A special DC circuit breaker for arc starting and a constant current control method for a DC arc heater

Abstract

The present invention relates to a dedicated DC circuit breaker for arc initiation, which breaks the DC current by resonating a commutation capacitor with a pre-charged voltage and a commutation inductor, uses the charging voltage of the capacitor by the DC current as the transient breaking voltage to break down the cathode long arc, and absorbs the excess energy of the commutation capacitor with an energy-dissipating resistor. The present invention also relates to a constant current control method for a DC arc heater. According to the reference current required by a high-power DC arc generator and the mathematical model of a rectifier bridge, the random arc impedance voltage obtained by real-time sampling in the circuit is added, the trigger angle of the required thyristor is calculated, a feed-forward link is formed, and it is combined with a PI feedback control link to form a feed-forward-feedback composite control strategy. When the arc heater of the present invention starts, the inrush current is small and the current following speed is fast; when the arc is initiated, the current fluctuation is small, the current interruption does not occur, and the arc initiation success rate is high; the established transient breaking voltage is stable, the voltage level is high enough, and the cathode long arc initiation success rate is high.

Description

Technical Field

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

[0002] DC arc heaters are mainly applied in arc wind tunnels, and the key devices therein are thyristor DC power supplies and dedicated DC circuit breakers for arc initiation. The thyristor DC power supply is used to output a stable DC current, and the dedicated DC circuit breaker for arc initiation is used to establish a stable transient breaking voltage to break through a long arc.

[0003] The existing constant current control strategy for thyristor DC power supplies is PI feedback control. The difference between the current reference value and the actual value is passed through a PI control link, and the complementary angle thereof is taken as the trigger angle of the thyristor of the commutation valve. In the starting stage of the arc heater, a single PI feedback control will have a large overshoot in the current rise, and the inrush current will be large. At the same time, it is difficult to respond in time to the sudden change of the loop impedance voltage at the moment of arc initiation, and the current will drop greatly under the 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 dedicated DC circuit breaker for arc initiation is an air-type DC circuit breaker. When the air-type circuit breaker operates, the direct release operates, the moving contact is separated from the static contact, an arc is generated between the contacts, and the arc blowing system quickly blows the arc into the arc extinguishing grid, and the arc extinguishing system extinguishes the arc to establish a transient breaking voltage. In the application scenario of a 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, and the arc initiation failure rate is relatively high, which may cause the circuit breaker and the electrode to burn out. Therefore, the present invention proposes a dedicated DC circuit breaker for arc initiation. Summary of the Invention

[0006] The purpose of the present invention is to overcome the deficiencies of the prior art, and provide a dedicated DC circuit breaker for arc initiation 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, the constant current control in different working stages of the arc heater is realized, which has the characteristics of rapid current response and good following characteristics. The dedicated DC circuit breaker for arc initiation of the present invention can quickly and stably break the DC current, establish a transient breaking voltage, and has a high arc initiation success rate.

[0007] The present invention solves its technical problems through the following technical solutions:

[0008] A dedicated DC circuit breaker for arc initiation, comprising a commutation inductor L, a commutation capacitor C, mechanical vacuum switches K1, K2, isolating switches K3, K4, a lightning arrester MOV and a discharge resistor. The mechanical vacuum switch K1 forms a current-carrying branch. The mechanical vacuum switch K2, the commutation inductor L and the commutation capacitor C are connected in series to form a commutation branch. The MOV, the discharge resistor and a charging power supply are connected in parallel across the two ends of the commutation capacitor. The lightning arrester MOV limits the voltage of the commutation capacitor. The discharge resistor absorbs the excess energy of the commutation capacitor. The charging power supply pre-charges the voltage of the commutation capacitor C. The charging power supply and the discharge resistor branch are connected in series with the isolating switches K3, K4 to control their switching on and off. After the mechanical vacuum switch K1 operates to initiate an arc, the resonant current of the pre-charged commutation capacitor C 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 commutation to break down the long cathode arc.

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

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

[0011] 2) During the working stage of the arc heater, after the anode short arc burns for a period of time, through the cooperation of the opening command of K1 and the closing command of K2, K1 and K2 are simultaneously opened and closed, and the commutation branch is connected into the circuit.

[0012] 3) When the resonant current I os is greater than the DC current I dc , the arc current of the mechanical vacuum switch passes through zero and extinguishes, and 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 is initiated, to avoid the oscillation caused by the reverse flow of capacitor energy into the arc branch, K2 opens to cut off the commutation branch from the working circuit. At the same time, K4 closes, and the capacitor energy is discharged to the high-voltage energy-discharging resistor.

