Overcurrent Suppression Method and Device for Voltage Source Converter for Impact Load

Abstract

The present invention relates to an overcurrent suppression method and device for a voltage source converter facing impact loads. The method includes the following steps: when it is recognized that there is an overload current caused by an impact load, a trigger signal for the switch tube to lock is generated; the trigger signal is sampled, and a first voltage drop is calculated based on the number of high-level times captured; a second voltage drop is generated based on the effective value of the inductor current; the sum of the first voltage drop and the second voltage drop is used as the total voltage drop, and a modulation wave is generated based on the total voltage drop, and then a drive pulse signal for the switch tube is generated to suppress the overload current. Compared with the prior art, the present invention has the advantages of reliable overcurrent protection, effectively improving the speed of the current limiting action, and being able to stably output a constant current in response to impact loads, with good waveform quality.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronic control, and particularly to an overcurrent suppression method and device for a voltage source converter facing impact loads. Background Art

[0002] In modern power electronics, voltage source converters are increasingly widely used, especially in high-power power supply systems, such as uninterruptible power supply systems, distributed generation systems, etc. Therefore, the stable operation of the load side places high demands on the reliability of the converter when dealing with overload or short-circuit conditions. When an impact load is connected to the converter power supply system, both the response time and output stability of the converter overcurrent suppression strategy are closely related to the control effect.

[0003] In existing research, a double closed-loop control strategy based on output voltage and current or an independent current loop control strategy based on inductor current is often adopted. For example, Chinese Patent Application CN107612034A discloses a method for suppressing transient overcurrent of an energy storage converter, which samples the inductor current of the energy storage converter and realizes the suppression of transient overcurrent of the energy storage converter under grid fault conditions through converter port voltage feedforward and variable virtual impedance methods. Although the above two control strategies have certain effects in limiting overload current, it is difficult to ensure the output stability of the converter with the output current waveform. At the same time, meeting the fast overcurrent suppression and output stability poses new challenges to the existing overcurrent suppression technologies. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above-mentioned defects existing in the prior art and provide an overcurrent suppression method and device for a voltage source converter facing impact loads, which can achieve reliable overcurrent protection and effectively improve the speed of current limiting action.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] An overcurrent suppression method for a voltage source converter facing impact loads includes the following steps:

[0007] When an overload current caused by an impact load is identified, a trigger signal for the switch tube to lock is generated;

[0008] Sample the trigger signal, and calculate the first voltage drop based on the number of high-level times captured;

[0009] Generate a second voltage drop based on the effective value of the inductor current;

[0010] Take the sum of the first voltage drop and the second voltage drop as the total voltage drop, generate a modulation wave based on the total voltage drop, and then generate a drive pulse signal for the switch tube to suppress the overload current.

[0011] Further, identifying the overload current specifically includes:

[0012] Obtain the inductor current , and determine the inductor current Whether it satisfies , if so, it is identified that there is an overload current, is the overcurrent protection threshold current.

[0013] Further, the calculation formula for the first voltage drop is:

[0014]

[0015] In the formula, is the first voltage drop, is the adjustment step size, is the number of high-level times.

[0016] Further, the adjustment step size is determined according to the output-side reference voltage.

[0017] Further, based on the effective value of the inductor current, the second voltage drop is generated through a PI controller.

[0018] Further, the total voltage drop is limited between 0 - v p , v p is the rated output voltage.

[0019] Further, the method further includes:

[0020] Continuously sample the trigger signal, record the number of sampling periods when the trigger signal is continuously at a low level, and determine that the number of sampling periods reaches a preset upper limit. If so, generate a voltage recovery signal, control the first voltage drop to drop to 0, and at the same time control the second voltage drop to increase synchronously to keep the total voltage drop unchanged, thereby realizing constant current control.

[0021] Further, when controlling the first voltage drop to drop, the voltage drop of the first voltage drop is: , where K re is the voltage recovery step size, is the voltage recovery duration, is the first voltage drop.

