Abstract:
The invention relates to a diagnostic method for multiple-stage excess voltage protection apparatuses that include at least one gas discharge distance between an input and a reference potential as a first stage, at least one diode path between an output and the reference potential as a second stage, and at least one decoupling inductance interposed between the input and the output. The diagnostic method is characterized in that a secondary voltage applied to a secondary inductance, which is actively connected, inductively, to the decoupling inductance, is measured and evaluated with a view to excess voltage events in the excess voltage protection apparatus. The invention also relates to a two-stage excess voltage protection apparatus.

Description:
TECHNICAL FIELD  
       [0001]    In a first aspect, the present invention relates to a diagnostic method for multi-stage excess voltage protection apparatuses, and in a second aspect, it relates to an excess voltage protection apparatus, by means of which the diagnostic method can be carried out. 
       RELATED ART  
       [0002]    An excess voltage protection apparatus is disclosed, for example, in DE 10 2009 004 673 A1, and comprises the following components: an input and an output, at least one gas discharge distance between the input and a reference potential as the first stage, at least one diode path between the output and the reference potential as the second stage and at least one decoupling inductance interposed between the input and output. Such excess voltage protection apparatuses serve the purpose of barring excess voltage events that arise between the input and the reference potential from reaching electrical equipment interposed between the output and the reference potential. The gas discharge distance is also referred to as the first stage and the diode path as the second stage. For this reason, excess voltage protection apparatuses of this kind are referred to as two-stage excess voltage protection apparatuses. 
         [0003]    A general problem to be solved in the case of such excess voltage protection apparatuses resides in the fact that after one or more events of excess voltage it is necessary to decide whether the excess voltage protection apparatus is still in working order or is already defective, in which case the excess voltage protection apparatus must either be completely replaced or only individual parts of the excess voltage protection apparatus. For this purpose, accurate information is required concerning the condition of components used in the excess voltage protection apparatus. 
         [0004]    In this context, some solutions have already been proposed in the prior art, each of which is in need of improvement. First of all, a temperature in the casing and/or a temperature on the diode or diodes forming the diode path can be monitored by the use of one or more temperature control fuses. The response of the diode path can then be verified in that a voltage drop across the diodes can be detected using one or more opto-couplers. Furthermore, the response of gas arrester or arresters, seen in the form of flashes, can be detected by the use of photodiodes. Finally, a peak value of the total amount of current withdrawn can be measured. 
         [0005]    These solutions in themselves make it possible to draw conclusions on the condition of the components used. However, such conclusions are not satisfactory as regards the unambiguity thereof. In addition, some of the monitoring methods mentioned do not provide useful information in all cases of an excess voltage event. For example, relevant information can be obtained by means of temperature control fuses only when, as a result of a load on, or possibly the destruction of, protective elements, a sufficiently large amount of power is transposed in the equipment to enable a threshold temperature of the temperature control fuses to be reached, if at all. This is not the case with many of the events occurring in instrumentation and control engineering, in particular with 4 mA to 20 mA-circuits or thermocouples. 
         [0006]    When using opto-couplers in the diode path, it is not possible to ascertain whether the detected impulse has damaged or destroyed the diodes. There is no information available whatever as regards the condition of the gas discharge distance. 
         [0007]    Finally, the ascertainment of a peak value of the total amount of current withdrawn does not provide any data on the state of the components of the diode path. 
       SUMMARY  
       [0008]    The present invention provides a diagnostic method for two-stage excess voltage protection apparatuses and a two-stage excess voltage protection apparatus of the kind indicated above, by means of which a maintenance and an error display for two-stage excess voltage protection apparatuses is provided and wherein significantly more information concerning the components used in the excess voltage protection apparatus is made available than in the prior art. 
         [0009]    Exemplary embodiments of the excess voltage protection apparatus according to the invention and advantageous variants of the diagnostic method of the invention are described below, in particular with reference to the dependent claims and the figures. 
