Abstract:
A test vessel assembly comprises a central test vessel defining a chamber in which a sample to be tested may be stored. A pair of side adjustable electrodes is received in the chamber and immersed in the sample under test to determine the breakdown voltage of the sample. A gap between the electrodes can be adjusted by respective electrode adjusting moved in and out of the test vessel by rotation of an associated adjusting wheel. To prevent the breakdown in air rather than in the sample, care is taken to ensure sufficiently large creepage and clearance distances between the connections to the electrodes and from the connections to the electrodes to a wall of the test chamber. To this end, the adjusting wheels include convolutions that mesh with corresponding convolutions in the test vessel.

Description:
This application claims priority to British Patent Application 1008285.7, filed May 18, 2010, which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to high voltage liquid dielectric test vessels, and in particular such vessels having side adjustable electrodes. 
     BACKGROUND TO THE INVENTION 
     Oil is frequently used as an insulator in high voltage equipment, such as transformers. Over time, the insulating properties of the oil degenerate due to contamination by water, dust, or other products emanating from other materials within the transformer. It is known to test the insulating properties of this oil in an instrument called an oil test set. A sample of oil to be tested is contained within a test vessel within the oil test set. In such a test vessel, a steadily increasing voltage is applied across the sample of the oil until a spark flashes through the oil as a result of it breaking down electrically. The voltage at which this occurs is known as the breakdown voltage. 
     In some designs, the test vessel has a pair of electrodes mounted at the sides of the vessel in an adjustable manner so that a gap between the electrodes can be adjusted. In alternative designs, with the electrodes depending downwardly from a lid, for example, the electrodes are removed together with the lid when it is removed to replace the oil sample in the test vessel. Since the electrodes have been immersed in the oil sample, they are prone to dripping when removed. 
     Care must be taken in designing oil test vessels, because the breakdown voltage of air is considerably less than that of the oil under test. As a result, if there is insufficient insulation between the electrodes and the air surrounding the test vessel as compared to the gap between the electrodes in the oil, then the spark will not flash across the oil as desired, but will discharge through the atmosphere. This would lead to erroneous test results. 
     SUMMARY OF THE INVENTION 
     According to the invention, there is provided a high voltage liquid dielectric test vessel assembly, comprising:
         a vessel having a sidewall defining a chamber for receiving a liquid dielectric under test;   a first electrode assembly including a first electrode disposed within the chamber; and   a second electrode assembly including a second electrode disposed within the chamber;   wherein the first electrode assembly comprises a first adjustment mechanism for moving the first electrode relative to the second electrode for adjusting a gap therebetween, the first adjustment mechanism comprising:
 
a first conductive shaft extending through the sidewall and having a distal end attached to the first electrode; and
   a first adjusting wheel interengaged with the first shaft such that rotation of the first wheel is translated into axial reciprocation of the first shaft;   wherein the first adjusting wheel is mounted to the vessel, with convolutions in the first adjusting wheel meshing with convolutions in the vessel.       

     The arrangement of the adjusting wheel, particularly the meshing of its convolutions with those of the vessel, is such that the air path from the conductive shaft to the periphery of the vessel is significantly increased, resulting in a much reduced likelihood of an inadvertent discharge through the air. This means that the test vessel assembly can be reduced in size whilst retaining protection against discharge through the air. Additionally or alternatively, the test voltage may be increased. The arrangement is such that the gap between the electrodes may be adjusted with great accuracy yet mitigating against breakdown in air. 
     Typically, the test vessel assembly further comprises a first horn cover connected to the test vessel and shrouding the proximal end of the first conductive shaft, which proximal end is adapted to be conductively connected to a first electrical contact of an insulated horn within a test assembly to which the test vessel is mounted, in use, the first horn cover including convolutions that mesh with convolutions of the first adjusting wheel. 
     Preferably, the second electrode assembly comprises a second adjustment mechanism for moving the second electrode relative to the first electrode for adjusting the gap therebetween, the second adjustment mechanism comprising:
         a second conductive shaft extending through the sidewall at a point substantially opposite the first conductive shaft and having a distal end attached to the second electrode; and   a second adjusting wheel interengaged with the second shaft such that rotation of the second wheel is translated into axial reciprocation of the second shaft relative to the vessel;   wherein the second adjusting wheel is mounted to the vessel, with convolutions in the second adjusting wheel meshing with convolutions in the vessel.       

