阅读说明：本技术 一种储能内置式换流器的柔性直流配电网故障快速恢复方法 (Rapid fault recovery method for flexible direct-current power distribution network of energy storage built-in converter ) 是由 刘忠 王冰冰 詹昕 许扬 李培培 梅军 陈武 范光耀 于 2019-12-05 设计创作，主要内容包括：一种储能内置式换流器的柔性直流配电网故障快速恢复方法。涉及电力系统,尤其涉及一种储能内置式换流器的柔性直流配电网故障快速恢复方法。提供了一种通过换流器与储能之间的协调配合,以他励方式来实现故障快速恢复,针对双极短路故障下配电网的快速恢复,以及在故障清除期间不间断供电的储能内置式换流器的柔性直流配电网故障快速恢复方法。本发明一方面为重要负荷提供功率以保障其在故障清除期间的不间断供电；另一方面,对闭锁期间的子模块电容电压进行调节以使其在解锁前达到正常运行时的额定值,减小换流器重新解锁时的电压波动,为换流器重启动缩短时间,这样的系统协调控制策略更加能保证提高整个配电网系统的可靠性。(A method for rapidly recovering faults of a flexible direct-current power distribution network of an energy storage built-in converter. The utility model relates to an electric power system, especially relate to a flexible direct current distribution network fault quick recovery method of built-in transverter of energy storage. The fault rapid recovery method for the flexible direct-current power distribution network of the built-in energy storage converter is characterized in that the fault rapid recovery is realized in a separate excitation mode through the coordination and the coordination between the converter and the energy storage, the rapid recovery of the power distribution network under the bipolar short-circuit fault is aimed at, and the uninterrupted power supply is realized during the fault clearing period. The invention provides power for important loads to ensure uninterrupted power supply during fault clearing; on the other hand, the sub-module capacitor voltage during the locking period is adjusted to reach a rated value during normal operation before unlocking, voltage fluctuation when the converter is unlocked again is reduced, time is shortened for restarting the converter, and the system coordination control strategy can better guarantee improvement of the reliability of the whole power distribution network system.)

1. A method for rapidly recovering faults of a flexible direct-current power distribution network of an energy storage built-in converter is characterized by comprising the following steps:

1) energy storage built-in converters are respectively arranged at two ends of the flexible direct current distribution network;

2) detecting whether a bipolar short-circuit fault occurs on the direct current side;

3) if the fault occurs, locking the energy storage built-in converter;

4) and carrying out fault recovery by using the built-in energy storage converter.

2. The method for rapidly recovering the fault of the flexible direct-current power distribution network of the built-in energy storage converter according to claim 1, is characterized in that the built-in energy storage converter comprises a full-bridge type modular multilevel converter, a double-active full-bridge converter and an energy storage device; the full-bridge type modular multilevel converter comprises an upper bridge arm and a lower bridge arm, wherein the upper bridge arm of each phase consists of N full-bridge submodules and corresponding bridge arm resistors R_aBridge arm inductance L_aAre connected in series;

the lower bridge arm of each phase consists of N full-bridge submodules and corresponding bridge arm resistors R_aBridge arm inductance L_aAre connected in series;

wherein, the output end of the first full-bridge submodule in each phase upper bridge arm and the medium-voltage direct-current bus u_uaThe output end of the last full-bridge submodule in each phase of lower bridge arm is connected with the medium-voltage direct-current bus u_laThe negative electrode of (1) is connected;

and on each phase of bridge arm, an output capacitor of each full-bridge submodule is connected with a double-active full-bridge converter, and 2N double-active full-bridge converters are connected in parallel and then serve as outlets of low-voltage side direct current circuits and are connected with an energy storage device.

