Overload settings for medium voltage (1 KV+)

[mbrooke]

Suggest you look into distance relaying because that was specifically invented for this exact scenario. Within a fairly short distance 50/51 relaying, let alone fuses, cannot detect faults on overhead lines effectively. Distance relaying relies on changes in impedance and can work at significantly longer distances.

At MV, fuses are almost exclusively used for short circuit protection only. They are considerably expensive to replace, often costing hundreds of dollars each. Almost all protective functions including short circuit (fuse saver) functions are implemented with relays. I would never even consider using fuses for overhead line protection in general. That's just not what it's meant for.

Right, but they do have their place in mobile, temp, or small specialty subs:

https://www.reddit.com/r/SubstationTechnician/comments/i84lp1

I figure, if you can, why not?

 

[mbrooke]

Impedance through Earth (a 2D conductor) is proportional to the inverse of the distance. This is pretty counter intuitive. See IEEE Green book. But it's the 1D impedance of the line that drastically and quickly makes detection and clearing line faults with fuses difficult except over short distances.

Most overhead line faults are "self repairing" but not "self clearing". A fault is usually initiated by vegetation, wildlife, or weather. The air is heated to the point where the critical flashover voltage drops well below the nominal line voltage so arcing becomes self sustaining. Once power is removed since there is really nothing there (or it has been destroyed) to continue a fault, unlike a downed line, the majority of the time power can be restored immediately with no repairs required. After the first attempt a few faults can be recovered even on the second or third attempt. Thus utilities frequently use reclosers for this purpose. This is a special circuit breaker that is intended to attempt to close into a fault. After the first couple attempts some utilities set up the reclosers in a "fuse blowing" mode...it purposely rides through a fault long enough to attempt to clear the fault via fuses located on the power line after the first couple cycles attempt to clear a fault without triggering fuses. After that point if the fault does not clear the recloser opens a final time and locks out until it is manually reset. Most people have "experienced" the effect of a recloser during storms. If you have ever seen power "blink" on and off, you have seen the effect of utility reclosers in operation.

I've drawn up recloser loop schemes, so I know a thing or two about them. Tree wire and spacer cable also helps with wildlife and weather related outages, though talking to guys here who work South and midwest tree wire is not something they approve of on the count of burn clear and not being able to clear when down.

Your right the about everything you say, however, even at a short distance it is possible to have a high impedance fault such that 50/51 doesn't "see it" or in this case a fuse.

 

[mbrooke]

Sure, many examples. Not saying this is the best way to do it but U.S. Pipe's Lynchburg, VA facility was maintained at one point by an ex-utility guy. In a plant expansion facing the difficulty of running a lot of distribution through an existing plant in a foundry environment, he elected to think a little outside the box. He installed what amounts to a "platform" arrangement for the transformers and then ran overhead lines to masts located in the areas they needed power (a total of 3 or 4 areas) fed from a new utility feed (also via an overhead line). It is a pretty clever design to deal with congestion issues. The plant itself is not a "utility" and so it clearly falls under NEC despite the fact that this is effectively a private (though short) utility.

Many conveyor systems often have extremely long distribution systems and at some point, running conduit over thousands of feet doesn't make sense so a lot of them have overhead lines.

And then we have the one that I have the most knowledge and personal experience with, Nutrien's phosphate mining operation in North Carolina. Incoming power is via two 230 kV lines fed from a Duke ring bus substation. Load is 90 MW. They have an on site 50+ MW cogen. There are over a dozen chemical plants, a port, an air strip, and a large mining operation. There is over 70 miles of 23 kV power lines. Some of it obviously falls outside NEC and in those areas such as the mine NEC or NESC are merely engineering standards but the chemical plant areas are clearly NEC, whether or not OSHA or others define things differently in terms of work rules. This one is particularly tricky because the facility spans so many different jurisdictions (FAA, maritime, MSHA, NEC, NESC).

Outside of these examples of extensive use of overhead conductors, medium voltage is very prevalent in many industries. Glass plants, steel mills, compressed gas plants, paper mills, wood chip mills, to name a few. Even a few of the larger sewage and water plants are medium voltage. Tire plants use it extensively in their mixing areas on what they call Banberry machines. Gas and oil pipelines use it on their pumps and compressors. I'd comment on refineries but there aren't a lot of those around here. Even Smithfields and Butterball use it extensively for operating their large chiller plants in their meat packing plants. Larger feed mills use it on their hammer mills. My current job is a field service engineer for a large regional motor shop. Most of the medium voltage plants are customers. I work with medium voltage equipment at least weekly. In addition to starters and motors such as these examples you see it even in plant distribution systems. It is pretty common to see larger plants with say a 12,470 service from the utility that is distributed throughout the plant and then transformed down to 480 before it hits distribution panels to service individual equipment. There are lots of advantages of this over one or two large transformers going to 480 and then distributing thousands of amps via bus duct.

