Help on SC Calculation: Second xfmr downstream with motor loads 1

[Gene1203]

Can anyone help me on this one: How to coordinate the CB protecting a low-voltage dry-type transformer serving motors.

The overview of the system is this: 2 grids of MV utility = 34.5kV each; 2 nos. of unit substation transformers feeding each end of a double-ended LV switchgear = 3000kVA, 34.5kV-0.48kV/0.277kV, 5.75%Z each; the double-ended switchgear (480V/277V) with a branch circuit breaker feeding the 1000kVA, 480V-208V/120V 3-phase dry-type transformer approximately 75 meters away.

At the primary side of the said dry-type transformer, I provided a circuit breaker. However, I am a bit lost in coordinating:

(1)the branch circuit breaker of the switchgear, where the feed comes from,
(2)the circuit breaker for the primary of the dry-type transformer and
(3)the circuit breakers of the MCC.

This dry-type transformer serves an MCC with 3-phase motors at 208V (mostly pumps - the owner purchased pumps rated at 208V instead of 480V).

Shall these pump motors still contribute to the short-circuit current to be considered in the switchgear? If so, how do I reflect the possible contributions of these motors upstream to the mains?

Your immediate response shall be highly appreciated. Thanks and more power to you all.

 

[rhatcher]

Gene1203, try this site out. Near the bottom of the list is a 3.5Mb download called "Short Circuit Current Calculations". I can't improve on their presentation of the material so I won't try. You will have to read it for yourself.

http://www.geindustrial.com/industrialsystems/publications/switchgear.htm

Before going further...is the tie for your substation closed or open?? If it is closed then the short circuit contribution of both transformers must be considered.

For coordination of the circuit breakers, the idea is that you want to interrupt a fault as close to the fault as possible while maintaining service to the rest of the system. This is as much an art as a science. I do not have a good web reference for you on this and I recognize that I could write all day without scratching the surface of this topic, so I will wait in hopes that a good web reference is presented.

 

[jbartos]

Suggestions:
1. Usually, the motor contributions are not considered upstream of main and upstream on the transformer primary since they are mitigated be the transformer impedance.
2. See my markups ///\\(1)the branch circuit breaker of the switchgear, where the feed comes from,
///This circuit breaker shall be protecting its branch circuit load. If load happens to be transformer it may be slightly delayed on the short circuit downstream of transformer.\\(2)the circuit breaker for the primary of the dry-type transformer and
///This circuit breakers are usually delayed, e.g. 0.3 sec.\(3)the circuit breakers of the MCC.
///The MCC circuit breakers (CBs) will see motor contributions and ICs or IRs of those CBS shall be sized for those.
However, modeling of this system by some Electrical Analysis software shall be applied.

 

[Gene1203]

Thank you very much for your replies. I really appreciate them. However, if you have some more time, I have further inquiries.

What I did for my initial calculations is that I isolated first the branch circuit. First, I considered the main switchgear as my system where I was able to calculate the short-circuit current from the utility and other motors at 480V. After gathering all X/R ratios, I now have the "system Z" including the impedance of the feeder conductors supplying the said dry-type transformer. The second step I did was to gather the impedances after and including dry-type transformer and applying the proper formulas for each motor (type) short-circuit contributions. After reducing to a single impedance, I now have the available short-circuit current for the CB at the primary of the DT transformer and subsequently for the MCC. Is this satisfactory?

I came to a point where I noticed that while I was considering the 480V motors, I did not consider the 208V motors.

As for rhatcher's tie-breaker question, it is normally open. It will close only during: an outage of any of the two power grids; a failure on any of the two transformers. Basically, the two main transformers are typically sized to carry all loads without paralleling.

Thanks for the link. I have downloaded it too. I browsed the pages but did not find a similar case example. I will still read it thoroughly though.

As for jbartos's suggestions:

"1. Usually, the motor contributions are not considered upstream of main and upstream on the transformer primary since they are mitigated be the transformer impedance."

Please confirm whether the motors on the downstream will not affect the CB protecting the dry-type transformer however large the motor loads are (the dry-type transformer is 1000kVA).

