Special direct-current circuit breaker for arcing and direct-current arc heater constant-current control method

Abstract

The invention relates to a direct-current circuit breaker special for arcing, which is characterized in that direct current is cut off through resonance of a commutation capacitor with pre-charging voltage and a commutation inductor, the charging voltage of the direct current to the capacitor is used as transient breaking voltage to break down a cathode long arc, and excess energy of the commutation capacitor is absorbed by a discharge resistor. The invention further relates to a direct-current arc heater constant-current control method which comprises the following steps: adding random arc impedance voltage obtained by real-time sampling in a loop according to reference current required by a high-power direct-current arc generator and a mathematical model of a rectifier bridge, calculating a trigger angle of a required thyristor, forming a feed-forward link, and combining with a PI feedback control link to realize constant-current control of the direct-current arc heater. And a feedforward-feedback composite control strategy is formed. When the arc heater is started, the surge current is small, and the current following speed is high; during arc starting, the current fluctuation is small, the current cutoff condition is avoided, and the arc starting success rate is high; the established transient breaking voltage is stable, the voltage grade is high enough, and the arc starting success rate of the cathode long arc is high.

Description

Technical Field

[0001] The invention belongs to the technical field of electrical equipment, and in particular relates to a DC circuit breaker dedicated for arc starting and a constant current control method for a DC arc heater. Background Art

[0002] DC arc heaters are mainly used in arc wind tunnels, and the key components are thyristor DC power supplies and arc-starting DC circuit breakers. The thyristor DC power supply is used to output stable DC current, and the arc-starting DC circuit breaker is used to establish a stable transient breaking voltage to break down the long arc.

[0003] The existing constant current control strategy of thyristor DC power supply is PI feedback control, which takes the difference between the current reference value and the actual value through the PI control link and takes its complementary angle as the trigger angle of the thyristor of the converter valve. In the startup phase of the arc heater, the single PI feedback control will have a large overshoot in the current rise and a large surge current; at the same time, it is difficult to respond to the sudden change of the loop impedance voltage in time at the moment of arc starting, and the current will drop significantly under impulse, and the arc will be interrupted, which is not conducive to the operation of the system.

[0004] Therefore, the present invention proposes a composite constant current control strategy based on real-time feedforward of loop impedance voltage.

[0005] The existing DC circuit breaker dedicated to arc starting is an air-type DC circuit breaker. When the air-type circuit breaker is actuated, the direct release is actuated, the moving contact and the static contact are separated, an arc is generated between the contacts, and the arc blowing system quickly blows the arc into the arc extinguishing grid, the arc extinguishing system extinguishes the arc, and a transient breaking voltage is established. In the application scenario of the DC arc heater, the transient breaking voltage established by the air-type circuit breaker is unstable, the voltage level is not high, it is difficult to stably break the DC current, the arc starting failure rate is high, and the circuit breaker and electrode may be burned. Therefore, the present invention proposes a DC circuit breaker dedicated to arc starting. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a DC circuit breaker dedicated to arc starting and a constant current control method for a DC arc heater. By adding a real-time feedforward link based on loop impedance voltage to improve the PI feedback control strategy, constant current control of the arc heater in different working stages is achieved, and the method has the characteristics of rapid current response and good following characteristics. The DC circuit breaker dedicated to arc starting of the present invention can quickly and stably interrupt the DC current, establish a transient interruption voltage, and has a high arc starting success rate.

