Power conversion unit direct current side capacitance online detection method based on signal extraction method

Abstract

The invention discloses a power conversion unit direct current side capacitance online detection method based on a signal extraction method. For a power conversion unit, a capacitor is gradually aged along with the increase of operation time, and the reduction of the capacitance value of the capacitor enables the ripple of the capacitor voltage of a sub-module to be increased, thereby causing the corresponding increase of the voltage stress and loss of a device, and possibly endangering the reliable operation of a system. Under the condition that no extra hardware circuit is added, a capacitor is controlled to completely absorb a specific subharmonic current component to increase the harmonic component of the corresponding number of times of the capacitor voltage, and the capacitor voltage regulation module is added to keep the capacitor voltage stable and other subharmonic suppression modules are added to suppress other subharmonics. And finally, online detection of the capacitance value of the capacitor is realized by measuring the voltage of the capacitor and performing online pre-estimation calculation. According to the invention, the health state of the capacitor can be accurately and effectively monitored and evaluated.

Description

Technical Field

[0001] The invention relates to the technical field of power electronic converters, and in particular to an online detection method for a DC side capacitor of a power conversion unit based on a signal extraction method. Background Art

[0002] In the power conversion unit, the capacitor is one of the key components prone to failure. As the running time goes by, the capacitor will gradually age, which is specifically manifested as a decrease in capacitance and an increase in equivalent series resistance (ESR). The decrease in capacitance will cause the voltage ripple of the submodule capacitor to increase, thereby increasing the device voltage stress and power conversion unit loss. The increase in ESR will cause the capacitor loss to increase, which will in turn cause the temperature to rise, further accelerating the aging of the capacitor. When the capacitor aging reaches the failure threshold, its aging rate will increase sharply. At this time, although the system can still operate, the negative impact of capacitor aging will endanger the reliability of the system. Capacitor aging detection usually detects the capacitance of the capacitor. The conventional solution is to record the increment of the capacitor voltage and the corresponding charging time when the system starts, and use the RC charging curve of the capacitor to estimate the capacitance of the capacitor. This method is simple and easy, but it cannot monitor the health status of the submodule in real time. Summary of the invention

[0003] The purpose of the present invention is to provide an online detection method for the DC side capacitance of a power conversion unit based on a signal extraction method. Without adding any additional hardware circuits, only the bus capacitor voltage needs to be measured to achieve high-precision online detection of the capacitance value.

[0004] The present invention provides an online detection method for the DC side capacitance of a power conversion unit based on a signal extraction method, wherein the main circuit comprises an AC power grid input, a front-stage AC / DC converter, a bus capacitor, a rear-stage DC / DC converter, and a DC load output; the input side of the front-stage AC / DC converter is connected to the AC power grid, the bus capacitor is connected in parallel between the AC / DC converter and the DC / DC converter, and the output side of the rear-stage DC / DC converter is connected to the DC load; the method of the present invention is to control the front-stage AC / DC converter control module to make the input power only include the second harmonic component and the fourth harmonic component in addition to the fundamental component without adding additional hardware circuits, and suppress the fourth harmonic component of the bus capacitor voltage and the second harmonic component of the current flowing into the rear-stage DC / DC converter by the rear-stage DC / DC converter control module, so that the second harmonic component in the input power is completely absorbed by the bus capacitor, and the fundamental and fourth harmonic components are transmitted to the DC / DC converter. The second harmonic component of the input power can be obtained by measuring the input voltage and current, and the generated voltage ripple is only related to the capacitance value of the bus capacitor. Therefore, by measuring the capacitor voltage, the current bus capacitance value can be calculated, realizing online detection of the capacitor capacitance value.

