Correct use of power transformers and energy-saving measures

Correct use of power transformers and energy-saving measures

Our hospital originally had a 630KVA transformer to supply power to the entire hospital. With the reconstruction of the CNC, lathe, milling and grinding workshops of the training center and the expansion of the canteen and reading room, the power consumption has increased significantly. Since the original transformer can no longer meet the power supply requirements, it is necessary to transform the power supply system. Starting from the calculation of transformer capacity, the selection of power supply mode and operation mode, a reasonable and economical operation plan was selected to achieve the expected purpose.

1 Load calculation results

The total installed capacity of our hospital is 2252KW. The following data are calculated by using the utilization coefficient method: Active power calculation value PJS = 838KW Reactive power calculation value QJS = 786KVAr Apparent power calculation value SJS = 1148KVA Total power factor COSф1 = 0.73

2 Reactive power compensation data

It is not difficult to see from the above figures that the reactive power of the system is relatively low, and the total power factor of the system COSф is only 0.73, which cannot meet the requirements. According to the management regulations of the Shanghai power grid, the power factor must be above 0.9, so reactive compensation is required. According to the power factor of 0.9, the compensation capacity can be easily determined. Let's take: COSф2=0.9, then TANф2=0.484, and the reactive power compensation amount can be determined as: QJS-PJSTANф2=786-838*0.484=398Kvar. The actual reactive power compensation capacity is 400Kvar, and the calculated load after compensation becomes: If the winning coefficient is 0.8, the actual calculated load is 922*0.8=737kva. This value can be used as the basis for our selection of transformers.

3 Determination of economical operating capacity of transformer

With the above calculations, the choice of transformer capacity seems unnecessary to discuss. In fact, before we selected the solution, we had three opinions. Some people advocated the use of 1000KVA transformers, while others proposed the use of 800KVA transformers. In response, the following arguments and calculations were carried out, and finally the solution of two 630KVA transformers was selected.

3.1 Relationship between transformer load rate and efficiency

An important economic operation indicator of a transformer is its efficiency. The efficiency of a transformer is not only related to manufacturing parameters, but also closely related to the load rate of the transformer. According to the definition of efficiency (η), the following formula should hold: η = output/(output+loss)*100%.

It can be seen from the above formula that the efficiency of the transformer changes with the load rate.

3.2 Capacity selection justification

From the above calculation, it is not difficult to know that the total capacity of our hospital's transformer should be around 800KVA to meet the requirements. However, it cannot achieve economical operation, so what capacity should be selected? Assuming that we hope the load rate of the transformer is 60%, according to the formula of load rate:

M= *100%=60% then S=1228KVA

According to national standards, the capacity of the transformer should be selected

1250KVA.

Therefore, the actual load factor of the transformer is

M = *100% = 59%

It just meets the requirements of economic operation. The conclusion is that the capacity of the transformer should be 1250KVA, which also takes into account future development.

4 Power supply solution selection

After the capacity of the transformer was determined, the question became whether to install one 1250KVA transformer or two 630KVA transformers. Based on the school's uneven load during the day and night and long holidays, the following calculations were performed and the plan was finally determined.

4.1 Transformer energy consumption comparison

The product selected is S9 energy-saving fully enclosed transformer, and the following calculation is based on this. The loss of the transformer includes two parts: no-load loss WI and load loss WE. Its no-load loss depends only on the material and manufacturing process of the transformer and has nothing to do with the load; while the load loss is proportional to the square of the load rate M. Based on this, let's take a look at the energy consumption calculation of the above two schemes.

4.1.1 Energy consumption when using a 1250KVA transformer If a transformer is used, the transformer will be in operation as long as there is electricity consumption. Its working time is 365 days a year. Therefore, its energy consumption is calculated as follows: From the equipment manual, the loss rating of the 1250KVA transformer is

Wi=2KW

We=11.8KW

So, W = W1 + We * M2 = 2 + 11.8 * ( 59%) 2 = 6.10 KW

The amount of electricity consumed in 365 days of a year is;

N=6.10*365*24=53436KW.h

4.1.2 Energy consumption when using two 630KVA transformers When the transformer is fully loaded, its loss is mainly load loss, and no-load loss can be ignored. When it is lightly loaded or no-loaded, since no-load loss is constant, its proportion increases greatly. In order to reduce the impact of no-load loss, we can adopt a strategy of alternating operation in different time periods, that is, during the light load period (PM21: 00~AM8: 00) or holidays, one transformer is used to centrally supply power, and during the heavy load period, two transformers are used to independently supply power to reduce the proportion of no-load loss. Let's not talk about the daily time-sharing operation, and only calculate the operation during the three-month statutory holiday of the school. Since two transformers are used for power supply, a single transformer works for a total of 5315 hours during the March holiday each year. Therefore, its loss can be calculated as follows: From the equipment manual, it is found that the loss rating of the 630KVA transformer is:

Wi=1.2KW

We=6KW

If the load factor is also approximately 59%, then the loss of a 630KVA transformer is;

W=Wi+We*M2=1.2+6*(59%) 2=3.2KW

The loss of the two transformers is: W = 3.2 * 2 = 6.4KW

Therefore, the energy consumption of the two transformers in one year can be calculated as:

N=6.4*365*240- 3.2*5315=39056KW.H

As follows: Close the oil filter outlet pipe valve, start the oil pump, the pressure gauge pressure indication should increase, if the pressure does not increase, the motor may be rotating in the wrong direction, then the motor power supply line should be replaced.

4.2 Work Operation

The online oil filter device should be put into operation manually every 15 days or for a certain period of time after the tap changer is operated.

4.2.1 Taking samples for oil analysis

After the online oil filtration device is put into operation, the oil service test personnel should conduct oil service test analysis at regular intervals based on the operating conditions and the requirements of the oil service supervision guidelines.

4.2.2 Oil filling

When the oil level in the tap changer oil chamber drops to a certain level, appropriate oil replenishment should be carried out.

4.2.3 Filter element replacement

The filter element is the main working component in the online oil filter device. After the filter element has been used for a certain period of time, it needs to be replaced to prevent clogging caused by excessive impurities.

5. Inspection and maintenance measures

Since the transformer on-load tap changer online oil filter device is an accessory of the tap changer, it should be inspected and maintained in a condition-based manner. Inspection and maintenance should be carried out in the following situations.

a. When the pressure indication exceeds the specified value.

b. Take oil samples for test and the insulation strength is less than 30MV/2.5M.

c. When the tap changer gas relay is activated.

d. When other abnormal situations occur.

e. Oil sample testing should be carried out in accordance with the provisions of the transformer oil supervision project. Take oil samples for oil testing 24 hours, 7 days, and 1 month after the online oil filter device is put into operation to check whether the oil samples are normal. Sampling and testing should be carried out every six months to check the filtering performance of the online oil filter device. Under normal operation of the live oil filter device of the on-load voltage-regulating tap changer, the tap changer room does not need to change oil on a periodic basis. Due to the application of the online oil filter device, the free carbon in the switch room is reduced, which has less impact on the switching of the switch, and correspondingly extends the core maintenance time of the on-load voltage-regulating tap changer, improving the ability of the transformer to be safely transported.