Zero-cost active discharge method for power electronic converter capacitor

Abstract

The invention discloses a zero-cost active discharge method for a power electronic converter capacitor. According to the method, the maximum available switching frequency is solved by introducing the junction temperature estimation model under the fixed duty ratio and controlling the junction temperature of the switching tube in the safe working area as a starting point, so that the discharge speed and the equipment safety are ensured. On the premise of not increasing any hardware circuit, the energy of the direct-current support capacitor is rapidly and controllably released by innovatively utilizing an original switching device, a topological structure and a control strategy of the system, and on the premise of not increasing any hardware cost, the capacitor discharge efficiency and the system safety are remarkably improved; and the defects of the traditional capacitor discharge technology are effectively overcome.

Description

Technical Field

[0001] The present invention relates to an active discharge control method for capacitors of a power electronic converter, belonging to the technical field of power electronics. Background Art

[0002] In a power electronic converter, the DC bus capacitor, as a key energy storage component, is mainly used to filter out voltage fluctuations, buffer instantaneous energy spikes, and provide a stable power supply. However, when the device is shut down, fails, or is maintained, if the electrical energy stored in the DC support capacitor cannot be quickly released, it will cause the voltage across the capacitor to remain at a high level for a long time, which may lead to electric shock hazards, damage to the device, or hinder safe operation. Therefore, the rapid and reliable discharge of the capacitor is an important part of the safety design of the power electronic system.

[0003] Currently, the conventional methods for capacitor discharge can be divided into two categories: passive discharge and active discharge.

[0004] Passive discharge usually realizes the release of electrical energy by connecting a fixed discharge resistor in parallel across the capacitor and using the natural energy consumption of the resistor. Although this method has a simple structure, it has significant defects: the continuous connection of the discharge resistor to the circuit will increase the standby loss of the system. Especially in low-power or high-voltage applications, it will cause a large amount of energy waste and increase the additional heat generation of the device. In addition, the volume and cost of the discharge resistor are difficult to ignore in compact or low-cost devices.

[0005] Active discharge schemes achieve on-demand discharge through control strategies or additional circuits (such as dedicated discharge circuits, combinations of switching tubes and resistors, etc.). Although such methods can reduce standby losses, they require the introduction of additional power devices (relays, IGBT modules, etc.), detection circuits, or isolation components, resulting in an increase in hardware costs, an increase in system complexity, and potential reliability risks.

[0006] In the active discharge scheme, by adjusting the control strategy to trigger the switching tube pulse, the energy on the DC capacitor is consumed through the existing circuit, without the need to increase additional hardware costs. It is a DC capacitor discharge method with great engineering application value.

[0007] However, there are also certain problems with the active discharge method of triggering the switching tube pulse by adjusting the control strategy. When the switching frequency is too high, it is easy to cause a sharp increase in the switching tube loss, the junction temperature of the switching tube exceeds the safety threshold, and device failure occurs. When the switching frequency is too low, the equivalent impedance of the discharge circuit will increase, and the discharge speed will drop significantly. Summary of the Invention

[0008] The object of the present invention is to realize the rapid and controllable release of the energy of the DC support capacitor by using the original switching devices and topological structure of the system without adding any hardware circuits, and solve the defects of traditional capacitor discharge technologies.

[0009] To achieve the above object, the technical solution of the present invention discloses a zero-cost active discharge method for the capacitor of a power electronic converter, which is characterized by including the following steps:

[0010] Step S1: Offline calculation of the duty cycle of the switching tube

[0011] According to the device parameters and the maximum discharge time that can be accepted, calculate the duty cycle of the switching tube, and draw a corresponding table of the initial DC capacitor discharge voltage and the duty cycle, which is embedded in the control system;

[0012] Step S2: Trigger the active discharge command

[0013] When detecting the shutdown instruction, emergency fault signal or abnormal DC bus voltage of the power electronic converter, the main controller starts the active discharge process;

[0014] Step S3: Cut off the external electrical connection to completely isolate the power electronic converter from the power grid, DC power supply and load;

[0015] Step S4: Calculate the maximum switching frequency

[0016] Select a fixed duty cycle D according to the corresponding table of the initial DC capacitor discharge voltage and the duty cycle embedded in Step S1, and calculate the maximum switching frequency f of the PWM according to the detected ambient temperature T a and the technical parameters of the switching tube; sw_max ;

[0017] Step S5: Construct a discharge energy loop

[0018] According to the duty cycle D and the modulation signal, send a switching instruction to the switching tube by calculating the PWM pulse signal at the maximum switching frequency f sw_max ;

[0019] Step S6: Feedback and adjustment

[0020] Continuously sample the DC voltage, discharge current and switching tube junction temperature of the discharge loop: if the switching tube junction temperature rises too fast, appropriately reduce the maximum switching frequency f sw_max ; if the discharge current is greater than the upper limit of the discharge current, jump to Step S4 to recalculate the duty cycle D; if the DC capacitor voltage U dc <U safe , then jump to Step S7; if the switching tube junction temperature is greater than the protection value, then jump to Step S7;

[0021] Step S7: Discharge completion feedback

[0022] Lock the PWM pulses of all bridge arms, the system outputs a "discharge completed" or "fault alarm" status signal, and enters the standby mode.

