Shigemi Harada

A new formulation for FACTS allocation for security enhancement against voltage collapse

This paper proposes a new formulation for var planning problems, including the allocation of FACTS devices. A new feature of the formulation lies in the treatment of security issues. Different from existing formulations, we directly take account of the expected cost for voltage collapse and corrective controls, in which the control effects by the devices to be installed are evaluated together with the other controls such as load shedding in contingencies to compute an optimal var planning. The inclusion of load shedding into the formulation guarantees the feasibility of the problem. The optimal allocation by the proposed method implies that the investment is optimized, taking into account its effects on security in terms of the cost for power system operation under possible events occurring probabilistically. The problem is formulated as a mixed integer nonlinear programming problem of a large dimension. The Benders decomposition technique is tested where the original problem is decomposed into multiple subproblems. The numerical examinations are carried out using AEP-14 bus system to demonstrate the effectiveness of the proposed method.

A method to approximate a closest loadability limit using multiple load flow solutions

A new method is proposed to approximate a closest loadability limit (CLL), or closest saddle node bifurcation point for power systems, using a pair of multiple load flow solutions. More strictly, the obtainable points by the method are the stationary points including not only CLL but also farthest and saddle points. An operating solution and a low voltage load flow solution are used to efficiently estimate the node injections at a CLL as well as the left and right eigenvectors corresponding to the zero eigenvalue of the load flow Jacobian. They can be used in monitoring loadability margin, in identification of weak spots in a power system and in the examination of an optimal control against voltage collapse. Most of the computation time of the proposed method is taken in calculating the load flow solution pair. The remaining computation time is less than that of an ordinary load flow.

A method of voltage stability evaluation for branch and generator outage contingencies

This paper proposes a fast computation method to evaluate the load power margins with respect to saddle node bifurcations under branch outage as well as generator outage contingencies. The voltage stability evaluation for generator outages is a new topic, while there exist various studies for the branch outage contingencies. The proposed method is based on the conventional linear sensitivity method. The linear estimations are improved by taking into account nonlinearities of the power flow equations and by using a fast line search technique. Compared with existing methods, the accuracy is considerably improved. The effectiveness of the proposed method is demonstrated through IEEE 14-, 30-, and 57-bus systems. The estimation errors in load power margins are not more than 5% for generator outage and 15% for all branch outage (route open outage) contingencies.