# What is a slack bus?



## ElecPwrPEOct11 (Oct 12, 2011)

This comes from CI Variation 1 #49. It states that in a power flow study a slack bus typically doesn't have a fixed voltage angle or magnitude.

I haven't come across this term in any of my review books. Can anyone explain it? Have I missed a major topic in my review work and now need to panic? Thanks for any help.

I found a humorous description here but it didn't help a ton.


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## mudpuppy (Oct 12, 2011)

http://en.wikipedia.org/wiki/Power_flow_study


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## ElecPwrPEOct11 (Oct 12, 2011)

mudpuppy said:


> http://en.wikipedia....ower_flow_study


Thanks, I had found that. I was hoping for a more common-sensical explanation, especially as it pertains to the PE.


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## R2KBA (Oct 12, 2011)

I don't know if this will help you, but I do lots of studies with software at work. I'll explain how I understand it. It is not really a bus, but it is a generator or utility source. Every system has to have at least one slack bus (we call it swing bus) which does just like it sounds...it picks up the "slack" that the controlled generators don't supply, or it absorbs the extra, if any. For example, If a system has a demand of a certain number of MW and MVAR, and you choose set your controlled generator(s) to only output half of that, the slack bus will automatically adjust to make up the difference by outputting the other half. The study will not run unless there are enough Watts and Vars available and if you force too much the slack bus will absorb either both. There are two non-slack bus generators I know of. One you can manually set exactly how many Watts and Vars are produced, and the other type you can manually set exactly how many watts, and you set a max/min for Vars which the generator will automatically adjust. Also, on the slack bus generator you can specify the voltage setpoint in p.u., but you cannot tell it to output a specified number of Watts or Vars.

All this applies to the specific software I use, so it may not apply in general or make sense to everyone.


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## Wolverine (Oct 12, 2011)

MP, you are cruel for posting that!

I think the restaurant explanation is a pretty good one. Modern day power flow studies are vast multi-variable systems solved by computer in a series of iterations that invariably leaves some remainder at the end that can only be solved with one more iteration. This is true at any given point in time. The slack bus operates as just that - it takes up the "slack" - so the model will solve, or at least solve a little faster once it gets close enough.

It's sort of like the plot to Matrix II if you think about it.


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## mudpuppy (Oct 12, 2011)

Yeah, I'm bad. . . it's been so long since I've done a power flow that I'm a bit rusty on the concept, glad you guys were able to chime in.

One thing I might add, correct me if I'm wrong. . . the iterations of a power flow solution continue until the swing bus P &amp; Q values fall below a set point, right? To minimize the error.


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## Electroizol.Ru (Oct 12, 2011)

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## Flyer_PE (Oct 12, 2011)

Electroizol.Ru said:


> Электроизол купить в Армавир (Краснодарский край) Лента ЛСКН-100-ТТ, ЛСКН-135-СПл, ЛСКН-160-ТТ, ЛСКО-180-Т поставки во все регионы России


Hell, I can barely handle English.


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## Wolverine (Oct 13, 2011)

mudpuppy said:


> One thing I might add, correct me if I'm wrong. . . the iterations of a power flow solution continue until the swing bus P &amp; Q values fall below a set point, right? To minimize the error.


That's correct. If you run enough iterations, the error approaches zero, but that level of accuracy is time consuming and unproductive when you can just let the slack bus take it up when it gets close enough to whatever set point you determine.


Electroizol.Ru said:


> Электроизол купить в Армавир (Краснодарский край) Лента ЛСКН-100-ТТ, ЛСКН-135-СПл, ЛСКН-160-ТТ, ЛСКО-180-Т поставки во все регионы России


If that one quarter of Russian I took in the last century is good for anything, I believe Electroeezal bought Armavir in the Krasnodar Territory and you can get in on that deal now, with deliveries to all areas of Russia. Something like that.


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## ElecPwrPEOct11 (Oct 13, 2011)

Great, thanks to all for the help. Between the restaurant analogy and the response by R2KBA I feel pretty good about it.

As I have never performed a power flow study for work, are there other essentials that I should be familiar with for the test?


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