[0023] Preferably, in step S1, the duty cycle is taken between 40% and 80%.

[0024] Preferably, in step S1, the duty cycle of the switching tube is calculated by the following formula:

[0025]

[0026] In the formula, C dc is the capacitance value of the DC capacitor, U dc (0) is the voltage at the starting moment of the DC capacitor discharge, U safe is the safe voltage of the DC capacitor, t discharge is the discharge time, I discharge [[ID=ID=19]]is the discharge current, R eq is the equivalent resistance of the circuit.

[0027] Preferably, in step S4, the maximum switching frequency f sw_max of the PWM is calculated by the following formula:

[0028]

[0029] In the formula, T j_max is the upper limit value of the switching tube junction temperature, ΔT safe is the safe threshold of the switching tube junction temperature, R th_jc is the thermal resistance of the switching tube, E on [[ID=ID=40]]、E off is the switching energy of the switching tube, V ce is the conduction voltage drop of the switching tube.

[0030] Preferably, in step S5, the modulation signal adopts an AC sine signal.

[0031] By introducing a junction temperature prediction model under a fixed duty cycle, starting from controlling the switching tube junction temperature within the safe operating area, the maximum available switching frequency is obtained, thus ensuring both the discharge speed and the equipment safety. Without adding any hardware circuits, the present invention innovatively utilizes the original switching devices, topology structure, and control strategy of the system to achieve rapid and controllable release of the energy of the DC support capacitor. Without increasing any hardware costs, the capacitance discharge efficiency and system safety are significantly improved, effectively solving the defects of traditional capacitance discharge technology. Compared with the existing technical solutions, the specific beneficial effects are as follows:

[0032] It completely relies on the existing switching devices (such as IGBT, MOSFET, IGCT, etc.) and controller resources of the converter, without the need to add a discharge resistor, relay, or dedicated discharge circuit, avoiding both the hardware cost and volume problems of traditional active discharge solutions and eliminating the continuous energy loss of passive discharge. It is particularly suitable for cost-sensitive or space-constrained application scenarios (such as energy storage converters, static var generators, photovoltaic inverters, etc.).

[0033] By optimizing the control algorithm, the high-frequency modulation mode of the switching device is actively triggered during system shutdown or failure, a low-impedance discharge loop is constructed, and the rapid directional release of capacitor energy is achieved. The discharge speed can be flexibly adjusted according to safety standards (for example, reducing the DC capacitor voltage from 1500V to below 60V within 15 seconds), which is significantly better than the slow decay characteristic of passive discharge, effectively reducing the electric shock risk and potential equipment maintenance hazards.

[0034] The present invention is applicable to a variety of mainstream power electronic topologies (such as two-level, three-level, multi-level inverters, etc.), and has no special requirements for the load type (resistive, inductive or capacitive loads are all compatible). Only software upgrade is required to implement the function embedding, without changing the hardware circuit or sensor configuration, greatly reducing the implementation difficulty and promotion cost.

[0035] The discharge process is automatically triggered by software logic and system operating status (such as fault signal, shutdown instruction), without complex mode switching control or additional isolation protection circuit. During discharge, the controller monitors the capacitor voltage in real time to ensure that it automatically enters the safety lock state after discharge is completed, avoiding interference with the main circuit or load and ensuring the overall stability of the system.

[0036] Compared with the passive discharge scheme, the present invention completely eliminates the standby loss and heating problem of the discharge resistor, improving the system energy efficiency; compared with the traditional active discharge scheme, since no new power devices are required, potential fault points such as component aging and contact failure are reduced, extending the service life of the equipment, meeting the requirements of green and low-carbon and high-reliability design.

