A power topology circuit for a deep-depth airborne electromagnetic detection transmission system

Abstract

The present invention provides a power topology circuit for a deep-depth airborne electromagnetic detection transmission system. The transmission current has a half-sine shape and a maximum peak value of 800A. The circuit uses a helicopter's onboard power supply as its input power source, with a DC voltage of 28V. Through a DC / DC boost circuit, a resonant capacitor charging circuit, and an RLC resonant transmission circuit, it can ultimately transmit a half-sine current wave with a maximum peak value of 800A. The DC / DC boost circuit is placed in the helicopter cabin, and the RLC resonant transmission circuit and the under-cabin transmitting coil are placed together under the aircraft cabin and connected with a 60m thin wire to charge the resonant capacitor. A high-ratio boost circuit is used to achieve a single-stage conversion from low voltage and high current to high voltage and low current. A full-bridge circuit is then used to charge the resonant capacitor using high-frequency chopping during the idle period of transmission to replenish electrical energy. Finally, the RLC resonant transmission circuit converts the energy in the resonant capacitor into a half-sine current waveform, emitting an electromagnetic field into space.

Description

Technical Field

[0001] The present invention relates to the technical field of airborne geophysical exploration, and in particular to an excitation source of a deep-depth airborne electromagnetic exploration system, namely, a power topology circuit of a transmitting subsystem. Background Art

[0002] Mineralized zones not only involve vast survey areas and a significant workload, but also complex and varied terrain, making it difficult for conventional equipment and personnel to access. Therefore, a geophysical prospecting method is needed that can overcome these complex terrain conditions and conduct efficient exploration. The key feature of an airborne electromagnetic survey system is its use of aircraft as a carrier, overcoming the inconvenience of operating in complex terrain. It also offers high exploration efficiency, low costs, and the ability to cover a wide range of areas. The system comprises a transmitting subsystem, a receiving subsystem, and a data interpretation subsystem. The transmitting subsystem is the excitation source for the entire system, and its performance significantly impacts the depth and accuracy of mineral resource exploration.

[0003] The operating process of a helicopter-mounted airborne electromagnetic survey is as follows: the transmitting system draws energy from the helicopter's DC output power supply. After electrical energy conversion, it is converted into an excitation source pulse current. This pulsed magnetic field is generated by the transmitting coil and radiated into space. This pulsed magnetic field is called the primary magnetic field. Since the transmitting coil remains parallel to the ground during the survey, the primary magnetic field is primarily concentrated in a direction perpendicular to the ground. When an underground ore body exists, the periodically alternating primary magnetic field excites eddy currents within the ore body. These eddy currents, over time, generate a secondary induced magnetic field of the same frequency around the ore body. Due to eddy current losses in the eddy currents within the ore body, the secondary induced magnetic field gradually decays over time until it reaches zero. The receiving system collects the decay data and rate of change of the secondary induced magnetic field through the receiving coil, converts them into electrical signals, and records and stores them. Through data processing, imaging, and inversion, the distribution and reserves of underground mineral resources are ultimately determined. The entire HTEM transmitting system is powered by the helicopter, but the helicopter's output power is very limited. A typical helicopter has a rated DC output voltage of 28V and a current of 100A, while the instantaneous power of the transmitted excitation source pulse current can reach tens of kilowatts. Therefore, it is necessary to develop power conversion circuits, energy storage technology and excitation source pulse modulation circuits to improve the overall efficiency of the power topology circuit as much as possible while meeting the performance index requirements and maximize the power density. Summary of the Invention

[0004] To address the shortcomings of existing airborne electromagnetic transmission systems, this paper proposes a power topology circuit for a deep-depth airborne electromagnetic detection transmission system. The circuit's experimental goal is to transmit a half-sine wave current waveform with a fundamental frequency of 25Hz, a half-sine wave pulse width of 4ms, and a peak current of 800A. The circuit uses a helicopter's onboard power supply as its input power source, with a DC input voltage of 28V, a maximum input current of 100A, and a maximum input power of 2.8kW. Through a DC / DC boost circuit, a resonant capacitor charging circuit, and an RLC resonant transmission circuit, it can ultimately transmit a half-sine current wave with a maximum peak value of 800A. The DC / DC boost circuit and resonant capacitor charging circuit are located in the helicopter cabin, while the RLC resonant transmission circuit is located on the transmitting coil. The two circuits are 60m apart, and power is transmitted via a power transmission line.

