High Voltage Surge Arrestors

What do these look like? I've tried to figure out what all of those gizmo's were in the substations I've seen. Some are easy to figure out while others are not.

I'm thinking that for high voltage, your current will be fairly low. If the current increases too much, the voltage will drop and that may be one way to extinguish an arc in the air. I would assume that it would be hard on the step up transformer as well.

A "Jacob's Ladder" requires high voltage at a controlled current. You want to sustain that as long as it dosen't burn out the step up transformer for that. But with any surge that turns into an arc, the plasma will ionize the air(making it more conductive) and heat the air surrounding the arc thereby creating a bouyency effect eventually breaking the curent path through the air. But sometimes they pursist!

They look like the the three vertical post insulators on the extreme right of the picture, the units with the "hula hoop" restrike ring suspended from the top. Nice picture of the arc, but I doubt its authenticity unless it was part of a "staged fault" test.

That picture is just a good photo of the arcing being stretched and extinguished as a 500kV switch (see photo file name if you save it) opens. I have seen similar photos and even videos online. It is quite impressive, but it doesn't necessarily take a lot of current to make this happen. Once the arc is diffused enough, it uaually breaks down.

The "hula hoops" you mention are only corona rings on the ends of electrical conductors, to reduce the voltage gradient around "pointy" ends of equipment. Otherwise, at these high voltages the gradient becomes too severe and you get too much corona breakdown that leads to equipment damage.

Peter,

Here's what ABB has to say about them: "...The design of long arresters often requires external grading rings to maintain a uniform and acceptable voltage stress along their length..."

Not to dispute ABB but when I witnessed factory testing to destruction they usually flashed over via the hoops as the MOV blocks were consumed and the internal impedance became higher than the external air BDV. The goal was to have the power follow through current continue flowing at that point rather than have it find the next arrester, at least that's what my boss told me, and he set the BILs system wide.

Corona rings would be placed higher and around the downcomer connection point, such as you see on the post insulators at the extreme left, not hanging over the side well below that point. It's those low hanging hoops that make it easy to distinguish the arresters from other equipment.

Alan

Two entirely different operational philosophies are involved.

Station class surge arresters are there to both protect millions of dollars worth of equipment and to ensure that the overall system maintains its integrity while interrupting as few customers as possible.

To that end surge arresters are sacrificial elements, meaning that they have to continue operating until they can no longer perform their intended function. At that point they self-destruct just like a fuse and the remaining power follow through current is conducted through the "hula hoop" arcing ring to ground, and if it continues backup relaying will de-energize both ends of the line to extinguish the arc.

All of that protection is designed into the arrester and the surrounding system, like most power system components it is not a unit that operates in isolation, it is part of an overall scheme to keep the system up as long as possible without destroying it.

Regarding your question about fusing them, if you had a fuse in series and it blew to protect the arrester, where would the surge go next? To the next weakest link, etc., etc. And what would happen during a really severe lightning storm after the fuse blew on the first strike and multiple restrikes occur? And how would you know it even blew?

Very good answer. The other reason LV are fused is that they are usually located within the electric panels. If the arrestor destructed the panel could be destroyed and it is better to fuse them and loose some other part that could be replaced rather than the entire panel. I have witnessed a lot of occasions where substation arrestors operated and have seen many high numbers on the discharge counters, but have never seen failures on HV arrestors unless hit directly by the lightning. If the substation protection is correct that will not happen, It will hit the ground tower or whatever before it gets to the arrestor. Most arrestors respond to lightning coming in from the transmission lines.

Do not confuse MV distribution lines, (1kV-132kV and transmission lines, 132kV and above), with panels in an MCC or such like. On lines you may find a surge arrestors and directly below them a set of drop out fuse on the cross arm. These fuse do not protect surge arrestors, only equipment such as the Tfx or items connected after the drop out fuses.

A surge arrestor is made up, internally, of resistor blocks, top side connected to line, bottom side connected to earth.

The photo shown was a disconnecter being opened on load to produce an arc on a test. Normally the breaker would have tripped in under 2 milliseconds and the arc would not have been produced.

BIL change around the world, depending on the area, altitude, lightning strikes per year. The amps vary too as a standard, some countries allow 10KA as a strike amperage, others allow 20KA, so one would need to know the requirement for ones area. This may prove difficult to obtain a fuse to suit the application.

I have never installed a fuse on any transmission line distribution line or sub station to protect a surge arrestor. AC or DC systems.

They are entirely different devices, designed for entirely different functions. Fuses are for protection against overcurrent and surge arresters for overvoltage protection. Arresters are normally installed ahead of fuses to protect the fuse from overvoltage surges.

The surge arresters in LV panels are not meant to discharge lightning or such large current surges. Hence, providing fuses / MCBs could be possible.

Yet, I think even in LV panels, surge arresters should not be fused considering that these surge arresters are meant to be sacrificial.

Quote from Raghun: "Yet, I think even in LV panels, surge arresters should not be fused considering that these surge arresters are meant to be sacrificial."

That is not the point, the reason for fusing the surge supressors in switchgear is not to protect the surge surpressors, but rather to prevent the surge suppressor from failing and causing a release of ionized gas within the closed spaces, which would act like a short circuit between energized components. When they are not fused they are place in a separate compartment vented externally, with all conducting surfaces covered. This is done on MV switchgear but on LV switchgear it is more common to just fuse the supressor to protect the rest of the gear.

I understand when you say, the failure of surge arrester in LV panels could cause problems. This is true even with HV surge arresters and it is important to select right discharge capacity for surge arresters (kA current that the arrester can discharge without failure). Does this not apply to LV surge arresters!

I am not much familiar with LV surge arresters and appreciate if you could clarify.

LV arrestors are only designed for attenuated surges. Once the transmission line voltage is transformed to the lower distribution voltage it is usually protected physically from the possibility of a lightning surge getting into the arrestor. Most of the the voltage spikes that the LV arrestors are designed to protect for are switching surges on the LV and the attenuated spikes that the transformer will propagate. Hence the reason for calling them surge suppressors instead of lightning arrestors, since they should not see lightning surges. This means they can be much less capable of thru-put currents. If that current is exceeded it will blow the fuse which should be rated for a high enough voltage to contain the arc and protect the gear. The surge may in that case cause some damage to the switchgear, but will not destroy it, since the arc will be contained in the fuse. That's the theory anyway. Of course with all electrical equipment there will be variations in the design and intent and I'm sure others can quote variations they have seen. Please note that the definition for LV is usually 0-600 volts but can actually go as high as 1150 volts on some systems.

what you need is basics on HV arresters

http://electrical-engineering-portal.com/complete-overview-of-lightning-arresters-part-2

http://www.hubbellpowersystems.com/7A8EEDF1-F0F3-4A55-9328-93665C59BA18/FinalDownload/DownloadId-9220F3A0F6E34F0924EF7B8C83E151AE/7A8EEDF1-F0F3-4A55-9328-93665C59BA18/literature/arresters/EU1377-H.pdf

http://ees.etf.bg.ac.rs/7A8EEDF1-F0F3-4A55-9328-93665C59BA18/FinalDownload/DownloadId-15437F94B8AC049FADBF17765C74973C/7A8EEDF1-F0F3-4A55-9328-93665C59BA18/predmeti/31/ArresterBook_1400107%20Siemens.pdf

http://www05.abb.com/7A8EEDF1-F0F3-4A55-9328-93665C59BA18/FinalDownload/DownloadId-F573559C6B20A6C34C1BB31E8F871E2B/7A8EEDF1-F0F3-4A55-9328-93665C59BA18/global/scot/scot245.nsf/veritydisplay/70e9fd6933c8c644c12578d200333cb5/$file/952_abb_awr_mittelspannung_e_low.pdf

http://www.emersonnetworkpower.com/7A8EEDF1-F0F3-4A55-9328-93665C59BA18/FinalDownload/DownloadId-7F7F5DB100225EFDE11EE91ADB607105/7A8EEDF1-F0F3-4A55-9328-93665C59BA18/documentation/en-us/brands/surgeprotection/documents/sl-30119_surgereferenceguide_11-11_singlepgs.pdf