On May 5, 2008, at 12:28 PM, [email protected] wrote:
I have some confusion about the output of the generator's
reactive power
Qg. In the power flow calculation with Matpower, if I change the Qg,
for
example in case30 Qg2(the 2nd column,the 3rd row of the gen Matrix),
it is
supposed that the results will change. But they don't. I think the
reason is
that in the calculation we do not calculate ÄQ2,because bus 2 is a
PV bus, so
we just calculate ÄV2. So the changes of Qg will not change the
results. Am I
right?
I think you mean row 2, column 3 of the gen matrix, right? You are
correct that it is a PV bus, so P and V are specified (in columns 2
and 6 of the gen matrix) and Qg and the voltage angle are computed by
the PF. So changing the initial value of Qg or the voltage angle will
not change the solution. Changing Pg or V will.
But with the same reason,if in the Optimal Power Flow
calculation,then the
change of these values of Qg will also not influence the results.
And then it
is meaningless to use Qg as control variables. It obviously seems to
be
contradictory. How to explain it?
In the PF, there is a specific value of Qg2 that will satisfy the
power flow equations *given* that their is a subset of all of the
voltages, angles and real and reactive injections that are pre-
determined and fixed (P & Q for load buses, P & V for generator buses,
V and theta for the reference bus). In the OPF, many of these
variables have been freed up, so there are many possible values of Qg2
can satisfy the power flow equations *if* the proper tradeoffs are
made in other variables. In particular, an increase in Qg2 will likely
require an increase in V2 as well. If V2 is at a limit, it will
probably require some other redispatch of generation. All of these
changes have cost implications, so the OPF makes all the appropriate
tradeoffs to find the optimal solution that satisfies the power flow
equations (and the other constraints) while minimizing cost.
Hope this helps,
--
Ray Zimmerman
Senior Research Associate
428-B Phillips Hall, Cornell University, Ithaca, NY 14853
phone: (607) 255-9645