[0005] The DC / DC boost adopts a full-bridge inverter circuit and a voltage doubler rectifier circuit, and uses a phase-shifted full-bridge soft-switching control method to achieve the conversion from low voltage and high current to high voltage and low current. The voltage doubler rectifier circuit can double the output voltage, thereby reducing the transformation ratio of the high-frequency transformer, eliminating the filter inductor of the traditional full-bridge converter, reducing the weight of the circuit, and alleviating the load pressure of the helicopter. The resonant capacitor is charged through the resonant capacitor charging circuit to compensate for the energy loss of each transmission cycle. The resonant capacitor charging circuit is a full-bridge inverter circuit that uses PWM modulation to control the charging direction and charging current peak of the previous circuit to the resonant capacitor. The onboard power supply is in the aircraft cabin, so the DC / DC boost circuit is in the helicopter cabin. The RLC resonant transmitting circuit is placed together with the under-cabin coil under the aircraft cabin, 60 meters away from the cabin. The RLC resonant transmitting circuit consists of a resonant capacitor C i The power supply is provided by triggering and shutting off the thyristor in the RLC resonant transmitting circuit and the switch in the resonant capacitor charging circuit at different time points, controlling the energy flow between the resonant capacitor and the transmitting coil, and finally modulating and generating a half-sine wave transmitting current with a maximum peak value of 800A. The main circuit of the transmitting system power topology used in this example is as follows Figure 2 shown.

[0006] Beneficial effects

[0007] While meeting performance requirements, the present invention improves the overall efficiency of the power topology circuit and maximizes power density, which has important application significance for further improving the detection performance of airborne transient electromagnetic exploration systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 , a structural block diagram of the topological main circuit of the present invention;

[0009] Figure 2 , a circuit diagram of the topological main circuit of the present invention;

[0010] Figure 3 , the system structure block diagram of the present invention. DETAILED DESCRIPTION

[0011] A power topology circuit of a deep-depth airborne electromagnetic detection transmission system includes a DC power supply, a DC / DC boost circuit, a resonant capacitor high-frequency charging circuit, and an RLC resonant transmission circuit.

[0012] The DC power supply adopts a helicopter onboard power supply as the input power supply, with an input DC voltage of 28V, a maximum input current of 100A, and a maximum input power of 2.8kW.

[0013] Among them, in the DC / DC boost circuit, V in is a DC 28V power supply; Q1-Q4 are electronic switching tubes, using IGBT switching devices, where the leading bridge arm consists of two groups of switching tubes Q1 and Q3, and the lagging bridge arm consists of switching tubes Q2 and Q4; D1-D4 are freewheeling diodes, integrated into the switching device; C1-C4 are resonant capacitors, connected in parallel to the switching tube; T is a power transformer; L r Represents the sum of the transformer primary leakage inductance and the external resonant inductance; D r1 and D r2 It is the high-frequency rectifier diode of the transformer secondary output; C r1 and C r2 is the rectifier capacitor; C o The resonant voltage doubler circuit output capacitor, also known as the energy storage capacitor, is used. A drive circuit controls the on / off switching of the switching device, converting DC power into AC power at a frequency of 10kHz. To reduce switching losses in the electronic switching devices and improve circuit efficiency, a phase-shifted full-bridge soft-switching control method is employed. However, due to the low input voltage and large transformer size, as the output power increases, the secondary voltage duty cycle is severely lost, reducing the circuit's transmission efficiency. Therefore, a resonant voltage doubler circuit is employed to address this severe duty cycle loss issue, enabling the DC / DC boost circuit output voltage to reach approximately 560V, meeting the requirements of the boost circuit.

[0014] Among them, in the resonant capacitor charging circuit and the RLC resonant transmitting circuit, the resonant capacitor charging circuit includes: electronic switch tubes Q5-Q8, D5-D8 are freewheeling diodes integrated in the switch tube device; the parasitic inductance L of the charging line w ; The RLC resonant transmitting circuit includes: resonant capacitor C i , forward thyristor T1, reverse thyristor T2, coil inductance L coil , coil parasitic resistance R coilTo reduce the weight of the transmitter, the resonant capacitor charging circuit was placed in the aircraft cabin, and the RLC resonant transmitting circuit was placed together with the coil under the cabin. A 60-meter wire was used to connect the RLC resonant transmitting circuit and the resonant capacitor charging circuit as an energy replenishment path.

