CN113036737B - Flexible current limiter capable of being connected in series on direct current circuit and control method thereof - Google Patents

Abstract

The invention relates to a flexible current limiter that can be connected in series on a DC line, which is characterized in that it includes a bridge circuit, a current limiter and a current limiter voltage-controlled rectifier module; the bridge circuit includes a first diode D _1. The _second diode D2, the third diode _D3 , the _fourth diode D4, the current limiting inductor L, the controllable DC bias power supply _Ib ; the cathode of the _first diode D1 It is connected with the negative electrode of the controllable DC bias power supply I _b and the negative electrode of the second inductor D ₂ respectively; the anode of the first diode D ₁ is connected with the negative electrode of the third diode D ₃ ; the third diode D ₃ The anode is connected to one end of the current limiting inductor L and the anode of the _fourth diode D4 respectively; the cathode of the _fourth diode D4 is connected to the anode of the _second diode D2; the current limiting inductor L is connected to the controllable DC The positive pole of the bias power supply I _b is connected away. By raising the voltage at the fault point, the invention simultaneously handles the single-pole grounding fault and the inter-pole short-circuit fault, and can flexibly control the voltage of the current-limiting inductor, thereby effectively suppressing the fault current.

Description

A flexible current limiter that can be connected in series on a DC line and its control method

technical field

The invention relates to the technical field of power electronics, in particular to a novel flexible current limiter and a control method based on a DC distribution network.

Background technique

With the development of power electronic technology, the control of DC distribution network is more flexible, and the power quality is improved. In application scenarios such as Distributed Generations (DGs) and Electric Vehicles (EV), the DC distribution network improves the efficiency of power transmission. However, due to the characteristics of "low inertia and low impedance" in DC systems, system failures have great harm, rapid development and wide influence. In particular, due to the extensive use of Voltage Source Converters (VSCs), the fault current of inter-pole short-circuits has a large amplitude and changes rapidly, which brings huge challenges to system protection. Therefore, DC distribution network protection has become a research hotspot in recent years.

When a short-circuit fault occurs in the DC line, the outlet capacitor of the AC/DC converter station and the AC system will both discharge to the fault point. At present, the effective protection of converter stations is one of the main research directions of DC system fault protection. However, if the fully-controlled transistor (Insulated Gate Bipolar Transistor, IGBT) inside the converter station is blocked due to self-protection, the AC system will directly discharge to the fault point, and the value of the fault current will be maintained at a high level, and because there is no overcurrent Zero point and difficult to break. Hybrid DC Circuit Breaker (HDCCB) can break the fault current of short circuit between poles. However, it is necessary to take into account the fault response time of the circuit breaker and the application cost of the circuit breaker. The traditional hybrid DC circuit breaker disconnects the fault line through a mechanical switch, but it takes a certain amount of time to improve the insulation strength inside the circuit breaker after a fault occurs, which cannot meet the quick-action requirement of protection. Therefore, the research of fault current limiter in DC distribution network is of great significance.

In the past 10 years, domestic and foreign scholars have carried out relevant research on DC current limiters, which can be divided into two categories: superconducting current limiters and solid-state current limiters. The existing literature introduces the technical principles of resistance type, saturated core type, magnetic shielding and bridge type superconducting current limiter respectively. Among them, the superconducting current limiter has 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, and the operating cost is high. After the fault is removed, it needs to be equipped with a capacity The large circulating liquid nitrogen cooling system ensures that the superconducting current limiter returns to the state before the action; the self-bypass current limiter switches the fault current loop to the current limiting branch through the load transfer switch and the ultra-fast mechanical switch, and considers The measures to realize the repeated use of the current limiter; the new type of capacitor commutation hybrid DC current limiter realizes the on and off of the corresponding bridge arm through the charging and discharging of the capacitor, which saves the investment of power electronic devices. It can be seen that the solid-state current limiter is mainly composed of power electronic devices, and its on-state loss is relatively high. At the same time, the increase in the number of power electronic devices increases the failure rate of the current limiter, and also increases the investment cost.

SUMMARY OF THE INVENTION

In view of this, the purpose of the present invention is to provide a flexible current limiter suitable for the DC distribution network, which can simultaneously handle the single-pole grounding fault and the inter-pole short-circuit fault by raising the fault point voltage, and can flexibly control the current-limiting inductance. voltage, effectively suppressing the fault current.

To achieve the above object, the present invention adopts the following technical solutions:

A flexible current limiter that can be connected in series on a DC line, including a bridge circuit, a current limiter and a current limiter voltage-controlled rectifier module, the current limiter is located on the bridge arm of the bridge circuit, and the bridge circuit is connected in parallel with the voltage control The rectifier modules are connected in parallel; the bridge circuit includes a first diode D ₁ , a second diode D ₂ , a third diode D ₃ , a fourth diode D ₄ , a current limiting inductor L, and a controllable DC Bias power supply _Ib ; the cathode of the _first diode D1 is connected to the cathode of the controllable DC bias power supply _Ib and the cathode of the _second inductor D2 respectively; the anode of the _first diode D1 is connected to the third and second The cathode of the diode _D3 is connected to the cathode; the anode of the third diode _D3 is connected to one end of the current limiting inductor L and the anode of the _fourth diode D4 respectively; the cathode of the _fourth diode D4 is connected to the second diode _The anode of the tube D2 is connected; the current limiting inductor L is connected to the anode of the controllable DC bias power supply _Ib .

Further, a closed-loop iron core with a controlled source is introduced into the flexible current limiter to eliminate the saturation of the current-limiting inductance, and the primary side of the iron core is the current limiter, and the secondary side is the controlled source.

Further, during normal operation, the bridge circuit bypasses the current limiter, which does not affect the operation of the system;

During the fault, through the bridge circuit switching, the current limiter is connected to the system and provides a variable clamping voltage to the fault circuit, thereby controlling the voltage drop of the capacitor at the outlet of the AC/DC converter station and suppressing the excessive fault current.

Further, the flexible current limiter uses the linear current provided by the rectifier, the clamping voltage of the current limiting inductor is controlled at a stable value, and the voltage U1 of the current limiting inductor is used as the control target of the flexible current limiter converter station;

The expression for double loop control is as follows:

(1)；

(1);

In formula (1), K is the amplification gain, K _vf is the feedback coefficient, u _set and i _q2 are the input quantity and disturbance quantity of the control system; where

(2)。

(2).

Further, the fault judgment is specifically: according to the protection criterion, if the DC side voltage satisfies U _dc ≤ 0.8 U _dcn , the fault of the DC system can be defined as an inter-pole short-circuit fault;

The fault can be defined as a single-pole earth fault if the voltage satisfies the following equation

(3)；

(3);

Under different fault conditions, the input of the flexible current limiter control system is set to different values

(4)；

(4);

Among them, U _dcn is the rated voltage of the DC system, U ⁺ _dc , U ^- _dc are the voltages of the positive and negative poles of the DC side, respectively, and U ⁺ _dcn and U ^- _dcn are the rated values of the positive and negative voltages, respectively. m ₁ , m ₂ and m ₃ are control coefficients input by the system and m _i =0∼1( i =1,2,3).

Further, during the fault period, the current limiter changes the clamping voltage provided by the current limiter by changing the value of the control coefficient m _i for different fault positions, thereby changing the fault voltage of the fault circuit, and finally realizing the reduction of the fault current. Flexible control.

Compared with the prior art, the present invention has the following beneficial effects:

1. The control method of the novel flexible current limiter provided by the present invention is divided into three parts: normal operation of the line where the novel flexible current limiter is located, detection of an inter-pole short-circuit fault or one-way ground fault, and degaussing the current-limiting inductance after the fault persists for a period of time. In this case, the new flexible current limiter can be controlled, and the new flexible current limiter can improve the stability of the DC distribution network in the event of a fault, and simplify the traditional current limiter topology structure;

2. The flexible current limiter of the present invention can be used to deal with single-pole grounding faults and inter-pole short-circuit faults at the same time, and has a wider application range;

3. On the basis of effective current limiting, the fault voltage of the DC system can be flexibly adjusted by the flexible current limiter. In the case of an inter-pole short-circuit fault, the fault current of the system can also be adjusted according to the current withstand value of the relevant devices, and the controllability of current limiting is greatly improved;

4. The present invention prolongs the effective action time of the current limiting inductor due to the function of the closed-loop iron core. Ensure the stability of the fault current limit before the fault disappears;

5. During the operation of the current limiter, the energy drawn by the current limiting inductor from the AC system can be returned to the power grid through the inverter loop of the flexible current limiter, reducing the energy loss.

