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Star - Delta Transformer Protection

05/29/2011 6:52 PM

Hi guys,

If I have the following configuration:

- DELTA-STAR transformer 690/415V
- Fuses installed (no circuit breaker or RCD) upstream of the transformer on each phase.
- Transformer secondary is solidly earthed

If I have an earth fault on say phase C to ground on the secondary side (external to transformer) will I take out all 3 fuses on the primary or only the fuse installed on phase C upstream? (presuming they were correctly rated etc.).

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#1

Re: Star - Delta Transformer Protection

05/29/2011 10:09 PM

Owing to phase shift between primary and secondary (i.e., vector group), there might not be a clear correlation between primary phases ABC and secondary phases A'B'C'. (I'm not sure of this--more like wondering about it.)

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#2

Re: Star - Delta Transformer Protection

05/29/2011 11:00 PM

If the fuses were absolutely equal you would take out two fuses. In practice one will blow first (just don't ask me which ones).

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#3
In reply to #2

Re: Star - Delta Transformer Protection

05/30/2011 5:20 AM

It means we need to take out three fuses including the blown one.

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#4
In reply to #3

Re: Star - Delta Transformer Protection

05/30/2011 5:35 AM

I used a bad analogy, instead of "take out" I should have said blow. If any fuse blows in a 3 phase circuit it's good practice to change all 3 fuses.

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#5

Re: Star - Delta Transformer Protection

05/30/2011 7:57 AM

If all the fuses are of correct rating, not required to change all the fuses. Change only which was blown-off.

Vinu_Answers Sure _Answers.

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#8
In reply to #5

Re: Star - Delta Transformer Protection

05/30/2011 8:51 AM

If any fuse blows in a 3 phase circuit it's good practice to change all 3 fuses. One fuse blowing will place stress on the other two, weakening them. Over time the two fuses are liable to fail. So if one of the remaining two fails you change that, but the last one to be changed has been stressed. The original fuse has now been stressed twice so that will be the next to go. You end up chasing your tail!

It's a false economy!

Practical answers usually out weigh theoretical ones.

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#20
In reply to #8

Re: Star - Delta Transformer Protection

05/31/2011 10:38 AM

I think your answer is more than good enough for a GA.

Many here obviously do not understand that the fuse blows because of always stressing two out of the three fuses with overcurrent, even though only one of them usually blows.

Which other one was stressed as well is a difficult question, as it changes 50 or 60 times a second..... so replacing all three is highly recommended.....

May I remind the other people here that some of you seem to be the type of person that "saves money no matter what it costs!" (Old German saying!!)

In this case, it is fully possible that the two unblown fuses are fully OK, or at least one will be, but without taking it/them apart, how will you tell?

Hint, look at the way an HRC fuse is made before replying!!! Then you may understand the quandary better....

Tony S is dead right!!!

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#6

Re: Star - Delta Transformer Protection

05/30/2011 8:24 AM

Thanks for the replies guys, anyone care to back up the above statement about taking out 2 fuses? Can anyone explain why only two fuses go?

My original thoughts lead me to 2 fuses blowing and I have a very high level reason for it, but I'd like to see what others think first before I reveal mine, I'm probably wrong.. :)

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#13
In reply to #6

Re: Star - Delta Transformer Protection

05/30/2011 10:24 PM

your question - - - "Can anyone explain why only two fuses go?"

. . . since you have a Delta Connected Primary, if two Lines are open then there will be no return path for the current in the third Line . . . . if there is a better answer then I shall be glad to learn . . . .

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#7

Re: Star - Delta Transformer Protection

05/30/2011 8:24 AM

Incidental question: did you really mean 690/415 volt, or did you mean 6.9kv/415 volt?

Fly in the ointment: Ground faults vary in magnitude. You could have a very low magnitude ground fault that would not blow any fuses on the primary. (This used to be a problem with mining trailing cables--you could get a (ground) fault that arc'ed through the trailing cable insulation at a fairly low magnitude, and burn the cable from the point of the initial arc to the breaker feeding that cable.)

