CN116819403A - DC capacitor aging monitoring method and system for two-stage inverter - Google Patents
DC capacitor aging monitoring method and system for two-stage inverter Download PDFInfo
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Abstract
The invention discloses a method and a system for monitoring direct-current capacitor aging of a two-stage inverter, wherein the method comprises the steps of changing control parameters of a pre-stage conversion device, controlling output voltage of the pre-stage conversion device to generate a transient voltage change curve, wherein the minimum value of the output voltage when the output voltage drops is larger than a network side voltage peak value, and the system modulation degree of the output voltage rising process is not lower than the system minimum modulation degree; recording the transient voltage change curve by using a voltage sensor at the DC capacitor side, and selecting transient characteristic points; predicting the capacitance value of the direct current capacitor by utilizing the transient characteristic points; according to the invention, the capacitor aging monitoring can be realized by using the DC capacitor voltage sensor of the inverter, so that the use of a high-precision voltage sensor is avoided, and the monitoring cost is saved.
Description
Technical Field
The invention relates to the technical field of electrical engineering and machine learning, in particular to a method and a system for monitoring direct-current capacitance aging of a two-stage inverter.
Background
The two-stage inverter consists of a front-stage conversion unit (commonly DC-DC, rectifier) and a rear-stage inversion unit, the method is widely applied to occasions such as photovoltaic power generation, motor driving systems, traction converters and the like. While capacitors are an important component of most electronic converters, they help to suppress dc-link voltage ripple, absorb harmonics, balance the front-end and back-end instantaneous power differences of the converter system, and in some applications they also provide sufficient energy for transients and abnormal operation, and therefore, typically, shunt capacitance on the dc bus side of the two-stage inverter, suppressing disturbances. However, capacitors are sensitive to thermal and electrical stresses, and the main disadvantage is limited lifetime and high degradation failure rate. About 30% of faults in the current transformer are caused by degradation of the capacitor, which is considered to be the weakest link in the power electronic system, and therefore, how to monitor the health of the capacitor in real time has important theoretical and practical significance. It is generally considered that the end of life of the aluminum electrolytic capacitor is a 20% decrease in capacitance or a double increase in equivalent series resistance (Equivalent Series Resistance, ESR). The loss of 2% -5% of the MPPF capacitance represents a capacitance failure, and the threshold is typically set to 5% of the capacitance loss.
A scholars proposed a scheme based on capacitive voltage and current ripple to estimate capacitive parameters of a motor drive system (ASD), directly measuring capacitive voltage ripple and current ripple through a sensor, then analyzing a sampled waveform by using a Goertzel algorithm, and estimating capacitance values C and ESR by using intermediate frequency and low frequency components in the extracted capacitive current ripple and voltage ripple. According to the scheme, due to the fact that capacitive voltage and current ripples are required to be measured, a high-precision voltage/current sensor is required, and the monitoring cost is high.
In some schemes, a switch function is utilized to construct a direct-current side current so as to construct a capacitance current, and a capacitance value in the H4 photovoltaic grid-connected inverter is estimated according to the constructed capacitance current and a capacitance differential formula; this solution results in a larger error in capacitance estimation due to the need to construct the capacitance current through a switching function.
Still scholars propose a quasi-on-line monitoring technique, which injects current of H harmonic frequency into the power grid when the H5 photovoltaic grid-connected inverter is not operating at night, which results in (H-1) times and (h+1) times of voltage and current ripple appearing on the dc-link capacitor; using a least mean square algorithm (Least Mean Square, LMS), the voltage and current ripple at (h-1) times or (h+1) times can be used to estimate ESR and C; however, the scheme can only estimate capacitance value at night, so that the scheme is a quasi-online method and has poor monitoring real-time performance.
The patent application document of publication number CN114792073A proposes a novel method for predicting the residual life of the supercapacitor based on a support vector machine, and combines an NSGA-II algorithm with SVR to accurately predict the residual life of the supercapacitor; the method has the key points that the super capacitor is repeatedly aged in advance, the change rule of the external characteristics along with aging is recorded, then the support vector machine training is carried out, the work of the method is ideal, default data can be accurately obtained, and the artificial intelligence training is directly carried out. However, it is very difficult to obtain accurate external characteristics, and a high sensing unit is required to obtain the external characteristics, which clearly increases the cost and is not beneficial to practical application.
Therefore, the current detection of the health state of the capacitor, especially the field of the two-stage inverter device, has obvious defects, and has a lifting space in the aspects of real-time performance, low cost and high precision.
Disclosure of Invention
The invention aims to solve the technical problem of obtaining effective capacitance monitoring data by means of the existing low-precision sensor so as to realize capacitance aging monitoring.
The invention solves the technical problems by the following technical means:
in one aspect, the invention provides a method for monitoring dc capacitance aging of a two-stage inverter, wherein the two-stage inverter comprises a front-stage conversion device and a rear-stage inverter, and the method comprises the following steps:
changing control parameters of the pre-stage conversion device, and controlling the output voltage of the pre-stage conversion device to generate a transient voltage change curve by fast transient, wherein the minimum value of the output voltage is larger than the network side voltage peak value when the output voltage is reduced, and the system modulation degree of the output voltage increasing process is not lower than the system minimum modulation degree;
recording the transient voltage change curve by using a voltage sensor at the DC capacitor side, and selecting transient characteristic points;
and predicting the capacitance value of the direct current capacitor by utilizing the transient characteristic points.
