Three-phase four-wire power supply failure

Update date:2018-04-08 Source:MAXGE

Three-phase four-wire power supply failure

Three-phase four-wire power supply common faults


1, single-phase short circuit fault

At present, in many occasions, in order to take power conveniently, three-phase phase voltages are directly supplied. Including many current rural power grid designs, the three phases of the three-phase power are divided equally among three groups of users, thereby eliminating the need for three-phase transformers. Although this kind of power supply method saves some equipment investment, it brings a great deal of hidden trouble to the user's power equipment. ( In practical applications, the probability of occurrence of a single-phase short-circuit ground fault can be up to 65%, that of a two-phase short circuit is about 10%, and that of a two-phase short circuit is about 20%. Three-phase short circuit accounts for about 5%. The following simple analysis of the threat of single-phase short circuit.


As shown in the above figure, once a single-phase short-circuit phenomenon occurs, the midline potential will be raised, posing a greater threat to the safety of the power-using personnel (equipment with a zero-line connection to the housing). At the same time, at the short circuit moment, load 2 and load 3 need to withstand transient large voltage shocks. In severe cases, the voltage value rises directly to the line voltage (380VAC). Caused by the use of electrical equipment over voltage damage.


 2. Open line of transmission line

In the actual electricity environment environment, the neutral line is often disconnected due to improper installation of the line or improper installation of the fuses and switches. If the neutral line breaks, the neutral point potential of the three-phase load will shift. The neutral point potential displacement directly leads to the unbalance of the output voltage of each phase, and the too high phase voltage will cause the device to overvoltage and directly burn, while the phase of the low phase voltage may damage the device due to the voltage drop and current increase. Because the calculation of the three-phase electric voltage is very complicated, due to the introduction of the load vector, the final detailed calculation formula is also very difficult to understand. The following is a simple way to explain the effect of the neutral short circuit on the line voltage.


As shown above, suppose load 3 is open and the midline is interrupted. At this time, load 1 and load 2 are connected in series and connected to line voltage UUV (380VAC). The voltages on the two loads mainly depend on the sizes of Z1 and Z2. If Z1 is much larger than Z2, the voltage of the load 1 will be close to the line voltage of 380VAC. At this time, the load 1 is likely to be damaged due to overvoltage, and the load 2 may stop working due to the voltage being too low. Under normal conditions, the phase voltage has less influence and can be used normally.


3, equipment power supply open circuit

In addition to power line equipment failures, power equipment input and socket failures may cause equipment damage. Because most occasions use three-phase four-wire power supply, and three-phase four-wire power supply also has a more specific application, and when using three-phase four-wire full-wave rectification, as long as any one phase of electrical equipment can be normal Operation.


As shown in the above figure, the advantage of this three-phase four-wire full-wave rectification circuit is that the power supply circuit can continue to work even when there is a problem with any two-phase three-phase power. However, once the middle line in the rectifier circuit is interrupted or not connected, this circuit becomes a three-phase three-wire rectifier circuit. At this time, the original voltage of 310VDC rises to 538VDC. If the rear-stage equipment cannot withstand the 538VDC high voltage, it will be damaged later. equipment.


Three-phase power supply improvement measures

Because there are many limitations in practical applications, it is impossible to avoid the occurrence of many power failures, but we can reduce the probability of equipment damage through some means to improve the reliability of products. The specific improvement measures are as follows:


1. Single-phase short-circuit fault improvement measures


This fault can properly improve the impact resistance of the input end of the power supply. Generally, it needs to resist the impact of 335VAC. This protects the post-circuit from damage due to overvoltage during transient short-circuits. In order to reduce the voltage increase caused by zero-floating, the cross-sectional area of the zero line can be properly increased to reduce the zero drift, to reduce the voltage increase of the other two phases.


2. Measures to Improve the Openness of Power Lines


From the fault analysis, we can see that the open circuit of the neutral line is mainly a current loop that affects the phase voltage, so that the current does not return to the neutral point. The loop can only be formed by two phase lines, which increases the risk of overvoltage in the equipment. In order to provide a reliable current loop for the phase voltage, the three-phase three-six-wire power supply mode can be adopted in the wiring, and the three-phase three-zero work independently. The disadvantage of this wiring is to increase the zero line investment and line loss, but this can effectively suppress zero drift and reduce the mutual influence of each phase voltage.


3, equipment power supply open circuit improvement measures


General equipment to take three-phase four-wire full-wave rectifier circuit, mainly to consider the redundant design of its power supply, as long as the three-phase power any one phase of electrical equipment can work properly. However, once the device is not connected to the neutral line, the voltage of the rectifier circuit will rise sharply. To solve this problem, it is necessary to cut off the post-stage circuit when the voltage rises, thereby protecting the post-stage circuit from damage. However, it is necessary to ensure that the detection control circuit is stable in power supply during design.

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