CN113036737A - Flexible current limiter capable of being connected in series on direct current circuit and control method thereof - Google Patents

Flexible current limiter capable of being connected in series on direct current circuit and control method thereof Download PDF

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CN113036737A
CN113036737A CN202110317326.7A CN202110317326A CN113036737A CN 113036737 A CN113036737 A CN 113036737A CN 202110317326 A CN202110317326 A CN 202110317326A CN 113036737 A CN113036737 A CN 113036737A
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current
fault
diode
current limiter
voltage
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CN113036737B (en
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郑峰
林佳壕
张锦松
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Fuzhou University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
    • H02H9/02—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess current
    • H02H9/021—Current limitation using saturable reactors
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

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Abstract

The invention relates to a flexible current limiter which can be connected in series on a direct current circuit, which is characterized by comprising a bridge circuit, a current limiter and a current limiter voltage-controlled rectifying module; the bridge circuit comprises a first diode D1A second diode D2A third diode D3A fourth diode D4Current-limiting inductor L, controllable DC bias power supply Ib(ii) a The first diode D1Cathode and controllable DC bias power supply IbNegative pole and second inductance D2The cathodes are respectively connected; the first diode D1Anode and third diode D3Connecting a cathode; the third diode D3Anode and one end of current-limiting inductor L and fourth diode D4The anodes are respectively connected; the fourth diode D4Cathode and second diode D2Connecting an anode; the current-limiting inductor L and the controllable DC bias power supply IbPositive pole is connectedAnd (4) connecting and disconnecting. The invention can simultaneously process single-pole grounding fault and interelectrode short-circuit fault by lifting fault point voltage, flexibly control the voltage of the current-limiting inductor and effectively restrain fault current.

Description

Flexible current limiter capable of being connected in series on direct current circuit and control method thereof
Technical Field
The invention relates to the technical field of power electronics, in particular to a novel flexible current limiter based on a direct-current distribution network and a control method.
Background
This along with the development of power electronics technology, the control of direct current distribution network is more nimble, and the electric energy quality obtains promoting. In application scenarios such as Distributed Generation (DGs) and Electric Vehicles (EV), the efficiency of Electric energy transmission is improved by a direct current distribution network. However, due to the characteristics of low inertia and low impedance of the direct current system, the system has large damage, rapid development and wide influence range. Especially, due to the use of a large number of Voltage Source Converters (VSC), the fault current amplitude of the inter-electrode short circuit is large and changes quickly, which brings a great challenge to system protection. Therefore, protection of dc distribution networks has become a research hotspot in recent years.
When a short-circuit fault occurs in a direct-current line, both the outlet capacitor of the AC/DC converter station and an alternating-current system can discharge to a fault point. At present, effective protection of the commutation station is one of the main research directions for fault protection of the dc system. However, if an Insulated Gate Bipolar Transistor (IGBT) inside the converter station is locked due to self-protection, the ac system will directly discharge to the fault point, the value of the fault current will be maintained at a high level, and it is difficult to turn off because there is no zero crossing point. A Hybrid DC Circuit Breaker (HDCCB) is capable of breaking a fault current of an inter-electrode short Circuit. But the fault response time of the circuit breaker and the application cost of the circuit breaker need to be considered. Traditional mixed type direct current circuit breaker passes through mechanical switch and cuts off the trouble circuit, but needs certain time to promote the inside dielectric strength of circuit breaker after the trouble takes place, can't satisfy the quick-action nature requirement of protection. Therefore, the research on the fault current limiter of the direct current distribution network has important significance.
In the last 10 years, domestic and foreign scholars have made relevant research on dc current limiters, which can be divided into two main categories: superconducting current limiters and solid-state current limiters. The prior documents respectively describe the technical principles of resistive, saturated core, magnetic shielding and bridge superconducting current limiters. The superconducting current limiter has the advantages of small on-state loss, simple structure and good current limiting effect, however, the existing superconducting material can maintain the superconducting state in a relatively ideal environment, the operation cost is high, and a circulating liquid nitrogen cooling system with large capacity needs to be equipped after the fault is removed to ensure that the superconducting current limiter is restored to the state before action; the self-bypass current limiter switches the fault current loop to the current-limiting branch circuit through the load change-over switch and the ultra-fast mechanical switch, and the measure of realizing the repeated use of the current limiter is considered; the novel capacitance phase-change hybrid direct current limiter realizes the on-off of the corresponding bridge arm through capacitance charge and discharge, and saves the investment of power electronic devices. It can be seen that the solid state current limiter is composed primarily of power electronics, with high on-state losses. Meanwhile, the number of power electronic devices is increased, the fault rate of the current limiter is increased, and the investment cost is also increased.
