Technical Improvement of 220KV Transformer Cooling System

1. Overview of the operation of the cooling system of 220KV No. 1 transformer

Huashan, our province The No. 1 main transformer of the 220KV transformer station of the substation is a product of the 1990s and has been in operation for more than 20 years. In 2003, the main transformer was changed from no-load voltage regulation to on-load voltage regulation after technical improvements. After years of operation, the cooling system of the transformer gradually exposed a series of problems: the cooling efficiency of the cooling system decreased, resulting in a high oil temperature of the transformer body, which was 9 to 10 degrees Celsius higher than the upper oil temperature of the No. 2 main transformer; the submersible pump and fan motor entered a high-incidence stage of failure, and the cooling system had many oil leakage defects. In recent years, the defects of the cooling system have reached more than ten times a year, seriously affecting the safe and reliable operation of the main transformer, and at the same time, increasing the maintenance workload. The burning of the submersible pump also caused traces of acetylene in the main transformer body oil; the fan noise of the cooling system was too loud, and the No. 1 main transformer was close to the main control room, which worsened the working environment of the on-duty personnel.

2. Cooling system improvement process

2.1 Requirements to be met after cooling system improvement

After the cooling system is improved, when the transformer is running at full load and the ambient temperature is +40°C, the upper oil temperature of the main transformer does not exceed +70°C, and the upper oil temperature is not greater than 30K. The total noise of the cooling system is greatly reduced compared with the original noise.

2.2 Determination of cooler model and number of groups

Technical parameters of main transformer No. 1: model OSFPSZ7-120000/220, product code 1CB710.001, cooling method ODAF, load loss Pg-z=312.64KW, Pg-d=260.6KW, Pz-d=361.77KW, no-load loss Po=49.13KW.

Originally, seven groups of YF-120 coolers were used, arranged along the long axis of the transformer, with three groups on the right side and four groups on the left side facing the high voltage side of the transformer. The modified cooling system is ready to use YF1-200 coolers, and the number of coolers required is calculated as follows:

(1) Ng-z = 1.15P1/KQ+1 (spare) = 1.15X(312.64+49.13)/0.8X200+1 = 2.6+1 = 3.6 (group)

(2) Ng-d = 1.15P2/KQ+1 (spare) = 1.15X(260.6+49.13)/0.8X200+1 = 2.2+1 = 3.2 (groups)

(3) Nz-d = 1.15P3KQ + 1 (spare) = 1.15X (361.77 + 49.13) / 0.8X200 + 1 = 2.95 + 1 = 3.95 (group) = 4 groups.

Note: P—total loss KW (P1=Pg-z+Po, P2=Pg-d+Po, P3=Pz-d+Po)

Q—Nominal cooling capacity of the cooler KW.

K—is the correction system: considering that the altitude of the substation is greater than 1000M and the maximum ambient temperature in summer is 40*C, the maximum upper oil temperature of the No. 1 main transformer is 70*C, and K=0.8.

According to the above calculation results, it was decided to use four groups of YF1-200 coolers to transform the cooling system. The layout of the coolers remains the same as before, that is, two groups of coolers are placed on both sides of the main transformer long axis direction, and each two groups of coolers use a set of centralized brackets, which are connected to the main transformer oil tank with a φ125 pipe.

2.3 Theoretical calculation of cooling system transformation effect

2.3.1 Determination of the ideal working condition point of the original cooling system

The ideal working condition point of the cooling system can be determined by the relationship curve between the cooling system resistance and the submersible pump flow rate and the head characteristic curve of the submersible pump. The layout of the cooling system before the transformation is to place four groups of YF-120 coolers on the left side of the long axis direction of the main transformer facing the high-pressure side, and three groups of YF-120 coolers on the right side. A total of seven groups of YF-120 coolers are used to cool the main transformer. The resistance calculation is based on the four groups of centralized coolers on the left (ignoring the internal resistance of the main transformer body). The reference oil flow rate of each group of YF-120 coolers is selected as Ql=40m³/h; the total oil flow rate of the four groups of coolers is Q=4X40=160m³/h. The pipelines from the main transformer to the four groups of centralized coolers are two inlets and two outlets, and the nominal diameter of the pipeline is Φ125, and the flow rate of each pipeline is 80m³/h. After calculation, it can be obtained that the relationship between the system resistance ΔP and the total flow rate Q′V1 of each pipeline is (calculation process omitted):

Δ P1=1.643X10Q′Vl

According to the above formula, the relationship between the system resistance ΔP1, the total flow rate of each pipeline Q′v1 and the flow rate of a single oil pump Qvl can be obtained as shown in Table 1

The relationship curve between system resistance ΔP1 and oil pump flow QVl is plotted from the data in Table 1. Combined with the lift characteristic curve of the submersible oil pump (QB40-160/3.0T) used, it can be determined that the ideal working condition point of the original YF-120 cooling system is G1 (ignoring the internal resistance of the main transformer body), see Figure 1 (the graph represented by the red numbers and curves in the figure).

When all the coolers on one side of the four groups of YF-120 coolers are running, the ideal working flow of each group of coolers is G1=49.6m³/h. According to "Selection and Calculation of Strong Oil-Air Coolers", the reference cooling capacity of YF-120 is 110KW.