[0007] The present invention solves the technical problem by the following technical solutions:

[0008] A DC circuit breaker dedicated to arc starting comprises a commutation inductor L, a commutation capacitor C, mechanical vacuum switches K1, K2, disconnectors K3, K4, a lightning arrester MOV and a discharge resistor, wherein the mechanical vacuum switch K1 constitutes a flow branch, the mechanical vacuum switch K2, the commutation inductor L and the commutation capacitor C are connected in series to constitute a commutation branch, the two ends of the commutation capacitor are connected in parallel to the MOV, the discharge resistor and a charging power supply, the lightning arrester MOV limits the commutation capacitor voltage, the discharge resistor absorbs the excess energy of the commutation capacitor, the charging power supply is the pre-charge voltage of the commutation capacitor C, and the charging power supply and the discharge resistor branch are connected in series with disconnectors K3, K4 to control the switching thereof; after the mechanical vacuum switch K1 is actuated to start arcing, the resonant current of the commutation capacitor C after the pre-charge voltage and the commutation inductor L is used for commutation, and a transient breaking voltage is established by the charging voltage of the commutation capacitor C after commutation to break down the long cathode arc.

[0009] Moreover, the working sequence of the DC circuit breaker is:

[0010] 1) Before the arc heater works, K3 is closed to charge the capacitor. After the capacitor is fully charged, K3 is opened and the charging circuit is cut off from the arc generator working circuit;

[0011] 2) During the arc heater working stage, after the anode short arc burns for a period of time, the K1 opening command and the K2 closing command are coordinated to make K1 and K2 open and close at the same time, and the commutation branch is connected to the circuit;

[0012] 3) Resonant current I os Greater than the DC current I dc When the arc current of the mechanical vacuum switch passes through zero and extinguishes, the current is transferred to the commutation branch and charges the commutation capacitor. The capacitor voltage is the transient breaking voltage.

[0013] 4) After the cathode arc starts, in order to prevent the capacitor energy from flowing back into the arc branch and causing oscillation, K2 is opened to cut off the commutation branch from the working circuit, and K4 is closed at the same time to discharge the capacitor energy to the high-voltage discharge resistor;

[0014] 5) After the arc heater has been running for a long period of time, the power supply pulse is blocked. After all arcs are extinguished and the capacitor energy is discharged, K1 is closed and K4 is opened, and the DC circuit breaker dedicated to arc starting is restored to the state before the arc heater was operated.

[0015] A method for controlling a DC arc heater with constant current, the DC arc heater comprising a thyristor DC power supply, a water-cooled resistor, a smoothing reactor, an anode, a cathode, a trigger pole and a DC circuit breaker dedicated for arc starting, wherein the anode output of the thyristor DC power supply is connected to the anode after passing through a water-cooled resistor, the cathode is connected to the cathode after passing through a smoothing reactor and is grounded, and the DC circuit breaker dedicated for arc starting is connected between the trigger pole and the cathode; the thyristor DC power supply is composed of three 6-pulse rectifier bridges V1, V2 and V3 connected in series, the AC side input voltage is obtained by a phase-shifting transformer to obtain three voltages U1, U2 and U3 with the same amplitude and a phase difference of 20°, respectively, which are input into the rectifier bridges V1, V2 and V3 to form an 18-pulse power supply, the thyristor DC power supply works in a constant current output mode, and outputs a constant DC current under the modulation of a composite constant current control strategy based on real-time feedforward of loop impedance voltage, specifically:

[0016] 1) Based on the mathematical model of 18-pulse controlled rectifier bridge and adding the real-time sampling of loop arc voltage, the feedforward input trigger angle α1 is calculated. The calculation basis is:

[0017] α1=arccos{[I out (R1+d x )+V arc ] / 4.05V1}

[0018] Where: V1 is the effective value of the input AC voltage of the 18-pulse controlled rectifier bridge;

[0019] α1 is the trigger angle of the feedforward input thyristor;

[0020] R1 is the load resistance;

[0021] d x The equivalent commutation resistance when the commutation overlap angle is taken into account is d x =3X r / π, where X r is the leakage reactance of the converter transformer;

[0022] V arc is the arc voltage;

[0023] I out It is the output current of 18-pulse controlled rectifier bridge;

[0024] 2) Current reference value I ref With the output current I out After the difference is made, the error obtained is passed through the PI control link to obtain the output β, and the feedback input thyristor trigger angle α2 is obtained by α2=π-β. The addition of α1 and α2 obtains the thyristor trigger angle α of the input 18-pulse rectifier bridge.

