Utility transformer primary fuse opening time

[nhee2]

mayanees,

did you enter the utility impedance data and transformer data exactly as described in the original post?

I've recently gotten access to use EasyPower so I was using the example to compare results to your SKM TCC, and I am not getting the primary / secondary 3P fault values that your curve is showing (closed to 30K at the secondary). It could be I am doing something wrong.

 

[mayanees]

mayanees,

did you enter the utility impedance data and transformer data exactly as described in the original post?

I've recently gotten access to use EasyPower so I was using the example to compare results to your SKM TCC, and I am not getting the primary / secondary 3P fault values that your curve is showing (closed to 30K at the secondary). It could be I am doing something wrong.

... busted. No I did not!
I think I came up with your numbers in my first pass, when using Utility impedance and the minimum transformer impedance, then adjusted the transformer impedance to get close to the reported Utility contribution of 37k. So it's probably the technically correct way to do it with the info given, but could require further investigation if the situation warranted it.

 

[Bugman1400]

I only mentioned SCADA because that's what gets put up by most utilities involving new reclsoers. The SCADA part itself isn't important, however what mitigates the risk is the 3 phase switching and 3 phase protection provided by the recloser itself.


The recloser is responsible for protecting the transformer in addition to being a 3 phase disconnect. The micro processor controller can be configured with just about any time current curve including ones that mimic fuse links, like a 140T or 200K for example. In fact because the controller is so versatile curves can be chosen that will protect the transformer under any fault condition from sustained chronic over load to an asymmetrical bolted fault on the secondary without exceeding the transformer's damage curve. Incident and arc flash energy can also be reduced as well on the secondary.

Primary VTs allow the recloser to be programmed for loss of phase (an open phase) response during a broken conductor event (out on the line) which could put the transformer into ferroresonance. Sometimes its done by enabling the loop scheme mode in the controller and setting a trip lockout if phase voltages are imbalanced beyond 85% exceeding 10 seconds. Even if ferroresonance never takes place the facility will not be subjected to costly single phasing.


The cost may not be worth it for a small 30kva padmount, but its routinely done around here for large facilities like malls and factories working as intended.

I would definitely go with a delta primary for dozens of reasons involving power quality including an inability to pass zero sequence currents like mentioned and the ability to use any type of grounding configuration as desired on the secondary.

At 1,500 kva this is definitely an option worth considering.

What type of facility is this btw?

Perhaps we are referring to different animals. I coordinate reclosers all the time whether they are controlled by a SEL-651, 351 or whatever but, they are typically installed on the backbone of the distribution system or on a major 3PH tap that has frequent faults. I've never seen them used to protect a xfmr since, typically, you would not want to reclose for a xfmr fault.

I have never used a recloser to mitigate or eliminate a ferro issue. However, since I am only used to doing it the way my utility prefers it, perhaps you can point me to some literature that describes how reclosers are used to mitigate ferro issues. I learn something new every day.

 

[mbrooke]

Perhaps we are referring to different animals. I coordinate reclosers all the time whether they are controlled by a SEL-651, 351 or whatever but, they are typically installed on the backbone of the distribution system or on a major 3PH tap that has frequent faults. I've never seen them used to protect a xfmr since, typically, you would not want to reclose for a xfmr fault.

Who said anything about re-energizing a faulted transformer? :blink: All the controls you mention, be it a Cooper forum 6, SEL, ABB ect have the option of programing the pole-mounted controller to disable the reclose attempts to a single trip and lockout. And no Im not talking about the hotline tag feature.

A micro processor recloser is nothing more then an oil or vacuum circuit breaker controlled a programmable protective relay. They can be programed to do just about anything with just about any time current curve, even those resembling fuse links (curve 163 or 165 if I remember correctly resemble a link).

I have never used a recloser to mitigate or eliminate a ferro issue. However, since I am only used to doing it the way my utility prefers it, perhaps you can point me to some literature that describes how reclosers are used to mitigate ferro issues. I learn something new every day.

