Webinar "Frequently asked questions about SPDs"
The thirteenth webinar of a series "Grounding and lightning protection: issues and problems arising in the design"(was held on February 10, 2016 at 11:00)
This year we are holding another webinar on the use of SPDs to limit surges in low-voltage electrical circuits. The reason for this is the numerous questions from the designers of internal lightning protection. Their cause is trivial enough. Russian lightning protection regulations cover the practical application of SPDs in very general terms, and only in SO 153-34.21.122-2003 instruction. There are no specific requirements, so the designers are left alone with the recommendations of SPD manufacturers. The situation is disparate. Manufacturers are primarily interested in the mass sales of products that are easy to manufacture and are expensive. For this purpose, they issue brochures with the basic assessment of storm surge levels and lightning currents coming through the SPDs, which clearly tend to turn designers' attention to the "right" protection elements.
Today, due to the serious financial complications in the country, such a situation can hardly be considered acceptable. An evidence-based guidance for the selection of SPDs based on the reliable calculations of thunderstorm surges and current loads is desperately needed. The upcoming seminar will form a basis for solving this problem.
It is worth reminding once again that the placement of SPDs in an electrical circuit is a very strong tool. Its use is justified only in the case if all the possibilities to limit dangerous lightning effects through the installation of external lightning protection devices have been exhausted. Much-hyped typical examples with hundreds of SPDs installed in the circuits of the protected facility cannot be considered a constructive solution, as any SPD is quite a complex appliance with a very specific reliability. Its very presence in the circuit may adversely affect the reliability of the whole facility and its technical specifications.
Designers should always pursue a task of minimizing the number of SPDs. This is hardly possible without a reliable assessment of hazardous lightning effects on the protected equipment and their comparison with the permissible parameters specified by the manufacturer.
To demonstrate the said above, we relied on specific questions asked by the audience. Most of these questions were answered either in the form of specific recommendations or as guidelines to the quantitative method of solving the problem.
Full-screen mode with the quality of "720p" is recommended.
УЗИП – SPD is very serious and is not cheap at all
- Today we hold the eleventh webinar with the professor and we will discuss surge protection devices, it is an acute issue. These are surge protection devices or as we call them - SPDs. Previously, we collected questions from designers to understand, what are the problems in design, selection and installation of surge protection devices. These questions were sent to the professor and he will answer them during the meeting today. I think my introduction speech is over here. I give the floor to Eduard Meerovich. Hello! May we begin?
- Good afternoon, Alexey! Good afternoon, dear audience, colleagues. Maybe I should say I'm very happy to meet you. Unfortunately, the joy is only one sided. This is the second webinar on this subject and I understand the sitatuaion of SPDs remains complicated for designers. I know why. SPDs are designed by companies engaged in power supply to buildings and facilities. These are the people knowing power industry well. They are able to design required circuits properly, but installation of SPDs relates to another kind of specialization. Any power supply company has a high voltage laboratory engaged in suppression of surges. These people have been trained absolutely in other way, because overvoltage is always a pulse transient process to be calculated by using other methods. And the experience we have is quite another practice. When I've been getting ready for the seminar, I tried to see the following thing. Now I take our national regulatory documents and see if the designer can do something he does not understand based on relevant codes and standards.
International Electro technical Commission Standard
Low voltage surge protection devices;
- The principal document is the standard of the International Electro technical Commission, IEC Standard which was translated into Russian, it is named as surge protection device in high-voltage networks. I looked through this standard and found, that any designer addressing to it will get no useful information at all. What should the designer do? He is forced to do the following. Search on the Internet trying to select relevant articles and recommendations. I tried to do the same. What did I find? I found many articles with recommendations. The authors of them are SPD manufacturers but not designers of SPD installation. These articles have one only goal- sell more and more SPDs. Because of these recommendations, they confuse designers again. What to say? Once looking at all these articles, SPDs seem to be the only thing protecting against overvoltage. I have a question, "is it really so?". Yes it is, if you trust recommendations of companies. Companies give certain recommendations, even specific standard designs. For example, I counted about 1,500 SPDs in one of such standard design for an industrial enterprise. Firstly, SPD price is not much affordable today, look at one shown in the poster here, the price of SPD is approaching 500$. 500$ is quite a high price, such costs shall be reasonable. Especially, when your number of SPDs is many dozens or hundreds. But, stop, who needs it? Can this situation be taken for granted? That the only thing to limit overvoltages is an SPD. I have strong doubts in this regard. The same was at our first webinar devoted to SPD, wasn't it?
SPD is an effective medicine
SPD – in low voltage electric circuit is not a cheap protective mean and is not safe at all;
- We discussed that issue and agreed that SPD is a state-of-the-art device for surge protection, but like any effective medicine, SPD is not safe for the following reasons. Having dozens or hundreds of electronic devices installed in any electrical circuit will result in change of reliability hereof. This will be changed at least because of reliability of SPDs themselves, which certainly has the coefficient other than one. Installation of dozens and hundreds of SPDs in high-voltage circuit will result in change of reliability hereof, moreover, in change of technical characteristics hereof. These will be changed because SPD can change the delivery capacity of the circuit, frequency characteristics hereof and all this will result in deterioration of safety after SPDs installation. This is the reason why you shall always take the following thing into account when installing SPDs: is it necessary to install them and is there anything to be protected in SPDs circuit.