[0025] Moreover, the working process of the DC arc heater is as follows: before the DC arc heater starts working, the DC circuit breaker dedicated to arc starting is in a closed state; when the arc heater starts working, the thyristor DC power supply triggers a pulse, and the short arc between the anode and the trigger pole starts and burns under the power supply output voltage; after the anode short arc burns stably for a period of time, the DC circuit breaker dedicated to arc starting operates to interrupt the DC current, and a stable transient interrupting voltage is established between the cathode and the trigger pole to break through the long cathode arc; after the arc burns stably for a long period of time, the thyristor DC power supply blocks the pulse, the arc is extinguished, the DC circuit breaker dedicated to arc starting is disconnected, and the arc heater stops working.

[0026] The advantages and beneficial effects of the present invention are:

[0027] 1. Under the control of the constant current control strategy, when the arc heater is started, the surge current is small and the current following speed is fast; when the arc is struck, the current fluctuation is small, there will be no current interruption, and the arc striking success rate is high;

[0028] 2. The transient breaking voltage established by the arc-starting dedicated DC circuit breaker of the present invention is stable, the voltage level is high enough, and the arc starting success rate of the cathode long arc is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the topological structure diagram of high-power DC arc generator;

[0030] Figure 2 This is a control block diagram of a constant current control strategy based on real-time voltage feedforward according to an embodiment of the present invention;

[0031] Figure 3 A comparison diagram of the control effect of the control strategy before and after the improvement of the embodiment of the present invention;

[0032] Figure 4 The waveform diagram of capacitor voltage and arrester energy absorption when cathode arc does not break down in the embodiment of the present invention. DETAILED DESCRIPTION

[0033] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0034] like Figure 1As shown, a DC arc heater includes a thyristor DC power supply, a water-cooled resistor, a smoothing reactor, an anode, a cathode, a trigger electrode and a DC circuit breaker dedicated to arc starting. The anode output of the thyristor DC power supply is connected to the anode after passing through a water-cooled resistor, the cathode is connected to the cathode and grounded after passing through a smoothing reactor, and the DC circuit breaker dedicated to arc starting is connected between the trigger electrode and the cathode; a constant current control strategy based on real-time feedforward of loop impedance voltage of the present invention is used to modulate the thyristor DC power supply and connect the DC circuit breaker dedicated to arc starting of the present invention into the loop.

[0035] The above control strategy includes the following two parts:

[0036] 1) Based on the mathematical model of 18-pulse controlled rectifier bridge and adding the real-time sampling of loop arc voltage, the feedforward input trigger angle α1 is calculated. The calculation basis is:

[0037] α1=arccos{[I out (R1+d x )+V arc ] / 4.05V1}

[0038] Where: V1 is the effective value of the input AC voltage of the 18-pulse controlled rectifier bridge;

[0039] α1 is the trigger angle of the feedforward input thyristor;

[0040] R1 is the load resistance;

[0041] d x The equivalent commutation resistance when the commutation overlap angle is taken into account is d x =3X r / π, where X r is the leakage reactance of the converter transformer;

[0042] V arc is the arc voltage;

[0043] I out It is the output current of 18-pulse controlled rectifier bridge;

[0044] 2) Current reference value I ref With the output current I out After the difference is made, the error obtained is passed through the PI control link to obtain the output β, and the feedback input thyristor trigger angle α2 is obtained by α2=π-β. The thyristor trigger angle α of the input 18-pulse rectifier bridge is obtained by adding α1 and α2.

[0045] The above-mentioned arc-starting DC circuit breaker includes a commutation inductor L, a commutation capacitor C, a mechanical vacuum switch K1, K2, an isolating switch K3, K4, a lightning arrester MOV and a discharge resistor. The mechanical vacuum switch K1 constitutes a flow branch, and the mechanical vacuum switch K2, the commutation inductor L and the commutation capacitor C are connected in series to constitute a commutation branch. The two ends of the commutation capacitor are connected in parallel with MOV, a discharge resistor and a charging power supply. The lightning arrester MOV limits the commutation capacitor voltage, and the discharge resistor absorbs the excess energy of the commutation capacitor. The charging power supply is the pre-charge voltage of the commutation capacitor C. The charging power supply and the discharge resistor branch are connected in series with the isolating switches K3 and K4 to control their switching. After the mechanical vacuum switch K1 is actuated to start the arc, the resonant current of the commutation capacitor C after the pre-charge voltage and the commutation inductor L is used for commutation, and the transient breaking voltage is established by the charging voltage of the commutation capacitor C after the commutation to break down the long cathode arc. The working sequence is as follows:

[0046] 1) Before the arc heater works, K3 is closed to charge the capacitor. After the capacitor is fully charged, K3 is opened and the charging circuit is cut off from the arc generator working circuit;

[0047] 2) During the arc heater working stage, after the anode short arc burns for a period of time, the K1 opening command and the K2 closing command are coordinated to make K1 and K2 open and close at the same time, and the commutation branch is connected to the circuit;

[0048] 3) Resonant current I os Greater than the DC current I dc When the arc current of the mechanical vacuum switch passes through zero and extinguishes, the current is transferred to the commutation branch and charges the commutation capacitor. The capacitor voltage is the transient breaking voltage.

[0049] 4) After the cathode arc starts, in order to prevent the capacitor energy from flowing back into the arc branch and causing oscillation, K2 is opened to cut off the commutation branch from the working circuit, and K4 is closed at the same time to discharge the capacitor energy to the high-voltage discharge resistor;

[0050] 5) After the arc heater has been running for a long period of time, the power supply pulse is blocked. After all arcs are extinguished and the capacitor energy is discharged, K1 is closed and K4 is opened, and the DC circuit breaker dedicated to arc starting is restored to the state before the arc heater was operated.

[0051] From the above, the working principle of the DC arc heater is: during the startup phase of the arc heater, the dedicated DC circuit breaker for arc starting is in a closed state, and the thyristor DC power supply triggers a pulse to establish a stable DC current to break down the short arc between the anode and the trigger electrode; after the anode arc burns for a period of time, the dedicated DC circuit breaker for arc starting breaks the DC current, and establishes a transient breaking voltage to break down the long arc between the cathode and the trigger electrode to complete the arc starting process of the DC arc heater; when the arc heater finishes working, the thyristor DC power supply blocks the pulse, the arc is extinguished, and the dedicated DC circuit breaker for arc starting returns to a closed state.

[0052] The constant current control strategy of a DC arc heater and a DC circuit breaker dedicated for arc starting of the present invention are simulated, and the simulation parameters are set as follows: the effective value of the AC side line voltage of a six-pulse rectifier bridge of the thyristor DC power supply is set to V1=6kV, and the DC side constant current reference current is set to I ref =3kA; rated voltage of AC side transformer V B =230kV, capacity S = 200MVA, transformer leakage reactance per unit value X r* =0.1; load resistance R = 0.5Ω, smoothing reactor L = 12mH, impedance voltage drop after arcing V arc =15kV, the cathode long arc breakdown voltage is 10kV. Mechanical DC circuit breaker commutation inductance L h =50μH, commutation capacitor C h =5μF, pre-charge voltage U h =15kV.

[0053] In the simulation example, after the plasma generator is started, the anode short arc is set to burn for 0.02s, the DC circuit breaker works for 0.04s, the mechanical vacuum switch starts the arc, and the commutation branch is put into the loop to generate resonance. After 2μs, the current in the flow branch passes through zero and the arc is extinguished. The current is transferred to the commutation branch, and the current transfer time is 2μs; the DC current is the capacitor C h Charging, establishing transient breaking voltage U TIV , voltage rise rate dU c / dt=I dc / C, until it rises to 10kV, the anode long arc breaks down and ignites the arc.