         [0010]    The diagnostic method of the kind mentioned above is developed, according to the invention, in that a secondary voltage connected to a secondary inductance that is actively connection, inductively, to a decoupling inductance is measured and evaluated with a view to excess voltage events. 
         [0011]    The excess voltage protection apparatus of the kind described above is developed, according to the invention, in that a secondary inductance is present, which is actively connected, inductively, to a decoupling inductance, and an evaluation unit is present, which is connected to the secondary inductance and adapted to evaluate a secondary voltage connected to the secondary inductance. 
         [0012]    It may be regarded as being a concept of the invention to supplement the decoupling inductance with the aid of a suitably positioned secondary inductance to form an inductive transducer. In this way, direct diagnostic access to electric currents and voltages is made possible within the excess voltage protection apparatus. An arrangement is regarded as being suitable, in which the secondary inductance is actively connected, inductively, to the decoupling inductance. 
         [0013]    In particular, precise information can be obtained with the aid of the excess voltage protection apparatus of the invention and the method of the invention as to whether the excess voltage protection apparatus is in good condition, has been frequently used, or is defective. 
         [0014]    Basically, Zener diodes can be used for the diode path. However, because of their steeper characteristics and higher discharge capacity, suppressor diodes are preferred, which are also referred to as TVS diodes. The diode path can, if desired, have a plurality of diodes, which can be connected in series and/or in parallel. 
         [0015]    The gas discharge distance has at least one gas protection apparatus and can, if desired, comprise a plurality of gas protection apparatuses, which can be connected in series and/or in parallel. 
         [0016]    Arrangements involving a plurality of diodes and/or a plurality of gas protection apparatuses can be referred to as a multi-stage diode path or a multi-stage gas discharge distance, respectively. 
         [0017]    The decoupling inductance and secondary inductance can each have a plurality of individual coils. 
         [0018]    The reference potential can basically be any potential. In an embodiment, the reference potential used is zero potential. 
         [0019]    In the case of a first variant of the method of the invention, ignition events are counted and, following a number of a specifiable plurality of ignition events, a service request is issued. In the evaluation unit of the excess voltage protection apparatus of the invention, a double flank detector can be formed for this purpose, by means of which a check is carried out as to whether an ascending and a descending flank have arisen in a fixed period of time, which event is then regarded and counted as an ignition event. An excess voltage event during which a gas discharge has taken place is referred to as an ignition event. 
         [0020]    Since the resistance is negligible across the diode path in conjunction with such ignition events, the secondary voltage connected to the secondary inductance is substantially equal to the voltage across the gas discharge distance. The double flank detector can therefore simply evaluate the course of the secondary voltage, with respect to time, when using the excess voltage protection apparatus of the invention. 
         [0021]    The ignition events in the gas discharge distance differ quantitatively, because the higher the voltage increases across the gas discharge distance the steeper is the ascending flank. An important parameter of an ignition event can accordingly be determined by setting the maximum value for the secondary voltage. 
         [0022]    Another important parameter in this respect is the maximum value of a derivative of the secondary voltage with respect to time, because this maximum value, which may also be referred to as the edge steepness, is equally characteristic of a certain ignition event. In addition, the maximum of the derivative, with respect to time, of the secondary voltage can be ascertained with the aid of the excess voltage protection apparatus of the invention. An important development of the method of the invention consists, therefore, in ascertaining a derivative, with respect to time, of the secondary voltage, in order then to ascertain a maximum value of this derivative, with respect to time, of the secondary voltage. 
         [0023]    In addition, a useful parameter is the so-called dynamic response voltage. This is the quotient of the maximum value of the derivative, with respect to time, of the secondary voltage and the maximum value of the secondary voltage. Accordingly, in a further particularly preferred variant of the method of the invention, a quotient of the maximum value of the derivative, with respect to time, of the secondary voltage and the maximum value of the secondary voltage itself is formed, this quotient is compared with the dynamic response voltage as specified in the gas protection apparatus&#39; specification, and a signal is generated, when the quotient does not conform to the specification. If this occurs, the excess voltage protection apparatus must be replaced. 
