FIELD OF THE INVENTION

The present invention relates to apparatus for protecting a medium voltage distribution transformer (MVDT) and a three-phase medium voltage distribution line from a phase to phase and/or single phase failure in the MVDT. BACKGROUND OF THE INVENTION

The need for improving the performance of the three-phase medium voltage (1-45 kV) distribution network has increased rapidly over the last few years. This has been mainly due to the increased use and sophistication of electrical and electronic equipment in every part of human activity.

In existing ungrounded or in the Petersen coil grounded medium voltage distribution networks the identification of single phase earth fault {i.e. detection of zero sequence currents) is particularly problematic since the zero sequence current is very low (often lower than the load current of the distribution transformer. Therefore, detecting these currents (and hence faults) is very difficult requiring specialist equipment.

One known system is the use of a zero sequence relay of an upstream breaker. This breaker is usually the main feeder breaker located in the high voltage/medium voltage power substation. The tripping operation of this breaker caused by such faults results in disconnection of at! the distribution transformers connected within the distribution line for that particular breaker and hence penalizes all the customers connected to the particular distribution line with a black-out till the failed distribution transformer is identified and disconnected from the network. This is undesirable.

Since more than 96% of the distribution transformer failures are caused by such earth faults, there is a much felt need for finding a cost-effective and practical solution to this problem. SUMMARY OF THE INVENTION

The present invention seeks to provide an effective technique for protecting a MVDT and a three-phase medium voltage distribution line from single phase earth fault in the MVDT.

This is achieved according to a first aspect of the present invention by apparatus for protecting a medium voltage distribution transformer (MVDT) and a three-phase medium voltage distribution line from a phase to phase fault and/or single phase earth fault in the MVDT, the MVDT being connected to the three-phase medium voltage distribution line, for supplying power to the MVDT, the apparatus comprising: a protection arrangement for protecting the MVDT and the three-phase medium voltage distribution line from the phase to phase faults in that the apparatus further comprises: a zero sequence three phase current interrupting device connected in series between outputs of the protection arrangement and the MVDT; a zero sequence sensor for detecting a zero sequence current in the three-phase medium voltage distribution line upon occurrence of a single phase earth fault in the MVDT; a triggering mechanism for triggering the opening of the zero sequence three phase current interrupting device if a zero sequence current is detected by the zero sequence sensor, disconnecting the MVDT from the three-phase medium voltage distribution line.

The apparatus may be located inside a container accommodating the MVDT or alternatively, it may be located inside a separate container using a dielectric, the dielectric comprising one of mineral or silicone oil, esters, SF6 and solid dielectric material. Further the zero sequence sensors, the triggering mechanism and the zero sequence three phase current interrupting device may be located in a separate container to that accommodating the protection arrangement.

All electrical medium voltage and low voltage interconnections between the container accommodating the apparatus and the container accommodating the MVDT are made via through bushings.

The triggering mechanism may further trigger the opening of the zero sequence three phase current interrupting device on the basis of an input signal of at least one of: a heavily unbalanced low voltage load sensor; an overload sensor; and a low voltage overcurrent sensor. The triggering mechanism may further trigger the opening of the three phase zero sequence current interrupting device on the basis of an input signal of at least one of: a pressure sensor; a temperature sensor; a dielectric level sensor; and a light sensor.

The protection arrangement may comprise three fuses, each of said fuses being connected within each phase of said three-phase medium voltage distribution line, such as for example, a full range High Rupturing Capacity (HRC) type or a partial range HRC type.

The protection arrangement may comprise medium voltage switchgear. It may further comprise a medium voltage breaker electrically connected in series or in parallel across the terminals of at least part of the medium voltage switchgear, and the breaker may also be electrically connected in series with the medium voltage electrical distribution line, wherein the medium voltage breaker may be interlocked with the medium voltage switchgear manually and/or electrically and/or automatically.

The medium voltage switchgear may comprise at least one load break-fault-make and grounding switch and/or a three-position switch.