Further, the changing the control parameter of the pre-stage conversion device controls the output voltage of the pre-stage conversion device to generate a transient voltage change curve, including:
changing control parameters of the pre-stage conversion device, and controlling the output voltage of the pre-stage conversion device to drop and meet a first constraint condition when the output voltage drops;
changing control parameters of the pre-stage conversion device, and controlling the output voltage of the pre-stage conversion device to rise and meet a second constraint condition when the output voltage rises;
and reading the voltage value of the direct-current side capacitor, and generating the transient voltage change curve.
Further, the first constraint includes:
1) The output voltage of the front-stage conversion device is larger than the voltage peak value at the network side:
2) When the parameter of the pre-stage conversion device is controlled to change, the pre-stage conversion device is ensured to work in a stable working interval;
3) Meets the minimum output power limit of the system:
V dcmin ·I≥P min
wherein: v (V) dc An output voltage of the current level conversion device; v (V) g Is the peak voltage of the network side; v (V) dcmin The lowest value of the output voltage of the current-stage conversion device; i is direct-current side current; p (P) min Is the minimum output power of the system.
Further, the second constraint includes:
1) Minimum modulation system constraint:
wherein: v (V) dcmax Is the maximum value of the output voltage; v (V) g Is the peak value of the power grid voltage; r is R min The minimum modulation degree of the system is set;
2) When the parameter of the pre-stage conversion device is controlled to change, the pre-stage conversion device is ensured to work in a stable working interval.
Further, when the preceding-stage conversion device is an LLC converter, a condition for ensuring that the LLC converter operates in a stable operation region thereof is an inductive operation region in which the switching frequency is adjusted according to a gain curve so as to operate on the right side of a resonance point of the gain curve, and a gain calculation formula of the LLC converter is as follows:
wherein: m is LLC converter gain; q is the LLC converter quality factor; f (F) x Normalizing the switching frequency for the LLC converter; m= (L) m +L r )/L r ，L m Exciting inductance value for LLC converter, L r Resonant inductance value for the LLC converter.
Further, when the control parameters of the pre-stage conversion device are changed and the output voltage of the pre-stage conversion device is controlled to generate a fast transient, the control mode of the pre-stage conversion device is open-loop control, and the post-stage inversion device adopts a voltage outer loop and current inner loop double-loop control mode.
Further, the transient characteristic points comprise the lowest point on the transient voltage change curve under different capacitance values, a transient voltage step point at a certain moment and a point with a difference value larger than the voltage resolution of the voltage sensor.
Further, after the voltage sensor on the side of the direct current capacitor is used for recording the transient voltage change curve and selecting the transient characteristic point, the method further comprises the following steps:
and when the voltage of the capacitor controlled by the voltage outer loop of the rear-stage reverse inversion device returns to the normal operation value of the system, adjusting parameters and a control structure of the front-stage conversion device to enable the system to recover to normal operation.
Further, the predicting the capacitance value of the dc capacitor by using the transient characteristic point includes:
and constructing a training set by utilizing the transient characteristic points, and training a support vector machine to obtain a prediction model, wherein a kernel function of the support vector machine adopts rbf.
In a second aspect, the present invention further provides a dc capacitor aging monitoring system of a two-stage inverter, where the two-stage inverter includes a front-stage conversion device and a rear-stage inverter, and the system includes:
the parameter control module is used for changing control parameters of the pre-stage conversion device, controlling the output voltage of the pre-stage conversion device to generate a transient voltage change curve, wherein the minimum value of the output voltage when the output voltage drops is larger than the network side voltage peak value, and the system modulation degree of the output voltage rising process is not lower than the system minimum modulation degree;
the characteristic point selection module is used for recording the transient voltage change curve by utilizing a voltage sensor at the DC capacitor side and selecting transient characteristic points;
and the capacitance prediction module is used for predicting the capacitance value of the direct current capacitance by utilizing the transient characteristic points.
The invention has the advantages that:
(1) Because the voltage sensor arranged in the two-stage inverter is a low-precision voltage sensor, the low-precision voltage sensor cannot directly measure the capacitor voltage ripple and the current ripple, and the high-precision voltage sensor is generally directly used for measuring the capacitor voltage ripple in the related technology, so that the monitoring cost is relatively high; the invention changes the control parameter of the front-stage conversion device to make the output voltage of the front-stage conversion device generate quick transient, namely, a transient process is inserted when the circuit operates normally, so that the DC side voltage generates jitter, and the transient fluctuation which can be identified by the low-precision voltage sensor is generated in the safety margin range. Compared with a method adopting capacitive voltage ripple, the method provided by the invention avoids using a high-precision voltage sensor, does not increase extra hardware, saves monitoring cost, and is beneficial to practical application and popularization.
Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
Drawings
Fig. 1 is a schematic flow chart of a method for monitoring dc capacitance aging of a two-stage inverter according to an embodiment of the present invention;
FIG. 2 is a topology of a two-stage inverter in an embodiment of the present invention;
fig. 3 is an overall flowchart of a method for monitoring dc capacitance aging of a two-stage inverter according to an embodiment of the present invention;
fig. 4 is a schematic block diagram of a dc capacitor aging monitoring method of a two-stage inverter according to an embodiment of the present invention;
fig. 5 is a block diagram of a two-stage inverter device with a DC/DC device as a front stage in an embodiment of the present invention;
FIG. 6 is a diagram showing an exemplary construction of an application of a pre-stage conversion apparatus according to an embodiment of the present invention;
fig. 7 is an exemplary configuration diagram of a post-inverter application in an embodiment of the present invention;
FIG. 8 is a topology of an LLC+H bridge inverter circuit of the present invention;
fig. 9 is a control block diagram of a post H-bridge inverter circuit in the present invention;
FIG. 10 is a graph showing the system grid-tie current variation during transients in an embodiment of the present invention;
FIG. 11 is a graph showing the capacitance voltage change during transients in an embodiment of the present invention;
FIG. 12 is a graph showing a predicted capacitance value according to an embodiment of the present invention;
fig. 13 is a schematic structural diagram of a dc capacitor aging monitoring system of a two-stage inverter according to an embodiment of the present invention.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions in the embodiments of the present invention will be clearly and completely described in the following in conjunction with the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
As shown in fig. 1, a first embodiment of the present invention provides a method for monitoring dc capacitance aging of a two-stage inverter, where the two-stage inverter includes a front-stage conversion device and a rear-stage inverter, and the method includes the following steps:
s10, changing control parameters of the pre-stage conversion device, and controlling the output voltage of the pre-stage conversion device to generate a transient voltage change curve in a rapid transient manner, wherein the minimum value of the output voltage in the process of falling is larger than the network side voltage peak value, and the system modulation degree of the output voltage rising process is not lower than the system minimum modulation degree;
it should be noted that, as shown in fig. 2, by changing the control parameters of the prior-stage inverter, such as the output voltage setting, the control frequency, etc., the prior-stage inverterThe output voltage of the pre-stage conversion device is the DC capacitor C due to the fast transient dc Therefore, the direct-current capacitor voltage generates transient change process to generate transient characteristic points related to the capacitance value.
S20, recording the transient voltage change curve by using a voltage sensor at the DC capacitor side, and selecting transient characteristic points;
s30, predicting the capacitance value of the direct current capacitor by utilizing the transient characteristic points.
In the embodiment, a transient process is interposed when a circuit normally operates, so that the voltage at the direct current side is dithered, instantaneous fluctuation which can be identified by a low-precision voltage sensor is generated in a safety margin range, and as the output voltage of a previous-stage conversion device is the voltage of a direct current capacitor, the capacitance of the capacitor is monitored by recording a transient voltage change curve, extracting transient characteristic points related to the capacitance, and performing data analysis on the transient characteristic points by using a machine learning algorithm (such as a support vector regression algorithm); compared with a method adopting capacitive voltage ripple, the method provided by the invention avoids using a high-precision voltage sensor, does not add extra hardware, and saves monitoring cost.
In one embodiment, the step S10: the method comprises the following steps of:
s11, changing control parameters of the pre-stage conversion device, and controlling the output voltage of the pre-stage conversion device to drop and meet a first constraint condition when the output voltage drops;
s12, changing control parameters of the pre-stage conversion device, and controlling the output voltage of the pre-stage conversion device to rise and meet a second constraint condition when the output voltage rises;
s13, reading the voltage value of the direct-current side capacitor, and generating the transient voltage change curve.
It should be noted that, in this embodiment, the transient characteristic point related to the capacitance value of the capacitor is created by changing the output voltage of the pre-stage conversion device, and meanwhile, the influence of the transient process on the ac side is ensured to be minimum, and the controller needs to be ensured to capture the transient characteristic point relatively easily. In the process, the lowest point of the direct-current capacitor voltage needs to be larger than the peak value of the alternating-current side grid voltage, otherwise, an overmodulation phenomenon occurs, and inversion failure is caused; and the lowest point of the capacitor voltage cannot be too low, otherwise, the output active power of the voltage loop in the transient process is given to be negative, and the system is unstable.
Further, the first constraint includes:
1) In the two-stage grid-connected inverter, the post-stage DC capacitor voltage V dc I.e. the output voltage of the front-stage converter needs to be larger than the voltage peak value V at the grid side of the inverter g That is, the output voltage of the pre-stage conversion device is larger than the peak voltage of the network side:
2) When the parameter of the pre-stage conversion device is controlled to change, the working characteristics of the pre-stage conversion device are considered in the voltage falling range of the pre-stage conversion device, so that the pre-stage conversion device is ensured to work in a stable working interval;
3) Considering the minimum output power limit of the system, if the output voltage of the front-stage converter is reduced, the output power of the system is reduced, and the output voltage of the front-stage converter is adjusted according to the minimum output power of the system, the minimum output power limit of the system is satisfied:
V dcmin ·I≥P min
wherein: v (V) dc An output voltage of the current level conversion device; v (V) g Is the peak voltage of the network side; v (V) dcmin The lowest value of the output voltage of the current-stage conversion device; i is direct-current side current; p (P) min Is the minimum output power of the system.