[0022] Further, the method further includes:

[0023] When the impact load starts to return to the normal operating state, the second voltage drop gradually decreases until it is limited to 0.

[0024] Further, generating a modulation wave based on the total voltage drop is specifically as follows:

[0025] Subtract the total voltage drop from the rated output voltage and perform normalization to obtain the amplitude of the sine modulation wave, and generate the modulation wave.

[0026] The present invention also provides an overcurrent suppression device for a voltage source converter facing impact loads, including:

[0027] A fast voltage suppression loop, configured to generate a trigger signal for the switching tube to lock when an overload current caused by an impact load is identified, sample the trigger signal, and calculate a first voltage drop based on the number of high-level times captured;

[0028] An effective current loop, configured to generate a second voltage drop based on the effective value of the inductor current;

[0029] A constant voltage loop, configured to sum the first voltage drop and the second voltage drop as the total voltage drop, generate a modulation wave based on the total voltage drop, and then generate a drive pulse signal for the switching tube to suppress the overload current.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. When an impact load is connected to the converter power supply system, the present invention can quickly identify the overload current, and generate the total voltage drop based on the trigger signal of the switching tube locking action and the effective value of the inductor current, which can cope with different operating conditions to accurately output the voltage drop, realize reliable overcurrent protection, and effectively improve the speed of the current limiting action.

[0032] 2. The present invention generates the total voltage drop based on the trigger signal of the switching tube locking action and the effective value of the inductor current at the same time, performs closed-loop control on the output current, improves the stability of the current when dealing with impact loads, and has a low harmonic content, avoiding mis-triggering of the overcurrent protection action, and further optimizing the waveform quality of the output current.

[0033] 3. When the impact load starts to return to the normal operating state, the second voltage drop gradually decreases until it is limited to 0, and the output voltage continuously rises until it completely returns to the rated value, which can realize the recovery of the output voltage under the condition of maintaining stable power supply, and effectively improve the reliability of the converter power supply system. Description of the Drawings

[0034] Figure 1 It is an example of the control block diagram of the overcurrent suppression method for a voltage source converter in an embodiment of the present invention;

[0035] Figure 2It is the topological structure diagram of the T-type three-level inverter adopted in the embodiment of the present invention;

[0036] Figure 3 It is the flow chart of the overcurrent suppression method for the voltage source converter in the embodiment of the present invention;

[0037] Figure 4 It is the waveforms of the phase-a output voltage and the three-phase output current in different control stages in the embodiment of the present invention;

[0038] Figure 5 It is the THD analysis result of the phase-a output voltage in the constant current control stage in the embodiment of the present invention. Specific implementation manners

[0039] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and the detailed implementation manners and specific operation processes are given, but the protection scope of the present invention is not limited to the following embodiments.

[0040] Embodiment 1

[0041] This embodiment provides an overcurrent suppression method for a voltage source converter for impact loads. When dealing with impact loads, this method can quickly suppress the overload current through drive locking, and stabilize the output of a constant current by limiting the rated value of the output voltage, which can improve the speed of the current limiting action while reliably achieving overcurrent suppression.

[0042] The voltage source converter applied in this embodiment is a T-type three-level inverter, and its topology is as Figure 2 shown. Its DC bus capacitor is connected to the inverter, and the AC side is connected to the motor load through an LC filter. In this embodiment, the inverter adopts a T-type neutral point clamped topology structure, including 12 switching tubes, 2 DC bus capacitors and an LC filter.