This is achieved according to another aspect of the present invention by a

transformation substation comprising a MVDT; and apparatus according to the first aspect above. The zero sequence three phase current interrupting device, the zero sequence sensor and the triggering mechanism may be located in a container separate from the containers accommodating the MVDT and/or the protection arrangement. Alternatively, the apparatus may be located inside a container accommodating the MVDT. The apparatus may be located inside a container using a dielectric, the dielectric comprising one of mineral or silicone oil, esters, SF6 and solid dielectric material, the container being separate of a container accommodating the MVDT.

All electrical medium voltage and low voltage interconnections between the container accommodating the apparatus and the container accommodating the MVDT may be made via through bushings. The container housing said apparatus may be connected through at least one first pressure vent to a first sub-container. The transformation substation may further comprise a second sub-container connected through at least one second pressure vent to the first sub-container, the first and second sub-containers being filled with oxygen- free gas or air, the second sub-container comprising a cooling labyrinth and a third pressure vent, to cool the oxygen-free gas or air between the at least one second pressure vent and the third pressure vent.

The first and/or second sub-containers may be filled with nitrogen.

The invented solution is to provide in addition to the conventional H C fuses or the breakers, which are required for the phase to phase faults (over current protection), a protection assembly which can identify the zero sequence current and trigger the opening of a zero sequence current interrupter (at a competitive cost) and thus isolating the damaged transformer instantly without causing the tripping of the main feeder upstream breaker of the distribution line.

Further the triggering of the operation of this zero sequence current interrupter can be activated in case of transformer overload or low voltage short circuits or heavily unbalanced loads and in case of increased pressure or temperature or loss of dielectric fluid or arcing (light sensors) in the transformer tank, preventing severe damage or explosion or fire of the transformer.

By locating the zero sequence current interrupter in series and after the fuses or the breaker the protection of this zero sequence current interrupting device by the fuses or the breaker is accomplished in case there will be a requirement for interrupting currents higher then the interrupting capacity of this zero sequence current interrupter.

By using partial range HRC fuses, the low current/overload protection element of the HRC fuses is removed and therefore the main cause of the erratic and spurious operations of the HRC fuse will be eliminated.

BRIEF DESCRIPTION OF DRAWINGS For a more complete understanding of the present invention, reference is made to the following description in conjunction with the accompanying drawings, in which: Figure 1 is a simple schematic of apparatus according to a first embodiment of the present invention;

Figure 2 is a simple schematic of apparatus according to a second embodiment of the present invention;

Figure 3 is a simple schematic of apparatus according to a third embodiment of the present invention; Figure 4 is a simple schematic of apparatus according to a fourth embodiment of the present invention;

Figure 5 is a simple schematic of apparatus according to a fifth embodiment of the present invention;

Figure 6 is a simple schematic of apparatus according to a sixth embodiment of the present invention;

Figure 7 is a simple schematic of apparatus according to a seventh embodiment of the present invention;

Figure 8 is a simple schematic of apparatus according to an eighth embodiment of the present invention; Figure 9 is a simple schematic of apparatus according to a ninth embodiment of the present invention; and

Figure 10 is a simple schematic of apparatus according to a tenth embodiment of the present invention;

Figure 11 is a simple schematic of apparatus according to an eleventh embodiment of the present invention;

Figure 12 is a simple schematic of an existing MVDT cable network installation using conventional Ring Main Units; Figure 13 is a simple schematic of a VDT cable network installation incorporating apparatus of an embodiment of the present invention; and

Figure 14 is a simple schematic of apparatus of a further embodiment of the present invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

With reference to Figure 1 , a first embodiment of the present invention will be described in detail below.

The apparatus 100 comprises a zero sequence interrupting device 105. A first three phase terminal of the zero sequence interrupting device 105 is connected to three- phase input terminals of a HVDT 106. A second three phase terminal of the zero sequence interrupting device 105 is connected to an output of a protection

arrangement 108. The protection arrangement 108 comprises a three-phase medium voltage breaker 112 connected in series with a three-phase load break fault make switch 110 and three fuses 102 (for simplicity only one fuse within one phase is shown in the figure). Each of the fuses 102 are connected within each of the phases of a three-phase distribution line. As a result the three-phase zero sequence current interrupting device 105 is connected in series between the output of the protection arrangement 108 and the MVDT 106.

The fuses 102 maybe of a full range High Rupturing Capacity (HRC) type or a partial range HRC type.

