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How much kW of electricity can a 1000 kVA transformer handle?

Release time:

2021-02-25

Question

"There is an old transformer with a capacity of 1000kVA, currently with a load of about 200kW. If we want to add a new load of about 600kW, can this transformer handle it?"

This question first involves a concept, which is the relationship and difference between kVA and kW.

kVA (kilovolt-amperes) isthe unit of apparent power.kW (kilowatts) is the unit of active power. In addition to apparent power andactive power,there is also reactive power.Reactive power.The unit is kvar (kilovars).

So what are the differences between active power, reactive power, and apparent power?

active power,Active power is the actual power consumed by electrical appliances, that is, the energy converted into other forms of energy.

For example, the electricity bill that everyone pays is for active power.

Reactive power.Reactive power refers to the part of power that certain devices do not actually consume electricity; it is just temporarily storing electricity.

For example, if an electrical device has capacitors/coils, when this device is working, the capacitors/coils will always be in a charging and discharging state. Because the capacitors/coils are continuously charging and discharging without actually consuming electricity, this part of power is called reactive power.

the unit of apparent power.Apparent power refers to the total power provided by the power source.

The power source (generally referring to transformers or generators) not only needs to provide active power to electrical devices but also needs to provide reactive power. The reason is simple: although the capacitors in electrical devices do not consume electricity, they are always charging and discharging, so they also need to occupy part of the capacity of the power source.

After clarifying these, let's look at the relationship between them, which leads to another concept—power factor.The amount of active power a power source can provide depends on the power factor.

[Power Factor]

The power factor refers to the ratio of active power to apparent power, typically represented by cosφ.

For example, a 1000 kVA transformer can output 600 kW of active power when the power factor cosφ is 0.6; however, when the power factor cosφ is 0.9, it can output 900 kW of active power.

If we assume 1 kilowatt-hour costs 1 yuan, at a power factor of 0.6, this transformer can generate an economic benefit of 600 yuan/hour; when the power factor reaches 0.9, it can generate an economic benefit of 900 yuan/hour. In fact, the benefits of improving the power factor are far more than this simple calculation, but I won't elaborate further here.

[Analysis of This Question]

With the above foundation, explaining this question becomes much easier.

The capacity unit of a transformer is kVA (kilovolt-amperes), while the power unit of electrical devices is kW (kilowatts). The difference is that when calculating the power in kW, it is necessary to multiply by the power factor. This means that a transformer with a capacity of 1000kVA can only output 1000kW of power when the power factor is 1, but this is practically impossible in real applications.

During the design phase, a certain margin needs to be left, generally calculated at a compliance rate of 90%, which is more economical and reasonable, i.e., 1000×0.9=900kVA. If we use power compensation to raise the power factor to 0.95 or above, then this transformer can output 900×0.95=855kW of active power.

Note: The power company requires the power factor to be above 0.9; otherwise, there will be penalties. However, the power factor cannot exceed 1, or else the system voltage will rise, affecting the normal operation of the system.

The question states that the 1000kVA transformer originally supplied power to a 200kW electrical device, and now an additional 600kW electrical device has been added, making the total active power of the electrical devices reach 800kW, which still does not exceed the calculated value.

Therefore, the 1000kVA transformer originally supplied power to a 200kW electrical device, and now with the addition of a 600kW electrical device, as long as we can improve the power factor to the required value, the transformer can operate safely and reliably for a long time.

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