Patent Publication Number: US-2023135970-A1

Title: Energy storage transient power coordinated control method for restraining subsequent commutation failures

Description:
TECHNICAL FIELD 
     The present invention falls in the technical field of multi-fed ultra-high voltage (UHV) AC/DC hybrid power grids, and more specifically, relates to an energy storage transient power coordinated control method for restraining subsequent commutation failures. 
     BACKGROUND TECHNIQUE 
     Commutation failure is one of the most common faults in high voltage direct current (HVDC) transmission system based on power grid commutation converter. After the commutation failure occurs, the DC voltage drops rapidly and the DC current increases sharply, which has a serious impact on the system, and even leads to the interruption of transmission power. With the successive investment of HVDC transmission projects, the characteristics of “strong DC and weak AC, multi-loop DC centralized feed” in the Chinese power grid have become increasingly prominent, and the supporting capacity of the receiving end of the power grid has declined. Ac system failure may induce multiple commutation failures of a single converter station, and even cause DC lock-up, which seriously threatens the safety of AC/DC hybrid power grid under the new form in China. 
     With the construction of large-scale energy storage power stations and 100-megawatt energy storage power stations on the grid side, the energy storage power stations can play a role in peak regulation, frequency regulation and stabilizing the power fluctuation after the new energy is connected to the grid. it can also provide transient active and reactive power emergency support for power grid faults. Compared with other forms of energy storage, electrochemical energy storage has the advantages of fast response, strong dynamic active and reactive power support, high environmental adaptability, short construction period, decentralized configuration, etc. Therefore, it can be regarded as a very high-quality regulation resource of the power grid. 
     After the first commutation failure of DC system, it is of great theoretical and practical significance to adopt an appropriate control strategy to make full use of the fast response and transient support capacity of electrochemical energy storage on the grid side, to provide transient power support for the AC system after fault, to reduce the number of subsequent commutation failures, and to improve the recovery ability of the power grid after a fault. 
     SUMMARY 
     Aiming at the defects and improvement requirements of the existing technology, The present invention provides an Energy Storage Transient Power Coordinated Control Method for Restraining Subsequent Commutation Failures, it is of great theoretical and practical significance to adopt appropriate control strategy to make full use of the fast response and transient support capacity of electrochemical energy storage on the grid side, to provide transient power support for the AC system after fault, to reduce the number of subsequent commutation failures, and to improve the recovery ability of the power grid after fault. 
     To achieve the above object, according to one aspect of the present invention, The present invention provides an Energy Storage Transient Power Coordinated Control Method for Restraining Subsequent Commutation Failures, include: 
     S 1 : Continuously detect the DC current on the inverter side, the AC bus voltage and the arc quenching angle on the inverter side. When the DC current is greater than the first current threshold or the AC bus voltage is less than the voltage threshold or the arc quenching angle is less than the first arc quenching angle threshold, the energy storage power station is controlled to enter the transient control mode; 
     S 2 : The output time instruction corresponding to the active output of energy storage is determined by using the relationship between the DC current and the first current threshold and the second current threshold. The first current threshold is greater than the second current threshold. The output time instruction corresponding to the reactive power output of energy storage is determined by using the relationship between the inverter arc quenching angle and the first arc quenching angle threshold and the second arc quenching angle threshold. The first arc quenching angle threshold is less than the second arc quenching angle threshold; 
     S 3 : The output amplitude instruction is determined by the relationship between the AC bus voltage and the voltage threshold, and the output amplitude instruction corresponding to the reactive power output of the energy storage is determined by using the constraint of the total energy storage capacity ; 
     S 4 : The energy storage active power instruction and the energy storage reactive power instruction of the target output range are issued to the energy storage power station at the target output time, the output time instruction carries the target output time, and the output amplitude instruction carries the target output amplitude. 
     Further, The step S 2  comprises: 
     S 201 : when it is detected that the DC current is greater than the first current threshold, an active power descending edge instruction is issued, and the value of the active power output time instruction P T  is −1. The active power descending edge instruction is used to control the energy storage power station to absorb the active power of the inverter side of the HVDC transmission system. When it is detected that the arc quenching angle is less than the first arc quenching angle threshold, a reactive power rising edge instruction is issued, and the value of the reactive power output time instruction Q T  is 1. The reactive power rising edge instruction is used to control the energy storage to release reactive power to the inverter side of the HVDC system. 
     S 202 : when it is detected that the DC current decreases and is successively lower than the first current threshold and the second threshold, the P T  is maintained as −1; when the arc quenching angle is detected to increase and is larger than the first arc quenching angle threshold and the second arc quenching angle threshold, the Q T  is maintained to be 1. 
     S 203 : when it is detected that the DC current rises to exceed the second current threshold, the PT is changed to 0; when the detected arc quenching angle is reduced to less than the second arc quenching angle threshold, the Q T  is changed to 0. 
     S 204 : when it is detected that the DC current rises to exceed the first current threshold, or when the real-time arc quenching angle is reduced to less than the first arc quenching angle threshold, stp S 201  is performed. 
     Further, After stp S 203 , step S 2  also includes: 
     S 205 : when the DC current I dci  exceeds the second current threshold I drefL  for the first time during the recovery process after the first commutation failure on the inverter side, an active power rising edge instruction is issued and the duration of the value PT of the output time instruction exceeds the active time threshold, so as to avoid short-term repeated trigger caused by high-frequency oscillation of the DC current near the second current threshold. When the arc quenching angle is lower than the second arc quenching angle threshold for the first time during the recovery process after each commutation failure on the inverter side, a reactive power falling edge instruction is issued and the duration of the value Q T  of the output time instruction exceeds the reactive power time threshold, so as to avoid short-term repeated triggers caused by high-frequency oscillation of the arc quenching angle near the second arc quenching angle threshold. 
     Further, Before stp S 205 , it also includes: Setting the active power time threshold to 8 ms and the reactive power time threshold to 5 ms. 
     Further, Step S 3  includes: 
     S 301 : When the AC bus voltage is lower than the voltage threshold and the DC current exceeds the preset range, the output active output amplitude instruction value PA is recorded as P 1 ; otherwise, the output active output amplitude instruction value P A  is recorded as P 2 , where, P 1 &gt;P 2 ; 
     S 302 : the reactive power output amplitude instruction is determined according to the active power output amplitude instruction and the capacity constraint of energy storage itself. 
     Further, 
     
