Abstract:
The invention relates to an arrangement ( 10 ) for ground fault monitoring of an AC circuit between, on one hand, a neutral conductor (N′) of the AC circuit and, on the other hand, a protective ground wire (PE) or a ground wire with a series connection of ohmic resistors (R 1 , R 2 , R 3 ), which has a first terminal ( 101 ) for the neutral conductor (N′) and a second terminal ( 102 ) for the protective ground wire (PE) or ground wire, wherein the arrangement includes at least one means for detecting a break of one of the resistors (R 1 , R 2 , R 3 ) of the series connection.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an arrangement for ground-fault detection in an AC circuit between, on one hand, a neutral conductor of the AC circuit and, on the other hand, a protective ground wire or a ground conductor. Such arrangement with the reference number  10  is illustrated in  FIG. 1 . The arrangement has a series connection of ohmic resistors R 1 , R 2 , R 3  with a first terminal  101  for the neutral conductor N′ and a second terminal  102  for the protective ground wire PE or the ground conductor. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention also relates to a current supply arrangement  1  for a polysilicon reactor which is also illustrated in  FIG. 1 . In the polysilicon reactor, a series connection of silicon rods or thin silicon rods for producing polysilicon according to the Siemens process is arranged. The series connection  2  is also shown in  FIG. 1 . The series connection  2  can be connected to the current supply arrangement  1  as a load. The current supply arrangement  1  includes a transformer  11  having a primary side  111 , which can be connected to a power grid L 1 , N, and a secondary side  112  with several taps  1121 ,  1122 ,  1123 , a controller  12 , and an arrangement  10  for ground fault monitoring. A tap  1123  of the secondary side  112  is connected with a neutral conductor terminal of an output of the current supply arrangement  1 , and at least two tabs  1121 ,  1122  are connected via power controllers  13  with a phase terminal of an output of the current supply arrangement. The terminals of the output are connected with a neutral conductor N′ and with a phase conductor L 1 ′. The power controllers  13  of the current supply arrangement  1  can be controlled with the controller  12  using voltage sequence control. 
     In the conventional arrangement for ground fault monitoring, as employed in the current supply arrangement  1  for a polysilicon reactor, a current flows in the event of a ground fault via the protective ground wire PE, the second terminal  102  of the arrangement  10 , the series connection of ohmic resistors R 1 , R 2 , R 3  to the first terminal  101  of the arrangement  10  for ground fault monitoring and hence to the neutral conductor N′ of the current supply arrangement  1 . The arrangement  10  for ground fault monitoring includes means I max  for detecting a current, which detects a current flowing from the second terminal  102  to the first terminal  101  through the series connection of ohmic resistors R 1 , R 2 , R 3 . If a sufficiently high current flows through the series connection R 1 , R 2 , R 3 , then a ground fault is detected by the means I max  for detecting the current. 
     The current flowing through the series connection of resistors R 1 , R 2 , R 3  as a result of the ground fault causes at each resistor R 1 , R 2 , R 3  of the series connection a voltage drop relative to the second terminal  102  of the arrangement for ground fault monitoring. In the conventional arrangements for ground fault monitoring, a means U max  for detecting a voltage is therefore connected to at least one node  103  located between two resistors R 2 , R 3  of the series connection, with the means U max  detecting a voltage across the at least one of the resistors R 3  and the second terminal  102  of the arrangement for ground fault monitoring. 
     The means I max  for detecting the current and the means U max  for detecting the voltage have outputs which are connected with the controller  12  of the current supply arrangement via a bus and an interface  103 . 
     The signals supplied from the arrangement  10  via the interface  103  can be evaluated in the controller so as to initiate suitable measures in the event of a ground fault, for example switching the current supply arrangement  1  off. 