In this embodiment, when the converter is an inverter, if the output voltage of the preceding-stage converter is reduced, it is necessary to ensure that the minimum value of the preceding-stage output voltage is greater than the peak voltage of the network side, that is, the modulation degree of the system is ensured to be less than 1, so as to prevent overmodulation of the system.
Further, the second constraint includes:
1) When the parameters of the prior-stage converter are changed to enable the output voltage to rise, as the voltage peak value at the network side is kept unchanged, the system modulation degree is reduced, and the system modulation degree is not lower than the minimum modulation degree of the system, namely the constraint of the minimum modulation degree of the system is avoided:
wherein: v (V) dcmax Is the maximum value of the output voltage; v (V) g Is the peak value of the power grid voltage; r is R min The minimum modulation degree of the system is set;
2) When the parameters of the prior-stage converter are controlled to change, the rising range of the voltage of the prior-stage converter needs to consider the working characteristics of the prior-stage converter, and when the parameters of the prior-stage converter are changed, the prior-stage converter needs to be ensured to work in a stable working range.
It should be noted that, as shown in fig. 3, when the output voltage of the preceding converter drops according to the adjustment parameter, the output voltage needs to satisfy the first constraint condition, otherwise, the readjustment parameter readjusts the output voltage. If the control parameters of the pre-stage conversion device are changed to enable the output voltage to rise, the highest point of the output voltage needs to meet a second constraint condition, otherwise, the parameters are readjusted to readjust the output voltage.
In an embodiment, when the control parameters of the pre-stage conversion device are changed and the output voltage of the pre-stage conversion device is controlled to generate a fast transient, the control mode of the pre-stage conversion device is open loop control, and the post-stage inversion device adopts a voltage outer loop and current inner loop double loop control mode.
It should be noted that, in the process of creating the transient characteristic points, the response speed of the system voltage outer loop cannot be too fast, so that the problem that the transient characteristic points cannot be captured by the controller due to the fact that the output voltage change track of the pre-stage DCDC converter is not obvious is avoided.
In order to eliminate the influence of the pre-stage voltage closed-loop control on the output voltage, the pre-stage DCDC converter needs to be changed into open-loop control during transient. But in order to ensure the normal operation of the pre-stage DCDC converter, the control parameters of the pre-stage converter need to be recorded before the pre-stage converter is operated in an open loop.
In an embodiment, the transient characteristic points include a lowest point on the transient voltage change curve at different capacitance values, a transient voltage step point at a certain moment, and a point where a difference value is greater than a voltage resolution of the voltage sensor.
When the transient characteristic points are selected, points with obvious difference of values on the capacitance voltage change tracks under different capacitance values are selected as the transient characteristic points, and the difference of the transient characteristic points under different capacitance values is larger than the voltage resolution of the adopted voltage sensor, so that the precision of the required voltage sensor is reduced, and the system monitoring cost is reduced.
In one embodiment, as shown in fig. 4, in the step S20: after the transient voltage change curve is recorded by utilizing the voltage sensor at the DC capacitor side and the transient characteristic points are selected, the method further comprises the following steps:
and when the voltage of the capacitor controlled by the voltage outer loop of the rear-stage reverse inversion device returns to the normal operation value of the system, adjusting parameters and a control structure of the front-stage conversion device to enable the system to recover to normal operation.
In this embodiment, only the pre-stage conversion device is required to create a controllable transient voltage change process, and most of the control strategies of the pre-stage conversion device can meet the requirement. The pre-stage conversion device can be a DC/DC converter, a three-phase rectifier bridge and the like according to different application occasions, and is commonly a DC/DC converter. The latter inverter may be a single-phase inverter or a three-phase inverter.
Taking a DC/DC converter as an example for analysis, fig. 5 is a block diagram of a two-stage inverter with a DC/DC converter as a front stage, and the front stage DC/DC converter may be a Buck converter, a Boost converter, a flyback converter, an LLC converter, a double active bridge converter (DAB), or the like, as shown in fig. 6. The latter inverter may be an H-bridge, a three-level converter, a cascaded H-bridge, etc., as shown in fig. 7. Therefore, the method has strong engineering applicability.
Taking LLC+H bridge two-stage topology as an example to illustrate the working principle of the proposed method:
fig. 8 is a topology of a two-stage inverter device, in which a DC/DC converter selects an LLC topology, and fig. 9 is a control method of a later-stage H-bridge inverter circuit. The front-stage LLC converter adopts an open-loop fixed-frequency control mode, namely PWM waves with the resonant frequency and 180-degree phase difference are sent to the pair of pipeline switching tubes, and the gain of the LLC converter is 1.
