With the rapid expansion of the market demand for ESS, the application scenarios are getting more extensive, resulting in more load types that storage inverter has to cope with. The most common question we’ve been asked is that when there are both single-phase and three-phase load at the project site, how should the storage inverter be connected? As we all know, connection of single-phase load will cause output power difference in each phase, which leads to imbalance between the three phases, bringing some negative effects to the inverter such as:
1. Increase cable power loss
2. Increase power loss of the built-in isolation transformer
3. The output of built-in isolation transformer is reduced
4. Built in isolation transformer generates zero-sequence current
5. Affect safe operation of the equipment
Considering the inevitability of the presence of single-phase and three-phase load in various applications, ATESS innovates the following topology of Δ/ Y-shaped transformer for three phase inverter.
A Δ/Y-shaped transformer is added between the 3-leg inverter and the load. The secondary Δ/ Y-shaped connection can create current path for the neutral current generated by unbalanced load, while the primary connection provides circulating current path for the zero-sequence current caused by unbalanced load or triplen subharmonics.
We use this topology to combine with the ATESS control logic: during grid connection, the software will control the inverter to perform phase lock referring to grid frequency and voltage, and output synchronously; when off-grid, ATESS inverter can precisely control the output voltage and frequency, so as to ensure that the voltage of each phase is consistent within the allowable tolerance. With all this, ATESS energy storage inverter turns out to be able to handle 100% unbalanced three-phase load, which means that a 150kw three-phase inverter can run with single-phase load of 50KW in any phase, providing an ideal solution for load variability. (If there are respectively more inductive loads, it needs to be recalculated according to the actual load operating parameters.)