08-12-2012, 02:24 PM
POWER FLOW ANALYSIS OF AC/HVDC SYSTEMS
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Modelling of HVDC systems
The representation of the dc systems requires consideration of the
following:
• Converter model
• DC transmission line/network model
• Interface between ac and dc systems
• DC system controls model
Representation of the converters is based on basic assumptions
Representation for power – flow solution
From the analysis presented earlier the converter equations may
be summarized as follows.
AC/DC interface at the HT bus cont………
Here Eacr and Eaci are considered to be input quantities for the
solution of dc system equations. They are known from the previous
step in ac solution.
Variables Pr ,Qr, Pi and Qi are considered to be the outputs from the
solution of the dc system equations. They are used in the next
iteration for solving the ac system equations. The dependent and
independent variables in the solution of DC equations depend on
rectifier and inverter control modes. The three possible modes of
operation are:
Mode 1: rectifier on CC control; inverter on CEA control
Mode 2: inverter on CC control; rectifier on CIA control
Mode 3: rectifier on CIA control; inverter on modified characteristic.
Power Flow Analysis in AC/DC Systems
The computations are further simplified using fast decoupled load
flow method in which the corrections to the bus voltage estimates
are found from solving the following equations:
ΔP/V = [B’] Δ ϴ
ΔQ/V = [B’’] ΔV
where ΔPi, ΔQi are mismatches of real and reactive powers at
bus i, Δϴ and ΔV are the correction vectors to bus angles and
voltages magnitudes. B’ and B’’ are constant matrices of
appropriate sizes and consist of elements that are related to the
reactances of the elements of the network.
While the modelling of DC systems for power flow is fairly
standard, the solution methodology varies. The sequential or
alternating method which does not require major changes in the
software available for the power flow analysis of AC systems, is
widely used.
Solution of DC Load Flow
• There are four basic variables per converter, Vd , Id , α(γ) and T.
• If the voltages at all the converters that form tree branches (in
addition to the conductances of the network) are specified, currents
at the remaining converters are specified, then it is possible to solve
for the remaining variables (voltages at the current controlled
converters and currents at the voltage controlled converters ).
• Once this is done ,the power factor is computed from the
appropriate equations.
• The power and reactive power at each converter station are then
obtained from the use of corresponding equations.
• The knowledge of the AC voltages allows the calculation of taps.