Lightning protection is a special industry that requires workers in this sector to have a comprehensive knowledge of electrical, electromagnetic compatibility, circuit analysis and analogue circuits. In this article, we hope that a simple introduction will enable you to select the power supply lightning arrester based on the selection, to facilitate the selection of suitable models.
1.the introduction of lightning protection parameters
When we get a power supply lightning protection module, there are many parameter values, such as: operating voltage, maximum sustainable voltage, nominal flux, maximum flux, voltage protection level, response time and so on, the following are introduced to these parameters in order to lightning protector selection.
1.1 Working voltage U0
The working voltage here is the working voltage of the equipment, not the working voltage of the lightning arrester. The power supply lightning arrester can be understood as a voltage limiting switching element, so it does not need any working voltage. Common operating voltages are 380V and 220V.
1.2 Maximum sustainable voltage Uc
The maximum AC voltage RMS or DC voltage that can be applied to the SPD on a permanent basis is called the maximum sustainable voltage. Many people think that this voltage should be the operating voltage of the equipment, or the voltage value after taking into account the voltage deviation, this understanding is wrong. The voltage value between the line and the ground varies considerably between different earthing methods, and if the maximum sustainable voltage cannot be selected reasonably, a reasonable voltage sensitivity cannot be selected. If the voltage is too high, the residual voltage will be too high; if the voltage is too low, there will be a continuous leakage current in the varistor, and as the varistor deteriorates, the continuous leakage current will become larger and larger, leading to a fire.
1.3 Nominal flux In
The nominal flux refers to the current value that can pass through the 8/20 waveform shock many times without causing changes in the performance of the varistor, this value is determined by the thickness of the lightning protection components, the size is generally equal to 1/2 of the maximum flux, this is not much practical significance in lightning protection selection, mainly for lightning shock testing.
1.4 Maximum flux Imax
The maximum through-flow is the maximum current value at which a lightning arrester can be passed once without damage or performance change under an 8/20 waveform, but may be damaged under two or more such waveform shocks. The usual term for the throughput of a lightning arrester is also the maximum throughput rather than the nominal throughput.
1.5 Voltage protection level Up
To understand the voltage protection level it is important to first understand the residual voltage and the limiting voltage. The residual voltage is also the voltage across the lightning protector when it is subjected to a lightning current. The voltage protection level is not measured, but is given by the measured limiting voltage plus a certain error, the voltage protection level value is greater than the limiting voltage level value. When selecting a lightning arrester it is required that the voltage protection level is less than the value of the equipment's withstand voltage capability.
1.6 Response time Ta
The response time of the lightning arrester is determined by the components of the lightning arrester, the general response time of the gas discharge tube is relatively long for 100ns, which also leads to its impact discharge voltage value is much higher than the DC discharge voltage, so the discharge tube can not be used as fine protection; the response time of the varistor is 25ns, according to this time difference requires multi-level lightning arrester in order of action, it requires a reasonable length of wire, if the wire If the length of the wire is not sufficient, inductors need to be added as decouplers.
2.Low-voltage distribution system grounding method power supply lightning protector maximum sustainable voltage selection
Most of the lightning protection salesmen and a few lightning protection technicians are only concerned about two issues when choosing lightning protection: one issue is single-phase or three-phase, single-phase on the choice of maximum sustainable voltage 275V, three-phase on the choice of 385V; the other issue is the installation location, that is, the choice of flux flow. Few people will be concerned about the grounding type of the system, many people do not understand what grounding system type, in this case to the customer elected lightning protection device may have problems, the following is to introduce a variety of grounding mode how to choose the power supply lightning protector.
2.1 China's low-voltage power distribution system classification
In China, according to the grounding state of the neutral point of the power supply and the configuration of the neutral line and protection line, the low-voltage distribution system grounding is divided into TN system, TT system and IT system three types, of which the TN system is divided into TN-C system, TN-C-S system, TN-S system three.
2.2 TN-C system how to choose power supply lightning protector
TN-C system as shown in the diagram, in the TN-C system we can see that the N line and PE line is merged together, so in the N line and PE line between the protection is not needed, only the L line to PE line between the protection can be taken, such a system first of all the number of modules is different from the three-phase five-wire system. Then look at the choice of the maximum sustainable voltage, assuming that the system occurs single-phase ground fault, the system line to ground voltage will increase to more than the phase voltage, such a situation we take Uc greater than or equal to 1.55U0, usually take the Uc value of 385V.
2.3 How to choose a power supply lightning protector for a TN-S system
The TN-S system is shown in the diagram. As the N and PE lines are separated, a voltage limiting element is required between the N and PE lines, and the number of modules required for such a system is 4. There are two options, one is the "4+0" option and the other is the "3+1" option. The following is an analysis of how to choose the maximum sustainable voltage in each of these two cases.
2.3.1 The "4+0" scheme
The "4+0" scheme is shown in the diagram. In the "4+0" scheme, if a single-phase grounding occurs, the voltage between the line and ground will rise above the phase voltage, generally taking Uc greater than or equal to 1.55U0, usually taking Uc value of 385V, appearing in the N line and PE between the Voltage difference depends on the displacement of the neutral point of the system, generally will not exceed the system phase voltage, so that the case to take Uc greater than or equal to 1.15U0 can, 10% to take into account the voltage deviation, 5% to consider is the component aging, generally take Uc for 255V or 275V.
2.3.2 The "3+1" scheme
The "3+1" scheme is shown in the diagram, the protection mode has changed, the L1 line, L2 line and L3 line are first protected against the N line, then the N line is protected against the PE line through the NPE module, if a single phase earth situation occurs, the operating voltage between the L line and the N line will not exceed the phase voltage, in this case take Uc greater than or equal to 1.15U0 can be.
2.4 How to select power supply lightning protectors for TN-C-S systems
TN-C-S system is a combination of TN-C and TN-S system, the selection can be made with reference to the above analysis。
2.5 How to choose a power supply lightning protector for a TT system
The TT system is shown in the diagram. It can be seen that the N line is only earthed on the power side, it is separate from the equipment protection ground, so a voltage limiting element is needed between the N and PE lines. There are also two options: the "4+0" option and the "3+1" option. In such a system choose the "3+1" scheme is better, the following analysis of the "4+0" scheme in such a grounding type may appear under the problem.
When the module is degraded, the circuit resistance of the current through the module consists of the power side earth resistance, transformer resistance, wire resistance, module resistance and protective earth resistance, such resistance is very large and the current will be very small, such current continuously added to the varistor element will cause it to heat up and eventually cause a fire. In most cases, the "3+1" scheme is chosen. If the "4+0" scheme is to be used, the mains lightning protector needs to be installed behind the RCD, so that small currents can be interrupted in time.
As for the choice of the maximum sustainable voltage, you can refer to the method described earlier to choose.
2.6 How to choose a power supply lightning protector for an IT system
The IT system is shown in the diagram. As there is no neutral line in the IT system, only protection between the L line and the PE line is required. This means that only three modules are required.
To see the choice of Uc value, when a single-phase earth fault occurs, the voltage between the sound and ground will rise to the line voltage, and then taking into account the voltage deviation and component ageing, the maximum sustainable voltage UC taken when using is greater than or equal to 1.15x380=437V, generally take Uc for 440V.