All literature says ferroresonance can occur with single phase switching and single phase protection which a recloser eliminates. Opening and closing is 3 phase and a trip is 3 phase, or in the case of 3 ganged singles the controller can be set to trip all 3 simultaneously.

 

[Bugman1400]

Who said anything about re-energizing a faulted transformer? :blink: All the controls you mention, be it a Cooper forum 6, SEL, ABB ect have the option of programing the pole-mounted controller to disable the reclose attempts to a single trip and lockout. And no Im not talking about the hotline tag feature.

A micro processor recloser is nothing more then an oil or vacuum circuit breaker controlled a programmable protective relay. They can be programed to do just about anything with just about any time current curve, even those resembling fuse links (curve 163 or 165 if I remember correctly resemble a link).











All literature says ferroresonance can occur with single phase switching and single phase protection which a recloser eliminates. Opening and closing is 3 phase and a trip is 3 phase, or in the case of 3 ganged singles the controller can be set to trip all 3 simultaneously.

Typically, a recloser is used to reclose. Yes, I know you can program it to have zero shots. If you meant vacuum or oil breaker, I would've expected you to say that instead of using the term recloser......I thought you were inferring that there was some correlation with reclosing and mitigating a ferro issue. Perhaps you were just inferring that its better for all 3 poles to open up for an SLG fault instead of just the faulted phase.

If all the literature says ferroresonance can occur with single phase switching and single phase protection then, can you point me to the one that best explains this.....I would like to learn more.

Also, you mention the term MGN in your previous post.....is that an acronym for something?

Thanks.

 

[GoldDigger]

The term MGN (Mult-Grounded or Multiple Grounded Neutral) refers to the system where single common neutral wire for the low voltage side of distribution transformers is grounded many times (typically every second pole) to a ground electrode at the base of the pole. It is used to provide a redundant low impedance path to earth potential without relying on that earth path to carry local fault current.
It has been argued to be more dangerous than an ungrounded or singly grounded (per transformer) neutral system).

 

[mbrooke]

Typically, a recloser is used to reclose. Yes, I know you can program it to have zero shots. If you meant vacuum or oil breaker, I would've expected you to say that instead of using the term recloser......I thought you were inferring that there was some correlation with reclosing and mitigating a ferro issue. Perhaps you were just inferring that its better for all 3 poles to open up for an SLG fault instead of just the faulted phase.

Yes, but the name for a vaccum or oil breaker mounted a top a pole is typically called a "recloser". At least that's what we call it around here. As for its function that's based on where its applied in the system and to what.

My point was, if the transformer fault for what ever reason, or needs to be shut off, tripping all 3 poles is the way to do it over pulling individual fuse links.

Also I am aware that most reclosers are applied mid line or in loop scheme having a continuous current ratting of 600 amps, but its not uncommon for them to be applied to 3 phase transformer primaries or co-generation having a full load current of under 200 amps.

If all the literature says ferroresonance can occur with single phase switching and single phase protection then, can you point me to the one that best explains this.....I would like to learn more.

http://www.hdelectriccompany.com/as...lained_Incident Prevention Article_100212.pdf

The best way to avoid suspect resonant circuits is to avoid opening three-phase transformers one phase at a time from potheads or lateral taps. Some utilities are using an AB switch to isolate the coil before switching potheads. Opening loads by elbows tends to isolate cable capacitance. Other corrective methods include shortening the primary run to change capacitance or replacing transformers with different inductive impedance values.

Most conditions from single phasing:

http://www.ece.mtu.edu/faculty/bamork/FR_WG/Panel/DuganPanel.pdf

https://www.youtube.com/watch?v=U7dNdQc1NL4

Also, you mention the term MGN in your previous post.....is that an acronym for something?

Thanks.

Multi grounded neutral

 

[Bugman1400]

Yes, but the name for a vaccum or oil breaker mounted a top a pole is typically called a "recloser". At least that's what we call it around here. As for its function that's based on where its applied in the system and to what.

My point was, if the transformer fault for what ever reason, or needs to be shut off, tripping all 3 poles is the way to do it over pulling individual fuse links.

