Instantaneous setting on a relay upstream of MV Switchgear lineup

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mull982

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I had a question regarding what is the best practice to set the Instantaneous setting on a relay which feeds a downstream MV switchgear lineup. For instance if I have a 4.16k Swichgear Bus A which has a feeder relay which feeds a downstream 4.16kV Switchgear Bus B containing a Main, Tie, and Feeder relays what should the feeder relay at Bus A have its Instantaneous setting set to?

I know that at the downstream Bus B the Main relay and Tie relay will have its Instantanous setting turned off so that it does not nusiance trip for Instantaneous faults downstream of the feeder relays on that bus, but what about the upstream feeder relay in Bus A? Should it also have its Inst setting turned off so not to nusiace trip for faults downstream of feeders in Bus B or should it be set so that it is above fault current at Bus B so that it does not pickup for faults at Bus B but still protects the feeder cable between Bus A and Bus B?

I'm looking for some setting guidelines for this type of scenario.

Thanks
 
You may exercise your last option but check that the thermal withstand capacity of equipment downstream to bus A is not exceeded during any heavy fault.
 
I don't like to eliminate instantaneous from a MV bus unless there's bus differential on it, otherwise there's the potential of significant equipment damage from a bus fault, and higher levels of arc flash incident energy.

One approach to setting the instantaneous so that it's fault selective is to look out to the downstream load to see where there's a higher incidence of faults, and see what the resultant fault current through your switchgear A is when that downstream bus is faulted; then adjust beyond that level, which should leave time for the devices in that anticipated-fault area to be selective.

It's more easily envisioned when your adjusting the MV Inst level for faults in a downstream LV system. In the case of a 15kV distribution loop serving multiple padmounts, fault the secondary of the largest padmount, then see what the resultant current flow is in your "A" source breaker, and adjust the instantaneous to allow about 150% of that current to flow, thereby giving the low voltage devices ample room to operate.

Also, if the source breaker in Swgr A is an electronic relay equipped with definite delay, you could elect to set the instantaneous level to protect for faults at bus B (~90%), and initiate a 150-200 ms delayed instantaneous response to give the downstream breakers a chance to operate.
 
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mayanees said:
I don't like to eliminate instantaneous from a MV bus unless there's bus differential on it, otherwise there's the potential of significant equipment damage from a bus fault, and higher levels of arc flash incident energy.

One approach to setting the instantaneous so that it's fault selective is to look out to the downstream load to see where there's a higher incidence of faults, and see what the resultant fault current through your switchgear A is when that downstream bus is faulted; then adjust beyond that level, which should leave time for the devices in that anticipated-fault area to be selective.

It's more easily envisioned when your adjusting the MV Inst level for faults in a downstream LV system. In the case of a 15kV distribution loop serving multiple padmounts, fault the secondary of the largest padmount, then see what the resultant current flow is in your "A" source breaker, and adjust the instantaneous to allow about 150% of that current to flow, thereby giving the low voltage devices ample room to operate.

Also, if the source breaker in Swgr A is an electronic relay equipped with definite delay, you could elect to set the instantaneous level to protect for faults at bus B (~90%), and initiate a 150-200 ms delayed instantaneous response to give the downstream breakers a chance to operate.
Click to expand...

Are you suggesting that I set the BusA feeder relay INST setting above the Bus B maximum fault current, or above the maximum fault current at buses downstream of Bus B. For instance if the maximum fault current at Bus B or buses downstream of Bus B is 5000A then perhaps I set the Bus A feeder relay at 7500A or so? This way it will protect the feeder cable to Bus B but not trip for faults downstream of Bus B? This sounds similar to setting an MV tranformer feeder relay above the secondary fault current as you mentioned below.

So do you always have an INST setting on Switchgear main breaker relays? I always thought that you do not want to have an INST setting on main or tie relays for coordination purposes. However it sounds like you are suggesting to put INST setings on main breakers but just make sure they are set to pickup above the fault current at buses downstream of feeder relays in the same switchgear. What about for calbe feeder faults close to the switchgear, cant this possibly lead to mis-coordinated tripping of the main and feeder breakers?

Thanks
 
.. the Art and Science

.. the Art and Science

mull982,

Your interpretation of my inclination for the system you described is correct.
And I do think that generally instantaneous needs to be considered for Switchgear mains, at the expense of reliability. But I would eliminate it if the bus was differential protected. And consideration would be given to the nature of the load and the impact of an outage, balanced with the consequences of equipment damage.

I think the most important part of the analysis is that it be tailored to the installation, such that settings are developed to take into consideration the actual parameters of the system, to include things like: O/H or U/G; critical nature of the load; age, condition, location of the gear; etc.

I like the title of "Art and Science of Protective Relaying" because there's the technical or scientific aspect that must be satisfied that's objective, but the application is somewhat subjective and dependent on the engineer's personal interpretation and translation of the system requirements. That's to say that we'll all do it a little different.
 
Consider zone interlocking protection which is available on most LV circuit breakers and can be applied to most modern relays.

The feeder breaker's protection has an output contact or signal that asserts when it senses a fault. That signal is used to block the main breaker's instantaneous trip or to delay the trip long enough to allow the feeder breaker to clear the fault. For bus faults, the Main trips on instantaneous with no delay since no blocking signal is present.