       
         
           
             
               
                 P 
                 1 
               
               = 
               
                 
                   7.5 
                   % 
                   ⁢ 
                       
                   
                     P 
                     dc 
                   
                 
                 + 
                 
                   
                     
                       P 
                       dc 
                       2 
                     
                     ⁢ 
                     
                       fU 
                       
                         f 
                           
                         0.1 
                       
                     
                   
                   
                     20 
                     ⁢ 
                     
                       U 
                       N 
                       2 
                     
                   
                 
               
             
             ; 
           
         
       
     
     Among them, P dc  is the rated DC transmission power of HVDC system, rated frequency of f AC system, U N  is the rated effective voltage of AC bus on the inverter side, and U f0.1  is the AC bus voltage of 0.1 s after the AC bus fault on the inverter side. 
     P 2 =7.5% P dc . 
     Further, The instruction of reactive power output amplitude is Q A =√{square root over (S BESS   2 −(P T P A ) 2 )}, where S BESS  is the energy storage capacity. 
     Further, before stp S 302 , it also includes: 
     Set the sample hold time of U f0.1  to 1 s. 
     Further, step S 4  includes: 
     S 401 : determining the target output time from the rising edge time or falling edge time in the output time instruction; taking the output amplitude instruction corresponding to the output value as the target output amplitude; 
     S 402 : the energy storage active power instruction corresponding to the target output amplitude is output at the time of target output, and the value of the energy storage active power instruction is the algebraic product of the value of the active power output time instruction and the value of the active power output amplitude instruction. When the target output time, the energy storage reactive power instruction corresponding to the target output amplitude is output, and the value of the energy storage reactive power instruction is the algebraic product of the reactive power output time instruction value and the reactive power output amplitude instruction value. 
     Further, before step S 2 , the method also includes: 
     when the DC current is less than or equal to the first current threshold, or when the AC bus voltage is greater than or equal to the voltage threshold, the control energy storage power station continues to maintain the energy storage active power instruction to 0. When the real-time arc quenching angle is greater than or equal to the first arc quenching angle threshold the energy storage power station is controlled to keep the value of the energy storage reactive power instruction to 0. 
     In general, compared with the prior art, the above technical proposal conceived by the invention can achieve beneficial effects including: 
     The invention determines the energy storage active power output time instruction and the reactive power output time instruction respectively by detecting the DC current and the arc quenching angle on the inverter side. Then, the active power output amplitude instruction is determined by detecting the AC bus voltage, and the reactive power output amplitude instruction is determined by using the constraint of the total energy storage capacity. thus, the energy storage active power instruction and reactive power instruction of the target output amplitude are issued to the energy storage power station at the time of target output. In this way, the invention is designed from three dimensions of energy storage output time, output amplitude and active and reactive power coordination, so as to make full use of the rapid response and transient support capacity of electrochemical energy storage on the grid side. so that the coordinated output of energy storage active and reactive power can be applied in the field of commutation failure suppression, which can effectively improve the transient control effect of energy storage power station on UHV AC/DC hybrid system. In particular, this can not only effectively prevent the serious impact on the power grid system caused by the rapid drop of DC voltage and the sharp increase of DC current after commutation failure, but also restrain the subsequent commutation failure after the first commutation failure after DC fault. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a flow chart of a transient active and reactive power coordinated control method for suppressing subsequent commutation failures in an embodiment of the present invention. 
         FIG.  2    is a flow chart of step S 2  in a transient active and reactive power coordinated control method for energy storage power stations to suppress subsequent commutation failures in an embodiment of the present invention. 
         FIG.  3    is a schematic diagram of the active time output instruction of an energy storage power station in an embodiment of the present invention. 
         FIG.  4    is a schematic diagram of the reactive time output instruction of an energy storage power station in an embodiment of the present invention. 
         FIG.  5    is a flow chart of step S 3  in a transient active and reactive power coordinated control method for energy storage power stations to suppress subsequent commutation failures in an embodiment of the present invention. 
         FIG.  6    is a flow chart of step S 4  in a transient active and reactive power coordinated control method for energy storage power stations to suppress subsequent commutation failures in an embodiment of the present invention. 
         FIG.  7    is a logical block diagram of a transient active and reactive power coordinated control method for suppressing subsequent commutation failures in an energy storage power station provided by the present invention. 
         FIG.  8    is a schematic diagram of the Henan multi-infeed DC system model provided by the present invention. 
         FIG.  9    schematic diagram of the lithium battery electrochemical energy storage power station model provided by the present invention. 
     
    
    
