Principles of Power Systems Protection Part 1

7/25/2019 Principles of Power Systems Protection Part 1

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Principles of Power System Protection1

PRINCIPLES

OF POWER SYSTEM

PROTECTION

Part 1

PRINCIPLESPRINCIPLES

OF POWER SYSTEMOF POWER SYSTEM

PROTECTIONPROTECTION

Part 1Part 1

Bob Coulter

Power System Protection


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Purpose of Protection EngineeringPurpose of Protection EngineeringPurpose of Protection Engineering Protection engineering is the branch of electrical power

engineering concerned with the design and operation of

protection schemes

The purpose of protection is to minimise the effects of faults

on electrical power systems - note faults can never be

entirely avoided

Protection schemes are specialised control systems that

monitor the power system - detect faults or abnormal

conditions and then initiate corrective action In this context the power system is considered all the plant

and equipment necessary to generate, transmit, distribute

and utilise electrical power


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The Electrical Power SystemThe Electrical Power SystemThe Electrical Power System

G

G

G

G

GENERATION TRANSMISSION SUBTRANSMISSION

PRIMARY or HV DISTRIBUTIONSECONDARY

or LV DISTRIBUTION

CUSTOMER

LOAD

Power StationTerminal Station or

Transmission Substation

500kV, 330kV

275kV, 200kV,

132kV

Zone

Substation

132kV, 110kV,

66kV, 33kV, 22kV

33kV, 22kV, 11kV, 6.6kV, SWER415/240V, 480/240V

Distribution

Substation

GEmbedded

Generation

Embedded

Generation

Embedded

Generation

Standby Generation

CUSTOMER

INSTALLATION


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Power System Secondary Systems includePower System Secondary Systems includePower System Secondary Systems include Protection

Auto control for voltage, frequency, reactive power compensation,

power flow, network configuration and stability

Metering for billing, operational control and statistical data

Local manual control (plant status, voltage level reactive power

support, network configuration)

Remote manual control via communications links (SCADA)

Plant condition monitoring and alarming (temperature,

malfunction, maintenance need, operating duty)

Communications infrastructure

Instrument transformers current and voltage transformers


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Types of Fault and AbnormalityTypes of Fault and AbnormalityTypes of Fault and Abnormality Short-circuit faults (3, 2, g, 2g)

Open-circuit faults (open conductor)

Complex faults (inter-circuit, broken conductor, cross-country etc)

Inter-turn faults in windings

Abnormalities: Overload and excessive operating temperature

Real power deficit - underfrequency

Power swings

Power frequency overvoltage or undervoltage

Underexcitation of synchronous machines

Overfluxing of power transformers

Asynchronous operation of synchronous machines

Overfrequency

Mechanical defects ie. leaking oil, tapchanger mechanism faults etc


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Causes of Short-Circuit FaultsCauses of ShortCauses of Short--Circuit FaultsCircuit Faults Insulation breakdown due to inherent weakness

Lightning

Birds and animals bridging insulators

Dig-ups for underground cables

Poles collapsing

Conductors breaking

Willful damage

Vehicle impact

Wind borne debris

Incorrect operation by personnel

etc


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Effects of Short-Circuit Type FaultsEffects of ShortEffects of Short--Circuit Type FaultsCircuit Type Faults Large or very large currents can flow through parts of the network

thousands or tens of thousands of Amps can be involved

These large currents can only be allowed to flow for a very short

time otherwise equipment and generators would be damaged,

most likely terminally allowable short-circuit current flow

duration could range from as short as 10 milliseconds up to say 3

seconds.