Awesome info!

Right- or in the case of Chicago and NYC, you've got 13.8 or 27.6kv running up the building to electrical rooms with transformers in a networked configuration. You can loose two MV feeder cables and still have full power everywhere through out the building. Highly compact, economical and service continuity that exceeds many data centers.

 

[xptpcrewx]

Right, but they do have their place in mobile, temp, or small specialty subs:

https://www.reddit.com/r/SubstationTechnician/comments/i84lp1

I figure, if you can, why not?

Distance relaying is used for primary and/or backup protection for phase faults and/or ground faults on transmission, sub- transmission and distribution lines. Whether it is a mobile, temporary or specialty substation (whatever that is) doesn’t matter.

 

[mbrooke]

Distance relaying is used for primary and/or backup protection for phase faults and/or ground faults on transmission, sub- transmission and distribution lines. Whether it is a mobile, temporary or specialty substation (whatever that is) doesn’t matter.

Universal in transmission typically accompanied with POTT, DUTT, ect, common in subtransmission, sometimes fround on distro. (around me distro tends to be is GE IAC relays, heading over to SEL 351 primary SEL 751 back-up.)

However, nothing stops you from using fuses for something temp, in theory. I've seen it done.

 

[petersonra]

215.3 Overcurrent Protection. Feeders shall be protected
against overcurrent in accordance with the provisions of Part I
of Article 240. Where a feeder supplies continuous loads or any
combination of continuous and noncontinuous loads, the
rating of the overcurrent device shall not be less than the
noncontinuous load plus 125 percent of the continuous load.
Exception No. 1: Where the assembly, including the overcurrent devices
protecting the feeder(s), is listed for operation at 100 percent of its
rating, the ampere rating of the overcurrent device shall be permitted to
be not less than the sum of the continuous load plus the noncontinuous
load.
Exception No. 2: Overcurrent protection for feeders between 600 and
1000 volts shall comply with Parts I through VIII of Article 240. Feeders
over 1000 volts, nominal, shall comply with Part IX of Article 240.

So, I would suggest that OCP for feeders over 1000 V have to comply only with part IX of article 240. I came to this conclusion because it is what the code actually says.

This makes sense to me because the code is not a design manual. It says that too somewhere.

 

[xptpcrewx]

So, I would suggest that OCP for feeders over 1000 V have to comply only with part IX of article 240. I came to this conclusion because it is what the code actually says.

Where does it say “only with part IX of article 240”?

 

[wwhitney]

240.1 Scope

Cheers, Wayne

 

[xptpcrewx]

240.1 Scope

Cheers, Wayne

Thanks.

 

[mbrooke]

Does anyone have a full link to OSHA 1910.269? Is this it?

https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.269

 

[jdsmith]

So, I would suggest that OCP for feeders over 1000 V have to comply only with part IX of article 240. I came to this conclusion because it is what the code actually says.

This makes sense to me because the code is not a design manual. It says that too somewhere.

I agree - it is possible to design MV systems to not have overloads, so it makes sense that overload protection is not required.


Sent from my iPhone using Tapatalk

 

[paulengr]

I've drawn up recloser loop schemes, so I know a thing or two about them. Tree wire and spacer cable also helps with wildlife and weather related outages, though talking to guys here who work South and midwest tree wire is not something they approve of on the count of burn clear and not being able to clear when down.

Your right the about everything you say, however, even at a short distance it is possible to have a high impedance fault such that 50/51 doesn't "see it" or in this case a fuse.

Distance relays increase reach. It’s not a perfect solution. On transmission/sub transmission with very few “taps” around here Duke is using a lot of “pilot wire” (using fiber communications) 87 schemes which is obviously superior to any impedance or 50/51/fuse method but it’s not very practical in distribution…too many taps.

But yes a high impedance fault is essentially impossible to detect. Even the “third zone” distance relays used in power plants as a last resort don’t easily distinguish it. On the other hand if it’s not tripping overload protection even if a fault exists it’s not a threat as such. I’m sure you e seen plenty of vines growing on poles or wildlife hanging off things. Since I mostly work inside I see a lot of arcing and tracking damage that is a fault in the making but won’t trip anything until the final stage of a failure.