[0005] The control system of the whole circuit includes a front-stage AC / DC converter controller and a rear-stage DC / DC converter controller; for the front-stage AC / DC converter controller, according to a specific example of the present invention, it may include a line-to-phase voltage conversion module, a PLL phase-locked loop module, a third harmonic injection module, a feedforward module, an output voltage control module, an input current control module, and an SPWM modulation module; sampling a three-phase grid input line voltage v AB 、v BC 、v CA , converted into phase voltage v through the line-to-phase voltage conversion module AN 、v BN 、v CN , and then connected to the input of the PLL phase-locked loop module, the third harmonic injection module and the feedforward module respectively; the PLL phase-locked loop module outputs the grid voltage phase angle ωt; the third harmonic injection module outputs the adjustment amount m of the third harmonic modulation wave A (3) ~m C (3) ; The feedforward module outputs the adjustment amount m of the feedforward modulation wave A (1) ~m C (1) The output voltage V is sampled by the output voltage control module. o The reference value obtained by the PI controller is V o-ref Output d-ref ; The input current control module samples the three-phase input current i A 、i B 、i C Connected to the input end of the abc / dq coordinate transformation module, the d-axis component i of the input current fundamental wave is obtained after filtering. d and the q-axis component i q , the fundamental current reference values ​​of d-axis and q-axis are i d-ref and 0, and the PI controller outputs m d and m q , the adjustment value m is output by the dq / abc coordinate transformation module A (1*) ~m C (1*) ; All adjustments, i.e. m A (1) ~m C (1) 、m A (3) ~m C (3) and m A (1*) ~m C (1*) The corresponding addition results in the three-phase modulation wave adjustment amount m A~m C After modulation by the SPWM modulation module, the switching signals of each switch tube of the previous AC / DC converter are obtained.

[0006] The post-stage DC / DC converter controller includes a DC capacitor voltage regulation module, a current second harmonic suppression module, and a voltage fourth harmonic suppression module. Taking the current second harmonic suppression module as an example, the current is delayed by T / 6 and T / 3 and then combined with itself to form abc three-phase. After the abc / dq coordinate transformation module with phase θ=2ωt, only the second harmonic component is output as a DC quantity, and the fundamental wave and other harmonics are AC quantities. After filtering, the current second harmonic component d-axis component i d (2) and the q-axis component i q (2) , through the PI controller i d (2) and i q (2) The PI controller output is adjusted to zero, and the second harmonic compensation frequency f is obtained through the dq / abc coordinate transformation module. s2 The idea of ​​voltage fourth harmonic suppression module is the same. The voltage is delayed by T / 12 and T / 6 and then combined with itself to construct abc three-phase. In order to make the fourth harmonic component a DC quantity after transformation, the coordinate transformation module θ=4ωt; Similarly, the fourth harmonic regulation frequency f is obtained s4 ; The DC component output by the DC capacitor voltage regulation module controls the frequency f s0 , the second harmonic compensation frequency f output by the current second harmonic suppression module s2 And voltage fourth harmonic suppression module fourth harmonic compensation frequency f s4 Added to the initial switching frequency F, the switching frequency f of the subsequent DC / DC converter is obtained s , the switching signal of the subsequent DC / DC converter switch tube is obtained through the PFM modulation module.

[0007] Sample the bus capacitor voltage, input current, and input voltage, and calculate the capacitance value at the measurement time using the formula:

[0008]

[0009] Where V ph is the fundamental voltage amplitude of the power grid; I ph is the amplitude of the fundamental current of the power grid; k3 is the amplitude coefficient of the injected third harmonic; ω is the working angular frequency of the power grid voltage; v bus-max is the maximum value of the bus capacitor voltage, and V is the DC component of the bus capacitor voltage. By comparing the measured capacitance value with the reference value, online diagnosis of the capacitance value can be achieved.

[0010] Compared with the prior art, the present invention has the following beneficial effects:

[0011] By adopting the control method of the present invention, the health status of the capacitor on the DC side of the power conversion unit can be detected in real time with high accuracy during operation. This solution does not require additional circuits, can be applied to different power conversion module topologies, and supports online real-time detection. At the same time, in this solution, only the low-frequency capacitor voltage, input voltage and current need to be sampled, which is not easily affected by sampling noise, and the sampling is relatively simple, and the capacitance value at the measurement time can be directly calculated. Closed-loop control is adopted to reduce the impact of disturbances on the measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is the main circuit diagram of the power conversion module;

[0013] Figure 2 This is a schematic diagram for capacitor aging detection and judgment;

[0014] Figure 3 A schematic diagram of a front-stage AC / DC converter controller of a power conversion module;

[0015] Figure 4 A schematic diagram of a post-stage DC / DC converter controller of a power conversion module proposed by the present invention; DETAILED DESCRIPTION

[0016] The present invention is described in detail below in conjunction with specific embodiments and accompanying drawings. The following embodiments will help those skilled in the art to further understand the invention, but are not intended to limit the present invention in any form. It should be noted that, without departing from the concept of the present invention, the modifications or improvements made are within the scope of protection of the present invention.

