Akihiro Ametani

Lightning-Induced Voltage Over Lossy Ground by a Hybrid Electromagnetic Circuit Model Method With Cooray–Rubinstein Formula

This paper presents the calculation of lightning-induced voltages over lossy ground, generated by a lightning strike to a flat ground and a tall object. A hybrid electromagnetic circuit model method adopting an approximation formula, i.e., Cooray-Rubinstein expression, is employed in this paper. A comparison of the simulation results with experimental data shows that the Cooray-Rubinstein formula is still good enough for the calculation of a lightning-induced voltage. Electric fields calculated by the proposed method, and by equations derived by Master and Uman and a conventional dipole technique, are compared. The two approaches yield the same total electrical fields but different components of electrostatic, induction and radiation.

An Investigation of Earth-Return Impedance Between Overhead and Underground Conductors and Its Approximation

This paper investigates numerical instability issues in the mutual impedance formula between an overhead conductor and a buried conductor proposed by Pollaczek with special reference to the integrand of Pollaczeks infinite integral. The integrand shows a negative real part and positive imaginary part at a high frequency, which result in negative resistance and inductance. The reason for this is the assumption of TEM-mode propagation in Pollaczeks formula as is well known. The fact suggests that numerical instability of the infinite integral is caused by incorrect integrand in a high-frequency region. Based on the observations, the applicable limit of Pollacezks formula is analytically derived. Finally, a simple closed-form formula is developed, which approximates Pollaczeks formula with satisfactory accuracy.

Automatic equivalent-circuit estimation system for lithium-ion battery

An automatic equivalent-circuit parameter-estimation system for a Lithium-ion battery including battery temperature controller is developed in this paper. The system consists of a thermostatic chamber and estimating programs for parameters of an internal voltage and of an internal impedance, which is expressed by two resistors and a capacitor. These resistances increases by more than 60% and above, while the temperature decreases from 20 to 0degC, and the capacitance becomes about 80% of that at 20degC. The proposed equivalent circuit reproduces the transient characteristics including the temperature dependent effect. An estimating method of SOC (State of Charge) using instantaneous terminal voltage and current of the battery is proposed in this paper. The internal voltage is a function of the SOC and it can be evaluated by the equivalent circuit. The system is applicable to the other types of secondary batteries. The proposed numerical model and its parameters including the temperature dependent effect is useful for simulations of the apparatuses, and for an estimation of SOC without any integration of a battery current.

A control method of charging and discharging lithium-ion battery to prolong its lifetime in power compensator for DC railway system

Charging and discharging methods of lithium-ion battery installed in a power compensator for a DC railway system are developed in this paper. The power compensator has been developed to reduce its line voltage fluctuation caused by powering and braking trains. A number of charges and discharges deteriorates the lithium-ion battery and reduces the efficacy of the compensator. The proposed methods reduce the number of charges and discharges for prolonging the battery life by an insertion of a lowpass filter to the control circuit. The control method with the filter reduces the number of charging and discharging by 34%. Although the efficacy of the line voltage stabilization to rapid voltage change is decreased by the method, a parallel operation with an EDLC unit, whose capacity of 3% to that of the battery, complements the stabilization. The operations of the proposed methods are confirmed by numerical simulations using the electromagnetic transients program (EMTP) and by experiments using a scaled model.

An optimal operating point control of lithium-ion battery in a power compensator for DC railway system

An optimal operating point control method of lithium-ion battery for a power compensator of a DC railway system is developed in this paper. The compensator consists of a DC/DC converter and a lithium-ion battery bank. The regenerative energy is stored to the battery, and the energy is released to powering trains. The charging and discharging currents are determined according to the feeding-line voltage. The stored energy is maximized by controlling the maximum discharging current according to the state-of-charge (SOC) and the internal resistance of the battery. The maximum charging current of the DC/DC converter is identical to the maximum current of the battery, for improving the efficiency of the train system. Numerical simulations are carried out to confirm the proposed maximum power point tracking (MPPT) method. An experiment is also carried out to prove the effectiveness of the proposed method. Both results show that the proposed method improves the stored energy by 20% compared with that of a conventional system.