Another important hall-effect current sensor in hybrid inverter is neutral current sampling for midpoint potential balancing. In a designed for 3-phase system, the load on each phase needs to stay the same. However, in some 3-phase household or commercial application scenarios, such as in Germany and Austria, both 3-phase and single-phase loads are used, which can cause unbalanced power consumption among the three phases. This means that one or two phases can have higher power demands than the other phases. This can lead to neutral line voltage imbalances, which can cause problems for grid and grid equipment. To power single phase load in the system, which means the output power on each phase depends on the corresponding load consumption and can not be the same, hybrid inverter usually has unbalanced output function. Solar inverter vendors usually have some feature description, such as support 100% unbalance output in backup and on grid mode (even 110%) in the vendors data sheets. Read more in this blog What is 100% or 110% unbalanced output inverter?

If the loads in 3-phase are balanced, then there needs to have no current in the neutral line and the midpoint potential is balanced, for example, half of the BUS voltage. On the contrary, the neutral line source or sink current if there has load imbalance, which can cause the midpoint potential variation. This needs to compensate the midpoint potential imbalance.

Figure 2-6 shows the conventional way of 2 split capacitors. The neutral point is the midpoint of the two bulky electrolytic capacitors C1 and C2 with equivalent capacitance. The neutral current keeps charging one split capacitor while discharging another one for a certain period to keep midpoint potential balancing. Though in fact there has some small capacitance or voltage mismatching between the two split capacitors, this design is easy for implementation and still widely used in string inverter and residential inverter where 3-phase output must be balanced to the grid. However, for obvious unbalanced output, DC component in the neutral current can cause severe voltage mismatching, then further leads to inverter fault shutdown protection.

Figure 2-6 2 Split Capacitors Design for Midpoint Potential Balancing in 3-phase Inverter

Unlike string or residential inverter, hybrid inverter has a fourth-leg (also known as balancing bridge, the inverter is then called 3-phase 4-leg inverter) to actively control the midpoint voltage that allows the inverter to support unbalanced output, as shown in Figure 2-7. The control of the fourth switching leg is decoupled from the 3-phase inverter. The balancing bridge control involves neutral current sampling in where hall-effect current sensor can be used.

Figure 2-7 Balancing Bridge Design for Midpoint Potential Balancing in 3-phase Inverter