And the later-stage H bridge adopts a voltage outer ring and current inner ring double-ring control mode. And the voltage outer ring is used for controlling the direct current capacitor voltage of the H bridge module to be constant, sampling the direct current capacitor voltage, comparing the sampled direct current capacitor voltage with the given capacitor voltage, outputting the sampled direct current capacitor voltage through the PI regulator, giving the d-axis current of the current inner ring, and controlling the reactive power of the system. The current inner loop controls the same phase of the power grid current and the power grid voltage, namely the unit power factor. After 180 DEG of power grid current delay, the power grid current is transformed into d-axis and q-axis current feedback sums through dq coordinates, and the current feedback and the d-axis current are subjected to difference, and then the current feedback and the d-axis current are output into modulation waves through a PI regulator. The modulated wave enters a carrier horizontal phase shift (CPS-SPWM) modulation module to generate SPWM waves.
According to the above embodiment, the proposed method is elaborated by taking the llc+h bridge topology as an example, and Matlab simulation results are given to verify the validity of the proposed method.
Simulation parameters:
the LLC resonant frequency of the two-stage grid-connected inverter is 48.2k, the DC bus capacitor is 15mF, the rated value of the DC capacitor of the H bridge module is 6.6mF, the DC bus voltage is 405V, and the voltage outer ring controls the voltage of the DC capacitor of the front stage of the H bridge to be 400V.
Step (1) sends out a capacitance monitoring instruction to change the pre-stage DC/DC voltage:
and sending out a capacitance monitoring instruction to improve LLC switching frequency. Because the front-stage LLC adopts open-loop fixed-frequency control, if the LLC switching frequency is changed from 48.2kHz to 52.5kHz, the gain of the LLC converter is reduced, at the moment, the front-stage DC/DC output voltage is reduced, and the DC capacitance voltage of the H-bridge module is reduced to be lower than 400V. Due to the equalizing ring, the DC capacitor voltage starts to rise after a period of time is reduced, and returns to 400V after a period of time. The dc capacitor voltage variation trace of the H-bridge is shown in fig. 11.
Step (2) reading the capacitor voltage at the direct current side, recording the voltage change track, and selecting proper characteristic points:
fig. 11 shows the capacitance voltage variation trace for rated values 6600uf and 5280uf (20% reduction), respectively. According to fig. 11, when the capacitance is reduced, the lowest point of the dc capacitance voltage variation trace is reduced, and the lowest point of the trace is different for different capacitance values. Therefore, the lowest point of the capacitance-voltage variation trace is selected as the feature point. Changing the capacity value records the numerical value of the characteristic point under different capacity values, and using the numerical value as a training set of a data processing method such as a neural network or machine learning.
Step (3) analyzing curve characteristic points by using a neural network or a machine learning method, and predicting capacitance voltage values:
according to the characteristic points, training is carried out by adopting an artificial neural network or a machine learning method, training data are fitted, and a relation curve of the capacitance and the transient characteristic points, namely a capacitance value prediction model is obtained. After model training is completed, the method can be used for capacitance monitoring, transient characteristic points recorded by a voltage sensor are input into a prediction model, and a predicted capacitance value can be obtained. When the capacitance value is below 20% nominal, the capacitance needs to be replaced.
Specifically, the above steps will be described below by taking an LLC converter and a Boost converter as a preceding conversion device, respectively. The constraint condition when the output voltage of the preceding stage decreases is described by taking an LLC converter as an example, and the constraint condition when the output voltage of the preceding stage increases is described by taking a Boost converter as an example.
(1) The preceding stage is an LLC converter:
1) When the preceding stage is an LLC converter, the output voltage can be changed by changing the switching frequency of the preceding stage LLC converter, and thus the gain of the LLC converter. First, the switching frequency of the LLC converter in the closed loop state of the LLC converter is recorded, the LLC converter is operated in an open loop at the switching frequency, and then the switching frequency of the LLC converter is changed so that the output voltage of the preceding converter is reduced.
2) When the current stage is an LLC converter and the output voltage thereof is reduced, the following constraint condition needs to be satisfied:
A. post-stage DC capacitor voltage V dc I.e. the output voltage of the preceding converter needs to be larger than the peak voltage Vg at the inverter network side.
B. In order to ensure stable operation of the preceding stage LLC converter, a gain curve of the LLC converter needs to be calculated in advance, a gain calculation formula of the LLC converter is utilized, and the switching frequency is adjusted according to the gain curve so as to enable the LLC converter to work in an inductive working interval on the right side of a resonance point of the gain curve, so that stable operation of the preceding stage converter is ensured, and the gain calculation formula of the LLC converter is as follows:
wherein: m is LLC converter gain; q is the LLC converter quality factor; f (F) x Normalizing the switching frequency for the LLC converter; m= (L) m +L r )/L r ，L m Exciting inductance value for LLC converter, L r Resonant inductance value for the LLC converter.
C. If the output voltage of the pre-stage converter is reduced, the output power of the system is reduced, and the output power of the system is required to be ensured to be larger than the minimum output power of the system.
3) And selecting a point with a difference value larger than the voltage resolution of the adopted voltage sensor on the output voltage change track under different capacitance values as a transient characteristic point, wherein the lowest point on the transient voltage change curve can be selected as the transient characteristic point by reducing the output voltage of the previous stage.
4) And recording the selected transient characteristic points through a voltage sensor, and adjusting parameters and a control structure of the pre-stage DCDC converter when the voltage of the capacitor controlled by the post-stage voltage loop returns to the normal operation value of the system, so that the system is restored to normal operation.
