What is the difference between three-phase four-wire and three-phase five-wire refrigeration unit?
Three-phase four-wire and three-phase five-wire, for many people, three-phase is easy to understand, but there are not many people who study four-wire and five-wire, so what is the difference between them?
1. four wire and wireless structure difference
The four wires include three phase wires: L1---(A) phase, L2---(B) phase, L3---(C) phase, neutral line PEN, which is set by the combination of working neutral line and protective neutral line The zero protection system is a TN-C system.
The five wires include three phase wires: L1---(A) phase, L2---(B) phase, L3---(C) phase, a neutral wire N, and a ground wire PE, which are working neutral wires The zero-connection protection system that is installed separately from the protection neutral line or partly installed separately, namely TN-S system (including TN-CS system).
2.The difference between PEN line and PE line in two, four and five lines
PEN line: It belongs to the TN-C grounding system and is designed to obtain 220V phase-to-phase voltage from 380V phase-to-phase voltage. There is working current passing through the PEN line, and the PEN building should be grounded repeatedly.
PE line: Connect it to the N line at the power supply transformer side, but cannot be used as a neutral line after entering the user side. There is working current through the N line, while the PE line usually has no current; in the 380V low-voltage distribution network, the N line is set to obtain 220V phase voltage from the 380V line voltage. In some cases, it can also be used for zero sequence current detection for Monitoring of three-phase power supply balance.
3.Comparison of advantages and disadvantages of three, four and five lines
1, five-wire system
Advantages: The protection sensitivity and reliability are higher than those of the three-phase four-wire system, because the PE line (that is, the grounded neutral line) is set separately and is the neutral point of the directly grounded power transformer. The neutral point of the transformer is reliable and direct Grounding, the grounding resistance is low, meeting the system protection requirements.
can be used in an explosive environment
Disadvantages: the price is more expensive.
2, four-wire system
Advantages: The protection line and the neutral line are merged into the PEN line, which has the advantages of simplicity and economy. When a fault occurs, the fault current is large, and general overcurrent protection appliances can be used to cut off the power supply, which is relatively safe.
a. For single-phase loads or three-phase unbalanced loads and lines with harmonic current loads, the normal PEN line has current, and the resulting voltage drop appears on the metal casing of the electrical equipment and the metal sleeve of the line. Sensitive electronic equipment is disadvantageous.
b. The weak current on the PEN line can also cause an explosion in an explosive environment. When the PEN line is broken or the phase line is directly short-circuited with the earth, it will present a very high ground fault voltage, which may expand the scope of the accident.