instanneous settings and short circuit levels 1

[victa1288]

Hi,

I have a tricky question. When we do the realy coordiantion, we always expect the protective device can operate at less than the minimum short circuit, like L-L faul.

In my case, I am using an extremely inverse curve to protect against Minimum fault to max. fault, when the fault increases to a higher level, I want the instaneous element to operate.But this setting should be less than the available maxmum fault current and higher than the max. fault at the downstream.

The problem is, the fault level at the downstream is slightly less than the fault level. It looks like, the only thing I can do to achieve the coordiantion is to delay the ins. tripping of upstream device. Is it Ok when the fault level is close to max. fault level?

I checked the cable damage curve, the cable can be protected at the max.fault level.

I can't set the Ins. too low, because there would be a miscordiantion with downstream fuse.


Thank you for your valuable inputs.

 

[nightfox1925]

Are you indicating a source connected to a step-down transformer and further connected to a downstream switchgear and you have both primary and secondary 50/51 relays? Is the transformer solidly grounded? What are the voltage levels and what are your CT ratios? Can you provide us a better picture?

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[victa1288]

It is a step down transformer 138kV/4160V) and further connected to a downstream switchgear and we have both primary and secondary 50/51 relays. SEL 587 provides overcurrent protection for low side fault

The transformer is high resistance grounded. Sec. CT ratio is 1500/5.

 

[nightfox1925]

The maximum fault level at the 4.16kV switchgear is slightly less (when converted in terms of primary current) than the maximum primary fault current, is this what you mean? What is your primary phasec CT ratio? What are the maximum and minimum fault values you got in the primary and secondary?

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[RajT]

You can delay the Ins of the upstream CB as long as the delay meets following criteria.
The upstream swgr/CB are designed to withstand the max sc current for a time ( usually 1-3 secs) in excess of your desired delay.
The feeder cable is also able to withstand the sc current for the desired delay which as you say it does.

 

[victa1288]

01201972,you misunderstood my question.I only talked about the sec. of transformer the cable to 4.16Kv swgr is short and so is the cable to downstream load. That's why the s-c levels at the 4.16kV level are not much different.

RajT, thank you for your response. Relay co-ordination is really an art. We always expect the trip as early as possible, even before the minimum short circuit occurs. But in a lot of cases, we have to allow upstream short circuit to incraese to be close to max. s-c level and sustain for a while in order to achieve a better co-ordiantion. Because we don't want to trip the incomers so early.

 

[wareagle]

If the distance is that short you probably can not coordinate the OC devices. Why is it necessary if the locations are so close?

 

[DiscoP]

Where I work, we try to avoid inst elements when possible to avoid the co-ordination problems you are referring to.

We only use an inst element if it is necessary to keep below the damage curve of the equipment we are protecting.

You must protect the equipment first, co-ordinate second.

 

[victa1288]

Actually, to be accurate, we are using delayed ins. settings to coordiante with downstream fuses. Delayed ins.
trip is the back up of fuse protection for downsteam fault and the primary protection of swgr. The main point is, we don't want the main incomers to trip so early so long as the swgr and cable can withstand until the main CB trips. That's why I want to delay the ins, setings to the permitanble limt which is less than the available max. fault level.

 

[nightfox1925]

Most of the substation we have dealt in the past consist of double ended switchgears supplying MCCs which are very close to each other. Further, these switchgears are fed from identical step down power transformers. It is natural that the fault levels will almost be identical on the switchgears and MCCs if installed close to each other dur to very small impedance between them.

I recommend that you deactivate the instantaneous settings of your back up protection. The incomer CBs, Bus Tie CBs and outgoing feeders of your 4.16kV switchgears should have their instantaneous settings deactivated to be able to ccordinate well with the downstream fuse protection wherein the fault levels do not vary that much to ensure proper coordination. Fuse curves should be plotted and the proper IDMT curve of the associated upstream 4.16kV outgoing feeder CBs should be selected and set accordingly. Then O/C relays of these outgoing CB will be coordinated with both bus tie (if applicable) and incomer CB. The only part where instantaneous setting would be set are:

1) At the primary O/C relay (138kV) wherein the
instantaneous setting will be higher than the maximum
4.16kV fault as seen at the 138kV level.

2) At the downstream feeders say the motor feeders or other
feeders protected by fuses.

The selected IDMT should be set to ensure coordination with the downstream fuses and fast clearance at maximum fault. Normally, we consider 0.35~0.50 sec time grading between relays and 0.25~0.35 between fuses and upstream O/C relays. (This is what we normally do).

Now, if you wish to set the instantaneous setting, it should be between the 4.16kV outgoing feeder and the fuse only. I do not recommend this but if you can't get the proper time interval using the IDMT, then set the instantaneous in such a way that it would not intersect the Fuse curve to maintain selectivity. Bear in mind that faults may vary depending on how the system is loaded. Ensure that the zones of protection (selectivity) is maintained at the same time make the call of coordination compromise where you find that the equipment protection is jeopardize. Make the fault clearance time to minimum as much as possible so as not to get stability problems (especially where on-line generators and synchronous motors are running).




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[victa1288]

02101972,Thank you for your point. I might have used the wrong wordings. When I said Ins. settings, I meant the definite settings, which I set at less than the max fault levels. Minimum fault can be tripped by extremly inverse curve.So any faults between Min and Max can be cleared by a short time delay. I am not using any inst(without delay) to protect the main incomers. That's the best I can do so far. What do you think?

