Knowledge point: power surge protector
Lightning protection is a special industry, requiring workers in this industry to have very comprehensive knowledge, including: electrical, electromagnetic compatibility, circuit analysis and analog circuits. Through a simple introduction, this article hopes to give you a basis for selecting the power surge protector, and to facilitate the selection of the suitable model.
1. Introduction to the parameters of the lightning arrester
When we get a power supply lightning protection module, there are many parameter values on it, such as: working voltage, maximum sustainable voltage, nominal flow rate, maximum flow rate, voltage protection level, response time, etc. These parameters are introduced below For the selection of lightning arresters.
1.1 Working voltage U0
The working voltage here is the working voltage of the equipment, not the working voltage of the surge protector. The power surge protector can be understood as a voltage limiting switching element, so no working voltage is required. Common working voltages are 380V and 220V.
1.2 Maximum sustainable voltage Uc
The maximum AC voltage rms or DC voltage that can be permanently applied to the SPD is called the maximum sustainable voltage. Many people think that this voltage should be the working voltage of the equipment, or the voltage value after considering the voltage deviation. This understanding is wrong. Under different grounding methods, the voltage value that will appear between the line and the ground is very different. If the maximum sustainable voltage cannot be reasonably selected, a reasonable varistor voltage cannot be selected. If the varistor voltage is selected too large, the residual voltage value will be high; fire.
1.3 Nominal flow In
The nominal flow rate refers to the current value that can pass through the 8/20 waveform impact for many times without causing changes in the performance of the varistor. This value is determined by the thickness of the lightning protection component, and its size is generally equal to 1/2 of the maximum flow rate. There is not much practical significance in the selection of lightning protection, and it is mainly used for lightning impact tests.
1.4 Maximum flow rate Imax
The maximum flow rate refers to the maximum current value that the surge protector will not be damaged and its performance will change when it passes through the 8/20 waveform once, and it may be damaged under the impact of such waveforms for 2 or more times. The flow rate of the lightning arrester usually referred to is the maximum flow rate rather than the nominal flow rate.
1.5 Voltage protection level Up
To understand the voltage protection level, we must first understand the residual voltage and limit voltage. The residual voltage is the voltage value at both ends of the lightning arrester when the lightning arrester is subjected to lightning current. The voltage protection level is not measured, it is the value given by adding a certain error to the measured limit voltage, and the voltage protection level value is greater than the limit voltage level value. When selecting a surge protector, it is required that the voltage protection level value should be less than the withstand voltage capability value of the equipment.
1.6 Response time Ta
The response time of the lightning arrester is determined by the components of the lightning arrester. The response time of the general gas discharge tube is relatively long, which is 100ns, which causes its impulse discharge voltage to be much higher than the DC discharge voltage, so the discharge tube cannot be used For fine protection; the response time of the piezoresistor is 25ns. According to this time difference, the multi-stage SPDs are required to operate in sequence, and a reasonable wire length is required. If the wire length is not enough, an inductance should be added as a decoupler.
2. The selection of the maximum sustainable voltage of the power surge protector for the grounding mode of the low-voltage power distribution system
Most lightning protection salesmen and a few lightning protection technicians only care about two issues when choosing a lightning protection device: one is whether it is single-phase or three-phase. Just choose 385V; another problem is the installation location, that is, the problem of choosing the flow rate. Few people care about the grounding type of the system, and many people don't know the type of grounding system. In this case, there may be problems with the lightning protection device selected for customers. The following is an introduction to how to deal with various grounding methods. Choose a power surge protector.
2.1 Classification of my country's low-voltage power distribution system
In my country, according to the grounding state of the neutral point of the power supply and the configuration of the neutral line and the protection line, the grounding of the low-voltage power distribution system is divided into three types: TN system, TT system and IT system. The TN system is further divided into TN-C system, TN-C-S system, and TN-S system.
2.2 How to choose power surge protector for TN-C system
The TN-C system is shown in the figure. In the TN-C system, we can see that the N line and the PE line are merged together, so there is no need for protection between the N line and the PE line, and only between the L line and the PE line. It is enough to take protection between the lines. The number of modules in such a system is different from that of the three-phase five-wire system. Look at the selection of the maximum sustainable voltage. Assuming that a single-phase ground fault occurs in the system, the line-to-ground voltage of the system will increase to exceed the phase voltage. In this case, we take Uc greater than or equal to 1.55U0, and usually take the value of Uc is 385V.
Figure1 TN-C system
2.3 How to choose power surge protector for TN-S system
The TN-S system is shown in the figure. Since the N line and the PE line are separated, a voltage limiting element needs to be set between the N line and the PE line. The number of modules required for such a system is 4. There are 2 schemes, one is "4+0" scheme and the other is "3+1" scheme. How to choose the maximum sustainable voltage in these two cases will be analyzed separately below.
Figure2 TN-S system
2.3.1 "4+0" scheme
The "4+0" scheme is shown in the figure. In the "4+0" scheme, if a single-phase grounding occurs, the voltage between the line and the ground will rise to exceed the phase voltage. Generally, Uc is greater than or equal to 1.55U0. Usually In this case, the value of Uc is 385V. The voltage difference between the N line and PE depends on the displacement of the neutral point of the system. Generally, it will not exceed the phase voltage of the system. In this case, Uc should be greater than or equal to 1.15U0, 10 %Considering the voltage deviation, 5% considers the component aging, generally take Uc as 255V or 275V.
Figure 3 "4+0" Plan
2.3.2 "3+1" scheme
The "3+1" scheme is shown in the figure, and the protection mode has changed. The L1 line, L2 line, and L3 line first protect the N line, and then the N line passes through the NPE module to the PE line. If a single-phase grounding occurs, it will appear at The operating voltage between the L line and the N line will not exceed the phase voltage. In this case, Uc should be greater than or equal to 1.15U0.
Figure 4 "3+1" Plan
2.4 How to choose power surge protector for TN-C-S system
The TN-C-S system is a combination of TN-C and TN-S systems, and the selection can refer to the above analysis.
2.5 How to choose power surge protector for TT system
The TT system is shown in the figure. In such a system, it can be seen that the N line is only grounded on the power supply side, and it is separated from the equipment protection ground. Therefore, a voltage limiting element needs to be set between the N line and the PE line. There are also two schemes: "4+0" scheme and "3+1" scheme. It is better to choose the "3+1" scheme under such a system. The following analyzes the possible problems of the "4+0" scheme under such a grounding type.
Figure5 TT system
As shown in the figure, a 4P power supply lightning protection module is installed under a TT system. When the module is degraded, the loop resistance of the current passing through the module is composed of the grounding resistance of the power supply side, the transformer resistance, the wire resistance, the module resistance and the protective ground resistance. , such a resistance is very large, and the current will be very small. Continuously adding such a current to the piezoresistive element will cause it to heat up and eventually cause a fire. In most cases, choose the "3+1" scheme. If you want to use "4 +0” plan, the power surge protector needs to be installed behind the RCD, which can block the small current in time.
As for the selection of the maximum sustainable voltage, it can be selected by referring to the method described above.
Figure 6 "4+0" scheme under the TT system
2.6 How to choose power surge protector for IT system
As shown in the figure of the IT system, since there is no neutral line in the IT system, it is only necessary to protect between the L line and the PE line. That is, only three modules are required.
Let’s look at the choice of Uc value. When a single-phase ground fault occurs, the voltage between the sound phases will rise to the line voltage. Considering the voltage deviation and component aging, the maximum sustainable voltage UC taken during use is greater than or equal to 1.15x380 =437V, generally take Uc as 440V.
Figure7 IT system