Protection of Power Systems 1. Introduction

Faults A fault is an inadvertent, accidental connection, and flashover between the phase wires or from the phase wires to ground. Short circuits (faults) occur in power systems when equipment insulation fails, due to: system overvoltages caused by lightning or switching surges, insulation contamination, or other mechanical and natural causes.

Careful design, operation, and maintenance can minimize the occurrence of short circuits but cannot eliminate them.

Lightning Striking Power Lines

Fault currents can be several orders of magnitude larger than normal operating currents and, if allowed to persist, may cause insulation damage, conductor melting, fire, and explosion. Windings and busbars may also suffer mechanical damage due to high magnetic forces during faults.

Clearly, faults must be quickly removed from a power system. Standard EHV protective equipment is designed to clear faults within 3 cycles. Lower-voltage protective equipment typically operates within 5–20 cycles.

Types of Faults Most faults in an electrical utility system with a network of overhead lines are one-phase- to-ground faults resulting primarily from lightning-induced transient high voltage and from falling trees and tree limbs. In the overhead distribution systems, momentary tree contact caused by wind is another major cause of faults. Ice, freezing snow, and wind during severe storms can cause many faults and much damage.

These faults include the following, with very approximate percentages of occurrence: Single phase-to-ground: 70%–80% Phase-to-phase-to ground: 17%–10% Phase-to-phase: 10%–8% Three-phase: 3%–2%

Series unbalances, such as a broken conductor or a blown fuse, are not too common, except perhaps in the lower-voltage system in which fuses are used for protection. Fault occurrence can be quite variable, depending on the type of power system (e.g., overhead vs. underground lines) and the local natural or weather conditions.

Protection Protection is defined as ‘‘the science, skill, and art of applying and setting relays and/or fuses to provide maximum sensitivity to faults and undesirable conditions, but to avoid their operation on all permissible or tolerable conditions’’

The basic idea is to define the undesirable conditions and look for differences between the undesirable and permissible conditions that relays or fuses can sense. It is also important to remove only the faulted equipment from the system while maintaining as much of the unfaulted system as possible in service, in order to continue to supply as much of the load as possible.

Although fuses and reclosers (circuit breakers with built-in instrument transformers and relays) are widely used to protect primary distribution systems (with voltages in the 2.4– 46 kV range), we focus primarily in this course on circuit breakers and relays, which are used to protect HV (115–230 kV) and EHV (345–765 kV) power systems.

Relay The Institute of Electrical and Electronic Engineers (IEEE) defines a relay as: ‘‘an electric device that is designed to respond to input conditions in a prescribed manner and, after specified conditions are met, to cause contact operation or similar abrupt change in associated electric control circuits.’’

Inputs are usually electric, but may be mechanical, thermal, or other quantities or a combination of quantities. Relays are used in all aspects of activity: home, communication, transportation, commerce, and industry, to name a few. Wherever electricity is used, there is a high probability that relays are involved.

They are used in heating, air conditioning, stoves, dishwashers, clothes washers and dryers, elevators, telephone networks, traffic controls, transportation vehicles, automatic process systems, robotics, space activities, and many other applications. In this course we focus on one of the more interesting and sophisticated applications of relays, the protection of electric power systems.

Protective Relay The IEEE defines a protective relay as: ‘‘a relay whose function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and to initiate appropriate control circuit action’’

Fuse Fuses are also used in protection. IEEE defines a fuse as: ‘‘an over-current protective device with a circuit-opening fusible pat that is heated and severed by the passage of the overcurrent through it’’

Thus, protective relays and their associated equipment are compact units connected to the power system to sense problems. Protective relaying is a nonprofit, nonrevenue-producing item that is not necessary in the normal operation of an electrical power system until a fault—an abnormal, intolerable situation—occurs.

A primary objective of all power systems is to maintain a very high level of continuity of service, and when intolerable conditions occur, to minimize the extent and time of the outage. Loss of power, voltage dips, and overvoltages will occur, however, because it is impossible, as well as impractical, to avoid the consequences of natural events, physical accidents, equipment failure, or misoperation owing to human error.

Relay Technologies Electromechanical relays Analog type electronic relays Microprocessor-based electronic relays

Electromechanical Relays Originally, all protective relays were of the electromechanical type. Electromechanical type relays are still in widespread use and continue to be manufactured and applied.

Electromechanical Relays

Analog Type Electronic Relays Analog type electronic relays using discreet electronic components were introduced in the 1970s.

