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d3j4vu

Mon Oct 27 2008, 08:25AM

Registered Member #1782 Joined: Mon Oct 27 2008, 08:03AM
Location:
Posts: 4

Hi all,

I was wondering whether anyone has any suggestion on how to go about finding the remaining capacity of a substation. The substation capacity is 2MVA. There's a big project coming up and I want to know whether there's still capacity to run motors from that substation or not.

What I have done:
1. Went and got all kW ratings of motors, fans currently running from that sub. But since they are full load rating, the power factor (=P/S) that I got exceeded 1.
2. Asked the power supplier what the maximum demand for that particular power station. But, they only have 1-month worth of data. (Was 1.2MW when I asked)

Any suggestions?

------------------------
I am a young engineer in training, your patience is much appreciated

HV Enthusiast

Mon Oct 27 2008, 10:44AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Again with the cryptic postings. I have to be honest with everyone here, this is one of my biggest pet peeves.

People ask such open ended questions like "whats the best way to generate 1000A of current) and say its for a project, but don't even bother to describe what the project is.

If you want people to help you with your response, at least show some courtesy and write some background information about your project so members here can better appreciate what you are doing. The more they appreciate your project, the more apt they will be in helping you with it.

Also, if you are working on a project where you feel the remaining capacity of a substation may be an *actual* issue, then you don't want to be asking some forum on the internet about it. You *need* to contact your local utility company and discuss this with them.

Steve Conner

Mon Oct 27 2008, 11:00AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Oh, I get it, this is a question about diversity. So go read up on diversity:

and ask the other engineers at your plant about it too.

d3j4vu

Mon Oct 27 2008, 10:15PM

Registered Member #1782 Joined: Mon Oct 27 2008, 08:03AM
Location:
Posts: 4

Steve McConner: Thanks for that, that is exactly what I was trying to ask about. All connected loads are not operating simultaneously. Therefore, I can't just sum up all loads installed and assume that it is running as such.

Dr. GigaVolt: Are you saying that there is no way to calculate how much capacity I have left from knowing all connected loads? Like using a rule of thumb? How about monitoring? Other than asking the utility company, is there anything else that I can do?

HV Enthusiast

Mon Oct 27 2008, 10:29PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

d3j4vu wrote ...

Dr. GigaVolt: Are you saying that there is no way to calculate how much capacity I have left from knowing all connected loads? Like using a rule of thumb? How about monitoring? Other than asking the utility company, is there anything else that I can do?

Firstly, your assuming you know everything there is to know about the subsystem. Your question is like asking, "how much remaining capacity will my power supply have if i connect a 10A load to it", without telling us all the specifications of the power supply, what loads are already present, and so forth . . .

The only way to really find out is to discuss this with the utility company, and i'm almost 100% sure they aren't going to tell you anything, especially in this day of age, unless you were some large corporation or business wanting to connect to it for power.

rp181

Mon Oct 27 2008, 11:57PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

if if this puerly educational, im pretty sure they will give you some information, if you have a good reason.

Mathias

Tue Oct 28 2008, 12:00AM

Registered Member #1381 Joined: Fri Mar 07 2008, 05:24PM
Location: Hungary
Posts: 74

I would say that a monthly average data can be "useful" for determinating the actual "avarage load" of the system.
The power factor can change drasticly at times (and there for the reactive power would raise , substation side so power companys are trying hard to regulate it) if lets say some guy would operate a couple of concrete mixers for long hours. But you can't asume that he will do that every day of the month. But if he would keep this up for long then there would be a need to correct the power factor for the substation.
The "best" sollution would be if the individual would be useing pfc caps when he's running his mixers.

d3j4vu

Tue Oct 28 2008, 08:20AM

Registered Member #1782 Joined: Mon Oct 27 2008, 08:03AM
Location:
Posts: 4

I am from big corp, so when I asked the utility company, they did give me an answer which was 1.2MW. But, that figure was only the average of "the peak of electricity flow into the site during half-hour periods" in the period of a month. I am not sure whether it is an accurate enough depiction of the load, since I do not know what ran and what did not during the month.

Should I actually get a monitoring device and ask the operators to run absolutely everything that is fed from the substation? I think this is also pointless, because it still is not showing your normal operating condition.

Dr. GigaVolt: I would gladly provide more information if it helps you in helping me.

Steve Conner

Tue Oct 28 2008, 10:23AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I'd say that 1.2MW figure is fairly useful. Maybe you could ask them to monitor for another month or two while you get on with the project. Also ask them for the peak of the peak, rather than the average of the peak.

Now for a little gentle math, if the substation capacity is 2MVA, and it's loaded to 1.2MW at 0.8PF, then 1.2/0.8= 1.5MVA is already in use, and you have another 500kVA to play with.

But you just learnt about diversity. The diversity factor for your loads is 1.2MW divided by the sum of all those kW ratings. So if the new loads you're adding are the same kind of loads as what's there already, then you can bolt on (500kVA/diversity factor) worth.

For example, if all those ratings added up to 4MW, then the diversity factor is 1.2/4 = 0.3 and you can add extra plant rated at 0.5/0.3 = 1.67MW full load.

Note that you should do your homework to validate the assumptions on diversity. If you have other information telling you the new load runs 24/7 at full load, then it can only be 500kVA. Also note that I've used W and VA interchangeably assuming all the plant has PF=0.8.

You also need to check out reactances and voltage drops: adding a big motor and starting it direct on-line can cause serious volt-drops that brown out other plant, even though the full-load power is within limits.

Also, this information is provided without warranty.

Dr. Slack

Tue Oct 28 2008, 02:20PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Diversity theory says that the power supply required to power the installed loads will rarely be equal to the sum of rated loads, and will usually be a lot less. But this is a statistical cost/benefit tradeoff. It's easy to quantify the benefits (2MVA stastion rather than 4MVA to supply the district), but what are the costs, and the likelyhood of incurring those costs, of exceeding the 1:thousand or 1:million chance of exceeding the supply?

So 1 month's figures from a monitor may give you an idea of the CCDF (Google if don't know) of the power for that month. From that you can estimate the level not exceeded more than 0.01% of the time (or some other threshhold that the power company prefer). If the power consumption comes from a large number of independant random processes, then you can be confident using high school probability to extrapolate that data. However, I'll bet the processes are neither random nor independant. I'll bet a lot of motors get turned on at the same time in the factory at clocking on time. That month is not going to include peak HVAC time in summer, nor heating time in winter. Statisitically the power demand is not an ergodic process, and no model is going to help anybody determine the 0.01% power level, or the peak.

What is the cost of drawing more power than the station's rated power? Does it overload gracefully with a voltage reduction, or hard by cutting supply. Are they temperature limited so you can actually peak to high values and it's only the 5 minute (or whatever) average that matters? Do you have the inconvenience of losing your supply, or do you get sued by other users for taking theirs down as well? Can you sue the other users if they take you out by going over their 1.2MVA?

This is not really an electrical problem, this is a contractual risk management problem. What is the relationship between the operators of the substation and you?

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