[0037] In summary, the present invention realizes the efficient and controllable release of capacitor energy with zero hardware cost, combining economy, safety and universality, providing an innovative solution for the safety design of power electronic equipment and having significant engineering application value. Description of the Drawings [[ID=I8]]

[0038] Figure 1 Schematically shows the typical topology of a power electronic converter with a voltage-type two-level LCL filter topology;

[0039] Figure 2 Schematically shows the instantaneous discharge loop of the two-level LCL topology;

[0040] Figure 3 Is the active discharge control flow chart. Detailed Implementation Manner

[0041] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0042] I. Typical Topologies of Power Electronic Converters

[0043] Taking the topology of a power electronic converter with a voltage-source two-level LCL filter topology as shown in Figure 1 as an example, it includes the following parts:

[0044] AC switch: used to connect / disconnect the power grid or load;

[0045] Filter: suppress high-frequency harmonics, and the more common ones are LC-type and LCL-type filters;

[0046] Converter module: a two-level, three-level, or multi-level topology circuit composed of switching devices such as IGBTs, MOSFETs, and IGCTs;

[0047] DC support capacitor: stabilize the DC bus voltage;

[0048] DC switch: control the on / off of the DC bus and the external power supply.

[0049] The method disclosed in the present invention can form different devices such as a power conversion system (PCS), a static var generator (SVG), a variable-frequency drive (VFD), a PV inverter, and a wind power converter for different application scenarios by adjusting the topology structure, parameters, and control strategy.

[0050] The method disclosed in the present invention does not require additional hardware during the discharge process, and actively discharges the DC support capacitor by fully utilizing the original switching devices of the converter and its own topology loop. The discharge control logic is integrated into the device main control program and implemented through software. Taking the power electronic converter with a voltage-source two-level LCL filter topology as an example, the instantaneous discharge loop is shown in Figure 2 .

[0051] II. Junction Temperature Prediction Model at Fixed Duty Cycle

[0052] 1. Relationship between Discharge Time and PWM Duty Cycle

[0053] The capacitance energy release equation of the discharge loop is:

[0054]

[0055] C eq —— The line equivalent capacitance, including the DC capacitance C dc and the AC filter capacitance. The AC filter capacitance is small and can be ignored;

[0056] U dc (0)—— The voltage at the starting moment of the DC capacitance discharge;

[0057] U safe —— The safe voltage of the DC capacitance, that is, the voltage at the completion of DC discharge;

[0058] i(t)—— The discharge current;

[0059] R eq —— The line equivalent resistance, including the inductor internal resistance, the switch tube conduction resistance, and the line resistance;

[0060] D(t)—— The duty cycle of the switch tube.

[0061] In the constant current mode of the discharge process, the duty cycle D(t) is fixed at D, then the discharge time t discharge can be simplified as:

[0062]

[0063] Among them:

[0064] 2. Relationship between the switch tube junction temperature and the switching frequency

[0065] The conduction loss equation is:

[0066]

[0067] P cond —— The conduction loss of the switch tube;

[0068] V ce —— The conduction voltage drop of the switch tube, obtained from the manual;

[0069] R on —— The conduction resistance of the switch tube, obtained from the manual.

[0070] The switching loss equation is:

[0071] P sw = f sw ×(E on + E off ) Equation (5)

[0072] P sw —— The switching loss of the switch tube

[0073] fsw —— Switching frequency

[0074] E on 、E off —— The switching energy of the switch tube, obtained by looking up the table through the switching energy - current curve in the manual or by fitting.

[0075] The steady - state junction temperature equation of the switch tube is:

[0076] T j =T a +(P cond +P sw )×R th_jc Equation (6)

[0077] T j —— The junction temperature of the switch tube;

[0078] T a —— The ambient temperature;

[0079] R th_jc —— The thermal resistance of the switch tube.

[0080] According to equations (4), (5), and (6), we have:

[0081]

[0082] It can be obtained that:

[0083]

[0084] f sw_max —— The maximum switching frequency of the switch tube;

[0085] T j_max —— The upper limit value of the junction temperature of the switch tube, according to the switch tube manual;

[0086] ΔT safe —— The safety threshold of the junction temperature of the switch tube, which can be selected between 10 - 15 °C.

[0087] III. Operating steps, as Figure 3 shown, include:

[0088] Step S1: Offline calculation of the duty cycle of the switch tube

[0089] According to the device parameters and the maximum discharge time that can be accepted, calculate the duty cycle D of the switch tube according to equations (2) and (3), draw the corresponding table of the initial DC capacitor discharge voltage and the duty cycle, and embed it in the control system. In the embodiment of the present invention, the duty cycle D is preferably taken between 40% and 80%.

[0090] Step S2: Trigger the active discharge command

[0091] When a shutdown instruction, an emergency fault signal, or abnormal DC bus voltage of the power electronic converter is detected, the main controller starts the active discharge process.

[0092] Step S3: Cut off the external electrical connection

[0093] Control the AC switch (K1) to cut off (which may have been cut off due to a shutdown instruction or a fault signal trigger) and confirm the contact feedback signal (if any). Control the DC switch (K2) to cut off (which may have been cut off due to a shutdown instruction or a fault signal trigger) and confirm the contact feedback signal (if any), so that the converter is completely isolated from the power grid, DC power supply, and load.