[0017] According to a specific embodiment of the present invention, the power conversion unit is composed of two stages, the front-stage AC / DC converter 2 uses a CHB converter, the rear-stage DC / DC converter 4 uses an LLC converter, and the bus capacitor 3 is connected in parallel between the AC / DC converter 2 and the DC / DC converter 4. The online diagnosis method of the DC side capacitor of the power conversion unit based on the signal extraction method of the present invention includes suppressing the fifth and seventh harmonics through the front-stage AC / DC converter 2 without additional circuits, injecting the third harmonic to reduce the current stress and making the power flowing into the bus capacitor 3 and the rear-stage DC / DC converter 4 only contain the fundamental component, the second harmonic component and the fourth harmonic component. The fourth harmonic component of the bus capacitor 3 voltage and the second harmonic component of the current flowing into the rear-stage DC / DC converter 4 are suppressed by the rear-stage DC / DC converter 4, so that the second harmonic component of the input power is completely absorbed by the bus capacitor 3, and the fundamental and fourth harmonic components are all transmitted to the rear-stage DC / DC converter 4. The second harmonic component of the input power can be obtained by measuring the input voltage and current. The voltage ripple generated by the bus capacitor is related to the capacitance value. Therefore, sampling the input voltage, input current, and bus capacitor voltage can realize online diagnosis of the capacitor. Specifically:

[0018] Reference Figure 1 The main circuit includes an input AC grid 1, a front-stage AC / DC converter 2, a bus capacitor 3, a rear-stage DC / DC converter 4, and a DC load 7; the AC side of the front-stage AC / DC converter 2 is connected to the AC grid, the bus capacitor 3 is connected in parallel between the AC / DC converter 2 and the DC / DC converter 4, and the output side of the rear-stage DC / DC converter 4 is connected to the DC load 7; the AC / DC converter control module 5 controls the front-stage AC / DC converter 2, and the DC / DC converter control module 6 controls the rear-stage DC / DC converter 4. Where v AB 、v BC 、v CA is the three-phase grid line voltage, i A 、i B 、i C is the three-phase grid current, v o is the output voltage, which is sampled to the front-stage AC / DC converter controller 5 for control, C bus is the DC bus capacitance, v bus is the DC bus capacitor voltage, i bus is the current flowing into the subsequent DC / DC converter 4, where v bus and i bus The sampled data is sent to the subsequent DC / DC converter controller 6 for control.

[0019] In the case of injecting the third harmonic, considering the three-phase voltage symmetry, the three-phase grid phase voltage v AN (t), v BN(t), v CN (t) can be expressed as:

[0020]

[0021] Where V ph is the amplitude of the fundamental voltage of the power grid; k3 is the amplitude coefficient of the injected third harmonic; ω is the operating angular frequency of the power grid voltage.

[0022] Three-phase input current i A (t), i B (t), i C (t) can be expressed as:

[0023]

[0024] Where I ph is the amplitude of the fundamental current of the input current.

[0025] Since the high-order power harmonic content is extremely small and can be ignored, the three-phase input power p A (t), p B (t), p C (t) can be calculated as:

[0026]

[0027] Since the three phases are symmetrical, taking phase A as an example for analysis, the equivalent input power p(t) of the power conversion unit is the input power of phase A:

[0028]

[0029] The front-stage AC / DC converter 2 is composed of four switching tubes, and basically consumes no input power during operation. Therefore, the sum of the power transmitted to the bus capacitor 3 and the rear-stage DC / DC converter 4 is the input power p(t) of the power module. The rear-stage DC / DC converter controller 6 includes a current second harmonic suppression module 603 and a voltage fourth harmonic suppression module 602. The current second harmonic suppression module 603 suppresses the second harmonic power from being transmitted to the DC / DC converter 4, so that the second harmonic power is completely absorbed by the bus capacitor 3. The voltage fourth harmonic suppression module 602 suppresses the bus capacitor 3 from absorbing the fourth harmonic power. In steady state, the capacitor does not absorb the fundamental power. Therefore, the power absorbed by the bus capacitor 3 is:

[0030]

[0031] The busbar capacitor 3 can be an ideal capacitor C bus It is connected in series with the capacitor equivalent resistor ESR, and the secondary ripple power absorbed by ESR is much smaller than C bus, so without considering ESR, the power absorbed by the bus capacitor can be expressed as the product of voltage and current, which is:

[0032]

[0033] Combined to get v bus (t) is expressed as:

[0034]

[0035] Where V is the fundamental component of the bus capacitor voltage.