     An arrangement  10  for ground fault monitoring of the aforedescribed type in a current supply arrangement  1  for a polysilicon reactor of the aforedescribed type operates particularly reliably with a high-resistance connection between the neutral conductor N′ and the protective ground wire PE across at least a portion of the series connection  2  of the silicon rods or thin silicon rods powered by the current supply arrangement  1 . Fundamentally, a low-resistance connection between the neutral conductor N′ and the protective ground wire PE via a portion of the series connection  2  of the silicon rods or silicon thin rods can also be detected with this type of arrangement  10  for ground fault monitoring. However, in this situation, the voltage drop between the neutral conductor N′ and the protective ground wire PE may be so small that only a small current is driven through the series connection of the ohmic resistors R 1 , R 2 , R 3  of the arrangement  10  for ground fault monitoring. Although the means I max  for detecting the current flowing from the second terminal  102  to the first terminal  101  through the series connection of ohmic resistors R 1 , R 2 , R 3  of the arrangement  10  for ground fault monitoring can fundamentally measure such small current, there is still the increased risk that the entire current supply arrangement  1  switches off due to short-circuits unrelated to safety or other extremely brief events, causing a termination or at least an undesirable interruption of the process for producing polysilicon. Therefore, the threshold which the current through the series connection of the ohmic resistors R 1 , R 2 , R 3  must cross in order to trigger suitable safety devices and to switch the current supply arrangement off is set so high that short-circuits unrelated to safety or other extremely brief events do not cause the current supply arrangement to switch off. The arrangement  10  for ground fault monitoring therefore is not triggered by a low-resistance connection between the neutral conductor N′ and the protective ground wire PE via a portion of the series connection  2  of the silicon rods or the thin silicon rods powered by the current supply arrangement. 
     Another disadvantage of the conventional arrangement  10  for ground fault monitoring is that the ground fault can no longer be detected if the series connection of resistors R 1 , R 2 , R 3  breaks. It is therefore desirable to switch the current supply arrangement  1  off when a break in the series connection of resistors R 1 , R 2 , R 3  of the arrangement  10  for ground fault monitoring is detected, because the ground fault monitoring of the current supply arrangement  1  then no longer functions properly. 
     The underlying problem to be solved by the invention is to improve an arrangement for ground fault monitoring so that low-resistance ground faults can be reliably differentiated from non-critical states and failures of the arrangement for ground fault monitoring through breaks in the series connection of ohmic resistors can also be detected. 
     The problem relating to the detection of breaks in the series connection of ohmic resistors is solved in that the arrangement has at least one means for identifying a break of one of the resistors of the series connection. If a break in the series connection occurs, then this break can be detected and a failure of the arrangement for ground fault monitoring causing the current supply arrangement to switch off can be indicated. 
     With respect to reliably identifying low-ohmic ground faults, the problem is solved in that the arrangement has a means for integrating a current flowing through the series connection from the first terminal to the second terminal. The energy dissipated through the ground fault can be determined by integrating the current. If this energy reaches a critical value, then a low-resistance ground fault can be reliably differentiated from a non-safety-related event. 
     Both solutions can be employed in conjunction or in parallel in an arrangement for ground fault monitoring. 
     A first of the means for detecting a break may include a first voltage source and a first coupling network, via which at least a first resistor of the resistors is connected to the first voltage source. The coupling network may be a transformer which galvanically separates a first voltage source from the first resistor. The first means for identifying a break may include a first means for detecting a current, which detects a current driven by the first voltage source through the first resistor. Alternatively, the first means for detecting a current may also measure the current flowing through the first voltage source. 
     The first means for detecting a break may also include a second voltage source and a second coupling network by which at least a second of the resistors is connected to the second voltage source. The second coupling network may also be formed by a transformer. The first means for detecting a break may include a second means for detecting a current which detects a current driven by the second voltage source through the second resistor. The second means for detecting a current may alternatively also detect the current flowing through the second voltage source. 
     The first resistor and the second resistor may be arranged in the series connection of the resistors directly one after the other. A common node is then provided between the resistors. 
     The sum of the voltage provided by the first voltage source and dropping across the first resistor and the voltage provided by the second voltage source and dropping across the second resistor is preferably equal to zero or approximately equal to zero. Since a break was detected, no voltage drop occurs across the series connection of the first resistor and the second resistor. However, a voltage drop occurs via the series connection of the first resistor and the second resistor in the event of a ground fault, which can be detected by a means for detecting a voltage, which may be part of the arrangement according to the invention. 
