Online monitoring method of aluminum electrolytic capacitor life based on auxiliary discharge network

Abstract

The present invention discloses an online life monitoring method for aluminum electrolytic capacitors based on an auxiliary discharge network. First, the voltage-sharing resistance is calculated according to the leakage current of the aluminum electrolytic capacitor and the structure of the capacitor group. Then, the auxiliary discharge network is connected in parallel to both ends of the aluminum electrolytic capacitor. According to the obtained voltage-sharing resistance R, the highest voltage at both ends of a single aluminum electrolytic capacitor, and the voltage V at both ends of the capacitor group, the voltage of the capacitor group is calculated. in and the structure of the capacitor bank to calculate the auxiliary discharge resistor R sup The midpoint voltage u of the capacitor bank is obtained by the timer mid (t) and the discharge time constant τ of the aluminum electrolytic capacitor below the midpoint of the capacitor group; compare the capacitance of the aluminum electrolytic capacitor at this time with their respective initial values; when the failure criterion of the aluminum electrolytic capacitor is met, it is determined that the aluminum electrolytic capacitor has failed. The present invention realizes the function of measuring the capacitance of the aluminum electrolytic capacitor through an auxiliary discharge network, samples a large signal, and does not need to sample the capacitor current.

Description

Technical Field

[0001] The invention belongs to the field of power electronics, and in particular is an online life monitoring method for aluminum electrolytic capacitors based on an auxiliary discharge network. Background Art

[0002] Capacitors are widely used in the DC link of power converters, and have the functions of suppressing DC voltage ripple and balancing instantaneous power changes between the input and output sides. DC link capacitor banks are usually composed of aluminum electrolytic capacitors, metallized polypropylene film capacitors, and multilayer ceramic capacitors. In practical applications, about 30% of power converter failures are caused by capacitor failure, and the construction of aluminum electrolytic capacitors makes them more sensitive to electrical and thermal stress than other capacitors in the capacitor bank, making them more prone to failure. Therefore, monitoring the life of aluminum electrolytic capacitors and repairing or replacing them before they fail is of great significance to ensure the reliable operation of power converters.

[0003] During the aging process of aluminum electrolytic capacitors, the capacitance value gradually decreases and the equivalent series resistance value gradually increases. The existing specifications stipulate that the failure criterion for aluminum electrolytic capacitors is that the capacitance value decreases by 20% or the equivalent series resistance value increases to 2.8 times the initial value. Based on this failure criterion, many aluminum electrolytic capacitor life monitoring methods have been derived: according to whether the monitoring is real-time, the monitoring methods can be divided into two categories, one is offline monitoring and the other is online monitoring. The offline monitoring method requires pausing the converter operation and disassembling the capacitor for measurement, which will interrupt the continuous operation of the converter. The online monitoring method can obtain the health status data of the aluminum electrolytic capacitor in real time without stopping the converter operation, which is more flexible and convenient.

[0004] The existing online monitoring methods for the life of aluminum electrolytic capacitors can be divided into three types:

[0005] 1) Small signal ripple monitoring technology: measure capacitor voltage ripple and current to calculate capacitance value or equivalent series resistance value. Measure capacitor voltage ripple, infer capacitor current based on topology, and calculate capacitance value and equivalent series resistance value. However, this method requires additional current sensor, and voltage sensor is affected by measurement range, so its accuracy is limited.

[0006] 2) Large signal transient monitoring technology: Calculate the capacitance value or equivalent series resistance value based on the waveform when the circuit is started, stopped or transient. This method is a quasi-online measurement method and is only suitable for online monitoring under specific working conditions.

[0007] 3) Black box model monitoring technology: collect input voltage, current, output voltage and other data for data training to obtain capacitance and equivalent series resistance values. However, this method requires a large amount of measurement data and complex algorithms, and occupies a large amount of computing resources.

[0008] In high voltage and high current situations, aluminum electrolytic capacitors often use a series-parallel structure to improve the withstand voltage and current carrying capacity, but the existing online monitoring methods are difficult to meet the needs of online monitoring of the life of single-branch capacitors. Summary of the invention

[0009] In view of the deficiencies in the prior art, the technical problem to be solved by the present invention is to provide an online monitoring method for the life of an aluminum electrolytic capacitor based on an auxiliary discharge network.

