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This paper reports simple and effective control logic for connecting abrupt, steep and short-term power sources (for example, regenerative power from electric railway vehicles) to distribution lines with renewable energy units connected. With conventional centralized control systems, power conditioner units were unable to keep voltage of the distribution line within an acceptable range when extremely steep regenerative power flowed backward in the line, due to control delay. This new logic has power conditioner units exchange calculated control amounts between themselves repeatedly in fixed intervals. When voltage of the distribution line rises relatively slowly from renewable energy generators such as PV, the power conditioner unit can suppress the voltage rise immediately with no control delay, if regenerative power flows backward to the lines and voltage departure from an acceptable range is foreseen. This logic needs no central control unit in particular, as its power conditioner units can effectively execute and coordinate control between themselves. We verified the effectiveness of this logic through an experiment at our demonstration facility.

This paper investigates the economic benefits of multi-terminal VSC HVDC systems where large scale wind farms are installed. The comparison between multi-terminal VSC HVDC systems and point-to-point VSC HVDC systems in economic aspects is the main topic. Security-constrained optimal power flow analysis is carried out considering the N-1 criterion in mixed AC / VSC HVDC power system models in Japan. Payback period index is adopted for the economic evaluations. A new idea of both wind power and AC grid power transmission through multi-terminal systems is suggested. The initial investment costs of the multi-terminal VSC HVDC systems can be paid back earlier than those of the point-to-point VSC HVDC systems due to higher system capacity factor.

This paper proposes a centralized voltage control system for distribution system to meet the impact of PV growth. The system consists of a server machine at control center and many field devices like voltage regulators, line switches with sensor to measure power flow and voltage, and Information and Communication Technologies (ICT) network under Smart Grid paradigm. The system gathers measurement values from switches through the ICT network and chooses the best voltage profile and voltage regulators condition to achieve the profile with use of our advanced Optimal Power Flow. The test results at our Smart Grid facilities with 4MW photovoltaic generators are also given to prove the effectiveness of the developed system.

Dispersed generators such as wind power systems, photovoltaic systems, and cogeneration systems are expected to mitigate the environmental burden of energy consumption, and their installation has been promoted recently. Micro-Grid is focused on as a method to solve some problems in a commercial electric power line when installing a large number of dispersed generators, and some demonstrative research on Micro-Grid for large-scale systems is being carried out now. Also, small cogeneration systems for houses, such as gas engines and fuel cells, are expected to improve CO2 emissions. However, if the power and heat demand of a family are relatively small or are unbalanced, the cogeneration system does not operate effectively. The authors have studied the application of Micro-Grid for home energy supply, and have developed a control system to solve this problem. The system achieves a reduction of CO2 emissions and energy costs by sharing electric power and heat among some houses with cogeneration systems. This paper presents an outline of the newly developed system, and in particular describes the effect of the reduction in CO2 emissions compared with a conventional energy supply method, and the case in which dispersed generators are installed in some houses and operate independently. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(3): 19–27, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20879

To meet the global warming, renewable energy sources like wind, solar and biomass generations are dramatically increasing. Cogeneration systems are also ever-growing to save consumers' energy costs among factories, buildings and homes where lots of thermal loads are expected. According to these dispersed generators growth, their negative impacts to commercial power systems quality become non-negligible, because their unstable output causes network voltage and frequency fluctuation. Micro-grid technology comes to the front to solve the problem and many demonstrative field tests are now going all over the world. This paper presents the control paradigm and its application to Hachinohe micro-gird project, especially focusing on the power quality at isolated operation on which strict condition is imposed.

The expansion of the renewable energy introduction is examined as measures for controlling global warming. Wind power generation is expected as effective power resource, but the negative impact from the difficulty of an unstable output is concerned. In recent years, WT generation with contract of cut-of with shorting adjusting power and with rechargeable battery for stabilizing control are examined, but the introduction has not been accelerated yet because there is an influence in WT generation entrepreneur's business. In this paper, we make a brief summary of relation between the fluctuation of wind power generation and stability of electric power operation, and two types of approach; cut-off contract and stabilization using rechargeable battery. For the stabilization using battery, there are two methods, one is reduction control and the other is constant control. We propose a new control method for constant control based on profit optimization considering WT generation forecast and its risk of deviation. We also propose the estimation method for the .limitation of battery installation. Simulation results show the efficiency of our proposed methods.