3. The method for rapidly recovering the fault of the flexible direct current power distribution network of the built-in energy storage converter according to claim 1, wherein the step 2) comprises the following steps:

calculating short-circuit current i before and after fault of direct-current power distribution network_f(t) and submodule capacitor voltage u_c(t) analytical formula:

capacitor voltage u before locking of energy storage built-in converter_cAnd fault loop discharge current i_fThe mathematical expression of (a) is:

in the formula, L_e＝2L_a/3+L₀，C_e＝6C_SM/N，R_e＝2R_aThe/3 + R is an equivalent inductance, an equivalent capacitance and an equivalent resistance of a fault current path before ESBC locking respectively; u shape_d1＝u_c(t₀+)＝u_c(t₀-) is the initial value of the DC voltage at the time of the fault, I_d1＝i_f(t₀+)＝i_f(t₀-) is the initial value of the direct current; τ 2L_e/R_eIs the discharge time constant of the equivalent circuit,is the angular frequency of the equivalent discharge circuit,

capacitor voltage u after locking of energy storage built-in converter_cAnd fault loop discharge current i_fThe mathematical expression of (a) is:

wherein L is_e＝2L_a/3+L₀，C_e1＝3C_SM/2N，R_e＝2R_a/3+R₀Respectively an equivalent inductor, an equivalent capacitor and an equivalent resistor of a fault current path after the ESBC is locked; u shape_f＝u_c(t₁+)＝u_c(t₁-) is the initial value of the voltage at latch-up, I_f＝i_f(t₁+)＝i_f(t₁-) is the initial value of the current at latch-up;

4. The method for rapidly recovering the fault of the flexible direct current power distribution network of the energy storage built-in converter is characterized in that in the step 4), after the energy storage built-in converter is locked, the energy storage device directly provides power support for the load.

5. The method for rapidly recovering the fault of the flexible direct-current power distribution network of the built-in energy storage converter according to claim 2 or 4, wherein in the step 4), after the built-in energy storage converter is locked, the energy storage device performs voltage regulation control on the capacitor of the full-bridge submodule through the double-active full-bridge converter.

6. The method for rapidly recovering the fault of the flexible direct-current power distribution network of the converter with the built-in energy storage device as claimed in claim 5, wherein when the energy storage device performs voltage regulation control on the capacitor of the full-bridge submodule through the dual-active full-bridge converter, the voltage of the full-bridge submodule is lower than the rated voltage U in the first stage_CrefThe voltage of the full-bridge submodule is higher than the rated voltage U in the second stage_CrefDischarging the submodules of (a);

and in the sub-module discharging process of the second stage, the energy storage device absorbs electric energy.

Technical Field

The invention relates to an electric power system, in particular to a method for quickly recovering faults of a flexible direct-current power distribution network of a built-in energy storage converter.

Background

With the gradual deepening of the industrial application degree of power electronics and the large-scale application of various distributed energy sources such as photovoltaic energy, wind energy and the like, compared with the traditional alternating current power distribution network, the direct current power distribution network has the advantages of high power supply reliability, low line loss, low harmonic content, narrow required power transmission corridor, low line cost, convenience in new energy power and energy storage access and the like, has wide application prospects in the fields of realizing distributed energy grid connection, urban power distribution networks, offshore platform power supply and the like, and has attracted extensive attention and research of experts and scholars at home and abroad.

The direct-current power distribution network is one of application occasions of the direct-current power distribution network in the field of medium and low voltage, and the fault handling capacity of the direct-current power distribution network under the condition of bipolar short-circuit fault is an important index for measuring the system performance of the flexible direct-current power distribution network. As one of the approaches for processing the dc fault, the converter (MMC) with the fault self-clearing capability has the advantages of fast response speed, relatively low engineering cost and the like, and is very suitable for a dc distribution network system without the configuration of a dc breaker with lower voltage level and smaller capacity.

Disclosure of Invention

Aiming at the problems, the invention provides a method for rapidly recovering the fault of the flexible direct-current power distribution network of the built-in energy storage converter, which realizes rapid recovery of the fault in a separate excitation mode through coordination and coordination between the converter and the energy storage, aims at rapid recovery of the power distribution network under the condition of bipolar short-circuit fault and uninterruptedly supplies power during fault clearing.

The technical scheme of the invention is as follows: the method comprises the following steps:

1) energy storage built-in converters are respectively arranged at two ends of the flexible direct current distribution network;

2) detecting whether a bipolar short-circuit fault occurs on the direct current side;

3) if the fault occurs, locking the energy storage built-in converter;

4) and carrying out fault recovery by using the built-in energy storage converter.

