In June 2023, the Protection Relay System Technical Committee planned a special issue labeled as “Protection Relay System Technology in Power Distribution Stations”. Protection relay systems play a vital role in the stable operation of power systems. In recent years, they have also been responsible for solving various problems in power systems, such as the realization of mass introduction of renewable energy, expansion of the operational capacity of power distribution facilities, and rationalization of facility configuration. The following topics were gathered and discussed as a result of the call for this special issue.: (1) Proposals for distribution line protection methods (short circuit protection, ground fault protection), (2) Case studies using IEDs (IEC 61850), N-1 inter trip scheme, bus bar protection relays, and comprehensive retrofits, (3) Optimizing accident recovery during distribution line accidents, (4) Voltage adjustment methods using storage battery capacity operation, (5) Bank Recorder and switchboard configuration when dealing with reverse power flow. A total of 10 papers were submitted. This commentary touches on the contents of the technical reports as an aid to understanding these papers. It also outlines the history of distribution substation protection relays and the latest technology in distribution substation protection relay systems.

This document provides information on typical European practices for protection of MV distribution networks, including reference to distributed energy sources It primarily focuses on MV network protection but includes HV.MV substation aspects. It addresses then implications of various MV neutral treatment options on earth fault protection. Further it deals with the impact of embedded energy resources upon protection and presents modern techniques, known as Advanced Networks Protection Solutions, to handle these challenges.

The service restoration to sound sections within a short period as much as possible, performed after the permanent fault occurs in a distribution system, is significant for maintaining power supply reliability. This paper proposes an efficient method to find service restoration procedures based on fault searching and the optimal work scheduling of crews, considering the power supply priorities of loads and the probability of occurrence of faults. The proposed method is composed of two-stage problems to reduce the outage sections efficiently: (i) the problem to find a series of searching procedures for unknown fault locations in the first stage, and (ii) the optimization problem of work schedules of crews in the second stage. In the first stage, the order of operation of switches opened for searching for the fault direction is determined, constituting the searching tree. After that, the proposed optimization method can efficiently solve the work scheduling problem by formulating it as a mixed-integer linear programming problem in the second stage. The computational experiments using a large scale distribution system model with many remote and manual switches show that the proposed method can provide efficient service restoration procedures within a reasonable computational time.

The increase of capacitance to earth of distribution system would make it difficult for the conventional earth fault protection system for distribution lines to locate and isolate an earth fault. Therefore, the authors propose a new method in consideration of the increase of capacitance to earth. The proposed method combines three relays with different protectable ranges to detect an earth fault. Specifically, earth faults are detected by measuring zero-phase voltage, zero-phase current, and residual voltage. In addition, the use of both zero-phase voltage and line-to-line voltage ensures the selection of the fault line. This paper clearly shows the usefulness of the proposed system by graphing the protectable range of each protection relay. In addition, this paper implements the proposed system using IEDs, which are general-purpose protective relay devices and reports the results of the verification test by Real Time Simulator.

In order to realize the retrofit method applying IEC 61850 compliant relay units (IEDs), it is necessary to avoid operational failures caused by the deterioration of existing facility and control cables to be used. Therefore, it is necessary to apply a method to reduce the number of existing cables and to improve the condition diagnosis function of existing facility. In this paper, we report on the study of communication networking of control signals, partial wireless communication, and upgrading of facility condition diagnosis functions in a protection control system using the new MELPRO-i series.

With the increase in the amount of interconnection of renewable energy, a transformer reverse power flow has occurred. In the past, transformer reverse power flow was restricted, but with the revision of the law in May 2013, it has permitted to operate the system on the premise of transformer reverse power flow by installing the necessary protection device. Along with this, the bank reverse power flow fault recorder is installed at the distribution substation for power system failure recording and analyzing at the time of transformer reverse power flow. On the other hand, the occurrence of voltage flicker phenomenon due to renewable energy has become a problem in recent years. This voltage flicker has the property of changing rapidly, and it is difficult to measure with the conventional ΔV10 flicker meter. Therefore, we implemented a short-time flicker measurement function that can measure voltage flicker with rapidly changing in the bank reverse power flow fault recorder. The outline and measurement examples are introduced.

To expand the introduction of renewable energy, it is necessary to efficiently utilize existing power systems without relying on system enhancement. One of the efforts is N-1 inter trip. N-1 inter trip is a mechanism to expand the operating capacity in normal times by controlling power generation in the event of a single failure in power transmission and conversion equipment. However, since it is difficult to realize an N-1 inter trip scheme with existing equipment, new development is required. Therefore, in this paper, we report an N-1 inter trip scheme that implements an IEC 61850 compliant MMS communication system and is highly functional and can be realized at low cost.