     Although it is preferable for each of the electrode assemblies to include a respective adjustment mechanism, it will be understood that one of the electrodes may instead be fixed. 
     Where the test vessel assembly includes a second adjustment mechanism, a second horn cover may be connected to the test vessel and shroud the proximal end of the second conductive shaft, which proximal end is adapted to be conductively connected to a second electrical contact of an insulated horn within a test assembly to which the test vessel is mounted, in use, the second horn cover including convolutions that mesh with convolutions of the second adjusting wheel. So, as with the meshing convolutions of the first adjustment mechanism, the meshing convolutions of the second adjustment mechanism are such that the air path from the second conductive shaft to the periphery of the vessel is significantly increased. 
     Optionally, one or both adjustment mechanisms may include a resilient member mounted to the proximal end of the respective shaft for urging the shaft, ergo the associated electrode, towards the other electrode. This arrangement mitigates against backlash that might be present between the adjusting wheel and the associated shaft. 
     Where there is a resilient member, this may comprise a conductive spring, in which case the assembly may further comprise a conductive end cap connected to the proximal end of the associated shaft and encasing the spring. The conductive end cap helps to prevent corona discharge from the thin wire diameter of the spring by enclosing it in metal of the same voltage. 
     The test vessel may further comprise a pin extending transversely through one or each shaft, the pin received in an associated groove in the vessel so as to prevent rotation of the associated shaft whilst allowing the required axial reciprocation. In order to set the electrode gap, a feeler gauge may be inserted between the first and second electrodes. By preventing rotation of the shaft, scuffing of the electrodes by the feeler gauge is reduced. 
     An electrode may be removably attached to the distal end of the associated shaft. The provision of easily removable electrodes facilitates maintenance of the test vessel assembly as a whole and replacement of old electrodes, and fitting of different electrode shapes for different standards. Also, where the rotation of the shafts is prevented by the pin and groove arrangement, replacement of each electrode is facilitated because it can simply be unscrewed from the respective shaft without having to otherwise clamp or hold the shaft. 
     An adjustment mechanism may further comprise a lock for releasably preventing rotation of the adjusting wheel, thereby preventing reciprocal movement of the associated shaft. In this manner, once a desired electrode gap has been set by relative reciprocal movement of an electrode, the gap can be fixed. 
     An adjusting wheel may be concentrically arranged about the respective shaft. Whereas other arrangements are possible, the concentricity will help ensure that the air path to the periphery of the vessel is consistent about the circumference of the shaft. 
     An adjusting wheel may be rotatably free, yet axially fixed to the associated shaft, and mounted by a threaded interconnection to the vessel. Alternatively, an adjusting wheel may be rotatably free, yet axially fixed to the vessel, and mounted by a threaded interconnection to the associated shaft. In the first arrangement, the adjusting wheel moves axially relative to the vessel as it is rotated. Because the wheel is axially fixed in this embodiment to the shaft, the shaft and wheel move together in the axial direction (although the wheel may rotate relative to the shaft). In the second arrangement, the shaft moves axially relative to the adjusting wheel as the adjusting wheel is rotated. Because the wheel is axially fixed to the vessel in this embodiment, the shaft moves axially relative to the vessel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view from above of a test vessel embodying an aspect of the invention; 
         FIG. 2  is a front side elevation view of the test vessel of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view through the test vessel of  FIG. 2 ; and 
         FIG. 4  is an end side elevation view of the test vessel of  FIG. 1 , in which a horn cover is removed to show a latch detail. 
     
    
    
     DETAILED DESCRIPTION 
     By reference in particular to  FIGS. 1 to 3 , a test vessel assembly  10  comprises a central vessel  12  having a sidewall  14  that, together with a base  16 , defines an internal chamber  18  for receiving a sample for testing. The vessel  12  has a substantially elliptical, or oval, plan profile, which provides compressive strength. A removable lid  20  rests on a horizontal shelf  21  of a rim  22  at the top of the vessel sidewall  14 . The rim includes an oval riser  23 . The lid includes a handle  24 , a flat bottom portion  25  and a lip  26 . The flat bottom  25  lies on top of the horizontal shelf  21  of the vessel, and the lip  26  sits within the oval riser  23  of the vessel. 