In a preferred embodiment, the energy storage built-in converter comprises a full-bridge type modular multilevel converter, a double-active full-bridge converter and an energy storage device; the full-bridge type modular multilevel converter comprises an upper bridge arm and a lower bridge arm, wherein the upper bridge arm of each phase consists of N full-bridge submodules and corresponding bridge arm resistors R_aBridge arm inductance L_aAre connected in series;

the lower bridge arm of each phase consists of N full-bridge submodules and corresponding bridge arm resistors R_aBridge arm inductance L_aAre connected in series;

wherein, the output end of the first full-bridge submodule in each phase upper bridge arm and the medium-voltage direct-current bus u_uaThe output end of the last full-bridge submodule in each phase of lower bridge arm is connected with the medium-voltage direct-current bus u_laThe negative electrode of (1) is connected;

and on each phase of bridge arm, an output capacitor of each full-bridge submodule is connected with a double-active full-bridge converter, and 2N double-active full-bridge converters are connected in parallel and then serve as outlets of low-voltage side direct current circuits and are connected with an energy storage device.

Comparing parameters before and after the fault in the step 2);

calculating short-circuit current i before and after fault of direct-current power distribution network_f(t) and submodule capacitor voltage u_c(t) analytical formula:

capacitor voltage u before locking of energy storage built-in converter_cAnd fault loop discharge current i_fThe mathematical expression of (a) is:

in the formula, L_e＝2L_a/3+L₀，C_e＝6C_SM/N，R_e＝2R_aThe/3 + R is an equivalent inductance, an equivalent capacitance and an equivalent resistance of a fault current path before ESBC locking respectively; u shape_d1＝u_c(t₀+)＝u_c(t₀-) is the initial value of the DC voltage at the time of the fault, I_d1＝i_f(t₀+)＝i_f(t₀-) is the initial value of the direct current; τ 2L_e/R_eIs the discharge time constant of the equivalent circuit,

is the angular frequency of the equivalent discharge circuit,

is the initial phase angle of the discharge current;

capacitor voltage u after locking of energy storage built-in converter_cAnd fault loop discharge current i_fThe mathematical expression of (a) is:

wherein L is_e＝2L_a/3+L₀，C_e1＝3C_SM/2N，R_e＝2R_a/3+R₀Respectively an equivalent inductor, an equivalent capacitor and an equivalent resistor of a fault current path after the ESBC is locked; u shape_f＝u_c(t₁+)＝u_c(t₁-) is the initial value of the voltage at latch-up, I_f＝i_f(t₁+)＝i_f(t₁-) is the initial value of the current at latch-up;

is the angular frequency of the equivalent charging circuit,

is the initial phase angle of the charging circuit.

In a preferred embodiment, storeAfter the built-in converter is locked, the energy storage device can directly provide power support for the load, so that uninterrupted power supply of important loads during fault clearing is guaranteed. In another preferred embodiment, after the energy storage built-in converter is locked, the energy storage device performs voltage regulation control on the capacitor of the full-bridge submodule through the double-active full-bridge converter. In another preferred embodiment, after the energy storage built-in converter is locked, the energy storage device directly provides power support for the load, and meanwhile, voltage regulation control is carried out on the capacitor of the full-bridge submodule through the double-active full-bridge converter. When the energy storage device performs voltage regulation control on the capacitor of the full-bridge submodule through the double-active full-bridge converter, the voltage of the full-bridge submodule is lower than the rated voltage U in the first stage_CrefThe voltage of the full-bridge submodule is higher than the rated voltage U in the second stage_CrefDischarging the submodules of (a);

and in the sub-module discharging process of the second stage, the energy storage device absorbs electric energy.

