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  • Users: mbk2k3
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  1. mbk2k3

    Transformer Insulation Class Selection

    hello folks! I have a good understanding of transformer insulation classes, the base ambient temp, the temperature rise, hot spots,etc. However what I'm not finding easily is how to select an insulation class for a transformer? Is it as simple as: outdoor, hot environment, pick the highest...
  2. mbk2k3

    Ventilated Tray + Ventilated Covers = Free-Air Rating or De-rate?

    Sometimes I hate the NEC. So I know that if I have a ladder or ventilated tray that has a SOLID cover top, I need to de-rate by almost 60% or so. But the NEC rules don't cover the scenario of a ventilated cable tray used with a VENTILATED tray cover. In this scenario, can I use free-air...
  3. mbk2k3

    DER: Connecting to 4 wire system at the same voltage

    I have a 13.8kV 3 phase generator that I'm connecting to the utility as part of a DER project. So I've been pouring over the interconnection requirements documents from several local utilities and all of them show a transformer between the generator and the utility. What if my generation is...
  4. mbk2k3

    TCC Cut-off Question

    I'm ashamed to admit that I should know the answer to this question, but I'm blanking.... In popular tools like ETAP, when I plot inverse time current curves, why does the curve cut-off in the inverse time (long time) region? [top left side of the curve] Kind of like this, or this ?
  5. mbk2k3

    Help with Fence and Gate Grounding Layout

    I've gone through IEEE 80, local code requirements and design guidelines, but I'm struggling with the implementation of ground loops around a substation fence that has a host of interferences, different surface materials, etc. I'm attaching a photo of the fence in question. The fence in...
  6. mbk2k3

    3 wire connection to 4 wire transformer (WYE-WYE)

    Hey guys, Here is the islanded setup: I have a 3 phase generator (with a NGR in its neutral) connected to a step-up transformer. I asked for a DELTA-WYE transformer (delta on the generator side). However the only available transformers are WYE-WYE. I suspect I can't just leave the XO on the...
  7. mbk2k3

    Excavation work around a in-service substation ground grid?

    44kV system (3 phase, 3 wire). This is an existing substation plant that is in-service and we can't de-energize due to customer uptime requirements. While doing some excavation work for ductbanks/concrete pads/etc, the civil contractor has run into the existing ground grid (done years ago by...
  8. mbk2k3

    Generator Fault Current Contribution

    Say you are doing a hand-calc for the fault contribution of a generator. If an alternator is rated for 2000kVA, but the genset max output kW is 1500kW at a worst case PF of 0.8: Which number do you use for short circuit fault contributions? The 2000kVA or the 1875kVA (1500/0.8)? I would...
  9. mbk2k3

    HVAC for E-Houses with Swgr + Dry Type Tx

    I have a 750kVA Dry Type transformer inside an e-house. I also have 13.8kV swgr in the same room. How do I cool this room? (A) ventilation louvers only with blower fans? or (B) enclosed room with HVAC? Transformer manufacturer requires ventilation openings on the wall near the transformer...
  10. mbk2k3

    HVAC For E-House

    For a pre-fabricated e-house that has switchgear/control panels/etc, one of the items is a large 750kVA dry type transformer located inside the e-house that dissipates 7kW of heat when running at full load. The transformer manufacturer says to provide 12" of clearance on the back of the...
  11. mbk2k3

    13.8kV System BILs (Dry-Type Tx)

    Hey folks, this is for a small cogen plant: I have a 13.8kV switchgear with a 1200A, 95kV BIL vacuum breaker that feeds a 750kVA dry-type 13.8kV/600V transformer via a short 20' cable run. Vacuum breaker has surge arrestors on the load side. The upstream supply is two large 10MVA transformers...
  12. mbk2k3

    CT Sizing

    I have a 600:5A CT, Rated C200. Total burden on CT circuit is 1.008Ohms. C200 means, it holds 10% accuracy upto 20x secondary rated current. If my fault level at this bus is 20kA, 20kA translates to 166A on the secondary, which is >20x secondary rated current. Is this a clear-cut case that I...
  13. mbk2k3

    Generators + Zig-Zag Tx: What to do with the neutrals?

    Hey folks, I have 3x 13.8kV generators connected in parallel to a 13.8kV bus with a zig-zag transformer. I'm trying to see what my options are here for the generator neutrals. See attached PDF for a sketch of what I'm trying to get at. The way I see it I have 3 options: OPTION 1 - transformer...
  14. mbk2k3

    Is this a legal connection? Does it violate any code rules?

    I'm attaching a sketch to make it 100% clear what I am trying to do. See attached JPG. https://imgur.com/a/1kLdd2p This is for an 8 month rental project where they are renting generators and are looking to parallel the 3. There is no space for a new switchgear / 'splitter' to tie the 4.16kV...
  15. mbk2k3

    Temporary Transformer + Oil Containment

    I know the answer to this is very subjective, depending on locales, local ministry/inspection guidelines, etc, but I wanted the general group's thoughts on this scenario: If I have an oil-filled transformer with 700 gallons of non-PCB mineral oil, installed outdoors, but its only a temporary...
  16. mbk2k3

    600V Generator with HRG + Surge Arrestors

    Do 600V generator suppliers/packagers typically include surge arrestors with HRG grounded units? I have a system where: generator is 600V, delta, with the neutral point grounded via a 5A NGR. 2 days ago the generator tripped offline: Phase B / C PTs failed and Phase C surge arrestor...
  17. mbk2k3

    Utility / Service Entrance Transformers - Wye or Delta?

    My projects are always on the service/consumer/industrial load end user. Often I have to run by the utility whether the main transformer I'm procuring is WYE or DELTA, and utilities will give me their preferences / requirements. Can someone walk me through why a utility would want a WYE on...
  18. mbk2k3

    Generator Modelling Question

    I'm doing a crash course into understanding generator impedances/reactances that the utility for our project is requesting to do their study. The generator manufacturer has given us all the reactances/impedances/time constants. My question: - If its a salient pole generator, is it...
  19. mbk2k3

    Current Transformers Voltage Class?

    I have a reputable switchgear manufacturer that has provided 4.16kV switchgear lineups (multiple instances, different projects) where the current transformers supplied are rated for 600V. For example the CT103 from Arman here: http://www.chesselectric.ca/wp-content/uploads/2015/07/CT103.pdf...
  20. mbk2k3

    Generator Reactances

    I often run into power system studies where I have the following setup: generator that is rated at a certain kVA (lets say 1375kVA) some refer to it as "type load". genset output, usually a lower kVA obviously (lets say 846kVA). the generator manufacturers provide reactances in the format of...

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