ACCELERATED AGEING TESTS
To shorten the test cycle, accelerated ageing tests were designed, including a voltage accelerated ageing test and a temperature accelerated ageing test.
To research the degradation characteristics of the thyristor, four typical electrical characteristics of thyristors were selected, including the reverse recovery characteristic, on-state characteristic, blocking characteristic and gate characteristic. Since the bipolar press pack thyristor is used in HVDC projects, a bipolar press pack high voltage thyristor, of the type KP03XY8500, was selected as the test object. The parameters are shown in Table 1.
Table 1. Main Electrical Parameters of Thyristor - KP03XY8500
Characteristics
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Symbol
|
Ratings
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Repetitive peak off-state voltage
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VDRM
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8.5kV
|
Repetitive peak reverse voltage
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VRRM
|
8.5kV
|
Average on-state current
|
IT(AV)
|
300A
|
Peak on-state voltage
|
VTM
|
2.8V
|
Peak one-cycle surge on-state current
|
ITSM
|
2.9kA
|
TEMPERATURE ACCELERATED AGEING TEST
The ageing rate k of thyristors conforms to the Arrhenius law as follows
Where, T is the temperature, and the A, E and a are the constants related to the material. Therefore, the thyristor ageing process is exponentially related to the junction temperature.
According to the thyristor data sheet, the thyristor should operate under a maximum junction temperature of 125 ℃, while in practice, the thyristor can operate above 200 ℃ for a certain time13. Therefore, a preliminary experiment was carried out to determine the temperature for the accelerated ageing test. The results indicated that if the junction temperature is higher than 190℃ for the selected thyristor, then the gate structure can be damaged in a short time, causing device failure. In contrast, when the junction temperature is 170 ℃, the selected thyristor can operate for a long time. Therefore, a junction temperature of 170 ℃ was selected for the accelerated ageing test.
The junction temperatures of selected thyristors were found to have certain differences under the same conduction current, and as the conduction current increases, the temperature differences become more obvious. Therefore, a preliminary experiment was carried out to select thyristors that have similar temperature characteristics.
Finally, 10 thyristors (TB1~TB10) with the same temperature characteristics were selected for the temperature accelerated ageing test. Because the average junction temperature is approximately proportional to the current, the junction temperature was controlled by the thyristor current using a DC source. The testing time was 1000 hours, and the electrical characteristics of the thyristors were measured every 100 hours. The degradation characteristics were obtained as follows.
recovery characteristic
There is a decreasing trend of the reverse recovery charge Qr and reverse recovery time ts of the thyristors with increasing testing time, as shown in Fig. 1. After 1000 h of testing time, Qr decreases by approximately 16.8% and ts decreases by approximately 17% on average. Moreover, the decreasing rate is relatively large at the beginning of the test. Qr and ts have large increases immediately before device failure, for example in TB2, TB3, TB4, and TB6.
On-state characteristic
The on-state voltage of the thyristors increases by less than 2% during the whole test. However, large increases in the on-state voltage of TB2, TB3 and TB6 occur immediately before device failure, as shown in Fig. 2.
Blocking characteristic
The forward leakage current Id and the reverse leakage current Ir of the thyristors have no obvious change trend, as shown in Fig. 3. However, the forward leakage current of TB5 obviously increases before failure.
Because the failed device at 170 ℃ cannot reach the forward blocking withstand voltage of 8500 V, there are no measurement data after the failure. However, the reverse blocking withstand voltage is still maintained after the forward blocking failure, such as in TB2.
Gate characteristic
There is a decreasing trend of the gate trigger voltage Vgt and the gate trigger current ts of thyristors at the beginning of the test from 0 h to 200 h in Fig. 4. However, as the testing time increases, Vgt and Igt slowly decrease by less than 2% during the remaining test. In addition, TB4 fails, accompanied by a significant increase in the gate trigger voltage.
In summary, during the temperature accelerated aging test, the reverse recovery characteristic and gate characteristic exhibit obvious and consistent degradation.
Voltage acceleratedageingtest
The voltage accelerated ageing test adopted a sine voltage wave equal to 1.06 UD/Ur, where UD is the rated forward voltage and Ur is the rated reverse voltage. The testing time was 1000 hours, and the test was suspended every 100 hours to measure the electrical characteristics. To protect the test platform, if the leakage current of the thyristors was greater than 220 mA, the test automatically stopped.
Six thyristors were tested. Two of them failed at the early stage. The other thyristors did not fail until the test finished. The four key electrical characteristics were obtained as follows.
Reverse recovery characteristic
The reverse recovery charge and reverse recovery time have no obvious change trend during the test, as shown in Fig. 5. The reverse recovery characteristic mainly reflects the speed of carrier dissipation in the chip under the same external circuit and test conditions. It is related to the doping, structure and defects in the chip. However, under blocking voltage stress, the thyristor does not turn on or turn off. Therefore, the stress has little influence on the carrier dissipation inside the chip, which may not change the reverse recovery characteristic.
On-state characteristic
The on-state voltage of the thyristor has no obvious change trend, and there is no obvious change before failure, as shown in Fig. 6. Because the chip does not turn on or turn off during the test process, there is almost no impact on the on-state characteristic of the thyristors.
Blocking characteristic
The forward leakage current Id significantly increases, by approximately 16.8% on average, during the test, as shown in Fig. 7, while the reverse leakage current Ir exhibits no obvious change. The leakage current also increases, and the blocking voltage decreases before device failure. Therefore, the voltage stress has an important influence on the blocking characteristic.
Gate characteristic
Fig. 8 shows that the gate trigger voltage and gate trigger current of the thyristors have no obvious change trend. Therefore, the voltage stress can be considered to have little influence on the gate trigger characteristic of the thyristors.
In summary, during the voltage accelerated aging test, the blocking characteristic exhibits obvious degradation, especially the forward leakage current. In addition, note that voltage and temperature stresses on thyristors always accompany each other at the same time in the actual operation of HVDC transmission systems.