There are two ways of load connection in three-phase circuit of biogas generator: star connection and triangle connection. The way of star connection is divided into three-phase three-wire system and three-phase four-wire system.
Three-phase three-wire system refers to three wires drawn from one end of the three-phase coil of the generator respectively, and the other end of the coil is connected together but not drawn out; three-phase four-wire system refers to three wires drawn from one end of the three-phase coil of the generator respectively, and the other end of the coil is connected together and drawn out as a neutral line.
The characteristics of three-phase three-wire system:
When the three-phase load is symmetrical, the phase angle between the phases is 120 degrees, the sum of the currents in the load neutral line is zero, and the neutral line does not work at this time, so it can not be connected out. This is the three-phase three-wire circuit, which is suitable for pure three-phase load circuit (only 380V power supply). This method is widely used in industrial consumption, because the three-phase load on consumption (usually seen as three-phase motor) is generally symmetrical.
The characteristics of three-phase four-wire system:
When the load is asymmetrical or a phase is disconnected, current will be generated in the neutral line, which ensures that the load voltage and generator voltage are equal and symmetrical. If there is no neutral line at this time, the load voltage of each phase will be unequal, which will lead to some electrical appliances operating voltage exceeding the rated voltage, which is not allowed. In our daily life, the loads we come into contact with, such as lamps, TV sets, refrigerators, fans and other household appliances and single-phase motors are all single-phase loads. When there are single-phase loads and three-phase loads in the circuit, we should adopt three-phase four-wire system (which can provide 380V power supply, but also 220 V power supply).
Three-phase three-wire insulation system is commonly used in ship power grid. One of the advantages is that when one of the three-phase lines touches the hull, it will not produce intense short-circuit current and impact circuit system. The other advantage is that it can avoid casualties caused by electric shock. The three-phase three-wire insulation system is that the neutral point of the generator is not grounded, that is, the neutral point is not connected with the hull. If one of the three phases is not carefully touched, because the human body is standing on the hull, and the hull is not connected with the power supply system, the human body will not have current flowing through, so it will not be shocked.