Solid State Relay (Static Relay)

Microprocessor-based Relays In recent years, microprocessor-based electronic relays have been developed and are being applied at an increasing rate. Microprocessor- based relays are sometimes referred to as numerical type relays since the analog inputs are converted to digital numbers that are then processed within the relay.

Numerical Relay

Even with this trend toward the utilization of micro processor -based relays, however, it may be a long time before electromechanical devices are completely replaced. With electronic relays, the protection principles and fundamentals are essentially unchanged as are the issues regarding protection reliability.

Benefits of Microprocessor Type Relays Microprocessor type relays do provide many benefits: Higher accuracy Reduced space Lower equipment and installation costs Wider application and setting capabilities

Other desirable supplemental features: Control logic Remote and peer-to-peer communications Data acquisition Event recording Fault location Remote setting Self monitoring and checking

Many modern micro processor relays utilize a liquid crystal display (LCD) on the front panel. Such displays typically show setting, metering, event, and relay self-test status information. Relay settings can also be changed through the LCD interface without the need for a data terminal.

Target information is typically displayed on microprocessor relays with the use of LEDs that identify the protective functions that had operated to initiate tripping along with other information such as the type of fault that had been detected (i.e., A- phase-t o-ground), recloser status, etc.

Terminal blocks are normally provided on the back of the relay for connecting the various input s that are require d and outputs that are provided by the relay. Communication ports are provided for transmitting digital data.

Classification of Relays Relays may be classified in several different ways, such as by function, input, performance characteristics, or operating principles. Classification by function is most common. There are five basic functional types: 1.Protective. 2.Regulating. 3.Reclosing, synchronism check, and synchronizing. 4.Monitoring. 5.Auxiliary.

Protective Relays Protective relays and associated systems (and fuses) operate on the intolerable power system conditions. They are applied to all parts of the power system: generators, buses, transformers, transmission lines, distribution lines and feeders, motors and utilization loads, capacitor banks, and reactors.

For the most part, the relays discussed are separate devices that are connected to the power system through CT and VTs from the highest system voltage (765 kV, at present) down to service levels of 480 V. In general, distribution equipment below 480 V is protected by fuses or protection devices that are integral with the equipment.

Regulating Relays Regulating relays are associated with tap changers on transformers and on voltage regulators of generating equipment to control the voltage levels with varying loads. Regulating relays are used during normal system operation and do not respond to system faults unless the faults are left on the system for too long. This is not normal.

Reclosing, Synchronism Check, and Synchronizing Relays Relays of this type are used in energizing or restoring lines to services after an outage, and in interconnecting preenergized parts of systems.

Monitoring Relays Monitoring relays are used to verify conditions in the power system or in the protective system. Examples in power systems are fault detectors, voltage check, or directional- sensing units that confirm power system conditions but do not directly sense the fault or trouble.

In a protection system, they are used to monitor the continuity of circuits, such as pilot wires and trip circuits. In general, alarm units serve as monitoring functions.

Auxiliary Relays Auxiliary units are used throughout a protective system for a variety of purposes. Generally, there are two categories: contact multiplication and circuit isolation. In relaying and control systems there are frequent requirements for: 1.more outputs for multiple tripping, alarms, and operating other equipment, such as recording and data acquisition, lockout, and so on,

2.contacts that will handle higher currents or voltages in the secondary systems, and 3.electrical and magnetic isolation of several secondary circuits. The seal-in (CS ) relay of Figure 1.9 is an auxiliary relay application. The trip and closing relays used with circuit breakers are auxiliary relays.

Other Relay Classifications Protective relays classified by input are known as current, voltage, power, frequency, and temperature relays. Those classified by operating principle include electromechanical, solid-state, digital, percentage differential, multirestraint, and product units.

Those classified by performance characteristics are known as distance, reactance, directional overcurrent, inverse time, phase, ground, definite, high-speed, slow-speed, phase comparison, overcurrent, undervoltage, overvoltage, etc.

Backup Protection Problems with the protection equipment itself can occur. A second line of defense, called backup relays, may be used to protect the first line of defense, called primary relays. These can be at the same location (primary backup), at the same station (local backup), or at various remote stations (remote backup).

All three are used together in many applications. In HV and EHV systems, separate current- or voltage-measuring devices, separate trip coils on the circuit breakers, and separate batteries for the trip coils may be used.

Relay Coordination Also, the various protective devices must be properly coordinated such that primary relays assigned to protect equipment in a particular zone operate first. If the primary relays fail, then backup relays should operate after a specified time delay.