(2) The front stage is a Boost converter:
1) When the front stage is a Boost converter, the output voltage of the front stage Boost converter can be changed by changing the duty cycle of the front stage Boost converter. Firstly, the duty ratio of the Boost converter under the current voltage closed loop is recorded, then the voltage loop of the Boost converter is opened, the duty ratio of the Boost converter is changed at the moment, and the output voltage of the Boost converter can be changed.
2) When the current stage is a Boost converter and the output voltage of the Boost converter is increased, the following constraint condition needs to be met:
A. when the output voltage of the front stage rises, the modulation degree of the system is reduced, and the system needs to be ensured to meet the minimum modulation degree of the stable operation of the minimum system.
B. The current stage is a Boost converter and the adjustment limitation of the duty cycle needs to be considered. The Boost converter duty cycle is not more than 0.9 in engineering, namely: d is less than or equal to 0.9.
3) And selecting a point with a difference value larger than the voltage resolution of the adopted voltage sensor on the output voltage change track under different capacitance values as a transient characteristic point, and increasing the output voltage of the previous stage can select a step value at a certain moment on the transient voltage change curve as the transient characteristic point.
4) And recording the selected transient characteristic points through a voltage sensor, and adjusting parameters and a control structure of the pre-stage DCDC converter when the voltage of the capacitor controlled by the post-stage voltage loop returns to the normal operation value of the system, so that the system is restored to normal operation.
In one embodiment, the step S30: predicting the capacitance value of the direct current capacitor by utilizing the transient characteristic points specifically comprises the following steps:
and constructing a training set by utilizing the transient characteristic points, and training a support vector machine to obtain a prediction model, wherein a kernel function of the support vector machine adopts rbf.
Taking the method of using support vector machine regression (SVR) as an example, the curve characteristic points are analyzed:
assume that for a given training data (x 1 ,y 1 ),(x 2 ,y 2 ),...,(x n ,y n )∈R n X R, using linear regression functionFitting the data and solving for ωAnd b can obtain a fitted curve, and the complexity of the function is controlled by making the regression function as flat as possible, which is equivalent to minimizing ω 2 Comprehensively considering the fitting error to obtain constraint conditions for solving omega and b
By introducing a Lagrangian function and saddle-pointing the Lagrangian function, the dual problem of equation (1) can be obtained.
Solving the formula (2) to obtain:
based on the KKT condition of the original problem formula (1), the threshold b can be calculated by the following formula
Linear SVR can be generalized into nonlinear high-dimensional space by introducing a kernel function K (x, x'), constructing a fitting function in high-dimensional space by solving the following optimization problem:
in this embodiment, the SVR algorithm is written by Python, the SVR library function is called, rbf is selected as the kernel function, and the constant c=400. Training is carried out by using the constructed training set to obtain a fitting curve as shown in fig. 12, the round dots in the figure represent training data in the training set obtained through experiments, the curve is a capacitance prediction model which is the relationship between transient characteristic points and capacitance values obtained through data fitting of the training data, the ordinate is the value of the transient characteristic points, and the abscissa is the capacitance value. As can be seen from fig. 12, the SVR algorithm has a better fitting effect. The fit score was 0.996 (full score 1). The rated capacitance was 6600uf. The capacitance value of the training set is from 4800uf to 7000uf, and the capacitance failure can be considered to be unusable as the rated capacitance value of the capacitor is reduced by 20% from 6600uf. Thus, curve segments 6600uf to 7000uf and 4800uf to 5000uf are not employed. The fitting model actually selected is the 6600uf curve segment from 5280uf to 6600uf.
Further, after model training is completed, the method can be used for capacitance monitoring, transient characteristic points recorded by the voltage sensor are input into a prediction model, and a predicted capacitance value can be obtained. When the capacitance value is below 20% rated value, the capacitance needs to be replaced, and the predicted result, actual value and error are given in table 1.
TABLE 1
| Rating/uf | Predicted value/uf | Error of |
| 6600 | 6575.60 | 0.37％ |
| 6300 | 6326.09 | 0.41％ |
| 6000 | 6038.83 | 0.65％ |
| 5700 | 5693.63 | 0.11％ |
| 5400 | 5378.87 | 0.39％ |
It can be seen from table 1 that the maximum error of the proposed method is not more than 1%, the estimation accuracy is better, and the method is an online monitoring method and has better engineering application value.
Aiming at the characteristic that the two-stage inversion topology is difficult to model and the direct-current capacitance is difficult to monitor, the embodiment creates and creates the instantaneous controllable voltage mutation of the direct-current capacitance by changing the pre-stage DC/DC voltage command, generates transient characteristic points related to capacitance values of the capacitance, and carries out capacitance value information fitting so as to monitor the capacitance values of the capacitance. Compared with a method adopting capacitive voltage ripple, the method does not need to use a high-precision voltage sensor, does not increase extra hardware, saves monitoring cost, and has accurate monitoring result.
