most European countries, usually consists of the following: a post source 1 which supplies the network, in which there are a high voltage transformer / medium-voltage 2, a neutral point impedance 3, and different line departures 4, equipped with means of protection against short circuits, including protections homopolar 5 which in particular occur in case of a fault between a phase line 6 and the earth,
MT 6 different lines, which can be either air-type bare conductor, either underground or overhead type insulated conductor with earth screen
Medium voltage transformer stations / low voltage 7, intended to provide the electricity needs of the clients served by the network.
In the past, the MT 6 lines were essentially air and consist of bare conductors. Such lines are exposed to climate constraints (storms, storms ...). To make them less vulnerable networks .and thus improve the quality of service, electricity distributors have replaced partially or in full, these airlines by underground lines consisting of cable with MT device screen grounded. By cons, if the reliability of networks is greatly improved as a result, it appeared that the capacitive effect of these cables could be detrimental to the protection of people and property, due to a strong increase of the fault current in case of short-circuit between a phase of the network and earth. To overcome this drawback, electricity distributors have an inductor inserted between the neutral of the MV network and the earth, so as to compensate the capacitive current occurring during fault between phase and earth.
As shown Figure 2 This inductor 8 is located in the post source high voltage / medium voltage, typically connected to the neutral point of the secondary winding of high voltage / medium voltage transformer 2. In parallel with this inductor 8 and also connected a resistor 9 for allowing fault detection between phase and ground. In this same figure, the capacitive effect of the opposite cable earth is symbolized by the capacitors 10 value C. In case of a fault between a phase 11 and earth, a capacitor current Ic will flow into the defect. This current can reach values of the order of hundreds of amperes and cause tension to touch or step voltages dangerous depending on the quality of ground connections instead of the default.

Insofar as the value L of the compensating inductance 8 is such that the following equality is met, 30. LC 0 © .2 = I or co = 2 0 n HI f
The mains frequency, the currents IL and 1C will cancel and the phase to earth fault current Id does not understand that the current ]R, of generally less than or equal to 10 A, is essentially dependent on value of the resistor 9 and chooses to value just sufficient to allow operation of zero sequence protections substation. The capacitive effect of the cables is thus deleted and the value of the fault current Id is controlled by the operator.
In order to meet the above equation, irrespective of the network configuration that can change according to the operating constraints or due to extensions, the inductor 8 is usually adjustable and tuned permanently to the actual value of the capacitive earth fault current of the network using an automatic device.
However, such inductors, also called coils Petersen, are particularly expensive devices whose setting range can quickly be insufficient in case of network expansion. It follows that the power distributor, wanting to continue an operation reliability of its network by progressive burial of the lines, or wanting to expand its network to ensure the supply of new clients, faces additional economic constraints due the need to intervene at the source position and replace the existing Petersen coil.
The object of the present invention is to avoid this drawback and to allow network extensions or progressive network lines burial without having to intervene in the substation, or at least minimizing further development. It relates to a medium processing position voltage / low voltage (or the MV / LV), wherein is integrated a means of compensating the homopolar current generated by the network portion to which is connected the left.
More particularly, the invention relates to an electric transformer station MV / LV comprising a MV / LV transformer, said station including means for compensating the homopolar capacitive current generated by a network portion MV fixed length to which said transformer is capable of being connected, in the event of a fault between a phase and the earth located on this network.

For example, a voltage of 10 kV underground network realized with cables with earth screen generates a capacitive current of 1.5 A / km in case of a solid fault between a phase and earth. The transformer station according to the invention ensures an economical manner the compensation of a current for values up to 15 A. It thus covers a portion length array equal to 10 km. The density of settlement in Western countries is such that the number of transformer stations tends to be greater than or equal to 1 MV / LV per km of network. Consequently, with the object of the invention, it becomes economic to consider underground MV network extensions, inserting eg within each group of 10 new MV / LV an MV / LV substation that provides.compensation of the capacitive earth fault current of the network portion needed to supply this group of positions. In this way, the network expansion can be achieved by avoiding to make changes in the source position, or, as discussed further, by limiting the changes in a simple adjustment of the neutral resistor.
In a particular embodiment, the MV / LV transformer is an oil-filled transformer.
More particularly, the compensating means can be integrated into the oil bath of the transformer.
Also in a particular embodiment, the current compensation means is a zero-sequence inductor.
More particularly, the characteristics of the inductor can be selected for on the one hand, ensure the compensation of the homopolar current of a portion of the MV network to which is connected the post and, on the other hand, does not disturb the operation homopolar protections located in the substation that feeds the MT network.
The inductance can be single- and connected between the neutral point formed by the MV coils of the transformer and the earthing of the position.
More particularly, the single-phase inductance may consist of a solenoid wrapped with a magnetic shield.
Also in a particular embodiment, the coupling of the MV coils of the transformer is . of the zig-zag.