Physical mechanism of lightning overvoltages. Part 1
- First of all, I shall begin with the explanation of the nature of these overvoltages occurring in electrical circuits. We can talk about two types of overvoltages. The first overvoltage from a direct lightning strike. The lightning strikes, for example, into power transmission line, current runs by cables hereof, by grounding of supports and the voltage appears on the support's ground electrodes and its inductivity, which acts through the wires to the electric network. The peculiarity of these overvoltages is that rather high current runs through protective devices intended for surge suppression. This current may form part of the lightning current, being the most part hereof. One shall be capable to determine which this lightning current part will appear in this case. It is a very critical and serious issue to be resolved.
Physical mechanism of lightning overvoltages. Part 2
EMF induced by the lightning magnet field
- The second thing is about the following. The second thing concerns induced overvoltages. You have the lightning current running somewhere. It can run in the lightning channel, in the lightning rod, in down conductors, and everywhere there is magnet field around this current. This magnet field causes EMF of magnetic induction in all the contours connected to that field. This is another kind of overvoltages. It is specific by, as a rule, a decreased level of overvoltage, whereas it of course represents high risk for low-voltage electrical circuits. And the current running in this case through the protective device is considerably lower.
Surge suppression means
- optimal harnessing and orientation of electrical circuits of the object;
- Before understanding which SPDs shall be installed and in what quantity, one shall determine the following: maybe we can suppress overvoltage by other means, not SPDs? What these means can be? First of all, it is harnessing of those electrical circuits to be protected. Overvoltages will vary depending on layout of electrical circuits. This especially concerns induced overvoltages. The second thing, try to transport lightning currents into the ground in a safe way. "Safe" here means the following: if you split the lightning current into many ways with high number of down conductors, more likely, that overvoltage will be decreased inside the building space to which these down conductors are connected. Finally, there is an old-fashioned simple way- is shielding of those electric circuits you are planning to protect. Proper shielding will help to reduce overvoltage by the order of magnitude, and even by two orders. Then, before saying "Let's install SPDs", be sure to check all affects you could win from the three mentioned methods.
Harnessing of electrical circuits
Подземные коммуникации… - Underground utilities should be remote from the lightning rod ground electrodes;
- You know that this issue was discussed at the workshop No.3, with many details. However, one thing I should say. At least one thing I should say. First of all, harnessing. What is meant by that? Two different factors. The first factor, you have a grounding electrode with the lightning current going over it. For example, this can be the grounding electrode of a "well-arranged" lightning rod, through which the lightning current runs into the ground, but there is a route of underground cable laid near. Let it be data cable from a microelectronic device or reliable power supply of that microelectronic device. You install the rod by observing distances between the grounding electrode and that cable according to the document AD-34 or to the departmental regulatory documents, for example, IS standard. In any way, this distance will be at least several meters. And what can we see from the experiments carried out properly? It is the following thing. Instead of 300 kV per meter which is considered to be breakdown distance in the ground, in fact, the ground is broken through by high lightning currents with the voltage of about 10 kV per meter. It is not a missay. The difference between the calculated value and the real one is about 30 times. This is a real experiment. Of course, in order to protect these cables, one shall take into account this fact.
Spatial orientation of electric circuits
- There is the second option. The situation is as follows. The lightning current runs, for example, over the down conductor or the lightning rod, it doesn't matter. You have an electrical circuit. You can arrange this electrical circuit along the lightning current path as shown in the figure on the left, or you can arrange it crosswise, as shown in figure on the right. Overvoltage will also be different in this case. Whereas, when the channel is generated crosswise, overvoltage level will differ from one with the parallel arrangement, again, by dozens of times, even hundred-time difference. And if you arrange protected electrical circuit relative to the current conductor in a proper way, you may decrease overvoltages to such a serious extent that no SPDs will be required at all.
Safe transportation of lightning current into the ground
High-rise building of a cylindrical shape with the radius of 25 m;
- Now, let's take a look at one more thing, which was also discussed at the recent webinar, what is safe lightning current transportation into the ground. Safe lightning current transportation into the ground is always splitting the current by high number of down conductors. This chart shows the change of magnet field strength when, for example, fifty down conductors are taken with the spacing of 1-2 meter instead of two down conductors. Where these down conductors will be taken from? The reason is simple. For example, the building includes metal columns which may function as down conductors. The building is equipped with glass units, which also may function as down conductors if properly installed, and by combining them you will get a solution. Overvoltages will be decreased sharply. Such a sharp decrease of overvoltages again means there is no need for SPDs installation.
Metal shields. Part 1
Disturbance forming current flows over the shield;
- Finally, one more thing. Metal shield also mentioned at the recent webinar. Metal shields: in any situation, the best metal shield is round metal pipe, round shape is important. Rectangular section conduit or any ellipsoidal structure drawn here. Unfortunately, they limit these voltages significantly. If you really want to make a shielded structure, then round cross-section will give an optimal effect. Optimal effect, even if the current is running over that structure. Imagine the situation, for example, when the current is running over the shield. For example, there is a lamp in an airport or railway reversing area. The lamp is hanging on the pole. The pole is used as the lightning rod; the lamp is powered by means of a shielded cable. If the shielded cable will be connected to the round metal pipe, then, not looking at that fact, almost the whole lightning current is running over it, overvoltages will be reduced sharply. If you take a rectangular cross-section conduit. The reduction won't be much less.
Metal shields. Part 2
Помехообразующий ток идет вне экрана – disturbance producing current goes outside the shield;
- Finally, shielding against external fields is also rather effective, if you use round cross-section metal shields, even made of steel. Copper and aluminum ones are far more effective. If all possibilities are exhausted and you see that the overvoltage is still dangerous for the electric circuit to be protected or for the equipment in it, this is the only case when SPDs using is reasonable and these need to be designed.
Useful materials for designers: