Abstract:
A series spark gap includes at least three partial spark gaps. In a continuous operation mode, a supply voltage acting over the partial spark gaps is divided by a first voltage dividing mechanism. During triggering, more than half of the partial spark gaps are bypassed by using a bypass mechanism such that more than half of the supply voltage acts over the partial spark gaps that are not bypassed, whereby the partial spark gap or the partial spark gaps that are not bypassed ignite. The voltage acting over the bypassed partial spark gaps is divided asymmetrically using the bypass mechanism, whereby the bypassed partial spark gaps ignite sequentially in the order determined by the asymmetric voltage division.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a method for triggering a series spark gap. 
         [0002]    The invention further relates to an arrangement for triggering a series spark gap. 
       BACKGROUND OF THE INVENTION 
       [0003]    In connection with high voltage lines, for instance, there are employed series capacitor banks to compensate for line inductance. In parallel with a capacitor bank, in protection thereof, there is generally coupled a metal oxide varistor and/or a spark gap. Solutions for triggering a spark gap are disclosed in publications SE 8205236, FI 88012 and WO 2007/003706, for example. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    The object of the present invention is to provide a new solution for triggering a series spark gap. 
         [0005]    The invention is characterized by the features of the independent claims. 
         [0006]    According to an embodiment, a series spark gap comprising at least three partial spark gaps or air gaps is provided. In steady state conditions a supply voltage acting over the partial spark gaps is divided by first voltage dividing means. During triggering, more than half of the partial spark gaps are bypassed by using bypass means such that more than half of the supply voltage acts over the partial spark gaps that are not bypassed, whereby the partial spark gap or the partial spark gaps that are not bypassed ignite. The voltage acting over the bypassed partial spark gaps is divided asymmetrically using the bypass means whereby the bypassed partial spark gaps ignite sequentially in the order determined by the asymmetric voltage division. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which 
           [0008]      FIG. 1  shows schematically a solution for protecting a series capacitor bank. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]      FIG. 1  is a single line diagram of a solution for protecting a series capacitor bank. The series capacitor bank  1  is shown in  FIG. 1  only schematically. Series capacitors are used to compensate for the inductance of a transmission line  2 . Series capacitors will increase a transmission capacity and stability of the line  2 . Series capacitors are also used to share the load between parallel lines. 
         [0010]    The series capacitor bank  1  may be protected by a spark gap  3 , for example. Further, a metal oxide varistor (MOV)  4  may be used for protecting the series capacitor bank  1 . The series capacitor bank  1  may be a thyristor controlled series capacitor bank. However, for the sake of clarity such thyristor control means are not shown in  FIG. 1 . 
         [0011]      FIG. 1  further shows switches K 1 , K 2  and K 3 , which switches are used for bypassing the series capacitor bank  1 , if needed. 
         [0012]    The spark gap  3  comprises a first enclosure  5   a  and a second enclosure  5   b.  The first enclosure  5   a  may also be called a lower enclosure and the second enclosure  5   b  may also be called an upper enclosure. 
         [0013]    The first enclosure  5   a  comprises main electrodes  6   a  and  6   b . Correspondingly, the second enclosure  5   b  comprises main electrodes  6   c  and  6   d.  In connection with the main electrode  6   b  there is an auxiliary electrode  7   a  which is in the same potential as the main electrode  6   b.  Correspondingly there is a second auxiliary electrode  7   b  in connection with the main electrode  6   d . The second auxiliary electrode  7   b  is in the same potential as the main electrode  6   d.    
         [0014]    The first enclosure  5   a  further comprises a middle electrode  8   a . Correspondingly the second enclosure  5   b  comprises a middle electrode  8   b . Thus the spark gap  3  is a series spark gap comprising four partial spark gaps. The first partial spark gap is between the main electrode  6   a  and the middle electrode  8   a.  The second partial spark gap is between the middle electrode  8   a  and the auxiliary electrode  7   a.  The third partial spark gap is between the main electrode  6   c  and the middle electrode  8   b.  The fourth partial spark gap is between the middle electrode  8   b  and the second auxiliary electrode  7   b.    
         [0015]      FIG. 1  further shows a main bushing  9   a  in connection with the first enclosure  5   a  and, correspondingly, a main bushing  9   b  in connection with the second enclosure  5   b.    FIG. 1  further shows a middle bushing  10   a  in connection with the first enclosure and a middle bushing  10   b  in connection with the second enclosure  5   b.    
