Designing a Substation Ground Grid Near a Power Plant

Hi,

have you done this before? Don't mean to insult you at all! - but it can be a tricky subject (with catastrophic consequences if you get it wrong...). Here are a couple things to follow up:

- get the plans for the existing ground grid (see below**)

- if you don't have recent ground measurments (in the 'old days' made with a device called a 'megger', which measures the ground's conductivity), make them or have a consultant make them for you

- if your project is close to the power plant, then (unless there's something very strange going on) normally you would connect the 2 ground grids. Normally you do this by (more or less) extending the existing GG into yours - in other words don't connect them at just one point, make a distributed interconnect and make sure it's done right - i.e. clean (esp. the existing GG!) metal (rough it then chemically etch it), bolt the grids together using suitable metals (NON voltaic!!!), then coat the connections with any number of commerically available underground paint/goops which prevent the connections corroding.

- However, BEFORE you connect them (which means before anything else is connected to your new substation), you (or a qualified contractor) should make some
measurements to see if there is any current flowing between the grids...not always possible due to the way construction proceeds but a very good idea. If there is more than say some volts and and amp or two then you should get with the power station electrician to figure out why. (normally it means your new grid is working and the old one isn't and then you have to think about what to do about it)

**In fact, saving the most appropriate point till last: it would in general be a good idea to get with the chief electrical engineer from the power plant before you get very far - he or she should have a lot of info about what you need (esp regarding local grounding conditions which are usually very important inputs to the design)

I'm experienced EMC & high voltage etc but I never actually designed a whole grid. Hopefully some other people will pick up on this and you'll get more 'hands-on' info. there are *certainly* simple calculators available. The inputs would typically comprise: ground conductivity, how much short-circuit current you expect to dump, lightning statistics for your area, physical area of the substation, etc.

Good luck & have fun!

I suggest you refer to IEEE 80.

It is the best way to design a substaiton grid.

Please note that a major point will be the phase to ground fault.

Regards

I don't mean to pry; but as an EE inside a power plant, I really must ask why are you designing a ground system for a substation especially near a power house if you are not exactly clear on what must be done? Secondly if you are unsure of what to do here; whom is going to stamp the plans when you are done?

the previous post has given you a good start with IEEE 80, but that is only about 20% of the story. If you are doing this in the US you must also pay attention to NFPA 70 NEC 250. You should seek help with properly sizing your NGR, transformer sizing, and over current protection. You are most certianly goning to need to review the environmental laws that deal with raptor protection and there is going to be a significant work with ground resistance testing; just to name a couple of things.

as I noted above why are you designing a substation if you are not perfectly clear on how? This kind of thing is usually handled by very competent consulting firms with hundreds of years of combined experience. Not to mention you are going to spend months working with the plant and the utility to make sure you have met their specifications.

I am not trying to be insulting here but the lives of everyone on that power line are riding on your design.

Hello There . Yes IEEE 80 is a good start . CYMGRD from CYME is a good software for designing the Grid . I know some versions of the software are avilable for under $3000. The key is to get the right fault levels to size the conductors and calculate the right grid current to calculate the step and touch potentials. I stress the word fault level because this is what will be your main Input to the design. Since your substation is near the powerplant , i assume your fault levels to be high. It will be tricky to keep your touch and step potentials low if the total area of your substation is small .Offcourse you will also need the ground resitivity measurements based on IEEE 81.

If I am designing the substation I will follow the the earth grid and earth pit design for the substation as per the guide lines given in IEEE Std 80-2000 .

I will also consider interpretation for IEEE Std 80-1986
Clause: 14.1 (IEEE Std 80-2000) where in IEEE Guide for Safety in AC Substation Grounding is given .

The acceptable level of ground resistance is usually about 1 ohm or less for a transmission substation and 1 to 5 ohms for a distribution substation is the normal practice adopted and I have designed ,constructed and maintained substations for the past 45 years in in very hot and dry weather tropical country where temp rises beyond 45to 50 degree centigrade .After maintaining the above values I never experienced any safety hazard in those substations. Separate earth pits and grid should be maintained for Body Earth , Neutral Earth, Transformer Neutral points,and LP ( Lightening Protection)system due to safety reasons .The size of the earth bus should be designed as per IEEE standards. Grounding is based on the safety criteria of acceptable touch and step potentials.

Sufficient watering funnel arrangement should be made to the earth pits to maintain the earth values as per required level during summer season. Earth Resistance value should be measured periodically and maintained with in limits since your substation is close to power plant which will have have very large fault current levels .