Disclosure of Invention
In view of the above, an object of the present invention is to provide a flexible current limiter suitable for a dc power distribution network, which can handle a single-pole ground fault and an inter-pole short circuit fault by raising a fault point voltage, and can flexibly control a voltage of a current-limiting inductor to effectively suppress a fault current.
In order to achieve the purpose, the invention adopts the following technical scheme:
a flexible current limiter which can be connected in series on a direct current circuit comprises a bridge circuit, a current limiter and a current limiter voltage-controlled rectification module, wherein the current limiter is positioned on a bridge arm of the bridge circuit, and the bridge circuit is connected in parallel with the voltage-controlled rectification module; the bridge circuit comprises a first diode D1A second diode D2A third diode D3A fourth diode D4Current-limiting inductor L, controllable DC bias power supply Ib(ii) a The first diode D1Cathode and controllable DC bias power supply IbNegative pole and second inductance D2The cathodes are respectively connected; the first diode D1Anode and third diode D3Connecting a cathode; the third diode D3Anode and one end of current-limiting inductor L and fourth diodeD4The anodes are respectively connected; the fourth diode D4Cathode and second diode D2Connecting an anode; the current-limiting inductor L and the controllable DC bias power supply IbThe positive electrode is connected.
Furthermore, the flexible current limiter introduces a closed-loop iron core with a controlled source to eliminate saturation of a current-limiting inductor, wherein the primary side of the iron core is the current limiter, and the secondary side of the iron core is the controlled source.
Furthermore, when the system runs normally, the bridge circuit bypasses the current limiter, and the running of the system is not influenced;
during a fault period, the bridge circuit is switched, the current limiter is connected into a system and provides variable clamping voltage for a fault loop, and therefore voltage drop of an outlet capacitor of the AC/DC converter station is controlled, and overlarge fault current is restrained.
Further, the flexible current limiter controls the clamping voltage of the current limiting inductor to be a stable value through the linear current provided by the rectifier, and the voltage Ul of the current limiting inductor is used as a control target of the flexible current limiter converter station;
the expression for the dual loop control is as follows:
Figure DEST_PATH_IMAGE002
(1)；
in the formula (1), the reaction mixture is,Kis the gain of the amplification and is,K vf is the coefficient of the feedback that is,u set andi q2 the input quantity and the disturbance quantity of the control system; wherein
Figure DEST_PATH_IMAGE004
(2)。
Further, the fault determination specifically includes: according to the protection criterion, if the DC side voltage satisfiesU dc ≤0.8U dcn Then the fault of the dc system may be defined as an inter-electrode short fault;
if the voltage satisfies the following formula, the fault can be defined as a single pole ground fault
Figure DEST_PATH_IMAGE006
(3)；
The input of the flexible current limiter control system is set to different values under different fault conditions
Figure DEST_PATH_IMAGE008
(4)；
Wherein, UdcnIs the rated voltage of the direct current system,U + dc 、U - dc the voltages of the positive and negative electrodes on the direct current side respectively,U + dcn andU - dcn the rated values of the positive and negative voltages are respectively.m 1 ，m 2 Andm 3 is a control coefficient input into the system andm i =0～1(i=1，2，3)。
further, during the fault, the current limiter aims at different fault positions by changing the control coefficientm i The clamping voltage provided by the current limiter is changed, so that the fault voltage of a fault loop is changed, and finally the flexible control of the fault current is realized.