[0094] Step S4: Calculate the maximum switching frequency

[0095] Select a fixed duty cycle D according to the initial DC capacitor discharge voltage and duty cycle correspondence table embedded in Step S1, and according to the detected ambient temperature T a and the switching device technical parameters, use Equation (8) to calculate the PWM maximum switching frequency f sw_max .

[0096] Step S5: Construct a discharge energy loop

[0097] According to the duty cycle D and the modulation signal, send switching instructions to the switching devices according to the PWM pulse signal at the maximum switching frequency f sw_max The modulation signal can be an AC sine signal.

[0098] Step S6: Feedback and adjustment

[0099] Continuously sample the DC voltage, discharge current, and switching device junction temperature of the discharge loop. Usually, sampling is available in the original control system. If the switching device junction temperature rises too fast (temperature rise rate > (T j_max -ΔT safe- T a ) / t discharge ), appropriately reduce the maximum switching frequency f sw_max . If the discharge current is greater than the discharge current upper limit, jump to Step S4 to recalculate the duty cycle D. If the DC capacitor voltage U dc <U safe , then jump to Step S7. If the switching device junction temperature is greater than the protection value, then jump to Step S7. These protection parameters and protection logics can be reset according to the discharge mode or executed according to the default values of the normal operating conditions of the device.

[0100] Step S7: Discharge completion feedback

[0101] Lock the PWM pulses of all bridge arms, the system outputs a "discharge completed" or "fault warning" status signal, and enters the standby mode.

Claims

1. A zero-cost active discharge method for capacitors in a power electronic converter, characterized in that, Including the following steps: Step S1: Calculate the duty cycle of the switching tube offline Calculate the duty cycle of the switching tube according to the device parameters and the maximum allowable discharge time, and draw a corresponding table of the initial DC capacitor discharge voltage and the duty cycle, which is embedded in the control system; Step S2: Trigger the active discharge command When the shutdown instruction, emergency fault signal or abnormal DC bus voltage of the power electronic converter is detected, the main controller starts the active discharge process; Step S3: Cut off the external electrical connection to completely isolate the power electronic converter from the power grid, DC power supply and load; Step S4: Calculate the maximum switching frequency According to the initial DC capacitor discharge voltage and duty cycle correspondence table embedded in step S1, a fixed duty cycle D is selected, and the detected ambient temperature T is set. a And the technical parameters of the switch tube, calculate the maximum PWM switching frequency f sw_max ; Step S5: Construct the discharge energy circuit According to the duty cycle D and the modulation signal, at the maximum switching frequency f sw_max calculate the switching instruction sent to the switching transistor by the PWM pulse signal; Step S6: Feedback and adjustment Continuously sample the DC voltage, discharge current, and switch junction temperature of the discharge circuit: If the switch junction temperature rises too fast, appropriately reduce the maximum switching frequency f sw_max ; If the discharge current is greater than the upper limit of the discharge current, jump to step S4 to recalculate the duty cycle D; If the DC capacitor voltage U dc < U safe , then jump to step S7; If the switch junction temperature is greater than the protection value, jump to step S7; Step S7: Feedback on completion of discharge Lock the PWM pulses of all bridge arms, the system outputs a "discharge completed" or "fault warning" status signal, and enters the standby mode.

2. The zero-cost active capacitor discharging method for a power electronic converter according to claim 1, characterized in that, In step S1, the duty cycle is taken between 40% and 80%.

3. A zero-cost active discharge method for capacitors of a power electronic converter according to claim 1, characterized in that, In step S1, the following formula is used to calculate the duty cycle of the switching tube: Where C dc is the capacitance value of the DC capacitor, U dc (0) is the voltage at the starting moment of the DC capacitor discharge, U safe is the safe voltage of the DC capacitor, t discharge is the discharge time, I discharge is the discharge current, R eq is the equivalent resistance of the circuit.

4. A zero-cost active capacitor discharge method for a power electronic converter according to claim 1, characterized in that, In step S4, the maximum PWM switching frequency f sw_max is calculated using the following formula: Where, T j_max is the upper limit value of the switching transistor junction temperature, ΔT safe is the safety threshold value of the switching transistor junction temperature, R th_jc is the thermal resistance of the switching transistor, E on , E off are the switching energies of the switching transistor, V ce is the on-state voltage drop of the switching transistor.

5. A zero-cost active capacitor discharge method for a power electronic converter according to claim 1, characterized in that In step S5, the modulation signal uses an AC sine signal.

Images