     An arrangement according to the invention may have at least one second means for detecting a break. The second means for detecting a break may have a third voltage source and a third coupling network, via which at least a third resistor of the resistors is connected to the third voltage source. The third coupling network can be formed by a transformer. The second means for detecting a break may include a third means for detecting a current, which detects a current driven through the third resistor by the third voltage source or a current through the third current source. 
     An arrangement according to the invention may include a fourth means for detecting a current, which detects a current flowing from the first terminal to the second terminal through the series connection. 
     The arrangement may have a branch which connects the first terminal, a node between two resistors of the series connection, an additional node between two resistors of the series connection and/or the second terminal and which includes a fourth voltage source and a controllable switch which can be controlled by a controller at discrete times to close. Advantageously, the first means for detecting a break and the second means for detecting a break are not operating at the discrete times. When the switch is closed, a current can be driven via the switch, the first terminal, a node between two resistors of the series connection, an additional node between two resistors of the series connection and/or the second terminal and hence via at least a portion of the series connection of the ohmic resistors. A fifth means for detecting a current can then detect a current flowing across the switch. If a current driven by the fourth voltage source does not flow in spite of the fact that the controllable switch is closed, then a break may be present in the series connection of the ohmic resistors of the arrangement according to the invention. The controllable switch may be a relay. 
     The means for detecting a current in an arrangement according to the invention may be current relays. 
     Additional features and advantages of the present invention will be described with reference to the appended drawings, which show in: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         FIG. 1  a current supply arrangement for a polysilicon reactor, 
         FIG. 2  a first arrangement for ground fault monitoring according to the invention, and 
         FIG. 3  a second arrangement for ground fault monitoring according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The arrangement is illustrated in  FIGS. 1 ,  2 ,  3  for ground fault monitoring and current supply arrangements with ground fault monitors, respectively, have a large number of identical or functionally identical elements and components. These elements or components are labeled in the Figures with identical reference symbols. Elements, components and/or parts of the two arrangements according to the invention illustrated in  FIGS. 2 and 3  may be entirely or partially combined with each other so as to create additional arrangement according to the invention. 
     The arrangements  10  for ground fault monitoring illustrated in  FIGS. 2 and 3  may be employed in the current supply arrangement illustrated in  FIG. 1  instead of the arrangement  10  for ground fault monitoring illustrated in  FIG. 1 . 
     The arrangement  10  for ground fault monitoring illustrated in  FIG. 2  includes first means for detecting a break. The first means for detecting a break includes a first voltage source U m1  and a first coupling network K 1  formed by a transformer. The first voltage source U m1  is connected to the primary side of the transformer K 1 . A first resistor R 1  of the series connection of the resistors R 1 , R 2 , R 3  of the arrangement  10  for ground fault monitoring is connected to the secondary side. 
     The first means for detecting a break further includes a second voltage source U m2  and a second coupling network K 2  also formed by a transformer. The second voltage source U m2  is connected to the primary side of the transformer K 2 . A second resistor R 2  of the series connection of the resistors R 1 , R 2 , R 3  of the arrangement  10  for ground fault monitoring is connected to the secondary side. 
     The first voltage source U m1 , the second voltage source U m2 , the transformer K 1  and the transformer K 2  are arranged and the voltages supplied by the voltage source U m1 , U m2  are dimensioned such that the secondary-site voltages cancel each other and no voltage drop of the sum voltage produced by the first voltage source U m1  or the second voltage source U m2  occurs across the series connection of the first resistor R 1  and the second resistor R 2 . A voltage drop caused by an external voltage source for the first means for detecting a break may occur across the series connection of the first resistor R 1  and the second resistor R 2 . 
     The first means for detecting a break furthermore includes a first means I m1max  for detecting a current, for example a current relay, which detects a current I m1  driven by the first voltage source U m1  through the first resistor R 1 . For this purpose, the first means I m1max  for detecting the current includes a measuring element  106  arranged in the circuit formed by the secondary side of the transformer K 1  and the first resistor R 1 . The measuring element  106  is arranged in series with the secondary side of the transformer. 
     If the current I m1  through the first resistor R 1  is sufficiently large, then it is certain that the first resistor R 1  is not broken. However, if there is no current I m1 , then it must be assumed that the first resistor R 1  is defective. 