As shown in fig. 13, a second embodiment of the present invention provides a dc capacitor aging monitoring system of a two-stage inverter, where the two-stage inverter includes a front-stage conversion device and a rear-stage inverter, and the system includes:
the parameter control module 10 is configured to change a control parameter of the preceding-stage conversion device, control an output voltage of the preceding-stage conversion device to generate a transient voltage change curve, where a minimum value of the output voltage when the output voltage drops is greater than a network-side voltage peak value, and a system modulation degree of the output voltage rising process is not lower than a system minimum modulation degree;
the characteristic point selection module 20 is configured to record the transient voltage change curve by using a voltage sensor at the dc capacitor side, and select a transient characteristic point;
the capacitance prediction module 30 is configured to predict a capacitance value of the dc capacitance by using the transient characteristic point.
According to the embodiment, the control parameters of the pre-stage conversion device are changed, so that the output voltage of the pre-stage conversion device is subjected to quick transient, namely, a transient process is interposed when the circuit normally operates, the voltage on the direct current side is dithered, the transient fluctuation which can be identified by a low-precision voltage sensor is generated within the safety margin range, and the capacitance value is monitored by recording a transient voltage change curve and extracting transient characteristic points related to the capacitance value; the high-precision voltage sensor is avoided, no extra hardware is added, and the monitoring cost is saved.
In one embodiment, the parameter control module 10 specifically includes:
the first control unit is used for changing control parameters of the pre-stage conversion device and controlling the output voltage of the pre-stage conversion device to drop and meet a first constraint condition when the output voltage drops;
the second control unit is used for changing control parameters of the pre-stage conversion device and controlling the output voltage of the pre-stage conversion device to rise and meet a second constraint condition when the output voltage rises;
and the curve generation module is used for reading the voltage value of the direct-current side capacitor and generating the transient voltage change curve.
In an embodiment, the first constraint includes:
1) The output voltage of the front-stage conversion device is larger than the voltage peak value at the network side:
2) When the parameter of the pre-stage conversion device is controlled to change, the pre-stage conversion device is ensured to work in a stable working interval;
3) Meets the minimum output power limit of the system:
V dcmin ·I≥P min
wherein: v (V) dc An output voltage of the current level conversion device; v (V) g Is the peak voltage of the network side; v (V) dcmin The lowest value of the output voltage of the current-stage conversion device; i is direct-current side current; p (P) min Is the minimum output power of the system.
In an embodiment, the second constraint includes:
1) Minimum modulation system constraint:
wherein: v (V) dcmax Is the maximum value of the output voltage; v (V) g Is the peak value of the power grid voltage; r is R min The minimum modulation degree of the system is set;
2) When the parameter of the pre-stage conversion device is controlled to change, the pre-stage conversion device is ensured to work in a stable working interval.
In an embodiment, when the preceding-stage conversion device is an LLC converter, a condition for ensuring that the LLC converter operates in a stable operating region thereof is an inductive operating region in which the switching frequency is adjusted according to a gain curve so as to operate on the right side of a resonance point of the gain curve, and a gain calculation formula of the LLC converter is as follows:
wherein: m is LLC converter gain; q is the LLC converter quality factor; f (F) x Normalizing the switching frequency for the LLC converter; m= (L) m +L r )/L r ，L m Exciting inductance value for LLC converter, L r Resonant inductance value for the LLC converter.
In an embodiment, when the control parameters of the pre-stage conversion device are changed and the output voltage of the pre-stage conversion device is controlled to generate a fast transient, the control mode of the pre-stage conversion device is open loop control, and the post-stage inversion device adopts a voltage outer loop and current inner loop double loop control mode.
In an embodiment, the transient characteristic points include a lowest point on the transient voltage change curve at different capacitance values, a transient voltage step point at a certain moment, and a point where a difference value is greater than a voltage resolution of the voltage sensor.
In an embodiment, the system further comprises a parameter adjustment module, specifically configured to:
and when the voltage of the capacitor controlled by the voltage outer loop of the rear-stage reverse inversion device returns to the normal operation value of the system, adjusting parameters and a control structure of the front-stage conversion device to enable the system to recover to normal operation.
In one embodiment, the capacitance prediction module 30 includes:
constructing a training set by utilizing the transient characteristic points, and training a support vector machine to obtain a prediction model, wherein a kernel function of the support vector machine adopts rbf;
and processing the currently extracted transient characteristic points by using the prediction model, and predicting the capacitance value of the capacitor.
It should be noted that, in other embodiments of the two-stage inverter dc capacitor aging monitoring system or the implementation method thereof according to the present invention, reference may be made to the above embodiments of the method, and no redundant description is provided herein.
In the description of the present specification, a description referring to terms "one embodiment," "some embodiments," "examples," "specific examples," or "some examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present invention. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiments or examples. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
Furthermore, the terms "first," "second," and the like, are used for descriptive purposes only and are not to be construed as indicating or implying a relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defining "a first" or "a second" may explicitly or implicitly include at least one such feature. In the description of the present invention, the meaning of "plurality" means at least two, for example, two, three, etc., unless specifically defined otherwise.
While embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and not to be construed as limiting the invention, and that variations, modifications, alternatives and variations may be made to the above embodiments by one of ordinary skill in the art within the scope of the invention.