     (a), (b) and (c) of  FIG.  10    are schematic diagrams of the simulation results of the commutation failure suppression effect of the invention when the grounding inductance L is 0.002H, 0.02H and 0.1H, respectively. 
     DESCRIPTION OF THE EMBODIMENTS 
     In order to make the purpose, technical scheme and advantages of the invention more clear, the invention is further described in detail in combination with the attached drawings and embodiments below. It should be understood that the specific embodiments described herein are used only to explain the invention and are not used to define the invention. In addition the technical features involved in the various embodiments of the invention described below can be combined with each other as long as they do not conflict with each other. 
     Referring to  FIG.  1   , combined with  FIGS.  2  to  7   , the invention provides an energy storage transient power coordinated control method for restraining subsequent commutation failures, which includes: 
     S 1 : continuously detect the real-time DC current I dci  on the inverter side, the AC bus voltage U aci  and the arc quenching angle y on the inverter side. When the real-time DC current I dci  is greater than the first current threshold I drefH  or the AC bus voltage U aci  is less than the voltage threshold U acirefA  or the real-time arc quenching angle γ is less than the first arc quenching angle threshold γ 1 , the energy storage power station is controlled to enter the transient control mode. 
     Specifically, the real-time DC current I dci  on the inverter side and the AC bus voltage U aci  on the inverter side are continuously collected and tested, and the first current threshold I drefH  is 1.2pu, the second current threshold I drefL  is 0.88ppu, U acirefA  is 0.95pu, the first arc quenching angle threshold γ 1  is 12°, and the second arc quenching angle threshold γ h  is 35°. If the amplitude of I dci  is greater than the threshold I drefL  or the amplitude of U aci  is less than the voltage threshold U acirefA  or the amplitude of γ is less than the threshold γ 1 , the energy storage power station is determined to enter the transient control mode. In one embodiment, before step S 2 , the transient active and reactive power coordinated control method for suppressing subsequent commutation failures of the energy storage power station also includes: when the real-time DC current I dci  is less than or equal to the first current threshold I drefH , or when the AC bus voltage is greater than or equal to the voltage threshold, the control energy storage power station continues to maintain the energy storage active power instruction to 0. When the real-time arc quenching angle y is greater than or equal to the first arc quenching angle threshold γ 1 , the energy storage power station is controlled to keep the value of the energy storage reactive power instruction to 0. 
     S 2 : the output time instruction corresponding to the active output of energy storage is determined by using the relationship between the real-time DC current I dci  and the first current threshold I drefH  and the second current threshold I drefL . The first current threshold I drefH  is greater than the second current threshold I drefL . The output time instruction corresponding to the reactive power output of energy storage is determined by using the relationship between the real-time inverter arc quenching angle γ and the first arc quenching angle threshold γ 1  and the second arc quenching angle threshold γ h . The first arc quenching angle threshold γ 1  is less than the second arc quenching angle threshold γ h . 
     Specifically, step S 2  is to determine the output time instruction corresponding to the energy storage active power output time according to the detected real-time DC current I dci , that is, when to release or absorb the active power to the HVDC system; and to determine the output time instruction corresponding to the energy storage reactive power output time according to the detected real-time arc quenching angle, that is, when to release or absorb the reactive power to the HVDC transmission system. For example, the process of determining the output time instruction is as follows: 1) when the DC current I dci  is greater than the threshold I drefH , the controller issues an instruction PT of −1 to control the energy storage to absorb the active power on the inverter side of the HVDC system; under the constant current control of the system, the DC current I dci  will first decrease and less than I drefH , I dreL  and then continue to rise. When it is detected that the real-time arc quenching angle γ is less than the first arc quenching angle threshold γ 1 , an instruction QT of reactive power rising edge is issued to control the energy storage to release reactive power to the inverter side of the HVDC transmission system. 2) when the DC current I dci  rises to the threshold I drefL , the controller issues an instruction that the rising edge PT is 0, and the control energy storage stops absorbing active power to the inverter side of the HVDC system; when it is detected that the real-time arc quenching angle γ decreases less than the second arc quenching angle threshold γ h  and exceeds the first arc quenching angle threshold γ 1 , the controller changes the QT to 0. 3) if the DC current I dci  crosses the threshold I drefL  and reaches the threshold I drefH  again, or when the falling real-time arc quenching angle γ is less than the first arc quenching angle threshold yi, continue to step 1); otherwise, the energy storage active power instruction and reactive power instruction of the energy storage power station are both 0. 
     S 3 : the output amplitude instruction is determined by the relationship between the AC bus voltage U aci  and the voltage threshold U acirefA , and the output amplitude instruction corresponding to the reactive power output of the energy storage is determined by using the constraint of the total energy storage capacity. 
     Specifically, step S 3  determines the output amplitude instruction corresponding to the active power output of energy storage according to the relationship between the AC bus voltage U aci  and the voltage threshold U acirefA , and determines the reactive power output amplitude instruction according to the active power output instruction and the capacity constraint of the energy storage itself. the specific process is as follows: 1) in the active power control, when the voltage U aci  falls beyond the threshold U acirefA , the high level is output to the and gate P A1  of the amplitude selection part. When the current I dci  no longer fluctuates in a certain period of time, it is judged that the system is stable, and when the system is unstable, the high level is output to the selector and gate P A2 . 2) when the energy storage active power amplitude selection part receives the high voltage from P A1  and P A2  at the same time, the selector switches to the optimal output amplitude determination module P 1 , otherwise it is in the P2 gear of the energy storage foundation output, and the final output is P A , that is, P A =P 1  or P A =P 1 ; 3) the reactive power output amplitude instruction Q A  is determined according to the active power output instruction and the capacity constraint of energy storage itself. In one of the embodiments, the output value 
     
       
         
           
             
               P 
               1 
             
             = 
             
               
                 7.5 
                 % 
                 ⁢ 
                     
                 
                   P 
                   dc 
                 
               
               + 
               
                 
                   
                     P 
                     dc 
                     2 
                   
                   ⁢ 
                   
                     fU 
                     
                       f 
                         
                       0.1 
                     
                   
                 
                 
                   20 
                   ⁢ 
                   
                     U 
                     N 
                     2 
                   
                 
               
             
           