2.3.2 Determination of the ideal working condition point of the cooling system after transformation

According to the modified method, the resistance calculation is based on the two groups of centralized coolers on one side of the main transformer (ignoring the internal resistance of the main transformer body). The reference oil flow rate of each group of YF1-200 coolers is selected as Q′12=80m³/h, and the total oil flow rate of the two groups of coolers is Q=2X80=160m³/h. The pipelines from the main transformer to the two groups of centralized coolers are two inlets and two outlets, and the nominal diameter of the pipeline is Φ150. The flow rate of each pipeline is 80m³/h. After calculation, it can be obtained that the relationship between the system resistance ΔP1 and the total flow rate Q′Vl of each pipeline is (calculation process omitted):

Δ P1=5.8X10﹣³Q′²Vl

According to the above formula, the relationship between system ΔP2, total flow rate of each pipeline Q′Vl and oil pump flow rate QVl is shown in Table 2.

The relationship between the system resistance ΔP2 and the oil pump flow QVl is plotted from the data in Table 2. The ideal working condition points of the modified YF1-200 cooling system are G2 and G3 (ignoring the internal resistance of the main transformer body) respectively, as shown in Figure 2. (See the curve represented by the black line in the figure)

When both groups of YF1-200 coolers on one side of the transformer are fully operational, the ideal operating flow rates of each group of coolers are:

G2=99.1m³/h; G3=85.8m³/h

2.3.3 Selection of submersible oil pump

According to the resistance calculation results, the oil flow rate of the pipeline between the main transformer and the centralized support is 80m³/h before and after the transformation, and the resistance of the pipeline before and after the transformation is 11.615KPa and 5.56757KPa respectively (the calculation process is omitted). It can be seen that the high resistance before the transformation is due to the existence of two right-angle elbows in the pipeline system. In addition, from the comparison of the resistance characteristic line diagram 1 and Figure 2, it can be seen that the resistance after the transformation is lower than that before the transformation.

From the resistance characteristic curve 2 (under the same ideal conditions), it can be concluded that when using the 6PB135-4.5/2.2V oil pump, the oil flow is 85.8m³/h; when using the 6PB135-4.6/3V oil pump, the oil flow is 99.1m³/h; if the internal resistance of the main transformer is taken into account, the oil flow of the 6PB80-4.5/2.2V oil pump will be lower, and the cooling capacity of the cooler cannot be fully utilized. Only the 6PB135-4.6/3V oil pump is the best choice, because the head characteristic of the oil pump is relatively stable, which can better exert the cooling capacity of the cooler.

2.3.4 Theoretical calculation of average temperature rise of main transformer oil after transformation

When the transformer cooling system is operating normally after the transformation, the number of YF1-200 coolers put into use is N=3 groups, the maximum ambient temperature is T1=40ºC, the top oil temperature of the main transformer is T2=70ºC, the total loss of the main transformer (take the group with the largest load loss) is P3=361.77+49.13=410.9KW, and the cooling capacity of each group of coolers is:

Q′=P3/N=410.9/3=136.97KW

Considering the internal resistance of the main transformer, the oil flow rate of each cooler group should vary between 60 and 100 m³/h. The heat transfer coefficient of the YF1-200 cooler is Kl = 5.898 W/K when the oil flow rate is 60 m³/h; Kn = 6.5618 W/K when the oil flow rate is 100 m³/h.

The average oil temperature rise is theoretically calculated as follows: When the oil flow rate is 60m³/h

ΔT1=Q′/Kl=136.97/5.898=23.22(K)

When the oil flow rate is 100m³h:

ΔTn=Q′/Kn=136.97/5.5618=24.627(K)

From this, it can be determined that: according to the maximum +40ºC in the cooling system environment, when the top oil temperature of the main transformer is +70ºC, the required temperature rise is 30K, and the calculated values ​​of the cooler temperature rise are all less than 30K; therefore, four groups of YF1-200 coolers (two working groups, one auxiliary group, and one spare group) are selected. Therefore, it is feasible to select 6PB135-4.6/3V oil pump to transform the cooling system of the main transformer.

3. Implementation and Effect of Cooler Renovation

In February 2010, the transformer cooling system was renovated according to the above plan. Taking advantage of the power outage opportunity of the cooling system renovation, the transformer was overhauled. After the cooling system renovation, the effect was good. By comparing the upper oil temperature of the No. 1 and No. 2 main transformers before and after the renovation, it can be seen (see Table 3) that the upper oil temperature of the No. 1 main transformer was 5-9ºC higher than that of the No. 2 main transformer before the renovation, but the upper oil temperature of the No. 1 main transformer was 1-6ºC lower than that of the No. 2 main transformer after the renovation; during the two months of operation after the renovation, the transformer carried an 80MW load, and the upper oil temperature of the transformer did not exceed 40ºC, and the temperature rise was less than 20ºC; after the cooling system renovation, the noise in the No. 1 main transformer site and the nearby main control room was greatly reduced, which changed the working environment of the operating staff on duty, and completely eliminated the safety hazards such as the high failure rate of the fan motor and oil pump motor of the original cooling system, and the frequent oil leakage defects.

IV. Conclusion

Through the practice of improving the cooling system of the 220KV No. 1 main transformer of Huashan Substation, we have learned how to correctly master the skills and calculations of transforming the transformer cooling system, ensuring the normal operation and operation of the substation, improving the efficiency and service life of the transformer, and saving the investment in substation equipment. At the same time, it provides a reference and reference for the technical transformation of similar transformer cooling systems in China.