Transformer capacity unit

The unit of power transformer capacity is KVA, that is, apparent power. Most electrical loads are inductive loads, and their apparent power is equal to the vector sum of active power and reactive power. Therefore, the apparent power of the inductive load is greater than the active power. If the transformer is selected with active power, the apparent power of the inductive load must be greater than the capacity of the power transformer, and the load current must exceed the allowable value of the transformer. Therefore, the inductive load must use the apparent power to select the transformer.

If all the loads on the transformer are resistive loads, the total power can be equal to the KVA value of the transformer.

What about KVA instead of KW? VA or KVA is the apparent power, used in AC lines, also known as capacity, which is the product of the voltage and the rms value of the current, the unit. But it will affect the load design of the transformer, so it is more accurate to use VA or KVA to calculate. And W or KW is what we usually call it

How to choose the capacity of the transformer

1. When we usually choose distribution transformers, if the capacity is too large, the phenomenon of "big horse-drawn trolley" will be formed, which not only increases equipment investment, but also keeps the transformer in a no-load condition for a long time. If the capacity of the transformer is too small, the transformer will be in an overloaded state for a long time, and it is easy to burn the transformer, whether it is an autotransformer or a three-phase transformer, it is the same. Therefore, the correct selection of transformer capacity is one of the important measures to reduce losses and save energy in the power grid. In practical applications, we can select the transformer capacity according to the following simple methods.

2. We should adhere to the principle of "small capacity, dense distribution", distribution transformers should be located in the load center as much as possible, and the power supply radius should not exceed 0.5 kilometers. The load rate of the distribution transformer is the highest between 0.5 and 0.6, and the capacity of the transformer at this time is called the economic capacity. However, if the load is relatively stable, the transformer capacity can be selected according to the economic capacity in the case of continuous production.

3. According to the characteristics of scattered rural power grid users, low load density, strong load seasonality and strong intermittent, etc., capacity-adjusting transformers can be used. The capacity-adjusting transformer is a transformer that can adjust the capacity without load according to the size of the load. It is suitable for use in places where the load seasonal changes are obvious. For substations or industrial and mining enterprises with a large power load, the mother and child transformers are generally used for power supply. One (mother) is configured according to the maximum load, and the other (secondary) is selected according to the low load state, which can greatly improve the power distribution. Transformer utilization, reduce no-load loss of distribution transformers. In view of the fact that some distribution transformers in rural areas are in low-load operation for a long time except for a small amount of peak electricity load in a year, for users with conditions, the power supply mode of parallel operation of mother-son transformers or transformers can also be used. When the load changes greatly, according to the principle of the lowest power loss, put into transformers of different capacities. For special transformers that only supply power to power loads such as irrigation and drainage, the capacity of the transformer can generally be selected according to 1.2 times the nameplate power of the asynchronous motor. Generally, the starting current of the motor is four to seven times the rated current, and the transformer should be able to withstand this impact. The capacity of the largest one of the directly started motors should generally not exceed about 30% of the transformer capacity. It should be pointed out that the special transformer for drainage and irrigation should generally not be connected to other loads, so as to stop the operation in time during the non-drainage and irrigation period and reduce the loss of electric energy.

4. For the selection of the capacity of comprehensive power transformers for power supply and lighting, agricultural and sideline product processing, etc., the simultaneous power of the electrical equipment should be considered. The capacity of the transformer can be selected according to 1.25 times the actual maximum load that may occur. In short, we Care should be taken when selecting the transformer capacity.

Transformer capacity calculation formula

1. Calculate the maximum power of each phase of the load

Add the load power of each phase of A-phase, B-phase and C-phase independently. For example, the total power of the A-phase load is 10KW, the total power of the B-phase load is 9KW, and the total power of the C-phase load is 11KW, and the maximum value is 11KW. (Note: The power of each single-phase device is calculated according to the maximum value on the nameplate, and the three-phase device power is divided by 3, which is equal to the power of each phase of this device.)

For example: C-phase load total power = (computer 300WX10 units) + (air conditioner 2KWX4 units) = 11KW

2. Calculate the three-phase total power

11KWX3 phase=33KW (transformer three-phase total power)

The three-phase total power/0.8 is the most important step. Currently, more than 90% of the transformers on the market have a power factor of 0.8, so it needs to be divided by the power factor of 0.8.

33KW/0.8=41.25KW (total transformer power)

The total power of the transformer is /0.85. According to the "Power Engineering Design Manual", the transformer capacity should be selected according to the calculated load. For a single transformer that supplies power with a stable load, the load rate is generally about 85%.

41.25KW/0.85=48.529KW (transformer power to be purchased), then you can choose a 50KVA transformer when purchasing.

Transformer capacity selection case

For example: the calculated load of electricity consumption of a construction site is 86.06KVA. Then the calculated capacity of the transformer is 100KVA, and the transformer of 100KAV can be selected according to the capacity level.

