Causes and Countermeasures of condensation in power switchgear

In recent years, condensation of switchgear has occurred in many units of power system. In order to solve the persistent problem of switchgear condensation, the intelligent operation and maintenance innovation team of substation operation and maintenance room of state grid power supply company, based on the prospect of substation operation and maintenance equipment owners, strengthened the concept of in-depth operation and maintenance of equipment owners, actively analyzed the problems in equipment operation, took 220kV substation as the research object, and carried out special research on condensation on the premise of fully completing various safety protection measures. Through the three-month special observation of condensation, the team extensively consulted the scientific and technological literature, accumulated observation and experimental data, analyzed the causes, drew the relationship curve between the dew point temperature of the switchgear and the ambient temperature and relative humidity, and formulated targeted control measures, which provided an effective reference method for other substations to deal with similar problems.

（1） Concept of condensation

Condensation refers to the phenomenon that water vapor condenses on the surface of the object when the surface temperature of the object is lower than the dew point temperature (condensation temperature) of the surrounding air, and the dew point temperature of the air is related to the ambient temperature and relative humidity. Therefore, whether the object condenses depends on the ambient temperature, the temperature of the object itself, the relative humidity and the dew point temperature.

（2） Relationship between dew point temperature and ambient temperature and relative humidity

After consulting scientific and technological literature, the relationship between dew point temperature and ambient temperature and relative humidity is as follows:

At the same ambient temperature, the higher the relative humidity is, the higher the dew point temperature is, and the dew point temperature is directly proportional to the relative humidity.

Under the same relative humidity, the higher the ambient temperature is, the higher the dew point temperature is, and the dew point temperature is directly proportional to the ambient temperature.

（3） Factors affecting condensation of switchgear

After specific analysis, the factors affecting the condensation of switchgear are as follows:

（1） Dew point temperature and ambient temperature of switch cabinet

Diagram of relative humidity

From the end of November 2018 to the first ten days of February 2019, the team members carried out special observation, experiment and comparative analysis on the condensation phenomenon of prospect transformer switchgear for nearly 3 months and more than 100 times, and drew the relationship between the dew point temperature of switchgear and ambient temperature and relative humidity.

Table 1: corresponding relationship between dew point temperature and ambient temperature and relative humidity

Special observation I: thorough investigation of condensation

(2018/11/26-2018/11/30)

Switch cabinet with excessive condensation at the outlet of dehumidifier

The on-site inspection found that there was condensation in the glass observation window of the front cabinet door of 16 Bay switchgear (unevenly distributed). When the front cabinet door was opened, it was found that there were water droplets on its inner wall and the rear cabinet door was in good condition.

The average temperature of the switch room is 16.1 ℃ and the average humidity is 65%. According to table 1, condensation can be formed when the temperature of the switchgear equipment is about 10 ℃.

During the observation period, the weather temperature is 9-12 degrees (i.e. the temperature of the switch cabinet), the air is humid, and the relative humidity in the switch room and the cabinet temperature meet the condensation conditions caused by the weather environment.

Special observation 2: effect of dehumidifier on condensation

(2019/01/10-2019/01/13)

Switch cabinet with excessive condensation at the outlet of dehumidifier

The field inspection found that after adding two dehumidifiers to the electric cabinet, there was no condensation in the observation window of the front cabinet door of 6 Bay switchgear, there was no water drop when opening the front cabinet door, and the rear cabinet door was in good condition.

The average temperature of the switch room is 16.3 ℃ and the average humidity is 60%. According to table 1, condensation can be formed when the temperature of the switchgear equipment is about 8.2 ℃.

During the observation period, the weather temperature is 6-10 degrees (i.e. the temperature of the switch cabinet), the air is humid, and the relative humidity in the switch room and the cabinet temperature meet the condensation conditions caused by the weather environment.

Special observation 3: influence of air conditioning mode on condensation

(2019/01/15-2019/01/19)

Position of dehumidifier of electric cabinet at the air conditioning position at the opening of switch cabinet where excessive condensation occurs

The field inspection found that the observation windows of the front cabinet doors of 8 Bay switchgear had condensation, concentrated on the air conditioning side of the heating mode, and the rear cabinet doors were in good condition.

The average temperature of the switch room is 15.8 degrees and the average humidity is 63%. According to table 1, condensation can be formed when the temperature of the switchgear equipment is about 9 degrees.

During the observation, the weather temperature is 7-9 degrees (i.e. the temperature of the switch cabinet), the air is humid, and the relative humidity in the switch room and the cabinet temperature meet the condensation conditions caused by the weather environment.

（3） Comparative analysis:

1. Influence of installation position and quantity of switch cabinet dehumidifier on condensation

The intelligent dehumidifier is installed at the middle and lower part of the space in the cabinet

The switch cabinet with condensation before adding the intelligent dehumidifier has no condensation after adding the intelligent electric cabinet dehumidifier

2. Influence of cabinet temperature on condensation

During the special observation period, it is found through statistics that the main transformer switchgear (high current cabinet) has never had condensation, and the switchgear close to the high current cabinet also has less condensation. The intervals of condensation are small load or empty charging lines.

Generally speaking, the higher the current, the higher the cabinet temperature. According to the mechanism of condensation, the higher the cabinet temperature, the smaller the temperature difference with the ambient temperature, and the less likely condensation is to occur.

3. Influence of air conditioning mode on condensation

On the premise that the dehumidification position is relatively balanced, only one switchgear condensation occurs at the air conditioning side of dehumidification mode, while seven switchgear condensation occurs at the air conditioning side of heating mode.