         
       
     
     of the first gear output. Among them, P dc  is the rated DC transmission power of HVDC system, rated frequency of f AC system, U N  is the rated effective voltage of AC bus on the inverter side, and U f0.1  is the AC bus voltage of 0.1 s after the AC bus fault on the inverter side. The output value of the second gear is P 2 =7.5% P dc . The instruction of reactive power output amplitude is Q A =√{square root over (S BESS   2 −(P T P A ) 2 )}, where S BESS  is the energy storage capacity. 
     It should be noted that the method is cycled in each control cycle (depending on the accuracy of the control system) of the transient control of the energy storage power station. In addition, the control parameters (U acirefA , I drefL , I drefH , γ 1 , γ h ) selected in the invention are artificially selected empirical values and can be adjusted appropriately. The selection principle of U acirefA  is to determine that the AC/DC system is in the transient process of large disturbance; the selection principle of I drefL  and I drefH  is to determine that the DC is in the high risk stage of subsequent commutation failure in the process of commutation failure recovery. The selection principle of γ 1  and γ h  is to ensure that the real-time turn-off angle is in the range of current deviation control in the recovery stage of commutation failure. 
     S 4 : the energy storage active power instruction and the energy storage reactive power instruction of the target output range are issued to the energy storage power station at the target output time, the output time instruction carries the target output time, and the output amplitude instruction carries the target output amplitude. Specifically, the target output time is determined according to the rising edge time or falling edge time in the output time instruction. Taking P 1  or P 2  as the target active output amplitude, the energy storage active power instruction of the target output amplitude is issued to the energy storage power station at the time of the target output. The value of the energy storage active power instruction P ref  is the algebraic product of the value P T  of the output time instruction and the value P A  of the output amplitude instruction, that is, P ref =P T *P A . The energy storage reactive power instruction of the target output amplitude is issued to the energy storage power station at the time of the target output. The value of the energy storage reactive power instruction Q ref  is the algebraic product of the value Q T  of the output time instruction and the value Q A  of the output amplitude instruction, that is, Q ref =Q T *Q A . 
     In one of the embodiments, as shown in  FIG.  2   . Step S 2  includes: S 201 : when it is detected that the DC current is greater than the first current threshold, an active power descending edge instruction is issued, and the value of the active power output time instruction P T  is −1. The active power descending edge instruction is used to control the energy storage power station to absorb the active power of the inverter side of the HVDC transmission system. When it is detected that the arc quenching angle is less than the first arc quenching angle threshold, a reactive power rising edge instruction is issued, and the value of the reactive power output time instruction Q T  is 1. The reactive power rising edge instruction is used to control the energy storage to release reactive power to the inverter side of the HVDC system. 
     S 202 : when it is detected that the DC current decreases and is successively lower than the first current threshold and the second threshold, the P T  is maintained as −1; when the arc quenching angle is detected to increase and is larger than the first arc quenching angle threshold and the second arc quenching angle threshold, the Q T  is maintained to be 1. 
     S 203 : when it is detected that the DC current rises to exceed the second current threshold, the PT is changed to 0; when the detected arc quenching angle is reduced to less than the second arc quenching angle threshold, the Q T  is changed to 0. 
     S 204 : when it is detected that the DC current rises to exceed the first current threshold, or when the real-time arc quenching angle is reduced to less than the first arc quenching angle threshold, stp S 201  is performed. 
