Abstract:
A method of verifying a protection apparatus is provided. The method includes: setting a plurality of relay elements for sensing an abnormal state of the protection apparatus; receiving an input regarding test information for testing each of the plurality of relay elements; and when at least one of the plurality of relay elements is in an abnormal state from the reception of the input regarding the test information, identifying whether the protection apparatus has created an abnormal state sensing signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Pursuant to 35 U.S.C. §119(a), this application claims the benefit of earlier filing date and right of priority to Korean Patent Application No. 10-2013-0077655, filed on Jul. 3, 2013, which is hereby incorporated by reference in its entirety into this application. 
       BACKGROUND 
       [0002]    The present disclosure relates to a protection apparatus and a method of verifying an operation thereof, and more particularly, to a method of verifying an operation of a protection apparatus for an AC filter or converter transformer used in a high voltage direct current (HVDC) system. 
         [0003]    A high voltage direct current (HVDC) system is used for long-distance transmission and transmission of electric power using underwater power cables. Since the HVDC system generates reactive power and harmonic waves while converting electric power using a converter, it needs an AC filter. 
         [0004]    To optimize operations of the AC filter that may be damaged by various factors and protect the AC filter, a protection apparatus including a specific relay and a protection panel is implemented. 
         [0005]    Particularly, the relay is a device to open or close an electric circuit depending on various input signals, such as temperature, light, and the like as well as electric signals, such as voltage, current, power, frequency, and the like, and has various purposes as well as a general control purpose. Such a relay is required to satisfy conditions, such as high reliability, long lifespan, high sensitivity, and the like, and for satisfying such conditions, tests for verifying operation characteristics according to subjects of application are required. 
         [0006]    The protection apparatus may realize high reliability of the HVDC system by protecting the AC filter or an inner circuit of a converter transformer bank through verification of relay elements, such as percentage differential, unbalanced current, harmonic overload, overcurrent, ground overcurrent relay elements. Therefore, a test for verifying performances of the AC filter or a protection relay and a protection panel of the converter transformer is essential and an important issue. 
       SUMMARY 
       [0007]    Embodiments provide a protection apparatus and a method of verifying an operation of the protection apparatus capable of simplifying a complicated test procedure for protecting an AC filter or a converter transformer bank in an HVDC system and at the same time enhancing the reliability. 
         [0008]    Embodiments also provide a protection apparatus and a method of verifying an operation thereof capable of simplifying a complicated test procedure of the protection apparatus to enhance economic feasibilities, such as time, costs, and the like for the test and increasing easiness of a correction test. 
         [0009]    In one embodiment, a method of verifying a protection apparatus includes: setting a plurality of relay elements for sensing an abnormal state of the protection apparatus; receiving an input regarding test information for testing each of the plurality of relay elements; and when at least one of the plurality of relay elements is has an abnormal state according to the reception of the input regarding the test information, identifying whether the protection apparatus generates an abnormal state sensing signal. 
         [0010]    According to various embodiments, the complicated test procedure of the protection apparatus may be simplified and at the same time, reliability may be enhanced. 
         [0011]    Also, according to various embodiments, the complicated test procedure of the protection apparatus may be simplified to enhance economic feasibilities, such as time, costs, and the like for the test, and easiness of the correction test may be increased. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a block diagram of a protection apparatus according to an embodiment. 
           [0013]      FIG. 2  is a circuit diagram of a protection apparatus according to an embodiment. 
           [0014]      FIG. 3  is a circuit diagram of a double-tuned filter according to an embodiment. 
           [0015]      FIG. 4  is a circuit diagram of a high pass filter according to an embodiment. 
           [0016]      FIG. 5  is a block diagram of a protection apparatus according to another embodiment. 
           [0017]      FIG. 6  is a block diagram of a protection apparatus according to still another embodiment. 
           [0018]      FIG. 7  is a circuit diagram of converter transformer included in a protection apparatus according to yet another embodiment. 
           [0019]      FIG. 8  is a flow diagram for explaining a method for verifying an operation of a protection apparatus according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0020]    Hereinafter, detailed description of embodiments related to the present invention will be made with reference to the accompanying drawings. A suffix “module” or “unit” used for constituent elements disclosed in the following description is merely intended for easy description of the specification, and the suffix itself does not give any special meaning or function. 
         [0021]    A structure of a protection apparatus according to an embodiment will now be described with reference to  FIG. 1 . 
         [0022]      FIG. 1  is a block diagram of a protection apparatus according to an embodiment. 
         [0023]    Referring to  FIG. 1 , a protection apparatus  10  may include an AC bus  100 , a double tuned filter (DTF)  200 , a high pass filter (HPF)  300 , a filter protection unit  400 , and a controller  500 . Since the elements shown in  FIG. 1  may not be essential, the protection apparatus  10  may be implemented to have elements more or less than the elements shown in  FIG. 1 . In an embodiment, the controller  500  may be included in the filter protection unit  400 . 