Actually, there are three CB's to be considered: 1)branch CB in the main switchgear; 2) the CB at the dry-type transformer primary; and 3)The main CB of the MCC at the secondary of this DT transformer.

"2. See my markups ///\\(1)the branch circuit breaker of the switchgear, where the feed comes from,
///This circuit breaker shall be protecting its branch circuit load. If load happens to be transformer it may be slightly delayed on the short circuit downstream of transformer.\\\"

I gives me comfort to hear this. However, the transformer's loads are mostly motors (about 80-90%) and the word "may"-can we use "shall" here instead?

"(2)the circuit breaker for the primary of the dry-type transformer and
///This circuit breakers are usually delayed, e.g. 0.3 sec.\\"

Could you please explain further why? My mindset is that, for any fault within this branch circuit, the available short-circuit currents shall come from the utility and motors.

"3)the circuit breakers of the MCC.
///The MCC circuit breakers (CBs) will see motor contributions and ICs or IRs of those CBS shall be sized for those.
However, modeling of this system by some Electrical Analysis software shall be applied."

(Do you mean IRs = interrupting rating and ICs = interrupting capacities?)Please confirm if you are saying that for this MCC, the short-circuit current to be considered shall be from the motors only - neglecting other sources?. Can I do my own evaluation also without the use of any software?

Please bear with me for my details. I would like to make certain all my calculations and I want to do them manually.

Again, thanks and more power.

 

[rhatcher]

Gene1203, sounds like you are on the right track. It is morning at my job now and I am expecting a bit of work here any minute so I will set aside time this afternoon to give a reply as I know this will take more than a few minutes.

In the meantime, will you provide the following info?:
- the conductor size and # of parallel conductors for the 480V feeder to the transformer
- same info for the 208V feed between transformer secondary and mcc main plus distance of the run
- info on substation main, feeder, transformer primary, and mcc main circuit breakers. If not to much trouble include manufacturer, model, current rating, trip unit type, and trip unit functions.
- your calculated values for sc current at
(1) load side of feeder breaker
(2) line side of primary breaker for stepdown transformer
(3) line side of mcc main breaker

 

[jbartos]

