Abstract:
A method and a device are provided for monitoring an electrical power supply network. A temporal frequency change value is determined for at least two sections or for at least two points of the power supply network. The frequency change value indicates the respective temporal frequency change of the network frequency. A conclusion is drawn regarding a possible islanding within the power supply network, and a warning signal which indicates the possible islanding within the power supply network is generated when the difference between the frequency change values, exceeds a predetermined frequency change threshold value.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method for monitoring an electrical power supply network. 
     As is known, in the event of an overload an electrical power supply network originally synchronous with respect to its network frequency can be separated into individual electrical islands which are no longer intrinsically synchronous as a consequence of automatic opening of corresponding switching connections after defined electrical threshold values have been determined. Owing to the equilibrium between electrical power generation and electrical power consumption no longer being guaranteed in the individual zones of the power supply network immediately after islanding, after a certain delay a network frequency individual to the island or characteristic of the respective island in each case is established in the individual islands—determined by the self-regulating effect of the power supply network and the use of primary regulation of the power generators present in the power supply network. The network frequency must be reduced again to the respective frequency target value of, by way of example, 50 Hz or 60 Hz as promptly as possible with the power generation units available within the respective island section in order to prevent complete breakdown of the electrical power supply network or damage to the large electrical components forming part of the power supply network. 
     BRIEF SUMMARY OF THE INVENTION 
     According to this the object of the invention is to disclose a method for monitoring an electrical power supply network which can quickly and reliably indicate possible islanding within the power supply network. 
     This object is achieved according to the invention by a method having the features as claimed. Advantageous embodiments of the inventive method are disclosed in the subclaims. 
     According to this it is provided according to the invention that a temporal frequency change value is determined for at least two sections or for at least two points of the power supply network, which value indicates the respective temporal frequency change in the network frequency, and a conclusion is drawn regarding possible islanding within the power supply network, and a warning signal which indicates the possible islanding within the power supply network is generated if the difference between the frequency change values exceeds a predetermined frequency change threshold value. 
     A fundamental advantage of the inventive method can be seen in that possible islanding can be detected and displayed especially quickly since the temporal frequency change in the network frequency is evaluated for different points of the power supply network and the situation is monitored to see if a predetermined frequency change threshold value is exceeded. The inventors have found that by monitoring the frequency change values of different points of the power supply network possible islanding can be detected especially quickly and reliably. 
     The power supply network is preferably a distribution network, for example a distribution network of the medium voltage level or a distribution network of the low voltage level, or a transportation network, for example a transportation network of the high voltage level or a high voltage network. 
     The frequency change values for the at least two sections or for the at least two points are preferably measured by direct measurement of the frequency change values in these at least two sections or at these at least two points. 
     With respect to the formation of the frequency change values, it is deemed advantageous if the network frequencies are each measured for the at least two sections or for the at least two points of the power supply network by forming a time-dependent frequency value, and the time-dependent frequency values are derived with respect to time by forming one of the temporal frequency change values respectively. 
     A conclusion is preferably drawn regarding the existence of islanding within the power supply network and an islanding signal indicating islanding within the power supply network is generated if the difference between the frequency values exceeds a predetermined frequency threshold value. 
     After forming the islanding signal, the islanding signal is preferably only reset again or switched off if the difference between the frequency values falls below a predetermined first resetting threshold value, and the difference between the temporal frequency change values falls below a predetermined second resetting threshold value. Delayed or controlled resetting of an islanding signal once formed has the advantage of an untimely and possibly premature all-clear signal being avoided. 
     It is deemed advantageous if the first resetting threshold value matches the frequency threshold value or is less than the frequency threshold value. 
     The second resetting threshold value preferably matches the frequency change threshold value or is less than the frequency change threshold value. 
     It is also deemed advantageous if a pre-warning signal indicating possible islanding within the power supply network is generated if the difference between the network frequencies exceeds a predetermined pre-warning threshold value. 
