With the rapid development of my country's economy, society, and science and technology, a large number of frequency converters are widely used in various industries. Due to the wide range of applications and my country's vast territory, a large number of frequency converters inevitably operate in humid environments, such as frequency converters installed in humid and rainy coastal and southern regions of my country, frequency converters for wind turbines installed near rivers, lakes, and humid mountainous areas, or frequency converter devices operating in rivers, lakes, and seaside areas.
Due to the influence of air humidity, once the ambient temperature changes significantly, condensation may occur in the frequency converter, causing a certain amount of liquid water to be generated in its internal power devices, circuit boards and other parts. When this liquid water mixes with the dust accumulated inside the frequency converter, it will seriously affect the electrical insulation of the frequency converter. In severe cases, it may even create a circuit, causing the frequency converter to malfunction and affecting normal operation.
For example, liquid water adhering to the heat sink of power devices can cause a circuit to form between the gate and drain of the IGBT, severely damaging the gate and causing the IGBT to lose its normal function. As another example, liquid water adhering to the circuit board can cause short circuits at the corresponding terminals, resulting in pulse disorder, and in severe cases, it can even lead to faults such as inter-bridge short circuits.
As can be seen from the above introduction, condensation can seriously affect and threaten the normal and stable operation of frequency converters. Once the frequency converter is working in a humid environment, it is necessary to take the correct measures to prevent and eliminate condensation.
I. The formation of condensation and its hazards to frequency converters
Air under natural conditions consists of a small amount of dust, water vapor, and completely dry air. The amount of water vapor that air can hold is directly proportional to the ambient temperature; that is, the higher the ambient temperature, the more water vapor the air can hold. The so-called dew point temperature refers to the highest temperature at which condensation occurs in air with a specific humidity level.
Water vapor trapped in the air at higher temperatures will condense as liquid water as the temperature drops, as it can no longer remain in the air. If air with high humidity and relatively high temperature comes into contact with the solid surface of a frequency converter, which is relatively cold (below the dew point temperature of the air under these conditions), condensation will occur, resulting in a certain amount of liquid water on the surface of the relevant components of the frequency converter.
When liquid water mixes with dust inside the frequency converter, it creates conductive channels, which in turn affects the electrical insulation of the frequency converter, causing areas that should not be conductive to become normally conductive.
For example, if dust and condensation adhere to the surface of an IGBT power device, it can create a circuit between the gate and drain of the IGBT, severely damaging the gate and causing the IGBT to lose its normal function. Similarly, if dust and condensation adhere to the control circuit board, it can create non-existent conductive channels, causing disordered logic pulses and resulting in power short circuits, electronic component failures, and other malfunctions.
Although some circuit boards have undergone corresponding coating treatments, condensation will always occur on the bottom of certain components, circuit connections, and other parts due to factors such as quality and blind spots.
II. Methods for eliminating condensation
By disrupting the conditions that cause condensation, such as temperature difference and humidity, the phenomenon of condensation can be fundamentally eliminated. If any one of these conditions is disrupted, condensation will not occur on the frequency converter.
Currently, the more common and frequently used methods for eliminating condensation are temperature control and humidity control. The former aims to reduce relative temperature, while the latter aims to reduce relative humidity.
1) Temperature control
Condensation can be prevented by disrupting the temperature difference that is a condition for condensation. Since the inside of the inverter cabinet is relatively enclosed, condensation will not occur if the cabinet temperature is kept above the dew point temperature.
Influenced by this idea, there are currently two main temperature control schemes:
The first approach includes ventilation openings and heaters. Ventilation openings are typically equipped with filters, which not only prevent large amounts of dust from entering the inverter but also ensure IP protection rating. The key to this approach is that heating begins when humidity is too high, and ventilation is increased as temperature rises. When humidity exceeds a preset value, heating is triggered to raise the internal temperature of the inverter, effectively controlling relative humidity. Once the temperature reaches a preset threshold, ventilation is activated, allowing a certain amount of fresh air from outside to enter the inverter, ensuring a consistent relative humidity and maintaining the temperature within a normal range both inside and outside the inverter. Generally, the ventilation system activates when the temperature exceeds 40°C, and the heater activates when the relative humidity exceeds 80%.
The main idea behind the second approach is to ensure that the internal cooling capacity of the frequency converter is relatively controllable, maintaining the cabinet temperature within a certain range. When humidity exceeds a threshold, the frequency converter's heat dissipation capacity is reduced, and the power consumption generated by the frequency converter is used to increase the cabinet temperature, thus preventing condensation. Conversely, when temperature exceeds a threshold, the heat dissipation capacity is increased to prevent excessive temperature from affecting the normal operation of the frequency converter. The frequency converter cabinets under this approach are mostly completely sealed, effectively preventing salt spray, harmful gases, and dust from entering the cabinet, facilitating long-term, reliable, and normal operation of the frequency converter.
2) Humidity control
By reducing water vapor content, the relative humidity of the air is effectively lowered, thus preventing condensation. This mainly includes three methods: temperature difference dehumidification, adsorption and membrane dehumidification, and condensation dehumidification.
Temperature difference dehumidification method: Install a heat sink inside the frequency converter that is conducive to condensation, so that condensation only forms on the heat sink and does not form in other parts of the frequency converter. The condensate formed on the heat sink is discharged to the outside through the outlet to ensure that the cabinet always maintains a relatively dry environment.