Description of drawings

1 is a topology diagram of a novel flexible current limiter bridge circuit in an embodiment of the present invention;

2 is an overall frame diagram of a novel flexible current limiter in a system according to an embodiment of the present invention;

3 is a judgment diagram of a current-limiting action after a fault is detected in an embodiment of the present invention;

4 is a diagram of a degaussing process of a current-limiting inductor when a fault lasts for a period of time in an embodiment of the present invention;

Fig. 5 is the action flow of the novel flexible current limiter in an embodiment of the present invention;

FIG. 6 is a diagram of the action of the novel flexible current limiter in two faults in an embodiment of the present invention.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and embodiments.

Referring to FIG. 1, the present invention provides a new type of flexible current limiter based on a DC distribution network, including a bridge circuit, a current limiter and a current limiter voltage-controlled rectifier module, the current limiter is located on the bridge arm of the bridge circuit, The bridge circuit is connected in parallel with the voltage-controlled rectifier module; the bridge circuit includes a first diode D ₁ , a second diode D ₂ , a third diode D ₃ , and a fourth diode D ₄ . Current inductance L, controllable DC bias power supply _Ib ; the cathode of the _first diode D1 is connected to the cathode of the controllable DC bias power supply _Ib and the cathode of the _second inductor D2 respectively; the first diode _The anode of D1 is connected to the cathode of the third diode _{D3 ;} the anode of the third diode D3 is connected to one end of the current limiting inductor L and the anode of the _fourth diode D4 respectively; the fourth diode D _4. The cathode is connected to the anode of the _second diode D2; the current-limiting inductor L is connected to the anode of the controllable DC bias power supply _Ib .

The operation mode when the system is in normal operation and in two fault states;

When the system is in normal operation, I _dc >0, the intermediate branch is connected in series with a forward DC bias power supply, so that I _b >I _dc . The current through diodes D ₁ and D ₄ is (I _b +I _dc )/2>0, and the current through diodes D ₂ and D ₃ is (I _b -I _dc )/2>0, so 4 diodes Both are forward conducting, the current limiter is bypassed, the current on the DC line will not flow through the middle branch, and the current in this branch is small. ;

Among them, when the inter-pole short-circuit fault occurs in the DC distribution network, the line current I _dc increases, I _b <I _dc , and the currents on the diodes D ₂ and D ₃ at this time are (I _b -I _dc )/2<0, Therefore, only diodes D ₁ and D ₄ are turned on at this time, and the DC side fault current flows through the current-limiting inductance of the intermediate branch, and its amplitude can be suppressed. As shown in Figure 5, connect the DC bias power supply in parallel with the fully controlled transistor. In the case of unipolar grounding fault, the G ₁ signal controls the transistor to be turned on, the DC power supply is short-circuited, only D ₁ and D ₄ are forwardly conducting, and the current limiting inductor is connected to the DC line;

As shown in Figure 2, the present invention provides a control method suitable for a novel flexible current limiter, and the specific process is as follows:

The clamping voltage of the current limiting inductor is controlled to a stable value by the linear current supplied by the rectifier. Therefore, the voltage _Ul of the current limiting inductor is used as the control target of the flexible current limiter converter station. The expression for double loop control is as follows:

(1)；

(1);

In formula (1), K is the amplification gain, K _vf is the feedback coefficient, u _set and i _q2 are the input quantity and disturbance quantity of the control system; where

(2)。

(2).

According to the protection criterion, if the DC side voltage satisfies U _dc ≤ 0.8U _dcn , the fault of the DC system can be defined as an inter-pole short-circuit fault. The fault can be defined as a single-pole earth fault if the voltage satisfies the following equation

(3)；

(3);

Under different fault conditions, the input of the flexible current limiter control system is set to different values

(4)；

(4);

Among them, U _dcn is the rated voltage of the DC system, U ⁺ _dc , U ^- _dc are the voltages of the positive and negative poles of the DC side, respectively, and U ⁺ _dcn and U ^- _dcn are the rated values of the positive and negative voltages, respectively. m ₁ , m ₂ and m ₃ are control coefficients input by the system and m _i =0∼1 (i=1, 2, 3).

