Patent Publication Number: US-9418806-B2

Title: Power switching apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the national stage of International Application No. PCT/EP2011/059400, filed Jun. 7, 2011, entitled, “Power Switching Apparatus,” the contents of which are incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     This invention relates to a power switching apparatus. 
     BACKGROUND 
     The operation of multi-terminal high voltage direct current (HVDC) transmission and distribution networks involves load and fault/short-circuit current switching operations. The availability of switching components to perform such switching permits flexibility in the planning and design of HVDC applications such as parallel HVDC lines with a tap-off line or a closed loop circuit. 
     A known solution for load and fault/short-circuit current switching is the use of semiconductor-based switches, which are typically used in point-to-point high power HVDC transmission. The use of semiconductor-based switches results in faster switching and smaller values of let-through fault current. The disadvantages of using such switches however include high forward losses, sensitivity to transients and the lack of tangible isolation when the devices are in their off-state. 
     Another known solution for load and fault/short-circuit current switching is a vacuum interrupter. The operation of the vacuum interrupter relies on the mechanical separation of electrically conductive contacts to open the associated electrical circuit. Such a vacuum interrupter is capable of allowing high magnitude of continuous AC current with a high short-circuit current interrupting capability. 
     The conventional vacuum interrupter however exhibits poor performance in interrupting DC current because of the absence of current zero. Although it is feasible to use the conventional vacuum interrupter to interrupt low DC currents up to a few hundred amperes due to the instability of an arc at low currents, such a method is not only unreliable but is also incompatible with the levels of current typically found in HVDC applications. 
     It is possible to carry out DC current interruption using conventional vacuum interrupters by applying a forced current zero or artificially creating a current zero. This method of DC current interruption involves connecting an auxiliary circuit in parallel across the conventional vacuum interrupter, the auxiliary circuit comprising a capacitor, a combination of a capacitor and an inductor or any other oscillatory circuit. The auxiliary circuit remains isolated by a spark gap during normal operation of the vacuum interrupter. 
     When the contacts of the vacuum interrupter begin to separate, the spark ignition gap is switched on to introduce an oscillatory current of sufficient magnitude across the vacuum interrupter and thereby force the current across the interrupter to pass through a current zero. This allows the vacuum interrupter to successfully interrupt the DC current. Such an arrangement however becomes complex, costly and space consuming due to the need to integrate the additional components of the auxiliary circuit. 
     OVERVIEW 
     According to an aspect of the invention there is provided a power switching apparatus comprising a vacuum interrupter assembly and a switching assembly connected in parallel between a pair of terminals, each of the terminals being connectable, in use, to an electrical circuit, wherein: the vacuum interrupter assembly includes at least one vacuum interrupter having first and second electrically conductive rods connected at a first end to a respective one of the terminals and extending at a second end into a vacuum-tight enclosure, a first electrode being mounted at or near the second end of a first of the electrically conductive rods and a slotted coil including a support base being mounted at or near the second end of a second of the electrically conductive rods, a second electrode being mounted on an inner surface of the slotted coil and a third electrode mounted on the support base, the second ends of the electrically conductive rods extending into the vacuum-tight enclosure such that the first and third electrodes define opposed contact surfaces and at least one of the electrically conductive rods is movable relative to the other electrically conductive rods to open or close a gap between the opposed contact surfaces; and the switching assembly includes at least one crossed-field plasma discharge switch that does not carry any current in its open state and conducts and carries current in its closed state, the switching assembly being controllable to switch between open and closed states, in use of the power switching apparatus, to modify a current flowing through the vacuum interrupter assembly. 
     The parallel connection of the vacuum interrupter and switching assemblies in the power switching apparatus has been found to improve current interruption carried out using the vacuum interrupter assembly. 