[0014] 5) After the arc heater operates for a relatively long period of time, the power supply blocks the pulse. After all the arcs are extinguished and the capacitor energy is completely discharged, K1 closes and K4 opens, and the dedicated DC circuit breaker for arc initiation returns to the state before the arc heater operates.

[0015] A constant current control method for a DC arc heater. The 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 dedicated DC breaker for arc starting. The anode output of the thyristor DC power supply is connected to the anode after passing through the water-cooled resistor. The cathode is connected to the cathode and grounded after passing through the smoothing reactor. The dedicated DC 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 obtains three voltages U1, U2, and U3 with the same amplitude and a phase difference of 20° in sequence through a phase-shifting transformer, and they are respectively input into the rectifier bridges V1, V2, and V3 to form an 18-pulse power supply. The thyristor DC power supply operates 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 the loop impedance voltage. Specifically:

[0016] 1) Based on the mathematical model of the 18-pulse controlled rectifier bridge and adding real-time sampling calculation of the loop arc voltage for feedforward input of the trigger angle α1, 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 AC input voltage of the 18-pulse controlled rectifier bridge;

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

[0020] R1 is the load resistance;

[0021] d x is the equivalent commutation resistance considering the influence of the commutation overlap angle, and its value is d x = 3X r / π, where X r is the leakage reactance of the commutation transformer;

[0022] V arc is the arc voltage;

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

[0024] 2) After subtracting the current reference value I ref from the output current I out , the obtained error passes through a PI control link to obtain the output β, and the feedback input thyristor trigger angle α2 is obtained by α2 = π - β. α1 and α2 are added to obtain the thyristor trigger angle α input to the 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 dedicated DC breaker for arc starting is in the closed state; when the arc heater starts working, the thyristor DC power supply triggers a pulse, and a short arc between the anode and the trigger electrode arcs and burns under the output voltage of the power supply; after the anode short arc burns stably for a period of time, the dedicated DC breaker for arc starting operates to interrupt the DC current, and a stable transient interruption voltage is established between the cathode and the trigger electrode to break down the cathode long arc; after the arc burns stably for a relatively long period of time, the thyristor DC power supply blocks the pulse, the arc extinguishes, the dedicated DC breaker for arc starting disconnects, and the arc heater stops working.

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

[0027] 1. Under the control of the constant current control strategy of the present invention, when the arc heater starts, the inrush current is small and the current following speed is fast; when the arc starts, the current fluctuation is small, there will be no current interruption, and the arc starting success rate is high;

[0028] 2. The transient interruption voltage established by the dedicated DC breaker for arc starting 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. Description of the Drawings

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

[0030] Figure 2 It 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 It is a comparison diagram of the control effects of the control strategies before and after improvement according to an embodiment of the present invention;

[0032] Figure 4 It is a waveform diagram of the capacitor voltage and the absorbed energy waveform of the lightning arrester when the cathode arc of the embodiment of the present invention is not broken down. Detailed Embodiment

[0033] The present invention will be further described in detail below through specific embodiments. The following embodiments are only descriptive and not restrictive, and the protection scope of the present invention cannot be limited thereby.

[0034] Such as Figure 1As shown in the figure, 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 dedicated DC breaker for arc ignition. The anode output of the thyristor DC power supply is connected to the anode after passing through the water-cooled resistor, and the cathode is connected to the cathode and grounded after passing through the smoothing reactor. The dedicated DC breaker for arc ignition is connected between the trigger electrode and the cathode; the thyristor DC power supply is modulated using a constant current control strategy based on real-time feedforward of loop impedance voltage in the present invention, and a dedicated DC breaker for arc ignition in the present invention is connected into the loop.

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

[0036] 1) Based on the mathematical model of an 18-pulse controlled rectifier bridge and adding real-time sampling and calculation of the loop arc voltage for feedforward input of the firing angle α1, 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 feedforward input thyristor firing angle;

[0040] R1 is the load resistance;

[0041] d x is the equivalent commutation resistance considering the influence of commutation overlap angle, and its value is d x = 3X r / π, where X r is the leakage reactance of the commutation transformer;

[0042] V arc is the arc voltage;

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

[0044] 2) After subtracting the current reference value I ref from the output current I out , the obtained error passes through a PI control link to obtain the output β, and the feedback input thyristor firing angle α2 is obtained by α2 = π - β. The sum of α1 and α2 gives the thyristor firing angle α input to the 18-pulse rectifier bridge.