When a fault occurs, the control coefficient can be artificially changed within the interval for different fault locations. In the case of inter-pole short-circuit fault, if the value of m ₁ 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 excessive fault current. When a single-pole ground fault occurs, the main problem of the system is that the positive and negative voltages are unbalanced, and the unbalance degree is negatively correlated with the values of m ₂ and m ₃ . Because the harm of single-pole grounding fault is small, in order to ensure the smooth operation of the system under fault conditions and prolong the time available for fault detection, the minimum value of m ₂ and m ₃ is selected to be 0.6. Therefore, the selection interval of the control coefficient m _i is

(5)；

(5);

By changing the value of the control coefficient _mi , the clamping voltage provided by the current limiter can be flexibly changed, thereby changing the fault voltage of the fault circuit, and finally realizing the flexible control of the fault current.

According to equations (1)-(2), the 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 due to the function of the bridge circuit, the influence of the state switching of the current limiter on the system can be ignored. The installation position and control system of the current limiter in the distribution network are shown in Figure 2. They are connected in series in the DC grid, and the voltage of the current limiter is rectified and controlled by the AC measurement.

Figure 2(A) sets the control system input values for positive ground fault and negative ground fault, which are m ₂ U ⁺ _dcn and m ₃ U ^- _dcn , respectively. Figure 2(B) shows the process of the system judging the single-pole grounding fault and the inter-pole short-circuit fault. When the positive voltage or negative voltage value is lower than 0.8 times the rated value, it can be considered that the system has a single-pole grounding fault, U _set =U _set1 . If the above single-pole grounding fault condition is not met, then U _set =U _set2 . Fig. 2(C) contains two states, when the DC side voltage satisfies the fault judgment condition of inter-pole short circuit, U _set2 =m ₁ U _dcn . Otherwise, the input of the current limiter control system is 0, that is, the voltage of the current limiting inductor is 0, and the system is in a normal operation state.

As shown in Figure 3, the clamping voltage U _l of the current limiting inductor is set by the input value of the control system. In different cases, the inductor voltage can be set to four values as shown in the figure. The rectifier provides a 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 will change, and the current limiting effect will be reduced. Therefore, during the use of the flexible current limiter, measures to eliminate the saturation of the current limiting inductor should also be considered.

As shown in Figure 4, the closed-loop iron core can be used to prevent the saturation of the current-limiting inductor, the N ₁ winding is the current-limiting inductor, and the N ₂ winding is the control-side inductor. The specific manifestation of current-limiting inductor saturation is that the magnetic flux inside the closed-loop iron core is saturated, and the inductance on the secondary side generates a reversed magnetic flux in the closed-loop iron core to suppress the increase in the magnetic flux of the current-limiting inductor and play a role in desaturation. The controlled voltage source is used as the secondary side power supply to flexibly adjust the magnitude of the reverse magnetic flux. The control quantity is I ₁ , and k is the control coefficient.

The basic equation of the iron core can be expressed as

(6)；

(6);

where φ1 is the magnetic flux _of the _{single -} turn coil, Ψ1 is the total magnetic flux, and e1 is the voltage _of the N1 winding. When the iron core magnetic flux is saturated, the inductor L ₁ is saturated, its inductive current no longer rises linearly, and the voltage cannot be stably controlled at the set value U _set .

If the magnetic induction intensity and permeability are close to saturation, the reverse magnetic flux of the iron core will increase, and the bias of the controlled voltage source can ensure that the current-limiting inductance is not saturated during the fault period.

As shown in Figure 5, the flexible current limiter can eliminate the imbalance of the positive and negative voltages of the single-pole grounding fault, and can also reduce the fault current amplitude of the inter-pole short-circuit fault. The operation process of the flexible current limiter can be divided into four stages: normal operation stage, fault current limiting stage, iron core saturation elimination stage and recovery stage.

Stage (1): When the system is in normal operation, the four diodes of the bridge circuit are all turned on, the DC line current will not flow through the current-limiting branch, and the current flowing through this branch is small. As shown in Figure 5(1), under normal conditions, the current limiter does not need to take power from the grid, and the controlled source on the secondary side of the iron core will not operate, and the current limiter does not affect the normal operation of the system.

