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 Post subject: Modeling Transmission Lines and Voltage RegulatorsPosted: Fri Aug 11, 2017 1:29 pm

Joined: Wed Jul 12, 2017 6:06 am
Posts: 35
We have a client we are doing an arc flash study for that owns several miles of 22KV transmission lines. On this line there are a handful of distribution voltage regulators. Along this line there are pole mounted transformers that feed equipment so obviously the transmission lines will need to be in the model. Our client prefers we use SKM for the model.
My questions are, 1. Will the voltage regulators affect the impedance along the line much? and 2. How critical is the configuration of the transmission lines to the impedance? Etap seems to have a much better library for transmission lines than SKM with a lot more detail as far as what the configuration is of the lines. What are you folks doing for transmission lines?

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 Post subject: Re: Modeling Transmission Lines and Voltage RegulatorsPosted: Sat Aug 12, 2017 5:16 am
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
I've done 23 kV transmission lines in SKM. The model is just fine. The big thing with transmission lines is that the impedance (distributed capacitance in particular) is highly dependent on the line configuration, particularly the spacing between phases and to a lesser extent the distance to the Earth. The best layman's book on the subject from what I've seen is the one from Southwire if you want to do/verify the calculations yourself by hand.

The first issue to cover is SKM's use of the Lee model for arc flash calculations, considering that this is the Arc Flash Forum. At 15 kV, the Lee model has been proven to be at least in error by 300%+ and the size of the error increases as the voltage increases. At least in the U.S., OSHA in one of the Annexes of 1910.269 has specifically recommended using ArcPro for modelling above 15 kV. The inherent problem with the Lee model is that it assumes that the arc power is proportional to the maximum power transfer or roughly V*I. In reality in a power arc the arc power is related closely with current but almost independent of voltage. There are a lot of other problems too but right now this is the best you can do. So to meet OSHA standards you can use SKM to get the current and clearing time but then you will have to enter everything into ArcPro to get the incident energy. ArcPro is kind of manual..you input every parameter and it does a calculation. There isn't a power system model like there is in SKM.

One of the most important factors that kind of plays havoc with the concept of sequence currents is whether or not the lines are periodically transposed. The particular system I was dealing with didn't do transposing so in reality sequence currents as calculated by SKM were wrong, but there's only so much you can do.

"Voltage regulators" whether of the capacitive or transformer variety definitely add their own capacitive/inductive/resistive impedances to the system and need to be modeled.

All that being said it's not terribly hard to model everything but there are four items that you have to tackle that can really throw a monkey wrench into everything:
1. First off, most transmission line systems are NETWORKS. So incident energy depends strongly on the impedance and thus the particular network configuration at the time of an incident. You will get maximum incident energy (although probably only a theoretical number) by modelling with all ties closed, and minimum with a radial only system. Real systems may adopt various configurations and somehow you've got to achieve a consensus as to what this layout actually is. In other words, short circuit current and thus incident energy may be any number of possible values.
2. With long distances and access anywhere along the line, somehow you are going to have to estimate incident energy as more of a range and not at discrete points. So this means for instance that you may be specifying that the incident energy from say mile 1 to mile 2 is X, from mile 2 to mile 4 is Y, and so forth, on the 22 kV side. Close to the power sources this matters a great deal but as you get further away, it doesn't matter.
3. Quite often distance elements are used for protection rather than simple overcurrent. The modelling isn't terribly hard but you will have to estimate the current first and then check clearing time (or more often clearing distance) and handle these values manually. Again, that pesky manual calculation creeps into things.
3. You will probably quickly notice that somewhat counterintuitively, arc flash gets more severe as you get further from the power sources, incident energy generally will grow to very high values and then at a certain point, it falls off as expected. What happens is that the clearing time of the overcurrent elements. Once the clearing time reaches 2 seconds then the fall off that you expect happens. Prior to that most transmission line type overcurrent elements have an inverse time relationship where the exponent is different from that of the incident energy curve. This means that the clearing time grows at a faster rate than the reduction in current and thus incident energy paradoxically grows as available fault current decreases. This actually happens...it is not just a calculation error. So paradoxically modelling towards the ends or at least at the point where clearing times are 2 seconds or less is more important than modelling at other points.

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 Post subject: Re: Modeling Transmission Lines and Voltage RegulatorsPosted: Mon Aug 14, 2017 7:54 am
 Sparks Level

Joined: Fri Apr 15, 2011 7:43 am
Posts: 178
Paul mostly hit it on the head. SKM works just fine for modeling, current values and protection time - if you can get the information. The Lee equation, as Paul also pointed out, has large errors. We do not do three phase calculations on distribution lines or transmission lines as the majority of the work on these lines is done one line at a time, therefore you need ArcPro. We have found the majority of the time the IE is less than 1 cal and about 1-5% it may jump higher. Keep in mind that OSHA does not require every point on a system but a representative sample.

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 Post subject: Re: Modeling Transmission Lines and Voltage RegulatorsPosted: Tue Aug 15, 2017 8:07 am

Joined: Wed Jul 12, 2017 6:06 am
Posts: 35
Thanks for the information guys! We do not anticipate calculating incident energy at 22KV everything fed off this line will have fuses on a pole and hit pad mount transformers below, although there is a small amount of 13.8KV and 4.16KV that will require calculations. We are concerned with having the correct transmission impedances for the model. I am seriously considering building the transmission lines in ETAP and carrying those impedances over to SKM.

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