Compared with the prior art, the invention has the following beneficial effects:
1. the control method of the novel flexible current limiter provided by the invention comprises the three conditions of normal operation of a line where the novel flexible current limiter is positioned, detection of an interelectrode short-circuit fault or a one-way grounding fault, and current-limiting inductance demagnetization after the fault lasts for a period of time, so that the novel flexible current limiter realizes control on the novel flexible current limiter, improves the stability of a direct-current power distribution network during the fault, and simplifies the topological structure of the traditional current limiter;
2. the flexible current limiter can be used for processing single-pole grounding faults and interelectrode short-circuit faults at the same time, and has wider application range;
3. on the basis of effective current limiting, the fault voltage of the direct current system can be flexibly adjusted by the flexible current limiter. If the inter-electrode short circuit fault occurs, the fault current of the system can be adjusted according to the current tolerance value of the related device, so that the current limiting controllability is greatly improved;
4. the invention prolongs the effective acting time of the current-limiting inductor due to the action of the closed-loop iron core. The stability of fault current limitation before the fault disappears is ensured;
5. during the action of the current limiter, the energy drawn by the current limiting inductor from the alternating current system can be returned to the power grid through the inversion loop of the flexible current limiter, and the energy loss is reduced.
Drawings
FIG. 1 is a diagram of a novel flexible current limiter bridge circuit topology according to an embodiment of the present invention;
FIG. 2 is an overall block diagram of the novel flexible flow restrictor in the system in one embodiment of the present invention;
FIG. 3 is a diagram illustrating the determination of current limiting after a fault is detected in an embodiment of the present invention;
FIG. 4 is a diagram illustrating the degaussing of the current limiting inductor during a period of time when a fault persists in an embodiment of the present invention;
FIG. 5 is a flow chart of the operation of the novel flexible flow restrictor in an embodiment of the present invention;
fig. 6 is a diagram of the effect of the novel flexible current limiter on two faults in one embodiment of the invention.
Detailed Description
The invention is further explained below with reference to the drawings and the embodiments.
Referring to fig. 1, the invention provides a novel flexible current limiter based on a direct current distribution network, which comprises a bridge circuit, a current limiter and a current limiter voltage-controlled rectification module, wherein the current limiter is positioned on a bridge arm of the bridge circuit, and the bridge circuit is connected in parallel with the voltage-controlled rectification module; the bridge circuit comprises a first diode D1A second diode D2A third diode D3A fourth diode D4Current-limiting inductor L, controllable DC bias power supply Ib(ii) a The first diode D1Cathode and controllable DC bias power supply IbNegative pole and second inductance D2Cathode electrodeAre respectively connected; the first diode D1Anode and third diode D3Connecting a cathode; the third diode D3Anode and one end of current-limiting inductor L and fourth diode D4The anodes are respectively connected; the fourth diode D4Cathode and second diode D2Connecting an anode; the current-limiting inductor L and the controllable DC bias power supply IbThe positive electrode is connected.
The operation mode of the system in normal operation and two fault states;
when the system is operating normally, Idc>0, the middle branch is connected in series with a forward DC bias power supply to enable Ib>Idc. Flow-through diode D1And D4Has a current of (I)b+Idc)/2>0, flow through diode D2And D3Has a current of (I)b-Idc)/2>0, so that 4 diodes are all conducting in the forward direction, the current limiter is bypassed, the current on the dc line does not flow through the middle branch, and the current in this branch is small. (ii) a
Wherein when the DC power distribution network has an interelectrode short-circuit fault, the line current IdcIncrease of Ib<IdcAt this time, the diode D2And D3The current at is (I)b-Idc)/2<0, so that there is only diode D at this time1And D4When the direct current side fault current is conducted, the amplitude of the direct current side fault current is suppressed by flowing through the current-limiting inductor of the middle branch circuit. As shown in fig. 5, a dc bias power supply is connected in parallel with the fully-controlled transistor. In case of monopolar earth fault, G1The signal controls the transistor to be conducted, the direct current power supply is short-circuited, and only D is1And D4The current limiting inductor is connected with a direct current circuit in a forward conduction mode;
as shown in fig. 2, the present invention provides a control method suitable for a novel flexible current limiter, which comprises the following specific processes:
the clamping voltage of the current-limiting inductor is controlled at a stable value by the linear current provided by the rectifier. Therefore, the voltage U of the current-limiting inductorlIs taken as a control target for the flexible current limiter converter station. The expression for the dual loop control is as follows:
Figure 990811DEST_PATH_IMAGE002
(1)；
in the formula (1), the reaction mixture is,Kis the gain of the amplification and is,K vf is the coefficient of the feedback that is,u set andi q2 the input quantity and the disturbance quantity of the control system; wherein
Figure 970268DEST_PATH_IMAGE004
(2)。
According to the protection criterion, if the voltage on the direct current side meets Udc≤0.8UdcnA fault of the dc system may be defined as an inter-electrode short fault. If the voltage satisfies the following formula, the fault can be defined as a single pole ground fault
Figure DEST_PATH_IMAGE009
(3)；
The input of the flexible current limiter control system is set to different values under different fault conditions
Figure DEST_PATH_IMAGE010
(4)；
Wherein, UdcnIs the rated voltage, U, of the DC system+ dc、U- dcVoltages, U, of positive and negative poles of the DC side, respectively+ dcnAnd U- dcnThe rated values of the positive and negative voltages are respectively. m is1，m2And m3Is a control coefficient of the system input and mi=0～1(i=1，2，3)。
When the fault occurs, the control coefficient can be artificially changed in the interval according to different fault positions. In case of short circuit between electrodes, if m1The value is small, the discharge voltage drop of the fault circuit is still large, so the minimum value is selected to be 0.5 to avoid generating excessive fault current. Single pole grounding deviceWhen the fault happens, the main problem of the system is that the voltages of the positive electrode and the negative electrode are unbalanced, and the unbalance degree is equal to m2、m3The magnitude of the values of (A) is inversely related. Because the damage of the single-pole grounding fault is less, m is selected to ensure the stable operation of the system under the fault condition and prolong the time for fault detection2、m3Is 0.6. So control coefficient miIs selected as
Figure DEST_PATH_IMAGE012
(5)；
By varying the control coefficient miThe clamping voltage provided by the current limiter can be flexibly changed, so that the fault voltage of a fault loop is changed, and finally the flexible control of the fault current is realized.
According to the expressions (1) to (2), a control block diagram of the current limiting device can be obtained. The current limiter can realize the switching from the normal operation state to the fault current limiting state, and the influence of the state switching of the current limiter on the system is negligible due to the action of the bridge circuit. The installation position of the current limiter in the power distribution network and the control system are connected in series in a direct current power grid as shown in figure 2, and the voltage of the current limiter is subjected to rectification control through alternating current measurement.
Fig. 2(a) sets control system input values of the positive ground fault and the negative ground fault as m2U+ dcnAnd m3U- dcn. Fig. 2(B) shows a process of determining a single-pole ground fault and an inter-pole short-circuit fault by the system. When the value of the voltage of the positive electrode or the voltage of the negative electrode is lower than a rated value of 0.8 times, the system can be considered to have a single-pole grounding fault, and Uset=Uset1. If the fault condition of the single-pole grounding is not met, Uset=Uset2. Fig. 2(C) includes two states, U, when the dc-side voltage satisfies the inter-electrode short-circuit fault determination conditionset2=m1Udcn. Otherwise, the input of the current limiter control system is 0, namely the voltage of the current limiting inductor is 0, and the system is in a normal operation state.
As shown in fig. 3, the current limiting inductor clampBit voltage UlSet by the input values of the control system. In different cases, the inductor voltages may be set to four values as shown in the figure, respectively. The rectifier provides linearly rising current to keep the clamping voltage stable, if the current-limiting inductor is saturated, the slope of the inductor current cannot be maintained, the inductor voltage changes, and the current-limiting effect is reduced. Measures to eliminate saturation of the current limiting inductance should also be considered during use of the flexible current limiter.
As shown in fig. 4, a closed-loop core may be used to prevent saturation of the current-limiting inductor, N1The winding being a current-limiting inductor, N2The winding is the control side inductance. The saturation of the current-limiting inductor is embodied as the saturation of magnetic flux inside the closed-loop iron core, and the inductor on the secondary side generates reverse magnetic flux in the closed-loop iron core to inhibit the increase of the magnetic flux of the current-limiting inductor, so that the saturation removing effect is achieved. The controlled voltage source is used as a secondary side power supply to flexibly adjust the size of the reverse magnetic flux, and the control quantity is I1And k is a control coefficient.
