Patent Publication Number: US-10785043-B2

Title: Load shedding system, communication method and access apparatus thereof

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to a Chinese patent application No. 201910104886.7 filed on Feb. 1, 2019, the disclosure of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     Embodiments of the present invention relate to the technical field of power systems, and particularly to a load shedding system, and a communication method and an access apparatus thereof. 
     BACKGROUND 
     One control terminal in a conventional safety and stability control system occupies a 2M channel. In the where the number of accessing control terminals is large, if each control terminal independently occupies a 2M channel, a large amount of communication bandwidth will be occupied, which is not conducive to saving investment. 
     A load shedding system can centralize interruptible loads of dispersed power users for millisecond level precision control, and can implement flexible adjustment, thereby achieving power supply and demand balance immediately. An existing load shedding system may be generally divided into three layers, i.e., a control master station layer, a control substation layer and a terminal user access layer. However, the existing load shedding system does not have an access apparatus specifically capable of achieving data interaction of the control substation layer and the terminal user access layer, and does not involve the reliability of the load shedding system. 
     SUMMARY 
     The present disclosure provides a load shedding system, and a communication method and an access apparatus for the load shedding system, capable of improving the reliability, saving cost and reducing the maintenance difficulty in addition to reducing the communication bandwidth occupied by a safety and stability control system solution. 
     In a first aspect, embodiments of the present invention provide an access apparatus. The access apparatus includes: two E1 interfaces, eight optical fiber interfaces, a central processing unit (CPU) and a field programmable gate array (FPGA). 
     The two E1 interfaces are respectively connected to a control apparatus A and a control apparatus B of a control substation. 
     The eight optical fiber interfaces are respectively connected to eight control terminals. 
     The FPGA includes eight optical fiber transceivers and two E1 transceivers. The eight optical fiber transceivers are respectively connected to the eight optical fiber interfaces through serial interfaces, and the two E1 transceivers are respectively connected to the two E1 interfaces through serial interfaces. Each optical fiber transceiver includes a reset submodule, and each E1 transceiver also includes a reset submodule. 
     The CPU is connected to the FPGA through a parallel bus. 
     In one or more embodiments, the FPGA further includes a watchdog module connected to the CPU. 
     The watchdog module is configured to reset the access apparatus in response to determining that the CPU fails. 
     The CPU is configured to reset the access apparatus in response to determining that the FPGA fails and in response to determining that the number of times the CPU resets the access apparatus is less than or equal to 3. 
     In one or more embodiments, a physical layer of the optical fiber transceiver adopts 8b10b encoding mode, and a communication rate is 32.768 Mbits/s. 
     A physical layer of the E1 transceiver conforms to ITU-T G.703 specification, a communication rate is 2 Mbits/s, and a bit error rate is less than 10′. 
     In one or more embodiments, the reset submodule of the optical fiber transceiver is configured to reset the optical fiber transceiver in response to determining that the optical fiber transceiver fails, and the reset submodule of the E1 transceiver is configured to reset the E1 transceiver in response to determining that the E1 transceiver fails. 
     In a second aspect, embodiments of the present invention further provide a load shedding system. The system includes: the access apparatus in the first aspect of embodiments of the present invention, a control substation, and at least one control terminal. 
     In a third aspect, embodiments of the present invention further provide a communication method applicable to the load shedding system in the second aspect of embodiments of the present invention. The method includes: receiving downlink data sent by a control substation; processing the downlink data with a multiplexing algorithm; and sending the processed downlink data to a control terminal. 
     The downlink data is two data frames or two command frames, the processed downlink data is two processed data frames or two processed command frames. The two data frames or the two command frames respectively correspond to the control apparatus A and the control apparatus B of the control substation. 
     In one or more embodiments, the communication method further includes: receiving uplink data sent by the control terminal; processing the uplink data with the multiplexing algorithm; and sending the processed uplink data to the control substation. 
     The uplink data is one data frame and the processed uplink data is one processed data frame. 
     In one or more embodiments, the command frame includes 12 words, and each word includes 16 bits. The command frame includes: a frame header with a length of 1 word, a control command with a length of 8 words, overall information with a length of 1 word, a sending sequence number with a length of 1 word and a cyclic redundancy check (CRC) code with a length of 1 word, and the control command includes a positive code and a negative code. 
     In one or more embodiments, the data frame includes 12 words, and each word includes 16 bits. The data frame includes: a frame header with a length of 1 word, data information with a length of 8 words, overall information with a length of 1 word, a sending sequence number with a length of 1 word and a CRC code with a length of 1 word. 
     In a fourth aspect, embodiments of the present invention further provide a computer readable storage medium which stores a computer program. The computer program, when executed by a processor, realizes the communication method of the load shedding system in the third aspect of embodiments of the present invention. 
     The access apparatus provided by the present invention includes: two E1 interfaces, eight optical fiber interfaces, a central processing unit (CPU) and a field programmable gate array (FPGA). The two E1 interfaces are respectively connected to the control apparatus A and the control apparatus B of a control substation. The eight optical fiber interfaces are respectively connected to eight control terminals. The FPGA includes eight optical fiber transceivers and two E1 transceivers. The eight optical fiber transceivers are respectively connected to the eight optical fiber interfaces through serial interfaces, and the two E1 transceivers are respectively connected to the two E1 interfaces through serial interfaces. Each optical fiber transceiver includes a reset submodule, and each E1 transceiver also includes a reset submodule. The CPU is connected to the FPGA through a parallel bus. The access apparatus can achieve data receiving and sending through the FPGA, the two E1 interfaces and the eight optical fiber interfaces, and thus, is applicable to actual engineering. Meanwhile, in the FPGA, each optical fiber transceiver is provided with the reset submodule, and each E1 transceiver is also provided with the reset submodule, so the reliability of the access apparatus is improved, thereby saving the investment cost and reducing the maintenance difficulty while reducing the communication bandwidth occupied by a conventional safety and stability control system solution. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a structural schematic diagram illustrating a load shedding system provided by an embodiment of the present invention; 
