Can single-phase and three-phase inverters be connected together
There is a customer who has already installed a three-phase 15kW inverter. Recently, they want to add 10 pieces of 300W solar panels, totaling 3kW, and they plan to use a single-phase inverter. May I ask if the electrical connection is feasible?
1. There is no problem with the connection in a system with a neutral wire.
According to the current national standard “Code for Design of Low Voltage Electrical Installations” (GB50054), there are three forms of grounding in low voltage distribution systems, namely IT system, TT system, and TN system. Among them, the IT system has no neutral wire, while the TT system and TN system have a neutral wire. In industrial, commercial, and civil systems, the vast majority are TN systems. When a grid-connected inverter is connected to the power grid, a three-phase inverter has 3 live wires, 1 neutral wire, and 1 ground wire, while a single-phase inverter has 1 live wire, 1 neutral wire, and 1 ground wire. If there is already a three-phase power grid, the single-phase inverter only needs to be connected to 1 phase wire (i.e., live wire), 1 neutral wire, and 1 ground wire. Therefore, there is no electrical problem.
2. There is no problem with the measurement using a three-phase four-wire electric meter.
A three-phase watt-hour meter is used to measure the electrical energy output by the power source (or consumed by the load) in a three-phase alternating current circuit. Its working principle is exactly the same as that of a single-phase watt-hour meter. It just adopts the method of multiple sets of driving components and multiple aluminum disks fixed on the rotating shaft in terms of structure to achieve the measurement of three-phase electrical energy. The difference between a three-phase four-wire active watt-hour meter and a single-phase watt-hour meter is that it is composed of three driving elements and three aluminum disks installed on the same rotating shaft, and its reading directly reflects the electrical energy consumed by the three phases. The three phases are measured separately, and it is allowed that the three phases are different. Therefore, if the power of one phase increases, it will not affect the other two phases.
Precautions
When a single-phase inverter is connected to the power grid, two issues should be noted. First, there is the problem of three-phase imbalance. Therefore, the single-phase inverter should be connected to the phase with the largest load as much as possible. If the three-phase load is balanced, the single-phase power should not be too large, and it is best not to exceed the load power. If the photovoltaic power is greater than the load power, when the photovoltaic system is connected to the power grid, if the local load cannot consume it, it will be sent to a remote load or the upper-level power grid, which will cause the inverter voltage to rise. The larger the power sent out, the higher the voltage will be. In severe cases, it may cause the inverter to activate the overvoltage protection and shut down.