         [0024]    Hereinafter, the above-described elements will be sequentially reviewed. 
         [0025]    The AC bus  100  may transmit AC power supplied from an AC power source to the double tuned filter  200  or high pass filter  300 . 
         [0026]    The double tuned filter (DTF)  200  is a filter having band characteristics tuned to two frequencies in a signal of the AC power supplied from the AC bus  100 . 
         [0027]    The high pass filter (HPF)  300  is a filter that passes through only a signal having a signal having a frequency higher than a specific frequency in the signal of the received AC power. 
         [0028]    The filter protection unit  400  may generate an abnormal state sensing signal through information regarding current supplied from the double tuned filter  2000  and the high pass filter  300 . The filter protection unit  400  may include a double tuned filter protection unit  410 , a high pass filter protection unit  430 , and an operation verifying unit  450 . In particular, the double tuned filter protection unit  410  may generate an abnormal state sensing signal through information on current supplied from the double tuned filter  200 . The high pass filter protection unit  430  may generate an abnormal state sensing signal through information regarding current supplied from the high pass filter  300 . 
         [0029]    In an embodiment, the abnormal state sensing signal may be a signal notifying occurrence of an abnormal state when any one of the elements constituting the protection apparatus  10  is in an abnormal state. The abnormal state sensing signal may include a trip signal and an alarm signal. The trip signal may be an interrupt signal for interrupting operations of elements constituting the protection apparatus  10  when any of the elements constituting the protection apparatus  10  is in an abnormal state. The alarm signal may be a signal for alarming occurrence of an abnormal state when any of the elements constituting the protection apparatus  10  is in an abnormal state. 
         [0030]    The operation verifying unit  450  may identify an inner line connection state of the protection apparatus  10  and also identify whether any of the elements constituting the protection apparatus  10  is in an abnormal state by setting a plurality of relay elements. Further, the operation verifying unit  450  may identify whether or not the filter protection unit  400  correctly generates an abnormal state sensing signal according to whether or not an abnormal state occurs. Detailed description of the operation verifying unit  450  will be made later. 
         [0031]    The controller  500  may control overall operations of the protection apparatus  10 . Detailed operations of the controller  500  will be described later. 
         [0032]    Next, a configuration of the protection apparatus according to an embodiment will be described with reference to  FIGS. 2 to 4 . 
         [0033]      FIG. 2  is a circuit diagram of a protection apparatus according to an embodiment,  FIG. 3  is a circuit diagram of a double-tuned filter according to an embodiment, and  FIG. 4  is a circuit diagram of a high pass filter according to an embodiment. 
         [0034]    Referring to  FIG. 2 , the protection apparatus  10  may include the AC bus  100 , the double tuned filter  200 , the high pass filter  300 , and the filter protection unit  400  as described with reference to  FIG. 1 . 
         [0035]    The double tuned filter  200  and the high pass filter  300  may be connected to each other by the AC bus  100 . In detail, the AC bus  100  may connect an input terminal of the double tuned filter  200  and an input terminal of the high pass filter  300  to each other, and may transmit AC power supplied from the AC power source to the double tuned filter  200  and the high pass filter  300 . 
         [0036]    The double tuned filter  200  may be connected to the double tuned filter protection unit  410  of the filter protection unit  400 , and the high pass filter  300  may be connected to the high pass filter protection unit  430  of the filter protection unit  400 . The double tuned filter protection unit  410  may generate an abnormal state sensing signal through information regarding current supplied from the double tuned filter  200 . 
         [0037]    Next, a configuration of the double tuned filter  200  will be described in detail with reference to  FIG. 3 . 
         [0038]    Referring to  FIG. 3 , the double tuned filter  200  may include a circuit breaker  210 , a capacitor bank  220 , a reactor  230 , an arrester  240 , a reactor current transformer  250 , and a ground current transformer  260 . 
         [0039]    The circuit breaker  210  may include an overcurrent measuring unit  211  and a gas insulated switchgear (GIS)  213 . The circuit breaker  210  may allow transmission current supplied through the AC bus  100  to flow or be interrupted. 
         [0040]    The overcurrent measuring unit  211  may measure current inputted into the double tuned filter  200  from the AC bus  100  in order to identify whether the current inputted into the double tuned filter  200  from the AC bus  100  corresponds to an overcurrent or a ground overcurrent. In an embodiment, the overcurrent measuring unit  211  may be a current transformer (CT). 