[0015] The control circuit mainly includes:

[0016] Sampling circuit: resonant capacitor C i Peak current, energy storage capacitor C O The voltage is sampled and transmitted to the DSP28335 controller as a feedback signal to adjust the controller output. In addition, the current voltage and current information is displayed on the host computer.

[0017] Protection circuit: A comparator compares the sampled signal with a reference signal and sends the comparison output signal to the main controller's IO port. If a fault occurs, the DSP controls the output drive signal to stop the DC / DC boost circuit. The protection circuit also collects power supply signals. If the power supply signal is abnormal, a manual emergency stop can be performed to cut off the power supply.

[0018] Drive circuit: includes IGBT drive circuit and thyristor drive circuit. Based on the drive signal of DSP28335, it generates voltage and current signals that meet the power requirements of the power tube drive power, and drives the switch tube to turn on and off according to the timing requirements.

[0019] Fiber-optic communication circuit: This includes the fiber-optic transmitting circuit inside the helicopter cabin and the receiving circuit located 60 meters away. Because the RLC resonant transmitting circuit is located on the coil and the control circuit is located inside the cabin, the transmission trigger signal is generated by the control circuit. Therefore, the fiber-optic communication circuit uses light to transmit signals, making it less susceptible to electromagnetic interference.

[0020] Logic control and time synchronization circuit: mainly composed of DSP28335 and GPS module. The GPS module provides the transmission time reference for DSP28335. DSP28335 completes the main algorithm operation, drive wave generation and communication functions, controls the output voltage of the DC / DC boost circuit; generates drive pulses for the RLC resonant transmission circuit according to the transmission timing; obtains the current transmission current data through the sampling circuit, and transmits the data to the controller DSP28335 through the communication module.

[0021] The following describes the transmission waveform process:

[0022] DC / DC boost circuit: First, Q1 and Q4 are turned on, Q2 and Q3 are turned off, and the DC 28V power supply V in The transformer T provides energy to the load. The secondary current starts from the same-name terminal of the transformer T and passes through the rectifier tube D. r1The current is divided into two parts, one part of the current passes through the rectifier capacitor C r1 C r1 Charge, make C r1 The voltage is equal to the secondary voltage of transformer T, and the other part is passed through the energy storage capacitor C o And the load circuit of the latter stage, the load circuit of the latter stage can be equivalent to a 100 ohm resistor, and the load current generated by the load current is applied to the capacitor C r2 Discharge is carried out through the capacitor C r2 Flows to the negative terminal of the transformer, and then turns off Q1. The direction of the secondary current of the transformer T remains unchanged. Since the voltage across the capacitor C1 cannot change suddenly, Q1 can achieve zero voltage shutdown. The inductor L r The current cannot change suddenly, the inductance L r The energy in the transformer will keep the primary current in its original direction, capacitor C1 starts to charge, and capacitor C3 starts to discharge. When the voltage across C3 drops to 0, D3 will turn on naturally, and the voltage across Q3 will be clamped to zero. At this moment, Q3 is turned on to achieve zero voltage turn-on. When L r The freewheeling ends, that is, the primary current of the transformer T drops to 0, and the primary winding cannot transfer energy to the secondary winding. At this time, the output capacitor C o Provide energy to the subsequent load circuit. Since the primary current is 0, the switch tube Q4 can achieve zero current shutdown, and finally turn on the switch tube Q2. The positive half cycle ends, and the working principle of the negative half cycle is similar to the upper half cycle.

[0023] Resonant capacitor charging circuit, RLC resonant transmitting circuit: First, the switch tubes Q6 and Q7 are turned off, and the switch tubes Q5 and Q8 charge the resonant capacitor C in the form of high-frequency chopping. i Forward charging, chopping frequency is 10kHz. After charging for 6ms, the switch tubes Q5 and Q8 are turned off and the circuit is in idle state. 2ms after charging is completed, the controller DSP28335 triggers the forward thyristor T1, the circuit resonates, and the resonant capacitor C i Energy is released, the inductor current rises, and after 2ms the parasitic inductance L w Current i Lcoil After reaching the forward peak, the current drops, and the thyristor current crosses zero and automatically turns off, and the positive pulse emission process ends. After waiting for 8ms, the switch tubes Q6 and Q7 charge the resonant capacitor in the form of high-frequency chopping, and the chopping frequency is 10KHz. After charging for 6ms, the switch tubes Q6 and Q7 are turned off, and the circuit is in an idle state. 2ms after the charging is completed, the reverse thyristor T2 is triggered, and the circuit resonates. After 2ms, the inductor current i Lcoil It reaches the negative peak, then the current drops, the thyristor T2 current passes through zero and automatically turns off, and the process of emitting negative pulses ends.