[0043] The above overcurrent suppression method controls the output voltage and current through the amount of output voltage drop. During the overall control process, the system operating conditions can be divided into three stages according to the circuit output and the load operating state: the buck control stage, the constant current control stage, and the load startup stage. Refer to Figure 2 and Figure 3 shown. The overcurrent suppression method includes the following steps:

[0044] Step S1, set the threshold current for overcurrent protection , after the impact load is applied, enter the buck control stage, detect the inductor current based on the threshold current, identify whether there is an overload current caused by the impact load, and if so, output the trigger signal for EMB (emergency break) braking, that is, the trigger signal for the switching tube locking action:

[0045]

[0046] Among them, i Lx is the sampled inductor current. When the EMB trigger signal outputs a high level, the IGBT is blocked, and the inductor current i Lx instantaneous value is limited within a safe range.

[0047] In this embodiment, the judgment of the overload current is realized through a hardware comparison circuit. In this embodiment, the output voltage is quickly suppressed according to the state of the EMB trigger signal. When the MCU captures the high level of the EMB trigger signal, the first voltage drop is calculated according to the number of captured high levels, that is, the calculation formula of the first voltage drop is:

[0048]

[0049] In the formula, is the first voltage drop, is the adjustment step, is the number of high levels. The setting of .

[0050] Step S2: Calculate the effective value of the inductor current I rms , generate the second voltage drop v 2. In this embodiment, it is realized through a PI controller, that is, the effective value of the inductor current I rms is subtracted from the reference value I ref of the current effective value loop to obtain the error err (k), and then sent to the PI controller to obtain the output v 2:

[0051]

[0052] Furthermore, calculate the total voltage drop v 3 in the buck control stage:

[0053]

[0054] Based on the above total voltage drop, a modulation wave is generated, and then a drive pulse signal of the switching tube is generated to suppress the overload current. Through the first voltage drop and the second voltage drop v The superposition of 2 can make up for the lack of control accuracy based on a single voltage control signal and improve control reliability.

[0055] It should be noted that in order to restore the output voltage to the rated value when the load starts, a limiting link is set to v 3 is limited between 0 - v p and v p is the rated output voltage.

[0056] Step S3: Set the output voltage recovery link and enter the constant current control stage. The purpose is: after realizing current protection through rapid voltage reduction, and when the load returns to the normal working condition, restore the output voltage to the rated value. Continuously sample the trigger signal, record the number of sampling periods when the trigger signal is continuously at a low level, and judge whether the number of sampling periods reaches the preset upper limit. If so, generate a voltage recovery signal, control the first voltage drop to drop to 0, and at the same time control the second voltage drop to increase to keep the total voltage drop unchanged, so as to realize constant current control.

[0057] Specifically, the voltage recovery signal is determined by the state of the EMB trigger signal:

[0058] Store the number of sampling periods when the EMB trigger signal is continuously at a low level in K sum : Judge the EMB trigger signal in each sampling period. If the trigger signal is detected to be at a low level, update K sum data:

[0059]

[0060] Among them, is the value stored in the next sampling period. Compare the value of K sum with its upper limit K max to generate the value of the voltage recovery signal:

[0061]

[0062] It should be noted that the upper limit K sum of the above K max is set depending on the opening time of the output voltage recovery link. In this embodiment, it is set to 100k sampling periods and needs to be flexibly adjusted according to specific working conditions in different embodiments.

[0063] When the voltage recovery signal outputs a high level, the overcurrent suppression strategy enters the constant current control stage. At this time, the first voltage drop is: , where K re is the step size of voltage recovery, is the duration of voltage recovery. The first voltage drop in the constant current control stage gradually decreases to 0; the second voltage drop increases synchronously to maintain the total voltage drop unchanged, realizing constant current control.

[0064] Step S4: When the impact load starts to return to the normal operating state, it enters the load startup stage, and the second voltage drop gradually decreases until it is limited to 0.

[0065] In the load startup stage, the impact load starts to return to the normal operating state, and the control enters the voltage recovery mode. At this time, the total voltage drop output by the control system is:

[0066]

[0067] As the load speed increases, the back electromotive force rises, and the output current continuously decreases, further causing the total voltage drop to continuously decrease until it is limited to 0 by the limiter; at the same time, the output voltage continuously rises until it completely recovers to the rated value, completing the startup.