[0045] The above-mentioned dedicated DC breaker for arc initiation includes a commutation inductor L, a commutation capacitor C, mechanical vacuum switches K1 and K2, isolating switches K3 and K4, a lightning arrester MOV, and a discharge resistor. The mechanical vacuum switch K1 constitutes a current-carrying branch. The mechanical vacuum switch K2, the commutation inductor L, and the commutation capacitor C are connected in series to form a commutation branch. The MOV, the discharge resistor, and the charging power supply are connected in parallel across the two ends of the commutation capacitor. The lightning arrester MOV limits the voltage of the commutation capacitor. The discharge resistor absorbs the excess energy of the commutation capacitor. The charging power supply pre-charges the 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 on and off. After the mechanical vacuum switch K1 operates to initiate an arc, the resonant current of the pre-charged commutation capacitor C and the commutation inductor L is used for commutation, and the transient interruption voltage is established by the charging voltage of the commutation capacitor C after commutation to break down the long cathode arc. The working sequence is as follows:

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

[0047] 2) During the working stage of the arc heater, after the anode short arc burns for a period of time, through the coordination of the opening command of K1 and the closing command of K2, K1 and K2 are simultaneously opened and closed, and the commutation branch is connected to the circuit;

[0048] 3) When the resonant current I os is greater than the DC current I dc , the arc current of the mechanical vacuum switch passes through zero and extinguishes, and the current transfers to the commutation branch and charges the commutation capacitor. The capacitor voltage is the transient interruption voltage;

[0049] 4) After the cathode arc is initiated, to avoid the oscillation caused by the reverse flow of capacitor energy into the arc branch, K2 opens to cut off the commutation branch from the working circuit. At the same time, K4 closes, and the capacitor energy is discharged to the high-voltage energy-dissipating resistor;

[0050] 5) After the arc heater operates for a relatively long period of time, the power supply blocks the pulse. After all the arcs are extinguished and the capacitor energy is completely discharged, K1 closes and K4 opens, and the dedicated DC breaker for arc initiation returns to the state before the arc heater operates.

[0051] Therefore, the working principle of the DC arc heater is as follows: During the start-up stage of the arc heater, the dedicated DC breaker for arc initiation is in the closed state. 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. After the anode arc burns for a period of time, the dedicated DC breaker for arc initiation interrupts the DC current and establishes a transient interruption voltage to break down the long arc between the cathode and the trigger to complete the arc initiation process of the DC arc heater. When the arc heater ends its operation, the thyristor DC power supply blocks the pulse, the arc extinguishes, and the dedicated DC breaker for arc initiation returns to the closed state.

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

[0053] In the simulation example, after the plasma generator is started, it is set that the anode short arc ignites for 0.02 s, the DC circuit breaker works for 0.04 s, the mechanical vacuum switch acts to start the arc, and at the same time the commutation branch is put into the circuit to generate resonance. After 2 μs, the current in the current-carrying branch passes through zero and extinguishes, and the current transfers to the commutation branch. The current transfer time is 2 μs; the DC current charges the capacitor C h to establish a transient breaking voltage U TIV , and the voltage rising rate dU c / dt = I dc / C until it rises to 10 kV and the anode long arc breaks down and ignites.

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

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

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

[0057] As Figure 3 shown, 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 scenarios of starting anti-surge current and starting anti-current impulse.

[0058] Figure 4 shows the capacitor voltage waveform when the cathode arc has not broken through, and designs the capacity of the arrester and the withstand voltage level of the entire system based on this data. Figure 4 shown, if the cathode long arc is difficult to break through under the current working condition, the peak value of the capacitor voltage rise is 21 kV, and then it remains stable at 16.7 kV. Considering a certain safety margin, the withstand voltage level of the entire system can be set to 25 kV.

[0059] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art can understand 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 content disclosed in the embodiments and drawings.

