Abstract:
The disclosure relates to a standby power supply system for connection to a superordinate power supply network and a local power distribution network, having a network isolation device for connecting the local power distribution network to the superordinate power supply network by means of at least two series-connected switching elements with respectively associated switching contacts and having a local power supply device with a separate-network detector for identifying a separation situation for the local power distribution network. A first of the two switching elements is connected to the superordinate power supply network and the second of the two switching elements is connected to the local power supply device. The disclosure furthermore relates to methods for isolating a local power distribution network from a superordinate power supply network.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to International Patent Application number PCT/EP2013/077101, filed on Dec. 18, 2013, which claims priority to German Patent Application number 10 2012 113 016.7, filed on Dec. 21, 2012, and is hereby incorporated in its entirety. 
     FIELD 
     The disclosure relates to a standby power supply system for connection to a superordinate power supply grid and a local power distribution grid. The standby power supply system has a grid disconnection device for connecting or disconnecting the local power distribution grid to or from the superordinate power supply grid by means of at least two series-connected switching elements, each having assigned switching contacts. The standby power supply system furthermore has a local power supply device comprising an island detector for detecting an island situation of the local power distribution grid. The disclosure furthermore relates to a method for disconnecting a local power distribution grid from a superordinate power supply grid. 
     BACKGROUND 
     Standby power supply systems are used for providing electric power for a consumer arrangement in a local power distribution grid in the case where the superordinate, possibly public power supply grid cannot provide this power. Reasons for this can be, for example, a failure of and/or a fault on the superordinate power supply grid. The standby power supply system comprises a disconnection device which, during normal operation of the superordinate power supply grid, connects the power supply grid to the local power distribution grid. In the event of a failure of and/or a fault on the superordinate power supply grid the disconnection device disconnects the local power distribution grid from the superordinate power supply grid. After disconnection of the local power distribution grid the local power supply device connects onto this local grid in order to supply power to the consumer arrangement. The local power generation device may be an inverter, which is coupled to an energy storage device, for example a battery. Such standby power supply systems for switching from a normal operating mode in the case of supply by the superordinate power supply grid to island operation in the case of supply by the local power generating unit are known, for example, from the documents EP 1 956 483 A1 and DE 20 2010 008 123 U1. 
     Such local power supply devices generally have an island detector in order to detect the island situation of the local power distribution grid and to implement the above-described steps. Methods for detecting such an island situation (“Anti-Islanding Detection”, AID) are known, for example from the documents EP 2 003 759 A1 and EP 0 810 713 B1. 
     Regarding such standby power supply systems, for safety reasons it is necessary to avoid a situation in which the local power supply device supplies power to the local power distribution grid while the grid is still connected to the superordinate power supply grid. In such a situation, the safety of personnel performing maintenance work on the actually voltage-free superordinate power supply grid would no longer be ensured, for example. Therefore usually so-called contactors with priority control and a checkback contact are provided as switching elements in the grid disconnection device. By these contactors it is ensured that the checkback contact is closed when and only when all of the main contacts are actually disconnected. The checkback contact is connected to the local power supply device so that the local power supply device only supplies power to the local power supply grid when the checkback contact is closed. In addition, often two series-connected contactors are provided in order to ensure additional safety by virtue of redundancy. Compared with conventional contactors which are not provided with priority control and which are also referred to as “installation contactors”, the mentioned contactors with priority control are firstly more expensive and secondly, owing to their size, cannot be used with conventional installation boxes (“service entrance boxes”, “domestic subdistribution cabinet”) used in domestic installation. 
     SUMMARY 
     In one embodiment a standby power supply system is disclosed which, even without any switching elements with priority control and checkback contacts, ensures that the local power distribution grid is disconnected from the superordinate power supply grid with the level of safety required by the relevant regulations before a power supply to the local grid by the local power supply device can take place. 
     A standby power supply system according to one embodiment of the disclosure comprises a first of the two switching elements of the grid disconnection device, for actuation thereof, connected directly to the superordinate power supply grid, and the second of the two switching elements of the grid disconnection device, for actuation thereof, connected to the local power supply device. 