         FIG. 2  is a structural schematic diagram illustrating an access apparatus provided by an embodiment of the present invention; and 
         FIG. 3  is a flow chart illustrating a communication method of a load shedding system provided by an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will be further described below in detail in combination with drawings and embodiments. It can be understood that specific embodiments described herein are only used for explaining the present invention, not used for limiting the present invention. In addition, it shall be indicated that for ease of description, drawings only show some structures related to the present invention rather than all structures. 
     It should be indicated that terms “system” and “network” in the present disclosure are often used interchangeably herein. “And/or” mentioned in embodiments of the present invention refers to any and all combinations including one or more related listed items. Terms, such as “the first” and “the second”, in the description, claims and drawings of the present invention are used for distinguishing different objects, rather than defining a specific sequence. 
     It should also be indicated that the following embodiments of the present invention can be executed independently, or in combination with each other, which is not specifically limited in embodiments of the present invention. 
     The load shedding system, the communication method, the access apparatus and technical effects thereof are described below in detail. 
       FIG. 1  is a structural schematic diagram illustrating a load shedding system provided by an embodiment of the present invention. The load shedding system includes: a control master station layer, a control substation layer and a terminal user access layer. 
     The control master station layer includes a control master station  10 , and the control master station  10  is divided into a control apparatus A and a control apparatus B. The control master station  10  is usually a 500 kV AC collecting station having a good channel condition near a converter station at a DC dropping location. The control master station  10  may receive a load shedding instruction sent by a superior stability control system, distribute loads, and issue the load shedding instruction. 
     The control substation layer includes a control substation  20 , and the control substation  20  is divided into a control apparatus A and a control apparatus B. The control substation  20  is connected to the control master station  10  through a Synchronous Digital Hierarchy (SDH) 2 M line. The control substation  20  is generally a 500 kV AC station and/or a 220 kV AC station arranged in a load lumped region. The control substation  20  may collect sheddable load information in the region, send the sheddable load information to the control master station  10 , and execute the load shedding instruction sent by the control master station. 
     The terminal user access layer includes an access apparatus  30  and at least one control terminal  31 . The access apparatus  30  is connected to the control substation  20  through a synchronous digital hierarchy (SDH) device, and each access apparatus may be connected to eight control terminals  31  at most. 
       FIG. 2  is a structural schematic diagram illustrating an access apparatus provided by an embodiment of the present invention. The access apparatus includes: two E1 interfaces  300 , eight optical fiber interfaces  301 , a central processing unit (CPU)  302  and a field programmable gate array (FPGA)  303 . 
     Each of the two E1 interfaces  300  is connected to a corresponding one of the control apparatus A and the control apparatus B of the control substation  20  through an E1 channel. 
     Each of the eight optical fiber interfaces  301  is connected to a corresponding one of the eight control terminals  31  through a special optical fiber channel. 
     The FPGA  303  includes eight optical fiber transceivers (denoted by OP-1, OP-2, . . . , OP-8 in  FIG. 2 ) and two E1 transceivers (denoted by E1-1 and E1-2 in  FIG. 2 ). The eight optical fiber transceivers are respectively connected to the eight optical fiber interfaces  301  through serial interfaces. The two E1 transceivers are respectively connected to the two E1 interfaces  300  through serial interfaces. Each optical fiber transceiver is provided with a reset submodule, and each E1 transceiver is also provided with a reset submodule. The reset submodule of the optical fiber transceiver is configured to reset the optical fiber transceiver in response to determining that the optical fiber transceiver fails, and the reset submodule of the E1 transceiver is configured to reset the E1 transceiver in response to determining that the E1 transceiver fails. Each reset submodule is independently arranged, and is controlled by the CPU to achieve separately resetting the optical fiber transceiver or the E1 transceiver, without affecting other communication links, thereby solving the communication link interruption problem of the FPGA being disturbed, and improving the reliability. 
     The CPU  302  is connected to the FPGA  303  through a parallel bus, and writes data to be sent in to the FPGA and controls data sending. 
     The access apparatus  30  may receive downlink data sent by the control apparatus A and the control apparatus B of the control substation  20 , and forward the downlink data to all control terminals  31  accessed to the access apparatus  30 . The access apparatus  30  can receive uplink data sent by all the control terminals  31  accessed to the access apparatus  30 , and forward the uplink data to the control apparatus A and the control apparatus B of the control substation  20 . The downlink data may be a data frame and/or a command frame, and the uplink data may be a data frame. 
     Specifically, the process that the access apparatus  30  receives the downlink data sent by the control apparatus A or the control apparatus B of the control substation  20  and forwards the downlink data to all the control terminals  31  accessed to the access apparatus  30  is as follows. The two E1 interfaces  300  of the access apparatus  30  receive the downlink data sent by the control apparatus A and the control apparatus B of the control substation  20 . The two E1 transceivers in the FPGA  303  read the downlink data, send the downlink data to the CPU  302  for processing, obtain data of a corresponding optical fiber channel, and then feed back the data to the FPGA  303 . The eight optical fiber transceivers in the FPGA  303  send the downlink data to all the control terminals  31  accessed to the access apparatus  30  through the eight optical fiber interfaces  301 . 
     The process that the access apparatus  30  receives the uplink data sent by all the control terminals  31  accessed to the access apparatus  30  and forwards the uplink data to the control apparatus A and the control apparatus B of the control substation  20  is as follows. The eight optical fiber interfaces  301  of the access apparatus  30  receives the uplink data sent by all the control terminals  31  accessed to the access apparatus  30 . The eight optical fiber transceivers in the FPGA  303  read the uplink data, send the uplink data to the CPU  302  for processing, obtain data of a corresponding E1 channel, and then feed back the data to the FPGA  303 . The two E1 transceivers in the FPGA  303  send the uplink data to the control apparatus A and the control apparatus B of the control substation  20  through the two E1 interfaces  300 . 
     A physical layer of the optical fiber transceiver adopts 8b10b encoding mode, and a communication rate is 32.768 Mbits/s. 
     The E1 transceiver is communicated with an SDH device in the unbalanced transmission manner through a 75Ω coaxial cable. The physical layer of the E1 transceiver conforms to ITU-T G.703 specification, a communication rate is 2 Mbits/s, and a bit error rate is less than 10 −8 . 
     In addition, a link layer protocol of communication between the FPGA  303  and the eight optical fiber interfaces  301  is a self-owned protocol, and a link layer protocol of communication between the FPGA  303  and the two E1 interfaces  300  is a high-level data link control (HDLC) protocol. 
     Further, as shown in  FIG. 2 , the FPGA  303  further includes a watchdog module  305  connected to the CPU  302 . 
     The watchdog module  305  is configured to reset the access apparatus  30  in response to determining that the CPU  302  fails. Specifically, the CPU  302  sends a feeding signal to the watchdog module  305  regularly according to a fixed period, and if the watchdog module  305  does not receive the feeding signal from the CPU  302  for more than a predetermined time, the watchdog module  305  determines that the CPU  302  fails and resets the access apparatus  30 , thereby effectively preventing long-term communication interruption because the CPU  302  is disturbed and crashes or a bus through which the CPU  302  access to the FPGA  303  is disturbed, and improving the reliability. 
     The CPU  302  is configured to actively reset the access apparatus  30  in response to determining that the FPGA fails and in response to determining that the number of times the CPU  302  resets the access apparatus  30  is less than or equal to 3. Specifically, by taking an initial state as an example, the CPU  302  has ever received any correct data sent by the two E1 interfaces  300  and/or the eight optical fiber interfaces  301 , and later, if the CPU  302  fails to receive a frame of correct data from the two E1 interfaces  300  and/or the eight optical fiber interfaces  301  for a continuous period Tnr, the CPU  302  determines that the FPGA  303  fails, actively resets the access apparatus  30 , and records one resetting in a nonvolatile memory. After this resetting, if the CPU  302  fails to receive a frame of correct data from the two E1 interfaces  300  and/or the eight optical fiber interfaces  301  for the continuous period Tnr, the CPU  302  actively resets the access apparatus  30  again, and also records another resetting in the nonvolatile memory. After this resetting, if the CPU  302  fails to receive a frame of correct data from the two E1 interfaces  300  and/or the eight optical fiber interfaces  301  for the continuous period Tnr again, the CPU  302  resets the access apparatus  30  again and updates the number of resetting in the memory. When the number of resetting is 3, even if the CPU  302  fails to receive a frame of correct data from the two E1 interfaces  300  and/or the eight optical fiber interfaces  301  for the continuous period Tnr, the CPU  302  does not reset the access apparatus  30 . If the CPU  302  receives a frame of correct data from the two E1 interfaces  300  and/or the eight optical fiber interfaces  301  after resetting, the CPU  302  clears the number of resetting recorded in the nonvolatile memory. 
     Embodiments of the present invention provide an access apparatus, the access apparatus includes: two E1 interfaces, eight optical fiber interfaces, a CPU and an FPGA. The two E1 interfaces are respectively connected to the control apparatus A and the control apparatus B of a control substation. The eight optical fiber interfaces are respectively connected to eight control terminals. The FPGA includes eight optical fiber transceivers and two E1 transceivers. The eight optical fiber transceivers are respectively connected to the eight optical fiber interfaces through serial interfaces. The two E1 transceivers are respectively connected to the two E1 interfaces through serial interfaces. Each of the eight optical fiber transceivers includes a reset submodule, and each of two E1 transceiver includes a reset submodule. The CPU is connected to the FPGA through a parallel bus. The access apparatus can achieve data receiving and sending through the FPGA, the two E1 interfaces and the eight optical fiber interfaces, and thus, is applicable to actual engineering. Meanwhile, in the FPGA, each optical fiber transceiver is provided with the reset submodule, and each E1 transceiver is also provided with the reset submodule, so the reliability of the access apparatus is improved, thereby saving the investment cost and reducing the maintenance difficulty while reducing the communication bandwidth occupied by a conventional safety and stability control system solution. 
       FIG. 3  is a flow chart illustrating a communication method of a load shedding system provided by an embodiment of the present invention. The method is applicable to the load shedding system in above embodiments. As shown in  FIG. 3 , the method includes the following steps. 
     In step S 101 , the access apparatus receives downlink data sent by the control substation. 
     The downlink data may be a data frame or a command frame. The data frame may be data information, such as sheddable load information, power information, state information; and the command frame may be instruction information, such as the load shedding instruction and conformance recovery instruction. 
     In step S 102 , the access apparatus processes the downlink data with a multiplexing algorithm. 
     In step S 103 , the access apparatus sends the processed downlink data to the control terminal. 
     When the downlink data is the data frame, the processed downlink data is a processed data frame; and when the downlink data is the command frame, the processed downlink data is a processed command frame. 
     Specifically, the access apparatus sends the processed downlink data to all the control terminals accessed to the access apparatus simultaneously. 
     In step S 104 , the access apparatus receives uplink data sent by the control terminal. 
     The uplink data is a data frame. 
     Specifically, the access apparatus receives the uplink data sent by all the control terminals accessed to the access apparatus simultaneously. 
     In step S 105 , the access apparatus processes the uplink data with the multiplexing algorithm. 
     In step S 106 , the access apparatus sends the processed uplink data to the control substation. 
     Since the uplink data is the data frame, the processed uplink data is a processed data frame. 
     In addition, the access apparatus sends processed uplink data to the control apparatus A and the control apparatus B of the control substation in a polling manner at a regular time interval Tval. In each time interval Tval, the data sent to the control apparatus A and the control apparatus B of the control substation is received by the same optical fiber interface, so as to avoid that information of a certain control terminal is not sent in time due to accidental loss of a frame on the channel, thereby improving the reliability. 
     Further, Table 1 shows basic information of the downlink data, the processed downlink data, the uplink data and the processed uplink data. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Downlink data to the 
                   
                 Uplink data to the 
               
               
                   
                 Downlink data from the 
                 control terminal after 
                 Uplink data from the 
                 control substation after 
               
               
                   
                 control substation to 
                 being processed by the 
                 control terminal to 
                 being processed by the 
               
               
                   
                 the access apparatus 
                 access apparatus 
                 the access apparatus 
                 access apparatus 
               
               
                   
               
             
            
               
                 Communication 
                 K 
                 N 
                 M 
                 L 
               
               
                 frequency (frame/s) 
                   
                   
                   
                   
               
               
                 Number of frames 
                 One frame for each 
                 Two frames for 
                 1 
                 One frame for each 
               
               
                 per communication 
                 of the control 
                 each control 
                   
                 of the control 
               
               
                   
                 apparatus A and the 
                 terminal 
                   
                 apparatus A and the 
               
               
                   
                 control apparatus B 
                   
                   
                 control apparatus B 
               
               
                 Length per 
                 12 
                 12 
                 12 
                 12 
               
               
                 frame (word) 
               
               
                   
               
            
           
         
       
     
     K, L, M and N may be specifically determined according to needs of actual engineering, but shall meet the following principles: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       K 
                       ≥ 
                       N 
                     
                   
                 
                 
                   
                     
                       
                         8 
                         ⁢ 
                         M 
                       
                       ≥ 
                       L 
                     
                   
                 
               
               . 
             
