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The influence of PFC on zero line current in LED outdoor lighting project

LED lamps used in the outdoor landscape lighting in has more than ten years of history, for the convenience of LED control, the overall size of this project has been more and more big, the whole project of tens of millions or even billions of yuan. In this case, some of the original technical blind spots are not beginning to appear.

In these technical blind spots, the influence of power factor and zero line current is one of them. Although this problem is simple, but because it is not paid attention to, the engineering problems, hidden dangers and waste is not small.

In the neon era, ballast power factor itself is generally relatively high, resulting in fewer problems. In the LED era, the use of low-voltage switching power supply usually does not have the PFC[L2] function, there will be the following consequences:

In large scale engineering, because the power factor is too low, it may seriously pollute the power grid.

At the same time, the power factor is too low, will increase the power load of the former equipment, increase user investment.

The zero line current is too large. In the old circuit due to high power factor, zero line current is small, usually the four core cable market is also based on this situation, the zero line of the line relative to the line will be smaller.

Serious excessive starting impulse current will be a strong impact on the power grid, resulting in closing difficulties is often the case.

A general switching power supply because of the larger input rectifier filter capacitor, which starts on the power grid caused by closing the impact of large, difficult, the engineering design unit has been considered, taking some measures, this is not the focus discussed in this paper. With PFC function of switching power supply, because the input capacitance is much smaller, help to reduce the impact current.

This paper describes and analyzes the use of three-phase four wire system of high-power LED project, the zero line current situation.

First, the three-phase four wire system phase separation using ordinary without PFC function of switching power supply, the total zero line current is equal to the sum of the current of each phase line.

The common misconception is that: the current phasor and the zero line in the current phase, the three phase current is equal to the frequency, the phase angle difference between 120 degrees, if the current size of three is equal, the three current phasor and cancel each other exactly zero, no zero line current in. Zero line is used to pass the unbalanced current.

In fact, these claims are only applicable to the switching power supply with PFC function.

Without PFC switching power supply, AC input rectifier, using a large capacitor filter. In this case, the input current is pulsed. A typical input current waveform of this switching power supply is shown in figure 1.

Figure 1

In such a typical current waveform, superposition of 120 degrees and 240 degrees of the same two phase current waveform, the waveforms of Figure 2 can be obtained, thus we get the three-phase equilibrium condition, three zero line together to zero line line four phase, the formation of the total zero line current.

Figure 2

It can be clearly seen that the total zero line current is a simple superposition of the three phase line currents, since all forward and negative currents do not occur simultaneously.

This waveform is certainly not the only possible case of a three-phase four wire system using a switching power supply with no PFC function. In this diagram, the forward current and negative current do not appear at the same time. If the filter capacitor switching power supply line after rectification is too small, or heavy load, different phase forward current and negative current may overlap, offset each other, then zero line current is not a simple superposition of three phase current, will be smaller.

For this reason, there is the following inference:

Two, three-phase four wire system phase separation using switching power supply, the total zero line must not be suspended.

The cost of the total zero line suspension, in accordance with our engineering experience, can be said that the engineering sector every day to pay. The reason why the PFC function is not removed, because even if there is a PFC function, unless the three-phase load balance, the total zero line current will not be zero, so it is not allowed to connect.

The normal three-phase alternating current, the voltage between the phase line and the zero line is 220V, the phase angle between each phase is 120 degrees. If a single phase of the zero line open, no problem, this is not only, if the zero line open, will result in the site of drift, phase between each phase angle change, and change the size of the three-phase access how much load, when the voltage of each phase of the vector and the adjacent two-phase voltage the resulting small load the voltage increases, the load phase voltage drop.

Figure 3 shows the case that the total zero line is not connected with the phase voltage. The use of the power supply without the PFC function, when the total zero line is not received, respectively, to adjust the use of three-phase power, the actual measurement of the voltage, the voltage value of the blue has exceeded the power supply voltage range. Such a situation in the project, to some extent, switching power supply may provide protection, but usually cause power damage.

Figure 3

The importance of the zero line is one of the key problems in the use of three-phase four wire AC system.

The use of switching power supply with PFC, although the current zero line is usually close to or equal to three and the phase current, but usually do not produce more than drop phase of each phase voltage, the reason is that this composite is basically the current;

Aluminum and copper must be connected with the use of aluminum wire clamp or wiring nose;

The zero line bus must be connected with the connecting wire, and the wire connection must be used correctly;

Establishment of zero line operation, regular inspection files;

The zero line connector shall be periodically oxidized and reinforced, and conductive adhesive shall be applied if necessary;

To prevent errors caused by the zero line connection total suspended. Sometimes in order to deal with other problems, the phase line to skip the circuit breaker, at this time must not forget the zero line, or when the circuit breaker is not closed, the zero line suspended.

Three, three phase four wire system with the use of PFC switching power supply, the total zero line current by the power factor of each phase and the difference of each phase of the current decision.

Assuming that the power factor of each phase is ideal, the total zero line current can be calculated by using a spreadsheet. A form of the form is shown in figure 4:

Figure 4

Which:

I1, I2, I3 are three phase current.

I1x= I1*COS (0*PI) (/180)

I2x=I2*COS (240*PI) (/180)

I3x=I3*COS (120*PI) (/180)

I1y=I1*SIN (0*PI) (/180)

I2y=I2*SIN (240*PI) (/180)

I3y=I3*SIN (120*PI) (/180)

Inx=I1x+I2x+I3x

Iny= I1y+I2y+I3y

Total zero line current effective value: In=SQRT (Inx*Inx+Iny*Iny)

In actual use, the right side of the equation of the electrical parameters, for the corresponding cell coordinates can be.

The following figure 5 is a simulation of a number of typical cases.

Figure 5

By comparing the calculated results with the measured results, it can be judged whether the switching power supply has a high power factor. We use the following circuit to do the verification test, the results are very close to the theoretical calculation.

Figure 6

Four. The use of residual current protector RCD or leakage protector related to this problem.

In the case of the power factor, especially the lack of understanding of the zero line current, some people will use the three-phase three wire residual current protector in the system, and then found that it will not work, can not switch on. This is no longer a reason. The conclusion is: even if you use a PFC switching power supply, as long as the three-phase imbalance, the protector will act.

On the leakage protector, do an extension: is not the use of three-phase four wire leakage switch, the zero line will be no problem?

The actual situation often or not, because the switching power supply in the project is required to enter the grounding wire, and switching power supply for EMC and safety considerations, the grounding and the AC line, usually by one or several small capacity high voltage capacitor. If you use a more sensitive leak protection device, the charging of the instantaneous current will trigger the leakage protector action.

In addition to the electric shock, it is also because of these reasons the capacitance of the switching power supply used in the drain current relative to the power frequency transformer, was much larger, while in large projects, the use of switching power supply quantity is usually very high, the leakage current together, will achieve great value, and it is the power is not grounded but not so big. In this case, the total leakage current value should be calculated, and the leakage current protector with larger action current is selected.

Five, concluding remarks:

This paper expatiates the theory and engineering practice, pointed out the necessity to use with the PFC function of switching power supply of large outdoor LED project, and sums up the application of LED in outdoor engineering since the zero line related problems has not been given full attention. The author hopes to provide a reference for the industry, as a way for LED outdoor engineering standardization process.

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