         [0041]    The overcurrent measuring unit  211  may convert an overcurrent to a current which is in proportional to a level of the current inputted to the double tuned filter  200  from the AC bus  100  and has a low level. The current transformer CT used as the overcurrent measuring unit  211  is a device extending the range of current measurement, and may convert a high current flowing through a circuit to a required low current value and then measure the converted current. 
         [0042]    The overcurrent measuring unit  211  may transmit information regarding the measured current to the double tuned filter protection unit  410  of the filter protection unit  400 . The double tuned filter protection unit  410  may generate an abnormal state sensing signal activating or inactivating the gas insulated switchgear  213  through the current measured by the overcurrent measuring unit  211 . In detail, when the current measured by the overcurrent measuring unit  211  corresponds to an overcurrent exceeding a preset current value, the double tuned filter protection unit  410  may generate an abnormal state sensing signal and transmit the generated abnormal state sensing signal to the controller  500 . The controller  500  may identify that the overcurrent is inputted into the double tuned filter  200  through the received abnormal state sensing signal and activate the gas insulated switchgear  213 , and the activated gas insulated switchgear  213  may interrupt the current flowing through the AC bus  100 . Meanwhile, when the current measured by the overcurrent measuring unit  211  does not exceed a preset current value, the double tuned filter protection unit  410  may not generate the abnormal state sensing signal, and the controller  500  may inactivate the gas insulated switchgear  213  to allow the current to be continuously inputted to the double tuned filter  200  from the AC bus  100 . 
         [0043]    When the current measured by using the current measured by the overcurrent measuring unit  211  corresponds to an overcurrent or a ground overcurrent, the gas insulated switchgear  213  may be activated to interrupt the current inputted into the double tuned filter  200  from the AC bus  100 . 
         [0044]    The capacitor bank  220 , the reactor  230 , the arrester  240 , and the reactor transformer  250  may play a role in removing a harmonic wave component of AC current flowing through the AC bus  100 . 
         [0045]    The capacitor bank  220  may identify whether an output current of the current breaker  210  is unbalanced or not. The capacitor bank  220  may include a first capacitor bank  221 , a second capacitor bank  223 , and an unbalanced current measuring unit  225 . Each of the first capacitor bank  221  and the second capacitor bank  223  may include a plurality of capacitive elements. The capacitive element may be a capacitor. The unbalanced current measuring unit  225  may measure a current at point ‘a’ of the first capacitor bank  221  and a current at point ‘b’ and transmit information regarding each of the measured currents to the double tuned filter protection unit  410 . The double tuned filter protection unit  410  may identify whether or not there is an unbalance between the current at point ‘a’ and the current at point ‘b’ through the information transmitted from the unbalanced current measuring unit  225 , and may generate an abnormal state sensing signal. In detail, when it is identified that there occurs an unbalance between the current at point ‘a’ and the current at point ‘b’, the double tuned filter protection unit  410  may generate an abnormal state sensing signal and transmit the generated abnormal state sensing signal to the controller  500 , and the controller may identify that an unbalanced current flows to the capacitor bank  220  through the received abnormal state sensing signal and perform a control such that a balanced current flows to the first capacitor bank  221  and the second capacitor bank  223 . 
         [0046]    The reactor  230  may remove a harmonic wave component from the output current of the capacitor bank  220 . The reactor  230  may include one inductor. 
         [0047]    The arrester  240  may prevent an instantaneous overvoltage or impulse voltage from being applied to the reactor  230 . In the case an instantaneous overvoltage or impulse voltage is applied to the reactor  230 , the arrester  240  may remove the instantaneous overvoltage or impulse voltage to protect the reactor  230 . 
         [0048]    The reactor current transformer  250  may remove a harmonic wave component from an AC output current of the reactor  230 . 
         [0049]    The reactor current transformer  250  may include a capacitive passive element  251 , an inductive passive element  253 , an arrester  255 , and a harmonic overcurrent measuring unit  257 . The capacitive passive element  251  may be a capacitor and the inductive passive element  253  may include an inductor. The arrester  255  may prevent an instantaneous overvoltage or impulse voltage from being applied to the inductive element  253 . The harmonic overcurrent measuring unit  257  may transmit information regarding an output current of the inductive element  253  to the double tuned filter protection unit  410  of the filter protection unit  400  in order to remove a harmonic component from the output current of the inductive element  253 . The double tuned filter protection unit  410  may identify whether or not the reactor current transformer  250  correctly removes a harmonic component through the information supplied from the harmonic overcurrent measuring unit  257 , and may generate an abnormal state sensing signal according to the identified states. In detail, when it is identified that the reactor current transformer  250  does not correctly remove a harmonic component, the double tuned filter protection unit  410  may generate an abnormal state sensing signal and transmit the generated abnormal state sensing signal to the controller  500 . The controller  500  may identify that a harmonic component is correctly removed from the output current of the reactor current transformer  250  through the received abnormal state sensing signal and may control the double tuned filter  200  such that the harmonic component is removed from the output current of the reactor current transformer  250 . 
