Formation hazards and preventive measures of conde

Analysis: the formation, harm and preventive measures of frequency converter condensation

with the rapid development of China's economy, society, science and technology, a large number of frequency converters are widely used in all walks of life in China, which is expected to drive the rapid development of the new generation of information technology, new energy, high-end equipment manufacturing and other fields. Due to the wide range of application fields and the vast territory of our country, a large number of frequency converters are bound to work in a humid environment, such as those installed in the humid and rainy coastal areas and southern regions of our country, or the frequency converters of wind turbines installed near rivers and humid mountains, or the frequency converter devices working in rivers, lakes and seaside

affected by the air humidity, once the working environment temperature changes greatly, it may cause the frequency converter to condense, causing a certain amount of liquid water to be produced in its internal power devices, circuit boards and other parts. After mixing with the dust accumulated inside the frequency converter, it will have a serious impact on the electrical insulation of the frequency converter, and in serious cases, there will be a path, leading to the failure of the frequency converter and affecting the normal operation

for example, liquid water attached to the heat dissipation plate of power devices will lead to the formation of a path between the grid and drain of IGBT, and seriously damage the grid of IGBT, resulting in the loss of normal function of IGBT; For another example, liquid water attached to the circuit board will lead to short circuit at the corresponding terminals, which will cause pulse chaos, and seriously lead to fault phenomena such as short circuit between bridges

through the above introduction, it can be found that condensation will seriously affect and threaten the normal and stable operation of the frequency converter. Once the frequency converter works in a humid environment, correct measures must be taken to prevent and eliminate condensation

first, the formation of condensation and its harm to the frequency converter

the air under natural conditions is composed of a small amount of dust, water vapor and absolutely dry air. The water vapor that the air can hold is 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 maximum temperature at which condensation occurs in air with a specific humidity

the water vapor contained in the air at higher temperature will be separated out in the form of liquid water due to the decrease of temperature. If the air with high humidity and relatively high temperature touches the solid surface of the frequency converter with relatively low temperature (lower than the dew point temperature of the air under this condition), condensation will occur, and then a certain amount of liquid water will be generated on the surface of the relevant parts of the frequency converter

when the liquid water is mixed with the dust inside the frequency converter, it will produce corresponding conductive channels, which will affect the electrical insulation of the frequency converter, making the non-conductive area into a normal conductive area

for example, once mixed with dust condensation and attached to the surface of IGBT power devices, a path will be formed between the gate and drain of IGBT, which will seriously damage the gate of IGBT and cause IGBT to lose its normal function; For another example, mixed dust condensation attached to the control circuit board will cause the circuit board to produce non-existent conductive channels, resulting in confusion of logic pulses, and then produce power short circuit, electronic component failure and other faults

although some circuit boards have been coated accordingly, condensation will always occur at the bottom of some components, circuit connections and other parts due to factors such as quality and blind spots

second, the elimination method of condensation

through the destruction of condensation formation conditions such as temperature difference and humidity, the purpose of fundamentally eliminating condensation can be achieved. If any formation condition is destroyed, the frequency converter will not appear condensation

at present, the more popular and commonly used methods to eliminate condensation are: temperature control method and humidity control method. The former aims to reduce the relative temperature, while the latter aims to reduce the relative humidity

1) temperature control

can prevent the formation of condensation by destroying the condensation formation condition of temperature difference. Since the inside of the inverter cabinet is relatively closed, if the cabinet temperature can always be higher than the dew point temperature, condensation will not occur

influenced by this idea, there are mainly two temperature control schemes at this stage:

the first scheme includes vents and heaters. Generally, the vent is equipped with a filter, which can not only prevent a large amount of dust from entering the inverter, but also ensure the IP protection level with processing properties such as color, texture, light weight, nailable, sawable, drillable, planed, and paintable. The main point of this scheme is to start heating once the humidity is too high, and increase ventilation when the temperature rises. When the humidity exceeds the preset value, promote the heating action, increase the internal temperature of the frequency converter, and then effectively control the relative humidity conditions. After the temperature reaches the preset threshold, start the ventilation, so that a certain amount of external fresh air enters the inside of the frequency converter, so as to ensure that the relative humidity of the air inside and outside the frequency converter is always consistent, and the temperature is always kept within the normal range. Generally, when the temperature exceeds 40 ℃, start the ventilation system, and when the relative humidity exceeds 80%, start the heater

the main idea of the second scheme is: the internal cooling capacity of the frequency converter is relatively controllable to ensure that the temperature in the cabinet is always maintained within a certain range. When the humidity exceeds the threshold, the heat dissipation capacity of the frequency converter will be reduced, and the temperature in the frequency converter cabinet will be increased through the power consumption generated by the frequency converter, so as to eliminate condensation; When the temperature exceeds the threshold, the heat dissipation capacity will be improved to prevent the normal operation of the frequency converter from being affected by excessive temperature. Most of the inverter cabinets under this scheme are completely sealed, effectively preventing salt mist, harmful gases and dust from entering the cabinet, which is convenient for the long-term, reliable and normal operation of the inverter

