Question: For which power do you have to dimension an isolation transformer or a generator to which one or several inverters are to be connected?
Answer: Experience has shown that the apparent power of the isolation transformer/generator must be at least
three times the input power of the connected inverters.
Relevant input power of an inverter:
S = U · I · 1.73 || U, I : Input voltage of inverter and rated mains current for operation with mains choke
Total apparent power of transformer with one or several inverters:
(S total) = 3x (S1 + S2 + ... + Sn)
It will not be enough only to consider the total rated powers of the connected motors and the power consumption of the controllers for dimensioning the transformer!
In that case, the harmonic reactive power occurring with bridge rectifiers would not be considered. With bridge rectifiers, as used in inverters with DC bus, the mains current is not absolutely sinusoidal. There are harmonics which do not contribute to the effective power of the drive. However, these higher-frequency current components still have to be transferred by the transformer (harmonic reactive power).
An insufficiently dimensioned transformer/generator can be overheated by harmonics. Voltage dips and oscillation tendency of the supply system may occur.
Tip:
For controllers, the mains current indicated for operation without mains choke (if permissible) is always higher than the mains current indicated for operation with mains choke. This is due to the fact that for transferring the same effective power there will occur a higher amount of harmonic reactive power (high re-charging current peaks).
Dimensioning of an autotransformer: see doc-ID
202300042Harmonic mains filter:If the transformer has not been sufficiently dimensioned, you can use a harmonic mains filter to reduce the harmonics, e. g.: Harmonic Filter Module HFM-A from the company Block.
Advantages of harmonic mains filters:
• The mains choke particularly prescribed for higher powers at the mains input of the inverter can be done without
• At rated load, the DC-bus voltage of the inverter will be up to 5 % higher
• Depending on the constellation, you may do without RFI filters.
Important for generator operation: A mains contactor must be provided at the output terminals of the generator which only connects the consumers when the generator has reached the rated speed.
