Teruo Ohno

Derivation of Theoretical Formulas of the Frequency Component Contained in the Overvoltage Related to Long EHV Cables

Recent studies of long extremely high-voltage (EHV) cables show the importance of performing temporary overvoltage analyses. As the switching of EHV cables can trigger temporary overvoltages, it is important to find the dominant frequency component contained in the switching overvoltages of these cables. Since there are no theoretical formulas to find the dominant frequency, it is generally found by means of time domain simulations or frequency scans. The derivation of theoretical formulas has been desired as the formulas would be useful in verifying the results of time-domain simulations or frequency scans. In addition, the formulas could eliminate the necessity of building simulation models of some network components. In order to address this need, this paper derives theoretical formulas to find the dominant frequency in long cable energization overvoltages. The accuracy and practical usefulness of the formulas are confirmed by comparing them with the results of time-domain simulations.

Statistical Distribution of Energization Overvoltages of EHV Cables

Statistical distributions of switching overvoltages have been used for decades for the determination of insulation levels of extremely high voltage (EHV) systems. Existing statistical distributions obtained in the 1970s are for overhead lines, and statistical distributions of switching overvoltages of EHV cables are not available to date. This paper derives the statistical distribution of energization overvoltages of EHV cables. Through the comparison of the statistical distributions of EHV cables and overhead lines, it has been found that line energization overvoltages on the cables are lower than those on the overhead lines with respect to maximum, 2%, and mean values. As the minimum value is almost at the same level, standard deviations are smaller for the cables. The obtained statistical distributions in this paper are of a great importance in considering insulation levels of cable systems.

Derivation of Theoretical Formulas of Sequence Currents on Underground Cable Systems

A reliable operation of a cable system necessitates an accurate calculation of sequence impedances of the system. It is recently even more important as longer cable lines are being constructed and planned. It has been a common practice that these sequence impedances or currents are measured after the installation and it is difficult to predict these values beforehand with good accuracy. This paper derives theoretical formulas of the sequence currents for a cross-bonded cable and a normal- bonded cable. The formulas give an important advantage in setting up transient overvoltage studies as well as planning studies. The accuracy of the proposed formulas is verified through a comparison with EMTP simulations.

Dynamic behavior of photovoltaic single-phase inverter following faults in medium-voltage networks using CRIEPI's power system simulator

The reverse power flow at distribution transformers caused by the large-scale integration of residential photovoltaics (PVs) might prevent the clearance of high-voltage transmission system faults within the designated time. The time to stoppage of PVs following short-circuit faults and the type of protective relays to stop PVs following single-line-to-ground faults were studied with a 66kV-emulated 1650V transmission line of the test system to examine the dynamic behavior of PVs following faults. All PVs stopped within two seconds following faults. The test results showed that the anti-islanding protection relays operated first when the pre-fault power flow in the 1650V transmission line was nearly zero, while the fault clearance protective relays led by the instantaneous overvoltage relay operated first when the pre-fault power flow in the same line was apart from zero. The maximum PV current was 130% of its rated current.

A study on dynamic behavior of coal-fired thermal power plant during significant system frequency rise after system separation

Power plants are increasingly located far from demand centers in Japan due to siting difficulties. As a result, these power plants are connected to a weak part of the network where severe contingencies could separate them from the main grid. This is a particular concern for coal-fired thermal power plants because of their larger footprints and environmental impact. Coal-fired thermal power plants, which have a relatively slow frequency response, can account for a majority of generation in an islanded system. In order to establish countermeasures for this problem and to maintain stable operation of the islanded system, the authors have developed models of coal-fired thermal power plants for system studies. This paper describes important findings obtained from analyses using the developed models, such as the necessity of modeling the power plant control system including main steam pressure dynamics which can limit the governor response to bring back frequency.