There are a lot of potentials to install photovoltaic panels along the railroad track and some systems have been already realized in Japanese railway network. If a lot of PV systems have installed along the railway premises in the future, the generated power has to be transmitted to the station for the self-consumption because the electric loads along the railroad track between the stations are, in many cases, small. To be cost effective, simple cooperative reactive power control of power conditioner for distribution line voltage control to realize inverse backflow from PV panels to station is required, proposed and tested in the practical trackside solar system. The results of the cooperative reactive power control by two PV-PCSs are shown in this paper.

This paper proposes a generalized admittance based method for fault location analysis of distribution systems. Based on the measurements collected from the feeder breakers and intelligent switches during a fault, the fault type and faulted feeder section are first determined by examining the over-voltages and over-currents on the breakers/switches. The load demands, faulted line segment and fault location are then determined sequentially by finding a set of loads, a line segment and a fault location that has minimal distance between the currents measured at the boundaries of the feeder section, and the estimated currents determined by multiplying the measured voltages by an equivalent admittance matrix determined for the feeder section when applying the given load and fault conditions to the feeder section. The proposed approach determines the equivalent admittance matrix for a feeder section with one or two measuring ports through topology and circuit analysis, and for a feeder section with more than two measuring ports through Kron reduction on nodal admittance matrix. Numerical examples are given to demonstrate the effectiveness of the proposed approach.

For future power outage management of a power distribution system, a smart meter is expected to be used. Disconnection of a high-voltage power distribution line during a power outage runs the risk of a major disaster, so it needs to be detected quickly. In this paper, we describe a power outage management system developed as a part of an in-house smart grid demonstration test for detecting disconnections of highvoltage power distribution lines. Then we propose and validate a disconnection detection method that does not need information of connection phases of pole transformers.

This paper proposes a hybrid current profile based fault location algorithm for double-line-to-ground (DLG) faults in a distribution system. The method uses both short-circuit fault current profile (average of fault currents recorded for the faulted phases) and during-fault load current profile (corresponding to the un-faulted phase) to estimate an accurate fault location. The method is extended to include the effects of fault resistance in determining the fault location. Both fault current profiles and load current profiles are simulated for different values of fault resistances. The profiles are also extrapolated for those fault resistances corresponding to which the simulated profiles are not available. Numerical examples on a sample distribution feeder with multiple laterals and load taps are provided to validate the proposed algorithm for its robustness.

This paper proposes a new method for determining locations of single-phase-to-ground faults in ungrounded distribution system using pre-fault and during-fault fundamental-frequency measurements. The pre-fault measurements along with the load profiles are used to determine load demands for each individual load in a feeder section. The during-fault measurements are used to determine possible faulted area and location. The faulted feeder and faulted feeder section are first determined based on the reactive power factors of residual powers determined for the feeder breakers and switches with sensors. The line segments in the faulted feeder section are then tested for a sign change of phase-to-ground voltage on the fault phase to determine the faulted line segment. At last, the location of the fault is determined by finding a location along the faulted line segment having a zero phase-to-ground voltage on the faulted phase. Numerical examples are given to demonstrate the effectiveness of proposed method.

This paper proposes a new method for determining single-phase-to-ground fault locations of ungrounded distribution systems based on the measurements collected from the feeder breakers and intelligent switches during the fault. The method first narrows down the possible faulted area into a specific feeder section based on the reactive power factors of residual powers determined for the breakers and switches. Then the power consumption of each individual load in the faulted section is determined based on measurements at the boundaries of the section, and load profiles for each individual load. The possible faulted line segment is determined using a forward sweep scheme to test a sign change of faulted phase voltages on terminal buses of line segments, and the possible fault point is determined by finding a location along the faulted segment having a zero voltage on the faulted phase. Numerical examples are given to demonstrate the effectiveness of proposed method.