     For measuring the network frequency and/or the frequency change values in the at least two sections or at the at least two points of the power supply network, it is deemed advantageous if what are known as pointer-type measuring instruments are used. It is therefore deemed advantageous if at least one pointer measured value, the respective frequency value pertaining to the pointer measured value and the respective temporal frequency change value are determined in each case by a pointer-type measuring instrument in the at least two sections or at the at least two points of the power supply network, the frequency values and the frequency change values are transferred to a central device, and the received frequency values and the received frequency change values are evaluated by the central device and the warning signal and/or the islanding signal is/are generated as a function of the received frequency values and/or the received frequency change values. 
     The phase difference by way of example between the phases of the voltage pointer in the at least two sections, its change (slip) and the change in slip (acceleration) of the voltage pointer are ascertained using the pointer-type measuring instruments. 
     The invention also relates to an arrangement for monitoring an electrical power supply network. With respect to such an arrangement it is provided according to the invention that arranged in at least two sections or at at least two points at least of the power supply network is at least one measuring instrument respectively which is capable of determining a frequency value indicating the respective network frequency and/or a temporal frequency change value indicating the respective temporal frequency change in the network frequency, and there is a central device which is capable of drawing a conclusion regarding possible islanding within the power supply network and of generating a warning signal indicating the possible islanding within the power supply network if the difference between the frequency change values exceeds a predetermined frequency change threshold value. 
     Reference is made with respect to the advantages of the inventive arrangement to the above statements in conjunction with the inventive method since the advantages of the inventive arrangement substantially match those of the inventive method. 
     The central device is preferably a device separate from the measuring instruments and is connected by a communications link to the measuring instruments. 
     Alternatively the central device can be integrated in a measuring instrument or can form part of one of the measuring instruments. 
     The invention also relates to a central device for an arrangement as has been described above. According to the invention it is provided with respect to such a central device that the central device has a computing device which is capable of receiving from at least two measuring instruments one temporal frequency change value respectively, indicating the respective temporal frequency change in the network frequency, and/or of determining the temporal frequency change values from measured values of the at least two measuring instruments and of generating a warning signal indicating possible islanding within the power supply network if the difference between the frequency change values exceeds a predetermined frequency change threshold value. 
     Reference is made with respect to the advantages of the inventive central device to the above statements in conjunction with the inventive method since the advantages of the inventive central device substantially match those of the inventive method. 
     The invention will be explained in more detail below with reference to exemplary embodiments, in which by way of example: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows an exemplary embodiment of an arrangement for monitoring an electrical power supply system with measuring instruments and a central device connected to the measuring instruments; 
         FIG. 2  shows by way of example the mode of operation of the central device of the arrangement according to  FIG. 1 , and 
         FIG. 3  shows by way of example a formation of resetting signals for the arrangement according to  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE INVENTION 
     For the sake of clarity, the same reference numerals will always be used in the figures for identical or comparable components. 
     A power supply network is illustrated only schematically in  FIG. 1  and identified by reference numeral  10 . Located within the power supply network  10  is a large number of measuring instruments, of which, for reasons of clarity, only three are shown in  FIG. 1 , and these are identified by reference numerals  20 ,  30  and  40 . The three measuring instruments  20 ,  30 ,  40  are located at different points or in different sections of the power supply network  10 . 
     A central device is connected to the three measuring instruments  20 ,  30 ,  40 , and this includes a computing device  60  and a memory  70 . A program P is stored in the memory  70 , and this determines the mode of operation of the computing device  60 . 
     The three measuring instruments  20 ,  30 ,  40  may by way of example be pointer-type measuring instruments, which are known a PMUs (Phasor Measuring Unit). Pointer-type measuring instruments of this kind are conventionally capable of determining pointer variables (i.e. complex measured values according to amount and phase), such as voltage pointer measured values or current pointer measured values by way of example. 
     It is assumed by way of example hereinafter that the three measuring instruments  20 ,  30 ,  40  each measure the network frequency at the point within the power supply network  10  associated with it and transfer the corresponding frequency values to the central device  50 . The frequency values are identified in  FIG. 1  by reference numerals f 1 , f 2  and f 3 . 
     Furthermore, the three measuring instruments  20 ,  30 ,  40  each also ascertain frequency change values f 1 ′, f 2 ′ and f 3 ′ which indicate the temporal frequency change in the network frequency at the respective point in the power supply system  10 . The three measuring instruments  20 ,  30 ,  40  can form the temporal frequency change values f 1 ′, f 2 ′ and f 3 ′ by way of example by a mathematical derivation of the time-dependent frequency values f 1 , f 2  and f 3  with respect to time, i.e. according to
 
