As Shiyang says, yes, it's ok to change the generator dispatches. In fact, as the load changes throughout the day, you will have to re-dispatch your generators to follow the load. There are many ways to re-dispatch the generators, and each option results in a different set of line flows and a different set of bus voltages. Simply re-running the PF without changing inputs means that the slack bus handles all of the redispatch to follow load (even if it has to absorb, rather than generate power) … probably not what you intend.
My suggestion, if you want to have something that approximates typical operation, would be to choose some reasonable costs for the generators and then use runopf() to compute the dispatch based on minimizing generation cost. -- Ray Zimmerman Senior Research Associate 419A Warren Hall, Cornell University, Ithaca, NY 14853 phone: (607) 255-9645 On Mar 13, 2013, at 12:48 AM, [email protected] wrote: > Hi Difei, > (1). See (2). > (2). Yes, it is. > (3). Why not? It's authorized. But I think your question is on dispatching > when loads increase, not the base case defined in case file. > > Shiyang Li > > > ----- 原始邮件 ----- > 发件人：TANG Difei <[email protected]> > 收件人：MATPOWER discussion forum <[email protected]> > 主题：new question about ref bus Pg when reduce and increase load > 日期：2013年03月13日 11点16分 > > Dear all, > > Thank you for Dr. Zimmerman and Li Shiyang 's help about my previous > question. I appreciate it. > > Here is a new question about ref bus Pg reduce and increase load. > I use runpf function for the power flow calculation for variable load in 24 > hours. My aims is to see how branch flow and bus voltage change when the bus > load change. So, I revised the bus load data to classify the buses into 3 > types of load: residential, industrial and commercial, which means the load > at each bus is different from the original case data. Please be noted that > the load is changed but the generator output (Pg) is not changed. > Then, the problem comes. > Although all 24 PF calculations converged successfully, the Pg of ref bus is > negative when the system load is low in the morning, while the Pg of ref bus > is very high (much higher than other gen) when the system load is high in the > evening. This result can not reflect the real world power system operation. I > know the problem is from the Pg of ref bus must archive system power balance. > (1)So, I want to know, how can I see the branch flow and bus voltage change > when all the generator Pg is between PMIN and PMAX? > (2)I refer to iman's thread about limits on the branch. > http://www.mail-archive.com/[email protected]/msg01475.html > Is it a good solution that I change the other generator Pg so the ref > bus Pg is kept in a reasonable scale? > (3) Is that OK if I change the original case data (bus load) to my own data? > > Thanks in advance. > > Difei > > > 发件人： Ray Zimmerman <[email protected]> > 收件人： MATPOWER Discussion List <[email protected]> > 发送日期： 2013年3月1日, 星期五, 5:04 上午 > 主题: Re: slack bus real power output (PG) is negative? > > (1) It depends what you mean by reasonable. Yes, this a perfectly reasonable > (i.e. correct) answer to this power flow problem. No, it is not reasonable to > operate this power system at this set-point, other generator set-points > should be reduced to make it feasible. > > (2) Again, it depends what you mean. What the result says, is that given that > network, under that loading condition, with those generator set-points, you > would need about 3 MW of load at that bus to achieve power balance. > > (3) You should not expect flows to increase uniformly throughout the system > as you scale loads at all buses and generation only at the slack bus. Some > may increase, some may decrease, some may even switch direction. > > -- > Ray Zimmerman > Senior Research Associate > 419A Warren Hall, Cornell University, Ithaca, NY 14853 > phone: (607) 255-9645 > > > > > On Feb 28, 2013, at 10:02 AM, TANG Difei <[email protected]> wrote: > >> Dear all, >> >> I ran 'results=runpf('case24_ieee_rts')' and found that >> results.gen(12,2)==-2.9536 MW, which means the real power slack bus (Bus 13) >> is a negative value. >> (1) Is that result reasonable? >> (2) Can the slack bus with negative PG be looked as a load? >> By checking the previous thread, I knew that PG of slack bus = all loads + >> all losses - all other generator real power outputs >> >> In my opinion, it is not reasonable. For example, I used function >> 'scale_load' to scale all the real and reactive loads from 10% to 100% of >> original value (I did not scale the generator output), by using >> >> mpc = loadcase('case24_ieee_rts'); >> for i = 1 : 10 >> multiple = i/10; >> mpc_m(i) = mpc; >> mpc_m(i).bus = scale_load(multiple, mpc_m(i).bus); >> [results_m(i), success_m(i)] = runpf(mpc_m(i)); >> end >> >> Then, I found that, for example, in the scenario of the multiple value from >> 0.1 to 1, the real power injected at "to" bus end (PT) of Branch 1 is >> reduced from 25.2679 MW to 11.9363 MW, and the PG of slack increase from >> -2322 MW to -2.9536 MW , which is not reasonable, because with the loads >> increase from 0.1 to 1, the power of each branch will increase too. >> (3) Am I correct? >> >> Any advice from anyone will be very much appreciated. >> >> >> Best wishes, >> Difei Tang > > >
