Fault Analysis of Abnormal Trip of 6kV High - Voltage Inverter Drive and Research on Preventive Measures

High-voltage inverters are critical devices for AC motor speed control and are widely used in high-power, high-voltage motor speed regulation applications in industries such as lifting, metallurgy, oil, and power generation. However, 6kV high-voltage inverters often experience abnormal drive tripping faults during operation due to factors like grid fluctuations and load impacts, significantly affecting the safety and reliability of motor speed control systems.

To ensure stable operation of high-voltage variable frequency drive (VFD) systems, improve industrial efficiency, and reduce energy consumption, the government has introduced a series of policies encouraging research and application of high-voltage inverter technology. Therefore, in-depth analysis of the causes of abnormal tripping faults in 6kV high-voltage inverters and the development of effective preventive measures are of great significance for advancing high-voltage VFD technology and sustaining industrial economic growth.

1 Overview of 6kV High-Voltage Inverters

A 6kV high-voltage inverter is a high-power power electronic device that uses IGBTs as switching elements and employs a multilevel topology to achieve variable-frequency speed control at 6kV and above. Its power units typically adopt three-level neutral-point-clamped (3L-NPC) or five-level active neutral-point-clamped (5L-ANPC) circuits, constructed by cascading multiple submodules. Each submodule contains 6–24 IGBTs and freewheeling diodes, generating a stepped waveform with 9–17 levels, which approximates a sine wave after filtering.

Typical capacity ranges from 3000 to 14,000 kVA, with voltage levels covering 6kV, 10kV, and 35kV. For higher capacity and voltage requirements, modular multilevel converter (MMC) topology can be used, where submodules employ half-bridge or full-bridge structures, with hundreds of submodules stacked per phase, enabling voltage levels up to 220kV and single-unit capacity up to 400 MVA, suitable for applications such as renewable energy grid integration, offshore wind power, and flexible DC transmission. The control strategy of high-voltage inverters is complex, involving key technologies such as carrier phase-shifted modulation, current balancing, sensorless detection, and field-weakening optimization.

2 Abnormal Drive Tripping Faults in 6kV High-Voltage Inverters

During operation, 6kV high-voltage inverters frequently trip due to abnormalities such as overcurrent, overvoltage, and overheating. Overcurrent faults typically occur during startup or sudden load changes, where instantaneous current may exceed 2–3 times the rated value. If the current exceeds 1600A for more than 100ms or 2000A for over 10ms, the inverter immediately blocks the IGBTs and disconnects the output contactor, triggering hardware protection tripping.

Overvoltage faults are usually caused by grid fluctuations or abrupt load changes. When the DC bus voltage exceeds 1.2 times the rated value (1368V), software overvoltage protection activates; if it exceeds 1.35 times (1026V), hardware protection directly trips. Overheating faults commonly occur in high-temperature environments or during prolonged overload operation. When IGBT temperature exceeds 90°C or heatsink temperature exceeds 70°C for more than 5 minutes, the system issues a high-temperature warning; tripping occurs directly if temperatures reach 100°C or 80°C, respectively. A common feature of these three fault types is the activation of the inverter’s self-protection mechanism, which rapidly cuts off output by blocking IGBTs and disconnecting contactors, resulting in phenomena such as motor emergency stop and flashing fault alarms.

3 Preventive Measures
3.1 Current-Limiting Resistor

To address overcurrent faults, a current-limiting resistor can be connected in series between the inverter output and the motor. Field measurements show that when a 6kV/1500kVA inverter starts a motor of 380kW or larger, the instantaneous starting current can reach 5–8 times the rated current, far exceeding the overcurrent protection setting.

To suppress starting current, a wire-wound resistor or nonlinear zinc-oxide varistor with a resistance of 1–3Ω and a rated power of 200–500W can be used. The latter has a cold-state resistance above 100Ω and rapidly decreases as current increases, limiting the peak starting current to within 2–3 times the rated value. After motor startup, when the inverter output frequency rises above 40Hz and current drops below the rated value, the voltage drop across the resistor is less than 50V.

At this point, a bypass contactor shorts the resistor to avoid continuous power loss. If current surges during startup, when the current transformer detects a value exceeding 1200A, the control system issues a warning; if it reaches 1500A, the inverter immediately blocks the IGBTs and opens the bypass contactor, reinserting the current-limiting resistor to rapidly reduce current. The bypass contactor is then reclosed to restore normal operation. The entire switching process takes less than 0.5s, effectively suppressing current spikes, ensuring smooth motor startup, and significantly enhancing inverter reliability.

3.2 Voltage Clamping Circuit

To suppress overvoltage faults, a voltage clamping circuit can be connected in parallel to the DC bus. This circuit mainly consists of a metal oxide varistor (MOV), a fast thyristor (GTO), and a detection circuit. Field data show that software overvoltage protection activates when grid voltage fluctuates more than 15% or when load reduction causes the DC bus voltage to exceed 1300V for over 20ms.

To prevent such faults, a TYN-20/141 MOV can be used, with a triggering voltage of 1420V, maximum discharge current of 20kA, and energy absorption capacity of 8800J per unit. When the bus voltage exceeds 1350V, the MOV begins to conduct and absorb excess energy; if the voltage rises to 1400V, the GTO triggers, rapidly diverting the overvoltage energy into a resistor to restore voltage to a safe level. The detection circuit continuously monitors the bus voltage.

