instanneous settings and short circuit levels
instanneous settings and short circuit levels
(OP)
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.
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.






RE: instanneous settings and short circuit levels
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RE: instanneous settings and short circuit levels
The transformer is high resistance grounded. Sec. CT ratio is 1500/5.
RE: instanneous settings and short circuit levels
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RE: instanneous settings and short circuit levels
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.
RE: instanneous settings and short circuit levels
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.
RE: instanneous settings and short circuit levels
RE: instanneous settings and short circuit levels
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.
RE: instanneous settings and short circuit levels
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.
RE: instanneous settings and short circuit levels
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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RE: instanneous settings and short circuit levels
RE: instanneous settings and short circuit levels
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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RE: instanneous settings and short circuit levels
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.
RE: instanneous settings and short circuit levels
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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RE: instanneous settings and short circuit levels
RE: instanneous settings and short circuit levels
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?
RE: instanneous settings and short circuit levels
RE: instanneous settings and short circuit levels
I will be glad if you could pour in some more details of this specific "facility" you are refering to.
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RE: instanneous settings and short circuit levels
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).
RE: instanneous settings and short circuit levels
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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RE: instanneous settings and short circuit levels
You're right, the breaking capacity of the contactor is not high enough to interupt the bolted fault. But the bolted fault is not high enough to trip the fuse before the contactor can trip. That's why I am thinking of allowing 469 to open contactor at lower fault level before it deveops to bolted fault to trip conatactor at higher fault level.
RE: instanneous settings and short circuit levels
For the "facility (pump station)" you were referring to:
You have a medium voltage MCC with several feeder starters having fuse-contactor arrangements with SR 469 MPRs. This medium voltage MCC is connected to an upstream medium voltage switchgear via outgoing feeder circuit breaker. I assume that the upstream switchgear is double ended with both incomers and bus ties. I also assume that the bus tie is closed. I also assume that the medium voltage switchgear and mendium voltage MCC are in one substation and hence there is no main disconnecting device on the medium voltage MCC.
Is this the scenario at hand?
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