         [0024]    The previously explained variants of the diagnostic method of the invention provide information on the gas discharge distance. Further, evaluations are also possible by means of which precise information on the diode path can be obtained. 
         [0025]    For example, the diagnostic method of the invention can, in order to obtain a value that is proportional to the energy inputted into the diode path, especially during an excess voltage event or an ignition event, be advantageously improved in that, in a first stage, a time integral is formed over the secondary voltage, that in a second step, an amount of this time integral is formed, and that a time integral of this amount is formed in a third step. This evaluation can be simply accomplished in the evaluation unit provided by the invention. This development of the method is based on the following: Integration of the secondary voltage provides a voltage that is proportional to the current passing through the primary coil and is also proportional to the current passing through the diode path, because the current that passes through the connected device to be protected is negligible compared with the current passing through the diode path. The integral of this voltage is then formed and integration is repeated to yield a voltage that is proportional to the energy inputted into the diode path, especially during the course of an excess voltage event or ignition event. 
         [0026]    With the aid of a further advantageous variant of the method of the invention, it is possible to ascertain the actual temperature of real interest of a suppressor diode forming the diode path in that a rise in temperature in the diode path is revealed by the fact that an energy release in the diode path to the environment is modeled, for example by a high pass, and that a temperature of the diode path is ascertained by adding the ambient temperature. 
         [0027]    The temperature thus obtained can be compared with a destruction temperature of the diode, that is, for example, the suppressor diode, as per data sheet. Once the destruction temperature has been exceeded, it is practically assumed that the diode or the whole piece of equipment is defective, which is advantageously externally indicated. 
         [0028]    The method described herein and, in particular, the concept of supplementing a decoupling activity with a secondary inductance to give an inductive transducer and of measuring and evaluating the reduced secondary voltage drop at the secondary inductance can also be extended to excess voltage protection apparatuses requiring protection for a plurality of conduction paths or distinguishable wire pairs. Then several transducers will be required and the existing branch currents and node voltages must be ascertained in a manner known per se while taking into account the Kirchhoff rules. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    Further advantages and features of the device according to the invention are described below with reference to the attached  FIGS. 1 to 5 . 
           [0030]      FIG. 1  is a diagram of an exemplary embodiment of the excess voltage protection apparatus of the invention. 
           [0031]      FIG. 2  shows typical time curves of the voltage across the gas discharge distance at various events of excess voltage. 
           [0032]      FIG. 3  shows the time curves of the current passing through the diode path corresponding to the time curves of the voltage across the gas discharge distance as shown in  FIG. 2 . 
           [0033]      FIG. 4  shows the time curves of the current through the gas discharge distance for the time curves of the voltage across the gas discharge distance as shown in  FIG. 2 . 
           [0034]      FIG. 5  is a diagram illustrating the evaluation possibilities of the secondary voltage using an evaluation unit in accordance with embodiments. 
       
    
    
       [0035]    Like or similarly effective components are usually marked by the same reference symbols in the figures. 
       DETAILED DESCRIPTION 
       [0036]      FIG. 1  shows an exemplary embodiment of a two-stage excess voltage protection apparatus  10  according to the invention, which has a gas discharge distance  20  and a diode path  22 . In the example shown, the gas discharge distance  20  is formed by a single gas arrester  13  and is connected between an input  15  and an input port  12  for a reference potential M. A voltage supplying the gas arrester  13  is marked with a double arrow UGE. In the shown example, the diode path  22  is formed by a single suppressor diode  17 , which is interposed between an output  16  of the excess voltage protection apparatus  10  and an output port  14  for the reference potential M. Just as the gas discharge distance  20  can have a plurality of gas arresters  13 , so can the diode path  22  comprise a plurality of diodes, more particularly suppressor diodes or TVS diodes.  FIG. 1  shows the current passing through the diode path  22  with an arrow ID. 
         [0037]    The input  15  and the output  16  are interconnected by a decoupling inductance  18 . Current passing through this decoupling inductance is designated by an arrow IP. 
