Recent content by mbk2k3

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...

thanks Gr8blu and 7anoter4! @gr8blu: which standard are you referring to? @7anoter4: I went through the NEMA standard, it looks like all the equations/etc assume a fully stacked (touching side-by-side) and no adjustment factors for 'spacing' between cables. or am i misreading it?

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...

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...

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 ?

its so difficult to explain without seeing the final expected layout. see attached. i've highlighted the fence in question, and also indicated where major switchgear equipment is located. (i.e. there are two grids interconnected with 2x #4/0 conductors and the fence in question runs right in...

@kiribanda, yes all the grounding studies have been completed. The GPR / step / touch potentials are all within tolerable limits (as per our local requirements), but the fence still has to be bonded/connected to the ground grid. Majority of the ground grid is not shown in the photo, and the...

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...

@dpc Wait, you can float a WYE winding / disconnect the X0? I thought this was a no-no.

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...

thanks for the responses guys. the stuff going in can't be installed around the ground loops. so we can't just vacuum truck out and install new ducts/etc. we def. have to cut and scrap the grounds. i'm planning on installing more wires/loops and rods in the area (to ensure that we have as...

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...

thanks guys for the input! I was confused because the impedance the genset provided gave was the adjusted impedance based on genset output.

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...

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...