 f 1′= df 1 /dt  
 
 f 2′= df 2 /dt  
 
 f 3′= df 3 /dt  
 
     The computing device  60  of the central device  50  receives the three frequency values f 1 , f 2  and f 3  and the three frequency change values f 1 ′, f 2 ′ and f 3 ′ and at the output side generates a pre-warning signal Svw, a warning signal Sw and/or an islanding signal Si if predetermined criteria are met in the program p and/or in the memory  70 . 
     An exemplary embodiment for the mode of operation of the central device  50  shall be explained below with reference to  FIG. 2 . It can be seen in  FIG. 2  that the central device  50  provides or processes three decision modules, of which a first decision module is identified by the reference numeral  80 , a second decision module by the reference numeral  90  and a third decision module by the reference numeral  100 . 
     In the first decision module  80  the central device  50  checks whether the difference between the frequency change values f 1 ′, f 2 ′ and f 3 ′ at the input side exceeds a predetermined frequency change threshold value Df′max or not. 
     If the first decision module  80  finds that at least one of the three differences exceeds the predetermined frequency change threshold value Df′max, then it generates the warning signal Sw at the output side, by way of example with logic “1”. 
     If, by contrast, the first decision module  80  finds that none of the three differences exceeds the predetermined frequency change threshold value Df′max, then a warning signal Sw by way of example with logic “0” is generated at the output side. 
     The logic level is therefore allocated by way of example as follows:
 
| f 1′− f 2′|&gt; Df ′max             Sw=“ 1”
 
| f 1′− f 3′|&gt; Df ′max           Sw=“ 1”
 
| f 2′− f 3′|&gt; Df ′max           Sw=“ 1”
 
| f 1′− f 2′|≦ Df ′max
 
and
 
| f 1′− f 3′|≦ Df ′max
 
and
 
≦| f 2′− f 3′|≦ Df ′max           Sw=“ 0”

     The second decision module  90  checks the three frequency values f 1 , f 2  and f 3 , which are supplied by the three measuring instruments  20 ,  30  and  40  according to  FIG. 1 . It forms differential values between the frequency values and generates the pre-warning signal Svw by way of example with logic “1” if at least one difference between the frequency values exceeds a predetermined pre-warning threshold value Dfpre. If, by contrast, the second decision module  90  finds that none of the differences between the network frequencies exceeds the predetermined pre-warning threshold value Dfpre, then a pre-warning signal Svw is generated at the output side, by way of example with logic “0”. 
     The following applies therefore by way of example for the allocation of the logic signal level:
 
| f 1− f 2|&gt; Df pre             Svw=“ 1”
 
| f 1− f 3|&gt; Df pre           Svw=“ 1”
 
| f 2− f 3|&gt; Df pre           Svw=“ 1”
 
| f 1− f 2|≦ Df pre
 
and
 
| f 1− f 3|≦ Df pre
 
and
 
| f 2− f 3|≦ Df pre            Svw=“ 0”

     The third decision module  100  serves to form an islanding signal Si if the frequency values f 1 , f 2  and f 3  supplied by the three measuring instruments  20 ,  30  and  40  according to  FIG. 1  suggest islanding; the third decision module  100  assumes this is the case if the differences between the frequency values f 1 , f 2  and f 3  are too great. The islanding signal Si is generated by way of example with logic “1” if at least one difference between the frequency values exceeds a predetermined frequency threshold value Dfmax. If, by contrast, the third decision module  100  finds that none of the three differences exceeds the predetermined frequency threshold value Dfmax, then an islanding signal Si with logic “0” is generated at the output side. 
     The following applies therefore by way of example for the allocation of the logic signal level:
 
| f 1− f 2|&gt; Df max             Si=“ 1”
 
| f 1− f 3|&gt; Df max           Si=“ 1”
 
| f 2− f 3|&gt; Df max           Si=“ 1”
 
| f 1− f 2|≦ Df max
 
and
 
| f 1− f 3|≦ Df max
 
and
 
| f 2− f 3|≦ Df max           Si=“ 1”