         [0038]    In accordance with the present invention, as shown in  FIG. 1 , a secondary inductance  24  is positioned in direct proximity to the decoupling inductance  18  such that there is an inductive active connection between the decoupling inductance  18  and the secondary inductance  24 , and due to the presence of these two components, an inductive transducer is formed, which is indicated in  FIG. 1  by a curly bracket  26 . The secondary inductance  24 , also referred to as a secondary coil, can be formed by means of a center tap, which can be connected to a reference potential. Rectifying the secondary voltage is then simpler. A secondary voltage drop across the secondary inductance  24  is indicated in  FIG. 1  by means of the double arrow US. This secondary voltage, i.e. that across the two terminals of the secondary inductance  24 , is then passed to an evaluation unit  30  present according to the invention, in which the secondary voltage US is evaluated with a view to excess voltage events in the excess voltage protection apparatus. 
         [0039]    In the case of excess voltage protection apparatuses of the type shown in  FIG. 1 , the excess voltage events can be divided into basically two groups. On the one hand, there are excess voltage events, during which the gas arrester  13  ignites. In the case of such events, which can also be referred to as ignition events, a majority of the current surge is absorbed by the gas arrester  13 . And there are other excess voltage events, during which the gas arrester  13  does not ignite. During these events, the surge current dissipates completely through the decoupling inductance  18  and the diode path, for example only one suppressor diode or only one ZVS diode. This is described in more detail with reference to  FIGS. 2 and 4 . 
         [0040]      FIG. 2  shows the voltage UGE connected to the gas arrester  13 . As evident from  FIG. 2 , the curves b, c and d show sharp downward gradients at different points in time b 1 , c 1  and d 1 . These are those points in time, at which the gas discharge begins, i.e. when the gas arrester  13  breaks through. In this context, it is of note that the voltage curves have different forms prior to the points in time b 1 , c 1 , and d 1 , and it is seen that the more the voltage across the gas discharge distance  20  increases, the faster it rises. In other words, for a steeply rising flank as, for example in curve d, a higher maximum value of the voltage UGE is reached than is the case with a less steeply rising, for example curve b. In this regard, the course of curve c lies between b and d. Further, it is to be observed that in curve “a” there is no sharply descending flank, which leads to the conclusion that there is no gas discharge in curve “a”. 
         [0041]      FIG. 3  shows the time curves A, B, C, and D, respectively, of the current passing across the diode path, pertaining to the voltage characteristics a, b, c, and d, respectively in  FIG. 2 . First, it is to be noted that in the case of the voltage curve “a” in  FIG. 2 , the time curve of the current passing through the diode path  50 , shown by the curve “A” in  FIG. 3 , rises during, about  18 ps, until it reaches a maximum value of just under  70  ampere, after which it falls again. The time curve of the current demonstrated by curve “A” illustrates an excess voltage event during which the gas discharge distance  20  does not break through. The curve “A” shows no discontinuity and is fundamentally different in this from the other time curves of the current B, C, and D passing through the diode path  50 , as shown in  FIG. 3 . In curves B, C, and D it is noticeable that the steeper the upward gradient, with which the current across the diode path  50  increases, the faster is the rise in voltage UGE present at the gas arrester  13 . Then it must be noted that the points in time b 1 , c 1 , and d 1 , at which the voltage breaks down across the gas arrester  13  (see  FIG. 2 ), correspond to the points in time B 1 , C 1 , and D 1 , respectively, at which the time curves B, C, and D, respectively, of current passing across the diode path  50  suddenly buckles to lower current values. Following this buckling, the curves B, C, and D descend with approximately the same negative gradient. Finally, it is evident from  FIG. 3  that the greater the maximum current passing through the diode path  50 , the more time it takes for the gas arrester  13  to break through. 