The zero sequence interrupting device 05 is connected to a triggering mechanism 104. The triggering mechanism 104 is connected to the output of a plurality of medium voltage zero sequence current sensors 103. The medium voltage zero sequence current sensors 103 are located within each phase of the three-phase distribution line between the zero sequence interrupting device 05 and the MVDT 06. The control output of the triggering mechanism 104 is connected to the control input of the zero sequence interrupting device 05. The protection arrangement 108 protects the MVDT 106 and the three-phase medium voltage distribution line from phase to phase failure (i.e. overcurrents). Upon occurrence of a phase to phase failure either the breaker 112, load break fault make switch 110 and/or the fuses 102 will be opened. Any one of these items being reset/replaced as required when the cause of the failure has been repaired. In the event of occurrence of a single phase failure, this is undetected by the protection arrangement 108 and the protection arrangement 108 continues to operate as normal. However, the zero sequence current in the three-phase medium voltage distribution line is detected by the plurality of medium voltage zero sequence current sensors 103. This is output to the triggering mechanism 104 whereupon the interrupting device 105 is triggered to open thus protecting the MVDT and the three-phase medium voltage distribution line from the single phase failure by disconnecting the MVDT from the three-phase medium voltage distribution line.

An alternative embodiment of the apparatus is shown in Figure 2 in which the protection arrangement 108 comprises a three-phase load break fault make switch 110 connected to three fuses 102.

The apparatus 100 may be placed within two separate containers C and C" containing a dielectric such as, for example mineral or silicone oil, esters, SF6 or a solid dielectric material. The first container C" containing the protection arrangement 108 and the second container C containing the interrupting device 105, triggering mechanism 104 and the sensors 103. The MVDT may be contained in a further separate container C as illustrated in Figure 2. A further embodiment is illustrated in Figure 3. The protection arrangement 108 is housed in the same container as the interrupting device 105, triggering mechanism 104 and the sensors 103 and the MVDT 106. In this embodiment, the protection assembly 108 comprises three fuses 102, each fuse connected within each phase of the three- phase medium voltage distribution line. The triggering mechanism 104 is also connected to the outputs of a plurality of low voltage zero sequence current sensors 03' and a plurality of sensors 107. These sensors 107 may include, for example, a heavily unbalanced low voltage load sensor, an overload sensor; a low voltage overcurrent sensor, a pressure sensor, a temperature sensor, a dielectric level sensor and a light sensor. In operation, the triggering mechanism 104 also triggers the interrupting device in response to abnormal conditions detected by any one of these sensors. This provides a competitive and compact solution by locating the transformer protection apparatus in a transformer tank.

Alternatively, as shown in Figure 4, the arrangement of Figure 3 may be arranged such that the protection arrangement 108, the interrupting device 105, the triggering mechanism 104 and the plurality of medium voltage zero sequence current sensors 103 are contained in a first container 401 and the MVDT 106 and the sensors 107 are contained in a second container 403. The plurality of low voltage zero sequence current sensors 103' are located outside of the containers 401 , 403. Alternatively, these may be located in the second container 103 as illustrated in Figure 5. The plurality of sensors 107 and low voltage zero sequence current sensors 103' are connected to the triggering mechanism 04 via a junction box 109 and bushings 407. The junction box 109 may be located outside of the containers 401 , 403 or located inside the second container 403 as shown in Figure 11.

A further embodiment is shown in Figure 5, in which a toroidal type of zero sequence sensor 103 is capturing the zero sequence current signal and feeding it to the triggering mechanism 104 for further processing and activation of the interrupting device 105 in case of occurrence of a zero sequence current. The number of individual phase windings is calibrated to match the sensitivity level of the zero sequence current required for the distribution system. in a further embodiment shown In Figure 6, the protection arrangement 108 comprises a first terminal 601 and a second terminal 602 connected in the medium voltage distribution line, as disclosed in more detail in PCT/EP2009/055845 . Each terminal 601 , 603 are connected to a terminal of a grounding switch 605, 607. The protection arrangement 108 further comprises a breaker 609 connected across the other terminals of each of the grounding switch 605, 607 and across two terminal of a three- position switch 61 1. The switching terminal of the three position switch is connected to the fuses 102.

