Types of Protection• There are two types of protection referred to in electric power systems. 1. System Protection2. Personal Protection• The first is system protection, having to do with protective relays, fault currents, effective grounding, circuit breakers, fuses, and so on.• The second is personal protection, having to do with rubber gloves, insulating blankets, grounding jumpers, switching platforms, tagging, and so on.

Chapter 7 SYSTEM PROTECTION

• The protection of power system equipment is accomplished by protective relaying equipment that is used to trip circuit breakers, reclosers, motorized disconnect switches, and self-contained protection devices. • The objective of system protection is to remove faulted equipment from the energized power system before it further damages other equipment or becomes harmful to the public or employees. • System protection protects power system equipment from damage due to power faults and/or lightning.• System protection uses solid-state and electromechanical protective relays to monitor the power system’s electrical characteristics and trip circuit breakers under abnormal conditions.

System Protection

• Another means for providing equipment protection is proper grounding.• Effective or proper grounding can minimize damage to equipment, cause protective relays to operate faster.System Protection Equipment and Concepts• System protection, often called protective relaying, is composed of relay devices in substations that monitor the power system’s voltages and currents through the CTs and PTs and are programmed to initiate “trip” or “close” signals to circuit breakers if the thresholds are exceeded.• The relays, trip signals, circuit breaker control systems, and the system control equipment are all battery powered.

System Protection……

• A protective relay is a device that monitors system conditions (amps, volts, etc., using CTs and PTs) and reacts to the detection of abnormal conditions.• The relay compares the real-time actual quantities against preset programmable threshold values and sends dc electrical control signals to trip circuit breakers or other opening devices in an effort to clear an abnormal condition on the equipment it is protecting.• When system problems are detected and breakers are tripped, alarm indications are sent to system control and sometimes other protection operations are initiated. • As a result, equipment may be deenergized, taken off line, and consumers will be out of power with minimal equipment damage.

Protective Relays

Protective relays are of two types: 1. Electromechanical2. Solid state• Electromechanical relays are composed of coils of wire, magnets, spinning disks and moving electrical switch contacts, and are very mechanical in nature.• Solid-state relays are electronic and have no moving parts.The basic differences are listed below.Solid State• Advantages: Multiple functionality, small space requirements, easy to set up and test, self-testing, remote access capability, and they provide fault location information.

Protective Relays……

Disadvantages: External power required, software can be complex, and may have many “functional eggs” all in one basket.

Electromechanical Relays• Advantages: Usually self-powered, simple and single-function design.• Disadvantages: Normally one relay per phase, difficult to set up and adjust, and require more frequent testing.

Protective Relays……

Protective Relays……

• Typically, protective relays are designed to follow the inverse current–time curve as shown in Figure 7-3.• In other words, the time to trip a circuit breaker shortens as the amount of fault current increases. • Therefore, a relay sensing a fault located near a substation would initiate a trip of the breaker faster than if the fault were located down the line because less current flows due to the additional resistance of the wire.• Between these two points, the relay engineer adjusts the shape of the curve to meet various system protection coordination objectives.

Inverse Current–Time Concept

Inverse Current–Time Concept……Upper limit

Lower limit

• Relay coordination is the term used to create a situation in which the most downstream clearing device from the source clears the fault first.• The coordination of all the protective relays in the transmission and distribution systems, or even a single power line is a very special art and science.

Inverse Current–Time Concept……

• A one-line diagram (also referred to as the single-line diagram) is a simplified drawing of the system or a portion of the system that shows the electrical placement of all major equipment. • Extra information is added to give the engineer or systems operator the full picture of the electrical system, including the system protection schemes.• One-line diagrams are very useful for planning maintenance activities, rerouting power after a fault, switching orders to change system configurations, and to view the relationships between smaller sections of the power system and the overall system.

One-Line Diagrams

One-Line Diagrams……

Some of the most common uses of one line diagram are discussed below:1. Line Crews refer to one-line diagrams to know what

protective relays are used on the power line being worked, to identify disconnect switch locations for load transfer operations, and to see the relationship to other nearby lines or equipment that are part of the system in question.

2. System Operators use one-line diagrams to identify the electrical placement of breakers, air switches, transformers, regulators, and so on in substations that may indicate alarms and/or needs corrective action. They use one-line diagrams to figure out how to switch the system equipment to restore power.

One-Line Diagrams……

3. Electrical Engineers use one-line diagrams to understand system behavior and to make changes to the power system to improve performance.

4. Consumers use one-line diagrams to identify their electrical equipment, circuits, and protection apparatus.

One-Line Diagrams……

Assignment 3

• What are the major factor involved in load shedding in Pakistan. Discuss briefly.

• How electric power system can be improved in Pakistan. Discuss it in term efficiency (minimum losses), voltage and frequency stability.

Note: Assignment will on maximum of 3 pages.

Due date: 29th November