Also I am aware that most reclosers are applied mid line or in loop scheme having a continuous current ratting of 600 amps, but its not uncommon for them to be applied to 3 phase transformer primaries or co-generation having a full load current of under 200 amps.




http://www.hdelectriccompany.com/as...lained_Incident Prevention Article_100212.pdf







Most conditions from single phasing:

http://www.ece.mtu.edu/faculty/bamork/FR_WG/Panel/DuganPanel.pdf

https://www.youtube.com/watch?v=U7dNdQc1NL4








Multi grounded neutral

So, does your utility install 3PH reclosers on every 3PH bank no matter the size? Seems like that would be cost prohibitive. I could understand perhaps installing a 3PH recloser on circuits with a known ferro issue (like after several bank failures). However, it seems like there are cheaper ways to de-tune the circuit.

 

[mbrooke]

So, does your utility install 3PH reclosers on every 3PH bank no matter the size? Seems like that would be cost prohibitive. I could understand perhaps installing a 3PH recloser on circuits with a known ferro issue (like after several bank failures). However, it seems like there are cheaper ways to de-tune the circuit.

There is no need to. Small banks (under 750kva) are connected to the 12.47 or 13.8kv distribution system which usually has an MGN allowing wye-wye transformers; or in small cases where delta-wye is used the risk is considered minimal at that voltage.

However, large facilities like malls and factories require transformer banks over 1000kva, feeding such intensive loads from 12kv distribution circuits is not practical for a variety of reasons. 23, 27 and 34.5kv sub-transmission circuits are used. These circuits do not have an MGN, therefore any load must be connected phase to phase. Delta-wye transformers are always used. Both the size of the customer (kwh), and the amount of equipment that might be damaged (CNC machines) make a 3 phase recloser cost justifiable.

Cost analysis is was done on cheaper methods like 3 phase air break switches (which are used on some older facilities rather than recloser for line men switching), and running an MGN on sub-transmission circuits but that idea was rejected due to cost and vector voltage shift associated with wye connected voltage regulators on long lines.


Also, this might be off scope, but some large critical facilities are forced to use reclosers anyways in auto source transfer applications, so having reclosers mitigate ferroresonance is a welcome compliment.

 

[JRW 70]

Higher voltages and CB's

Higher voltages and CB's

I can't think of any of our substations that have
a 69kV line coming in, that doesn't have a CB that
opens all phases on both ends of the line (source and
before the transformer at the sub) any taps along
these lines where transformers are used are also
covered by CB not necessarily to protect the xfmr.
but to keep the line on. The fusible disconnects are
often used in the same way, not to protect the
transformer, but to clear on reclose attempts to keep
the majority of the line in service. Most transformers
<500kVA are considered replaceable items. Plant and
Substation transformers are protected within or under
their full rating (since some of these are delivered by
rail and are a very difficult changeout) Some plant
transformers are 75 years old and still in service.
So there is a double standard on engineering practice
dictated out of the finance dept.

JR

 

[Bugman1400]

I can't think of any of our substations that have
a 69kV line coming in, that doesn't have a CB that
opens all phases on both ends of the line (source and
before the transformer at the sub) any taps along
these lines where transformers are used are also
covered by CB not necessarily to protect the xfmr.
but to keep the line on. The fusible disconnects are
often used in the same way, not to protect the
transformer, but to clear on reclose attempts to keep
the majority of the line in service. Most transformers
<500kVA are considered replaceable items. Plant and
Substation transformers are protected within or under
their full rating (since some of these are delivered by
rail and are a very difficult changeout) Some plant
transformers are 75 years old and still in service.
So there is a double standard on engineering practice
dictated out of the finance dept.

JR

I think the previous discussions were referring to distribution-level voltages, not sub-transmission. Not sure why a blown fuse on a highside phase at one tapped xfmr would prevent other tapped xfmrs on the same 69kV TL from receiving 3PH voltages. For smaller xfmrs on a 69kV system (<10MVA), I typically see fuses on the highside that generally do protect the xfmr.