The invention has the following beneficial effects: aiming at the rapid recovery of the power distribution network under the bipolar short-circuit fault, the topological structure based on the energy storage built-in converter is adopted, and during the locking period of the converter, the energy storage provides power for important loads on one hand so as to ensure the uninterrupted power supply of the loads during the fault clearing period; on the other hand, the sub-module capacitor voltage during the locking period is adjusted to reach a rated value during normal operation before unlocking, voltage fluctuation when the converter is unlocked again is reduced, time is shortened for restarting the converter, and the system coordination control strategy can better guarantee improvement of the reliability of the whole power distribution network system.

Drawings

Figure 1 is a diagram of an ESBC topology structure of the energy storage built-in converter of the invention,

figure 2 is a schematic diagram of the fault fast restart strategy based on energy storage built-in type of the invention,

FIG. 3 is a schematic diagram of the fault handling and recovery timing sequence of the two-terminal DC distribution network based on built-in energy storage of the invention,

figure 4 is a fault current path and its equivalent circuit before ESBC lockout,

figure 5 is a fault current path after ESBC lockout and its equivalent circuit,

figure 6 is a diagram of a dc distribution network topology,

figure 7 is a schematic diagram of a prior art dc distribution network fault handling and recovery sequence,

FIG. 8 (a-e) is the fast recovery strategy simulation waveform data after a DC fault according to the present invention,

figure 8(a) is based on the MMC1 dc bus voltage after energy storage,

in fig. 8(b) is the a-phase upper bridge arm sub-module voltage when not based on the energy storage function,

in fig. 8(c) is the voltage of the bridge arm submodule on the a phase based on the energy storage function,

in FIG. 8, (d) is the low-side DC voltage when not based on the energy storage function,

in FIG. 8, (e) is based on the low-side DC voltage during the energy storage,

FIG. 9 (a-i) is prior art post DC fault coordination control strategy simulation waveform data,

figure 9(a) is the MMC1 dc bus voltage,

figure 9 (b) is the MMC2 dc bus voltage,

figure 9(c) is MMC1 dc bus current,

figure 9 (d) is MMC2 dc bus current,

in figure 9(e) is MMC1 output active power,

in figure 9 (f) is MMC2 output active power,

in figure 9 (g) is the MMC1 net side ac system unity current,

FIG. 9 (h) shows the second net-side AC system current of MMC 2;

in figure 9(i) is the MMC1 lower leg current,

in the figure 7, MMC1 is a converter station I, MMC2 is a converter station II, S1 to S6 are fast isolating switches, and gs1 is a tie switch.

Detailed Description

In order to make the technical solutions in the present application better understood by those skilled in the art, the technical solutions of the present invention will be described in detail in the following with reference to the accompanying drawings and specific embodiments.

As shown in fig. 1 to 5, the method for recovering from a fault of a flexible dc power distribution network provided by the present invention includes the following steps:

1) energy storage built-in converters (namely ESBC) are respectively arranged at two ends of the flexible direct current distribution network;

2) detecting whether a bipolar short-circuit fault occurs on the direct current side;

3) if the fault occurs, locking the energy storage built-in converter;

4) and carrying out fault recovery by using the built-in energy storage converter.

In a preferred embodiment, the step 2) of detecting whether the bipolar short-circuit fault occurs on the direct current side includes comparing parameters before and after the fault, and if the current of the ESBC bridge arm exceeds 2 times of the rated operation, the ESBC is locked.

In a preferred embodiment, after the energy storage built-in converter is locked in the step 4), the energy storage device can directly provide power support for the load, so that uninterrupted power supply of important loads during fault clearing is guaranteed. In another preferred embodiment, after the energy storage built-in converter is locked, the energy storage device performs voltage regulation control on the capacitor of the full-bridge submodule through the double-active full-bridge converter. In another preferred embodiment, after the energy storage built-in converter is locked, the energy storage device directly provides power support for the load, and meanwhile, voltage regulation control is carried out on the capacitor of the full-bridge submodule through the double-active full-bridge converter.

In the process that the energy storage device charges the capacitors of the full-bridge sub-modules through the double-active full-bridge converter, a staged charging strategy is adopted, and in the first stage, the voltage of the full-bridge sub-modules is lower than the rated voltage U_CrefThe voltage of the full-bridge submodule is higher than the rated voltage U in the second stage_CrefDischarging the submodules of (a);

in the sub-module discharging process of the second stage, the energy storage device absorbs electric energy to supplement the power consumed in the first stage charging process.