[0024] Afterwards, the transmitter repeats the above process back and forth, generating a periodic bipolar sinusoidal current pulse signal on the transmitting coil.

Claims

1. A power topology circuit for a deep-depth airborne electromagnetic detection transmission system, characterized by: It includes a topology main circuit and a control circuit. The topology main circuit is used for power conversion and pulse emission, and ultimately emits a half-sine current wave with a maximum peak value of 800A. The control circuit is used to complete algorithm calculations, generate and communicate the drive wave of the switch tube, and realize overall control. The topology main circuit includes a DC 28V power supply, a DC / DC boost circuit, a resonant capacitor charging circuit, and an RLC resonant transmitting circuit. The DC / DC boost circuit includes electronic switch tubes Q1-Q4, the resonant capacitor charging circuit includes electronic switch tubes Q5-Q8, and the RLC resonant transmitting circuit includes a resonant capacitor C i , forward thyristor T1, reverse thyristor T2; the DC 28V power supply in the main circuit of the topology is provided by the helicopter, which serves as the energy supply of the circuit and can input a maximum current of 100A to the DC / DC boost circuit; the DC / DC boost circuit adopts a full-bridge inverter circuit and a voltage doubler rectifier circuit, and uses a phase-shifted full-bridge soft switch control method. Its input is a DC 28V power supply and the final output is a 560V voltage. The output high voltage flows to the resonant capacitor charging circuit to compensate for the energy loss of the RLC resonant transmitting circuit in each transmitting cycle; the resonant capacitor charging circuit adopts a full-bridge inverter circuit and uses PWM modulation to control the resonant capacitor C i The charging direction and charging current peak of the DC / DC boost circuit and the resonant capacitor charging circuit are located in the helicopter cabin, and the RLC resonant transmitting circuit is located on the transmitting coil. The two are 60m apart and transmit electrical energy through the power transmission line. The RLC resonant transmitting circuit consists of a resonant capacitor C i The resonant capacitor C is controlled by triggering and shutting down the thyristors T1 and T2 in the RLC resonant transmitting circuit and the switches Q5, Q6, Q7 and Q8 in the resonant capacitor charging circuit at different time points. i The energy flow between the transmitter and the transmitting coil is finally modulated to generate a half-sine wave transmitting current with a maximum peak value of 800A.

2. The power topology circuit of a deep-depth airborne electromagnetic detection transmission system according to claim 1, characterized in that: The control circuit includes: logic control and time synchronization circuit, voltage and current sampling circuit, drive circuit, protection circuit, and optical fiber communication circuit; The logic control and time synchronization circuit is composed of DSP28335 and GPS module. The GPS module provides the transmission time reference for the controller DSP28335. The voltage and current sampling circuit is used to measure the resonant capacitor C i Peak current, energy storage capacitor C O The voltage is sampled and transmitted to the controller DSP28335 as a feedback signal, which then adjusts the control signal output by the controller DSP28335; The drive circuit includes an IGBT drive circuit and a thyristor drive circuit. The drive circuit generates a voltage and current signal that meets the drive power requirements through the drive signal sent by the controller DSP28335, and drives the switch tube to open and close according to the timing requirements. Specifically, the controller DSP28335 sends a control signal to the switch tubes Q1, Q2, Q3, and Q4 in the DC / DC boost circuit through the IGBT drive circuit to adjust the duty cycle for controlling the energy storage capacitor C O The controller DSP28335 sends on / off signals to the switches Q5, Q6, Q7, and Q8 in the resonant capacitor charging circuit through the IGBT drive circuit to adjust the duty cycle to control the resonant capacitor C i Charging current peak; the controller DSP28335 sends the on / off signal to the thyristors T1 and T2 in the RLC resonant transmitting circuit in accordance with the transmitting timing through the optical fiber communication circuit and the thyristor drive circuit; The protection circuit collects the sampling signals obtained by the voltage and current sampling circuit, compares the collected signals with the reference signals, and sends the comparison output signals to the controller DSP28335 to control the controller DSP28335 to output the driving signal; the optical fiber communication circuit includes an optical fiber transmitting circuit in the helicopter cabin and an optical fiber receiving circuit 60m away from the helicopter cabin, which is used to send the driving signal of the thyristor.

