Tomihiro Takano

An approach to the dynamic verification of knowledge-based systems

Abstract This paper describes a method of formal dynamic verification of knowledge-based systems. Almost all verification methods that have been proposed can validate the structural and logical integrity of knowledge-based systems, but they are not semantic. In other words, they are all static and syntactic. In addition, this paper adopts a dynamic approach in which verification is carried out in the reasoning process. This methid enables the detection of incorrect rules in the knowledge base using a simple debugging technique, which traces the execution of inference. Following the debugging algorithm, the user is asked several questions. To reduce the number of questions, knowledge is added to the system. In this way, some of the checks for correctness can be done by the system. Specifications are the judgement criteria of correctness, which are made up of some constraints. Application of the method to a power system operation is given. Benefits of this approach include: the ability to evaluate systems conclusions in situations where the experts may judge reasoning process, and the ability to predict the performance of the knowledge-based system in untested cases or to identify other cases that should be tested.

Distributed three-phase reactive power control of distributed energy resources in distribution systems

This paper proposes a new distributed control method for three-phase reactive power control of distributed energy resources (DERs) in distribution systems. Each DER-connected bus has been equipped with a local bus controller which has bi-way communications with bus controllers at adjacent buses upstream and downstream to the bus under consideration. The reactive powers of a DER-connected bus are adjusted to prevent its monitored buses from voltage violations. When voltage violations occur, the required reactive powers for the bus are determined based on the voltage violations on its monitored buses, and equivalent impedances and phase angle shifting between the substation transformer and the study bus. If the required reactive powers of a DER-connected bus are beyond its capacities, the neighbor DER-connected buses are required to share the additional generations. A new iterative residual splitting distributed control algorithm is proposed to coordinate the reactive powers between DER-connected buses. Each DER unit at a DER-connected bus has been equipped with a local unit controller which has bi-way communications with unit controllers at adjacent units of the same bus as the unit under consideration. A consensus-based distributed control algorithm is used to allocate the total demand among units with respect to their capacities. The test results on a modified IEEE 123-node feeder are given to demonstrate the effectiveness of the proposed method.

Cooperative control of reactive power of distributed PV systems to suppress voltage of distribution line along railroad track

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.

Development of Centralized Voltage Control System for Distribution Systems

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.

Economic benefit evaluation of multi-terminal VSC HVDC systems with wind farms based on security-constrained optimal power flow

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.

Mitigating substation demand fluctuations using decoupled price schemes for demand response

In this paper, we propose a decoupled substation price model with separate charges for base energy production, up/down reserve usages and reserve usage variations. Using the decoupled price scheme, the flexible loads in the distribution system are scheduled to closely follow variations in renewable generation in order to mitigate demand fluctuations in substation. This reduces the need for reserve unit power production, thereby allowing transmission systems to operate at lower cost and higher efficiency. We formulate an optimization problem to determine the optimal scheduling of flexible loads subject to power balance, power flow and demand response constraints. The optimization problem is solvable by computationally efficient linear programming methods. Real-time and day-ahead models are considered. Numerical examples on a modified 13-node test feeder demonstrate the effectiveness of the proposed method.

Locating double-line-to-ground faults using hybrid current profile approach

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.

A generalized admittance based method for fault location analysis of distribution systems

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.

Disconnection detection method for power distribution lines using smart meters

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.

Locating of multi-phase faults of ungrounded distribution system

This paper proposes a new method for locating multi-phase faults in ungrounded distribution systems, including phase-to-phase faults, double-phase-to-ground faults, and three-phase faults. Through identification of the faulty feeder section, the fault location algorithm is only implemented on the faulty section of the feeder, thus avoiding the unnecessary computations. A two-ended fault location algorithm is implemented for both two and three phase faults on the mainline of the faulty section. Two different one-ended fault location algorithms are also implemented for handling two-phase faults and three-phase faults on the laterals of the faulty section respectively. The proposed fault location algorithms make no assumption regarding the fault impedance, thus estimation of fault location is invariant to fault impedances. Numerical examples are given to demonstrate the effectiveness of the proposed method.