My guess is that you want to use a relatively small price on reactive power,
but I’m afraid yours is a research question without a quick and easy answer.
Ray
> On Sep 21, 2018, at 4:02 AM, Weber, Jens <[email protected]> wrote:
>
> Dear Ray,
>
> at first, thank you very much for your reply to my question,
>
> I know that there is no classic slack generator in OPF and I know that I
> could model my cost functions by using the polynomial form (I tried that also
> before).
> To make it clear what I‘m trying to do: I want to maximize reactive power
> output from all generators except by one which I called „slack generator“.
> The voltage for this generator is fixed at 1 p.u. It models the busbar at the
> substation.
>
> I want all (non "slack") generators to maximize their active power output at
> highest priority and then maximize their reactive power output.
> So i set the slope for my active power cost function higher then the slope
> for my reactive power cost function.
>
> For little cases it works well, but for bigger cases it doesn't work anymore.
> I also tried to fix the active power output from all (non "slack") generators
> to maximum by setting PMAX and PMIN in the gen matrix to the same maximum
> active power value.
> I tried to use IPOPT solver instead of MIPS solver.
> But as you said it seems that there are many local minima in the problem I’m
> trying to solve.
> What would be the right way to find the global minimum? As you said trying
> different starting points. But is there any possibility to setup the problem
> better so that finding the global minima is easier for the OPF?
>
> Thank you very much,
>
> Jens W.
>
>
> Von: [email protected]
> <mailto:[email protected]>
> <[email protected]
> <mailto:[email protected]>> Im Auftrag von Ray
> Zimmerman
> Gesendet: Donnerstag, 20. September 2018 18:45
> An: MATPOWER discussion forum <[email protected]
> <mailto:[email protected]>>
> Betreff: Re: maximize the reactive power output from all generator
>
> A few comments and questions …
>
> - The concept of a slack generator only applies to the power flow problem,
> where the output of all other generators is specified and the “slack
> generator” output takes on whatever value is need to satisfy the total power
> balance. In an OPF, there is no distinction among generators, as each is
> dispatched to minimize cost.
>
> - I assume the cost function you are defining is for the reactive power, but
> what are you using for the active power costs? In your case, there is no need
> to use a piece-wise linear gencost since your function is strictly linear.
> Simply use the polynomial form with the desired linear coefficient.
>
> Now to your questions …
>
> 1. If you have non-zero active power costs, then the marginal cost of
> reactive power (the slope) does matter, since it changes the weighting of the
> active vs. reactive power cost. If not, then it should not make a difference
> in the solution, except for a simply scaling the objective function value and
> nodal prices.
>
> 2. It’s not clear to me what solution you are attempting to find. Do you want
> to maximize total reactive power generated? Or maximize reactive power
> generated at all gens except one which you are calling the “slack generator”,
> which you want to minimize its reactive output?
>
> 3. I’m not quite sure without knowing the answer to some of my other
> questions. Especially in the absence of costs on active power, am not
> surprised if there are many local minima in the problem you are trying to
> solve, so different starting points may even result in different local
> solutions.
>
> Ray
>
>
>
> On Sep 19, 2018, at 8:24 AM, Weber, Jens <[email protected]
> <mailto:[email protected]>> wrote:
>
> Dear MATPOWER-Community, dear Mr. Zimmerman,
>
> I would like to maximize the reactive power output from all generators in the
> net, one time for maximum induktive reactive power and one time for maximum
> kapacitive reactive power.
> So I use the MATPOWER-OPF with a customized piecewise linear cost function
> which I define in the gen_cost matrix.
> For example if i would like to maximize the reactive power output from all
> non slack generators so that they behave like an inductivity I set the data
> points for my cost function for all non slack generators like this:
> (-100 | 1000), (0 | 0), (100 | -1000)
>
> My first question is:
> Does it make any difference in OPF-behavior if I change the slope oft the
> segments of the cost function or is it irrelevant?
> For example is there a difference with regard to stability or performance if
> I use this cost function instead of the one above:
> (-10 | 1000), (0 | 0), (10 | -1000)
> If there is any difference, how do i find the best slope for the segments of
> the cost function?
>
> My second question is:
> I can define an opposite cost function for the slack generator:
> (-100 | -1000), (0 | 0), (100 | 1000)
> Is it better to set this cost function also for the slack with regard to
> stability or performance or is it irrelevant?
> An other option would be to only set the cost function for the slack and set
> costs for all non slack generators to zero.
> Is it better with regard to stability or performance or is it irrelevant?
>
> My third question is:
> With a casefile with about 90 buses and about 10 generators it works great
> but with a casefile with about 2500 buses and about 300 generators it doesn’t
> work anymore.
> With the big casefile there are some generators where the reactive power
> output is not maximized, though there are no voltage violations or power flow
> limits.
> Whats the reason for this behavior and is there a possibility to make it work
> with a big casefile?
>
> Do you have any general tips for me or is there another method to improve
> stability or performance?
>
> Thank you very much!
>
> Best regards
> Jens W.