Claims (10)
1. The method for monitoring the direct-current capacitance aging of the two-stage inverter device comprises a front-stage conversion device and a rear-stage inverter device, and is characterized by comprising the following steps of:
changing control parameters of the pre-stage conversion device, and controlling the output voltage of the pre-stage conversion device to generate a transient voltage change curve by fast transient, wherein the minimum value of the output voltage is larger than the network side voltage peak value when the output voltage is reduced, and the system modulation degree of the output voltage increasing process is not lower than the system minimum modulation degree;
recording the transient voltage change curve by using a voltage sensor at the DC capacitor side, and selecting transient characteristic points;
and predicting the capacitance value of the direct current capacitor by utilizing the transient characteristic points.
2. The method for monitoring dc capacitor aging of a two-stage inverter according to claim 1, wherein said changing control parameters of said pre-stage converter to control an output voltage of said pre-stage converter to generate a transient voltage change curve comprises:
changing control parameters of the pre-stage conversion device, and controlling the output voltage of the pre-stage conversion device to drop and meet a first constraint condition when the output voltage drops;
changing control parameters of the pre-stage conversion device, and controlling the output voltage of the pre-stage conversion device to rise and meet a second constraint condition when the output voltage rises;
and reading the voltage value of the direct-current side capacitor, and generating the transient voltage change curve.
3. The method for monitoring dc capacitor aging of a two-stage inverter according to claim 2, wherein the first constraint condition includes:
1) The output voltage of the front-stage conversion device is larger than the voltage peak value at the network side:
2) When the parameter of the pre-stage conversion device is controlled to change, the pre-stage conversion device is ensured to work in a stable working interval;
3) Meets the minimum output power limit of the system:
V dcmin ·I≥P min
wherein: v (V) dc An output voltage of the current level conversion device; v (V) g Is the peak voltage of the network side; v (V) dcmin The lowest value of the output voltage of the current-stage conversion device; i is direct-current side current; p (P) min Is the minimum output power of the system.
4. The method for monitoring dc capacitor aging of a two-stage inverter according to claim 2, wherein the second constraint condition includes:
1) Minimum modulation system constraint:
wherein: v (V) dcmax Is the maximum value of the output voltage; v (V) g Is the peak value of the power grid voltage; r is R min The minimum modulation degree of the system is set;
2) When the parameter of the pre-stage conversion device is controlled to change, the pre-stage conversion device is ensured to work in a stable working interval.
5. The method for monitoring dc capacitor aging of a two-stage inverter according to claim 3 or 4, wherein when the preceding-stage converter is an LLC converter, a condition for ensuring that the LLC converter operates in a stable operating region thereof is an inductive operating region in which a switching frequency is adjusted according to a gain curve so as to operate on the right side of a resonance point of the gain curve, and a gain calculation formula of the LLC converter is as follows:
wherein: m is LLC converter gain; q is the LLC converter quality factor; f (F) x Normalizing the switching frequency for the LLC converter; m= (L) m +L r )/L r ，L m Exciting inductance value for LLC converter, L r Resonant inductance value for the LLC converter.
6. The method for monitoring the aging of the direct current capacitor of the two-stage inverter according to claim 1, wherein when the control parameters of the pre-stage conversion device are changed and the output voltage of the pre-stage conversion device is controlled to generate a fast transient, the control mode of the pre-stage conversion device is open loop control, and the post-stage inverter adopts a voltage outer loop and current inner loop double loop control mode.
7. The method for monitoring the aging of the dc capacitor of the two-stage inverter according to claim 1, wherein the transient characteristic points include a lowest point on the transient voltage change curve, a transient voltage step point at a certain moment, and a point with a difference greater than the voltage resolution of the voltage sensor at different capacitance values.
8. The method for monitoring the aging of the dc capacitor of the two-stage inverter according to claim 1, wherein after the voltage sensor on the dc capacitor side records the transient voltage change curve and selects the transient characteristic point, the method further comprises:
and when the voltage of the capacitor controlled by the voltage outer loop of the rear-stage reverse inversion device returns to the normal operation value of the system, adjusting parameters and a control structure of the front-stage conversion device to enable the system to recover to normal operation.
9. The method for monitoring the aging of the dc capacitor of the two-stage inverter according to claim 1, wherein predicting the capacitance value of the dc capacitor by using the transient characteristic point comprises:
and constructing a training set by utilizing the transient characteristic points, and training a support vector machine to obtain a prediction model, wherein a kernel function of the support vector machine adopts rbf.
10. The utility model provides a two-stage type inversion device direct current capacitance aging monitoring system which characterized in that, two-stage type inversion device includes preceding level conversion device and rear level inversion device, the system includes:
the parameter control module is used for changing control parameters of the pre-stage conversion device, controlling the output voltage of the pre-stage conversion device to generate a transient voltage change curve, wherein the minimum value of the output voltage when the output voltage drops is larger than the network side voltage peak value, and the system modulation degree of the output voltage rising process is not lower than the system minimum modulation degree;
the characteristic point selection module is used for recording the transient voltage change curve by utilizing a voltage sensor at the DC capacitor side and selecting transient characteristic points;
and the capacitance prediction module is used for predicting the capacitance value of the direct current capacitance by utilizing the transient characteristic points.
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