[0068] Step S5: Input the total voltage drop v 3 obtained in the above steps S2, S3 or S4 into the SPWM modulation wave generator as an input quantity to generate a modulation wave. The specific steps are as follows:

[0069] Subtract the total voltage drop v p from the rated output voltage v 3 and perform normalization processing to obtain the modulation wave amplitude K s =( v p - v 3) / v p , and generate a sine modulation wave.

[0070] Step S6: Intercept the above modulation wave with a double triangular carrier wave to generate a drive pulse signal for the switching tube through comparison.

[0071] The above control strategy can quickly limit the overload current to a safe range after the impact load is applied. While ensuring the rapidity of the current limiting action and the good waveform quality of the output current, it has the ability to start the load. After the impact load returns to the normal operating state, it quickly restores the output voltage to the rated value to realize the load startup.

[0072] To verify the effectiveness of the above control method, a specific embodiment is given in this embodiment, and its main parameters are as follows:

[0073] DC voltage on the input side of the converter V;

[0074] Rated voltage (rms) on the output side of the converter V;

[0075] Switching frequency kHz;

[0076] Setting of the impact load power on the output side of the converter kW;

[0077] Setting of the power under normal operating conditions of the load on the output side of the converter kW;

[0078] Setting of the current protection threshold A;

[0079] Step-down step of the fast voltage suppression loop V;

[0080] Flag bit of the voltage recovery link ;

[0081] Voltage recovery coefficient of the fast voltage suppression loop ;

[0082] Reference value of the rms current loop A;

[0083] Proportional coefficient of the rms current loop ;

[0084] Integral coefficient of the rms current loop .

[0085] Output voltage of phase a on the output side and three-phase output current , , in different stages of the control strategy are as shown in Figure 4 . The THD analysis results of the output voltage of phase a are as shown in Figure 5 .

[0086] From Figure 4It can be seen that in the initial state, the load connected to the converter power supply system operates under normal conditions. At 0.2 seconds, an impact load is switched in, and the system enters the step-down control stage. The system detects the overload current and quickly suppresses it, and completes the overcurrent suppression action within half a fundamental wave period. After completing the overcurrent suppression action, the system enters the constant current control stage, where the inductor current is stably controlled at the set rated value and gradually transitions to the independent current effective value loop control. At 1 second, the impact load resumes normal operation, and the system enters the load startup stage. The output voltage fully recovers to the rated value within one fundamental wave period, reliably completing the overload suppression and load startup processes, thus verifying the effectiveness of the control method of the present invention. Figure 5 This is the harmonic analysis result of the a-phase output voltage of this embodiment. It can be seen from the figure that in the constant current control stage, the total harmonic distortion (THD) of its harmonic content is 2.94%, effectively proving the waveform quality under the overcurrent suppression strategy control.

[0087] Embodiment 2

[0088] This embodiment provides an overcurrent suppression device for a voltage source converter facing impact loads, as Figure 1 shown, including:

[0089] A fast voltage suppression loop, which is used to generate a trigger signal for the switching tube locking action when an overload current caused by an impact load is identified, sample the trigger signal, and calculate the first voltage drop based on the number of high-level times captured. This fast voltage suppression loop includes a hardware comparison circuit and an EMB (emergency break) control circuit;

[0090] A current effective value loop, which is used to generate a second voltage drop based on the inductor current effective value;

[0091] A constant voltage loop, which is used to sum the first voltage drop and the second voltage drop as the total voltage drop, generate a modulation wave based on the total voltage drop, and then generate a drive pulse signal for the switching tube to suppress the overload current.

[0092] Limiters are provided in the above-mentioned fast voltage suppression loop, current effective value loop and constant voltage loop to limit the output between 0 - v p .