Adsorption and membrane dehumidification methods: Appropriate adsorption materials are installed inside the inverter cabinet to adsorb water vapor and ensure that the cabinet is always kept in a relatively dry environment; alternatively, a membrane filter can be installed to block water vapor and allow only dry air to pass through the filter, so that only relatively dry air flows into the inverter.
Condensation dehumidification method: Set the lowest temperature point inside the frequency converter so that condensation only occurs at that point, thereby effectively reducing the relative humidity inside the frequency converter and keeping the inside of the frequency converter in a relatively dry environment.
III. Real-world Case Analysis
In my work, I encountered an incident where a certain type of transformer experienced a power module breakdown and burnout during operation due to the influence of humid air. The following section will describe the fault symptoms, analyze the causes, and propose corresponding preventative measures.
1) Accident Description
After discovering that the frequency converter was malfunctioning, the rectifier cabinet panel was opened. It was found that the R-phase buffer capacitor and IGBT were burnt out and exploded, and the trigger wire was completely burned. The insulation paper between the IGBT and the buffer capacitor showed signs of partial erosion and carbonization. The metal from the exploded IGBT severely burned the five electrolytic capacitors below it. Simultaneously, the DC fuse blew, the negative copper busbar was severely burned, and the busbar copper busbar and fixing screws were completely fused together. Reviewing the alarm history revealed that the DCF=1 DC fuse was open, and the fuses on the three-phase AC input T-phase and R-phase did not trip.
2) Analysis of the causes of the accident
Before powering on, the rectifier cabinet needs to go through a charging process of about 3 seconds. After the charging is completed, the main contactor is activated by a feedback signal, and then the charging resistor circuit is disconnected.
However, during operation, a short circuit occurred simultaneously with the control power supply, causing the main contactor to fail to engage. After the incident, inspection revealed that the charging resistor and contactor in the charging circuit were completely burned out, leading to the conclusion that a short circuit fault occurred during the charging process.
After reviewing the alarm history of the rectifier cabinet, it was found that the DCF=1 DC fuse was in an open circuit state, the contactor and charging resistor were burned out, the IGBT was broken down, and the 2000A DC fuse was blown. Therefore, it was concluded that no short circuit fault occurred in the inverter circuit, but a short circuit fault occurred in the rectifier section.
Further on-site inspection revealed obvious carbonization of the insulating paper between the positive and negative copper busbars, and obvious creepage signs between the busbars, leading to the conclusion that there was a short circuit between the DC busbars.
At the time of the accident, the area had experienced continuous rainfall for more than half a month, and the air humidity had exceeded 80%. During the investigation and analysis of the cause of the accident, obvious condensation was found inside the cabinet.
Because the rectifier cabinet had been out of service before the accident, condensation occurred on the copper busbars. In addition, the cabinet exhaust fan only operated normally when the inverter was working in the sealed cabinet environment, making it difficult to effectively remove the humid air inside the cabinet. This caused the insulation paper to become damp, which in turn significantly reduced the insulation capacity between the positive and negative busbars.
During the charging process of the frequency converter, due to the stray inductance of the components and circuits, a large instantaneous charging current is generated at the moment of switching. The positive and negative busbars, due to their weak insulation, will experience insulation arcing. A huge current from capacitor feedback and short-circuit current will be superimposed on the bus voltage, causing avalanche voltage breakdown in the PN junction of the IGBT, leading to complete malfunction, buffer capacitor failure, and IGBT short-circuit failure. Due to the contact adhesion caused by the instantaneous short-circuit current, the charging circuit contactor will suffer severe damage to the charging capacitor.
3) Proposal of specific preventive measures
The relationship curves between the three main factors contributing to condensation—dew point temperature, humidity, and ambient temperature—are shown in the figure below.
Figure 1. Curves of relative humidity, condensation temperature, and ambient temperature
Based on the actual site conditions and condensation formation conditions, the author proposes the following main condensation prevention measures:
Firstly, it's crucial to strengthen temperature and humidity control of the inverter cabinet's operating environment. Turn on the indoor air conditioner and set it to dehumidification mode. During normal operation, the inverter's own heat generation will cause the cabinet's internal temperature to be higher than the ambient temperature. However, once the inverter stops running, the cabinet's internal temperature will slowly decrease to the appropriate dew point temperature. Therefore, I believe that when the inverter is not running, the indoor air conditioner's temperature setting should be lowered accordingly to prevent the ambient temperature from exceeding the inverter cabinet's internal temperature.
Secondly, during normal operation of the frequency converter, it is essential to ensure that the heater inside the cabinet is off, and conversely, to ensure that the heater is in normal operation, so as to ensure that the ambient temperature is always lower than the temperature inside the frequency converter cabinet. To achieve the automatic constant temperature regulation function, the heater must use PTC material.
The existing condensation controller was modified by installing three temperature sensors: two inside the inverter cabinet and one outside, to monitor both the cabinet temperature and the ambient temperature. By ensuring the cabinet's internal temperature, condensation conditions were eliminated, effectively preventing condensation from occurring.
IV. Conclusion
Condensation can seriously affect the normal operation of frequency converters. The measures proposed by the author to prevent condensation can effectively reduce the occurrence of condensation, reduce the probability of accidents, and ensure that the frequency converter can operate stably and reliably for a long time.