         [0050]    The ground current transformer  260  may measure an output current of the double tuned filter  200 . The overcurrent measuring unit  211  may transmit information regarding current inputted from the AC bus  100  into the double tuned filter  200  to the double tuned filter protection unit  410 , and the ground current transformer  260  may measure the output current of the double tuned filter  200  and transmit information regarding the measured current to the double tuned filter protection unit  410 . The double tuned filter protection unit  410  may compare the input current of the double tuned filter  200  with the output current of the double tuned filter  200  by using the information regarding the current supplied from the overcurrent measuring unit  211  and the ground current transformer  260 . The double tuned filter protection unit  410  may compare the input current of the double tuned filter  200  with the output current of the double tuned filter  200  to identify that a difference between the input current and the output current of the double tuned filter  200  is within a preset difference. When the difference exceeds the preset difference, the double tuned filter protection unit  410  may generate an abnormal state sensing signal and transmit the generated abnormal state sensing signal to the controller  500 . In an embodiment, the present difference may differ according to a user&#39;s setup. 
         [0051]    Next, a configuration of the high pass filter  300  will be described with reference to  FIG. 4 . 
         [0052]    Referring to  FIG. 4 , the high pass filter  300  may include a current breaker  310 , a capacitor bank  320 , a reactor current transformer  330 , and a ground current transformer  340 . 
         [0053]    The circuit breaker  310  may include an overcurrent measuring unit  311  and a gas insulated switchgear (GIS)  313 . The circuit breaker  310  may allow transmission current inputted from the AC bus  100  to flow or be interrupted. 
         [0054]    The overcurrent measuring unit  311  may measure a current inputted to the high pass filter  300  from the AC bus  100  in order to identify whether the current inputted to the high pass filter  300  from the AC bus  100  corresponds to an overcurrent or a ground overcurrent. In an embodiment, the overcurrent measuring unit  311  may be a current transformer (CT). 
         [0055]    The overcurrent measuring unit  311  may convert an overcurrent to a current which is in proportional to a level of the current inputted into the high pass filter  300  from the AC bus  100  and has a low level. The current transformer CT used as the overcurrent measuring unit  311  is a device extending the range of current measurement, and may convert a high current flowing through a circuit to a required low current value and then measure the converted current. 
         [0056]    The overcurrent measuring unit  311  may transmit information regarding the measured current to the high pass filter protection unit  430  of the filter protection unit  400 . The high pass filter protection unit  430  may generate an abnormal state sensing signal activating or inactivating the gas insulated switchgear  311  through the current measured by the overcurrent measuring unit  313 . In detail, when the current measured by the overcurrent measuring unit  311  corresponds to an overcurrent exceeding a preset current value, the high pass filter protection unit  430  may generate an abnormal state sensing signal and transmit the generated abnormal state sensing signal to the controller  500 . The controller  500  may identify that the overcurrent is inputted into the high pass filter  300  through the received abnormal state sensing signal and activate the gas insulated switchgear  313 , and the activated gas insulated switchgear  313  may interrupt the current flowing through the AC bus  100 . Meanwhile, when the current measured by the overcurrent measuring unit  311  does not exceed a preset current value, the high pass filter protection unit  430  may not generate the abnormal state sensing signal, and the controller  500  may inactivate the gas insulated switchgear  313  to allow the current to be continuously inputted into the high pass filter  300  from the AC bus  100 . 
         [0057]    When the current measured by using the current measured by the overcurrent measuring unit  313  corresponds to an overcurrent or a ground overcurrent, the gas insulated switchgear  311  may be activated to interrupt the current flowing into the high pass filter  300  from the AC bus  100 . 
         [0058]    The capacitor bank  320 , the reactor current transformer  330 , and the ground current transformer  340  may play a role in removing a harmonic wave component of AC current inputted from the AC bus  100 . 
         [0059]    The capacitor bank  320  may identify whether an output current of the current breaker  310  is unbalanced or not. The capacitor bank  320  may include a first capacitor bank  321 , a second capacitor bank  323 , and an unbalanced current measuring unit  325 . Each of the first capacitor bank  321  and the second capacitor bank  323  may include a plurality of capacitive devices. The capacitive element may be a capacitor. The unbalanced current measuring unit  325  may measure a current at point ‘c’ of the first capacitor bank  321  and a current at point ‘d’ and transmit information regarding each of the measured currents to the high pass filter protection unit  430 . The high pass tuned filter protection unit  430  may identify whether or not there is an unbalance between the current at point ‘c’ and the current at point ‘d’ through the information transmitted from the unbalanced current measuring unit  325 , and may generate an abnormal state sensing signal. In detail, when it is identified that there occurs an unbalance between the current at point ‘c’ and the current at point ‘d’, the high pass filter protection unit  430  may generate an abnormal state sensing signal and transmit the generated abnormal state sensing signal to the controller  500 , and the controller may identify that an unbalanced current flows to the capacitor bank  320  through the received abnormal state sensing signal and perform a control such that a balanced current flows into the first capacitor bank  321  and the second capacitor bank  323 . 