         [0042]      FIG. 4  shows the diagrams of corresponding current curves pertaining to the voltage curves “a” to d shown in  FIG. 2 . As there is no gas discharge for curve “a” in  FIG. 2 , no current passes through the gas discharge distance  20 . The curve a in  FIG. 4 , which is pertains to curve “a” in  FIG. 2 , is coincident with the zero line. Then in  FIG. 4 , the current curves β, γ, and δ are shown, which correspond to the voltage curves b, c, and d, respectively, in  FIG. 2 . It should be noted that each of the current curves rises to a maximum value in a smooth curve and then drops again as the curve proceeds, but at a slower rate than its increase. Insofar consistent with the voltage curves shown in  FIG. 2 , it is seen also that the greater the maximum value of the current attained in each case in the curves β, γ, and δ the greater is the maximum value of the voltage across the gas discharge distance  20 . Finally, the current rise in each case begins at the points in time visible in  FIG. 2 . 
         [0043]    The excess voltage events which produce the curves b/B/β, c/C/γ, and d/D/δ are thus excess voltage events during which the gas arrester  13  ignites. Accordingly, they relate to ignition events. 
         [0044]    Only that event that produced the curves a/A/a belongs to the second group of excess voltage events, namely those during which the gas arrester  13  does not ignite. With these events the surge current dissipates completely via the decoupling coil  18  and the diode path  22 , in the situation as shown in  FIG. 1 , i.e. via the suppressor diode  17 . Nevertheless, the protection apparatus according to the invention works reliably also during the event of a/A/α, during which the surge current is discharged via the decoupling inductance or decoupling coil  18  and through the diode path  50 . 
         [0045]    The question as to whether the gas arrester  13  ignites or not depends, as explained, on the amplitude, i.e. a maximum value of the voltage, and the edge steepness of the excess voltage impulse, that is, a maximum of a derivative, with respect to time, of the secondary voltage. 
         [0046]    In the context of  FIG. 5 , three different evaluation approaches will now be described, which can be carried out in each case with the aid of the evaluation unit  30  provided by the invention. The evaluation unit  30  according to the invention can be, for example, a microcontroller or a comparable intelligent component. It should be generally noted, particularly in the following description, that values of physical parameters, such as voltage, current, temperature, etc., are represented in the evaluation circuit by voltages, which are proportional to the respective physical parameters. 
         [0047]    Firstly, an excess voltage protection apparatus  10  of the invention is diagrammatically shown in  FIG. 5  comprising the transducer  26  shown therein. Likewise, a conductor is diagrammatically shown, by means of which the secondary voltage US is conducted by the excess voltage protection apparatus  10  to the above-mentioned three evaluation branches. The first evaluation branch generates evaluation signals to the outputs  91  and  92 . The second evaluation branch generates signals to the outputs  93  and  94 . The third evaluation branch sends evaluation signals to the outputs  95  and  96 . 
         [0048]    The term “output” expresses in this context merely that it is an output of the evaluation branches described herein. However, this does not necessarily imply that these outputs will be connected to an external entity, for example an interface. This is indeed possible, but it is equally possible for the signals delivered at the outputs  91  to  96  to be passed to a further internal processing. 
         [0049]    The first evaluation branch is adapted to detect excess voltage events, during which the gas arrester  13  has ignited. This takes place with the aid of a double flank detector  71 , which monitors whether, in a specific time frame, two voltage flanks of the secondary voltage US, one with a positive gradient and the other with a negative gradient, can be ascertained in the secondary inductance  24 . Such double flanks occur, as is evident from  FIG. 2  for curves b, c, and d, when the gas arrester  13  ignites due to a sufficiently large maximum voltage of the excess voltage impulse. The number of ignition events thus detected are counted with the aid of a counter  72  and compared with a specifiable limit value in the query stage  73 . If the number of ignition events is higher than the specified limit value, then a service request can be signaled or displayed. For example, a signal “gas arrester worn out” can be given at the output  91  and a signal “gas arrester unspent” at the output  92 . 