[0010] The technical solution of the present invention to solve the technical problem is to provide an online monitoring method for the life of an aluminum electrolytic capacitor based on an auxiliary discharge network, characterized in that the method comprises the following steps:

[0011] Step 1: Determine the structure of the capacitor bank according to the input side voltage and the output side voltage of the power converter; in order to achieve the voltage balancing effect, the current flowing through the voltage balancing resistor must be 20 times higher than the leakage current of the aluminum electrolytic capacitor; calculate the voltage balancing resistor size according to the leakage current of the aluminum electrolytic capacitor and the structure of the capacitor bank. The empirical formula for selecting the voltage balancing resistor R is shown in formula (1):

[0012]

[0013] In formula (1), N is the number of aluminum electrolytic capacitors in parallel, C i is the rated capacitance of a single aluminum electrolytic capacitor;

[0014] Step 2: Select a single aluminum electrolytic capacitor whose maximum voltage across both ends is not higher than its rated voltage V rated 0.8 times;

[0015] Since the input voltage value is known, the voltage V across the capacitor bank can be obtained. in ;

[0016] For series-parallel capacitor banks, the auxiliary discharge network is connected in parallel at both ends of the aluminum electrolytic capacitor located below the midpoint of each capacitor series branch; for parallel-series capacitor banks, the auxiliary discharge network is connected in parallel at both ends of the aluminum electrolytic capacitor located below the midpoint of the overall capacitor bank; the auxiliary discharge network consists of an auxiliary discharge resistor R sup And a switch S is formed in series;

[0017] Then, based on the obtained voltage-equalizing resistor R, the maximum voltage across a single aluminum electrolytic capacitor, and the voltage V across the capacitor bank, in and the structure of the capacitor bank to calculate the auxiliary discharge resistor R sup The resistance value of auxiliary discharge resistor R sup The selection formula is shown in formula (2):

[0018]

[0019] In formula (2), R sup is the resistance of the auxiliary discharge resistor, V in is the voltage across the capacitor bank, V rated is the rated voltage of the aluminum electrolytic capacitor, and R is the resistance of the voltage-equalizing resistor;

[0020] Step 3: During the operation of the power converter, the on-off of the switch S in the auxiliary discharge network is controlled to realize the discharge of the aluminum electrolytic capacitor connected in parallel with the auxiliary discharge network; when the switch S is turned on, the timer is started, and when the aluminum electrolytic capacitor connected in parallel with the auxiliary discharge network is discharged to the threshold voltage, the timer is stopped, thereby obtaining the midpoint voltage v of the capacitor group. mid (t) and the discharge time constant τ of the aluminum electrolytic capacitor below the midpoint of the capacitor bank;

[0021] For a series-parallel capacitor bank, all the voltage-equalizing resistors have the same resistance value, and the midpoint voltage v of the capacitor bank is mid (t) is:

[0022]

[0023] In formula (3), τ is the discharge time constant, which can be obtained according to the timer result; the midpoint voltage v of the capacitor bank is mid (t) is obtained according to the midpoint voltage sampling result;

[0024] Then, the time domain analysis of the input side circuit of the power converter is carried out, and the following results are obtained:

[0025]

[0026] Then, by transforming formula (4), we can obtain the capacitance of the aluminum electrolytic capacitor in each capacitor series branch:

[0027]

[0028] In formula (5), C1, C2, ..., C 2n is the capacitance of each aluminum electrolytic capacitor;

[0029] For a parallel-series capacitor bank, all the equalizing resistors have the same resistance value, and the midpoint voltage v of the capacitor bank is mid (t) as shown in formula (3);

[0030] Then, the time domain analysis of the input side circuit of the power converter is carried out, and the following results are obtained:

[0031]

[0032] Then, by transforming formula (6), the capacitance of the aluminum electrolytic capacitor in the overall capacitor bank is obtained as follows:

[0033]

[0034] Step 4: Compare the capacitance of the aluminum electrolytic capacitor obtained in step 3 with the respective initial values; when the failure criterion of the aluminum electrolytic capacitor is met, it is determined that the aluminum electrolytic capacitor has failed.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] (1) The present invention realizes the function of measuring the capacitance of aluminum electrolytic capacitors through an auxiliary discharge network.

[0037] (2) The present invention does not need to sample the capacitor current, so there is no need to add an additional current sensor, which greatly reduces the cost and alleviates the intrusion problem caused by inserting the current sensor into the capacitor branch.

[0038] (3) The present invention samples large signals, thus eliminating the need for high-precision, high-bandwidth sampling circuits, reducing the complexity of the processing circuit and also reducing costs.

[0039] (4) The capacitor discharge of the present invention only affects the midpoint voltage and has no effect on the voltage across the capacitor bank. The measurement time is more flexible, and online monitoring of the capacitor life can be achieved instead of only when the converter is shut down or started.