I'm trying to remember how that problem was solved. Nowadays it would be solved by a ground fault detector (e.g., GFCI (or, I guess the more modern (or European?) term is RED?). I don't remember if (or when) we started using something like that in mining equipment. (I'm getting old.) (Aside: I know we started installing "ground check" (i.e., pilot wire) systems to make sure the ground wire in a cable maintained a connection to the piece of mining equipment powered from it.)

(After more thought, I guess the ground fault detection equipment existed before I started working in mining (ca. 1972)--I was involved in retrofitting ground check equipment, but not ground fault detection equipment.)

I wasn't really a power engineer, but when we talked about faults, we often talked about a theoretical "bolted" fault--maybe that's what you want to ask about. That would relate to the fault current that would flow if you had a solid connection (i.e., bolted together) across all three phases, between two phases, or from one phase to ground. Then you can calculate how much current would flow based on the impedance in various parts of the circuit (the transformer, cables, ...).

I guess I can imagine circumstances, especially for higher ratio transformers (like 6.9kv/415 instead of 690/415), where the fuses on the primary don't blow (or don't blow quick enough) in the event of a ground fault, especially if it is not a bolted phase to ground fault.

And, in that case, it is possible that the transformer is damaged (or destroyed) before a fuse blows. Especially in a case like this (i.e., a delta-wye or wye-delta transformer) where a ground fault on one phase on the secondary of the transormer is split between two phases on the primary of the transformer.

(That is less likely on a 690/415 volt transformer, but I'm not aware of circumstances where you'd use a 690/415 transformer, that's why I asked if it was really a 6.9kv/415 volt transformer. (And also why I think this is a homework / school problem rather than a real situation.))

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#9
In reply to #7

Re: Star - Delta Transformer Protection

05/30/2011 12:36 PM

British mining transformers used a star point earth choke to limit earth fault current to just 5A, it would then pass through high sensitivity earth detection (250mA) which would trip the HV incoming. Phase / phase faults would be detected on the LV side but trip the HV. FLP OCB's or ACB's would be used on the HV, never fuses. No switchgear was on the LV side, only detection.

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#10
In reply to #9

Re: Star - Delta Transformer Protection

05/30/2011 1:24 PM

Interesting.

That helps spark some of my memory (but it's still pretty vague). We did have "switchgear" (well, molded case circuit breakers) on the secondary side, with undervoltage coils that tripped the breaker on undervoltage but were also used to trip the breakers on other conditions (like loss of the pilot wire, and probably ground fault--that's part of what I can't remember very well).

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#12
In reply to #9

Re: Star - Delta Transformer Protection

05/30/2011 6:42 PM

Due to the location of the transformer, lack of realestate and logistical issues, we cannot put an enclosure to house protection equipment on the secondary side.

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#22
In reply to #9

Re: Star - Delta Transformer Protection

05/31/2011 10:41 AM

Interesting, I can understand the logic and the thinking easily for the choke, someone was using his grey cells on that day!!!

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#11
In reply to #7

Re: Star - Delta Transformer Protection

05/30/2011 6:40 PM

it is 690V to 415V, there was a design error on site (long story). You will take out the 6A fuses upstream on the 690V primary side with a ground fault no worries.

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#14

Re: Star - Delta Transformer Protection

05/30/2011 11:17 PM

The correct answer is,

if a single line to ground fault occurs on grounded star, only the phase C, on which the fault has occurred will trip.

There will be no fault current on line side of delta side.

The other two phases on star side will remain healthy.

If there is a neutral tripping device on star side, it will also trip and will have same current as the faulted phase current.

The above inferences can be derived from fundamental calculations ignoring pre-fault load current.

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#15
In reply to #14

Re: Star - Delta Transformer Protection

05/31/2011 12:04 AM

Re: if a single line to ground fault occurs on grounded star, only the phase C, on which the fault has occurred will trip.

The OP tells us that the fault occurs on the secondary side of the transformer, which is delta connected.

The OP also tells us that there are no fuses (or breakers) on the secondary side of the transformer, only on the primary side (upstream of the transformer).