     In use, the chamber  18  is filled with a sample to be tested, such as oil, or another dielectric liquid, for example (not shown). To aid the filling process, the vessel may be made of a transparent or translucent material. A fill line  27  provides a visual indication of a desirable sample level. The vessel rim  22  includes a spout  28 , which can be used for emptying the vessel after use. 
     First and second electrode assemblies are arranged on opposite sides of the vessel  12 . Since the second assembly is a mirror image of the first assembly, the following description of the first assembly applies  mutatis mutandis  to the second assembly, which will not be described individually. Where the description requires the common respective parts to be distinguished, the reference for the part of the second assembly will be that of the first assembly but suffixed with ‘a’. 
     The first electrode assembly comprises a first electrode  30  disposed within the chamber  18  and removably attached to a distal end of a shaft  32 , for example by a threaded connection. The shaft  32  extends through the vessel sidewall  14  in a horizontal orientation, entering at a point substantially mid-way up the vessel  12  and in about the centre of the flatter side of the oval sidewall  14 . 
     A reinforcing boss  34  projecting outwardly from the sidewall  14  supports the shaft  32  within a bore  36 . The shaft includes annular grooves  38 ,  39  containing O-rings  40 ,  41  that maintain a seal to prevent the sample within the chamber  18  from leaking through the bore  36 . 
     A proximal end of the shaft  32  projects outwardly from the boss  34 . A metal coil spring  42  is mounted on the proximal end of the shaft for a purpose to be described below. The spring  42  is encased within a conductive end cap  44 . 
     The first electrode assembly further comprises a horn cover  46 . The horn cover  46  comprises a body portion  48  that defines a central cavity  50  having an opening  52  at a bottom end. A flange portion  54  protrudes from an upper end of the body portion  48  for removable attachment of the horn cover  46  to the reinforcing boss  34  via fasteners  56 . 
     An adjusting wheel  60  is located between the vessel  12  and the horn cover  46 . As best seen in  FIGS. 3 and 4 , the adjusting wheel  60  is arranged concentrically about the shaft  32  and sandwiched between the vessel boss  34  and the flange portion  54  of the horn cover  46 . The wheel  60  comprises a central hub  62  mounted in an annular groove  80  on the shaft that allows relative rotation yet prevents relative axial movement between the wheel and the shaft. A further O-ring  81  is included in a deeper portion  82  of the groove  80 . 
     The adjusting wheel  60  has a convoluted profile, comprising a series of peaks  63  and troughs  65  that extend in a serpentine manner out from the central hub  62  to a knurled outer circumferential surface  66 . 
     The vessel boss  34  has a concentric series of annular ribs  70  that project out from the vessel sidewall  14  towards the horn cover  46 . The horn cover  46  has a concentric series of annular ribs  72  projecting towards the vessel  12  from the flange portion  54  of the horn cover  46 . The ribs  72  of the horn cover  46  nest within annular spaces between the ribs  70  of the vessel, whilst the ribs  70  of the vessel nest within annular spaces between the ribs  72  of the horn cover  46 . The interdigitating ribs  70 ,  72  together define a serpentine space. The serpentine peaks and troughs  63 ,  65  of the wheel  60  fit within the serpentine space defined by the interdigitating ribs  70 ,  72 . In other words, the adjusting wheel  60  has a convoluted profile that meshes with corresponding convoluted profiles in each of the reinforcing boss  34  of the vessel  12  and the flange portion  54  of the horn cover  46 . 
     The adjusting wheel  60  is mounted to the vessel  12  by a threaded connection between a female thread on the wheel and a male thread on a stud  90  forming part of the boss  34  and projecting from the centre of the sidewall  14 . Accordingly, the adjusting wheel  60  moves axially relative to the vessel  12  as it is rotated. Because the wheel  60  is axially fixed to the shaft  32 , the shaft and wheel move together in the axial direction (although the wheel may rotate relative to the shaft). In this manner, rotation of the wheel  60  results in a translation of the first electrode  30  and therefore an adjustment of the electrode gap. A stud  90   a  is included in the second electrode assembly for the same purpose. 