     Specifically, when it is detected that the DC current I dci  is greater than the threshold I drefH , the controller issues an instruction P T  of −1 to control the energy storage to absorb the active power on the inverter side of the HVDC system. Under the constant current control of the system itself, the DC current I dci  will first decrease and less than I drefH , I drefL , and then continue to rise. When it is detected that the real-time arc quenching angle γ is less than the first arc quenching angle threshold γ 1 , an instruction Q T  of reactive power rising edge is issued to control the energy storage to release reactive power to the inverter side of the HVDC transmission system. When it is detected that the DC current I dci  rises to the threshold I drefL , the controller sends an instruction that the rising edge P T  is 0, and the control energy storage stops absorbing active power to the inverter side of the HVDC system. When it is detected that the real-time arc quenching angle γ is reduced to less than the second arc quenching angle threshold γ h  and greater than the first arc quenching angle threshold γ 1 , the controller changes the Q T  to 0. If the DC current I dci  crosses the threshold I drefL  and reaches the threshold I drefH  again, the active power falling edge instruction P T  is −1. When the real-time arc quenching angle γ is less than the first arc quenching angle threshold γ 1 , the reactive power rising edge instruction Q T  is 1. Otherwise, the energy storage active power instruction and reactive power instruction are both 0, and the energy storage power station is in a state of waiting for output. 
     In one embodiment, after stp S 203 , step S 2  also includes: S 205 : when the real-time DC current I dci  exceeds the second current threshold I drefL  for the first time during the recovery process after the first commutation failure on the inverter side, an active power rising edge instruction is issued and the duration of the value PT of the output time instruction exceeds the active time threshold, so as to avoid short-term repeated trigger caused by high-frequency oscillation of the real-time DC current near the second current threshold. When the real-time arc quenching angle is lower than the second arc quenching angle threshold for the first time during the recovery process after each commutation failure on the inverter side, a reactive power falling edge instruction is issued and the duration of the value Q T  of the output time instruction exceeds the reactive power time threshold, so as to avoid short-term repeated triggers caused by high-frequency oscillation of the real-time arc quenching angle near the second arc quenching angle threshold. The two-time thresholds can be set according to the requirements, generally set at the millisecond level. In one embodiment, before stp S 205 , the transient active and reactive power coordinated control method for suppressing subsequent commutation failures of the energy storage power station also includes setting the active power time threshold to 8 ms and the reactive power time threshold to 5 ms. In this step, in order to avoid short-term repeated trigger caused by high-frequency oscillation near the threshold I drefL  when the I dci  rises to the threshold I drefL , when the I dci  exceeds the threshold I drefL  for the first time in the recovery process after the first commutation failure, the shortest time for the controller to issue the rising edge instruction PT is 8 ms. In order to avoid short-term repeated triggers caused by high-frequency oscillation near the threshold γ h , when γ is less than the threshold γ h  for the first time in the recovery process after the first commutation failure, the shortest time for the controller to issue the rising edge instruction Q T  is 5 ms. It should be noted that after the first commutation failure, because the DC system itself has the function of regulation and control, the commutation conditions of the system will be improved, such as DC current and AC voltage recovery, which is the first commutation failure recovery process. If the transition is restored, it will lead to a second or subsequent commutation failure. The schematic diagram of energy storage reactive power time output instruction and the schematic diagram of energy storage active power time output instruction are shown in  FIGS.  3  and  4    respectively. 
     In one of the embodiments, as shown in  FIG.  5   , step S 3  includes: S 301 : when the AC bus voltage is lower than the voltage threshold, the first amplitude value is high with the gate P A1  output level. When the real-time DC current exceeds the preset range, the instability of the energy storage station is determined, and the output level of the second amplitude and gate P A2  is high. S 302 : when both the first amplitude value and the gate P A1  and the second amplitude and gate P A2  output high voltage, the output amplitude instruction of the first gear output is selected, and the output value is recorded as P 1 . Otherwise, the output amplitude instruction of the second gear output is selected, and the output value is recorded as P 2 . Among them, P 1 &gt;P 2 , the reactive power output amplitude instruction is determined according to the active power output instruction and the capacity constraint of energy storage itself. 