3. The power topology circuit of a deep-depth airborne electromagnetic detection transmission system according to claim 1, characterized in that: The DC / DC boost circuit includes: a full-bridge inverter circuit, a power transformer T, an inductor L r , voltage doubler rectifier circuit; wherein, the full-bridge inverter circuit includes electronic switch tubes Q1-Q4, freewheeling diodes D1-D4, and resonant capacitors C1-C4, and the voltage doubler rectifier circuit includes transformer secondary output high-frequency rectifier diode D r1 and D r2 , rectifier capacitor C r1 and C r2 , and the resonant voltage doubler circuit output capacitor C o , C o That is, energy storage capacitor; inductor L r Represents the equivalent inductance of the transformer primary leakage inductance and the external resonant inductance; the electronic switch tubes Q1-Q4 use IGBTs as switch tubes, among which two groups of switch tubes Q1 and Q3 form a bridge arm, and the switch tubes Q2 and Q4 form a bridge arm. The two bridge arms are connected in parallel, and the collectors of Q1 and Q2 are connected to the positive pole of the input power supply, and the emitters of Q3 and Q4 are connected to the negative pole of the input power supply; the freewheeling diodes D1-D4 are body diodes integrated in the electronic switch tubes Q1-Q4, and the anodes of the freewheeling diodes are connected to the emitters of the electronic switch tubes, and the cathodes of the freewheeling diodes are connected to the collectors of the electronic switch tubes; the resonant capacitors C1-C4 are respectively connected in parallel between the emitters and collectors of the electronic switch tubes Q1-Q4; the primary-to-secondary turns ratio of the power transformer T is 1:10, and the primary terminals of the transformer T are connected via the inductor L r Connected to the emitter of the switch tube Q1, the opposite end of the primary side of the transformer T is connected to the collector of the switch tube Q2; the energy storage capacitor C o One end is connected to a high-frequency rectifier diode D r1 The anode and rectifier capacitor C r1 , where the high frequency rectifier diode D r1 The anode of the transformer is connected to the secondary side of the same terminal, and the rectifier capacitor C r1 The other end is connected to the opposite end of the secondary side of the transformer T, and the energy storage capacitor C o The other end is connected to a high-frequency rectifier diode D r2 The anode and rectifier capacitor C r2 , where the high frequency rectifier diode D r2 The cathode of the transformer is connected to the secondary side of the same terminal, and the rectifier capacitor C r1 The other end is connected to the opposite-name terminal of the secondary side of the transformer T; the control circuit controls the conduction and cutoff of the switching devices Q1, Q3, Q2 and Q4, converting the DC power into an AC power with a frequency of 10kHz, and then passing through the voltage doubling rectifier circuit to double the voltage and convert it into DC power, ultimately making the output voltage of the DC / DC circuit reach about 560V, meeting its boost requirement.

4. The power topology circuit of a deep-depth airborne electromagnetic detection transmission system according to claim 3, characterized in that: The specific working process of the DC / DC boost circuit is as follows: First, Q1 and Q4 are in the on state, Q2 and Q3 are in the off state, and the DC 28V power supply V in The transformer T provides energy to the load. The secondary current starts from the same-name terminal of the transformer T and passes through the rectifier tube D. r1 The current is divided into two parts, one part of the current passes through the rectifier capacitor C r1 C r1 Charge, make C r1 The voltage is equal to the secondary voltage of transformer T, and the other part is passed through the energy storage capacitor C o And the load circuit of the latter stage, the load circuit of the latter stage can be equivalent to a 100 ohm resistor, and the load current generated by the load current is applied to the capacitor C r2 Discharge is carried out through the capacitor C r2 Flows to the negative terminal of the transformer, and then turns off Q1. The direction of the secondary current of transformer T remains unchanged. Since the voltage across capacitor C1 cannot change suddenly, when Q1 is turned off, the voltage across Q1 is clamped to 0V, so that Q1 can achieve zero voltage shutdown. The inductor L r The current cannot change suddenly, the inductor L r The energy in the transformer will keep the primary current in its original direction, capacitor C1 starts to charge, and capacitor C3 starts to discharge. When the voltage across C3 drops to 0, D3 will turn on naturally, and the voltage across Q3 will be clamped to zero. At this moment, Q3 is turned on to achieve zero voltage turn-on. When L r The freewheeling ends, that is, the primary current of the transformer T drops to 0, and the primary winding cannot transfer energy to the secondary winding. At this time, the output capacitor C o Provide energy to the subsequent load circuit. Since the primary current is 0, the switch tube Q4 can achieve zero current shutdown, and finally turn on the switch tube Q2. The positive half cycle ends, and the working principle of the negative half cycle is similar to the upper half cycle.