2) humidity control

effectively reduce the relative humidity of the air by reducing the content of water vapor, so as to prevent condensation. It mainly includes the following three schemes: temperature difference dehumidification, adsorption and membrane dehumidification and condensation dehumidification

temperature difference dehumidification: install a radiator in the converter that is conducive to condensation, so that condensation only forms on the radiator, so that condensation will not form in other parts of the converter. The condensate formed on the radiator is discharged outward through the outlet, so as to ensure that the cabinet is always kept in a relatively dry environment

adsorption and membrane dehumidification: set up corresponding adsorption materials in the inverter cabinet to achieve the purpose of water vapor adsorption and ensure that the cabinet is always kept in a relatively dry environment; The purpose of blocking water vapor can also be achieved through the setting of membrane filter. Only dry air can pass through the filter, so that only relatively dry air can flow into the inverter

condensation dehumidification: set the lowest temperature inside the frequency converter, so that the condensation only occurs here, so as to effectively reduce the relative humidity inside the frequency converter and keep the relatively dry environment inside the frequency converter all the time

III. analysis of real cases

the author once encountered an accident of power module breakdown and burning in the working process of a certain type of transformer due to the influence of humid air. Next, on the basis of describing the fault phenomenon, the author will analyze the causes and put forward corresponding preventive measures

1) accident description

after finding that the frequency converter cannot operate normally, open the rectifier cabinet panel and see that the buffer capacitor and IGBT of phase R are burned and cracked, and the trigger line is completely burned out. The insulating paper between IGBT and buffer capacitor is partially ablated and carbonized. The metal produced by IGBT explosion seriously burns out the five electrolytic capacitors below it. At the same time, the DC fuse is fused, and the negative copper bar is seriously burned out, The busbar copper bar and the fixing screw are completely fused together. After reviewing the alarm history, it was found that dcf=1 DC fuse was in open circuit state, and the T-phase and R-phase fuses of three-phase AC incoming line did not produce any action

2) analysis of accident causes

because the rectifier cabinet needs to undergo a charging process of about 3 seconds before being powered on, after the charging is completed, the pull in action of the main contactor is realized through the feedback signal, and then the charging resistance circuit is disconnected

however, in the process of operation, a short circuit occurred while closing the control power supply, resulting in the failure of the main contactor to generate the pull in action. After the accident, the inspection found that the charging electricity of the charging circuit selectively enhanced that part of the resistor and contactor of the component were completely burnt out, so it was concluded that a short-circuit fault occurred during the charging process

after reviewing the alarm history of the rectifier cabinet, it is found that the dcf=1 DC fuse is in an open circuit state, the contactor and charging resistance are burned out, the IGBT is broken down, and the 2000a DC fuse is fused, so it is concluded that there is no short-circuit fault in the inverter circuit, and there is a short-circuit fault in the rectifier part

after further on-site inspection, it was found that there was obvious carbonization of insulating paper between positive and negative copper bars, and there were obvious signs of creepage between busbars, so it was concluded that there was a short circuit between DC busbars

at the time of the accident, the local area had experienced continuous rainfall for more than half a month, and the air humidity had exceeded 80%. During the inspection and analysis of the cause of the accident, it was found that there was obvious condensation in the cabinet

since the rectifier cabinet has been in the out of service state before the accident, the copper bar has condensation phenomenon. In addition, under the sealed cabinet environment, the cabinet exhaust fan can operate normally when the frequency converter works, so it is difficult to effectively remove the humid air in the cabinet, resulting in the moisture of the insulating paper, which greatly reduces the insulation capacity between the positive and negative electric bars

during the charging process of the frequency converter, due to the influence of stray inductance in the devices and lines, the switch will produce a large instantaneous charging current at the moment, while the positive and negative busbar will produce insulation arcing phenomenon due to weak insulation ability, and the bus voltage will superimpose the huge current and short-circuit current fed back by the capacitor, resulting in avalanche voltage breakdown of the PN junction in the IGBT, and then completely lose its normal function The buffer capacitor bursts and IGBT short circuit bursts. Due to the influence of the contact adhesion of the instantaneous short-circuit current, the charging circuit contactor will seriously burn out the charging capacitor

3 time-consuming and money-consuming) specific preventive measures proposed

the three factors of condensation: the relationship curve between dew point temperature, humidity and ambient temperature is shown in the figure below

temperature and humidity condensation fog

Figure 1 relative humidity, condensation temperature and ambient temperature curve

combined with the actual situation on site and the formation conditions of condensation, the author puts forward the following main condensation prevention measures:

first, strengthen the temperature and humidity control of the working environment of the inverter cabinet, turn on the indoor air conditioning, and adjust it to the dehumidification mode. During normal operation, due to the self heating of the frequency converter, the temperature in the cabinet will be greater than the external ambient temperature. Once the frequency converter stops running, the temperature in the cabinet will slowly reduce to the corresponding dew point temperature. Therefore, the author believes that when the frequency converter stops running, the temperature setting of the indoor air conditioner should be reduced accordingly to avoid the ambient temperature being higher than the temperature in the frequency converter cabinet

secondly, during the normal operation of the frequency converter, it is necessary to ensure that the heater in the cabinet is in the disabled state. On the contrary, it is necessary to ensure that the heater is in the normal operation state, so as to ensure that the ambient temperature is always lower than the temperature in the frequency converter cabinet. To realize automatic constant temperature