The invention adopts the energy storage built-in converter as the grid-connected converter between the direct current distribution network and the alternating current power grid, and the energy storage device can directly provide power support for the load during the locking period of the converter, thereby ensuring the uninterrupted power supply of the important load during the fault period.

Fig. 6 is a topology structure diagram of a typical two-terminal dc distribution network not based on energy storage, wherein the main bodies of two converter stations MMC1 and MMC2 connected to the ac large grid in the dc distribution network system employ converters with fault self-clearing capability, such as a full-bridge sub-module type converter, a double-clamping sub-module type converter, and the like. In practical engineering application, when a two-end power distribution network operates in a closed loop mode, if a fault occurs in a direct-current power distribution network without a direct-current breaker, converter stations at two ends are shut down due to overcurrent, and reliability is reduced on the contrary.

In normal operation, a converter station I MMC1 and a converter station II MMC2 both adopt a constant direct-current voltage control mode, and the rapid isolating switches (s 1-s 6) are in a closed state when the system operates normally; a gS1 which is a communication switch between the two converter stations of the MMC1 and the MMC2 is in an open state when the system normally operates; and loads or distributed power supplies are connected between the adjacent quick isolating switches (S1-S6).

Fig. 1 is a topological structure diagram of an energy storage built-in converter (i.e., ESBC) of the present invention, and the ESBC is mainly a full-bridge modular multilevel converter (i.e., FBMMC) having a fault self-clearing capability.

The FBMMC comprises an upper bridge arm and a lower bridge arm; each phase upper bridge arm is composed of N full-bridge submodules (FBSM) and corresponding bridge arm resistors R_aBridge arm inductance L_aAre connected in series; each phase lower bridge arm is also composed of N full-bridge submodules (FBSM) and corresponding bridge arm resistors R_aBridge arm inductance L_aAre connected in series; the output of the first full bridge submodule (i.e. FBSM) of each phase upper bridge arm is connected to the medium voltage dc bus u_uaIs connected with the output end of the last full-bridge submodule (namely, FBSM) of each phase lower bridge arm and the medium-voltage direct-current bus u_laThe negative electrode of (1) is connected;

on each phase of bridge arm (namely an upper bridge arm and a lower bridge arm), an output capacitor of each full-bridge submodule (namely the FBSM) is connected with a double-active full-bridge converter (namely DAB), and 2N groups of double-active full-bridge DABs are output in parallel and then serve as outlets of low-voltage side direct current circuits and are connected with an energy storage device.

And 2) when the direct-current side bipolar short circuit fault occurs and the reference quantity such as bridge arm current and the like exceeds a set value, a system locking instruction is sent out through certain time delay to lock the ESBC. Capacitor voltage u before ESBC lockout_cAnd fault loop discharge current i_fThe mathematical expression of (a) is:

in the formula, L_e＝2L_a/3+L₀，C_e＝6C_SM/N，R_e＝2R_aThe/3 + R is an equivalent inductance, an equivalent capacitance and an equivalent resistance of a fault current path before ESBC locking respectively; u shape_d1＝u_c(t₀+)＝u_c(t₀-) is the initial value of the DC voltage at the time of the fault, I_d1＝i_f(t₀+)＝i_f(t₀-) is the initial value of the direct current; τ 2L_e/R_eIs the discharge time constant of the equivalent circuit,

is the angular frequency of the equivalent discharge circuit,is the initial phase angle of the discharge current.

