Rating of any equipment means the maximum value it can bear. if we then equipment we got damage or fail if we talk about 100 kVA transformer it simply means 100000 volt ampere. Rating of transformer is determined by product of voltage and current. If it’s value exceed the 100kVA then transformer can overheat and failed. So hundred kVA transformer means that the product of voltage and current should not exceed 100 KVA.