Teruhisa Kumano

Fast monitoring and optimal preventive control of voltage instability

Abstract This paper discusses a new method for monitoring and controlling to prevent voltage instability in electrical power systems. Voltage instability phenomena or voltage collapse may occur at the critical loadflow point in a static sense. From the system operators viewpoint, this critical point must be carefully monitored in a heavily loaded power system. In addition, the operators need to take an adequate control action when the current operating state approaches the critical point. These processes require very fast computation. Hence special attention is paid to the required computing time and on-line applicability. Monitoring methodology proposed here is based upon the multiple loadflow solutions and sensitivity analysis. The complex-valued multiple loadflow calculation method, which has already been developed by the authors, is free from the numerical instability in calculating the lower voltage solution. By using this algorithm, the operating personnel can monitor the lower voltage loadflow solution corresponding to the current operating condition. The critical point is calculated approximately using these two loadflow solutions, and the load forecast very quickly for on-line usage. The estimation error of the critical point becomes very small as the operating state becomes closer to the critical point. The existing preventive control against voltage collapse is based on the operators experience, not on a theoretical basis. The preventive control proposed here gives the optimal operations of various kinds of control equipment in the sense that the total demand margin is enlarged most effectively. Control variables consist of voltage/active power setting values at generator terminals, switched capacitors, and tap ratios of OLTCs (on load tap chargers). The resultant control action satisfies all the constraints about bus voltages, power flows, etc. Its effectiveness is demonstrated by using several numerical examples.

Nonlinear stability indexes of power swing oscillation using normal form analysis

We propose stability indexes of power swing oscillation taking nonlinearity into consideration using normal form analysis. For a relatively large oscillation to which eigenanalysis cannot be applied, we approximately describe the effect of nonlinearity up to the third order in the form of the change in oscillation characteristics (damping factor and period) with respect to the change in amplitude. Consequently, we can obtain approximated stability regions of oscillation modes in state space. It is also possible to apply the proposed indexes under auto-parametric resonance condition, when a ratio of eigenvalues is close to a simple whole number. We demonstrate the effectiveness of the proposed indexes by comparing with time-domain simulation results in longitudinal power system models.

Observation of photo-stimulated detrapping currents of FEP teflon electrets

Photo-Stimulated Detrapping Currents (PSDCs) of FEP-teflon electrets charged by corona at 150°C were observed to understand the charge storage mechanism. Monochromatic light of a deuterium lamp (4–6.5 eV) or a mercury lamp (4.9 eV) was irradiated to the sample electrets at a room temperature. A threshold energy of light to generate PSDCs was found to be 4.9 eV. The experimental results indicate that there exists only single trapping level (4.9 eV) to be activated by a deuterium lamp. However, at a higher temperature, the another photo-sensitive detrapping current was also observed by the irradiation of the same light.