         [0050]    In the second evaluation branch, the maximum value MUS of the secondary voltage US of the transducer  26 , also referred to as the peak value of the secondary voltage US, is detected. This is done with the aid of a first peak value detector  79 , which displays the maximum secondary voltage MUS at its output. In addition, a derivative, with respect to time, DUS of the secondary voltage US is formed in the unit  78 , which is made available at an output of the unit  78 . Using a second peak value detector  74 , a peak or maximum value MDUS of this derivative, with respect to time, DUS is formed and then presented at an output of the second peak value detector  74 . This peak value MDUS, which, it will be remembered, is a voltage, is proportional to the maximum edge steepness of the first ascending flank. From these two parameters, that is to say, the maximum secondary voltage MUS and the maximum derivative, with respect to time, of the secondary voltage MDUS, a quotient Q is determined in the unit  75 . This is the so-called dynamic response voltage. The voltage MUS is the response voltage of the gas arrester  13  when it has ignited. The maximum value of the derivative, with respect to time, of the secondary voltage MDUS corresponds to the maximum edge steepness of the excess voltage impulse. The dynamic response voltage is thus the response voltage in relation to the maximum edge steepness. 
         [0051]    The quotient Q, i.e., the dynamic response voltage, is then passed to the unit  76 , which relays the value Q only when an ignition event has taken place, that is to say, when the gas arrester  13  has ignited. This information is made available to unit  76  via the output of the double flank detector  71 . If an ignition event has taken place, the quotient Q is then compared with the specification data, and in stage  77  a decision is made as to whether the dynamic response voltage Q of the gas arrester  13  conforms to the specification. For example, a signal “gas arrester defective” is displayed at the output  93  and a signal “gas arrester intact” at the output  94 . 
         [0052]    Using the third evaluation branch, information concerning the diode link can be obtained, especially concerning the particularly important temperature of the depletion layer of the suppressor diode  17 . First of all, the integral IUS, with respect to time, of the secondary voltage US of the transducer  26  is formed in the unit  81 . This integral IUS, with respect to time, is proportional to the current IP passing through the decoupling inductance  18  and thus proportional to the current passing through the suppressor diode  17 , see  FIG. 1 . The integral IUS, with respect to time, is then passed to the unit  82 , which forms the absolute value B of the integral IUS. As can be seen from  FIG. 5 , this amount is then fed to stage  83 , which ascertains the integral IB, with respect to time, therefrom. This integral IB, with respect to time, is proportional to the energy input into the suppressor diode  17  during a transient excess voltage event. 
         [0053]    The value of IB is then fed to a high pass HP, by means of which the derivative, with respect to time, of the thermal energy dissipation of the suppressor diode  17 , that is to say, the decrease in temperature of the suppressor diode  17  is modeled or simulated. In this case, the relevant mechanisms are thermal loss due to radiation and thermal loss due to convection. Thus, a voltage can be obtained at the high pass HP output, which voltage is proportional to the increase in temperature of the depletion layer of the suppressor diode  17  in relation to the environment. 
         [0054]    By way of a connection  85 , the ambient temperature is then supplied and added to the temperature increase of the depletion layer in stage  86 . There is thus obtained at the output of stage  86  a temperature value of the depletion layer of the suppressor diode  17 . This is then compared, in the comparator unit  88 , with a destruction temperature known from a data sheet which destruction temperature is fed to the comparator  88  via the connection  87 . The comparator  88  then provides information at its output as to whether the destruction temperature of the suppressor diode  17  has been exceeded or not. If it is found that the destruction temperature has been exceeded once, it is expedient to assume that the suppressor diode  17  or even the entire excess voltage protection apparatus  10  is destroyed. Thus, for example, a signal “suppressor diode defective” can be given at the output  95  and a signal “suppressor diode intact” at the output  96 . 
         [0055]    The present invention provides a novel two-stage excess voltage protection apparatus, which, unlike the prior art, provides clear statements on the condition of the excess voltage protection apparatus independently of the characteristics of any electric circuit to be protected. Using the method of the invention and the excess voltage protection apparatus of the invention, it is possible to monitor real component parameters. The method is independent of the parameters of the electrical equipment to be protected. It can be seen whether the ignition voltage of the gas arrester is still true to specification. Finally, it can be discerned whether the suppressor diode or other used diodes have been operated outside of their specification and, consequently, destruction thereof is to be assumed.