Claims

1. A dedicated DC circuit breaker for arc ignition, used in a DC arc heater, characterized in that: It includes commutation inductor L, commutation capacitor C, mechanical vacuum switches K1 and K2, isolating switches K3 and K4, lightning arrester MOV and discharge resistor. The mechanical vacuum switch K1 constitutes a current-carrying branch. The mechanical vacuum switch K2, commutation inductor L and commutation capacitor C are connected in series to form a commutation branch. The MOV, discharge resistor and charging power supply are connected in parallel across the two ends of the commutation capacitor. The lightning arrester MOV limits the voltage of the commutation capacitor. The discharge resistor absorbs the excess energy of the commutation capacitor. The charging power supply pre-charges the voltage of the commutation capacitor C. The charging power supply and the discharge resistor branch are in series with isolating switches K3 and K4 to control their switching on and off. After the mechanical vacuum switch K1 acts and arcs, the resonant current of the pre-charged commutation capacitor C and the commutation inductor L is used for commutation, and the transient breaking voltage is established through the charging voltage of the commutation capacitor C after commutation to break down the long cathode arc of the arc heater. The working time sequence of the DC circuit breaker is as follows: 1) Before the arc heater works, K3 closes to charge the capacitor. After the capacitor is fully charged, K3 opens, and the charging circuit is cut off from the working circuit of the arc generator. 2) During the working stage of the arc heater, after the short anode arc of the arc heater burns for a period of time, through the cooperation of the opening command of K1 and the closing command of K2, K1 and K2 are simultaneously opened and closed, and the commutation branch is connected to the circuit. 3) Resonant current I os is greater than the DC current I dc When this occurs, the arc current of the mechanical vacuum switch passes through zero and extinguishes, and the current transfers to the commutation branch to charge the commutation capacitor. The capacitor voltage is the transient interruption voltage; 4) After the cathode arc of the arc heater arcs, to avoid the oscillation caused by the reverse flow of capacitor energy into the arc branch, K2 opens to cut off the commutation branch from the working circuit. At the same time, K4 closes, and the capacitor energy is discharged to the high-voltage energy-discharging resistor. 5) After the arc heater operates for a relatively long period of time, the power supply blocks the pulse. After all the arcs are extinguished and the capacitor energy is completely discharged, K1 closes and K4 opens, and the dedicated DC circuit breaker for arcing resumes to the state before the arc heater operates.

2. A constant current control method for a DC arc heater, characterized in that: The DC arc heater includes a thyristor DC power supply, a water-cooled resistor, a smoothing reactor, an anode, a cathode, a trigger electrode and the dedicated DC circuit breaker for arcing as described in claim 1. The positive output of the thyristor DC power supply is connected to the anode after passing through the water-cooled resistor. The negative pole of the thyristor DC power supply is connected to the cathode through the smoothing reactor and grounded. The trigger electrode is connected to the dedicated DC circuit breaker for arcing between 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 obtains three voltages U1, U2 and U3 with the same amplitude and a phase difference of 20° in sequence through a phase-shifting transformer, and are respectively input into the rectifier bridges V1, V2 and V3 to form an 18-pulse power supply. The thyristor DC power supply operates 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 the loop impedance voltage. Specifically: 1) Based on the mathematical model of the 18-pulse controlled rectifier bridge and adding the real-time sampling calculation of the loop arc voltage for feedforward input of the trigger angle α1, its calculation basis is: α1 = arccos{[I out (R1 + d x ) + V arc / 4.05V1} Where: V1 is the effective value of the AC input voltage of the 18-pulse controlled rectifier bridge; α1 is the feedforward input thyristor trigger angle; R1 is the load resistance; d x The equivalent commutation resistance when considering the influence of commutation overlap angle, and its value is d x = 3X r / π, where X r is the leakage reactance of the commutation transformer; V arc is the arc voltage; I out is the output current of an 18-pulse controlled rectifier bridge; 2) Current reference value I ref After subtracting from the output current I out The obtained error is processed through a PI control link to obtain the output β, and the feedback input thyristor firing angle α2 is obtained from α2 = π - β. The thyristor firing angle α of the input 18-pulse rectifier bridge is obtained by adding α1 and α2.

3. The constant current control method of the DC arc heater according to claim 2, characterized in that: The working process of the DC arc heater is as follows: Before the DC arc heater starts working, the special DC breaker for arc starting is in the 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 electrode arcs and burns under the output voltage of the power supply; after the short arc of the anode of the arc heater burns stably for a period of time, the special DC breaker for arc starting operates to interrupt the DC current, and a stable transient interruption voltage is established between the cathode and the trigger electrode to break down the long arc of the cathode of the arc heater; after the arc burns stably for a relatively long period of time, the thyristor DC power supply blocks the pulse, the arc extinguishes, the special DC breaker for arc starting disconnects, and the arc heater stops working.

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