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#18
In reply to #15

Re: Star - Delta Transformer Protection

05/31/2011 4:36 AM

If fault is on ungrounded delta side, which has no grounded source, there will not be any fault current.

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#25
In reply to #18

Re: Star - Delta Transformer Protection

05/31/2011 11:17 AM

Oops, sorry, I got confused by contradictions in the OP's post: title says "Star - Delta", body says "DELTA-STAR transformer 690/415V". So, now I have to revisit what I said in my post #15.

I guess I believe that it is DELTA-STAR, but I'm not sure the OP has confirmed that.

You may be right, but I'm too confused about it to think straight atm--I think I'll find it easer to revist post #15 than to comment here.

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#23
In reply to #15

Re: Delta - Star (correction) Transformer Protection

05/31/2011 10:50 AM

Usually the primary is Delta and the secondary is Star, otherwise you would not have a neutral available at the secondary side.....so it is true that a secondary short to ground will usually blow a single fuse on the secondary side only...if fitted!! Due to the grounded neutral point (on any correctly installed system).

So this blog probably should have been named Delta - Star transformer, which I have done for this post only, for the sake of clarity!!

PS There are probably step up transformers that are primary star and secondary Delta, only I have never seen one myself.....so don't go jumping me for no reason!!!

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#26
In reply to #15

Re: Star - Delta Transformer Protection

05/31/2011 11:33 AM

I need to retract my comments in the post above (post #15)--there is a conflict in the original post, the title says Star - Delta, but the body of the post is Delta - Star. My comments were based on it being a Star - Delta, but I suspect that is wrong, so I should revise them based on a Delta - Star. So, re my post #15:

Re: Re: if a single line to ground fault occurs on grounded star, only the phase C, on which the fault has occurred will trip.

This is the statement (by raghunath7), which I had issues with.

Re: The OP tells us that the fault occurs on the secondary side of the transformer, which is delta connected.

Partially correct, partially wrong, I should have said:

The OP tells us that the fault occurs on the secondary side of the transformer, which is star connected.

Re: The OP also tells us that there are no fuses (or breakers) on the secondary side of the transformer, only on the primary side (upstream of the transformer).

Still correct.

Summarizing: I still have issues with the statement that "only the phase C, on which the fault has occurred will trip", because there are no fuses on the secondary (Y) side of the transformer. The ground fault current has to be "reflected back" to the primary side of the transformer and we have to consider the effect on the fuses there.

Someone else (rbnwatson) described that quite well in their post.

I still believe that there can be low current ground faults on the Y secondary that will not reflect enough current back to the primary side to blow any fuses there.

Sorry for the confusion I created by misunderstanding the OP.

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#27
In reply to #26

Re: Star - Delta Transformer Protection

05/31/2011 12:16 PM

I wonder why there is so much confusion about this topic. My statement that only one fuse on secondary side star connection will trip is correct, for a single earth fault on single phase on secondary star side [provided of-course there is a fuse/breaker on this phase, else no tripping can occur and the fault will be permanent].

On primary side delta no fuse should trip, as the zero sequence current due to secondary side earth fault, will circulate in the delta winding and will not appear on the line side of delta winding and therefore will not affect fuses on lines on delta winding side.

The above statements are correct answers for Delta primary - Star Secondary winding configuration.

If the Star is primary and Delta is secondary and is ungrounded without any source, there will not be any earth fault current on the secondary delta side.

These can be proved by simple sequence diagram connection.

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#31
In reply to #14

Re: Star - Delta Transformer Protection

06/01/2011 8:09 AM

bUT,

i HAVE COME ACROSS EVENTS, WHERE 2 FUSES NORMALLY BLOW, WITH A SINGLE GROUND FAULT. SOME TIMES, LESSER TIME, ONLY ONE FUSE BLOW!

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#16

Re: Star - Delta Transformer Protection

05/31/2011 12:29 AM

415 x √3 ≈ 718 volts. Thus I don't yet fully understand this transformer. As I recall, in the US NEC, both the primary and secondary should have fuses or other overcurrent devices.