Suggestions to Gene 1203 June 24, 2001 marked by ////\\\What I did for my initial calculations is that I isolated first the branch circuit. First, I considered the main switchgear as my system where I was able to calculate the short-circuit current from the utility and other motors at 480V. After gathering all X/R ratios, I now have the "system Z" including the impedance of the feeder conductors supplying the said dry-type transformer. The second step I did was to gather the impedances after and including dry-type transformer and applying the proper formulas for each motor (type) short-circuit contributions. After reducing to a single impedance, I now have the available short-circuit current for the CB at the primary of the DT transformer and subsequently for the MCC. Is this satisfactory?
////This appears to be satisfactory, if you do not have any motor contributions in this power distribution area. If you do, then they have to be counted in addition to the Utility/System source and its contribution to faults.\\\
I came to a point where I noticed that while I was considering the 480V motors, I did not consider the 208V motors.
////The 208 motors are downstream of 480V/208V transformer, most likely, since you do not mention it. They are not counted as fault current contributors on 480V power distribution side since the 480V/208V transformer impedance mitigates their contributions to fault noticeably. However, if you wish you may add them and see the difference in the fault magnitude by yourself.\\\
As for rhatcher's tie-breaker question, it is normally open. It will close only during: an outage of any of the two power grids; a failure on any of the two transformers. Basically, the two main transformers are typically sized to carry all loads without paralleling.
////Make sure that there is an interlock, ideally electrical and mechanical.\\\As for jbartos's suggestions:
"1. Usually, the motor contributions are not considered upstream of main and upstream on the transformer primary since they are mitigated be the transformer impedance."
Please confirm whether the motors on the downstream will not affect the CB protecting the dry-type transformer however large the motor loads are (the dry-type transformer is 1000kVA).
////The motor fault current contributions upstream through the transformer are relatively small; therefore, they are not considered. If you will you may consider them and find out the difference for yourself.\\\Actually, there are three CB's to be considered: 1)branch CB in the main switchgear; 2) the CB at the dry-type transformer primary; and 3)The main CB of the MCC at the secondary of this DT transformer.
////Usually, the selective coordination will require to delay the transformer primary switchgear trip on short circuit faults. The overload can be somewhat delayed too if it is not delay automatically by the protective device curves.\\\"2. See my markups ///\\(1)the branch circuit breaker of the switchgear, where the feed comes from,
///This circuit breaker shall be protecting its branch circuit load. If load happens to be transformer it may be slightly delayed on the short circuit downstream of transformer.\\\"
I gives me comfort to hear this. However, the transformer's loads are mostly motors (about 80-90%) and the word "may"-can we use "shall" here instead?
////This depends on sizes and may become relative. If sizes are tight it shall be delayed, else it will trip. However, it should not be delay a lot since the transformer and cable thermal or damage curves have to be watched and on right hand side of switchgear curve, i.e. still protected by the time delayed switchgear. This is almost by common-sense. However, the software packages are calculating this automatically. You may learn; however, you will probably be wasting plenty of time.\\\"(2)the circuit breaker for the primary of the dry-type transformer and
///This circuit breakers are usually delayed, e.g. 0.3 sec.\\"
Could you please explain further why? My mindset is that, for any fault within this branch circuit, the available short-circuit currents shall come from the utility and motors.
////If you do not delay this circuit breaker/switchgear, you will have a hard time to coordinate it with the downstream circuit breaker, unless they are far apart in sizes. If you intend to design the power distribution without any huge redundancies (fats), then you need to consider such delays that the protective devices curves are tightly selectively coordinated.\\\"3)the circuit breakers of the MCC.
///The MCC circuit breakers (CBs) will see motor contributions and ICs or IRs of those CBS shall be sized for those. However, modeling of this system by some Electrical Analysis software shall be applied."
(Do you mean IRs = interrupting rating and ICs = interrupting
capacities?)
////Usually, IR is on the protective device nameplate. It is usually associated with low voltage circuit breakers and fuses and their highest rms alternating current that are required to interrupt. IC is more associated with medium voltage switchgear. It is defined in IEEE dictionary as a highest current at rated voltage the device can interrupt (any device). Essentially, both have the same intent, to characterize rating of protective device.\\\
Please confirm if you are saying that for this MCC, the
short-circuit current to be considered shall be from the motors only -
neglecting other sources?.
////No. You have to consider all sources: Power supply source contribution (over transformer in your case), motor contribution to the fault, capacitor contributions, standby generator contributions, etc.\\\ Can I do my own evaluation also without the use of any software?
////Yes, you can; however, the software is usually much faster and more accurate. There are various constants imbedded that you may be not aware of, e.g. for circuit breakers and for motors, grouped motors, etc.\\\

 

[rhatcher]

From jbartos' previous post:
"This is almost by common-sense. However, the software packages are calculating this automatically. You may learn; however, you will probably be wasting plenty of time."

gene1203, I am sure that you are busy reading the 60 page download on short circuit calculations. There are no specifics on coordination there, but perhaps the way to understanding that will follow once you understand the short circuit factors for your system. If you need any guidance on coordination or any specifics on your breakers or their trip units let me know as I can give some good info on that. Otherwise, keep reading and come back if you have any questions or need assistance. When you are done you can walk away knowing what others 'almost' understand...............

I'll leave you guys a thought that crossed my mind today but could not be voiced for obvious reasons.

"If you are such an expert, why are you paying $200/hr for me to be here?......"

 

[Gene1203]

To rhatcher and jbartos, a big THANKS.

For rhatcher:

I would not like to consume much of your time but here goes:

"- the conductor size and # of parallel conductors for the 480V feeder to the transformer"

5 sets of 3-250mm sq. THW in cable galv. steel cable ladder

"- same info for the 208V feed between transformer secondary and mcc main plus distance of the run"

10 sets of 3-250mm sq. THW in cable galv. steel cable ladder
approximately 15m circuit length

"- info on substation main, feeder, transformer primary, and mcc main circuit breakers. If not to much trouble include manufacturer, model, current rating, trip unit type, and trip unit functions."