         [0016]    The arrangement shown in  FIG. 1  further comprises voltage divider capacitors  11   a,    11   b,    11   c  and  11   d.  The voltage divider capacitors  11   a  to  11   d  divide the supply voltage Ucap acting over the partial spark gaps in steady state conditions or, in other words, during continuous operation mode. The supply voltage Ucap is the voltage across the series capacitor bank  1 . The arrangement further comprises auxiliary spark gaps  12   a,    12   b  and  12   c.  The auxiliary spark gaps may be gas pressure spark gaps, i.e. trigatrons, for example. The first auxiliary spark gap  12   a  may be a trigatron that is force-triggered with a triggering pulse for igniting the first trigatron  12   a.  The arrangement may be provided with an ignition coil  13  for forming a triggering pulse for the second trigatron  12   b.  The third auxiliary spark gap  12   c  may be called an auxiliary trigatron. 
         [0017]    In series with the first auxiliary spark gap there is a first current-limiting damping resistor  14   a.  In series with the second auxiliary spark gap  12   b  there is a second current-limiting damping resistor  14   b  and in series with the third auxiliary spark gap  12   c  there is a third current-limiting damping resistor  14   c.    
         [0018]    The series resistors  14   a,    14   b  and  14   c  are of different sizes. In one embodiment the resistance of the first resistor  14   a  is lower than the resistance of the second resistor  14   b  and the resistance of the second resistor  14   b  is lower than the resistance of the third resistor  14   c.  In another embodiment the resistance of the second resistor  14   b  is 2 to 3 times higher than the resistance of the first resistor  14   a  and the resistance of the third resistor  14   c  is 2 to 3 times higher than the combined resistance of the resistors  14   b  and  14   a.  In a third embodiment the resistance of the first resistor  14   a  is 2 kΩ, the resistance of the second resistor  14   b  is 5 kΩ and the resistance of the third resistor  14   c  is 17.5 kΩ. The resistance of the resistors  14   a  to  14   c  may vary between 100Ω and 100 kΩ, for example. The resistance of the resistor must be high enough to limit the current through the trigatron. On the other hand, the resistance of the resistor must not be too high to ensure sufficiently fast discharge of the capacitors  11   a  to  11   c.    
         [0019]    In an embodiment, in steady state conditions voltage is divided equally between all partial spark gaps. Thus in that embodiment the voltage division is symmetric in steady state conditions because the voltage is divided symmetrically by the voltage divider capacitors  11   a  to  11   d.  During the triggering of the spark gap, an asymmetric voltage division is used. Thus in that embodiment symmetric and asymmetric voltage division is combined. 
         [0020]    In the beginning of an ignition process, before triggering, the voltage over the partial spark gaps is divided symmetrically such that the voltage across each partial spark gap is ¼* Ucap if the capacitors  11   a  to  11   d  are substantially equal in size. Dimensioning the capacitors to be equal in size may provide cost efficiency. The voltage division is determined by the high impedance of the voltage divider capacitors  11   a  to  11   d.  The capacitance of the capacitors  11   a  to  11   d  may vary between 100 pF and 100000 pF, for example. In another embodiment the capacitance of the capacitors  11   a  to  11   d  may vary between 2000 pF and 40000 pF, for example. In the present embodiment the capacitance of the capacitors  11   a  to  11   d  may be 3000 pF, for example. The capacitances of the capacitors  11   a  to  11   d  must be high enough to prevent line current, which is to pass through the series capacitor bank  1 , from passing through the capacitors  11   a  to  11   d.  Furthermore the reactances of the capacitors  11   a  to  11   d  must be substantially higher than the reactances of the series resistors  14   a  to  14   c.    
         [0021]    Next, the first auxiliary spark gap  12   a  is force-triggered. Thus the first auxiliary spark gap  12   a  is ignited by a control system command. The second auxiliary spark gap  12   b  is ignited by a voltage increase and ignition coil  13  current caused by the ignition of the first auxiliary spark gap  12   a.  The voltage Ucap acts then over the second enclosure  5   b  and therefore the third auxiliary spark gap  12   c  ignites. Because the auxiliary spark gaps  12   a,    12   b  and  12   c  are ignited, the voltage divider capacitors  11   a,    11   b  and  11   c  are discharged. 
         [0022]    Thereafter the whole voltage Ucap acts across the fourth partial spark gap between middle electrode  8   b  and the auxiliary electrode  7   b  which therefore ignites the partial spark gap. 
         [0023]    At this stage the current path runs through the fourth partial spark gap and the auxiliary spark gaps  12   a,    12   b  and  12   c.  At this stage the series resistors  14   a,    14   b  and  14   c  determine the voltage division between the auxiliary spark gaps  12   a,    12   b  and  12   c.    
         [0024]    In an embodiment the voltage division ratio of the series resistors  14   a,    14   b  and  14   c  is 2.5 between the resistors. This is achieved, for example, such that the resistance of the first resistor is 2 kΩ, the resistance of the second resistor is 5 kΩ and the resistance of the third resistor  14   c  is 17.5 kΩ. At that stage the voltage across the third partial spark gap that is between the main electrode  6   c  and the middle electrode  8   b  rises to 0.71 Ucap, which ignites the air gap. 
