"There is an old-fashioned transformer of 1000KVA 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 a unit of apparent power, and KW (kilowatt) is a unit of active power. In addition to apparent power and active power, there is reactive power, and 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, electrical energy is converted into other forms of energy.
For example, the daily electricity bills that everyone pays are active electricity;
Reactive power means that some devices do not actually consume electricity, it is only the part of the power that 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 charge and discharge. Because the capacitor/coil has been charged and discharged, it does not really consume electricity, so this part of the power is called reactive power.
Apparent power refers to the total power provided by the power supply.
In addition to providing active power to electrical equipment, the power supply (generally refers to a transformer or generator) also needs to provide reactive power. 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 a part of the power supply capacity.
After clarifying these, let's look at the relationship between them, which will talk about another concept - power factor. How much active power a power supply can provide depends on the power factor.
Power factor refers to the ratio of active power to apparent power, generally expressed as cosφ.
For example, a 1000KVA transformer can output 600KW of active power when the power factor cosφ=0.6; but when the power factor cosφ=0.9, it can output 900KW of active power.
If 1 kWh of electricity is 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 900 yuan/hour of economic benefits. In fact, the role of providing power factor is far more than that simple, there are many more, so I won't talk about it here.
【Analysis of this topic】
With the above foundation, it is easy to explain this topic.
The capacity unit of the transformer is KVA (kilovolt-ampere), and the power unit of the electrical equipment is KW (kilowatt). Only when the power factor is 1, can the power output of 1000KW be fully loaded, but it is basically impossible in practical applications.
When designing, it is necessary to leave a certain margin, which is generally calculated according to the compliance rate of 90%, 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 increase and affect the normal operation of the system.
The title says that the 1000KVA transformer originally supplied power to the 200KW electrical equipment, and now a 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 originally supplied power to the 200KW electrical equipment, and now the 600KW electrical equipment is added. As long as we can increase the power factor to the required value, the transformer can operate safely for a long time.