     In one of the embodiments, before stp S 302 , the transient active power control method of the energy storage power station also includes setting the sample and hold time of the U f0.1  to 1s by default. In order to make P1 change dynamically with the fault recovery degree of DC system, the sample and hold time of U f0.1 is set to 1s. After the end of sample and hold, whether to sample and sample size is decided according to whether the fault is removed and the fault degree. When the AC bus voltage is lower than the voltage threshold, the fault is judged to occur. The sampling time is 0.1s after the fault, and the sampling time is kept for 1s. 
     In one of the embodiments, as shown in  FIG.  6   , step S 4  includes: S 401 : determining the target output time from the rising edge time or falling edge time in the output time instruction; taking the output amplitude instruction corresponding to the output value as the target output amplitude. S 402 : the energy storage active power instruction corresponding to the target output amplitude is output at the time of target output, and the value of the energy storage active power instruction is the algebraic product of the value of the active power output time instruction and the value of the active power output amplitude instruction. When the target output time, the energy storage reactive power instruction corresponding to the target output amplitude is output, and the value of the energy storage reactive power instruction is the algebraic product of the reactive power output time instruction value and the reactive power output amplitude instruction value. 
     As shown in  FIG.  7   , after detecting the real-time DC current I dci , the AC bus voltage U aci  on the inverter side and the arc quenching angle γ, the whole process of energy storage active power instruction value Pref and reactive power instruction value Qref can be obtained according to I dci , U aci  and γ. Specifically, when the real-time DC current I dci  is greater than the first current threshold I drefH  or the AC bus voltage U aci  is less than the voltage threshold U acirefA , or the arc quenching angle γ is less than the first arc quenching angle γ 1  threshold, the energy storage power station is controlled to enter the transient control mode. The output time instruction PT corresponding to the active output of energy storage is determined by using the relationship between the real-time DC current I dci  and the first current threshold I drefH  and the second current threshold I drefL , and the first current threshold is greater than the second current threshold. The active power output amplitude instruction PA is determined by using the relationship between the AC bus voltage U aci  and the voltage threshold U acirefA . According to the output time instruction P T  and the output amplitude instruction P A , the energy storage active power instruction P ref  of the target output amplitude is issued to the energy storage power station at the target output time. The output time instruction Q T  corresponding to the reactive power output of energy storage is determined by using the relationship between the real-time inverter arc quenching angle γ and the first arc quenching angle threshold γ 1  and the second arc quenching angle threshold γ h . The first arc quenching angle threshold is less than the second arc quenching angle threshold. The output amplitude instruction Q A  corresponding to the reactive power output of energy storage is determined by using the constraint of total energy storage capacity. According to the output time instruction Q T  and the output amplitude instruction Q A , the energy storage reactive power instruction Q ref  of the target output amplitude is issued to the energy storage power station at the target output time. In the time control of active power output of energy storage, the purpose of delay 30 ms is to ensure that the controller maintains the original output mode when the DC current is in turn less than the first current threshold and the second current threshold. In the control of reactive power output of energy storage, the purpose of delay 10 ms is to ensure that the controller maintains the original output mode when the arc quenching angle jumps from less than the first arc quenching angle threshold to greater than the second arc quenching angle threshold. 
     In order to verify the technical effect of the control method provided by the invention, a simulation experiment is carried out by taking the Henan multi-feed DC system as a case of UHV AC/DC hybrid system. The corresponding simulation model is built in PSCAD/EMTDC, as shown in  FIG.  8   . Henan multi-infeed DC system includes two UHVDC transmission lines, Qinghai-Henan DC and Tianzhong DC. Their voltage levels are ±800 kV and the DC power transmitted in steady state is 8000 MW. The parameters in  FIG.  8    are shown in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 equivalent model impedance parameters of Henan multi- 
               