     It can be seen in  FIG. 2  that a signal memory  110  is downstream of the third decision module  100 , and this may be by way of example a flip-flop or the like. The function of the signal memory  110  consists in buffering the islanding signal Si. This shall be explained in more detail below. 
     If the islanding signal Si is generated with logic “1” by the third decision module  100 , then this logic “1” is transferred from the signal memory  110 , so the islanding signal Si is output at the output of the signal memory  110  with logic “1”. 
     The logic signal level “1” is stored by the signal memory  110  and passed at the output side even if the islanding signal Si is in the meantime no longer being generated by the third decision module  100 . The islanding signal Si output by the signal memory  110  is only reset to logic “0” if both a first resetting signal Sr 1  and a second resetting signal Sr 2  exhibit logic “1”. In other words, only if the output signal generated by the third decision module  100  exhibits logic “0” and at the same time the two resetting signals Sr 1  and Sr 2  exhibit logic “1” is the signal level of the islanding signal Si reset and the islanding signal Si output with logic “0”. 
     The first resetting signal can be formed by way of example with logic “1” if the difference between the frequency values exceeds a predetermined first resetting threshold value. 
     The second resetting signal Sr 2  can be generated by way of example with logic “1” if the difference between the temporal frequency change values falls below a predetermined second resetting threshold value. 
       FIG. 3  shows by way of example how the two resetting signals Sr 1  and Sr 2  may be easily formed: 
     The frequency threshold value Dfmax by way of example, which is used by the third decision module  100 , can be employed to form the first resetting signal Sr 1 . In this case the first resetting signal Sr 1  can be easily generated by inverting the islanding signal Si, output by the third decision module  100 , by means of an inverter  120 . The inverter  120  inverts the islanding signal Si of the third decision module  100  and outputs the inverted islanding signal Si to the signal memory  110  as a first resetting signal Sr 1 . 
     In the case of the second resetting signal Sr 2  the frequency change threshold value Df′max by way of example can be taken into account, and this is used by the first decision module  80  in the course of generating the warning signal Sw. A second inverter  130  by way of example can invert the output signal of the first decision module  80 , i.e. invert the warning signal Sw, and feed the inverted warning signal Sw as a second resetting signal Sr 2  into the signal memory  110 . 
     In the course of the described generation of the two resetting signals Sr 1  and Sr 2 , there is a resetting of an islanding signal S 1 , set previously to logic “1”, at the output of the signal memory  110  only if the islanding signal Si emitted at the output side by the third decision module exhibits logic “0” and at the same time the warning signal Sw of the first decision module  80  exhibits logic “0”. 
     The explanations in connection with  FIGS. 2 and 3  should be understood only as examples with respect to the logic signal levels of the signals. The signal levels of the signals Sw, Svw, Sr 1  and Sr 2  can of course also be defined in another way. In this case it is only necessary for the evaluation logic within the central device  50  to be adapted accordingly. 
     Although the invention has been illustrated and described in detail by preferred exemplary embodiments it is not restricted by the disclosed examples and a person skilled in the art can derive other variations herefrom without departing from the scope of the invention. 
     LIST OF REFERENCE NUMERALS 
     
         
           10  power supply network 
           20  measuring instrument 
           30  measuring instrument 
           40  measuring instrument 
           50  central device 
           60  computing device 
           70  memory 
           80  decision module 
           90  decision module 
           100  decision module 
           110  signal memory 
           120  inverter 
           130  inverter 
         Df′max frequency change threshold value 
         Dfmax frequency threshold value 
         Dfpre pre-warning threshold value 
         f 1  frequency value 
         f 2  frequency value 
         f 3  frequency value 
         f 1 ′ frequency change value 
         f 2 ′ frequency change value 
         f 3 ′ frequency change value 
         P program 
         Si islanding signal 
         Sr 1  resetting signal 
         Sr 2  resetting signal 
         Sw warning signal 
         Svw pre-warning signal