The interconnection of a large amount of renewable energies to the power system has created new problems in terms of supply and demand, system protection, and power quality. System protection issues include reverse power flow in transformers at distributing substations and overload of power distribution equipment.

We report on examples of solving these problems by utilizing IEDs, which are becoming widely applied in Japan.

Because of a large number of facilities in distribution lines, the rationalization of power distribution equipment is an important issue. In recent years, photovoltaic power generation system (PV) has become increasingly interconnected, so the use of distributed power sources and storage battery to streamline facilities is also being studied. However, large capacity of the storage battery is required to reduce power flow in distribution lines using only the storage battery. Dynamic rating (DR), on the other hand, dynamically changes the current capacity of transmission lines and transformers according to weather conditions and other factors. In this study, the necessary capacity of the storage battery is evaluated by distribution lines according to DR. Furthermore, considering transitional characteristics of the wire temperature about current that it takes several minutes to change after varying current can increase the allowable current capacity in distribution lines based on DR. Since downsizing distribution lines to rationalize distribution facilities increases the impedance in distribution system, there is a problem that the voltage fluctuation becomes large. Therefore, this paper proposes a method for calculating the necessary capacity of storage battery according to DR considering transient temperature change and voltage fluctuations in the distribution system. Then, the necessary capacity of the storage battery is evaluated through numerical simulations based on actual annual data.

A fault current measured at a distribution substation is small in case of a two-phase short-circuit fault near the end of a long distribution line. This characteristic makes it difficult to settle the overcurrent relay installed at the root of line because the load current could hardly be distinguished from the fault current. On the other hand, a negative-sequence current relay likely isolates such a two-phase short-circuit fault, but it may not operate properly due to effects of load on the distribution line. This study sets a proposal of short-circuit protection system considering the negative sequence of fault current that can be applied to long distribution lines. It also demonstrates how useful the proposed system is for isolating such a two-phase short-circuit fault on a long distribution line.

In December 2020, the “Green Growth Strategy for Carbon Neutral” was launched, and it is expected that more distributed power sources will be introduced into distribution system in large quantities. It is necessary to streamline equipment and reduce costs. Efforts include centralizing the functions of protection relay units and automatically setting the target voltage of transformers for distribution substations. By realizing these, we aimed to slim down the equipment and reduce the cost. Therefore, in this paper, we report on the specific contents of the integrated protection relay function.

This paper proposes a new method for detecting electrical abnormal signs of photovoltaic module only by measuring the voltage value and current value, which are the power generation output values on the DC side in a photovoltaic power generation system. The proposed method is an abnormal sign detection method for photovoltaic power generation systems that uses the frequency difference distribution index of power generation output in the power generation region for evaluation. This paper first outlines the proposed power generation region as an evaluation window in the anomaly detection method. Next, as an example of an anomaly sign detection method using the proposed power generation region, we show the effectiveness of a quantitative evaluation using a frequency difference distribution index focusing on the occurrence of measured values in the power generation region. Finally, the validity and effectiveness of the proposed method is demonstrated using measured data from actual mega solar data.

In recent years, breakdown failures occur in joint boxes of oil-filled cables associated with the adhesion of copper compounds, such as copper sulfide or organocopper compounds. This is the adhesion degradation of copper compounds due to electric charging. Therefore, the degradation should be diagnosed for keeping its high reliability. However, the degradation cannot be predicted by the conventional diagnostic methods based on dissolved gas analysis, because the definition of degradation aspect in the method does not assume this degradation. Therefore, a new degradation diagnosis method is developed based on insulating oil analysis and the trend of the concentration of dissolved copper in the oil and the concentration of combustible gas generated due to the degradation. However, it is not possible to measure the concentration of dissolved copper in the past 30 years, because the majority of oil-filled cables are over 30 years old. Therefore, the past concentration of dissolved copper is calculated by the dielectric loss tangent value of oil, which has been traditionally measured. As a result, the consistency rate of the diagnostic methods was confirmed to be more than 80% by comparison with the results of demolition investigations of the using joint boxes.

Arc extinguish grids are used to increase the arc voltage and limit the short circuit current at load-break switches and/or molded case circuit breakers (MCCB). The extinction grid much effects to the arc and the breaker's interrupting performance. When the breaker has an arc-runner, the arc has a magnetic force from the external magnetic field. But, when the breaker does not have an arc-runner, the arc also is got a magnetic force and is driven to the grid. This driving force is not same as the arc-runner.

In this paper, the magnetic force working to the arc was theoretically calculated by mirror image current and vector potential. In the iron grid, eddy currents flow to cancel the magnetic fluxes made by the arc. Therefore, the magnetic force between the grid and the arc is generated and the arc is drive to the grid.