December 2009

Multi-agent protection

By Sri Kolla and Dhadbanjan Thukaram

Power systems occasionally experience faults resulting from insulation failures caused by atmospheric disturbances and switching surges. Occurrence of a fault can cause expensive damage to the equipment and substantial loss of revenue due to interruption of service. Therefore, it is important to disconnect without delay a faulted element of a power system. For this reason, protective relays monitor power system apparatus continuously and isolate faulted elements by operating circuit breakers.

The initial design of protective relays used electromechanical devices. Rapid advances in digital processor technology have prompted the application of microprocessors to protective relays. Artificial intelligence techniques such as neural networks and intelligent agents are seeing use in power system protective relays these days due to the recent interest in Smart Grid technologies.

Multi-agent systems

Multi-agent systems (MASs) are typically distributed systems in which several distinct components, each of which is an independent problem-solving agent, come together to form some coherent whole. A MAS is simply a system comprising two or more agents or intelligent agents. A communication protocol enables agents to exchange and understand messages. The behavior of a MAS depends not just on its component agents, but also on how they interact. The agents have the ability to act in an environment and have different spheres of influence. This means they will have control over or will have the ability to influence different parts of the environment. In some cases, these spheres of influence may overlap or coincide.

Power system protective relaying

An electric power system consists of generators, transformers, transmission lines, and loads such as induction motors. Most of the time, the power system operates in a steady state, but disturbances occur occasionally due to faults caused because of natural calamities, human errors, and aging. Occurrence of a fault can cause expensive damage.

You must detect and eliminate the faulted part of a power system quickly. That is why protective relays monitor the power system continuously and isolate faulted parts by operating circuit breakers. Protective relays have gone through major transitions with the changes in technology. Electromechanical relays, the oldest in the family of protective relays, served the power system reliably. With the advancement in analog electronics, solid-state relays came along. Small size, light weight, and quiet operation are the advantages of solid-state relays over the electromechanical relays. Rapid advances in digital computer technology have prompted the application of microprocessors in protective relays. Microprocessor technology made the relays even more compact, multifunctional, and flexible. A microprocessor-based power system relay scheme consists of several subsystems, such as analog processing, analog-to-digital conversion, digital processor, relay output, and power supply subsystems.

Different types of relays see use to protect various power system apparatus against faults. A three-phase transmission line may experience 10 types of shunt faults. Distance relays are common to protect transmission lines against these faults. These relays compute the fault impedance, which is a function of fault distance, as a ratio of fundamental frequency voltage to current to determine whether the fault is within its protected zone.

In the cases when the distance relaying and other schemes are not sufficiently effective, a differential relay scheme protects transmission lines. The differential relay scheme generally sees use for transmission lines that have difficult application requirements, such as series compensation or multi-terminal configuration. In this type of relaying, we can compare the measurements from two ends of a transmission line. Transmitting the measurements from one end to the other enables comparison. The decision to trip occurs once the comparison indicates a significant difference between the measurements taken at the transmission line ends.

A MAS-based relay for transmission line distance protection is composed of five different types of agents; each agent assumes one of the roles that appear in a microprocessor relay and others.

  • Input Agent (IA): This type of agent reads the current and voltage data at the selected sampling period and checks the status of circuit breakers. This information transfers to relay algorithm agent.
  • Relay Algorithm Agent (RAA): This agent calculates fundamental frequency phasor components of voltages and currents from the data received from IA using a relay algorithm. There are several types of algorithms, such as discrete Fourier transform, lease error square, and correlation functions.
  • Relay Logic Agent: (RLA): This agent uses the phasors information from the RAA, detects if there is a fault based on appropriate logic, and transmits trip signal to performer agent. In case of a distance relay, the logic computes the impedance seen at relay location, which is a function of fault distance and determines if the fault is within the protected zone.
  • Performer Agent (PA): This type of agent receives trip signal from local RLA and RLAs in neighboring stations, then trips circuit breakers isolating the fault section if it detects a fault.
  • Management Agent (MA): This type of agent manages data and functions proper to the piece of the equipment, calculates the settings of the relay according to the changes of power system, and passes back to RLA.
ABOUT THE AUTHORS

Sri Kolla is a professor in electronics and computer technology at Bowling Green State University in Bowling Green, Ohio (skolla@bgnet.bgsu.edu). Dhadbanjan Thukaram is with the department of electrical engineering at the Indian Institute of Science in Bangalore, India (dtram@ee.iisc.ernet.in).