     By directly connecting the control lines of the first switching element to the superordinate power supply grid, the switching element opens when the voltage (holding voltage) in the superordinate power supply grid is no longer sufficient for actuating the switching element, i.e. when the superordinate power supply grid fails. At this instant a disconnection of the two grids is implemented. The second of the two switching elements is actuated by the local power supply device, which identifies an island situation via the island detector and can correspondingly likewise open safely the second switching element, completely independent of the correct operation of the first switching element. Thus, in relation to the first switching element, the “single-fault safety” which is often required by grid operators is ensured without the need to use contactors with priority control. 
     In the case of a method according to the disclosure for disconnecting a local power distribution grid from a superordinate power supply grid, the two mentioned grids are connected by a grid disconnection device comprising at least two series-connected switching elements, each comprising switching contacts. In this case, a control contact or coil of a first of the switching elements is connected directly to the superordinate power supply grid. At least one local power supply device is provided in the local power distribution grid, the local power supply device comprising an island detector for detecting an island situation of the local power distribution grid and an internal switching element for disconnecting the local power supply device from the local power distribution grid. Furthermore, a control contact or coil is provided, via which the second switching element is connected to the superordinate power supply grid. The local power distribution grid is monitored for the presence of an island situation by means of the island detector. If an island situation is detected, the switching contacts of the second switching element are opened by opening the control contacts, and the local power supply device is disconnected from the local power distribution grid by means of the internal switching element in order to open the switching contacts of the first switching element. 
     In this way, the method ensures the “single-fault safety” with respect to the second switching element. Should the disconnection from the superordinate power supply grid by the second switching element fail, it is necessary to ensure that the first switching element opens safely. For this purpose, the local power supply device is disconnected from the local power distribution grid. So the local power supply device cannot feed into the local grid and no holding voltage for the first switching element is generated. 
     Ideally, the local power supply device monitors that the voltage in the local power distribution grid collapses for a short period of time to ensure that further power feeders, for example PV generators with the associated inverters, which may be present in the local power distribution grid also disconnect themselves from the local power distribution grid. It is thus ensured that the first switching element no longer receives a holding voltage and thus, even in the case of a fault of the second switching element, safe disconnection of the local power distribution grid from the superordinate power supply grid is realized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be explained in more detail below with reference to an exemplary embodiment with the aid of three figures, in which: 
         FIG. 1  shows a part of a superordinate power supply grid, to which a local power distribution grid is connected via a standby power supply system; 
         FIG. 2  shows a detailed schematic illustration of the standby power supply system shown in  FIG. 1 , and 
         FIG. 3  shows a schematic illustration of a disconnection device for a standby power supply system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows in a schematic illustration, a standby power supply system  10 , which is connected firstly to a superordinate power supply grid  20  and secondly to a local power distribution grid  30 , to which consumers  31  are connected, inter alia. 
     Only one section, a so-called secondary distribution grid  23 , of the superordinate power supply grid  20  is shown. Power is supplied to the secondary distribution grid  23  via a medium-voltage connection  21  and a secondary distribution grid transformer  22 . A plurality of domestic installations  24  are connected to the secondary distribution grid  23 . 
     The right-hand part of  FIG. 1  illustrates in more detail one of the domestic installations  24  starting from a house transfer point  25 . This domestic installation  24  comprises the standby power supply system  10  and the local power distribution grid  30 . The standby power supply system  10  comprises a grid disconnection device  11  and a local power supply device  12 , in this case a battery inverter with connected battery  13  as energy storage device, by way of example. The grid disconnection device  11  is actuated by the local power supply device  12  via a control line  14 . In addition to the already mentioned consumers  31 , furthermore a photovoltaic (PV) inverter  32  with assigned PV generator  33  is provided in the local power distribution grid  30 . The PV inverter here represents, by way of example, local power feeders or local power generating units which can feed into an existing power grid but cannot build up a grid by themselves, however. The inclusion of any other desired sources is possible. 
       FIG. 1  shows, by way of example, two possible grid fault situations within the secondary distribution grid  23 . Both situations relate to a grid failure on the basis of a disconnection of the secondary distribution grid line. Such a disconnection can occur, for example, accidently as a result of construction work owing to the secondary distribution grid cable being severed or during the course of maintenance work. With respect to the house transfer point  25 , the location of a first illustrated grid fault  26  is relatively far away with respect to a second illustrated grid fault  27 , which occurs in the direct physical vicinity of the house transfer point  25 . The first grid fault  26  is therefore also referred to as remote grid fault and correspondingly the second grid fault  27  is referred to as close grid fault  27 . In the case of a close grid fault  27 , a high-resistance disconnection of the superordinate power supply grid  30  is present, when viewed from the house transfer point  25 . In the case of the remote grid fault  26 , a series of domestic installations  24  are still connected to the severed secondary distribution grid cable, when viewed from the house transfer point  25 . When viewed from the house transfer point  25 , the remote grid fault  26  has a low resistance owing to a probably large number of consumers within the domestic installation  24  or comes close to a short-circuited secondary distribution grid cable. 