         [0060]    The reactor current transformer  330  may remove a harmonic wave component from an AC output current of the capacitor bank  320 . 
         [0061]    The reactor current transformer  330  may include a passive element  331 , an inductive passive element  333 , an arrester  335 , and a harmonic overcurrent measuring unit  337 . The passive element  331  may be a resistor and the inductive passive element  333  may include an inductor. The arrester  335  may prevent an instantaneous overvoltage or impulse voltage from being applied to the inductive element  333 . The harmonic overcurrent measuring unit  337  may transmit information regarding an output current of the inductive element  333  to the high pass filter protection unit  430  of the filter protection unit  400  in order to remove a harmonic component from the output current of the inductive element  333 . The high pass filter protection unit  430  may identify whether or not the reactor current transformer  330  correctly removes a harmonic component through the information supplied from the harmonic overcurrent measuring unit  337 , and may generate an abnormal state sensing signal according to the identified states. In detail, when it is identified that the reactor current transformer  330  does not correctly remove a harmonic component, the high pass filter protection unit  430  may generate an abnormal state sensing signal and transmit the generated abnormal state sensing signal to the controller  500 . The controller  500  may identify that a harmonic component is correctly removed from the output current of the reactor current transformer  330  through the received abnormal state sensing signal and may control the high pass filter  300  such that the harmonic component is removed from the output current of the reactor current transformer  330 . 
         [0062]    The ground current transformer  340  may measure an output current of the high pass filter  300 . The overcurrent measuring unit  311  may transmit information regarding the current inputted from the AC bus  100  into the high pass filter  300  to the high pass filter protection unit  430 , and the ground current transformer  340  may measure the output current of the high pass filter  300  and transmit information regarding the measured current to the high pass filter protection unit  410 . The high pass filter protection unit  410  may compare the input current of the high pass filter  300  with the output current of the high pass filter  300  by using the information regarding the current supplied from the overcurrent measuring unit  311  and the ground current transformer  340 . The high pass filter protection unit  430  may compare the input current of the high pass filter  200  with the output current of the high pass filter  300  to identify that a difference between the input current and the output current of the high pass filter  300  is within a preset difference. When the difference exceeds the preset difference, the high pass filter protection unit  430  may generate an abnormal state sensing signal and transmit the generated abnormal state sensing signal to the controller  500 . In an embodiment, the present difference may differ according to a user&#39;s setup. 
         [0063]    Next, a protection apparatus and a method of verifying an operation thereof according to another embodiment will be described with reference to  FIGS. 5 to 7 . 
         [0064]      FIG. 5  is a block diagram of a protection apparatus according to another embodiment,  FIG. 6  is a block diagram of a protection apparatus according to still another embodiment, and  FIG. 7  is a circuit diagram of converter transformer included in a protection apparatus according to yet another embodiment 
         [0065]    First, referring to  FIG. 5 , a protection apparatus  20  according to another embodiment may include an AC bus  600 , a converter transformer (CT) bank  700 , a valve  800 , a transformer protection unit  900 , and a controller  1000 . Since the elements shown in  FIG. 5  may not be essential, the protection apparatus  20  may be implemented to have elements more or less than the elements shown in  FIG. 5 . In an embodiment, the controller  1000  may be included in the transformer protection unit  900 . 
         [0066]    Hereinafter, the above-described elements will be sequentially reviewed. 
         [0067]    The AC bus  600  may transmit an AC power supplied from an AC power source to the CT bank  700 . 
         [0068]    The CT bank  700  may be connected to the AC bus  600  and the valve  800 . 
         [0069]    The valve  800  may convert an AC power to a DC power and vice versa. That is, the valve  800  is a power electronic element to convert an AC voltage to a DC voltage and vice versa. 
         [0070]    The transformer protection unit  900  may generate an abnormal state sensing signal through information regarding current supplied from the CT bank  700 . In an embodiment, the abnormal state sensing signal may be a signal notifying occurrence of an abnormal state when any one of the elements constituting the protection apparatus  20  is in an abnormal state. The abnormal state sensing signal may include a trip signal and an alarm signal. The trip signal may be an interrupt signal for interrupting an operation of an element of the protection apparatus  20  when the element of the protection apparatus  20  is in an abnormal state. The alarm signal may be a signal for alarming occurrence of an abnormal state when an element of the protection apparatus  20  is in an abnormal state. 