When the voltage drops below 1250V and remains there for 50ms, a release signal is sent, turning off the GTO and restoring normal system operation. If the bus voltage remains above 1400V for more than 100ms, a severe overvoltage fault is identified, and the inverter enters a software lockout state, requiring manual reset before restart. Practice shows that with this clamping circuit, a 6kV inverter can withstand 35% instantaneous overvoltage and suppress overvoltage to within 1.05 times the rated voltage within 100ms. The response is fast and reliable, effectively preventing frequent overvoltage tripping and significantly improving system continuity and reliability.

3.3 Current-Sharing Design

To address overheating faults, current-sharing technology can be used to reduce heat generation in critical components such as IGBTs and heatsinks, preventing thermal tripping.

Specific measures include connecting 1–2 electrolytic capacitors in parallel across the positive and negative DC bus terminals of each power unit. The capacitors should have a capacitance of 1000–2200μF, voltage rating ≥1600V, and continuous ripple current ≥100A. When the inverter output current exceeds 1.2 times the rated value (e.g., 900A), these parallel capacitors can provide 10%–20% current sharing capability, reducing the actual current through the IGBTs to 720–810A. Given that IGBT conduction losses are proportional to the square of the current, this approach effectively reduces temperature rise.

In the formula: PCPC​ is the IGBT conduction loss (W); VCEVCE​ is the IGBT saturation voltage (V), which has a linear relationship with the current ICIC​ (A); UηUη​ is the turn-on voltage of the IGBT (V); $K$ is the current amplification factor of the IGBT.

It can be seen that after taking shunt measures, the conduction loss of the IGBT can be reduced by 19% to 36%, and the chip junction temperature can decrease by 10°C to 25°C, thus greatly alleviating the heating problem of the inverter.

In addition, install 1 to 2 electric fans in parallel at the inlet and outlet of the inverter heat sink, with a rated air volume of ≥ 3000 m³/h, which can effectively enhance the cooling effect of the heat sink. Set up 6 to 8 temperature sensors inside the control cabinet to monitor the temperatures of various power units, Mother board, IGBT drive board, etc., in real time. When any point's temperature exceeds 65°C, the control system immediately starts the electric fan at full speed and sends a "load reduction warning" signal to the inverter control unit.

If the temperature continues to rise to 75°C and lasts for more than 10 minutes, the system issues an "over-temperature alarm" signal, limiting the maximum output current of the inverter to below 50% of the rated value until the temperature drops below 60°C, at which point the "over-temperature alarm" is lifted.

If any measurement point's temperature exceeds 85°C and the motor current does not drop below 30% of the rated value, the inverter immediately locks out hardware and stops output. To further improve the cooling effect, apply nanomaterials such as graphene or carbon nanotubes on the IGBT heat sinks of each power unit, utilizing their ultra-high thermal conductivity to accelerate the heat dissipation of the IGBT chips, thereby reducing the junction temperature.

4 Effectiveness of Preventive Measures
4.1 Experimental Design

The ZINVERT-6kV/1500kVA intelligent high-voltage inverter was used as the test object, and a grouped control experiment was conducted to verify the effectiveness of the three proposed preventive measures. The experiments were carried out under rated operating conditions (input voltage: 6kV±5%; ambient temperature: 25°C±2°C; relative humidity: 65%±5%). The experiment was divided into four groups: the control group adopted no preventive measures; Group A employed a 2.2Ω/350W current-limiting resistor with an MSC-500 fast bypass switch; Group B used a voltage clamping circuit formed by a TYN-20/141 varistor and an IXYS-GTO connected in parallel, with the clamping voltage set at 1420V; Group C utilized a 2000μF/1600V electrolytic capacitor (Hitachi HCG series) connected in parallel for current sharing, combined with a 3500 m³/h variable-speed fan (EBM-W3G450) for forced cooling.

Each group operated continuously for 72 hours, with key parameters—such as inverter output current, DC bus voltage, and IGBT junction temperature—recorded every 6 hours. Data were collected using a Fluke 435-II power quality analyzer and an HIOKI 8847 data logger. During the experiment, three typical fault scenarios were simulated: inrush overcurrent (8 times rated current / 0.5s), grid voltage fluctuation (+20% / 1s), and full-load operation (ambient temperature 35°C / 2h). The experimental setup is shown in Figure 1.

4.2 Result Analysis

After 72 hours of continuous operation, data from the four groups were collected and analyzed, with results presented in Table 1. The control group experienced tripping under all three fault conditions, whereas the experimental groups with preventive measures demonstrated effective fault suppression. In Group A, the peak starting current was reduced from 7.8 to 2.2 times the rated value, effectively preventing overcurrent tripping.

In Group B, the voltage clamping circuit limited the maximum DC bus voltage fluctuation to 1368V, well below the 1420V protection threshold. In Group C, the combination of current sharing and forced cooling maintained the maximum IGBT junction temperature below 87.5°C, significantly lower than the 100°C tripping threshold. Furthermore, the response time of all three preventive measures was within 100ms, meeting the requirement for fast protection. No false triggering occurred during the experiment, indicating stable and reliable system performance.

5 Conclusion

This study systematically analyzed the causes of abnormal tripping in 6kV high-voltage inverters and proposed targeted preventive measures. Experimental results confirm that the current-limiting resistor effectively controls inrush current, the voltage clamping circuit significantly suppresses DC bus overvoltage, and the combination of current sharing with forced cooling greatly reduces the risk of IGBT overheating, thereby enhancing the overall reliability of the system.