     Instead of the wheel  60  being threaded to the vessel  12  via the post  90  and being rotatably free yet axially fixed on the shaft  32 , the wheel  60  may be rotatably free yet axially fixed to the vessel  12  and mounted by a threaded interconnection to the shaft  32 . In this alternative arrangement, the shaft  32  moves axially relative to the adjusting wheel  60  as the adjusting wheel is rotated. Because the wheel  60  is axially fixed to the vessel  12  in this embodiment, the shaft  32  moves axially relative to the vessel  12 , again resulting in a translation of the first electrode  30  and therefore an adjustment of the electrode gap. 
     The O-ring  81  also allows for a little articulation between the shaft  32  and the stud  90 , affording smoother adjustment of the electrode gap. The relatively large diameter of the adjusting wheel  60  enables accurate electrode adjustment. 
     The springs  42 ,  42   a  pre-load the adjusting assemblies to avoid backlash while adjusting the electrode gap 
     The shelf  21  of the vessel rim  22  is wider in the direction of the electrodes  30 ,  30   a  in order to stiffen up the assembly, maintaining a more constant electrode gap. Further stiffening is also provided by the concentric ribs  70  that mesh with the adjusting wheel  60 . The lid  20  has been designed not to exert pressure in the direction of the electrodes  30 ,  30   a , so it should not distort the test vessel  12  and upset the electrode gap. 
     As best seen in  FIG. 4 , a lock mechanism  100  comprises a latch  102  pivotally mounted on a pivot post  104  that projects from the vessel sidewall  14  at a position outside the circumference of the adjusting wheel  60 . A tab  106  and an associated pawl  108  are located on a free end of the latch, opposite to the pivot post  104 . In a locked position, the pawl  108  resiliently engages a latch post  110  and a middle portion  112  of the latch engages a part of the outer circumference  66  of the wheel  60 . Friction between the middle portion  112  of the latch  102  and the outer circumference of the wheel  60  resists rotation of the wheel and thus locks the shaft  32  and the associated electrode  30  in position. 
     A user may release the lock mechanism  100  by pushing down on the tab  106  which releases the pawl  108  from engagement with the latch post  110 . The latch  102  can thus be rotated (arrow A) out of engagement with the wheel  60  to an unlocked position. A protuberance  114  projecting from the horn cover  46  provides an end stop to the rotation of the latch  102 . 
     The skilled person will understand that other locking mechanisms could be employed to prevent rotation of the adjusting wheel  60  to lock the position of the electrode  30 . 
     The shaft  32  is prevented from rotating by means of a pin  120  mounted in a transverse hole  122  through the shaft. The pin  120  is longer than the shaft diameter, so that a portion of the pin  120  projects out of either side of the shaft  32 . The projecting portions of the pin  120  are located in a slotted groove  124  in the post  90  so as to prevent rotation of the shaft whilst allowing the required axial reciprocation. This arrangement helps in the case where the electrode  30  is removably attached to the shaft  32  by a threaded connection, because the electrode can be screwed on to or off of the shaft without having to clamp or otherwise hold the shaft. 
     In use, the test vessel assembly  10  would be placed in a test chamber having a pair of insulated horns that each shield a respective spring-loaded electrical contact at their upper end (not shown). The horn covers  46 ,  46   a , together with the vessel  12 , would be lowered into position over the horns with a snug fit. An electrical connection would be established between the contacts and the respective shafts  32 ,  32   a , possibly via the respective end covers  44 ,  44   a . The thickness and height of the horn covers  46 ,  46   a  has been selected to complement the air gap from the adjusting rings to a metal wall of the test chamber. 
     In use, the level of the sample under test would be sufficient to cover both of the electrodes  30 ,  30   a.    
     In order to set the electrode gap, a feeler gauge (not shown) may be inserted between the first and second electrodes  30 ,  30   a . An adjusting wheel  60 ,  60   a  would be rotated to move the associated shaft  32 ,  32   a  in or out of the vessel  12  to move the respective electrodes  30 ,  30   a . Scuffing of the electrodes  30 ,  30   a  by the feeler gauge is reduced by virtue of the fact that rotation of the shaft is prevented. Once the electrode gap has been set, the user would push an locking mechanism  100  to the locked position to prevent inadvertent further adjustment. 