[0036] The bus capacitor voltage peak value is obtained by sampling. Taking the maximum value as an example, the bus capacitor voltage maximum value v bus-max for:

[0037]

[0038] Therefore, the busbar capacitance value is C bus for:

[0039]

[0040] Since k3 is a coefficient given in the third harmonic injection link, the grid operating frequency ω is known, and the current bus capacitor capacitance can be calculated by sampling the bus capacitor voltage, inputting the grid voltage, and inputting the grid current. Capacitor aging diagnosis is usually performed by detecting the capacitance value, and the health of the capacitor can be determined by comparing the calculated value after sampling with the initial value. According to a specific embodiment of the present invention, a capacitor aging detection and judgment schematic diagram is shown as follows Figure 2 shown.

[0041] According to a specific example of the present invention, using reference Figure 3 The control method of the front-stage AC / DC converter 5 shown in the figure, the front-stage AC / DC converter controller 5 includes a line-to-phase voltage conversion module 501, a PLL phase-locked loop module 502, a third harmonic injection module 503, a feedforward module 504, an output voltage control module 505, an input current control module 506, and an SPWM modulation module 507; sampling a three-phase grid line voltage v AB 、v BC 、v CA , the three-phase voltage v is obtained through the line phase voltage conversion module 501 AN 、v BN 、v CN, respectively connected to the input ends of the PLL phase-locked loop module 502, the third harmonic injection transformation module 503 and the feedforward module 504; the PLL phase-locked loop module 502 outputs the grid voltage phase angle ωt, and is connected to the input ends of the third harmonic injection module 503, the abc / dq coordinate transformation module and the dq / abc coordinate transformation module; the third harmonic injection module 503 outputs the adjustment amount m of the third harmonic modulation wave with an amplitude coefficient of k3 A (3) ~m C (3) The feedforward module 504 introduces the grid voltage into the control loop to improve the dynamic performance and output the feedforward modulation wave m A (1) ~m C (1) ; Sample three-phase grid current i A 、i B 、i C and the output voltage v o , output voltage v o The reference value is V o-ref The PI controller outputs the three-phase grid current fundamental component d-axis reference value i d-ref . Three-phase grid current i A 、i B 、i C After the abc / dq coordinate transformation module of ωt and filtering, the current fundamental wave d-axis and q-axis components i are obtained. d and i q ,i d and i q The reference values ​​are i d-ref The adjustment amount m of the input current fundamental modulation wave is obtained by the adjustment amount output by the PI controller through the dq / abc coordinate transformation module. A (1*) ~m C (1*) The total adjustment amount m is obtained by adding up all modulation wave adjustment amounts. A ~m C , after being modulated by the SPWM modulation module 507, the switching signals of each switch tube of the front-stage AC / DC converter 2 are obtained.

[0042] The specific implementation of each module of the front-stage AC / DC converter controller 5 is as follows:

[0043] The PLL phase-locked loop module 502 obtains the phase angle ωt of the grid voltage through a software phase-locked algorithm.

[0044] The feedforward link 504 introduces the grid voltage into the control loop to improve the dynamic performance. The modulation wave m generated by the feedforward link 504 is A (1) ~m C(1) It is the ratio of the phase voltage input to each phase to the sum of the intermediate bus voltages of each module in that phase.

[0045] The third harmonic injection module 503 is used to inject a given third harmonic m into each phase modulation wave. A (3) ~m C (3) . Phase voltage v AN 、v BN 、v CN The abc / dq coordinate transformation module performs dq coordinate transformation according to the grid voltage phase ωt to obtain the d-axis component i of the voltage. d The phase of the third harmonic is the same as that of the fundamental wave, and its amplitude is the product of the amplitude of the fundamental wave and the given third harmonic injection coefficient k3.

[0046] The output voltage control module 505 uses the output voltage reference value V o-ref Subtract the output voltage sampling, and the error value is obtained through the PI controller to output the current fundamental wave d-axis reference value i d-ref .

[0047] The input current control module 506 performs dq coordinate transformation according to the grid voltage phase ωt through the abc / dq coordinate transformation module, and obtains the d-axis and q-axis components i of the fundamental current after filtering. d and i q ,i d The reference value is i d-ref ,i q The reference value of is 0, and the d-axis and q-axis errors are subtracted and input into the PI controller. The output of the PI controller is transformed into the abc coordinate according to the grid voltage phase ωt by the dq / abc coordinate transformation module to obtain the adjustment amount m of the fundamental modulation wave A (1*) ~m C (1*) .