         [0071]    The transformer protection unit  900  may include a first transformer protection unit  910 , a second transformer protection unit  930 , and an operation verifying unit  950 . 
         [0072]    The first transformer protection unit  910  may generate an abnormal state sensing signal through information regarding current supplied from the CT bank  700  in a normal environment. 
         [0073]    The second transformer protection unit  930  may operate when the first transformer protection unit  910  has a problem, and may perform the same role as the first transformer protection unit  910 . 
         [0074]    The operation verifying unit  950  may identify an inner line connection state of the protection apparatus  20  and also identify whether an abnormal state occurs by setting a plurality of relay elements. Further, the operation verification unit  950  may identify whether or not the transformer protection unit  900  correctly generates an abnormal state sensing signal according to whether or not an abnormal state occurs. 
         [0075]    The controller  1000  may control overall operations of the protection apparatus  20 . 
         [0076]    Next, a configuration of a protection apparatus  20  according to another embodiment will be described with reference to  FIG. 6 . 
         [0077]    Referring to  FIG. 6 , the protection apparatus  20  may include an AC bus  600 , a converter transformer (CT) bank  700 , a valve  800 , a transformer protection unit  900 , and a controller  1000  as described with reference to  FIG. 5 . 
         [0078]    The CT bank  700  may be connected to the AC bus  600  to receive an AC power from the AC bus  600 , and be connected to the valve  800 . 
         [0079]    An input or output of each of elements constituting the CT bank  700  may be connected to the transformer protection unit  900 . 
         [0080]    The transformer protection unit  900  may generate an abnormal state sensing signal by using information regarding current delivered from the CT bank  700 . 
         [0081]    Next, a configuration of the CT bank  700  will be described in detail with reference to  FIG. 7 . 
         [0082]    Referring to  FIG. 7 , the CT bank  700  may include a circuit breaker  710 , a bus voltage measuring unit  730 , and a converter transformer  750 . 
         [0083]    The circuit breaker  710  may include an overcurrent measuring unit  711  and a gas insulated switchgear (GIS)  713 . The circuit breaker  710  may allow transmission current inputted from the AC bus  100  to flow or be interrupted. 
         [0084]    The bus voltage measuring unit  730  may measure a bus voltage which is outputted from the circuit breaker  710  and is supplied through the AC bus  600 . In an embodiment, the bus voltage measuring unit  730  may include a potential transformer (PT). The bus voltage measuring unit  730  may be a configuration for identifying an overvoltage relay element and an overexcitation current relay element. The overvoltage relay element may be a relay element for identifying whether or not a voltage applied to the converter transformer  750  exceeds a preset voltage, and the overexcitation current relay element may be a relay element for identifying an inrush current generated when a voltage is applied in a state that a voltage is not applied to the converter transformer  750 . 
         [0085]    The converter transformer  750  may convert an AC voltage inputted through a three phase power source to an AC voltage having a predetermined level. 
         [0086]    The converter transformer  750  may include a current transformer connected to each of two Y power sources and one delta power source constituting the three phase power source. 
         [0087]    Next, a method of verifying an operation of the protection apparatus will be described with reference to  FIG. 8 . 
         [0088]      FIG. 8  is a flow diagram for explaining a method for controlling distributed generators according to an embodiment. The descriptions regarding  FIGS. 1 to 7  will be considered for describing the embodiment of  FIG. 8 . While respective operations in the following embodiment will be described with an example of the protection apparatus  10  according to an embodiment, these respective operations may be also applied to the protection apparatus  20  according to another embodiment. Therefore, description thereof will be described at the end of each operation. 
         [0089]    Referring to  FIG. 8 , the operation verifying unit  450  of the filter protection unit  400  identifies a line connection state of an inner circuit of the protection apparatus  10  (S 101 ). The filter protection unit  400  may separately store information in advance on the line connection state in which the double tuned filter  200 , the high pass filter  300  and the filter protection unit  400  are connected, and the operation verifying unit  450  may identify the line connection state of the protection apparatus  10  through a current outputted from the high pass filter  300  when a test current having a predetermined level is inputted into the AC bus  100 . In detail, in the case the output currents of the double tuned filter  200  and the high pass filter  300  with respect to the input current having the predetermined level are all zero (0), the operation verifying unit  450  may identify that there no exist misconnection in the protection apparatus  10 , and in the case the output currents are not zero (0), the operation verifying unit  450  may identify that there exists a misconnection in the protection apparatus  10 . 
         [0090]    In another embodiment, the operation verifying unit  450  may identify at which point a misconnection occurs through a current measured by the current transformer included in each of the double tuned filter  200  and the high pass filter  300 . 