[0093] The overcurrent suppression process based on the above overcurrent suppression device is as Figure 3 shown, including:

[0094] Step S1. In the overall control process, both the fast voltage suppression loop and the current effective value loop control the output voltage and current by outputting the voltage drop. According to the output of the control loop and the operating state of the load, the system operating conditions are divided into three stages: step-down control stage, constant current control stage, and load startup stage.

[0095] Set the threshold current for overcurrent protection , after applying an impact load, enter the step-down control stage, use a hardware comparison circuit to detect the inductor current, and output a trigger signal for EMB braking.

[0096] In this embodiment, the output voltage is quickly suppressed according to the state of the EMB trigger signal. When the MCU captures the high level of the EMB trigger signal, it will immediately adjust the output of the voltage loop and control the output of the voltage loop in the next switching cycle to:[[]]

[0097]

[0098] where v 1(k) is the output of the current voltage suppression loop,[[]] v 1(k + 1) is the output of the voltage suppression loop in the next cycle,[[]] is the step size for each adjustment.

[0099] Step S2: Calculate the effective value of the inductor current I rms , subtract it from the reference value of the current effective value loop I ref to obtain the error err (k), and then send it to the PI controller to obtain the output v 2 of the current effective value loop:[[]]

[0100]

[0101] Calculate the total voltage drop in the step-down control stage according to the total number of braking triggers m v 3:[[]]

[0102]

[0103] Note that to restore the output voltage to the rated value during load startup, a limiter is set to limit v 3 between 0 - v p , v p which is determined by the output of the constant voltage loop.

[0104] Step S3: Collect the voltage recovery signal . When the voltage recovery signal outputs a high level, the overcurrent suppression strategy enters the constant current control stage. At this time, the voltage drop output by the voltage suppression loop is:[[]] , where K re is the step size of voltage recovery,[[]] is the duration of voltage recovery. The output of the voltage suppression loop in the constant current control stage gradually decreases to 0; the error input of the PI loop in the effective current value loop increases, and the output increases synchronously, so as to maintain the unchanged total voltage drop and achieve constant current control.

[0105] Step S4: To ensure that the inverter power supply system can meet the requirement that the output voltage recovers to the rated value during load startup, there are two operating modes for different loads: 1. Voltage suppression mode: For impact loads, the control strategy accurately outputs the voltage drop , and maintains constant current operation; 2. Voltage recovery mode: For loads under normal operating conditions, the voltage loop is fully closed, and the control system is controlled through the effective value loop of the independent inductor current. As the load returns to the normal operating condition, the output of the effective current value loop gradually decreases and is finally limited to 0 by the limiting link, so that the output voltage gradually recovers to the rated value.

[0106] During the load startup stage, the impact load starts to return to the normal operating state, and the control enters the voltage recovery mode. At this time, the total voltage drop output by the control system is:

[0107]

[0108] As the load speed increases, the back electromotive force rises, the output current continuously decreases, further causing the total voltage drop to continuously decrease until it is limited to 0 by the limiter; at the same time, the output voltage continuously rises until it completely recovers to the rated value, completing the startup.

[0109] At the same time, as Figure 1 shown, in this embodiment, a constant voltage loop control is set, and its purpose is: after the load enters the normal operating condition, the effective value of the output voltage is closed-loop controlled to avoid the output voltage fluctuation caused by the internal resistance of the device.

[0110] Step S5: Take the total voltage drop v 3 as the input quantity and input it into the SPWM modulation wave generator to generate a modulation wave. The specific steps are as follows:

[0111] Subtract the total voltage drop v p from the rated output voltage v 3, and perform normalization processing to obtain the modulation wave amplitude K s =( v p - v 3) / v p , and generate a sine modulation wave.

[0112] Step S6: Intersect the above modulation wave with the double triangular carrier waves to generate the drive pulse signals of the switching tubes through comparison.

[0113] The rest is the same as in Embodiment 1.