Stage (2): The short-circuit fault of the DC system can be divided into ground short-circuit fault and inter-pole short-circuit fault. During the fault, only two diodes D ₁ and D ₄ are turned on in the bridge circuit, and the flow 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 ① in Figure 5(2). The current-limiting inductance starts to act at this stage and does not reach the saturation state, so the controlled voltage source will not act. The voltage of the current-limiting inductor is U _l , and there is U _l =m ₂ U ⁺ _dcn , the positive voltage will rise to U _l , and the negative voltage will decrease at the same time, which eliminates the imbalance of positive and negative voltages during single-pole grounding faults. The fault circuit with short-circuit between poles is shown as ② in Figure 5(2). At this time, the current-limiting inductor voltage U _l =m ₁ U _dcn can be used as a variable-sized clamping voltage in the fault circuit, thereby reducing the AC voltage. /DC converter station outlet capacitor discharge voltage.

Stage (3): After the current limiter acts for a period of time, if the fault has not been removed, it is necessary to extend the effective current limiting time of the current limiting inductor through the controlled source on the secondary side of the iron core. The installation position of the controlled current source is shown in Figure 5(3). According to the saturation condition of the current limiting inductor, the voltage of the controlled source can be adjusted adaptively to maintain the unsaturated state of the current limiting inductor and ensure the reliability of the current limiter.

Stage (4): When the fault disappears or is cleared, the four diodes on the bridge circuit resume conduction, the current limiter is bypassed again, and the process of its recovery state does not affect the system in normal state. The energy on the current-limiting inductor is released to the grid, and the system can be divided into a main loop and an energy release loop, as shown in Figure 5(4). In the main circuit, the voltage and current of the DC system are restored to their rated values. In the discharge loop, the converter in the flexible current limiter is transformed into an inverter state, and the inductive energy flows to the grid through the inverter to reduce energy loss.

Based on the above principles, the flexible current limiter can reduce the discharge voltage of the capacitor by increasing the reverse clamping voltage in the fault loop.

As shown in Figure 6,

Single pole ground fault:

The current limiter is installed at the outlet of the converter station. When a single-pole ground fault occurs, the current limiter acts between the positive capacitor and the fault point, as shown in Figure 6(1). The current limiter can provide a stable clamping voltage, so the negative voltage can be expressed as

(7)；

(7);

With the action of the flexible current limiter, the negative electrode voltage decreases with the increase of the positive electrode voltage. Therefore, if the current-limiting inductor voltage is set reasonably at this time, the system can isolate the single-pole ground fault point and operate normally, ensuring the stability of the DC distribution network power supply.

Interpole short circuit fault:

As shown in Figure 6(2), the discharge voltage of the capacitor decreases due to the clamp voltage, which can be expressed as

(8)；

(8);

Among them, U ^' _dc is the equivalent discharge voltage of the outlet capacitor. After the discharge voltage of the capacitor is reduced, the fault current will also be limited. Because U _l =U _set =m ₁ U _dcn , this value can be flexibly changed by changing the size of m ₁ . Therefore, the equivalent discharge voltage of the capacitor can also be changed flexibly, and the flexible control of the fault current can be realized.

The above descriptions are only preferred embodiments of the present invention, and all equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope of the present invention.

Claims (4)

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 current limiter voltage-controlled rectifying module, 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 D₄A current-limiting inductor L and a controllable DC bias power supply I_b(ii) a The first diode D₁Cathode and controllable DC bias power supply I_bNegative pole and second inductance L₂The 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 I_bConnecting the positive electrodes;

the control method of the flexible current limiter is specifically

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, so that the voltage drop of an outlet capacitor of the AC/DC converter station is controlled, and overlarge fault current is restrained; the current limiter controls the clamping voltage of the current-limiting inductor to be a stable value through the linear current provided by the rectifier, so that the voltage of the current-limiting inductor L is controlledu _l Is taken as a control target for the flexible current limiter converter station;

the expression for the dual loop control is as follows:

(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

(2)。

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 the current limiting inductor L, the core having a primary side as the current limiter and a secondary side as the controlled source.

3. The flexible current limiter according to claim 1, wherein the fault determination is specifically: 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

(3)；

The input of the flexible current limiter control system is set to different values under different fault conditions

(4)；

Wherein, U_dcnIs 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 the control coefficient of the system input.

4. The flexible current limiter according to claim 1, wherein during the fault, the current limiter changes the clamping voltage provided by the current limiter by changing the value of the control coefficient according to different fault positions, so as to change the fault voltage of the fault loop, and finally realize flexible control of the fault current.

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