           
         
       
     
     As shown in Table 1, the control substation sends the downlink data to the access apparatus with a communication rate of K frame/s, and the control apparatus A and the control apparatus B of the specific control substation send one frame respectively in each communication. Each frame has 12 words, and each word includes 16 bits. In each communication, the downlink data processed by the access apparatus and sent to control terminal includes two frames, i.e., the first frame is sent by the control apparatus A of the control substation, and the second frame is sent by the control apparatus B. The control terminal sends the uplink data to the access apparatus in a similar way, which is not repeated herein for simplicity. 
     Meanwhile, as shown in Table 1, the access apparatus sends and receives data using a fixed time interval, and the frame length is also fixed, which can effectively prevent saturated attack and overflow attack, thereby improving the reliability. 
     In addition, in embodiments of the present invention, data is sent and received at a fixed time interval, and the time interval is less than or equal to the reciprocal of the communication rate. 
     Further, embodiments of the present invention also define the frame contents of the downlink data, the processed downlink data, the uplink data and the processed uplink data respectively. 
     Table 2 shows the frame content of the downlink data sent by the control substation to the access apparatus. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Word 
                 Data frame 
                 Command frame 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 0 
                 Frame header (0x550a for the 
                 Frame header (0x990a for the 
               
               
                   
                 control apparatus A, and 0x550b 
                 control apparatus A, and 0x990b 
               
               
                   
                 for the control apparatus B) 
                 for the control apparatus B) 
               
               
                 1 
                 Time synchronization 
                 Control terminal 1 (command provided: 0x 
               
               
                   
                 mark (0xb2b2) 
                   xx xx; no command: 0x0000) 
               
               
                 2 
                 (BCD code) low-byte year, and 
                 Control terminal 2 (command provided: 0x 
               
               
                   
                 high-byte month 
                   xx xx; no command: 0x0000) 
               
               
                 3 
                 (BCD code) low-byte day, and 
                 Control terminal 3 (command provided: 0x 
               
               
                   
                 high-byte hour 
                   xx xx; no command: 0x0000) 
               
               
                 4 
                 (BCD code) low-byte minute, and 
                 Control terminal 4 (command provided: 0x 
               
               
                   
                 high-byte second 
                   xx xx; no command: 0x0000) 
               
               
                 5 
                 (BCD code) millisecond 
                 Control terminal 5 (command provided: 0x 
               
               
                   
                   
                   xx xx; no command: 0x0000) 
               
               
                 6 
                 0 
                 Control terminal 6 (command provided: 0x 
               
               
                   
                   
                   xx xx; no command: 0x0000) 
               
               
                 7 
                 0 
                 Control terminal 7 (command provided: 0x 
               
               
                   
                   
                   xx xx; no command: 0x0000) 
               
               
                 8 
                 0 
                 Control terminal 8 (command provided: 0x 
               
               
                   
                   
                   xx xx; no command: 0x0000) 
               
               
                 9 
                 Overall information 
                 Overall information 
               
               
                 10 
                 Sending sequence number (0 to (K − 1)) 
                 Sending sequence number (0 to (K − 1)) 
               
               
                 11 
                 Check code (CRC16 of above 11 words) 
                 Check code (CRC16 of above 11 words) 
               
               
                   
               
            
           
         
       
     
     Table 3 shows the frame content of the processed downlink data sent by the access apparatus to the control terminal. 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Word 
                 Data frame 
                 Command frame 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 0 
                 Frame header (0x330a) 
                 Frame header (0x770a) 
               
               
                 1 
                 Time synchronization 
                 (command provided: 0x  xx xx; 
               
               
                   
                 mark (0xb2b2) 
                 no command: 0x0000) 
               
               
                 2 
                 (BCD code) low-byte year, and 
                 0 
               
               
                   
                 high-byte month 
                   
               
               
                 3 
                 (BCD code) low-byte day, and 
                 0 
               
               
                   
                 high-byte hour 
                   
               
               
                 4 
                 (BCD code) low-byte minute, and 
                 0 
               
               
                   
                 high-byte second 
                   
               
               
                 5 
                 (BCD code) millisecond 
                 0 
               
               
                 6 
                 0 
                 0 
               
               
                 7 
                 0 
                 0 
               
               
                 8 
                 0 
                 0 
               
               
                 9 
                 Overall information 
                 Overall information 
               
               
                 10 
                 Sending sequence number (0 to (N − 1)) 
                 Sending sequence number (0 to (N − 1)) 
               
               
                 11 
                 Check code (CRC of above 11 words) 
                 Check code (CRC of above 11 words) 
               
               
                 12 
                 Frame header (0x330b) 
                 Frame header (0x770b) 
               
               
                 13 
                 Time synchronization 
                 (command provided:0x 
               
               
                   
                 mark (0xb2b2) 
                   xx xx: no command: 0x0000) 
               
               
                 14 
                 (BCD code) low-byte year, and 
                 0 
               
               
                   
                 high-byte month 
                   
               
               