[0040] (5) The present invention is not only applicable to the online monitoring of the life of the aluminum electrolytic capacitor of the Buck converter, but also applicable to other converter topologies. At the same time, the present invention is applicable to the online monitoring of the life of the aluminum electrolytic capacitors on the input side and the output side of the converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 A circuit topology diagram of a series-parallel capacitor bank connected in parallel with an auxiliary discharge network according to the present invention;

[0042] Figure 2 The circuit topology diagram of the parallel-series capacitor bank with an auxiliary discharge network in parallel according to the present invention

[0043] Figure 3 This is a main circuit topology diagram of the Buck converter according to Embodiment 1 of the present invention;

[0044] Figure 4 1 is a voltage waveform diagram of the aluminum electrolytic capacitor C2 according to the first embodiment of the present invention. DETAILED DESCRIPTION

[0045] The specific embodiments of the present invention are given below. The specific embodiments are only used to further illustrate the present invention in detail and do not limit the protection scope of the claims of the present invention.

[0046] The present invention provides an online life monitoring method for aluminum electrolytic capacitors based on an auxiliary discharge network (hereinafter referred to as the method), characterized in that the method comprises the following steps:

[0047] Step 1: Determine the structure of the capacitor bank according to the input side voltage and output side voltage of the power converter (such as Figure 1 and Figure 2 In order to achieve the voltage balancing effect, the current flowing through the voltage balancing resistor must be 20 times higher than the leakage current of the aluminum electrolytic capacitor. The voltage balancing resistor is calculated based on the leakage current of the aluminum electrolytic capacitor and the structure of the capacitor bank. The empirical formula for selecting the voltage balancing resistor R is shown in formula (1):

[0048]

[0049] In formula (1), N is the number of aluminum electrolytic capacitors in parallel (in this embodiment, for a series-parallel capacitor bank, N is 1; for a parallel-series capacitor bank, N is n), C i is the rated capacitance of a single aluminum electrolytic capacitor;

[0050] Step 2: The maximum voltage across a single aluminum electrolytic capacitor must be lower than its rated voltage V rated , and a certain margin must be left, and the maximum voltage across a single aluminum electrolytic capacitor is selected to be no higher than 0.8 times its rated voltage (in this embodiment, the maximum voltage across a single aluminum electrolytic capacitor is 0.8 times its rated voltage);

[0051] Since the input voltage value is known, the voltage V across the capacitor bank can be obtained. in ;

[0052] For series-parallel capacitor banks, the auxiliary discharge network is connected in parallel at both ends of the aluminum electrolytic capacitor located below the midpoint of each capacitor series branch; for parallel-series capacitor banks, the auxiliary discharge network is connected in parallel at both ends of the aluminum electrolytic capacitor located below the midpoint of the overall capacitor bank; the auxiliary discharge network consists of an auxiliary discharge resistor R sup And a switch S is formed in series;

[0053] Then, based on the obtained voltage-equalizing resistor R, the maximum voltage across a single aluminum electrolytic capacitor, and the voltage V across the capacitor bank, in and the structure of the capacitor bank (such as Figure 1 and Figure 2 As shown) to calculate the auxiliary discharge resistor R sup The resistance value of auxiliary discharge resistor R sup The selection formula is shown in formula (2):

[0054]

[0055] In formula (2), R sup is the resistance of the auxiliary discharge resistor, V in is the voltage across the capacitor bank, V rated is the rated voltage of the aluminum electrolytic capacitor, and R is the resistance of the voltage-equalizing resistor;

[0056] Step 3: During the operation of the power converter, the on-off of the switch S in the auxiliary discharge network is controlled by the digital controller to realize the discharge of the aluminum electrolytic capacitor connected in parallel with the auxiliary discharge network; when the switch S is turned on, the timer is started, and when the aluminum electrolytic capacitor is discharged to a threshold voltage (in this embodiment, the threshold voltage is set to 0.79 times the initial voltage), the timer is stopped, thereby obtaining the midpoint voltage v of the capacitor group. mid (t) and the discharge time constant τ of the aluminum electrolytic capacitor on the lower side of the midpoint of the capacitor bank (i.e., the aluminum electrolytic capacitor connected in parallel with the auxiliary discharge network);

[0057] The midpoint voltage of the capacitor bank is v mid (t) According to the structure of the capacitor bank, there are two cases:

[0058] Case 1: For series-parallel capacitor banks (such as Figure 1 As shown), the resistance values ​​of all voltage-equalizing resistors are equal (i.e. R1=R2=…=R 2n-1 =R 2n =R), the midpoint voltage of the capacitor bank v mid (t) is:

[0059]

[0060] In formula (3), τ is the discharge time constant, which can be obtained according to the timer result; the midpoint voltage of the capacitor bank is mid (t) is obtained according to the midpoint voltage sampling result;

[0061] Then, the time domain analysis of the input side circuit of the power converter is carried out, and the following results are obtained:

[0062]

[0063] Then, by transforming formula (4), we can obtain the capacitance of the aluminum electrolytic capacitor in each capacitor series branch:

[0064]

[0065] In formula (5), C1, C2, ..., C 2n is the capacitance of each aluminum electrolytic capacitor;

[0066] Case 2: For parallel-series capacitor banks (such as Figure 2As shown), the resistance values ​​of all the voltage-equalizing resistors are equal (ie, R1 = R2 = R), and the midpoint voltage of the capacitor group is v mid (t) as shown in formula (3);

[0067] Then, the time domain analysis of the input side circuit of the power converter is carried out, and the following results are obtained:

[0068]

[0069] Then, by transforming formula (6), the capacitance of the aluminum electrolytic capacitor in the overall capacitor bank is obtained as follows:

[0070]

[0071] Step 4: Compare the capacitance of the aluminum electrolytic capacitor obtained in step 3 with their respective initial values; when the failure criterion of the aluminum electrolytic capacitor is met (i.e., the capacitance of the aluminum electrolytic capacitor decreases by 20%), it is determined that the aluminum electrolytic capacitor has failed and needs to be repaired or replaced.

[0072] Example 1

[0073] A method of converting the high voltage side voltage V H = 300V DC step-down to V L =150V DC, full load power P max =1.5kW Buck converter, set the switching frequency fs = 200kHz. Buck converter main circuit topology Figure 3 As shown, the aluminum electrolytic capacitor C1=C2=600μF on the input side of the converter, and the online monitoring method includes the following steps:

[0074] Step 1: In order to achieve the voltage balancing effect, the current flowing through the voltage balancing resistor must be 20 times higher than the leakage current of the aluminum electrolytic capacitor. In practical applications, the resistance of the voltage balancing resistor R can be calculated according to formula (1). However, due to the limitation of simulation time, a large voltage balancing resistor will make the discharge time too long, resulting in the inability to obtain a complete waveform in the simulation. Therefore, the selected voltage balancing resistor has a lower resistance and a smaller tolerance rate, R1 = R2 = 10Ω.

[0075] Step 2: In this embodiment, the maximum voltage across the two ends of a single aluminum electrolytic capacitor is selected to be lower than 0.8 times of its rated voltage.

[0076] Since the input voltage value is known to be 300V, the voltage V across the capacitor bank can be obtained. in is 300V.

[0077] Then, based on the obtained voltage-equalizing resistor R, the maximum voltage across a single aluminum electrolytic capacitor, and the voltage V across the capacitor bank, in and the structure of the capacitor bank (such as Figure 3As shown) to calculate the auxiliary discharge resistor R sup1 In practical applications, the auxiliary discharge resistor R sup1 The resistance value can be calculated according to formula (2). Here, the resistance value of the auxiliary discharge resistor is R sup1 =10Ω.

[0078] Step 3: During the operation of the power converter, the digital controller controls the on and off of the switch S1 in the auxiliary discharge network to realize the discharge of the aluminum electrolytic capacitor C2 connected in parallel with the auxiliary discharge network; when the switch S1 is turned on, the timer is started, the voltage across the aluminum electrolytic capacitor C1 begins to rise, and the voltage across the aluminum electrolytic capacitor C2 begins to drop. When the aluminum electrolytic capacitor C2 is discharged to the threshold voltage (in this embodiment, the threshold voltage is set to 118.4V, as shown in FIG. Figure 4 As shown), stop the timer, the discharge time constant τ of the aluminum electrolytic capacitor on the lower side of the midpoint of the capacitor group (i.e., the aluminum electrolytic capacitor C2) is 4ms (as shown Figure 4 According to formula (5), C1+C2=1200μF can be calculated.

[0079] Step 4: According to formula (5), C1+C2=1200μF can be calculated, which is the same as the set value. After 3 to 5 time constants, the aluminum electrolytic capacitor C2 is discharged to 100V, and the aluminum electrolytic capacitor C1 is charged to 200V. At this time, it is necessary to ensure that the rated voltage of the aluminum electrolytic capacitor C1 is greater than the maximum voltage it can withstand.

[0080] Similarly, it can be proved that when aluminum electrolytic capacitors are used in parallel-series structure, this method is still feasible, and the total capacitance of the aluminum electrolytic capacitors can be calculated according to formula (8).