In an alternative embodiment illustrated in Figure 7, the protection arrangement 108 comprises two load-break fault-make switches 701 , 703 having a terminal connected to the terminal 601 , 603. The other terminal of the load-break fault-make switches 701 , 703 is connected to a terminal of a switch 705. The other terminal of the switch 705 is connected to the fuses 102. in an embodiment, shown in Figure 8, the protection arrangement 108 of Figure 7 is installed within a container G. The zero sequence interrupting device 105, the medium voltage zero sequence current sensors 103, the triggering mechanism 104, the MVDT 106 and low voltage zero sequence current sensors 103, are contained within the same container G. The container G is located on the top of a sub-container G' and communicates with G through one or more pressure relief valves V located between the containers G, G' and adapted to satisfy the usual internal arc test requirements. As disclosed in more detail in co-pending PCT application PCT/EP2009/055845. A second sub-container G" is located in communication with the first sub-container G' . The second sub-container G" comprises a labyrinth as disclosed in more detail in copending PCT application PCT/EP2009/055845. Figure 9 illustrates a variation of the embodiment of Figure 8 in that the MVDT 106 and low voltage zero sequence current sensors 103' are housed in a separate container H. Figure 10 illustrates another variation of the embodiment of Figure 9 in that the first sub container G' is omitted. A further embodiment is shown in Figure 11 in which the first container 401 co-joins the second container 403 and the first three phase terminals of a zero sequence interrupting device 105 are connected to the input terminals of the MVDT 106 via the through bushing 501 between the first and second containers 401 , 403 and the input of the triggering mechanism 104 is connected to the junction box 109 via a second through bushing 503 between the first and second containers 401, 403. A sub container G" is located in communication with the first container 401 via the pressure relief valve such that internal arc test requirements are met.

For improving the explanation of the embodiments of Figures 13 and 14, description is provided of an existing MVDT cable network installation using conventional Ring Main Units (RMU) a shown in Figure 12. Conventionally the input of the unit 1200 is connected to a first load break fault make ground switch 703 and the output 603 of the unit 1200 is connected to a second load break fault make ground switch 701. The first and second load break fault make ground switches 703,701 are connected to a third load break fault make ground switch 705 connected the input terminals of a MVDT (not shown here) via 3 fuses 102 in each phase. However, as outlined above this arrangement provides no protection against earth faults.

Therefore, electrical utilities considering to eliminate the use of fuses and in particular implementing the Peterson coil grounding can modify existing Ring-Main-Units by replacing the existing combination of fuses102 and the load break fault make switch 705 in Fig, 12 with a protection arrangement 108 comprising a breaker 609 connected across the first and second load break fault make switches 703,701, which now are used as off-load and off-voltage simple disconnect and grounding switches thus making them practically maintenance free, as shown in Figure 13 and 14. The breaker 609 is also connected across the terminals of the interrupting device 05. The zero sequence interrupting device 105 comprises a 3-position switch.

Operation of the apparatus 100 will now be described with reference to figures 1 to 7. A medium voltage is transferred via the medium voltage distribution line to the protection arrangement 108. In normal operation, the zero sequence interrupting device 105 is in its closed position and the medium voltage input on the input terminals 101 is fed to the input terminals of the MVDT where it is converted to a low voltage and output via output terminals of the MVDT 106 to a conventional distribution panel for distribution to consumers.

In the event of a fault, the protection arrangement 08 protects the MVDT from phase to phase faults, such as overcurrents. The medium voltage zero sequence current sensors 103 and the low voltage zero sequence current sensors 103' detect any zero sequence current in the three-phase medium voltage distribution line upon occurrence of a single phase failure. This is output to the zero sequence current interrupter triggering mechanism 104 which triggers the zero sequence interrupting device 105 into is open position disconnecting the MVDT from the three-phase medium voltage distribution line. The zero sequence current interrupter triggering mechanism 104 is also triggered by abnormal outputs of the plurality of sensors 107.

Although embodiments of the present invention have been illustrated in the

accompanying drawings and described in the foregoing detailed description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous modifications without departing from the scope of the invention as set out in the following claims.