A tertiary winding triangle coupling stabilization may further be added to the transformer.
Also in a particular embodiment, the transformer has a container and comprises, on a lateral face of said tank.a first cabinet in which there are connection terminals to the MV network authorizing a connection diagram in framing transient or antenna and a second cabinet comprising a LV distribution permitting the discharge of energy from several LV cables.
The transformer may further include a protective device associating three MV fuses with striker, a three-phase disconnector device, and a fault current sensing device between phase and earth of the active part of the transformer.
More particularly, the mass of the active part of the inductance can be isolated from the envelope of the post and electrically connected to the mass of the active part of the transformer.
The invention also relates to an electric network medium voltage, comprising at least one processing station as described above, and at least one processing station not comprising compensation means of the homopolar capacitive current will now be described by way of non limiting example, a particular embodiment of the invention, with reference to the accompanying diagrammatic drawings in which: -the figure 1 is a circuit diagram of an electrical distribution network average phase voltage of the prior art;
The Figure 2 illustrates the currents in the network of figure 1 ;
The Figure 3 is a diagram of a network portion MV including a transformer station according to the invention;
The Figure 4 illustrates the currents in the network of Figure 3 ;
The Figure 5 is a diagram of a transformer comprising the means of the invention;
The figures 6 and 7 are respectively a diagram and a perspective view of a transformer according to the invention.
The Figure 3 shows a network section MT 12, partially formed or integrally an underground cable. This section 12 is supplied by a source station A spotted 1 in

normal operating conditions. The capacitive earth fault current corresponding to this line section is compensated by the Petersen coil located in the source station A. However, due to unavailability or a damage to the line between the source position 1 and the section 12, it may be necessary to change the power source. By opening the switch 14 and closing switch 15, the section is fed back from the source station B. With a prior solution to that corresponding to the present invention, it was necessary to modify the Petersen coils agreement belonging respectively to the positions A and B sources, the capacitive earth fault current seen by the station A declined, the one seen by the station B increased. But in the case of not equipped with an automatic tuning system posts, it is necessary to do this manually, which can be restrictive for the operator in case of frequent network reconfiguration. With the solution corresponding to the present invention, the agreement of Petersen coils need not be changed, no human intervention is required.
Finally, if the structure of the MV network is suitable, it is even possible to replace the Petersen coil with a still much less expensive inductor that provides a lump sum compensation of the permanent part of the network supplied by the source terminal. Only sections likely to switch power source are then equipped with MV / LV incorporating a specific compensation device. It follows for the operator significant investment reduction.
Multiplying points grounded neutral MV network can lead to a loss of sensitivity of the earth-fault 5 that, each team starting line 4 at the substation 1. As shown Figure 4 , Part of the current due to default and initially closed through the substation will now be diverted to the compensating device 17 fitted to one or more transformer stations MV / LV 16, and therefore will not be seen by the protection homopolar 5.
Nevertheless, the case study of practical application shows that when it is necessary, it is easy to maintain the level of protection of the sensitivity simply by changing the value of the resistor 9. In the above example of a voltage distribution network 10 kV, comprising a total length of 100 km of MV cable, implying that the Petersen coil is set to a substantially value close to 150 a, such a resistor is usually dimensioned to provide a current IR equal to 5 a for failure franc between a phase of the network and the earth. Under the same conditions, the 17 compensation devices integrated in the processing units 16 each provide an

inductive current IL "15A and a resistive current IR" of the order of 0.2 A, due to ohmic losses in the device . By imagining a situation where the relevant network is extended with a total length of 100 km of cables, comprising about 100 MV / LV substations including 10 positions including a compensation device 17, the sensitivity of the protections will be maintained at a satisfactory level by dividing 2 simply by the value of the resistor 9. Now it is easy to make such a change, which corresponds to a very limited investment.
Will now be described the embodiment of processing MV / LV stations equipped with a compensation device of the capacitive earth fault current of the network. A transformer station includes among its components an MV / LV transformer, usually oil bath. The Figure 5 shows the electrical diagram of the transformer equipping the object position of the invention. The transformer 18 includes primary windings or MT 19 and secondary windings or BT 20. As a three-phase transformer, the windings.of the three phases can be connected in triangle, star or zigzag. The coupling zigzag is particularly interesting here because it offers a very low zero impedance and therefore can be used to create a neutral point. According to a preferred embodiment, the transformer station object of the invention therefore comprise a three phase transformer whose primary windings 19 are connected in a zigzag coupling. Can then be inserted between the neutral point 21 resulting in the transformer and the earth of the station a single-phase inductor 17 of selected characteristics that compensate a certain value of the capacitive earth fault current of the network for example by giving a reactive current of 15 A under full zero sequence voltage.
This inductance can be isolated in mineral oil, which has the advantage of allowing a reduction of its size and also to be inserted directly into the transformer tank. According to a preferred embodiment, it consists of a solenoid formed from the coil of an insulated conductor wire.
To protect the environment of the inductance against the effects of the magnetic field generated when it is crossed by a current, a magnetic shield is arranged around the solenoid so as to allow the lines of the field to close a controlled manner.