[0081] Any matters not described in the present invention are applicable to the prior art.

Claims

1. An online monitoring method for the life of an aluminum electrolytic capacitor based on an auxiliary discharge network, characterized in that: The method comprises the following steps: Step 1: Determine the structure of the capacitor bank according to the input side voltage and the output side voltage of the power converter; in order to achieve the voltage balancing effect, the current flowing through the voltage balancing resistor must be 20 times higher than the leakage current of the aluminum electrolytic capacitor; calculate the voltage balancing resistor size according to the leakage current of the aluminum electrolytic capacitor and the structure of the capacitor bank. The empirical formula for selecting the voltage balancing resistor R is shown in formula (1): In formula (1), N is the number of aluminum electrolytic capacitors in parallel, C i is the rated capacitance of a single aluminum electrolytic capacitor; Step 2: Select a single aluminum electrolytic capacitor whose maximum voltage across both ends is not higher than its rated voltage V rated 0.8 times; Since the input voltage value is known, the voltage V across the capacitor bank can be obtained. in ; For series-parallel capacitor banks, the auxiliary discharge network is connected in parallel at both ends of the aluminum electrolytic capacitor located below the midpoint of each capacitor series branch; for parallel-series capacitor banks, the auxiliary discharge network is connected in parallel at both ends of the aluminum electrolytic capacitor located below the midpoint of the overall capacitor bank; the auxiliary discharge network consists of an auxiliary discharge resistor R sup And a switch S is formed in series; Then, based on the obtained voltage-equalizing resistor R, the maximum voltage across a single aluminum electrolytic capacitor, and the voltage V across the capacitor bank, in and the structure of the capacitor bank to calculate the auxiliary discharge resistor R sup The resistance value of auxiliary discharge resistor R sup The selection formula is shown in formula (2): In formula (2), R sup is the resistance of the auxiliary discharge resistor, V in is the voltage across the capacitor bank, V rated is the rated voltage of the aluminum electrolytic capacitor, and R is the resistance of the voltage-equalizing resistor; Step 3: During the operation of the power converter, the on-off of the switch S in the auxiliary discharge network is controlled to realize the discharge of the aluminum electrolytic capacitor connected in parallel with the auxiliary discharge network; when the switch S is turned on, the timer is started, and when the aluminum electrolytic capacitor connected in parallel with the auxiliary discharge network is discharged to the threshold voltage, the timer is stopped, thereby obtaining the midpoint voltage v of the capacitor group. mid (t) and the discharge time constant τ of the aluminum electrolytic capacitor below the midpoint of the capacitor bank; For a series-parallel capacitor bank, all the voltage-equalizing resistors have the same resistance value, and the midpoint voltage v of the capacitor bank is mid (t) is: In formula (3), τ is the discharge time constant, which can be obtained according to the timer result; the midpoint voltage v of the capacitor bank is mid (t) is obtained according to the midpoint voltage sampling result; Then, the time domain analysis of the input side circuit of the power converter is carried out, and the following results are obtained: Then, by transforming formula (4), we can obtain the capacitance of the aluminum electrolytic capacitor in each capacitor series branch: In formula (5), C1, C2, ..., C 2n is the capacitance of each aluminum electrolytic capacitor; For a parallel-series capacitor bank, all the equalizing resistors have the same resistance value, and the midpoint voltage v of the capacitor bank is mid (t) as shown in formula (3); Then, the time domain analysis of the input side circuit of the power converter is carried out, and the following results are obtained: Then, by transforming formula (6), the capacitance of the aluminum electrolytic capacitor in the overall capacitor bank is obtained as follows: Step 4: Compare the capacitance of the aluminum electrolytic capacitor obtained in step 3 with the respective initial values; when the failure criterion of the aluminum electrolytic capacitor is met, it is determined that the aluminum electrolytic capacitor has failed.

2. The method for online monitoring of the life of an aluminum electrolytic capacitor based on an auxiliary discharge network according to claim 1 is characterized in that: In step 1, for a series-parallel capacitor bank, the value of N is 1; for a parallel-series capacitor bank, the value of N is n.

3. The method for online monitoring of the life of an aluminum electrolytic capacitor based on an auxiliary discharge network according to claim 1 is characterized in that: In step 2, the maximum voltage across a single aluminum electrolytic capacitor is 0.8 times its rated voltage.

4. The method for online monitoring of the life of an aluminum electrolytic capacitor based on an auxiliary discharge network according to claim 1 is characterized in that: In step 3, the threshold voltage is 0.79 times the initial voltage.

Images