     The first means for detecting a break also includes a second means I m2max  for detecting a current, likewise a current relay, which detects a current I m2  driven by the second voltage source U m2  through the second resistor R 2 . If the current I m2  through the second resistor R 2  is also sufficiently large, then it is certain that the first resistor R 2  is not broken. However, if there is no current I m2 , then it must be assumed that the first resistor R 2  is broken. 
     The second means I m2max  for detecting the current has a measuring element  107  arranged in the circuit formed by the secondary side of the transformer K 2  and the second resistor R 2 . The measuring element  107  is hereby arranged in series with the secondary side of the transformer K 2 , so that only the current I m2  can flow through the measuring element  107 . 
     The arrangement for ground fault monitoring illustrated in  FIG. 2  furthermore includes a second means for detecting a break. The second means for detecting a break includes a third voltage source U m3 , a third coupling network K 3  and a third means I m3max  for detecting a current, which detects a current driven through the resistor R 3  by the third voltage source U m3 . The third resistor R 3  is connected to the third voltage source U m3  via the coupling network K 3  which is also formed by a transformer. The third means I m3max  includes a measuring element  108  arranged in series with the secondary side of the transformer K 3 . The third means I m3max  for detecting the current may also be a current relay. 
     If the third resistor R 3  is not broken, then the third voltage source U m3  can drive a current through the circuit formed by the secondary side of the transformer K 3 , the measuring element  108  and the third resistor R 3 . Conversely, if the third resistor R 3  is broken, then this current can no longer flow. The third means I m3max  for detecting the current detects this and indicates the break to the controller  12  of the current supply arrangement  1  via the output  103  of the arrangement. 
     The arrangement  10  according to the invention according to  FIG. 2  further includes a fourth means I max  for detecting a current, which detects a current flowing from the second terminal  102  to the first terminal  101  through the series connection R 1 , R 2 , R 3 . The fourth means I max  includes a measuring element  109  arranged after the first terminal  101 . The fourth means I max  may also be a current relay. In the event of a ground fault, a current flows via the series connection of the resistors R 1 , R 2 , R 3 , which is detected by the fourth means I max  and is indicated to the controller  12  of the current supply arrangement  1  via the terminal  103  of the arrangement  10 . 
     The measuring element  109  is furthermore connected with a means Int for integrating, which integrates the measured current. The integration of the current flowing through the series connection of the resistors R 1 , R 2 , R 3  represents the energy flowing during a ground fault. The amount of the dissipated energies can be used to distinguish ground fault from another non-safety-critical phenomenon. 
     Lastly, the arrangement illustrated in  FIG. 2  includes a means U max  for detecting a voltage, which detects a voltage drop between a node  104  located between the resistors R 2  and R 3  and the second terminal, i.e., the protective ground wire PE. If a current flows via the series connection of the resistors R 1 , R 2 , R 3  due to a ground fault, then a voltage drop occurs across the series connection of the resistors R 1 , R 2 , R 3 . This voltage is detected by the means U max  and may be used by the controller as an indication of a ground fault. 
     The arrangement  10  according to the invention illustrated in  FIG. 3  corresponds substantially to the arrangement illustrated in  FIG. 2 . Unlike the arrangement illustrated in  FIG. 2 , the arrangement  10  illustrated in  FIG. 3  does not include first means for detecting a break. Only the second means for detecting a break is provided, with which the third resistor R 3  can be continuously monitored for a break. 
     Continuous detection of a break is then not performed for the resistors R 1  and R 2 . However, a means is provided for testing the resistors R 1  and R 2  for a break at discrete times, for example before the current supply arrangement  1  supplies current to the series connection  2 . 
     The means for detecting a break at discrete times includes a branch which connects the first terminal  101  and the node  104  with each other and which includes a fourth voltage source U t  and a controllable switch  109  which can be controlled by a controller  12  at discrete times so as to close. When the switch is closed, the fourth voltage source U t  drives a current I t  via the switch  109  and the resistors R 1  and R 2 . The current I t  flowing through the branch is detected by a fifth means I max  for detecting a current. In this way, it can be detected at the discrete times if the resistors R 1  and R 2  are broken or not. The output of the fifth means I max  for detecting the current is connected with the controller  12  via the interface  103 .