[0054] In the simulation example, a control group is added to verify the control characteristics of the constant current control strategy based on the real-time feedforward of the loop impedance voltage. The control group adopts the PI feedback control strategy. Figure 3 The output current waveform diagram of the high-power DC arc generator when it is started and the anode arc is struck is shown in the figure. fb The output current waveform is modulated by the constant current control strategy based on the real-time feedforward of the loop impedance voltage. b The output current waveform modulated by PI feedback control strategy.

[0055] Attached Figure 3 (a) shows the operation at the reference current I ref=The output current waveform under the working condition of 3kA. It is calculated that, under the modulation of PI feedback control strategy before improvement, the starting current overshoot σ1%=(9.7-3) / 3*100%=223.3%, and the current impulse decrease △I1=1.1kA during arc starting; under the modulation of constant current control strategy based on real-time feedforward of loop impedance voltage after improvement, the starting current overshoot σ2%=(3.2-3) / 3*100%=6.7%, and the current impulse decrease △I1=0.4kA during arc starting.

[0056] Attached Figure 3 (b) shows the operation at the reference current I ref =Output current waveform under 1kA working condition. It is calculated from the figure that, under the modulation of PI feedback control strategy before improvement, the starting current overshoot σ1%=(7.6-1) / 1*100%=660.0%, the current impulse drop △I1 at arc starting is greater than 1kA, the arc current drops to zero, the arc is extinguished, the whole system is cut off and stops running; under the modulation of constant current control strategy based on real-time feedforward of loop impedance voltage after improvement, the starting current has no overshoot, and the current impulse drop △I1=0.09kA at arc starting.

[0057] like Figure 3 It is shown that under the two working conditions, the improved constant current control strategy based on real-time feedforward of loop impedance voltage has great advantages in both the startup surge current protection and arcing current impulse protection scenarios.

[0058] Figure 4 The capacitor voltage waveform is shown when the cathode arc does not break down, and the capacity of the arrester and the withstand voltage level of the entire system are designed based on this data. Figure 4 It is shown that under the current working conditions, if the long cathode arc is difficult to break through, the capacitor voltage rises to a peak of 21kV, and then remains steadily at 16.7kV. Considering a certain safety margin, the withstand voltage level of the entire system can be set to 25kV.

[0059] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will appreciate that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A DC circuit breaker dedicated to arc starting, characterized in that: It includes a commutation inductor L, a commutation capacitor C, a mechanical vacuum switch K1, K2, an isolating switch K3, K4, a lightning arrester MOV and a discharge resistor. The mechanical vacuum switch K1 constitutes a flow branch. The mechanical vacuum switch K2, the commutation inductor L and the commutation capacitor C are connected in series to constitute a commutation branch. The two ends of the commutation capacitor are connected in parallel with MOV, a discharge resistor and a charging power supply. The lightning arrester MOV limits the commutation capacitor voltage. The discharge resistor absorbs excess energy of the commutation capacitor. The charging power supply is a pre-charge voltage for the commutation capacitor C. The charging power supply and the discharge resistor branch are connected in series with the isolating switches K3 and K4 to control the switching thereof. After the mechanical vacuum switch K1 is actuated and arcs are started, the resonant current of the commutation capacitor C after the pre-charge voltage and the commutation inductor L is used for commutation. The transient breaking voltage is established by the charging voltage of the commutation capacitor C after commutation to break down the long cathode arc.