This paper proposes a new method for line fault analysis of ungrounded distribution systems. The fault condition of a line fault is integrated into the nodal admittance matrix of the faulted line to be modeled. The zero-impedance branch is merged into adjacent impedance branches to be taken into account, and one of its terminal buses with zero neutral-to-ground voltage is chosen as a slave bus when it is an ideal transformer or a voltage regulator with ungrounded winding connection. The three-phase jointly regulation of a distributed generation source is embedded into nodal admittance model of its internal impedance branch by combining three phases of its internal bus into one equivalent phase. The distribution system is partitioned into a main network and a set of lateral networks to be solved. The main network is formed by the connected paths between the terminal buses of the faulted line, and generation sources, and solved by a Gauss-Seidel method. A lateral network is formed by one of the buses of main network and all buses and branches downstream to the bus, and solved by a backward and forward sweep method. The numerical examples are provided to prove the effectiveness of proposed method.

This paper proposes a new method for determining the location of single-phase-to-ground faults of ungrounded distribution systems based on residual voltage distribution. The possible faulted line segment and fault location are determined by comparing the phase angles of faulted phase voltages against a faulted phase reference angle, and the faulted phase voltages are determined as the difference between the fault caused residual voltage of all phases and the shunt caused residual voltage of un-faulted phases. The method determines the fault location solely based on the voltages and currents measured during the fault, and associated series impedance and shunt impedances of line segments. It does not require any measurements or information for the load demands and prefault conditions. Numerical examples are given to demonstrate the effectiveness of the proposed method.

This paper proposes a hybrid three-phase load flow method for ungrounded distribution systems. Based on topology connectivity analysis, the system is partitioned into a mainline system and multiple tap systems. A Newton method with constant admittance matrix is used to solve the mainline system, such that zero impedance branches are merged into adjacent impedance branches to be considered, and constant active-power and voltage-magnitude (PV) buses with three-phase balanced voltages are transformed into single-phase PV buses to be modeled. A backward/forward sweep with loop compensation is used to solve the tap systems, such that a transformer and a voltage regulator is modeled using line-to-line voltages, a distribution line is simplified as a series branch, and loop compensation current is initialized based on loop downstream loads and the impedances of loop paths. Test results of sample systems are given to demonstrate the effectiveness of the proposed method.

This paper proposes a decoupled three-phase load flow analysis method for unbalanced distribution systems. The power flows are solved through nodal current injection mismatch equations written in rectangular coordinates. The voltage changes resulting from nodal current injection mismatches and nodal admittance matrix have been decoupled into one contribution from the real part, conductance matrix, and the other contribution from the imaginary part, susceptance matrix. The method determines voltage changes resulting from conductance and susceptance matrices respectively, and determines the voltages of a node as a linear combination of those voltage changes. The relative contributions are determined based on the diagonals of the conductance and susceptance matrices. The constant active power and voltage magnitude(PV) nodes have been converted into constant active and reactive power(PQ) ones based on a sensitivity matrix determined through Kron reduction of the nodal admittance matrix, and the corresponding reactive powers are adjusted after each solution has converged. The zero-impedance branches are merged with adjacent impedance branches, and the three phases of a balanced-voltage PV bus are merged into one single-phase PV bus to be modeled in the nodal admittance matrix. Numerical examples on IEEE 37 node test feeder and IEEE 123 node test feeder are presented to demonstrate the effectiveness of the proposed method.

In Japan, power systems have become complicated due to load increases. Problems with power system stability and short circuit current have occurred in large power systems and must be resolved. This requires not only the installation of new equipment but also the effective utilization of existing equipment, while also considering economic costs. The de-regulation and requirements for cost reduction increase the demands of power accommodation between large areas and between small areas. Power electronics technology is a possible solution. In this paper, the authors describe how an installation of a large back-to-back (BTB) system guarantees the beneficial effects of reducing short circuit current, improving power system stability and controlling power flow easily in the bulk power system.