Most relays now have the ability to block a trip function when a digital input is active. It takes some wiring between the feeder and Main relays or a fiber communication link.
 
Perhaps no need for complicated protecti?n scheme to get INT relay co-ordination. A gradual and suitable reduction INT relay settings fr?m upstream to downstream may achieve the relay operation coordination.
 
Thanks for the responses. The switchgear in question does not have any sort of bus differential protection.

To aide the discussion I have attached a sample one-line with the buses in question to help me gain a better understanding.

So it sounds like we are saying that both the feeder relay at Bus A and the Main and Tie relays at Bus B should have their INST settings active in order to protect Bus B. So the question now becomes what to set the INST settings at. I know the feeder relay at Bus B whcih feeds Bus C should have its INST setting set above the max fault current at Bus C. Would it make sense to then set the INST setting of the Bus B main breaker somwhere above the Bus C feeder breaekr INST setting to try to provide some degree of coord? But what about a fault on the primary of the Bus C transformer that falls in both the Bus C feeder and Bus B main relay INST region?

For the Bus B main relay should you also take into consideration the fault current at Bus D which is the same voltage? How would you set the Bus B main relay INST setting to coord with the Bus D feeder relay INST setting?

Now for the Bus A feeder relay INST setting. Should this be set the same or above the Bus B main relay INST setting to coordinate? Should it be set above the 14.8kA fault current at Bus B so that it does not pickup for faults at Bus B or faults downstream of Bus B? Or should it be set above the fault currents at Bus C and Bus D so that it protects faults at Bus B but will not pickup for faults downstream of Bus B?

Thanks for the guidance.
 

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Take ratio of INST settings of any two adjacent upstream and downstream relays a minimum 2:1 and check by calculation if desired. Take transformer ratio into account as there is one.
 
mull982,

I can't directly answer the questions without spending some time on it, but I will suggest a method to go about developing the settings.
I would apply a fault to BUS C, where the resultant fault current at the 480 bus is now shown as 29.8 kA. If you're using SKM you do that in the Run Balanced System Studies, SC Setup, Fault selected buses, then select a bus. Paste a datablock format on the 1-line that shows branch fault currents through the devices as a result of that 480V, 29.8 kA fault at Bus C.
Then analyze your instantaneous settings accordingly, setting the upstream relays (BUS A and BUS B) high enough to ignore anything resulting from a BUS C fault.
Then I'd fault the primary of the SWGR-A 480V transformer, (Fault selected bus) and set the BUS B feeder relay to catch that fault current at around 90% of what's flowing for a fault at the primary of that 480V xfmr.
The nature of instantaneous is such that series devices don't coordinate so you'll only get coordination by using intentional delays, by faulting across transformers, or by taking advantage of long lengths of cable where there's an appreciable difference in magnitude from beginning to end.
I think Easy Power is set up better for faulting selected buses, as I recall that all you need to do is select a bus and fault it, and the resultant current flows show up throughout the system.

Good luck. I always advise that you document the development of settings in your report so it forces you to analyze it in a step-by-step manner based on specific fault conditions, and the logic is there to understand when you pick the report up 6 months later.
We're discussing instantaneous settings here, but there's also the other end of the curve - longtime pickups, which are much more straightforward.
 
Sahib said:
Take ratio of INST settings of any two adjacent upstream and downstream relays a minimum 2:1 and check by calculation if desired. Take transformer ratio into account as there is one.
Click to expand...

Is a 2:1 Instantaneous pickup ratio a typical rule of thumb when setting the Instantaneous settings for main and feeder breakers in the same switchgear lineup. In other words set you feeder Instantantous based on the downstream bus which it feeds and them make sure you main is set at least 2x higer than you feeder INST setting?

mayanees said:
Then analyze your instantaneous settings accordingly, setting the upstream relays (BUS A and BUS B) high enough to ignore anything resulting from a BUS C fault.

Then I'd fault the primary of the SWGR-A 480V transformer, (Fault selected bus) and set the BUS B feeder relay to catch that fault current at around 90% of what's flowing for a fault at the primary of that 480V xfmr.
Click to expand...

I have always set the transformer feeder relay (In this case feeder relay at Bus B) above the transformers secondary fault current according to secondary fault curret x .1 67x10%. Are you also suggesting to set it so that it catches 90% of the transformers primary fault current? Is there any reason why 90% is chosen?

For the Bus A relay and Bus B main relay it sounds like they just need to be set high enough to ingore fault current on the secondary of the transformer. What about for faults at Bus D where they system voltage is the same and there is no large impedance such as a transformer or long cable in between. Any rule of thumb for INSt setting for main and feeder breakers which feed downstream buses in close proximity at the same voltage level?
 
@mull982,
Here are my thoughts:
  1. Breakers S-22-OXY and OX-P3-5kV doesn't have to coordinate with each other. Both are serving the same bus. Losing either breaker will yield the same results.
  2. A simple rule I use is to protect the element just under the protective breaker. If it's a cable, I refer to the short-circuit withstand of that cable; if a transformer, the withstand rating of the transformer and the cables, whichever is weaker; etc.
  3. Since you mentioned that the system does not employ differential protection, you are right in taking each element individually and calculating the protection settings of each.
Hope that helps.
 
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