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, and the unit of reactive power is Kvar (kilovar).
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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 bills that everyone pays are active electricity;
Reactive power refers to the part of power that some equipment does not really consume electricity, but only temporarily stores electricity.
For example, if there is a capacitor/coil in an electrical device, when the device is working, the capacitor/coil will always be in a state of charging and discharging. Because the capacitor/coil is always charging and discharging, and does not really consume electricity, this part of the power is called reactive power.
Apparent power refers to the total power provided by the power supply.
A power supply (generally refers to a transformer or a generator) needs to provide reactive power in addition to active power to electrical equipment. The reason is very simple. Although the capacitor in the electrical equipment does not consume power, it is always charging and discharging, so it also needs to occupy part of the capacity of the power supply.
After figuring this out, let's look at the relationship between them. This is 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.
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For example, a 1000KVA transformer can output 600KW active power when the power factor cosφ=0.6; but when the power factor cosφ=0.9, it can output 900KW active power.
If 1 kWh of electricity costs 1 yuan, when the power factor is 0.6, the transformer can generate economic benefits of 600 yuan/hour; when the power factor reaches 0.9, the transformer can generate economic benefits of 900 yuan/hour. In fact, the function of providing power factor is far more than that simple, and there are many more, so I won’t go into details here.
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【Analysis of this topic】
With the above foundation in place, it will be easier to explain this question.
The capacity unit of the transformer is KVA (kilovolt-ampere), while the power unit of the electrical equipment is KW (kilowatt). The difference between the two is that when calculating the power KW of the equipment, the power factor needs to be multiplied, that is, a transformer with a capacity of 1000KVA, Only when the power factor is 1, will it be able to output a power of 1000KW at full load, but it is basically impossible in practical applications.
When designing, it is necessary to leave a certain margin. Generally, it is calculated according to the 90% compliance rate, which is more economical and reasonable, that is, 1000×0.9=900KVA. If we compensate the power factor to 0.95 and above through power compensation, then the transformer can output 900×0.95=855KW active power.
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 rise and affect the normal operation of the system.
The topic said that a 1000KVA transformer originally supplied power to 200KW electrical equipment, and now 600KW 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 used to supply power to 200KW electrical equipment, and now 600KW electrical equipment is added. As long as we can increase the power factor to the required value, the transformer can fully operate safely for a long time.