                 15 
                 (BCD code) low-byte day, and 
                 0 
               
               
                   
                 high-byte hour 
                   
               
               
                 16 
                 (BCD code) low-byte minute, and 
                 0 
               
               
                   
                 high-byte second 
                   
               
               
                 17 
                 (BCD code) millisecond 
                 0 
               
               
                 18 
                 0 
                 0 
               
               
                 19 
                 0 
                 0 
               
               
                 20 
                 0 
                 0 
               
               
                 21 
                 Overall information 
                 Overall information 
               
               
                 22 
                 Sending sequence number (0~(N − 1)) 
                 Sending sequence number (0~(N − 1)) 
               
               
                 23 
                 Check code (CRC16 of the 11 words) 
                 Check code (CRC16 of the 11 words) 
               
               
                   
               
            
           
         
       
     
     Table 4 shows the frame content of the uplink data sent by the control terminal to the access apparatus. 
     
       
         
           
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Word 
                 Data frame 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 0 
                 Frame header (0x440a) 
               
               
                 1 
                 Low word of Class I sheddable 
               
               
                   
                 load of the control terminal 
               
               
                 2 
                 High word of Class I sheddable 
               
               
                   
                 load of the control terminal 
               
               
                 3 
                 Low word of Class II sheddable 
               
               
                   
                 load of the control terminal 
               
               
                 4 
                 High word of Class II sheddable 
               
               
                   
                 load of the control terminal 
               
               
                 5 
                 Low word of Class III sheddable 
               
               
                   
                 load of the control terminal 
               
               
                 6 
                 High word of Class III sheddable 
               
               
                   
                 load of the control terminal 
               
               
                 7 
                 Abnormal information word of the 
               
               
                   
                 control terminal 
               
               
                 8 
                 Identification code of the control 
               
               
                   
                 terminal 
               
               
                 9 
                 Overall information 
               
               
                 10 
                 Sending sequence number (0 t (M − 1)) 
               
               
                 11 
                 Check code (CRC of above 11 words) 
               
               
                   
               
            
           
         
       
     
     Table 5 shows the frame content of the processed uplink data sent by the access apparatus to the control substation. 
     
       
         
           
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Word 
                 Data frame 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 0 
                 Frame header (0x660a for the A set, and 0x660b 
               
               
                   
                 for the B set) 
               
               
                 1 
                 Low word of Class I sheddable load of the control 
               
               
                   
                 terminal connected to the opticalfiber channel i (1-8) 
               
               
                 2 
                 High word of Class I sheddable load of the control 
               
               
                   
                 terminal connected to the optical fiber channel i (1-8) 
               
               
                 3 
                 Low word of Class II sheddable load of the control 
               
               
                   
                 terminal connected to the optical fiber channel i (1-8) 
               
               
                 4 
                 High word of Class II sheddable load of the control 
               
               
                   
                 terminal connected to the optical fiber channel i (1-8) 
               
               
                 5 
                 Low word of Class III sheddable load of the control 
               
               
                   
                 terminal connected to the optical fiber channel i (1-8) 
               
               
                 6 
                 High word of Class III sheddable load of the control 
               
               
                   
                 terminal connected to the optical fiber channel i (1-8) 
               
               
                 7 
                 Abnormal information word of the control terminal 
               
               
                   
                 connected to the optical fiber channel i (1-8) 
               
               
                 8 
                 Identification code of the control terminal connected 
               
               
                   
                 to the optical fiber channel i (1-8) 
               
               
                 9 
                 Overall information 
               
               
                 10 
                 Sending sequence number (0 to (L-1)) 
               
               
                 11 
                 Check code (CRC16 of above 11 words) 
               
               
                   
               
            
           
         
       
     
     It should be noted that, 0x  xx xx may be a load shedding category command, the low byte thereof indicates a positive code, the high byte thereof indicates a negative code; a specific command code is defined according to the engineering; and the access apparatus only needs to check whether the format is correct, without checking the specific content. 
     It may be known by comparing the contents of Table 2 and Table 3 that, when the control substation needs to send a load shedding command to any of the control terminals 1 to 8, for example, the control substation needs to send the load shedding command to the a-th control terminal, the control substation sends a command frame, the data in the a-th word in the command frame is 0x  xx xx, and other words of the rest control terminals are 0. 
     The sending sequence number may be increased by 1 whenever one frame is sent. If the sending sequence number is increased to the maximum value, the sending sequence number for sending the next frame becomes 0. The receiving side determines whether the data is repeated data according to whether the sending sequence number in the frame received this time and the sending sequence number in the frame received last time, to prevent a problem that repeated data is sent due to hardware malfunction and the receiving side cannot identify the hardware malfunction. 
     The length of the overall information is 1 word, that is, 16 bits, and different bits indicate different meanings. Table 6 shows a definition of the overall information (i.e., overall information corresponding to Table 2 and Table 5) of the frame transmitted between the access apparatus and the control substation. 
     