[0048] The SPWM modulation module 507 adds the three-phase modulation wave signal m A ~m C By comparing with the high-frequency carrier signal respectively, a square wave pulse signal whose duty cycle changes with the modulation signal is obtained, which is used as the switching signal of each switch tube of the previous AC / DC converter.

[0049] Use reference Figure 4 In the control method of the post-stage DC / DC converter 4 shown in the figure, the DC / DC converter control module 6 includes a DC bus capacitor voltage regulation module 601, a voltage fourth harmonic suppression module 602, a current second harmonic suppression module 603, and a PFM modulation module 604. The sampling bus capacitor C bus Voltage bus, and the reference value V bus-ref The difference is input into the PI controller to obtain the switching frequency fundamental compensation value f of the subsequent LLC converter s0 The current second harmonic suppression module 603 and the voltage fourth harmonic suppression module 602 are used to suppress the second harmonic in the current flowing into the subsequent LLC converter and the fourth harmonic of the bus capacitor voltage, respectively, and sample the current flowing into the subsequent LLC converter i bus and bus capacitance C bus Voltage bus The second harmonic compensation value f of the switching frequency of the subsequent LLC converter is obtained by the current second harmonic suppression module 603 and the voltage fourth harmonic suppression module 602. s2 And the fourth harmonic compensation f s4 .f s0 、f s2 、f s4 Add the rated switching frequency F of the LLC converter to get the switching frequency f of the subsequent LLC converter. s After passing through the PFM modulation module 604, a switching signal of the subsequent LLC converter switch tube is obtained.

[0050] The specific implementation of the post-stage DC / DC converter control module 6 is as follows:

[0051] The capacitor voltage regulation module 601 is used to control the voltage V across the bus capacitor. bus Stable, using capacitor voltage reference value V bus-ref Subtract the sampled value of the bus capacitor voltage v bus The error value is calculated by the PI controller to calculate the switching frequency fundamental compensation value f of the subsequent LLC converter s0 .

[0052] The current second harmonic suppression module 603 is used to suppress the current i flowing into the subsequent LLC converter bus The second harmonic in the sampled current i bus Delayed by T / 6 and T / 3 (T is the power frequency period) respectively, and with the current i bus The abc / dq coordinate transformation of the three-phase signal input phase is constructed to be 2ωt. After dq transformation, it is input into the low-pass filter. Since only the second harmonic d-axis component and q-axis component of the current are DC quantities, the d-axis component and q-axis component obtained from other subharmonics and fundamental waves are AC quantities, the d-axis component i of the second harmonic current is obtained after filtering. d (2) and the q-axis component i q (2) The reference values ​​of the second harmonic currents of the d-axis and q-axis are both 0, and the d-axis component i d (2) and the q-axis component i q (2)Subtracting from 0, the error value is calculated by the PI controller and the input phase is 2ωt. After the abc transformation, the second harmonic compensation value f of the switching frequency of the subsequent LLC converter is obtained. s2 .

[0053] The voltage fourth harmonic suppression module 602 is used to suppress the voltage V flowing into the bus capacitor bus The fourth harmonic in the sampled current V bus Delayed by T / 12 and T / 6 (T is the power frequency period) respectively, and with the current V bus The abc / dq coordinate transformation of the three-phase signal input phase is constructed to be 4ωt. After dq transformation, it is input into the low-pass filter. Since only the fourth harmonic d-axis component and q-axis component of the voltage are DC quantities, the d-axis component and q-axis component obtained by other harmonics and fundamental waves are AC quantities, the d-axis component i of the fourth harmonic current is obtained by filtering. d (4) and the q-axis component i q (4) The fourth harmonic current reference values ​​of the d-axis and q-axis are both 0, and the d-axis component i d (4) and the q-axis component i q (4) Subtracting from 0, the error values ​​are respectively calculated by the PI controller and the input phase is 4ωt dq / abc coordinate transformation. After abc transformation, the fourth harmonic compensation value f of the switching frequency of the subsequent LLC converter is obtained. s4 .

[0054] LLC converter switching frequency fundamental compensation amount f s0 , second harmonic compensation amount f s2 , fourth harmonic compensation amount f s4 The operating frequency f of the subsequent LLC converter is obtained by superimposing the rated operating frequency FR. s The PFM module 604 passes the frequency f s The frequency of the square wave pulse signal is changed to serve as the switching signal of the subsequent LLC converter switch tube.