     The above arrangement of the first electrode and the slotted coil in the vacuum interrupter assembly enables the generation of a self-induced axial magnetic field that is perpendicular to the arc current drawn between the first and second electrodes during the current interruption process. In the presence of the axial magnetic field, the arc voltage begins to rise while the arc current begins to drop rapidly until it reaches a value lower than the chopping current value of the electrode material. At this point the current drops instantly to zero, which results in full dielectric recovery and successful current interruption. The generation of the self-induced axial magnetic field removes the need to incorporate additional equipment into the vacuum interrupter assembly in order to generate the required axial magnetic field and thereby reduces the complexity of the layout of the vacuum interrupter assembly. 
     The switching assembly provides additional control over the current interruption process by enabling modification of the size of current flowing through the vacuum interrupter assembly during the current interruption process. For example, the size of current can be altered to minimise any adverse effects of high current densities on the electrodes to thereby improve the lifetime of the vacuum interrupter assembly. 
     The crossed-field plasma switch has rapid switching capability and can support a high voltage drop in its open state. This in turn renders the crossed-field plasma switch compatible for use in the switching assembly to aid current interruption in high voltage applications. 
     The parallel connection of the vacuum interrupter and switching assemblies in the power switching apparatus also results in a simple layout of the power switching apparatus, which in turn reduces the manufacturing and installation costs of such an apparatus. 
     The shape of the slotted coil and the material of each electrode may vary, depending on the design requirements of the vacuum interrupter. The slotted coil may, for example, includes either only a single slot, which preferably extends around the full perimeter of the coil, or a plurality of slots, while each electrode may, for example, be made from a refractory material, which may be selected from a group of chromium-chromium, copper-tungsten, copper tungsten carbide, tungsten, chromium or molybdenum. 
     In embodiments of the invention the switching assembly may be controllable to switch from an open state to a closed state in response to a formation of a gap between the opposed contact surfaces of the or each vacuum interrupter. 
     During normal operation, current only flows through the vacuum interrupter while the switching assembly remains in an open state and therefore does not conduct current. As soon as a gap between the opposed contact surfaces is formed, the switching assembly is switched to a closed state to divert part of the flow of current through the switching assembly. This not only limits the arc voltage across the or each vacuum interrupter, but also reduces the current density at the opposed contact surfaces to thereby minimise the damage to the opposed contact surfaces during the stage when the length of the gap is still very small and the current is flowing through molten globules of electrode material. 
     The power switching apparatus preferably further includes a control circuit, the control circuit being controllable to generate a control signal in response to the formation of the gap between the opposed contact surfaces of the or each vacuum interrupter and transmit the control signal to the switching assembly, the switching assembly being controllable to switch between a closed state and an open state in response to the control signal received from the control circuit. 
     The use of such a control circuit enables the switching assembly to respond rapidly and automatically to the formation of the gap between the opposed contact surfaces of the or each vacuum interrupter. 
     The switching assembly is preferably controllable to switch from an open state to a closed state at a predetermined length of the gap following the formation of the gap between the opposed contact surfaces of the or each vacuum interrupter. 
     Once the gap between the opposed contact surfaces reaches a sufficiently large length, current begins to flow transversely between the first and second electrodes. At this stage the switching assembly is switched back to an open state so that all of the current flows through the vacuum interrupter. This in turn allows the axial magnetic field generated in the slotted coil to act on all of the current flowing through the or each vacuum interrupter. 
     The switching assembly may be controllable to switch from an open state to a closed state at a predetermined level of current prior to the extinguishing of current in the or each vacuum interrupter and may be controllable to switch from a closed state to an open state following the extinguishing of current in the or each vacuum interrupter. 
     In the presence of the axial magnetic field, the arc voltage begins to rise while the arc current begins to drop rapidly until it reaches a value lower than the chopping current value of the electrode material. Switching the switching assembly to a closed state in the moments prior to the current being extinguished diverts the flow of any residual current through the switching assembly. The switching assembly is then switched to an open state to complete the current interruption process. 
     Such operation of the power switching apparatus not only allows each electrode to be made from material that are conducive to the high dielectric withstand requirements in the vacuum interrupter during open condition, but also has lower chopping current value which reduces any overvoltage generated during the current interruption process. 
     The number and arrangement of vacuum interrupters in the vacuum interrupter assembly may vary, depending on the design requirements of the power switching apparatus. The vacuum interrupter assembly may, for example, include a plurality of series-connected and/or parallel-connected vacuum interrupters. 