       
         
           
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                 bit 
                 Downlink data 
                 Processed uplink data 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 0 
                 Reception failure of E1 channel 
                 Reception failure of E1 channel 
               
               
                   
                 (after resetting for three times, 
                 (after resetting for three times, 
               
               
                   
                 the E1 channel is still not 
                 the E1 channel is still not 
               
               
                   
                 successfully received) 
                 success fully received) 
               
               
                 1 
                 0 
                 0 
               
               
                 2 
                 Reception of the E1 channel is 
                 0 
               
               
                   
                 abnormal (confirmed by a delay T1) 
                   
               
               
                 3 
                 0 
                 0 
               
               
                 4 
                 Reception of the optical fiber 
                 0 
               
               
                   
                 channel 1 is abnormal (confirmed 
                   
               
               
                   
                 by a delay T2) 
                   
               
               
                 5 
                 Reception of the optical fiber 
                 0 
               
               
                   
                 channel 2 is abnormal (confirmed 
                   
               
               
                   
                 by a delay T2) 
                   
               
               
                 6 
                 Reception of the optical fiber 
                 0 
               
               
                   
                 channel 3 is abnormal (confirmed 
                   
               
               
                   
                 by a delay T2) 
                   
               
               
                 7 
                 Reception of the optical fiber 
                 0 
               
               
                   
                 channel 4 is abnormal (confirmed 
                   
               
               
                   
                 by a delay T2) 
                   
               
               
                 8 
                 Reception of the optical fiber 
                 0 
               
               
                   
                 channel 5 is abnormal (confirmed 
                   
               
               
                   
                 by a delay T2) 
                   
               
               
                 9 
                 Reception of the optical fiber 
                 0 
               
               
                   
                 channel 6 is abnormal (confirmed 
                   
               
               
                   
                 by a delay T2) 
                   
               
               
                 10 
                 Reception of the optical fiber 
                 0 
               
               
                   
                 channel 7 is abnormal (confirmed 
                   
               
               
                   
                 by a delay T2) 
                   
               
               
                 11 
                 Reception of the optical fiber 
                 0 
               
               
                   
                 channel 8 is abnormal (confirmed 
                   
               
               
                   
                 by a delay T2) 
                   
               
               
                 12 
                 Low bit of terminal number of 
                 0 
               
               
                   
                 current uplink data (optical 
                   
               
               
                   
                 fiber channel number) 
                   
               
               
                 13 
                 Middle bit of terminal number of 
                 0 
               
               
                   
                 current uplink data (optical 
                   
               
               
                   
                 fiber channel number) 
                   
               
               
                 14 
                 High bit of terminal number of 
                 0 
               
               
                   
                 current uplink data (optical 
                   
               
               
                   
                 fiber channel number) 
                   
               
               
                 15 
                 0 
                 0 
               
               
                   
               
            
           
         
       
     
     The meaning of the reception failure of the E1 channel is as follows. If the receiving side fails to receive new effective data sent by the sending side for a continuous period Te, a hardware receiving module is reset. If the receiving side still fails to receive the new effective data after the hardware receiving module has been reset for three times, the hardware receiving module is not reset again, and the sending side is informed of such situation through the overall information. If the receiving side receives the new effective data, the corresponding bit in the overall information is immediately reset to 0. If the sending side is informed that the receiving side still fails to receive the new effective data after the hardware receiving module has been reset for three times, a hardware sending module is reset after a delay time period Te. After the hardware sending module is reset, the sending side determines whether the bit indicating the reception failure of the E1 channel still exists in the subsequent delay period T. if the bit indicating the reception failure of the E1 channel still exists, the hardware sending module is reset again, and the hardware sending module is reset for three times at most. The relationship between T1 and Te is T 1 &gt;6T e . 
     Table 7 shows a definition of the overall information (i.e., corresponding overall information in Table 3 and Table 4) of the frame transmitted between the access apparatus and the control terminal. 
     
       
         
           
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                 bit 
                 Processed downlink data 
                 Uplink data 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 0 
                 Reception failure of optical fiber 
                 Reception failure of optical fiber 
               
               
                   
                 channel (after resetting for three 
                 channel (after resetting for three 
               
               
                   
                 times, the optical fiber channel is 
                 times, the optical fiber channel is 
               
               
                   
                 still not successfully received) 
                 still not successfully received) 
               
               
                 1 
                 Reception of the optical fiber 
                 0 
               
               
                   
                 channel is abnormal (confirmed 
                   
               
               
                   
                 by a delay T3) 
                   
               
               
                 2 
                 E1 channel data in data sent to the 
                 0 
               
               
                   
                 optical fiber channel is effective 
                   
               
               
                 3 
                 0 
                 0 
               
               
                 4 
                 The E1 channel receives new effective 
                 0 
               
               
                   
                 data 
                   
               
               