The discharging of the sub-module capacitor before locking is an oscillating discharging process, after a direct-current side bipolar short-circuit fault occurs, the direct-current side fault current is rapidly increased in a short time, the sub-module capacitor is rapidly discharged to cause the bridge arm current to be rapidly increased in a short time, and the bridge arm current exceeds the overcurrent protection threshold value, so that the energy storage built-in converter ESBC is locked. Capacitor voltage u after ESBC locking_cAnd fault loop discharge current i_fThe mathematical expression of (a) is:

wherein L is_e＝2L_a/3+L₀，C_e1＝3C_SM/2N，R_e＝2R_a/3+R₀Respectively an equivalent inductor, an equivalent capacitor and an equivalent resistor of a fault current path after the ESBC is locked; u shape_f＝u_c(t₁+)＝u_c(t₁-) is the initial value of the voltage at latch-up, I_f＝i_f(t₁+)＝i_f(t₁-) is the initial value of the current at latch-up;is the angular frequency of the equivalent charging circuit,is the initial phase angle of the charging circuit.

In practical engineering, the fault decay time is very short, so the decay term e can be ignored^-t/τThen, the maximum value of the capacitor voltage of the final equivalent circuit is obtained as follows:

after the converter is locked, the fault current charges the sub-module capacitor through the fault loop, the fault current rapidly drops to zero and keeps zero unchanged until the converter is restarted and unlocked. During converter locking, the energy storage built-in converter ESBC can effectively isolate faults and realize direct-current side fault clearing through a reverse charging loop. The maximum value of the sub-module voltage during the locking is related to the capacitance voltage value and the fault current value when the converter is locked, and is related to the inductance parameter and the sub-module capacitance parameter of the fault loop. However, due to the difference of the discharging conditions of each bridge arm and the difference of the parameters of the sub-modules during locking, the difference exists between the capacitor voltages of the sub-modules after the converter is locked, and current impact can be brought when the converter is unlocked again, so that the bridge arms bear the risk of secondary overcurrent.

Fig. 7 is a schematic diagram of a fault handling and recovery timing sequence of a two-terminal dc power distribution network when a fast recovery strategy based on energy storage is not applied in the background art of the present invention. Taking the bipolar short-circuit fault occurring at the position of the medium-voltage direct-current bus F1 shown in fig. 6 as an example, the processing procedure after the fault occurs is as follows:

(1) initial stage of failure (t)_f1-t_f2): at the initial stage of fault occurrence, the electric quantity indexes of all phases do not exceed the set value, the system still operates normally, and fault current is fed into the alternating current system through the current converter at a fault point. The bridge arm current is rapidly increased but does not reach the threshold value of bridge arm overcurrent protection (generally 2 times of the rated current of the bridge arm), and the converter is not locked;

(2) fault handling (t)_f2-t_f3): when the fault current exceeds a threshold value when the system normally operates, the control protection system sends a locking instruction to the converter after a certain time delay after detecting the fault, and a converter station-MMC 1 reversely accesses an FBSM capacitor in the ESBC into a fault loop due to overcurrent locking to quickly block the fault current; the protection system rapidly detects and positions the fault point according to the fault alarm information, controls the protection system to control the quick disconnecting switches S2 and S3 at two ends of the fault point F1 to be disconnected, and isolates the fault point.

(3) Fault recovery (t)_f3-t_f4): after the fault point is isolated, the fault point upstream fast isolation switch S1 and S2 area and the downstream fast isolation switch S3 and S4 area lose power. Due to the isolation of the fault point, the fast disconnectors S3 and S4 area cannot be continuously supplied with power by the converter station-MMC 1, and the power supply of the fast disconnectors S1 and S2 area needs to be restored by re-unlocking the converter station-MMC 1; and (3) closing the contact switch gs1, and recovering the load power supply between the quick isolating switches S3 and S4 by the converter station two MMC2 through the contact switch gs1, so that the system can recover the load of the non-fault area which loses power as far as possible, and the influence of the fault on the power distribution network is reduced.

(4) Steady state operation (t)_f4-): and the system recovers normal operation, the isolated fault area is repaired, and the initial network architecture of the direct current power distribution network system is recovered as far as possible.

During fault recovery, the converter station-MMC 1 needs to be re-unlocked to restore power to the power loss zone. In the locking process, the capacitor voltage of the full-bridge sub-module FBSM is unbalanced and deviates from a rated value in normal operation, so that the bus voltage fluctuates in the restarting process of the converter station, and the restarting time is long.