     Multiple vacuum interrupters may be connected to define different configurations of the vacuum interrupter assembly in order to vary its operating voltage and current characteristics to match the requirements of the associated power application. 
     As with the vacuum interrupter assembly, multiple crossed-field plasma discharge switches may be connected to define different configurations of the switching assembly in order to vary its operating voltage and current characteristics to match the requirements of the associated power application. 
     In embodiments where the switching assembly includes a plurality of parallel-connected crossed-field plasma discharge switches, the switching assembly may be controllable to sequentially open or close the plurality of parallel-connected crossed-field plasma discharge switches. 
     Sequentially opening and closing the plurality of parallel-connected crossed-field plasma discharge switches allows discharge to be maintained in at least one crossed-field plasma discharge switch for a longer duration and thereby increases the overall duration of current conduction in the switching assembly. This in turn renders the switching assembly compatible for use in current interruption processes in which the time taken to initially separate the opposed contact surfaces and the time taken to diffuse the arc is longer than the allowed duration of current conduction in a single crossed-field plasma discharge switch. 
     Examples of applications that are compatible with the power switching apparatus according to the invention include, for example, AC power networks, AC high voltage circuit breakers, AC generator circuit breakers, railway traction, ships, superconducting magnetic storage devices, high energy fusion reactor experiments, stationary power applications, and high voltage direct current (HVDC) multi-terminal networks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described, by way of non-limiting examples, with reference to the accompanying drawings in which: 
         FIG. 1  shows a power switching apparatus according to an embodiment of the invention; and 
         FIG. 2  shows a vacuum interrupter assembly that forms part of the power switching apparatus of  FIG. 1 . 
         FIGS. 3 and 4  show a plurality of series-connected ( FIG. 3 ) and/or parallel-connected ( FIG. 4 ) vacuum interrupters. 
         FIGS. 5 and 6  show a plurality of series-connected ( FIG. 5 ) and/or parallel-connected ( FIG. 6 ) crossed-field plasma discharge switches. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     A power switching apparatus  6  according to an embodiment of the invention is shown in  FIG. 1 . 
     The power switching apparatus  6  comprises a pair of terminals  7 , a vacuum interrupter assembly  8 , a switching assembly  9  and a control circuit (not shown). 
     The vacuum interrupter assembly  8  is connected in parallel with the switching assembly  9  between the terminals  7 . 
     In use, the terminals are respectively connected to positive and negative terminals of a DC electrical circuit. 
     The vacuum interrupter assembly  8  comprises a single vacuum interrupter  10 . 
     The vacuum interrupter  10  includes a pair of cylindrical housings  12 , first and second end flanges  14 , 16  and an annular structure  18  assembled to define a vacuum-tight enclosure. Each end flange  14 , 16  is brazed to a first end of a respective cylindrical housing  12  to form a hermetic joint. The two cylindrical housings  12  are joined together at their second ends via the annular structure  18 . The annular structure  18  includes a central shield  20  that overlaps inner walls of the cylindrical housings  12  to protect the inner walls of the cylindrical housings  12  from metal vapour deposition arising from arc discharge, while each end flange  14 , 16  includes an end shield  22  to improve the electrostatic field line distribution along the length of the vacuum interrupter  10 . 
     Each cylindrical housing  12  is metallised and nickel-plated at both ends. The length and diameter of the respective cylindrical housing  12  varies depending on the operating voltage rating of the vacuum interrupter  10 , while the dimensions and shape of the first and second end flanges  14 , 16  and the annular structure  18  may vary to correspond to the size and shape of the cylindrical housings  12 . 
     The vacuum interrupter  10  also includes a tubular bellows  24  and first and second electrically conductive rods  26 , 28 . 
     The first end flange  14  includes a hollow bore dimensioned to accommodate the tubular bellows  24 , while the second end flange  16  includes a hollow bore dimensioned to accommodate the second rod  28  within its hollow bore. The tubular bellows  24  also includes a hollow bore for retention of the first rod  26 . 