2. The arc-starting DC circuit breaker according to claim 1, characterized in that: The working sequence of the DC circuit breaker is: 1) Before the arc heater works, K3 is closed to charge the capacitor. After the capacitor is fully charged, K3 is opened and the charging circuit is cut off from the arc generator working circuit; 2) During the arc heater working stage, after the anode short arc burns for a period of time, the K1 opening command and the K2 closing command are coordinated to make K1 and K2 open and close at the same time, and the commutation branch is connected to the circuit; 3) Resonant current I os Greater than the DC current I dc When the arc current of the mechanical vacuum switch passes through zero and extinguishes, the current is transferred to the commutation branch and charges the commutation capacitor. The capacitor voltage is the transient breaking voltage. 4) After the cathode arc starts, in order to prevent the capacitor energy from flowing back into the arc branch and causing oscillation, K2 is opened to cut off the commutation branch from the working circuit, and K4 is closed at the same time to discharge the capacitor energy to the high-voltage discharge resistor; 5) After the arc heater has been running for a long period of time, the power supply pulse is blocked. After all arcs are extinguished and the capacitor energy is discharged, K1 is closed and K4 is opened, and the DC circuit breaker dedicated to arc starting is restored to the state before the arc heater was operated.

3. A method for controlling a DC arc heater constant current, characterized in that: The DC arc heater comprises a thyristor DC power supply, a water-cooled resistor, a smoothing reactor, an anode, a cathode, a trigger electrode, and a DC circuit breaker for arc starting as claimed in claim 1 or 2, wherein the anode output of the thyristor DC power supply is connected to the anode after passing through a water-cooled resistor, the cathode is connected to the cathode after passing through a smoothing reactor and is grounded, and the DC circuit breaker for arc starting is connected between the trigger electrode and the cathode; The thyristor DC power supply is composed of three 6-pulse rectifier bridges V1, V2, and V3 connected in series. The AC side input voltage is obtained by a phase-shifting transformer to obtain three voltages U1, U2, and U3 with the same amplitude and a phase difference of 20°, which are respectively input into the rectifier bridges V1, V2, and V3 to form an 18-pulse power supply. The thyristor DC power supply works in a constant current output mode and outputs a constant DC current under the modulation of a composite constant current control strategy based on real-time feedforward of loop impedance voltage, specifically: 1) Based on the mathematical model of 18-pulse controlled rectifier bridge and adding the real-time sampling of loop arc voltage, the feedforward input trigger angle α1 is calculated. The calculation basis is: α1=arccos{[I out (R1+d x )+V arc ] / 4.05V1} Where: V1 is the effective value of the input AC voltage of the 18-pulse controlled rectifier bridge; α1 is the trigger angle of the feedforward input thyristor; R1 is the load resistance; d x The equivalent commutation resistance when the commutation overlap angle is taken into account is d x =3X r / π, where X r is the leakage reactance of the converter transformer; V arc is the arc voltage; I out It is the output current of 18-pulse controlled rectifier bridge; 2) Current reference value I ref With the output current I out After the difference is made, the error obtained is passed through the PI control link to obtain the output β, and the feedback input thyristor trigger angle α2 is obtained by α2=π-β. The thyristor trigger angle α of the input 18-pulse rectifier bridge is obtained by adding α1 and α2.

4. The method for controlling a DC arc heater constant current according to claim 3, characterized in that: The working process of the DC arc heater is as follows: before the DC arc heater starts working, the DC circuit breaker dedicated to arc starting is in a closed state; when the arc heater starts working, the thyristor DC power supply triggers a pulse, and the short arc between the anode and the trigger pole starts and burns under the output voltage of the power supply; after the anode short arc burns stably for a period of time, the DC circuit breaker dedicated to arc starting is actuated to interrupt the DC current, and a stable transient interrupting voltage is established between the cathode and the trigger pole to break through the long cathode arc; after the arc burns stably for a long period of time, the thyristor DC power supply blocks the pulse, the arc is extinguished, the DC circuit breaker dedicated to arc starting is disconnected, and the arc heater stops working.

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