Abstract:
Power exchanges between utilities improve reliability and increase economical savings
by accessing cheaper generation resources, taking advantage of peak diversity and time
zone difference between interconnected systems and time difference of forced outages of
generating units of two systems, reducing the overall spinning reserve and minimizing
interruption cost. As such, interconnection between utilities has become a part of power
system planning.
Interconnection planning requires the evaluation of location and capacity of tie lines that
ensures optimum operating cost and optimum rate of return on investment. The problem
is combinatorial i.e. it is a problem of selecting a solution from among a finite set of
possible solutions, which precludes closed form solution. Current practices in
interconnection planning use transmission expansion planning techniques that do not
meet these requirements.
A methodology, first of its kind, is introduced for tie line planning between
interconnected systems. The methodology is a heuristic one, using a full nonlinear power
system model. A cost function is used to evaluate the performance, during the planning
period of different tie line candidates. Instead of a single peak load, correlated demand is
used as the load model. This decomposes the planning problem into a number of smaller
optimization problems that are solved using a full ac optimal power flow (OPF)
technique. At first, at every load level, the OPF is used to search for local minima and to
evaluate the optimized capacity and location for a tie line. The cost function includes
both the tie line cost and the energy production cost for a particular load level. The energy production cost for the whole planning period is evaluated in the next stage
considering the evaluated optimum tie -line obtained for a -particular load level.
Appropriate penalty function is introduced to evaluate penalty cost, which calculates the
cost that would be incurred if the tie capacity determined at a load level is over or under
sized with respect to the optimum tie capacity obtained for other load levels. This penalty
cost is also added to the total cost. Proceeding in the same way the total cost for the
whole planning horizon for all optimum tie capacities for all load levels are evaluated.
Comparing these costs the tie set with the minimum cost is selected.
The proposed methodology is applied to the IEEE Two Area Reliability Test System
1996 and Bangladesh Power System. The results obtained using the proposed
methodology is compared with those obtained using the conventional transmission line
planning approach. The results obtained using the proposed methodology with multilevel
load model is also compared with those obtained using single peak load model.
