3.1. DCLC Sample Preparation
The DCLC sample selected for this experiment consists of four elements connected in parallel, and its capacitance is 200 ± 5% μF, as illustrated in
Figure 4. Each element is wound with two layers of metalized film. The thickness of the film is considered as 6 μm, the width of the film is 150 mm, and the average resistance value of the square area of the metal coating is 35 Ω/□. The diameter of the element is 76 mm, and the corresponding inside capacitor consists of two series of segment structures. The rated voltage (
UNDC) of the DCLC sample is set as 2800 V DC, and the rated working electric field strength (
E0) is 233.3 V/μm.
The tension coefficients (kT) in the winding process are set as 1.4, 1.5, 1.6, and 1.7, respectively, and the corresponding tension (Td) magnitudes are 1260, 1350, 1440, and 1530 gf, respectively. In addition, the winding pressure (TF) on the film during winding is set as 10,800 gf. The elements of the DCLC samples are heated at two temperature levels: T1 = 105 °C and T2 = 110 °C.
Under the premise that the HST of the DCLC is 105 °C and the values of WTC (kT), in order, are set equal to 1.4, 1.5, 1.6, and 1.7, respectively, and the corresponding tension magnitudes (Td) are 1260, 1350, 1440, and 1530 gf, respectively, and the winding pressure (TF) on the film during winding is set as 10,800 gf. The samples used for the aging tests are itemized as C1.1, C1.2, C1.3, and C1.4, respectively, and the samples for the insulation resistance measurements are itemized as C1.11, C1.12, C1.13, and C1.14, respectively, whereas the samples used for the voltage withstanding tests are itemized as C1.21, C1.22, C1.23, and C1.24, respectively.
Under the premise that the HST of the DCLC is 110 °C and the WTC (kT) are 1.4, 1.5, 1.6, and 1.7, respectively, and the corresponding tension (Td) values are 1260, 1350, 1440, and 1530 gf, respectively, and the winding pressure (TF) on the film during winding is set equal to 10,800 gf. The sample used for aging tests are itemized as C2.1, C2.2, C2.3, and C2.4, respectively, and the sample employed for insulation resistance measurements are itemized as C2.11, C2.12, C2.13, and C2.14, whereas the sample used for voltage withstanding tests are itemized as C2.21, C2.22, C2.23, and C2.24, respectively.
The number of samples tested associated with each number (C1.1, C1.2, …, C2.24) was four. Therefore, in total, ninety-six test samples were implemented for the above three tests.
3.2. Life Aging Test
A test platform was set up, consisting of the necessary equipment and measuring devices for the life aging test, as presented in
Figure 5. The thermostat is capable of controlling the temperature from 20 °C to 150 °C. The DC power supply is able to supply 0–8000 V DC. In addition, an LCR meter was utilized to measure the capacitance and tangent of the loss angle (usually, the testing frequency is set as 100 Hz).
The test method refers to the durability test of IEC 61071: 2017 Capacitors for Power Electronics [
23].
The test method procedures are as follows: First, the capacitance and tangent of the samples are measured at room temperature. Then, the samples are placed in the test thermostat until the temperature of the samples reaches 70 °C. Next, the voltage of 1.4
UNDC is applied to the samples. After 100 h, the samples are taken out and their capacitance and tangent of the loss angle are immediately measured. Then, they are placed into the test thermostat and 1.4
UNDC voltage is applied to them at 70 °C. After 150 h, the samples are taken out and the capacitance and tangent of loss angle values are immediately measured. Then, they are placed into the test thermostat and, again, 1.4
UNDC voltage is applied at 70 °C for a longer time duration. After the end, the samples are taken out, and the capacitance and tangent of the loss angle are immediately measured. After cooling the samples, the capacitance and the tangent of the loss angle are measured again. Before and after the test, the rate of change of the capacitance should not exceed 3% [
23]. The time value of the test with a capacitance change rate of −3% characterizes the life of the capacitor.
The capacitance (
Ct) measured at high temperatures can be converted to the capacitance (
CT) at room temperature according to the following equation:
where
Tt represents the high-temperature value,
T0 denotes the room-temperature value, and
kc is the temperature coefficient of the BOPP film capacitance (commonly set as −0.0002/K).
The capacitance change rate ∆
C (%) can be calculated based on the following equation:
where
C1 denotes the capacitance after the test, and
C0 represents the capacitance before the test.
3.3. Insulation Resistance Measurement
A test platform consisting of the necessary equipment and a measuring device was set up to measure the insulation resistance. The measurement circuit of the test platform is presented in
Figure 6. In the figure,
C is the equivalent capacitance of the sample,
RP is the insulation resistance of the sample,
UDC is the DC power supply,
S1 is the charging switch,
S2 is the discharge switch,
R is the discharge resistor, and
V represents the high-impedance digital voltmeter (the measurement range of DC voltage is 0–10,000 V, and the input impedance is 1000 MΩ).
The sample is placed in the test thermostat to maintain the test temperature. To eliminate the effect of the sample surface leakage current on RP, a ground wire is employed to bypass the surface leakage current. The surface conductance of a clean dry solid dielectric of the sample is very small; that is, the value of the surface insulation resistance is very high. Therefore, this test does not consider the measurement of surface insulation resistance, and the surface conductance is assumed to be zero.
The test procedure is as follows: Set the thermostat temperature to 20 °C and place the sample in the thermostat for at least 12 h. Connect the test circuit as shown in
Figure 6. First switch
S2 off, then switch
S1 on, and charge the sample with a test voltage of
U0 for 1 min. Then switch
S1 off, and let the sample self-discharge. After a period of time
t, use the V to measure the voltage to obtain
Ut and the LCR meter to measure the capacitance value. Finally, the insulation resistance (
RP) of the sample is calculated by employing Equation (7). After the test is completed,
S2 is closed, and the sample is discharged through the resistance
R:
where
U0 represents the initial voltage at the beginning of self-discharge,
Ut denotes the voltage at self-discharge to time
t,
t is the time of self-discharge, and
C is the capacitance of the sample.
The setting sequence of the initial voltage U0 is as follows: 1.0 UNDC for the first time. Then it gradually increases, with the voltage increasing by 0.1 UNDC each time, until U0 reaches 2.0 UNDC.
3.4. Withstand Voltage Test
A test platform was set up, consisting of necessary equipment and measuring devices for the withstand voltage test. The test circuit of the test platform is also illustrated in
Figure 6.
The test method refers to the voltage test between terminals of IEC 61071: 2017 Capacitors for Power Electronics.
The test method is as follows: First, set the test temperature at 20 °C in the thermostat. Then, place the sample into the thermostat, keep it for 12 h or more, charge the sample to a test voltage value of 5 min, and then use the LCR meter to measure its capacitance value and calculate the capacitance change rate. Setting of test sequence: the initial test voltage is 1.5 UNDC, and then the test voltage is gradually increased by 0.1 UNDC each time until the sample fails.
The capacitance change rate ∆C (%) can be calculated according to Equation (5).