Current differential protection relays are widely applied to the protection of electrical plant due to their simplicity, sensitivity and stability for internal and external faults. The proposed idea has the feature of unit protection relays to protect large power transmission grids based on phasor measurement units. The principle of the protection scheme depends on comparing positive sequence voltage magnitudes at each bus during fault conditions inside a system protection center to detect the nearest bus to the fault. Then the absolute differences of positive sequence current angles are compared for all lines connecting to this bus to detect the faulted line. The new technique depends on synchronized phasor measuring technology with high speed communication system and time transfer GPS system. The simulation of the interconnecting system is applied on 500 kV Egyptian network using Matlab Simulink. The new technique can successfully distinguish between internal and external faults for interconnected lines. The new protection scheme works as unit protection system for long transmission lines. The time of fault detection is estimated by 5 msec for all fault conditions and the relay is evaluated as a back up relay based on the communication speed for data transferring.

A Novel Back Up Wide Area Protection Technique for Power Transmission Grids Using Phasor Measurement Unit

As power systems grow larger, it is becoming difficult to achieve reach and time coordination in backup protection when using distance relays. To cope with those problems, current differential backup protection for a busbar and transmission circuits connected to that busbar on an inter-station or wide-area basis is proposed. The proposed system can operate better than conventional systems with respect to operation time, section selectivity, etc. A system configuration is also proposed to utilize ATM transmission networks and time synchronous systems which make the construction and operation of the proposed wide-area protection system easy and flexible.

Wide-area current differential backup protection employing broadband communications and time transfer systems

This paper proposes a multiagent-based adaptive wide-area current differential protection system. A power grid is divided into primary and backup protection zones dynamically online with the help of a developed expert system, and the relay agents of these zones cooperate to perform a differential relaying function. Predictive self-healing strategies have been proposed to prevent misoperation of the relay agent, and the communication system has been simulated. The performance of the proposed adaptive wide-area current differential protection system based on computer simulation is presented in this paper.

Adaptive Agent-Based Wide-Area Current Differential Protection System

This paper proposes an agent-based paradigm for self-healing protection systems. Numerical relays implemented with intelligent electronic devices are designed as a relay agent to perform a protective relaying function in cooperation with other relay agents. A graph-theory-based expert system, which can be integrated with supervisory control and a data acquisition system, has been developed to divide the power grid into primary and backup protection zones online and all relay agents are assigned to specific zones according to system topological configuration. In order to facilitate a more robust, less vulnerable protection system, predictive and corrective self-healing strategies are implemented as guideline regulations of the relay agent, and the relay agents within the same protection zone communicate and cooperate to detect, locate, and trip fault precisely with primary and backup protection. Performance of the proposed protection system has been simulated with cascading fault, failures in communication and protection units, and compared with a coordinated directional overcurrent protection system.

Agent-based self-healing protection system

A novel wide area backup protection algorithm to identify fault branch based on the fault steady state component is proposed. Under normal conditions of the power system, subsets of buses called protection correlation regions (PCRs) are formed on the basis of the network topology and phasor measurement unit (PMU) placement. After the fault occurs, by analyzing the fault steady state component of differential current in each PCR, the fault correlation region is confirmed and then a fault correlation factor (FCF), is calculated in real time to locate the fault branch. The simulation results for the 10-generator 39-bus system verify that this method is able to easily identify fault branch with limited measurement points.

A Fault Steady State Component-Based Wide Area Backup Protection Algorithm

Round-trip transmission delays in a 128.5-km optical ground wire (OPGW) link and 12.5-km and 12-km overhead fiber-optic cable links were measured to clarify the effect of meteorological conditions, dominantly atmospheric temperature, on the delay variation. Transmission delays measured at 5 a.m. can be well expressed only in terms of atmospheric temperature along the links, while those at 1 p.m. can be expressed using four meteorological parameters of atmospheric temperature, sunshine duration or solar radiation, wind speed and precipitation. The coefficients of delay variation with respect to atmospheric temperature range approximately from 60 to 100 ps/km//spl deg/C where the cable links are more affected by temperature than the OPGW link, and delays in the daytime vary more than in the nighttime due to solar radiation. The results could be used for estimating the increase of cable temperature due to solar radiation and for designing fiber-optic time transfer networks effective for power system protection.

Transmission delay variations in OPGW and overhead fiber-optic cable links

As precise time information is particularly useful for power system communications, a time transfer system using terrestrial fiber optic digital networks, or SDH (synchronous digital hierarchy)-based time synchronous networks, is proposed in addition to radio- or satellite-based time dissemination systems such as GPS (Global Positioning System). In the system, auxiliary synchronizing equipment is attached to existing networks composed of SDH transmission equipment and digital clock supplies, and not only frequency but also frame phase or time is synchronized. Experimental time synchronizing devices showed potentially satisfactory performance; synchronization errors between two devices were of the order of sub-microseconds.

M. Myoujin

Papers

Wide-area current differential backup protection employing broadband communications and time transfer systems

Transmission delay variations in OPGW and overhead fiber-optic cable links

SDH-based time synchronous system for power system communications