In the event of a secondary fault, which could blow one or two secondary fuses, I think we still don't know which primary phase(s) might also have overcurrent. It sounds to me like a vector group question.

[If only I had a transformer, an oscilloscope, and some fuses/CBs to try a few experiments...]

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#17

Re: Star - Delta Transformer Protection

05/31/2011 4:00 AM

If you have an earth fault on the transformer with no load, you will get a phase/phase overcurrent on the delta primary. so 2 fuses will experience the overcurrent, but only one might blow. The other may blow but will be degraded. If one fuse blows, you will have one energised phase winding supplying load, and the other two phase windings connected in series across the same 2 phases. one will be supplying no load because the fuse is blown, the other will be supplying full phase volts into its conductor in antiphase (180degrees) to other. If your load is 3 phase machinery there may be a abnormal current which will blow a second fuse, or cause the machinery protection to operate. If your load is all single phase load, 2 out of three phases will be supplying load , but one of them will have experienced a 60 degree phase shift.

So my answer is you will only blow one or possibly 2 fuses.

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#19
In reply to #17

Re: Star - Delta Transformer Protection

05/31/2011 5:00 AM

Sorry,

My explanation re the situation after one fuse has blown was incorrect. The blown fuse will cause all three phases to be energised, one phase correctly at 415v but the other two supplying at reduced voltage. If their load impedances are same they will have equal voltages, the higher impedance will experience the higher voltage, the the sum of both will be 415v.

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#21

Re: Star - Delta Transformer Protection

05/31/2011 10:40 AM

Before you all start to fall out read the OP

- DELTA-STAR transformer 690/415V
- Fuses installed (no circuit breaker or RCD) upstream of the transformer on each phase.
- Transformer secondary is solidly earthed

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#24
In reply to #21

Re: Star - Delta Transformer Protection

05/31/2011 11:16 AM

Ye gods, the dogs are loose and running wild!

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#28
In reply to #24

Re: Star - Delta Transformer Protection

05/31/2011 2:28 PM

Some of the confusion relates to not knowing the full info.

The original question related to the fuses upstream of the transformer. If fusing has been correctly applied, there should be discrimination between the primary and secondary fusing. That in itself can be complex because some agencies employ dual characteristic fuses on transformer primaries, providing good prolonged overload protection but quite fast clearance for fault current magnitudes.

The following comments relate to fault condition beyond the 3off secondary fuses:-

A single phase fault on the feeder beyond the secondary fuses will probably only blow the fuse on the faulted phase. There is a possibility (eg if the load is a heavily loaded motor) that the unbalanced voltage condition that will exist during the fault clearance, and after the fuse has blown, will produce overcurrents in the remaining energised phases, which will cause a further fuse to blow. If the load is delta connected the remaining fuse will not blow, however if there is an earthed or neutral connection (unlikely) the third fuse may blow, as the machine will invariably stall.

The following comments relate to a fault between the transformer secondary winding and the secondary fuses.

(1) the secondary fuse on that phase may blow. If the primary fuses are rated quite highly, there will be a major voltage unbalance applied to the machine. This will produce significant negative sequence torques and may reduce the positive torque available. The mechanical load is still engaged so all phase currents will increase. But current will flow to the fault position from the faulted phase connection and back through the secondary fuse by inductive coupling within the machine. That current may be sufficient to blow that fuse.

(2) in relation to a delta/star transformer with a 1/1 ratio, with the star point earthed, a 1 - 0 - 0 current condition on the secondary, will produce a 1 -1- 0 current condition on the primary, which will equally stress 2 fuses, but the weaker one will blow first. A 690/415 tranformer brings in some transformation ratios (690/(415 times SQRT 3) to consider. There will also be a minor difference between a DY1 or a DY11 transformer but one fuse is going to blow first, and the other one will degrade. If the conditions described in (1) take some time to clear and what is described in (2) is now proceeding without the ability to generate any positive torque, the machine will stall, but my guess is that the secondary fuses will operate to clear the condition.

These thoughts are me walking through the fault condition step by step, but I could be wrong. Would appreciate hearing what others think.