         [0091]    When description is made by applying operation S 101  to  FIGS. 5 to 7 , the operation verifying unit  950  of the transformer protection unit  900  identifies a line connection state of the inner circuit of the protection apparatus  20  (S 101 ). The transformer protection unit  900  may separately store in advance information regarding the line connection state in which the CT bank  700  and the valve  800  are connected, and the operation verifying unit  950  may identify the line connection state of the protection apparatus  20  through a current outputted from the CT bank  700  and the valve  800  when a current having a predetermined level is inputted into the AC bus  100 . In detail, in the case the output currents of the CT bank  700  and the valve  800  with respect to the input current having the predetermined level are all zero (0), the operation verifying unit  950  may identify that there no exist misconnection in the protection apparatus  20 , and in the case the output currents are not zero (0), the operation verifying unit  950  may identify that there exists a misconnection in the protection apparatus  20 . 
         [0092]    In another embodiment, the operation verifying unit  450  may identify at which point a connection is made through a current measured by the current transformer included in each of the double tuned filter  200  and the high pass filter  300 . 
         [0093]    In the case it is identified that an inner connection is made in the protection apparatus  10  (S 103 ), the operation verifying unit  450  identifies at which point a connection is made (S 105 ). Meanwhile, when description is made by applying operations S 103  and S 105  to  FIGS. 5 to 7 , in case it is identified an inner connection exists in the protection apparatus (S 103 ), the operation verifying unit  950  identifies at which point of the protection apparatus  20  a connection is made (S 105 ). 
         [0094]    When it is identified that an inner connection is made in the protection apparatus  10  (S 103 ), the operation verifying unit  450  sets each of the plurality of relay elements in order to sense an abnormal state of the protection apparatus (S 107 ). In an embodiment, the plurality of relay elements may include an overcurrent relay element, a ground overcurrent relay element, an unbalanced current relay element, a harmonic overcurrent relay element, and a percentage differential relay element. 
         [0095]    The overcurrent relay element may be a relay element to determine whether or not the current inputted from the AC bus  100  into the double tuned filter  200  or the high pass filter  300  exceeds a preset current. The operation verifying unit  450  may set the preset value in order to identify whether the current inputted into the double tuned filter  200  or the high pass filter  300  is an overcurrent. 
         [0096]    The ground overcurrent relay element may be a relay element to determine whether or not the current inputted from the AC bus  100  into the double tuned filter  200  or the high pass filter  300  exceeds a preset ground current. The operation verifying unit  450  may set the preset ground current value in order to identify whether the current inputted into the double tuned filter  200  or the high pass filter  300  is a ground overcurrent. 
         [0097]    The unbalanced current relay element may be a relay element to identify whether a current measured by the capacitor bank  220  of the double tuned filter  200  or the capacitor bank  320  of the high pass filter  300  has an unbalanced state. 
         [0098]    The harmonic overcurrent relay element may be a relay element to identify whether a harmonic component is removed from an AC current outputted from the reactor current transformer  250  of the double tuned filter  200  or the reactor current transformer  330  of the high pass filter  300 . 
         [0099]    The percentage differential relay element may be a relay element to identify whether a difference between input and output currents of the double tuned filter  200  and a difference between input and output currents of the high pass filter  300  exceed preset differences. 
         [0100]    Meanwhile, description will be made by applying operation S 107  to  FIGS. 5 to 7 . When it is identified that an inner connection is not made in the protection apparatus  20  (S 103 ), the operation verifying unit  950  sets each of the plurality of relay elements in order to sense an abnormal state of the protection apparatus (S 107 ). In an embodiment, the plurality of relay elements may include an overcurrent relay element, a ground overcurrent relay element, an overvoltage relay element, an overexcitation current relay element, and a percentage differential relay element. 
         [0101]    The overcurrent relay element may be a relay element to determine whether or not the current inputted from the AC bus  600  into the CT bank  700  exceeds a preset current. The operation verifying unit  950  may set a preset value in order to identify whether the current inputted into the CT bank  700  is an overcurrent. 
         [0102]    The ground overcurrent relay element may be a relay element to determine whether or not the current inputted from the AC bus  600  into the CT bank  700  exceeds a preset ground current. The operation verifying unit  950  may set a preset ground current value in order to identify whether the current inputted into the CT bank  700  is a ground overcurrent. 
         [0103]    The percentage differential relay element may be a relay element to identify whether a difference between input and output currents of the CT bank  700  exceeds a preset difference. 
         [0104]    The operation verifying unit  450  receives an input regarding test information for testing each of the set plurality of relay elements (S 109 ). In an embodiment, the test information may be information inputted into the double tuned filter  200  and the high pass filter  300  in order to identify whether any one of the plurality of relay elements has a problem. For example, it is assumed to test the percentage differential relay element of the double tuned filter  200 . In this case, the test information may be a micro current inputted into the double tuned filter  200 , and the operation verifying unit  450  may compare the level of the micro current inputted into the double tuned filter  200  and the level of a current outputted from the double tuned filter  200  to identify whether the difference exceeds a preset difference. 