The insertion of an inductance between the transformer neutral and earth still requires certain precautions, especially when the MV / LV station is likely to fuel an unbalanced load BT, which is usually the case in rural areas. This imbalance is reflected on the Figure 5 by a current flowing in the neutral BT equal to three times the zero sequence current Io. Indeed, this sequence current BT could result in a zero sequence current MV side, through inductance compensation 17, would create an undesirable sequence voltage. To avoid this drawback, according to another preferred embodiment, the transformer 18 is provided with tertiary windings 22, said stabilization windings which are connected in delta. In this way, the zero sequence current Io BT will be offset by a current flow in the delta stabilizing windings 22 without influencing the MV circuits.
A transformer station generally consists of an envelope, which contains various components, the main ones being a connecting apparatus and operating MT, a MV / LV transformer, a distribution table BT. According to a preferred embodiment, presented on figures 6 and 7 , The position of object of the invention processing does not include MV switchgear and consists essentially of the transformer, which comprises the greater side face of the tank 23, a first cabinet 24 wherein the network terminals are located MT 25, and a second cabinet 26 including the distribution of LV 27. the result is a set of highly compact, fully monolithic constitution, a reduced cost compared to conventional positions and enabling easy implementation .
The connection to the network can be done in a pattern antenna, the transformer having three crossings MT 25. But it can also be done in a pattern or passage called cutoff. In this case, the transformer is equipped with six bushings 25 MT associated in pairs.
The direct connection of the transformer to the MT network involves certain provisions against the effects of internal faults transformer. Indeed, the MV supply network being particularly intensive arcing occurring in the oil bath of the transformer may cause thermal and mechanical effects resulting in hazardous external events to the neighborhood of the position. It is therefore intended that the transformer integrates a protective device and cut as described in the Patent EP 0817346 and EP 1122848 filed by the Applicant. This device combines fuses MT 28 29 hammers fitted with a three-phase disconnection device MT 30, and a

device for detecting a fault current to earth 31. To integrate the inductance compensation in the protective device , the mass thereof, consisting of the magnetic shield which surrounds it, is isolated from the general earth of the station and is connected through an electrical connection 32 with the mass of the active part of the transformer to a detection device fault current to ground 31. Moreover, the electrical characteristic of the fuse 28 is selected so that they are insensitive to the current which crosses them when different faults between phase and earth to appear on the network.
Finally, such a transformer station, as shown in Figure 7 Can be either equipped or not an inductance compensation 17, according to the needs of network evolution project. Whatever the version, the vessel 23, cabinets 24 and 26, the equipment 25 and 27, the protective device and internal disconnection consisting of fuses 28, firing pin 29, the disconnector 30 and the'fault current detecting device to ground 31 remain common. The transformer 18 includes primary windings then MT 19 coupled in a triangle, the secondary windings in star BT 20, and does not include stabilization windings 22.

1. MV/LV electrical transforming station comprising a MV/LV transformer (18), characterized by the fact that it comprises connected between the neutral point and the MV coils of the transformer and the earth of the station, means (17) of compensating the homopolar capacitive current generated by a portion of the MV network of a set length to which said transformer may be connected, in the event of a fault between a phase and the earth located on this network.
2. Transforming station of claim 1, in which the MV/LV transformer is an oil-immersed transformer.
3. Transforming station of claim 2, in which the compensation means are integrated into the oil bath of the transformer.
4. Transforming station of any of claims 1 to 3, in which the compensation means of the homopolar current, is an inductance.
5. Transforming station of claim 4, in which the features of the inductance are selected so that they compensate the homopolar current of a portion of the MV network to which the station is connected, and so as not to disrupt the operation of the homopolar protections located on the source station which supplies the MV network. c-fr/\„ ". .

6. Transforming station of any of claims 4 and 5, in which the inductance is single phased and is connected between the neutral point formed by the MV coils of the transformer and the earth connection of the station.
7. Transforming station of claim 6, in which the single phased inductance is formed by a solenoid surrounded by magnetic shielding (earth of the single phased inductance).
8. Transforming station of claim 7, characterized by the fact that the coupling of the MV coils of the transformer is of the zig zag type.
9.Medium/low voltage transformer station in Substation area system herein substantially described with reference to and as illustrated through accompanying drawing. These drawing are for illustrative purpose only not intended to limit the scope of the invention.