Where did Phase Clearance above grade come from? 3

[NAZ55]

I am trying to find any research papers or articles on the topic of minimum phase-phase, phase to ground and particularly clearance above grade.

I know that for 138kV you need 13ft clearance above grade and for 345kV 18ft etc, but I don't know how these numbers are derived.

Any help or direction will be much appreciated.

 

[stevenal]

For clearance above grade, see the NESC Appendix A. Start with a truck height reference component of 14 ft, add a 2 ft mechanical component, then add an electrical component of 2.5 ft plus .4" per kV above 22 kV.

Your clearances seem low. Reduced values are allowed for pedestrian only areas, but anywhere a four wheel drive can possibly get to must be considered to have vehicle access.

Don't know about any research, though.

 

[racobb]

Agree with stevenal that your clearances seem very low. I would not put them that low in a fenced substation area.

Alan

 

[jghrist]

The clearances in the OP are from ANSI C37.32 and NEMA SG-6. The NESC has somewhat lower clearance requirements for 138 kV and the NESC clearances for 345 kV are either higher or lower, depending on the switching surge factor.

Also see 1427-2005 IEEE Guide for Recommended Electrical Clearances and Insulation Levels in Air Insulated Electrical Power Substations

"This guide, covering three-phase ac systems from 1 kV to 800 kV, provides recommended electrical operating and safety clearances and insulation levels in air-insulated electric supply substations; addresses insulation coordination procedures; provides design procedures for the selection and coordination of the insulation levels within the station as they relate to substation clearances; and addresses how reduced clearances in high-voltage ac substations will allow for compact bus arrangements and substation voltage uprating applications."

IEEE Std 1427 includes a discussion of the historical background of clearances as well as the technical requirements, but does not cover the electrical safety clearances of the NESC or ANSI C37.32 in detail.

 

[xxjohnh]

If you live in CA they have their own regulations, it's called GO95.

 

[NAZ55]

Thanks jghrist for clarifying my original post. I forgot to mention that I was referring to NEMA SG-6.

I also noticed that 1427 has a clause suggesting that there my be conflicts between it and NESC and in case of conflicts local laws and regulations prevail.

The reason for me asking this question is because I am working on a project where people are experiencing shocks while opening breaker cabinets or around the fence, and one possible scenario that has been suggested is that the 345kV bus height is at its minimum clearance of 18ft and might be causing some induced voltage. I am trying to find a way to verify that statement.

 

[stevenal]

http://www.nema.org/stds/sg6.cfm

SG6 has been rescinded. All these rules are for safety. The static like shocks you describe are not considered to be dangerous.

http://www.transmission.bpa.gov/NewsEv/pdfs/LivingAndWorking.pdf

 

[NAZ55]

I am looking for an equation that would describe the voltage drop in the air as you move away from the energized three phase conductors

 

[itsmoked]

stevenal; What a well written - frankly wonderful - brochure. Clear, concise, and it doesn't talk down to or at second grade level.

I especially liked the "run for your life" comment.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jghrist]

The reason for me asking this question is because I am working on a project where people are experiencing shocks while opening breaker cabinets or around the fence, and one possible scenario that has been suggested is that the 345kV bus height is at its minimum clearance of 18ft and might be causing some induced voltage. I am trying to find a way to verify that statement.

Sounds like a grounding problem to me. You shouldn't get any induced voltage on equipment that is bonded to the ground grid. Note that from stevenal's reference:

Under certain conditions, a perceptible electrostatic voltage can be induced on such objects as a
large vehicle, a fence, metal building or irrigation system. This can happen when the object is near a high-voltage transmission line and is insulated from the ground.

Induced voltage can happen when the object is insulated from the ground.

 

[NAZ55]

let's say for discussion sake the object on the surface is insulated from the ground. Now this object will experience a potential rise under normal conditions based on the vicinity to the transmission line. Do you know what potential rise is and how to calculate it?

I would assume that this induced voltage would be something equal to

E = Io x (Zap+Zbp+Zcp) + I1 x (Zap+ a^2 x Zbp +a x Zcp)+ I2 x (Zap+ a x Zbp + a^2 x Zcp)

 

[jghrist]

It's related to the voltage, not the current. Like a capacitive voltage divider.

 

[stevenal]

Usually the fences and breakers in the yard are very well grounded. It is the person walking around under the bus, on the crushed rock with his rubber soles who is not. He will start rising in potential until he touches a grounded object and discharges. Like a static shock, the voltage is high and the current is not. My suggestion is to avoid gasoline powered tools and get used to it, or wear rubber gloves. Calculation sounds difficult to me, since so many variables are involved.

 

[NAZ55]

As stevenal pointed out, the person or ungrounded object's potential rises up with the decrease in phase to grade clearance. It is this rise that I am trying to figure out.

jghrist,
I agree with you that the voltage would be like a capacitive divider, but I am trying to come up with an equation that would describe this voltage drop as one moves away from the phase conductors.

 

[jghrist]

It doesn't look simple. See

Induced voltages on fence wires and pipelines by AC power transmission lines

Abdel-Salam, M. Al-Shehri, A.
Dept. of Electr. Eng., King Fahd Univ. of Pet. & Miner., Dhahran, Saudi Arabia;
This paper appears in: Industry Applications, IEEE Transactions on
Publication Date: March-April 1994
Volume: 30 , Issue: 2
On page(s): 341 - 349
ISSN: 0093-9994
CODEN: ITIACR
INSPEC Accession Number:4708413
Digital Object Identifier: 10.1109/28.287525
Current Version Published: 2002-08-06


Abstract
A method is developed for calculating the induced voltages on fence wires/pipelines underneath AC power transmission lines. The method is based on the charge simulation technique and takes into account the disturbances of the electric field and potential due to the presence of the fence wire/pipeline underneath the line. The calculated values of the induced voltage on fence wires are compared with those measured before. Induced voltages on pipelines underneath 230- and 380-kV lines are measured, correlated to the calculated values and discussed in the light of electric field induction on objects adjacent to AC power transmission lines

 

[NAZ55]

you are right jghrist.

The process described by Abdel-Salam is very complicated and he just assume some numbers for several vriables without describing why. As such I am back to square one. I noticed that the topic has been discussed in the past and veritas suggested that he had a spreadsheet that does the calculation for him. I wonder if he could shed some light on the topic. The spreadsheet I have created only accounts for induced voltage due to phase imbalance and I am looking for one that would predict induced voltage even during normal operation.

 

[jghrist]

Getting accurate answers when the object in which the voltage is induced is as complex as a person would be difficult.

 

[NAZ55]

Using method of images, I have come up with Electric Field and Voltage for the isolated object certain distance away from the conductors, however it's an open circuit voltage. I have also calculated induced current flow b/w the isolated object and the ground conductor using I = wDA
where

w= 2*pi()*f
D = eo x E
A = surface area

however I am having difficulty correlating the two and trying to find out what the actual induced voltage would be.

Any suggestions?