Arcs, sparking and the heating effect of short-circuit currents can

start fires involving non-electrical assets/property

Very large mechanical forces can be caused by short-circuit

currents which have potential to break or damage equipment

Electric current can escape from the network conductors and

flow through paths where they could create a hazard to people or

livestock and cause damage to non-electrical assets/property


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Large Transformer FireLarge Transformer FireLarge Transformer Fire


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Short-circuit Type FaultsShortShort--circuit Type Faultscircuit Type FaultsR

W

B

R

W

B

R

W

B

R

W

B

R

W

B

R

W

B

R

W

B

R

W

B

3 Fault 2 Fault

2-e Fault-e Fault


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Short-circuit Type Faults with Fault ImpedanceShortShort--circuit Type Faults with Fault Impedancecircuit Type Faults with Fault Impedance

R

W

B

R

W

B

3 Fault 2 Fault

R

W

B

R

W

B

2-e Fault-e Fault

R

W

B

R

W

B

R

W

B

R

W

B

Zf

Zf1 Zf2 Zf3

Zf

Zf1 Zf2

Zf3


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Some Causes of Fault ImpedanceSome Causes of Fault ImpedanceSome Causes of Fault Impedance

Arc resistance

Pole, tower or structure footing resistance to earth

Resistance things that may come in contact with a

line i.e. tree, crane etc

Contact resistance where a conductor falls to the

ground

For protection design it is important to recognise thatfault impedance can significantly reduce the

magnitude of fault currents and take this into account


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Series Type FaultsSeries Type FaultsSeries Type Faults

1 Open 2 Open

R

W

B

R

W

B

R

W

B

R

W

B

R

W

B

R

W

B

Zf1R

W

B

R

W

B

Zf

Series impedance in 1 Series impedance in 2s

Zf2


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Some Complex Fault TypesSome Complex Fault TypesSome Complex Fault Types

Broken conductor fault Inter-circuit fault

R

W

B

R

W

B

R

W

B

R

W

B

Cross country fault

Zf

R

W

B

R

W

B

Circuit 1

Circuit 2Zf

R

W

B

-e Fault at

Location 1

-e Fault at

Location 2

R

W

B


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Protection Function - ComponentsProtection FunctionProtection Function -- ComponentsComponents

CB trip coilTr

Communications LinkPCL

Man-machine interfaceHMI

DC Auxiliary supplyDC Aux

Voltage TransformerVT

Current TransformerCT

Protected ItemEquip

Circuit BreakerCB

Protection RelayPR

Bus

CBCT

P

C

L

Equip

PR

Tr

VT

DC Aux HMI

Basic Arrangement of aProtection Scheme

Control


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Protection Schemes Key FactorsProtection SchemesProtection Schemes Key FactorsKey Factors

Automatic and correct diagnosis of faults and abnormalities

Quick response measured in milliseconds to a few

seconds at the most

Must operate reliably

Can be viewed as a form of active insurance intended to

maintain high degree of service continuity and limit lossesthrough equipment and plant damage


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Protection Application ConceptsProtection Application ConceptsProtection Application Concepts

Substation A Substation B

Substation C Substation D

Source

Network

PR

PR

PRPR

PR

PR

Short-circuit faulton Line B-C

Fault current flow Circuit Breaker Circuit breaker to be opened


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Protection Application Concepts - continuedProtection Application ConceptsProtection Application Concepts -- continuedcontinued Protection relays are associated with each network plant

equipment item or circuit (ie line, cable, transformer, generator,

busbar etc)

Protection relays closest to the fault location should operate first

Circuit breakers closest to fault should be opened first

Fault current components will flow in other (unfaulted) circuits protection relays associated with these should not operate in the

first instance

As protection relays or circuit breakers may fail to operate during

a fault on occasions, back-up is necessary

Concepts of protection function Availability, Reliability andSecurity


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Protection Application Concepts - continuedProtection Application ConceptsProtection Application Concepts -- continuedcontinued

Circuit

Busbar

PRProtection

Relay for

BusbarPR

Protection

Relay for

Circuit

Fault

Unprotected

Zone

No overlap of current transformersOverlapped current transformers

correct practice

Circuit

BusbarFault

PRProtection

Relay for

CircuitPR

Protection

Relay for

Busbar

Circuit

PRProtection

Relay for

CircuitPR

Protection

Relay for

Busbar

Fault

Fault between current transformers

and circuit breaker

PRProtection

Relay for

BusbarPR

Protection

Relay for

Circuit

Current transformers on both

sides of circuit breaker


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Impacts of Protection Scheme Performance onPower System PerformanceImpacts of Protection Scheme Performance onImpacts of Protection Scheme Performance onPower System PerformancePower System Performance