[0114] It should be noted that the above overcurrent suppression method can be used for voltage source converters, including but not limited to being applied in the Figure 2 topological structure shown; and the EMB control is derived from the switch tube drive locking function of the controller, including but not limited to the controller adopted in this embodiment.

[0115] The preferred specific embodiments of the present invention have been described in detail above. It should be understood that those of ordinary skill in the art can make many modifications and variations based on the concept of the present invention without creative labor. Therefore, all technical solutions that can be obtained by those skilled in the art in the technical field of the present invention through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the protection scope determined by the claims.

Claims

1. A method for overcurrent suppression of a voltage source converter facing impact loads, characterized in that It includes the following steps: When it is recognized that there is an overload current caused by an impact load, a trigger signal for the switch tube to lock is generated; Sample the trigger signal, and calculate the first voltage drop based on the number of high-level times captured; Generate a second voltage drop based on the effective value of the inductor current; Take the sum of the first voltage drop and the second voltage drop as the total voltage drop, generate a modulation wave based on the total voltage drop, and then generate a drive pulse signal for the switch tube to suppress the overload current; The calculation formula for the first voltage drop is: ， Wherein, is the first voltage drop, is the adjustment step, is the number of high-level times, and the adjustment step is determined according to the reference voltage on the output side.

2. The overcurrent suppression method for a voltage source converter facing impact loads according to claim 1, characterized in that, Identifying the overload current specifically is: Obtain the inductor current , and judge the inductor current whether it meets . If so, it is recognized that there is an overload current, which is the overcurrent protection threshold current.

3. The overcurrent suppression method for a voltage source converter facing impact loads according to claim 1, characterized in that Generate the second voltage drop through a PI controller based on the effective value of the inductor current.

4. The overcurrent suppression method for a voltage source converter facing impact loads according to claim 1, characterized in that The total voltage drop is limited to 0 - v p between v p which is the rated output voltage.

5. The overcurrent suppression method for a voltage source converter facing impact loads according to claim 1, characterized in that This method further includes: Continuously sample the trigger signal, record the number of sampling periods when the trigger signal is continuously at a low level, determine whether the number of sampling periods reaches a preset upper limit. If so, generate a voltage recovery signal, control the first voltage drop to drop to 0, and at the same time control the second voltage drop to increase synchronously to keep the total voltage drop unchanged, so as to achieve constant current control.

6. The overcurrent suppression method for a voltage source converter facing impact loads according to claim 5, characterized in that When controlling the decrease of the first voltage drop amount, the voltage drop amount of the first voltage drop amount is: , where K re is the voltage recovery step size, is the voltage recovery duration, is the first voltage drop amount.

7. The overcurrent suppression method for a voltage source converter facing impact loads according to claim 1, characterized in that This method further includes: When the impact load starts to return to the normal operating state, the second voltage drop gradually decreases until it is limited to 0.

8. The overcurrent suppression method for a voltage source converter facing impact loads according to claim 1, wherein Generating a modulation wave based on the total voltage drop specifically is: Subtract the total voltage drop from the rated output voltage and perform normalization processing to obtain the amplitude of the sine modulation wave, and generate a modulation wave.

9. A device for suppressing overcurrent of a voltage source converter facing impact loads, characterized in that, It includes: A fast voltage suppression loop, which is used to generate a trigger signal for the switch tube to lock when it is recognized that there is an overload current caused by an impact load, sample the trigger signal, and calculate the first voltage drop based on the number of high-level times captured; An effective current loop, which is used to generate a second voltage drop based on the effective value of the inductor current; A constant voltage loop, which is used to sum the first voltage drop and the second voltage drop as the total voltage drop, generate a modulation wave based on the total voltage drop, and then generate a drive pulse signal for the switch tube to suppress the overload current; The calculation formula for the first voltage drop is: ， In the formula, is the first voltage drop, is the adjustment step, is the number of high-level times, and the adjustment step is determined according to the reference voltage on the output side.

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