There is an old-fashioned 1000kVA transformer with an existing load of about 200kW. If a new load of about 600kW is to be added, can this transformer withstand it? ”

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

kVA (kilovolt-ampere) is the unit of apparent power, and kW (kilowatt) is the unit of active power. In addition to apparent power and active power, there is also reactive power. The unit of reactive power is kvar (kilovar).
What is the difference between active power, reactive power, and apparent power?

Active power is the power actually consumed by electrical appliances, that is, the conversion of electrical energy into other forms of energy.

For example, the daily electricity bill paid by everyone is the active electricity;

Reactive power refers to some equipment that does not really consume electricity, it is only the part of power that temporarily stores electricity.

For example, if there is a capacitor/coil in an electrical device, the capacitor/coil will always be in a state of charging and discharging when the device is working. Because the capacitor/coil has been charged and discharged without real power consumption, this part of the power is called reactive power.

Apparent power refers to the total power provided by the power supply.

The power source (generally refers to a transformer or a generator), in addition to providing active power to electrical equipment, it also needs to provide reactive power. The reason is simple. Although the capacitor in the electrical equipment does not consume power, it is always charging and discharging, so it also needs to occupy a part of the power supply capacity.

After clarifying these, let’s look at the relationship between them, which requires another concept—power factor. How much active power a power supply can provide depends on the power factor。

【Power factor】

Power factor refers to the ratio of active power to apparent power, generally expressed by cosφ.

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

If it is 1 yuan per kilowatt-hour of electricity, when the power factor is 0.6, the transformer can produce 600 yuan/hour economic benefits; when the power factor reaches 0.9, the transformer can produce 900 yuan/hour economic benefits. In fact, the effect of improving the power factor is far more than that simple, there are many more, so I won’t talk about it here.

[Analysis of this question]

With the above foundation, it will be easy to explain this question again.

The capacity unit of the transformer is kVA (kilovolt-ampere), while the power unit of the electrical equipment is kW (kilowatts). The difference between the two is that the power factor needs to be multiplied when calculating the power kW of the equipment, that is, a transformer with a capacity of 1000kVA. Only when the power factor is 1, it can output 1000kW of power at full load, but it is basically impossible in practical applications.

When designing, if we compensate the power factor to 0.95 through power compensation, then the active power output by the transformer should be: 1000×0.95=950kW.

Note: The power company requires that the power factor must be above 0.9, otherwise there will be penalties; but the power factor cannot exceed 1, otherwise the system voltage will increase and affect the normal operation of the system.

According to the question, the 1000kVA transformer originally supplied power to the 200kW electrical equipment, and now 600kW of electrical equipment has been added. The total active power of the electrical equipment has reached 800kW, which still does not exceed the calculated value.

Therefore, the 1000kVA transformer originally supplied power to the 200kW electrical equipment, and now 600kW electrical equipment has been added. As long as we can increase the power factor to the required value, the transformer can fully operate safely for a long time.

Post time: Jan-07-2022