                 5 
                 0 
                 0 
               
               
                 6 
                 The E1 channel receives a new command 
                 0 
               
               
                 7 
                 0 
                 0 
               
               
                 8 
                 Reception of the E1 channel is abnormal 
                 0 
               
               
                   
                 (confirmed by a delay) 
                   
               
               
                 9 
                 0 
                 0 
               
               
                 10 
                 0 
                 0 
               
               
                 11 
                 0 
                 0 
               
               
                 12 
                 Low bit of the optical fiber channel 
                 0 
               
               
                   
                 number of current transmission 
                   
               
               
                 13 
                 Middle bit of the optical fiber channel 
                 0 
               
               
                   
                 number of current transmission 
                   
               
               
                 14 
                 High bit of the optical fiber channel 
                 0 
               
               
                   
                 number of current transmission 
                   
               
               
                 15 
                 0 
                 0 
               
               
                   
               
            
           
         
       
     
     When the control substation needs to send the load shedding instruction to any of the control terminals 1 to 8, for example, send the load shedding command to the a-th control terminal, the access apparatus sends the command frame to the a-th control terminal, and sends the data frame (the bits of the data frame except the frame header, the overall information, the sending sequence number and the check code are set to 0) to other control terminals. In addition, the bit indicating “the E1 channel data in the data sent to the optical fiber channel is effective” in the overall information is set to 0. 
     The access apparatus checks a state of last received data which is sent by the control substation when sending data to the control terminal. If the new effective data is obtained, the bit indicating “the E1 channel receives the new effective data” is set to 1, otherwise, the bit is set to 0. If the last received data sent by the control substation is new command data, the bit “the E1 channel receives a new command” is set to 1, otherwise, the bit is set to 0. 
     In addition, the control terminal may independently conduct multi-frame confirmation when receiving the load shedding command sent by the control apparatus A and the control apparatus B of the control substation via the access apparatus. The multi-frame confirmation method is to check whether Nc effective command frames are received in a period of time TC, and whether the Nc effective command frames are consistent. The control terminal finally checks whether the load shedding commands of the control apparatus A and the control apparatus B of the control substation are consistent. If the load shedding commands of the control apparatus A and the control apparatus B are not consistent, one with more loads is executed. 
     Embodiments of the present invention further provide a computer readable storage medium which stores a computer program. The computer program, when executed by a processor, realizes the communication method of the load shedding system described by above embodiments. 
     The computer storage medium of embodiments of the present invention may use any combination of one or more computer readable media. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. The computer readable storage medium, for example, may be, but not limited to, a system, an apparatus or a device of electricity, magnetism, light, electromagnetism, infrared ray or semiconductor, or any combination thereof. More specific examples (non-exhaustive list) of the computer readable storage medium include: an electric connection of one or more wires, a portable computer hard disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or a flash memory), an optical fiber, a portable compact disk read-only memory (CD-ROM), a light storage device, a magnetic storage device, or any proper combination thereof. Herein, the computer readable storage medium may be any tangible medium including or storing a program, and the program may be used by an instruction execution system, an apparatus or a device or used in a combination thereof. 
     The computer readable signal medium may be included in a baseband or be a data signal which is spread as a part of carrier, and carries a computer readable program code. Such spread data signal may adopt multiple forms, including but not limited to an electromagnetic signal, a light signal or any proper combination thereof. The computer readable signal medium may also be any computer readable medium other than the computer readable storage medium, and the computer readable medium may send, spread or transmit a program used by the instruction execution system, the apparatus or the device or in the combination thereof. 
     The program code in the computer readable medium may be transmitted with any proper medium, including but not limited to wireless transmission, wire transmission, optical cable transmission, RF transmission, or any proper combination thereof. 
     The computer program code for executing the operation of the present invention may be written with one or more program design languages or a combination thereof. The program design language includes object-oriented program design languages, such as Java, Smalltalk and C++, and also includes conventional procedural program design languages, such as “C” language or similar program design languages. The program code may be completely executed on a user computer, partially executed on the user computer, executed as an independent software package, executed partially on the user computer or partially on a remote computer, or completely executed on the remote computer or a server. In case of the remote computer, the remote computer may be connected to the user computer through any kind of network, including a local area network (LAN) or a wide area network (WAN), or may be connected to an external computer (such as being connected through an Internet by means of an Internet service provider). 
     It should be noted that, the above only describes exemplary embodiments of the present invention and applied technical principles. Those skilled in the art shall understand that the present invention is not limited to specific embodiments described herein. For those skilled in the art, various apparent variations, readjustments and replacements can be made without departing from a protection scope of the present invention. Therefore, although the present invention is described in detail through the above embodiments, the present invention is not limited to the above embodiments and may further include more other equivalent embodiments without departing from the concept of the present invention, while the scope of the present invention is decided by a scope of attached claims.