 

[nightfox1925]

Yes you may use the definite time as long as you maintain SELECTIVITY on all the fuses, feeders, incomers and bus ties AT ALL FAULT LEVELS. The main point here is to make your protection system work to trip within their respective zones and do not cross the damage curves of your protected equipment and the fault clearance is fast as much as possible even though your equipment ratings are based from 1 sec or more. Protective relaying is an art and it does opens you up in many possible setting configuration. As long as the above mentioned criteria are met, I do not see any harm in doing so. The Definite Time setting if properly applied will provide backup but ensure that it is set fast enough at that specific maximum fault of yourS AND STILL MAINTAIN SELECTIVITY with upstream and downstream OCPDs. At what specific OCPD you are applying this Definite Time? Is it on the relay just above the downstream fuses. Even if you do not apply the definte time settings on the Incomers and Bus Ties, As long as the total time grading between incomer, bus tie, outgoing feeders are maintained at no more than say 0.5~0.7 sec. (less than 1 sec) at a bus fault at 4.16kV switchgear, then its ok. 0.25~0.35 sec time grading between the 4.16kV outgoing feeder and downstream fuse can be done since they are of a separate zone of protection.

Check your respective zones of protection and establish the acceptable selectivity and make your call. Protective relaying is really an ART as they say so.

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[JMatt]

victa1288,

I would strongly suggest that you do not set the primary xfmr winding instantaneous overcurrent to trip, especially considering that you will be coordinating with a fuse.

In industrial plants, it is common for plant personnel to replace fuses with whatever fuse they have on hand at the time of need. If you set the instantaneous too sensitive, you could wind up miscoordinating with that downstream fuse in the future. The instantaneous element usually does you no good in that application.

If someone is requiring that you set it to protect the cable, set it as high as possible.

 

[nightfox1925]

The short circuit level at the transformer primary is commonly much higher than that on the secondary side due to the opposing impedance of the transformer and the difference of the voltage levels. Set your Inverse Time Element to coordinate your 138kV primary 51 relay to the 4.16kV secondary main incomer 51 relays on a MAXIMUM FAULT ON THE 4.16KV MAIN SWITCHGEAR BUS. Then set your 138kV primary (feeding the 138/4.16kV transformer) 50 relay Instantaneous setting to trip at 138kV MAXIMUM FAULT (i.e. short circuit at the 138kV conductor run or at the transformer primary terminals. This 138kV Maximum Fault will normally be of a much higher value compared to the 4.16kV maximum fault level at a 138/4.16 ratio.

The main point of the matter is that the 138kV primary O/C relay should instantaneously trip on a 138kV fault and its inverse time delayed setting set to coordinate with the downstream 4.16kV maximum fault at around 0.35~0.50 sec. This will allow that the relay will trip to its dedicated protection zone and serve as a back-up on the other O/C relay on the secondary mains.

In addition, the 138kV primary relay instantaneous setting shall be coordinated with the associated upstream 138kV inverse time delayed O/C relay by 0.35sec.

The fuses is located way downstream and should not be intersecting with the primary protection. Of course, it is the plant electrical personnels' responsibility to replace this fuses with the proper acceptable ratings possible.



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[Electic]

Sometimes the instantaneous function must be turned off to achieve coordination. In such cases the available fault energy (and arc flash exposure) can be very high. This needs to be verified on a specific case, but has been done in other instances.

 

[victa1288]

Hi, Fellows,

I totally agree with 02101972. That's what I am exactly trying to achieve.I will set the settings of inverse OC of upstream backup relay and downstream incomer relay slightly less than Max fault current on 4.16kV swgr Bus so long as they can coordinate with downstream fuses.

02101972, Another problem with these facilities I am working on is, at one of the facilities, at max S-C level with two upstream relays and downstream fuse, there won't be enough s-c current to trip the fuse first, what will happen is, the upstream relays will operate first with inverse time elements. Because the fuses have been purchased already, which intersects with inverse time curve on the top portion. Even though, we set the settings higher than Max S-C level. The fuse won't trip first. In this situation, I am thinking of using SR469 S-C trip element to trip against fault within Contacor withstand limit.Who has a better solution?

 

[windie]

Can you use a different time characteristic on your relays - for example, extremely inverse may co-ordinate better with your downstream fuses.

 

[nightfox1925]

Hi again Victa,

I will be glad if you could pour in some more details of this specific "facility" you are refering to.

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[victa1288]

I am alraedy using extremely inverse curve.

02101972, sorry for the late response. I was extremely busy at work. The problem with this pump station we are dealing with is, the power supply side is very weak so that it doesn't have a high short circuit current. But we still have to make sure our downstream fuse or SR469 controlled contarctor will trip at such a lower fault level. The problem is. the client doesn't want to change the fuses, so I am planning to allow SR469 to operate first within within its withstand limit. I know its' not a common practice, but what can we do? If we don't allow the SR469 to trip first, Upstream SR750 will trip before the fuse clears when the fault occurs, beacuse there is not enough fault current to clear the fuse even though we set SR750 at the higher setting(I,E, higher than Max Fault levels).

 

[nightfox1925]

The withstand rating of the contactor may be quite high, but the breaking capacity is limited and may not be adequate for your application. The fuse total clearing time (Tc) is equal to the melting time (Tm) + arcing time (Ta). The value of Tc is a function of the fault current before 1/2 cycle value. Now, if the value of the let-through current of the fuse is higher than the contactor breaking capacity, then it the current limitting effect of the fuse won't help you justify tripping the contactor. Lower fault current values like overloads and phase unbalance is tripped by the contactor and the fuse clears faster than the contactor for a high S.C. current. if your contactor clearing time is within the fuse let-through zone and the let-through current is lower than the contactor breaking capacity, then you're in.

check the fuse curve against the S.C. current, determine the let-through current and compare it to your contactor breaking capacity. Check also contactor clearing time against the fuse clearing time (at fault value).

This is just for the contactor point of view...



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