[0055] The present invention aims to solve the problem that as the running time of a power conversion unit increases, the capacitor gradually ages, causing the device voltage stress and the power module loss to increase accordingly. Without adding additional hardware circuits, the present invention adopts control to make the capacitor completely absorb the specific harmonic components of power, thereby increasing the corresponding harmonic components of the capacitor voltage. A capacitor voltage control loop is added to keep the capacitor voltage stable and a harmonic suppression module is added to suppress other harmonics. By measuring the capacitor voltage, the online detection of the capacitor capacitance can be completed, thereby realizing accurate and effective monitoring and evaluation of the health status of the capacitor.

Claims

1. A method for online detection of DC side capacitance of a power conversion unit based on a signal extraction method, wherein the input side of the power conversion unit is connected to an AC power supply, and the output side is connected to a DC load; characterized in that: The method controls the front-stage AC / DC converter so that the input power includes only the second harmonic component and the fourth harmonic component in addition to the fundamental wave component without adding additional hardware circuits, and uses a rear-stage controller (6) to control the rear-stage DC / DC converter (4) of the power conversion unit to suppress the bus capacitor voltage harmonic components other than the second harmonic component, and suppress the second harmonic component of the current flowing into the rear-stage DC / DC converter, so that the double frequency ripple power is completely absorbed by the bus capacitor, and the current capacitor capacitance is calculated by measuring the generated capacitor voltage ripple, so as to realize online detection of the capacitor capacitance.

2. The method for online detection of DC side capacitance of a power conversion unit based on a signal extraction method according to claim 1, characterized in that: The post-stage controller (6) comprises a capacitor voltage regulation module (601), a current second harmonic suppression module (603), a voltage fourth harmonic suppression module (602), and a PFM modulation module (604); the capacitor voltage regulation module samples the voltage v of the bus capacitor (3) on the DC side of the power conversion unit. bus , and the reference value V bus-ref The difference is input into the PI controller to generate the fundamental compensation value f of the switching frequency of the subsequent DC / DC converter s0 The current second harmonic suppression module and the voltage fourth harmonic suppression module are used to suppress the second harmonic component of the current flowing into the subsequent DC / DC converter and the fourth harmonic component of the DC side bus capacitor voltage, respectively. The generated modulation signals are the second harmonic compensation amount f of the switching frequency obtained when the harmonic component reference value is zero. s2 And the fourth harmonic compensation f s4 ; Three compensation quantities f s0 、f s2 and f s4 At the same time, it is superimposed on the initial switching frequency F of the subsequent DC / DC converter to obtain the final switching frequency f of the subsequent DC / DC converter s , frequency signal f s After passing through the PFM modulation module, the switching signals of the four switching tubes S5 to S8 of the subsequent DC / DC converter are obtained.

3. The method for online detection of DC side capacitance of a power conversion unit based on a signal extraction method according to claim 2, characterized in that: The second harmonic compensation amount f s2 The determination method is as follows: after the current is delayed by T / 6 and T / 3, it is combined with itself to form abc three-phase, and the output of the abc / dq coordinate transformation module with phase θ=2ωt is that only the second harmonic component is DC, and the fundamental wave and other harmonics are AC. After filtering, the second harmonic component of the current d-axis component i d (2) and the q-axis component i q (2) , through the PI controller i d (2) and i q (2) The PI controller output is adjusted to zero, and the second harmonic compensation frequency f is obtained through the dq / abc coordinate transformation module. s2 The fourth harmonic suppression module 602 has the same idea. The voltage is delayed by T / 12 and T / 6 and then combined with itself to construct the abc three-phase. In order to make the fourth harmonic component a DC quantity after transformation, the abc / dq coordinate transformation module θ=4ωt. Similarly, the fourth harmonic adjustment frequency f is obtained. s4 .

4. The method for online detection of DC side capacitance of a power conversion unit based on a signal extraction method according to claim 1, characterized in that: Sample the bus capacitor voltage, input current, and input voltage, and calculate the capacitance value at the measurement time using the formula: Where V ph is the fundamental voltage amplitude of the power grid; I ph is the amplitude of the fundamental current of the power grid; k3 is the amplitude coefficient of the injected third harmonic; ω is the operating angular frequency of the power grid voltage; v bus-max is the maximum value of the bus capacitor voltage, and V is the DC component of the bus capacitor voltage. By comparing the measured capacitance value with the reference value, online diagnosis of the capacitance value can be achieved.

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