Fig. 2 is a schematic diagram of a fault fast restart strategy based on energy storage according to the present invention. As can be seen from the ESBC topology described in fig. 1, the energy storage device is configured between the parallel dual active full bridge converter (bidirectional DC/DC converter) and the low voltage load, so that, in operation, by using the intermittent operation characteristic of the energy storage device, the energy storage device can directly provide power support to the load during the converter lockout period without passing through the converter station-MMC 1, thereby ensuring uninterrupted power supply of the important load during the fault clearing period;

meanwhile, after the fault is isolated, the energy storage device carries out voltage regulation control on the capacitor of the FBSM through DAB, so that the voltage of the capacitor of the sub-module is quickly recovered to a rated value U in stable operation_Cref. Thereby enabling fast recovery when the system reboots.

Fig. 3 is a schematic diagram of a fault processing and recovery timing sequence of a two-terminal type direct-current power distribution network, in the fault clearing time after ESBC locking, energy storage is controlled to perform balance control on sub-module capacitor voltage of a full-bridge sub-module FBSM, fault isolation can be performed simultaneously with a sub-module capacitor voltage rebalancing process, after an unlocking instruction is received, a converter can quickly reach a stable state by establishing direct-current voltage, and the restart time is shortened by adjusting the restart timing sequence, so that the purpose of quickly recovering the fault is achieved.

In the initial stage of the fault, the system still works in a normal converter switching mode before correctly detecting the fault, the number of the sub-modules put into each phase of bridge arm of the ESBC is N, and because the switching frequency of the IGBT is very high, the capacitors of the upper bridge arm and the lower bridge arm are equivalently connected in parallel, and the value of the capacitors is 2 CSM/N. The fault current path is the RLC second order discharge loop as shown in figure 4. L0 and R0 are the equivalent inductance and resistance on the dc side before ESBC blocking.

After the ESBC is locked, the bridge arm inductor and the direct current reactor follow current, and the capacitor of the sub-module is charged through the follow current diode, so that the voltage of the capacitor of the sub-module is increased. The sub-module capacitors of the upper bridge arm and the lower bridge arm of each phase are equivalent to a series structure, and each phase of equivalent capacitorIs C_SMand/2N. The sub-module capacitors of all bridge arms form back electromotive force and are connected in series in the circuit, and an equivalent fault current path after ESBC locking and a simplified circuit thereof are shown in figure 5.

After the fault is isolated, if the sub-modules are put in simultaneously, due to the difference between the capacitor voltage values of the sub-modules, instantaneous current impact is formed between bridge arms when the sub-modules are conducted simultaneously, so that the capacitors of the sub-modules and the IGBT devices participating in the work bear overcurrent. Therefore, under the strategy of staged charging provided by the invention, after the system is locked, the capacitor voltages of all the sub-modules are detected, and the minimum voltage value is recorded as U_CminAnd the maximum voltage is denoted as U_CmaxRated voltage of U_Cref. The voltage U of the sub-module capacitor_CCan be expressed as:

wherein k is₁(t) and k₂(t) is the buffer function during charging.

k_C1And k_C2Ratio of sub-module capacitor voltage to nominal value, M, after blocking and before starting charging of the stored energy_sRepresenting the charging function of the stored energy charge, t_C1Starting the charging time for energy storage, t_C2Is at a voltage value of [ U_Cmin，U_Cref]Submodule end of charge time, t, of interval_C3Is a voltage value of (U)_Cref，U_Cmax]And the discharge end time of the sub-modules in the interval, namely the rebalancing end time of the whole sub-module.

t＝t_C1At the moment, the energy storage starts to charge, and the voltage of the sub-module is lower than U_CrefCharging the sub-modules of (1), namely charging in a first stage; t is t_C2First order of timeThe segment charging is completed, and the voltage of the sub-module is higher than U_CrefDischarging the sub-modules, namely charging in the second stage; t is t_C3And then, the charging in the second stage is finished, and the sub-module capacitor voltages of all bridge arms are stabilized at the rated value U_CrefNearby. In the sub-module discharging process of the second stage, the energy storage device absorbs electric energy to supplement the power consumed in the first stage charging process.