       FIG. 2  shows, in a block circuit diagram, the design of the standby power supply system  10  in more detail. The standby power supply system  10  is connected firstly to the superordinate power supply grid  20  and secondly to the local power distribution grid  30 .  FIG. 2  shows, by way of example, only two connecting lines here, namely a PEN conductor N and a phase conductor L 1  in each case. It goes without saying that a three-phase connection to a PEN conductor N and three separate phase lines L 1  to L 3  and possibly a connection to ground may also be provided. In the connection between the superordinate power supply grid  20  and the local power distribution grid  30 , in the region of a grid disconnection device  11 , a first switching element  111  and a second switching element  114  are arranged connected in series. The two switching elements are capable of disconnecting all poles of the two grids  20 ,  30  via switching contacts (not illustrated here). In addition, a grid monitoring unit  16  for the superordinate power supply grid  20  is provided, the grid monitoring unit continuing to monitor the superordinate power supply grid  20  in the event of failure of the superordinate power supply grid  20  after disconnection of the local power distribution grid  30  and the superordinate power supply grid  20 . On recovery of the superordinate power supply grid  20 , this recovery is signaled to the local power supply device  12  by the grid monitoring unit  16  so that, once the phase angle in the two grids  20 ,  30  has been synchronized, the grids can be connected to one another again. 
     The local power supply device  12  comprises an inverter  126 , which is connected to the battery  13  (not illustrated here). On the AC side, an internal switching element  123 , for example a contactor, is connected downstream of the inverter  126 , as well as grid monitoring device  127  for the local power distribution grid. The inverter  126  is connected on the AC-voltage side to the local power distribution grid  30  via the internal switching element  123 . This connection is in this case likewise single-phase corresponding to the configuration of the grids  20 ,  30 . It goes without saying, in the case of polyphase grids  20 ,  30 , that the inverter  126  and the connection to the local power distribution grid  30  may also be polyphase. 
     The local power supply device  12  furthermore comprises a data acquisition device  125 , which is connected to a controller  124 . The data acquisition device  125  receives information (measured values, status data) from the inverter  126 , the grid monitoring device  127  and possibly from a checkback contact of the second switching element  114  and makes this information available to the controller  124 . In an alternative configuration, it is possible for the data acquisition device  125  to be integrated in the controller  124 . The controller  124  controls the functions of the standby power supply system  10 , in particular the inverter  32  and the switching elements  123 ,  122 . 
     Embodied as part of the controller  124  or alternatively also separately, the local power supply device additionally comprises an island detector  121 . The island detector is data-connected to the grid monitoring device  127  of the local power supply grid, which monitors the grid conditions with respect to the permissible limits for voltage and frequency. These limits are sometimes differing from Nation to Nation. 
     The island detector  121  is capable of detecting an island situation present in the local power supply grid  30 . In order to detect a present island situation, in one embodiment the detector uses known anti-islanding detection (AID) methods. For this purpose it receives the necessary measured values (voltage, current, frequency of the connected grid  30 ) from the grid monitoring  127  via data acquisition  125  and actuates the inverter  126  correspondingly if an active AID method is used. 
     The island detector  121  is set up such that an island situation is detected both when a close grid fault  27  (cf.  FIG. 1 ) occurs and when a remote fault grid fault  26  is present. The detection of an island situation is passed on to the controller  124 , which actuates the second switching element  114  via a switching contact  122  in such a way that the switching element  114  opens. The first switching element  111  is actuated directly via the superordinate power supply grid  20  and opens since there is no holding voltage available to it. 
     If further local energy sources are arranged in the local power distribution grid  30 , for example the PV system illustrated in  FIG. 1  comprising PV inverter  32  and PV generator  33 , it should be ensured that the energy sources are disconnected in the event of the presence of an island situation. This maybe realized, for example, by the PV inverter  32  having an apparatus which is similar to the island detector  121  for detecting an island situation. Alternatively, it is also conceivable for the PV inverter  32  to be actuated via the island detector  121  of the standby power supply system  10 . 