Reliability of supply

Quality of supply voltage dip severity

Danger to public and livestock step and touch potentials

Hazards to other utilities assets gas, water and

telecommunications

Fire ignition bushfires, equipment fires and customer

installation fires

Operator safety

Network damage and repair cost


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Performance Requirements of ProtectionSystems - 1Performance Requirements of ProtectionPerformance Requirements of ProtectionSystemsSystems -- 11

Discriminate between load (normal) and fault (abnormal)

conditions Not be confused by non-damaging transient conditions

Be selective coordinate with other protection systems

Fast enough to prevent damage and hazards but not too

fast

Have no blind spots i.e. unprotected zones

Be able to handle maximum fault current duty


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Principles of Power System Protection

23

Performance Requirements of ProtectionSystems - 2Performance Requirements of ProtectionPerformance Requirements of ProtectionSystemsSystems -- 22

High degree of reliability and availability

Secure against incorrect operation (security)

An acceptable compromise between reliability and security

is required reliability should take precedence

Should not restrict rating of primary plant and equipment

Should be affordable


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Methods of Detecting FaultsMethods of Detecting FaultsMethods of Detecting Faults

Magnitude of current Overcurrent protection

Magnitude of current in earth or neutral Earth Fault protection

Magnitude and Phase Angle of current Directional Overcurrent protection

Magnitude and Phase Angle of current in earth or neutral Directional Earth Fault

protection

Magnitude and Angle of Impedance (Ratio V/I) Impedance protection

Difference between two or more currents Differential protection Difference between Phase Angles of two currents Phase Comparison protection

Magnitude of negative sequence current

Magnitude of Voltage Overvoltage or Undervoltage protection

Magnitude of Frequency Over or Underfrequency protection

Temperature Thermal protection

Specials i.e. transformer gas protection,


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Types of Protection RelayTypes of Protection RelayTypes of Protection Relay

Electromechanical

Electronic analogue using discrete components

Electronic analogue using ICs

Microprocessor

Microprocessor + DSP (Digital Signal Processor)


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Digital Protection RelaysDigital Protection RelaysDigital Protection Relays

Input measured quantities conditioned, filtered and digitised

All protection functions done via software in

microprocessor(s) or DSP(s)

Communications processors for HMI, control interfacing

and data transfer

Digital I/O for protection communications aspects

Software driven HMI

Relays for circuit breaker tripping


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Enhanced Performance Provided by Digital

Relays

Enhanced Performance Provided by DigitalEnhanced Performance Provided by Digital

RelaysRelays

Better accuracy of protection function measurements i.e.

less margin required for measuring errors (time, current,voltage, impedance, phase angle)

True RMS sensing and unaffected by transient components

of fault voltage and current signals

Tuning or rejection of non-power frequency currents in

sensitive earth fault protection

Complete catalogue of timing functions Result better protection coverage, shorter operating times,

better load carrying capability and enhanced security


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New FunctionsNew FunctionsNew Functions

Protection functions done in software

Once input voltages and currents are digitised, new and

additional functionality can be added at low cost

For example:

Negative sequence protection for distribution feeders

Impedance measurement instead of current

Circuit breaker fail logic Thermal modelling

Circuit breaker duty

Directional features

Other input signals can be added

Can use improved fault detection techniques

for much lower cost


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Communications Links Used for ProtectionCommunications Links Used for ProtectionCommunications Links Used for Protection

Utility owned metallic communications cables (called

supervisory or pilot cables

Optical fibre cables

Power Line Carrier (PLC)

UHF or Microwave radio

Leased links from a Telco


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SummarySummarySummary

Protection systems are critical elements of the power

system and their and their operating performance

impacts directly on the power systems safety and

reliability

Principles of Power Systems Protection Part 1

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