     The first and second rods  26 , 28  are respectively retained within the hollow bores of the tubular bellows  24  and the second end flange  16  so that the second ends of the rods  26 , 28  are located inside the enclosure and the first ends of the rods  26 , 28  are located outside the enclosure. The first and second rods  26 , 28  may be fabricated from, for example, oxygen-free high conductivity (OFHC) copper. 
     The vacuum interrupter  10  further includes first, second and third electrodes  30 , 32 , 34 , and a multiple slotted coil  38 . The multiple slotted coil  38  includes a plurality of slots (not shown). 
     It is envisaged that, in other embodiments, the multiple slotted coil may be replaced by a slotted coil that includes only a single slot. Preferably such a single slot would extend completely around the full perimeter, e.g. the circumference of the coil. 
     The first electrode  30  consists of first and second electrode portions  30   a,   30   b . The first electrode portion  30   a  is in the form of a rounded electrode portion that is mounted at the second end of the first rod  26 . The second electrode portion  30   b  is in the form of a annular electrode portion that is mounted around the circumference of the first rod  26  and is adjacent to the first electrode portion  30   a.    
     The second electrode  32  is mounted on an inner surface of the multiple slotted coil  38 . 
     The multiple slotted coil  38  includes a support base  36 . The support base  36  is mounted at the second end of the second rod  28 . 
     The third electrode  34  is mounted at the centre of the support base  36 . The rods  26 , 28  are coaxially aligned so that the first and third electrodes  30 , 34  define opposed contact surfaces. The third electrode  34  includes a recess  40  for receipt of the first electrode portion  30   a  and the shape of the recess  40  corresponds to the shape of the rounded first electrode portion  30   a  so as to maximise contact between the first and third electrodes  30 , 34 . 
     Each electrode  30 , 32 , 34  is made from a refractory material, which may be selected from a group of, for example, chromium-chromium, copper-tungsten, copper tungsten carbide, tungsten, chromium or molybdenum. These refractory materials not only exhibit excellent electrical conductivity, but also display high dielectric strength subsequent to the current interruption. Moreover, these refractory materials have relatively high chopping current values, which helps to rapidly extinguish the arc once the current has dropped below the chopping current value. 
     Corrugated walls of the tubular bellows  24  allow the tubular bellows  24  to undergo expansion or contraction so as to increase or decrease the tubular length of the tubular bellows  24 . This allows the first rod  26  to move relative to the second rod  28  between a first position where the first and third electrodes  30 , 34  are kept in contact, and a second position where only a portion of the first electrode portion  30   a  remains located inside the multiple slotted coil  38 . The second rod  28  is kept at a fixed position. 
     In use, the first ends of the first and second rods  26 ,  28  are respectively connected to the terminals  7  of the power switching apparatus  6  so that the first end of the first rod  26  is connected to the positive terminal  42  of the DC electrical circuit, while the first end of the second rod  28  is connected to the negative terminal  44  of the DC electrical circuit. 
     The switching assembly  9  includes a crossed-field plasma discharge switch. 
     The crossed-field plasma discharge switch includes an anode  46 , a cathode  48 , crossed-field magnets  50  and two grid electrodes  52 . The grid electrodes  52  are located between the anode  46  and the cathode  48 , the grid electrodes  52  being respectively adjacent to the anode  46  and the cathode  48 . The crossed-field magnets  50  are controlled to maintain a cold cathode discharge between the cathode  48  and the grid electrode  52  to the cathode. 
     The switching assembly  9  is in an open state and therefore does not conduct current when the grid electrode  52  adjacent to the anode  46  is maintained at a lower anode voltage to prevent electrons from passing through. The switching assembly  9  is in a closed state and therefore conducts current when the potential of the grid electrode  52  is increased to match the potential of the anode  46  to permit electrons to pass through. 
     Examples of crossed-field plasma discharge switches and their operation are described in U.S. Pat. No. 5,336,975. 
     The crossed-field plasma switch has rapid switching capability and can support a high voltage drop in its open state. This in turn renders the crossed-field plasma switch compatible for use in the switching assembly to aid current interruption in high voltage applications. 