By the way: the capacity of a single transformer should not be greater than 1000KVA. When the load is large, several transformers can be used to supply power in parallel. The parallel operation should meet the conditions of equal transformation ratio, the same connection group, and the same short-circuit voltage. Secondly, pay attention to the problem of load distribution. Generally, the ratio of the maximum capacity to the minimum capacity does not exceed 3:1.

Calculate the maximum power per phase of the load:

Add the load power of each phase of A-phase, B-phase and C-phase independently. For example, the total power of the A-phase load is 10KW, the total power of the B-phase load is 9KW, and the total power of the C-phase load is 11KW, and the maximum value is 11KW.

(Note: The power of each single-phase device is calculated according to the maximum value on the nameplate, and the three-phase device power is divided by 3, which is equal to the power of each phase of this device.)

For example: C-phase load total power = (computer 300WX10 units) + (air conditioner 2KWX4 units) = 11KW

Calculate the total three-phase power:

11KWX3-phase = 33KW (transformer three-phase total power) three-phase total power/0.8, this is the most important step, the power factor of more than 90% of the transformers currently on the market is only 0.8, so it needs to be divided by 0.8 power factor.

33KW/0.8=41.25KW (transformer total power) transformer total power/0.85, according to the "Power Engineering Design Manual", the transformer capacity should be selected according to the calculated load, and the load rate of a single transformer with stable load power supply is generally about 85%.

41.25KW/0.85=48.529KW (transformer power to be purchased), then you can choose a 50KVA transformer when purchasing.

Transformer capacity class

Transformers are usually divided into two types: oil-immersed transformers and dry-type transformers. Their representatives are S11-M transformers and SCB10 transformers. Since oil-immersed transformers use oil as the insulating medium, the capacity is often larger. The capacity specifications of dry-type transformers are easily smaller.

1. Capacity grade of immersion transformer

(1) 6kv and 10kva systems: 10kva, 20kva, 30kva, 50kva, 80kva, 100kva, 125kva, 160kva, 200kva, 250kva, 315kva, 400kva, 500kva, 630kva, 800kva, 1000kva, 1250kva, 1600kva, 20500kva 4000kva, 5000kva;

(2) 35kv and 60kv systems: 315kva, 400kva, 500kva, 630kva, 800kva, 1000kva, 1250kva, 1600kva, 2000kva, 2500kva, 3150kva, 4000kva, 5000kva;

(3) 110kv and above systems: 3150kva, 4000kva, 5000kva, 6300kva, 8000kva, 10000kva, 12500kva, 16000kva, 20000kva, etc.

2. Dry-type transformer capacity level

(1) 6kv and 10kva systems: 10kva, 20kva, 30kva, 50kva, 80kva, 100kva, 125kva, 160kva, 200kva, 250kva, 315kva, 400kva, 500kva, 630kva, 800kva, 1000kva, 1250kva, 1600kva, 20500kva 4000kva, 5000kva;

(2) 35kva system: 30kva, 50kva, 80kva, 100kva, 125kva, 160kva, 200kva, 250kva, 315kva, 400kva, 500kva, 630kva, 800kva, 1000kva, 1250kva, 1600kva; 2000kva, 25000kva, 310000kva

(3) 60kv system: 315kva, 400kva, 500kva, 630kva, 800kva, 1000kva, 1250kva, 1600kva, 2000kva, 2500kva, 3150kva, 4000kva, 5000kva;

(4) 110kv and above systems: 3150kva, 4000kva, 5000kva, 6300kva, 8000kva, 10000kva.

Transformer capacity calculation formula

The transformer capacity should be selected according to the load. For a single transformer with stable load power supply, the load rate is generally about 85%.

Namely: β=S/Se-------Formula (1)

In the formula: S is the calculated load capacity (kVA) Se is the transformer capacity (kVA); β is the load rate (usually 80% to 90%)

Some Questions about Transformer Capacity Calculation

1. The rated capacity of the transformer should be the maximum apparent power of the transformer that can ensure the normal operation of the transformer under the specified operating conditions;

2. This apparent power is the output power of the transformer, and it is also the apparent power that the transformer can carry with the maximum load;

3. When the transformer is in rated operation, the output apparent power of the transformer is equal to the rated capacity;

4. When the transformer is in rated operation, the input apparent power of the transformer is greater than the rated capacity;

5. Due to the high efficiency of the transformer, it is generally believed that when the transformer is in rated operation, the input apparent power of the transformer is equal to the rated capacity, and the calculations and results are basically accurate;

6. Therefore, when using the transformer, it is safe to observe that the current, voltage, power factor and its apparent power output by the transformer are equal to or less than the rated capacity (when the conditions of use are met);

7. Some people think that the transformer has losses and must operate below 90% of the rated capacity is wrong!

8. When designing and selecting the capacity of the transformer, it is correct to multiply the safety factor according to the calculated load.