27-02-2012, 04:22 PM
Usefulness of DC Power Flow for Active Power Flow Analysis
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I. INTRODUCTION
STATIC power system analysis has always been
performed using full power flow. It is one of the
fundamental tools for power system analysis and is used in
the operational as well as planning stages. Vertically
integrated companies have used it to control their systems, as
well as to plan the optimal economic operation of generation
resources, either by means of optimal power flow or unit
commitment. It is therefore extremely important to solve the
load flow problem as efficiently as possible. Since the
invention and widespread of computers, in the 1950’s and
1960’s, many methods for solving the load flow problem have
been developed [1],[2].
II. FORMULATION OF DC POWER FLOW
The classic power flow problem consists of active and
reactive power flow and it can be formulated using four
variables per each node - voltage angle, voltage magnitude,
active and reactive power injections. Active power losses are
not known in advance as they depend on active power
injection pattern and voltage profile. Other variables are also
interdependent, which makes the problem non-linear. This is
why it is often made linear and the solution is iterated. The
losses are re-estimated at each iteration based on all other
variables.
III. ASSUMPTIONS OF DC LOADFLOW
In order to simplify the power flow problem and make it
linear, a number of assumptions are made:
Voltage angle differences are small i.e. sin(δ) = δ
Line resistance is negligible i.e. R<<X, thus lossless lines
Flat voltage profile
However, such assumptions are not always realistic. Firstly,
the X/R ratio condition can be difficult to guarantee. The
influence of resistance increases with the decrease of voltage,
which means that only the high voltage transport networks can
withstand this condition. Moreover, voltages will most likely
not be flat but will vary among busses, causing the voltage
profile to be different from the assumed one. Each of these
assumptions has some influence on the accuracy of the power
flow calculations.
CONCLUSIONS
This paper has examined DC power flow as a power system
analysis tool. The method is increasingly used for technoeconomic
studies, related to electricity markets. The authors
identified indexes that quantify the assumptions underlying
the method. There are number of factors affecting the
accuracy of DC power flow. First of all, the voltage profile
has to be as flat as possible, meaning that there should be as
little voltage deviations as possible.