Lecture 4 Handouts

8/18/2019 Lecture 4 Handouts

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Lecture 4Power System Protection

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Course map


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Outline of the Lecture

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Control vs Protection

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Protection Principles

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Protection requirements

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Protection Schemes

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Control vs. Protection


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Why do we need to control?

• Control actions needed• Automatic

– Equipment Protection• Disonnection at fault

– Voltage control• Move a tapchanger in a transformer

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Human intervention– Frequency control

• Increase output in hydro plant

– Voltage control• Connect capacitor bank

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What can we control?

• Breakers

• Valves

• Tap changers

• Switches

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Drives• ……

All done using Relays


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Basic Relay concepts

• Electromechanical

• Static

• Digital

• “Numerical”

Pre 1970s

1970s

1980s

Present ->

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The numerical relay

• Current state ofthe practice

– A/D & D/Aconverters

– Dedicated CPU

for Digital SignalProcessing

– Programmable

– Real-timeoperating system


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Application examples

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Numerical relays - issues

• Software Version Control

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Same problem as for all software systems

• Relay Data Management

– Large amounts of parameters

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Vendors specific vs. standardisation• Testing & Comissioning

– Complex equipment needed for testing

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Too complex for field repairs


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Protection Schemes

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Purpose of the ProtectionSystem

• ProtectEquipment

• Protect People &Property

• Separate Faultysection frompower system

• Restore normaloperation


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Most basic type of protection?

• The Fuse

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Relay Protection system


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Different Types of Protection

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Assets in a Power Grid (value)


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Fault types

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Fault Statistics


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Line fault statistics

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Protection Schemes


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• The protection system must be

– Reliable

– Stable

– Sensitive

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Selective– Timely

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Reliability

• The protection system must provide itsfunction when required to avoid damage toequipment, people or property

• Reliability problems stem from

– Incorrect design

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Incorrect installation/testing

– Deterioration

• The study of the reliability of a protection

system is critical


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Stability

• The protection system shall not react tonon-fault situations

• The protection system must not react tofaults in neighboring zones or high loadcurrents.

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Sensitivity

• Sensitivity refers to the minimal changes inmeasured parameter that the system canreact to.

• For electromagnetic relays, this was a maindesign characteristic.

• Presently, the sensitivity is determined by

the CT/VT and design of the system


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Selectivity

• Only the effected parts of the powersystem shall be disconnected.

• Is achieved by two main methods

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Time-grading/Current Grading

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Relays are set to operate depending on

the time and current characteristics

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Unit systems

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Current is measured at several pointsand compared.

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Timeliness - Speed

• Faults must be isolated as fast as possible.

• Speed is necessary for two main reasons

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Maintain stability of the overall power

system–

Reduction of damage to equipment &

property


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Stability vs Reliability

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Fault Clearance Chain


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FCC with Local backup

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Breaker Failure Protection


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Protection Schemes

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Different Types of Protection


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Fundamentals of Protection

• Protection System

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A complete arrangement of equipment that

fulfills the protection requirements

• Protection Equipment

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A collection of devices excluding CT, CB etc

• Protection Scheme

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A collection of protection equipment

providing a defined function.

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Zones of Protection

• By dividing the powersystem into protectionzones the extent ofdisconnections can belimited


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Overlapping protection zones

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Backup Protection Zones


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Over-Current Protection

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Achieving Selectivity


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Discrimination by Time

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Discrimination by Time & Current


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Directional Relays

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Time grading

• Example from Strauss 4.4.2.


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Mathias Ekstedt

Example - Protection Scheme

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Mathias Ekstedt

Example – relay settings


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Distance Protection

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Principle of Distance Protection


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Power Line Characteristics

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Benefits of Distance Protection


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Differential Protection

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Mathias Ekstedt

Unit – Differential Protection


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Diffferent Protection schemes

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Protection Summary

• The Power System must be protected

– To avoid damage to equipment, people &property

• Protection systems are created using CT/VTs, relaysand circuit breakers

• Key characteristics are:

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Selectivity– Speed

– Reliability

– Stability

–

Sensitivity

• Numerical Relays are essentially small computers -The Intelligent Electronic Device

Lecture 4 Handouts

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Transcript of Lecture 4 Handouts