               
                 infeed DC receiver AC system 
               
            
           
           
               
               
               
               
            
               
                 Impedance 
                   
                 Impedance 
                   
               
               
                 No. 
                 Parameter value 
                 No. 
                 Parameter value 
               
               
                   
               
               
                 Z5(Ω) 
                 0.5992 + j6.8484  
                 Z45(Ω) 
                  3.3994 + j38.8554 
               
               
                 Z4(Ω) 
                 0.7977 + j9.1169  
                 Z35(Ω) 
                 3.86436 + j44.1699 
               
               
                 Z3(Ω) 
                 0.5864 + j0.02133 
                 Z34(Ω) 
                 1.39404 + j15.9340 
               
               
                   
               
            
           
         
       
     
     In addition, the electrochemical energy storage power station model of lithium battery is built in PSCAD/EMTDC, as shown in  FIG.  9   . The model mainly consists of three parts: lithium battery array, Boost boost circuit and grid-connected inverter. The fixed power control is adopted in the energy storage power station, and the power output of the energy storage power station can be changed flexibly by changing the active and reactive power instructions P ref  and Q ref . A simulation example is given to verify the suppression effect of the proposed active power control strategy on the subsequent commutation failure of Henan multi-infeed DC system. The AC bus on the DC inverter side shown in  FIG.  8    is set to have a three-phase ground fault through a certain size of grounding inductor at 0.5 s, and the duration is 0.1 s. The energy storage power station with the capacity of 1200 MVA is connected to the AC bus, and the active power control strategy of the energy storage power station adopts the active power control method proposed by the invention. 
     When the grounding inductance L is 0.002H, 0.02H and 0.1H respectively, the simulation results are shown in  FIG.  10   . It is generally believed that commutation failure occurs when the turn-off angle is less than 7°. It can be seen that when the energy storage power output is coordinated and controlled by the invention, the commutation failure can be suppressed under different fault degrees. 
     It should be added that, as shown in  FIG.  8   , a three-phase ground fault occurs through a certain size of grounding inductor on the AC bus on the DC inverter side, with a duration of 0.1s and an energy storage capacity of 1200 MVA. By changing the fault time and the size of the grounding inductor, several groups of working conditions are set for ergodic simulation, and the commutation failure suppression effects of energy storage under different working conditions are compared. Among them, Working condition 1: no energy storage access. Working condition 2: energy storage is connected and the coordinated control strategy of transient active power and transient reactive power is adopted. Working condition 3: energy storage is connected and only energy storage transient active power control strategy is adopted. Working condition 4: energy storage is connected and only energy storage transient reactive power control strategy is adopted, and the output amplitude is 1200 MVar. 
     Traversing the simulation results as shown in Table 2, we can see that in most working conditions, after energy storage is connected, it can effectively reduce the number of commutation failures after DC faults, and play a good control effect. Furthermore, under some working conditions, when energy storage is connected and the coordinated control mode of transient active power and transient reactive power is adopted, the number of DC commutation failures can be suppressed to a minimum, and the best control effect can be achieved by energy storage. From the ergodic simulation results, it can be seen that the proposed coordinated control strategy of transient active power and transient reactive power of energy storage power station has good applicability to different working conditions of the system. 
     In summary, the coordinated support of the transient power of the energy storage power station is an effective control method to suppress the subsequent commutation failure after the UHV DC system failure. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Traversal simulation results 
               