The fundamental equation of the core can be expressed as
Figure DEST_PATH_IMAGE014
(6)；
Wherein phi is1Is the flux of a single-turn coil, Ψ1As total magnetic flux, e1Is N1The voltage of the winding. When the magnetic flux of the iron core is saturated, the inductance L1Saturation, no linear increase of the inductive current, and unstable voltage control at the set value Uset。
If the magnetic induction intensity and the magnetic conductivity are close to saturation, the reverse magnetic flux of the iron core is increased, and the current-limiting inductor can be ensured to be in an unsaturated state in the fault period through the bias of the controlled voltage source.
As shown in fig. 5, the flexible current limiter can eliminate the imbalance of the voltages of the positive electrode and the negative electrode of the unipolar ground fault, and can also reduce the fault current amplitude of the inter-electrode short-circuit fault. The operation process of the flexible current limiter can be divided into four stages, namely: the method comprises a normal operation stage, a fault current limiting stage, an iron core saturation eliminating stage and a recovery stage.
Stage (1): when the system normally operates, the four diodes of the bridge circuit are all conducted, the current of the direct current line cannot flow through the current limiting branch circuit, and the current flowing through the branch circuit is small. As shown in fig. 5(1), in a normal state, the current limiter does not need to get power to the power grid, the controlled source on the secondary side of the iron core does not act, and the current limiter does not affect the normal operation of the system.
Stage (2): short-circuit faults of a direct-current system can be classified into ground short-circuit faults and inter-electrode short-circuit faults. During fault, only D in bridge circuit1And D4The two diodes are turned on, and the circulation path of the fault current is switched to the current-limiting branch. Taking the positive ground fault as an example, the fault loop is shown as (r) in fig. 5(2), the current-limiting inductor starts to operate at this stage, and does not reach the saturation state, so the controlled voltage source does not operate. The voltage of the current-limiting inductor is UlAnd is provided with Ul=m2U+ dcnThe voltage of the positive electrode will rise to UlMeanwhile, the voltage of the negative electrode is reduced, and the unbalance of the voltages of the positive electrode and the negative electrode during the single-pole ground fault is eliminated. The fault loop of the short circuit between electrodes is shown in the second step of FIG. 5(2), in which the current-limiting inductance voltage U isl=m1UdcnThe voltage can be used as a clamping voltage with variable size in a fault loop, and the discharge voltage of an outlet capacitor of the AC/DC converter station is further reduced.
Stage (3): after the current limiter acts for a period of time, if the fault is not cut off, the effective current limiting time of the current limiting inductor needs to be prolonged by a controlled source on the secondary side of the iron core. The installation position of the controlled current source is shown in fig. 5(3), and the secondary side current generates reverse magnetic flux in the iron core, so that the saturation state of the inductor is eliminated. According to the saturation condition of the current-limiting inductor, the voltage of the controlled source can be adjusted in a self-adaptive mode, the unsaturated state of the current-limiting inductor is maintained, and the reliability of the current limiter is guaranteed.
Stage (4): when the fault disappears or is cleared, the four diodes on the bridge circuit are recovered to be conducted, the current limiter is bypassed again, and the process of recovering the state does not influence the system in the normal state. The energy on the current-limiting inductor is released to the power grid, and the system can be divided into a main loop and an energy release loop, as shown in fig. 5 (4). In the main circuit, the voltage and current of the dc system are restored to nominal values. In the discharging loop, a converter in the flexible current limiter is converted into an inversion state, and inductive energy flows to a power grid through the inverter, so that energy loss is reduced.
Based on the above principle, the flexible current limiter can reduce the discharge voltage of the capacitor by adding a reversed clamping voltage in the fault loop.
As shown in figure 6 of the drawings,
single pole ground fault:
the current limiter is arranged at the outlet of the converter station, and when a unipolar ground fault occurs, the current limiter acts between the anode capacitance and the fault point, as shown in fig. 6 (1). The current limiter can provide a stable clamping voltage, so the cathode voltage can be expressed as
Figure DEST_PATH_IMAGE016
(7)；
The voltage of the negative electrode is reduced along with the increase of the voltage of the positive electrode through the action of the flexible current limiter. Therefore, if the voltage of the current-limiting inductor is set reasonably, the system can isolate a single-pole grounding fault point to normally operate, and the stability of power supply of the direct-current power distribution network is ensured.