       FIG. 3  shows the design of a grid disconnection device  11 , as may be used in the standby power supply system  10  in  FIG. 2 , for example, in more detail. 
     In this embodiment contactors without priority control, so-called installation contactors, are used as switching elements  111  and  114 . This advantageously makes it possible for the entire grid disconnection device  11  to be integrated in conventional domestic installation boxes (“service entrance boxes”, “domestic subdistribution cabinet”) without dispensing with safety standards. 
     In  FIG. 3 , the grids  20 ,  30  are three-phase grids plus neutral line. The switching elements  111 ,  114  correspondingly each have four switching contacts  113 ,  116 . Each of the switching elements  111 ,  114  therefore disconnects the connection between the grids  20 ,  30  at all poles. The switching contacts of the two switching elements  111 ,  114  are connected in series for each of the lines. 
     The switching elements  111 ,  114  have control coils, lines or contacts  112 ,  115  by which such elements may be signaled. The control coil  112  of the first switching element  111  is connected directly to the superordinate power supply grid  20 . The control coil  115  of the second switching element  114  taps off its supply voltage between the switching contacts  113  and  116  of the first and second switching elements  111 ,  114 , respectively, wherein the supply voltages are passed via the control line  14  via the switching contact  122  in the local power supply device  12 . In one embodiment the two switching elements  111 ,  114  are so-called “normally-open contacts”, i.e. if there is an insufficiently high voltage (holding voltage) present at the control coil of the switching element, the switching contacts are open. 
     Optionally, the second switching element  114  can be provided with a checkback contact (not shown), which is detected by the data acquisition device  125 , wherein provision is then made for the internal switching element  123 , via which the inverter  126  is connected onto the local power distribution grid  30 , to only be switched on when the checkback contact communicates non-actuation of the switching element  114 . This is an additional safety measure. 
     Even if, as in the embodiment in  FIG. 3 , switching elements  111 ,  114  which do not have priority control are used, the standby power supply system  10  illustrated has “single-fault safety” in respect of the mode of operation of the switching elements  111 ,  114 . 
     If one or more of the switching contacts  113  of the first switching element  111  are fused, for example, and do not open correctly, the switching element  114  nevertheless opens owing to the actuation via the switching contact  122  of the local power supply device  12  once an island situation has been detected. On the other hand, if one or more of the switching contacts  116  of the second switching element  114  are fused and do not open correctly, the switching element  111  opens since there is no holding voltage available to the control coil  112  owing to the failure of the superordinate power supply grid  20 . 
     In the case of a close grid fault  27  (cf.  FIG. 1 ), a case may arise in which generation and consumption (by the consumers  31 ) can be compensated for locally at least in the short term in the case of the local power supply grid  30  in the case of the connected local power supply device  12 , plus possibly further power generating units. Thus, the situation can occur in which, in the event of faulty operation of the second switching element  114 , the first switching element  111  does not open although the superordinate power supply grid  20  fails since the local power supply device  12  generates the holding voltage for the control coils  112 . For this case, the local power supply device  12  is disconnected from the local power distribution grid  30  by the internal switching element  123  in order to safely draw the holding voltage from the first switching element  111 . 
     In the case where further local power generating units are provided in the local power distribution grid  30 , the local power supply device  12  can, by means of its grid monitoring device  127 , determine a voltage collapse of the local power supply grid  30  and thus ensure that the first switching element  111  opens. In addition, with this measure, faulty operation of the control contacts  122  is also intercepted. Should the control contacts not open despite actuation by the power supply device, it is ensured by the controlled voltage dip that the second switching element  114  opens. The linking of the actuation to the superordinate power supply grid additionally results in the second switching element only being able to be switched on again on recovery of the superordinate power supply grid. 
     Once the local power distribution grid  30  has been disconnected from the superordinate power supply grid  20 , the local power supply device  12  can be connected again by means of its internal switching element  123  and build up a local voltage supply. The locally recovered grid  30  is also identified by the local power generating units  32  so that the energy generating units are likewise again switched on and feed into the local power distribution grid  30 . The recovery of the superordinate power supply is monitored by means of grid monitoring of the superordinate power supply grid  16  ( FIG. 2 ) so that once the local power distribution grid  30  has been synchronized with the superordinate power supply grid  20  as regards voltage and frequency, the two grids  20 ,  30  can be reconnected again to one another.