            
           
           
               
               
               
            
               
                   
                 (Failure occurs at 0.50 s) 
                 (Failure occurs at 0.52 s) 
               
               
                   
                 Number of commutation failures 
                 Number of commutation failures 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Working 
                 Working 
                 Working 
                 Working 
                 Working 
                 Working 
                 Working 
                 Working 
               
               
                 Ground 
                 condition 
                 condition 
                 condition 
                 condition 
                 condition 
                 condition 
                 condition 
                 condition 
               
               
                 inductance/H 
                 1 
                 2 
                 3 
                 4 
                 1 
                 2 
                 3 
                 4 
               
               
                   
               
               
                 0.002 
                 3 
                 2 
                 4 
                 3 
                 5 
                 3 
                 1 
                 4 
               
               
                 0.004 
                 3 
                 2 
                 2 
                 5 
                 4 
                 3 
                 3 
                 4 
               
               
                 0.008 
                 4 
                 3 
                 4 
                 3 
                 4 
                 1 
                 3 
                 4 
               
               
                 0.012 
                 4 
                 3 
                 4 
                 3 
                 4 
                 3 
                 4 
                 2 
               
               
                 0.016 
                 4 
                 2 
                 2 
                 5 
                 5 
                 4 
                 1 
                 4 
               
               
                 0.020 
                 4 
                 2 
                 4 
                 6 
                 2 
                 2 
                 3 
                 3 
               
               
                 0.025 
                 4 
                 3 
                 4 
                 3 
                 3 
                 2 
                 2 
                 3 
               
               
                 0.035 
                 3 
                 2 
                 3 
                 1 
                 3 
                 1 
                 2 
                 1 
               
               
                 0.070 
                 2 
                 2 
                 2 
                 2 
                 2 
                 2 
                 2 
                 2 
               
               
                 0.090 
                 2 
                 1 
                 1 
                 2 
                 2 
                 1 
                 2 
                 1 
               
               
                 0.100 
                 2 
                 1 
                 1 
                 1 
                 3 
                 1 
                 1 
                 1 
               
               
                   
               
            
           
         
       
     
     The invention provides a coordinated control strategy of transient active power and transient reactive power of energy storage power station suitable for UHV DC. The control strategy is designed from three dimensions: energy storage output time, output amplitude, active and reactive power coordination, so as to take into account the favorable timing and magnitude of energy storage, which enables the energy storage power station to play a better role in restraining the commutation failure of UHV AC/DC hybrid system. 
     Those skilled in the art should understand that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of complete hardware embodiments, complete software embodiments, or embodiments combining software and hardware. In addition, the present invention may take the form of a computer program product implemented on one or more computer available storage media (including, but not limited to, disk memory, CD-ROM, optical memory, etc.) containing computer available program codes. 
     The invention should understand that the combination of each flow and/or block in the flowchart and/or block diagram and the combination of the flow and/or block in the flowchart and/or block diagram can be realized by computer program instructions. These computer program instructions may be provided to a processor of a general-purpose computer, special purpose computer, embedded processor or other programmable data processing device to generate a machine. Accordingly, instructions executed by a processor of a computer or other programmable data processing device generate a device for implementing a function specified in a flowchart one or more processes and/or block diagrams. 
     It is easy for those skilled in the art to understand that the above are only better embodiments of the invention and are not used to limit the invention, and any modifications, equivalent replacements and improvements made within the spirit and principles of the invention, shall be included in the scope of protection of the invention.