Short-circuit fault between electrodes:
as shown in FIG. 6(2), the discharge voltage of the capacitor is reduced by the clamping voltage, which can be expressed as
Figure DEST_PATH_IMAGE018
(8)；
Wherein, U’ dcIs the equivalent discharge voltage of the outlet capacitance. After the discharge voltage of the capacitor is reduced, the fault current will also be limited. Because of Ul=Uset=m1UdcnThe value is obtained by changing m1Is flexibly variable. Therefore, the equivalent discharge voltage of the capacitor can be flexibly changed, and the flexible control of the fault current is realized.
The above description is only a preferred embodiment of the present invention, and all equivalent changes and modifications made in accordance with the claims of the present invention should be covered by the present invention.

Claims (6)

1. A flexible current limiter which can be connected in series on a direct current circuit is characterized by comprising a bridge circuit, a current limiter and a voltage-controlled rectifying module of the current limiter, wherein the current limiter is arranged on a bridge arm of the bridge circuit; the bridge circuit comprises a first diode D1A second diode D2A third diode D3A fourth diode D4Current-limiting inductor L, controllable DC bias power supply Ib(ii) a The first diode D1Cathode and controllable DC bias power supply IbNegative pole and second inductance D2The cathodes are respectively connected; the first diode D1Anode and third diode D3Connecting a cathode; the third diode D3Anode and one end of current-limiting inductor L and fourth diode D4The anodes are respectively connected; the fourth diode D4Cathode and second diode D2Connecting an anode; the current-limiting inductor L and the controllable DC bias power supply IbThe positive electrode is connected.
2. The flexible current limiter of claim 1, wherein the flexible current limiter comprises a closed-loop core with a controlled source for eliminating saturation of a current-limiting inductor, the core having a primary side as the current limiter and a secondary side as the controlled source.
3. A method for controlling a flexible current limiter which can be connected in series in a DC line according to claim 1 or 2,
when the system runs normally, the bridge circuit bypasses the current limiter, and the running of the system is not influenced;
during a fault period, the bridge circuit is switched, the current limiter is connected into a system and provides variable clamping voltage for a fault loop, and therefore voltage drop of an outlet capacitor of the AC/DC converter station is controlled, and overlarge fault current is restrained.
4. A method for controlling a flexible current limiter which can be connected in series on a dc line according to claim 3, wherein the flexible current limiter controls the clamping voltage of the current limiting inductor to be a stable value by a linear current provided by the rectifier, and the voltage Ul of the current limiting inductor is used as a control target of the converter station of the flexible current limiter;
the expression for the dual loop control is as follows:
Figure DEST_PATH_IMAGE001
(1)；
in the formula (1), the reaction mixture is,Kis the gain of the amplification and is,K vf is the coefficient of the feedback that is,u set andi q2 the input quantity and the disturbance quantity of the control system; wherein
Figure 382000DEST_PATH_IMAGE002
(2)。
5. The method according to claim 3, wherein the fault determination is specifically as follows: according to the protection criterion, if the DC side voltage satisfiesU dc ≤0.8U dcn Then the fault of the dc system may be defined as an inter-electrode short fault;
if the voltage satisfies the following formula, the fault can be defined as a single pole ground fault
Figure DEST_PATH_IMAGE003
(3)；
The input of the flexible current limiter control system is set to different values under different fault conditions
Figure 236824DEST_PATH_IMAGE004
(4)；
Wherein, UdcnIs the rated voltage of the direct current system,U + dc 、U - dc the voltages of the positive and negative electrodes on the direct current side respectively,U + dcn andU - dcn rated values of positive and negative voltages respectively;m 1 ，m 2 andm 3 is a control coefficient input into the system andm i =0～1(i=1，2，3)。
6. a method as claimed in claim 3, wherein during said fault, the current limiter is controlled by changing the control coefficient for different fault positionsm i The clamping voltage provided by the current limiter is changed, so that the fault voltage of a fault loop is changed, and finally the flexible control of the fault current is realized.
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