         [0025]    The current path runs through the second enclosure  5   b  and the first auxiliary spark gap  12   a  and the second auxiliary spark gap  12   b.  The voltage across the second partial spark gap between the first auxiliary electrode  7   a  and the first middle electrode  8   a  rises to 0.71 Ucap thus igniting the partial spark gap. 
         [0026]    Thereafter the whole voltage Ucap acts across the first partial spark gap between the first middle electrode  8   a  and the first main electrode  6   a  which ignites the partial spark gap. Thus the whole spark gap  3  is ignited and the series capacitor bank  1  is bypassed. 
         [0027]    A voltage across a single partial spark gap before forced triggering is thus 0.25 Ucap. During the ignition process the voltage rises to 0.71 to 1 Ucap for a single partial spark gap. Thus the voltage rush across a single partial spark gap is 2.84-4 pu (per unit). 
         [0028]    It is also possible to dimension the components such that the voltage rush across each partial spark gap during triggering is substantially equal in size. In one embodiment the components are dimensioned such that the voltage rush across one partial spark gap differs less than 10% from the voltage rush across other partial spark gaps. 
         [0029]    In one embodiment the voltage rush across each partial spark gap is dimensioned to be substantially equal such that the capacitances of the capacitors  11   a  and  11   d  are smaller than the capacitances of the capacitors  11   b  and  11   c.  In this embodiment the resistances of the resistors  14   a  to  14   c  may correspond to the resistances in the previous embodiment. The capacitances of the capacitors  11   a  and  11   d  may be 2143 pF and the capacitances of the capacitors  11   b  and  11   c  may be 3000 pF, for example. 
         [0030]    In a continuous operation mode the supply voltage Ucap is divided between the partial spark gaps such that voltage across the fourth partial spark gap between the middle electrode  8   b  and the auxiliary electrode  7   b  is 0.29 Ucap. The voltage across the third partial spark gap between the main electrode  6   c  and the middle electrode  8   b  is 0.21 Ucap. The voltage across the second partial spark gap between the first auxiliary electrode  7   a  and the first middle electrode  8   a  is 0.21 Ucap. The voltage across the first partial spark gap between the first middle electrode  8   a  and the first main electrode  6   a  is 0.29 Ucap. 
         [0031]    When the third, second and first partial spark gaps are bypassed, the whole voltage Ucap acts across the fourth partial spark gap which provides a voltage rush of 3.4 pu across the fourth partial spark gap. 
         [0032]    When the fourth partial spark gap has been ignited the voltage across the third partial spark gap rises to 0.71 Ucap, which provides a voltage rush of 3.4 pu across the third partial spark gap, thus igniting the third partial spark gap. 
         [0033]    Thereafter the voltage across the second partial spark gap rises to 0.71 Ucap, providing a voltage rush of 3.4 pu across the second partial spark gap, thus igniting the second partial spark gap. 
         [0034]    Thereafter the whole voltage Ucap acts across the first partial spark gap, providing a voltage rush of 3.4 pu across the first partial spark gap, thus igniting the first partial spark gap. 
         [0035]    Thus, when the components are dimensioned such that the voltage rush across each partial spark gap is substantially equal, the minimum voltage rush across a single partial spark gap may be rather high, which improves the ignition of the spark gap. The values of the components disclosed above are only examples and it is thus possible to dimension the components case by case to fulfill the required needs. 
         [0036]    The described solutions provide improvement in forced triggering performance also in extreme conditions. The forced triggering voltage of the spark gap can be lowered which enables to mitigate line circuit breaker transient recovery voltage. The described spark gap arrangement can be used in extreme climatic conditions because the arrangement is reliably ignited in cold conditions and self-firing of the spark gap can be avoided also in hot conditions. Lowering the forced triggering voltage also enables a high self-ignition voltage level, whereby a self-ignition in high pollution and during snow storms, for example, may be avoided. The above-described arrangement further provides a fast ignition and allows greater manufacturing tolerances. 
         [0037]    It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims. 
         [0038]    The series spark gap comprises at least three partial spark gaps. In the embodiment shown in  FIG. 1  the series spark gap comprises four partial spark gaps. The series sparks gap may also comprise more than four partial spark gaps. 
         [0039]    Instead of or in addition to capacitors, the first voltage dividing means for dividing the supply voltage acting over the partial spark gaps during continuous operation mode may also be coils, resistors or metal oxide varistors, for example. 
         [0040]    Instead of or in addition to a trigatron, the bypass means arranged to bypass the partial spark gaps may also comprise other force-triggered airgaps or semiconductor components, for example. 
         [0041]    Instead of or in addition to the series resistors, the second voltage dividing means comprised in the bypass means may also be metal oxide varistors or other non-linear or linear reactances or resistances, for example.