     During normal operation of the connected DC electrical circuit, the tubular bellows  24  is controlled to move the first rod  26  to the first position to bring the first and third electrodes  30 , 34  into contact. At the same time the crossed-field plasma discharge switch remains in an open state. This allows current to flow between the positive and negative terminals  42 , 44  of the connected DC electrical circuit via the electrically conductive rods  26 , 28  of the vacuum interrupter  10  whilst no current flows through the switching assembly  9 . The low contact resistance resulting from the contact between the first and third electrodes  30 , 34  means that there is no flow of current through the multiple slotted coil  38 . 
     In the event of a fault resulting in a high fault current flowing in the connected DC electrical circuit, the current must be interrupted in order to prevent the high fault current from damaging components of the DC electrical circuit. Interruption of the fault current permits isolation and subsequent repair of the fault in order to restore the DC electrical circuit to normal operating conditions. 
     The current interruption process is initiated by controlling the tubular bellows  24  to move the first rod  26  toward its second position so as to separate the opposed contact surfaces of the first and third electrodes  30 , 34 . The separation of the opposed contact surfaces results in the formation of a gap between the first electrode  30  and the third electrode  34 , which leads to the formation of an arc in this gap. The arc consists of metal vapour plasma, which continues to conduct the current flowing between the first and third electrodes  30 , 34 . 
     The control circuit detects the formation of the gap between the opposed contact surfaces and generates a control signal that is subsequently transmitted to the switching assembly  9 . Upon receipt of the control signal, the switching assembly  9  is controlled to switch from an open state to a closed state. The switching assembly  9  then begins to conduct current, which has the effect of diverting part of the current through the switching assembly  9 . 
     The diversion of current flow through the switching assembly  9  not only limits the arc voltage across the vacuum interrupter  10 , but also reduces the current density at the opposed contact surfaces to thereby minimise the damage to the opposed contact surfaces during the stage when the length of the gap is still very small and the current is flowing through molten globules of electrode material. This helps to minimise damage to the electrodes  30 , 34  during current interruption and thereby extend the lifetime of the vacuum interrupter  10 . 
     As the gap between the opposed contact surfaces increases and the magnitude of current increases, the multiple slotted coil  38  begins to draw current via the second electrode  32 . The shape of the multiple slotted coil  38  causes the drawn current to flow in a preferential direction within the multiple slotted coil  38 , which results in generation of an axial magnetic field in the gap between the first electrode  30  and the second electrode  32 . The direction of the generated axial magnetic field is perpendicular to the direction of current being drawn between the first electrode  30  and the second electrode  32 . 
     When the gap increases to the point where the multiple slotted coil  38  begins to draw current, the switching assembly  9  is controlled to switch from a closed state to an open state. The switching assembly  9  then stops conducting current, which has the effect of causing all of the current to flow through the vacuum interrupter  10 . This allows the axial magnetic field generated in the multiple slotted coil  38  to act on all of the current flowing through the power switching apparatus  6 . 
     In the presence of the axial magnetic field, the metal vapour plasma is forced away from the gap between the first electrode  30  and the second and third electrodes  32 , 34 . Subsequently the arc voltage begins to rise while the magnitude of the drawn current begins to drop rapidly. When the magnitude of the drawn current reaches a value lower than the chopping current value of the electrode material, the arc energy becomes insufficient to sustain the current, which leads to the arc becoming highly unstable and the current dropping instantly to zero. This allows full dielectric recovery and successful current interruption to take place. 
     The duration of current interruption is limited by the time required to mechanically move the first rod  26  from the first position to the second position, which is typically a few milliseconds. Once the first rod  26  reaches the second position, the current will typically drop to zero in about 10 to 20 μs. 
     The arrangement of the first rod  26  and the multiple slotted coil  38  therefore allows the separation of the first and third electrodes  30 , 34  to result in the generation of a self-induced axial magnetic field to assist in the extinguishing of the arc formed between the first electrode  30  and the second and third electrodes  32 , 34 . This removes the need to incorporate additional equipment into the vacuum interrupter assembly in order to generate the required axial magnetic field, and thereby reduces the complexity of the layout of the vacuum interrupter assembly. 