The overview of the system is this: 2 grids of MV utility = 34.5kV each; 2 nos. of unit substation transformers feeding each end of a double-ended LV switchgear = 3000kVA, 34.5kV-0.48kV/0.277kV, 5.75%Z each (dual temperature rise 55deg C/65deg C - 3000kVA/4200kVA - OA/FA); the double-ended switchgear (480V/277V) with 5000AT Mains and a branch circuit breaker (1600AT) feeding the 1000kVA, 480V-208V/120V 3-phase dry-type transformer approximately 75 meters away.

I used CH Magnum breakers MWI-C50, 100kAIC (drawout with Digitrip 1150i) for the main and MWI-C16, 100kAIC D-O with the same trip unit for the branches. (is this allowed here? - there is a "no-promotion" clause below) I guess you have info on the trip unit's characteristics if not please let me know.

"- your calculated values for sc current at"
(1) load side of feeder breaker = 93.4kA
(2) line side of primary breaker for stepdown transformer = 47.5kA
(3) line side of mcc main breaker = 96kA (800kVA motors)

I'm not quite sure if I did the right thing. Please point to me the errors.

Thanks and God bless.

 

[jbartos]

Suggestions:
1. Please, notice that the protection is more on the Art side than on The Science side.
2. The Art may or may not be easily learned.
3. Notice, that software producers imbed various proprietary items, which make one software more applicable than other or perhaps producing more conservative or liberal results than others. It is hard to challenge the software nowadays.

 

[rhatcher]

It sounds like you have a pretty good setup. The Digitrip 1150 is very powerful and flexible. Hopefully you have a copy of the I.L. (instruction literature) as it is not avalable online. Please read the I.L. and familiarize yourself with the following features of your trip unit:

- selectable short time, instantaneous, and ground fault trip functions
- selectable I2T or fixed time delay for short time and ground fault trips functions. (I2T or I-squared-T is inverse time delay meaning variable time delay with less time allowed as current level increases. Fixed time delay means that once you exceed the pickup level the time delay is the same regardless of how high the current goes.)
- zone interlocking

***Also note that the trip function settings are given in multiples of a base current as follows:
- long delay pickup is multiple of rating plug current
- short delay is multiple of long delay pickup
- instantaneous is multiple of rating plug current
- ground fault is multiple of rating plug current. Note that the letter A-K designate multipled of 0.25-1.0 but that 1200A cannot be exceed for any rating plug value.

***This is based on Digitrip 510 through 910 series. You will need to confirm whether the same convention is used on the 1150 series.

 

[corona]

Just a thought.
Depending on the number of motors involved , the characteristics of the MCC, and the distances of the motors from the MCC, have you given consideration to exchanging the 208v, 3p motors for 480 3p, motors.

 

[rhatcher]

gene1203, if you have familiarized yourself with the features and capabilities of your trip unit, then try this out for more understanding....Again, I cannot say the basics any better than what is presented here. Before clicking, read instructions below the link to be sure to find the info you want as this is not another useless web reference.

http://www.ch.cutler-hammer.com/cgi...tentServer?pagename=C-H/CHHome/CHHomeMainPage

1- select "Learning" from the top of the page
2- select "more" from the right of the page under "Other Learning Resources"
3- select "Click here to view the 101 basics online"
4- from here you may go at will...(the mind is a terrible thing to waste and there is much valuable info here...) The topic at hand is discussed in section "7+ Advanced Low Voltage Power Circuit Breakers"

 

[Gene1203]

I think we have gone too far here.

Thanks to all of you for the TIPS. We have different point of views obviously. What I was hoping to get from these posts is a common thinking of what was initially asked: whether the motors downstream of a second transformer may be considered for the short-circuit calculation of the system.

Anyway, I have reviewed a number of actual calculations from previous designs (existing and proposed) and I have found several similar situations.

From there, plus your TIPS, I feel I have stablished a common ground.

Can anyone suggest a popular software for short-circuit calculations?

Again, thanks so much and I hope that I could ask for further assistance in the future.

 

[jbartos]

Suggestion: The electrical power system analysis sotware frequently used:
SKM, Inc. (DAPPER, A-FAULT, etc.), OTI, Inc. (ETAP), EDSA, etc. These businesses have their websites.

http://www.skm.com

http://www.etap.com

http://www.edsa.com