5. The power topology circuit of a deep-depth airborne electromagnetic detection transmission system according to claim 1, characterized in that: The resonant capacitor charging circuit includes: electronic switch tubes Q5-Q8, freewheeling diodes D5-D8, and the parasitic inductance L of the charging line w The two groups of switch tubes Q5 and Q7 form a bridge arm, and the switch tubes Q6 and Q8 form a bridge arm. The two bridge arms are connected in parallel, and the collectors of Q5 and Q6 are connected to the high-frequency rectifier diode D r1 The cathode of Q7 and Q8 are connected to the high-frequency rectifier diode D r2 The anode of the freewheeling diodes D5-D8 are respectively integrated between the emitter and the collector of the electronic switch tubes Q5-Q8, and the cathode of the freewheeling diodes D5-D8 is connected to the emitter.

6. The power topology circuit of a deep-depth airborne electromagnetic detection transmission system according to claim 1, characterized in that: The RLC resonant transmitting circuit includes: resonant capacitor C i , forward thyristor T1, reverse thyristor T2, coil inductance L coil , and the coil parasitic resistance R coil ; The forward thyristor T1 and the reverse thyristor T2 are connected in reverse parallel and then connected to the coil parasitic resistance R coil , coil inductance L coil , and the resonant capacitor C i The series connection forms a closed loop, the anode of the forward thyristor T1, that is, one end of the resonant capacitor Ci, passes through the parasitic inductance L of the charging line. w It is connected to the emitter of the electronic switch tube Q5, and the other end of the resonant capacitor Ci is connected to the collector of the electronic switch tube Q8.

7. The power topology circuit of the deep-depth airborne electromagnetic detection transmission system according to claim 6, characterized in that: The resonant capacitor charging circuit is used to charge the resonant capacitor C during the idle period of transmission. i To replenish electric energy, the thyristors in the RLC resonant transmitting circuit and the switches in the resonant capacitor charging circuit are triggered and turned off at different time points, and finally a half-sine wave transmitting current with a base frequency of 25Hz and a maximum peak of 800A is modulated; the specific working process of the resonant capacitor charging circuit and the RLC resonant transmitting circuit is as follows: first, the switches Q6 and Q7 are turned off, and the switches Q5 and Q8 are used to chop the resonant capacitor C in the form of high-frequency i Forward charging, the chopping frequency is 10kHz; after charging for 6ms, the switch tubes Q5 and Q8 are turned off, and the circuit is in an idle state; 2ms after charging is completed, the controller DSP28335 triggers the forward thyristor T1, the circuit resonates, and the resonant capacitor C i Energy is released, the inductor current rises, and after 2ms the parasitic inductance L w Current i Lcoil When the positive peak is reached, the current drops, and the positive thyristor T1 is automatically turned off when the current passes through zero, and the positive pulse emission process ends. After waiting for 8ms, the switch tubes Q6 and Q7 chop the resonant capacitor C in the form of high-frequency chopping. i Charging, the chopping frequency is 10KHz; after charging for 6ms, the switch tubes Q6 and Q7 are turned off and the circuit is in an idle state; 2ms after charging is completed, the reverse thyristor T2 is triggered and the circuit resonates. After 2ms, the inductor current i Lcoil It reaches the negative peak, then the current drops, the thyristor T2 current passes through zero and is automatically turned off, and the process of emitting the negative pulse ends.

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