Fig. 9 (a-i) is response waveforms of recovery of a power distribution grid system after a dc fault occurs when dc fault post-coordination control strategy simulation waveform data not based on energy storage is not added to a fault rapid recovery strategy based on energy storage in the prior art. Initially, the direct current distribution network system is in a stable operation state, the active power output by the first converter station MMC1 is 4.8MW, and the active power output by the second converter station MMC2 is 2MW, as shown in fig. 9 (e); when t is 0.8s, if a bipolar short-circuit fault occurs as at F1 in fig. 7, the short-circuit resistance is 0.01 Ω. When a bipolar short-circuit fault occurs, the outlet direct-current bus voltage of the converter station-MMC 1 drops to zero rapidly from 20kV in normal operation, as shown in figure 9 (a); when the direct current bus current rapidly rises to 2kA within about 2.5ms, the bridge arm current is sharply increased to about 2.7 times of that in normal operation, as shown in fig. 9(c) and 9 (i); when the system detects that the bridge arm current is greater than 0.6kA, a locking instruction is sent out after the delay of 130us, and the converter station-MMC 1 is locked quickly; after locking, the fault current of the direct current side is rapidly reduced and is reduced to zero after about 1.8 ms; at the moment, the quick isolating switches S2 and S3 are opened, and the fault point is quickly isolated; after fault isolation, the converter station-MMC 1 is unlocked again, direct-current voltage at the outlet of the MMC1 is established, power supply to a power supply area between the quick isolating switches s1 and s2 is recovered, and active power output of the converter station-MMC 1 is adjusted to be 2 MW; and closing a tie switch gs1 between the first converter station MMC1 and the second converter station MMC2 to restore the power supply to the power supply area between the quick isolating switch S3 and the tie switch gs1, and adjusting the output power of the second converter station MMC2 to be 3 MW. For the converter station one MMC1, after the fault is cleared, the control protection system issues an unlocking instruction, the converter station one MMC1 is unlocked again, the voltage of the direct current bus is established, and meanwhile, the power supply to the low-voltage load is recovered. Throughout the process (as shown in fig. 9 a-i), there is about 75ms from the time the system issues an unlock command to the time the system re-reaches steady operation.

Fig. 8 (a-e) are simulation waveforms using the energy storage based fast failure recovery strategy proposed by the present invention. When the operation is performed until t is 0.8s, if a dc-side bipolar short-circuit fault occurs at F1 in fig. 6, the short-circuit resistance is 0.01 Ω. When the energy storage based fault rapid recovery strategy is not adopted, after the converter station MMC1 is locked, the sub-module capacitor voltage rises about 165V compared with that in normal operation, as shown in FIG. 8 (b); during the lockout period, the low-side dc power is in shortage and the dc voltage drops by about 50V, as shown in fig. 8 (d); when a fault quick recovery strategy based on energy storage is added, after the system is locked, the direct current bus current and the bridge arm current are quickly attenuated to zero, and at the moment, the energy storage carries out voltage balance control on the module capacitor of the FBSM through DAB so that the voltage balance control is stabilized at 2.5kV, as shown in fig. 8 (c); the stored energy supplies power to the low-voltage side load at the same time, so that the voltage of the low-voltage side load is stabilized at 750V, as shown in fig. 8 (e). After fault isolation, the system issues an unlocking instruction, the converter station MMC1 is unlocked again, and the system reaches a steady state and recovers normal operation in a short time, as shown in fig. 8 (a). The time required from the issuance of the system unlock command to the resumption of stable operation of the system is only about 0.4 ms.

The above results show that by adopting the energy storage based fault restart strategy, the energy storage device can stabilize the capacitance value of the FBSM near the rated value within several milliseconds after DAB, effectively stabilize the fluctuation of the capacitance and voltage, and have short regulation time and reliable modulation mode. And the uninterrupted power supply of important loads and the full-bridge submodule capacitor voltage rebalancing of the ESBC can be realized after the fault occurs. Thereby achieving rapid recovery from the fault in the power distribution grid system and shortening the system re-settling time by about 74.6 ms.

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.