         [0105]    Meanwhile, description will be made by applying operation S 109  to  FIGS. 5 to 7 . 
         [0106]    The operation verifying unit  950  receives an input regarding test information for testing each of the set plurality of relay elements (S 109 ). In an embodiment, the test information may be information inputted into the CT bank  700  in order to identify whether any one of the plurality of relay elements has a problem. For example, it is assumed to test the percentage differential relay element of the CT bank  700 . In this case, the test information may be a micro current inputted into the CT bank  700 , and the operation verifying unit  950  may compare the level of the micro current inputted into the CT bank  700  and the level of a current outputted from the CT bank  700  to identify whether the difference exceeds a preset difference. 
         [0107]    Then, the operation verifying unit  450  identifies whether an abnormal state is generated in any of the plurality of relay elements according to the input test information (S 111 ). For example, when the percentage differential relay element of the double tuned filter  200  is tested, when the level of a micro current inputted into the double tuned filter  200  with the level of a current outputted from the double tuned filter  200  are compared and a difference therebetween exceeds a preset difference, the operation verifying unit  450  may identify that an abnormal state is generated. When the level of the micro current inputted into the double tuned filter  200  and the level of the micro current outputted from the double tuned filter  200  are compared and a difference therebetween does not exceed a preset difference, the operation verifying unit  450  may identify that an abnormal state is not generated. 
         [0108]    Meanwhile, description will be made by applying operation S 111  to  FIGS. 5 to 7 . 
         [0109]    Then, the operation verifying unit  950  identifies whether an abnormal state is generated in any of the plurality of relay elements according to the input test information (S 111 ). For example, when the percentage differential relay element of the CT bank  700  is tested, when the level of a micro current inputted into the CT bank  700  and the level of a current outputted from the CT bank  700  are compared and a difference therebetween exceeds a preset difference, the operation verifying unit  950  may identify that an abnormal state is generated. When the level of the micro current inputted into the CT bank  700  and the level of the micro current outputted from the CT bank  700  are compared and a difference therebetween does not exceed a preset difference, the operation verifying unit  950  may identify that an abnormal state is not generated. 
         [0110]    When it is identified that an abnormal state is generated in any one of the plurality of relay elements, the operation verifying unit  450  identifies whether the filter protection unit  400  generates an abnormal state sensing signal (S 113 ). For example, when the percentage differential relay element of the double tuned filter  200  is tested, when the level of a micro current inputted into the double tuned filter  200  and the level of a current outputted from the double tuned filter  200  are compared and a difference therebetween exceeds a preset difference, it may be identified whether the double tuned filter protection unit  410  correctly generates an abnormal state sensing signal. 
         [0111]    Meanwhile, description will be made by applying operation S 113  to  FIGS. 5 to 7 . 
         [0112]    When it is identified that an abnormal state is generated in any one of the plurality of relay elements, the operation verifying unit  950  identifies whether the transformer protection unit  900  correctly generates an abnormal state sensing signal (S 113 ). For example, when the percentage differential relay element of the CT bank  700  is tested, when the level of a micro current inputted into the CT bank  700  and the level of a current outputted from the CT bank  700  are compared and a difference therebetween exceeds a preset difference, the operation verifying unit  450  may identify whether the transformer protection unit  900  correctly generates an abnormal state sensing signal. 
         [0113]    The operation verifying unit  450  outputs a relay element having an abnormal state among the plurality of relay elements on a display unit (not shown) through the generated abnormal state sensing signal (S 115 ). 
         [0114]    Meanwhile, when it is identified that an abnormal state is not generated in any one of the plurality of relay elements, the flow returns to operation S 109 . The test information input operation S 109  may be periodically performed. 
         [0115]    Meanwhile, description will be made by applying operation S 113  to  FIGS. 5 to 7 . 
         [0116]    The operation verifying unit  950  outputs a relay element having an abnormal state among the plurality of relay elements on a display unit (not shown) through the generated abnormal state sensing signal (S 115 ). 
         [0117]    Meanwhile, when it is identified that an abnormal state is not generated in any one of the plurality of relay elements, the flow returns to operation S 109 . The test information input operation S 109  may be periodically performed. 
         [0118]    According to an embodiment, the foregoing method may be implemented as codes readable by a process on a program-recorded medium. Examples of the processor-readable media may include ROM, RAM, CD-ROM, magnetic tape, floppy disk, and optical data storage device, and the like, and also include a device implemented in the form of a carrier wave (for example, transmission via the Internet).