Rbnwatson

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#29

Re: Star - Delta Transformer Protection

05/31/2011 9:17 PM

It depends on the internal wiring (or vector group) of the transformer. Each secondary winding of the transformer has a corresponding primary winding (or vice-versa). In this case, there is a corresponding winding in the Delta side for the Y side faulty winding (the one between the line C and neutral point). The lines which are connected to the corresponding delta side winding will be affected with the fault current introduced in Y side.

Regarding fuse blown, it may be two fuses or one fuse. If the fuses are of identical characteristics, two fuses will be blown due to same fault current flows through both lines and it is the most probable case. However, in the case of difference of the characteristics of the fuses, only one (the faster one) may blow.

In terms of sequence components, the fault current in the Y side are all zero sequence current but the fault current in the Delta side consists of positive, negative and zero sequence component of the fault current. The zero sequence component of the Delta side circulates within Delta winding (which I believe someone indicated in a comment) but the positive and negative sequence current flows through the two lines in the delta side. This diagram however doesn't show the zero sequence component of the fault current flowing through the Delta windings.

Other useful link for further analysis of the fault current:

http://www.gedigitalenergy.com/smartgrid/Mar07/article5.pdf

http://www.fecime.org/referencias/npag/chap5-46-77.pdf

http://www.hvpower.co.nz/TechnicalLibrary/Siprotec/Day_1_Part_1_Symmetrical_Components.pdf

Hope it helps.

- MS

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#30
In reply to #29

Re: Star - Delta Transformer Protection

05/31/2011 10:55 PM

The links provided appears to be related only to differential protection, which is meant for protection against winding faults. However, we are discussing earth fault external to the winding on star side of a Delta-Star winding. This situation is different from differential protection.

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#32
In reply to #29

Re: Star - Delta Transformer Protection

06/01/2011 8:40 AM

GA to you MS. Your references are helpful too. I agree, a ground fault in the wye secondary winding will cause current to flow in the associated winding of the delta primary, which will show up in the two lines associated with that winding. See the diagram on page 2 of the "gedigitalenergy.com" link for a good example. Although the references may focus on differential protection, the faults for which it protects include ground faults.

As for the magnitude of the current in the primary due to a ground fault on the secondary, see pages 18-19 of the following link:

http://www.sandc.com/edocs_pdfs/EDOC_025854.pdf

For a 1:1 ratio transformer, if 1.0I flows in a single secondary winding, 1.0I will flow in the associated primary winding, and 0.58I will flow in each of the two connected lines. If the fault on the secondary is high impedance, of course the fault current will be lower than maximum possible, so the primary will also be decreased and the fuses may not blow.

Fuses are just that - a fusible element which heats and eventually melts due to current above its rating. Fuse characteristics include a "minimum melt" curve and a "total clearing time" curve, and the fuse actually blows somewhere within that area bounded by the curves. However, when you approach minimum melt, it can affect the element's metal, so that it will not react to a future overcurrent event the same as a new element. That is why one should change all the fuses if one blows. Otherwise a lower than fault level current (such as a transformer inrush on energization, or a motor start) could cause the fuse to blow later, when it really shouldn't.

I don't know where the OP is located, but per the NEC (2005 edition in hand), Article 240 requires that transformer secondary conductors be protected by an overcurrent device except in very specific circustances that don't match the OP's description. Therefore he really should have at least fuses on the secondary of the transformer. The article actually states that the secondary conductors shall not be considered protected by the primary overcurrent protective device.

And a note on the transformer secondary voltage: My guess is that it is 415 volts line-line (= 415/√3 = 240V line-neutral).

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#33
In reply to #32

Re: Star - Delta Transformer Protection

06/01/2011 9:04 AM

I like your post, makes sense to me, therefore GA.

I would add that a high current in a secondary circuit cannot be relied on to generate a particularly high (just a "higher") current in the primary, the design of the transformer plays a part, saturation of the iron etc etc., which supports your post completely....The secondary also needs fusing or some sort of protection.....