     The comparatively simpler layout of the vacuum interrupter assembly has the effect of reducing the amount of space required for the assembly and the associated installation costs, while the reduced number of components in the vacuum interrupter assembly improves the reliability of the current interruption process. 
     Optionally, prior to the extinguishing of current in the vacuum interrupter  10 , the switching assembly may be controlled to switch from an open state to a closed state at a predetermined level of current. Switching the switching assembly  9  to a closed state in the moments prior to the current being extinguished diverts the flow of any residual current through the switching assembly  9 . This allows the current to become zero across the vacuum interrupter open contacts. This is followed by the switching assembly  9  being controlled to switch back to an open state to complete the current interruption process. 
     Such operation of the power switching apparatus  6  not only allows each electrode  30 , 32 , 34  to be made from material that are conducive to the high dielectric withstand requirements in the vacuum interrupter  10  during open condition, but also has been found to reduce any overvoltage generated during the current interruption process. 
     The parallel connection of the vacuum interrupter and switching assemblies  8 , 9  in the power switching apparatus  6  has been found to improve current interruption carried out using the vacuum interrupter  10 . 
     The switching assembly  9  provides additional control over the current interruption process by enabling modification of the size of current flowing through the vacuum interrupter  10  during the current interruption process. 
     The parallel connection of the vacuum interrupter and switching assemblies  8 , 9  in the power switching apparatus  6  also results in a simple layout of the power switching apparatus  6 , which in turn reduces the manufacturing and installation costs of such an apparatus  6 . 
     It is envisaged that, in other embodiments, the vacuum interrupter assembly may include a plurality of series-connected ( FIG. 3 ) and/or parallel-connected ( FIG. 4 ) vacuum interrupters. 
     Multiple vacuum interrupters may be connected to define different configurations of the vacuum interrupter assembly in order to vary its operating voltage and current characteristics to match the power requirements of the associated power application. For example, connecting multiple vacuum interrupters in series increases the dielectric strength of the vacuum interrupter assembly and thereby permits the use of the vacuum interrupter assembly at higher operating voltages, while connecting multiple vacuum interrupters in parallel permits the vacuum interrupter to interrupt higher levels of current. 
     It is also envisaged that, in other embodiments, the switching assembly may include a plurality of series-connected ( FIG. 5 ) and/or parallel-connected ( FIG. 6 ) crossed-field plasma discharge switches. 
     As with the vacuum interrupter assembly, multiple crossed-field plasma discharge switches may be connected to define different configurations of the switching assembly in order to vary its operating voltage and current characteristics to match the power requirements of the associated power application. 
     For example, multiple vacuum interrupters and crossed-field plasma discharge switches can be connected in series and parallel to interrupt continuous current≧6 kA and short-circuit current≧100 kA at an operating voltage of≧400 kV of a HVDC multi-terminal network. 
     Depending on the opening speed of the rods  26 , 28  and the time taken to diffuse the arc, the maximum duration of conduction of the crossed-field plasma discharge switch is typically 1 to 3 ms. 
     The use of parallel-connected crossed-field plasma discharge switches in a switching assembly allows the plurality of parallel-connected crossed-field plasma discharge switches to be sequentially turned off/opened. This in turn allows discharge to be maintained in at least one cross-field plasma discharge switch for a longer duration and thereby increases the overall duration of current conduction in the switching assembly. This in turn renders the switching assembly compatible for use in current interruption processes in which the time taken to initially separate the opposed contact surfaces and the time taken to diffuse the arc is longer than the duration of current conduction in a single crossed-field plasma discharge switch. 
     The power switching apparatus of  FIG. 1  is compatible for use, but is not limited to, applications such as AC power networks, AC high voltage circuit breakers, AC generator circuit breakers, railway traction, ships, superconducting magnetic storage devices, high energy fusion reactor experiments, stationary power applications, and high voltage direct current (HVDC) multi-terminal networks.