     A steadily increasing test voltage would be applied via the horns, through the shafts  32 ,  32   a , to the electrodes  30 ,  30   a  at their ends. The steadily increasing voltage is applied across the sample until a spark flashes through the sample as a result of it breaking down electrically. The breakdown voltage for that particular sample will thus have been determined. 
     The provision of the end caps  44 ,  44   a  enclosing the springs  42 ,  42   a  mitigates against corona discharge from the thin wire diameter of the springs by enclosing them in metal of the same voltage. 
     The shortest air distances between the connections to electrodes  30 ,  30   a , or between these connections and the wall of a metal enclosure within which the test vessel  12  is located during testing, has been optimised to accommodate 100 kVrms between electrodes at an altitude of 1 km, and 50 kVrms between an electrode and the test chamber wall at an altitude of 1 km. To avoid breakdown in air, rather than the test sample, the air path from the connections to electrodes  30 ,  30   a  to the outer edge of the test vessel assembly  10  is increased by providing the convolutions on the adjusting wheels  60 ,  60   a.    
     This is also the reason for a skirt  130  at the bottom of the test vessel  12 , which mounts, in use, on a corresponding raised area (not shown) of the test chamber floor beneath it. The raised area may house a temperature sensor. The base  16  of the internal vessel chamber  18  may include a thinned area to provide a low thermal resistance path to the vessel contents for the temperature sensor. An additional small skirt surrounding the thinned area of the base  16  may trap a very small volume of air between the raised area and the thinned area, providing a low thermal resistance and therefore improved temperature measurement accuracy. In addition, the skirt  130  provides increased rigidity to the assembly and prevents draughts under the vessel  12  from upsetting temperature measurements. 
     The meshing convolutions of the adjusting wheels  60 ,  60   a  and the respective horn covers  46 ,  46   a  and the vessel  12 , plus the skirt  130  each increase the creepage and clearance distances across the vessel from the connections from one electrode  30  to the connections from the other  30   a , and to the test chamber wall. As a result, the test vessel assembly  10  may be operated at increased voltages, and/or the assembly  10  and associated test chamber may be reduced in size, resulting in a lighter-weight instrument. 
     Although the vessel has been described in conjunction with two similar adjustable electrode assemblies, it will be understood that the objective of adjusting the electrode gap may be achieved by having just a single adjustment mechanism, the second electrode being fixed in position. 
     The test vessel assembly  10  has been designed to accommodate several standards at once with a common design. The vessel assembly might be used in conjunction with a motorised stirrer option or a magnetic bean stirrer option. 
     Preferably, the test assembly  10  is constructed using plastic mouldings. For example, the test vessel  12 , including the rim  22  and skirt  130  and the reinforcing bosses  34 ,  34   a  may be moulded as an integral unit from clear material. The horn covers  46 ,  46   a  may each be moulded integrally with the respective ribs  72  and protrusions  114 . Plastic materials are more robust and lighter in weight than glass, yet can also be chemically inert and resistant to cleaning agents. Moulding enables the smooth filleting of corners for ease of cleaning, and is also inexpensive. 
     An optional oval, plastic baffle (not shown) might be suspended from the test vessel lid  20  for example by suspension pillars depending downwardly from the underside of the lid, so as to rest on the surface of the oil in the vessel  12  thereby excluding air from the surface of the oil in the vessel. This satisfies a requirement of one particular test vessel standard in which there must be no air in contact with the surface of the oil in the vessel. 
     Typically, the shafts  32 ,  32   a  connecting the electrodes  30 ,  30   a  to the contacts in the horns are made of conductive material. However, the shafts  32 ,  32   a  could be made of non-conductive material per se but carry a conductor, for example through a hollow lumen. 
     Also, it will be understood that the principles enunciated could be modified for application to a flow-through test, such as for process monitoring (rather than the testing of a static sample as described). This might require modification of some parts and could require the lid to be sealed. 
     The exemplary internal chamber  18  has a capacity of 400 ml. It will be appreciated that other capacities could equally be employed, with appropriate modification of the remainder of the apparatus. For example, a 150 ml capacity chamber  18  is also envisaged.