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#34
In reply to #32

Re: Star - Delta Transformer Protection

06/01/2011 9:17 AM

What GE diagram is showing is an internal winding ground fault.

The figure shows two line currents on delta side which represents pre-fault load current and not fault current. This means primary Delta has only normal load current and the fault current on secondary "If" is not reflecting to the primary line side of Delta Winding. At least this is how , I have understood the same, in the absence of any explanations in the text.

When an external ground fault occurs on the line side of star winding [not a winding fault], We can compute line currents from the sequence diagrams both on Star and on Delta Side. May be we should do this to understand the flow of fault currents.

Perhaps, I should cross check by hand calculations before participating further in the discussions.

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#35
In reply to #34

Re: Star - Delta Transformer Protection

06/01/2011 10:40 AM

I did some quick hand calculations using sequence diagrams, ignoring vector group for simplicity.

This appeared to show the fault current in the same phase on delta winding side , as the faulted phase on Star grounded winding side.

The other two phase appeared to be healthy. Perhaps, others may do the calculations for clarity.

May be vector group consideration will result in a different solution.

Of course the source on the Delta winding side was considered as solidly grounded in my calculations. Else, there will not be any earth fault current on either side.

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#36
In reply to #32

Re: Star - Delta Transformer Protection

06/02/2011 12:00 AM

Thanks for the discussions.

I spent some more time calculating with and without phase shift. Delta side will have fault currents, in such a way their vector sum will match with the single phase fault current on star grounded side.

It appears my earlier opinion in this discussion forum was incorrect. I apologize for the same.

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#37
In reply to #36

Re: Star - Delta Transformer Protection

06/02/2011 7:30 PM

With having been involved with CR4 since February 2009, I still see this forum is an excellent learning platform. I am proud of seeing so many talented and experienced persons contributing their thoughts here. This is a discussion forum and there is nothing to apologize if some thought goes to other direction.

CR4 needs the involvement of the people like you. So, please keep your valuable contribution to this forum.

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#38

Re: Star - Delta Transformer Protection

06/06/2011 1:04 PM

Questions:

1_Is your transformer correctly earthed (i mean secondary star-earthed and transformer box-envelope- earthed)?

2_Are your delta connections on primary side, and is this side of 690 V ?

Observations:

A_If your secondary C 415 V phase is accidentally earthed, there will be a loop between your neutral and your secondary faulty phase. No incident will occurs onto the primary side ( But do not touch the transformer box!)

B_Nothing will occur on primary side, because they have not the same earth (on the two sides of the transformer - Primary side earthing is away from the transformer) C primary phase will see the failure after a long time ( 6 to 7 seconds)

C_As you have not a standard transformer protective device (Fuses are not a protection, they are only an instantaneous overheating - or let's say, an 'overcurrent protection'), it will be good to switch off the three fuses, to isolate the transformer, before to eliminate the earthing on the faulty phase. Because you will have a return from neutral, from earthed faulty phase and the two other phases will compensate the one which is missing ( so an overheating on the healthy phases and a blow out of their fuses).

D_You should have, at least, a fuse-protection on LV side (with a commoned trip action, in order to trip directly in 3-phase), in order to protect the transformer from downstream failures.

E_You should have, at least, an earthing failure relay on the star grounded neutral. This is not costly and is very efficient (more than fuses), in order to leave LV fuses to trip on dowstream failures only (your primary fuses do not represent a correct transformer protection).

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#39

Re: Star - Delta Transformer Protection

06/07/2011 9:46 PM

In theory, two fuses will clear (clear is more politically correct term then blow)

Protective relaying handbook, Blackburn 1987 page 322.

The star side get a 1.0 per unit fault to earth, neutral ground on phase a.

The delta phase lines A and C get .577 per unit fault current.

Reality, A fuse clears in microseconds ahead of fuse C, fuse C tests OK with ohmmeter.

Fuse C is made up of several fuse wires surrounded by glass beads, some burned open in the microseconds.

An x-ray test by the fuse manufacturer can be done for free if it's Ferraz Shawmut (spelling).

Dennis S, P.E.

Hope Creek Generating Station

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