Fundamentals of Power System Protection - EIT · 2020. 10. 22. · • The impedance of the earth...

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Fundamentals of Power

System Protection

by

Steve Mackay

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EIT Micro-Course Series• Every two weeks we present a 35 to 45 minute interactive course• Practical, useful with Q & A throughout• PID loop Tuning / Arc Flash Protection, Functional Safety, Troubleshooting conveyors presented so far• Upcoming: – Electrical Troubleshooting and much much more…..• Go to http://www.eit.edu.au/free-courses• You get the recording and slides

IDC Technologies and The Engineering Institute of Technology (EIT)

Fundamentals of Power System Protection - Free Webinar

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The nuts and bolts of electrical power

system protection

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Key Topics• Need for protection• Characteristics and components of a

protection system• Faults and protection• Earthing and its relevance to protection• Protective devices



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Protection fundamentals• What is protection?– Avoiding the undesirable effects of abnormal electrical systembehaviour by appropriate action• What are protected?– Equipment, personnel and system (stability)• Why are they protected?– Damage, injury (Shocks/Arc flash), collapse• How are they protected?– Isolating the abnormal part of a system from thehealthy parts with least delay

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Power System ProtectionIntroduction• Customers always demand power on a continuous basis without interruptions. • Hence it is necessary to foresee the likely interruptions thatmay occur in the distribution system to detect failures and to isolate only the faulty sections.• Protective equipment or protective relay is used in a power network to detect, discriminate and isolate the faulty equipment in the network to ensure that the rest of the system is fed with continuous power and at the same time, damage to faulty section is minimized.



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Role of Power system protection

1. To safeguard the entire system to ensurecontinuity of supply.

2. To minimize damage and repair costs.

3. To ensure safety of personnel.

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Power System Protection: Basic Attributes1. SelectivitySelectivitySelectivitySelectivity:::: To detect and isolate the faulty item only.2. StabilityStabilityStabilityStability:::: To leave all healthy circuits intact to ensurecontinuity or supply.3. SensitivitySensitivitySensitivitySensitivity:::: To detect even the smallest values of faultcurrent or system abnormalities and operate correctly at itssetting before the fault causes irreparable damage.4. SpeedSpeedSpeedSpeed:::: To operate speedily when it is called upon to doso, thereby minimizing damage to the surroundings andensuring safety to personnel.



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Protection philosophy• Emphasis on Speed for the followingreasons:– To minimise damage and repair costs.– To reduce production downtime.– To prevent undue thermal and magnetic overstressing of healthy equipment on through fault.– To keep voltage depressions as short as possible inthe interests of plant stability. – Above all, to enhance the safety of personnel due toarc flashes and electric shock.

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Protection system components• Measurement of electrical parameters• Sensing abnormal behaviour• Actuating the device for isolation• Isolating• Annunciating• Powering• Enabling (ex: earthing system)



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Possible faults• Cable Faults - Most common due to both external like

moisture, digging, etc., as well as fault currents beingcarried

• Transformer faults - Not always common but economicsrule the decision on the capacity of standby transformers

• Busbar Faults - Catastrophic but duplication is morefollowed in EHV substations.

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Types of faults• There are a number of different types of faults

• A protection system must work for all the types of faults itis meant to operate

• Protection must operate at the least possible value of thedesignated parameter– Note: Current is NOT the only protection parameter



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Active vs. Passive� Active fault types (solid and incipient)

� Solid

� Immediate, complete breakdown of insulation causing:

- High fault currents / energy

- Danger to personnel

- High stressing of all network equipment due to heating and electromechanical forces and possibility of combustion

- Dips on the network voltage affecting other parties

- Faults spreading to other phases

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Active vs. Passive� Active fault types (solid and incipient)

� Incipient

� A fault that takes a long time to develop into a breakdown of

insulation caused by:

� Partial discharge currents

� These faults normally become solid faults in time



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Active vs. Passive� Passive fault types

� These are not “real” faults but conditions that will

cause faults due to cumulative effects, such as:

� Overloading (over heating insulation)

� Overvoltage (over stressing insulation)

� Under frequency

� Power swings (damages generators)

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(A) Phase-to-ground (E) Three Phase-To-ground

(B) Phase-to-Phase (F) Phase-to-Pilot *

(C) Phase-to-Phase-to-ground (G) Pilot-to-ground *

(D) Three Phase * In mines

Types of Three-Phase Faults



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Magnitudes of fault currents• Normally impedance decides the value of fault currents -

But impedance can not be reduced below a certain value

• ground currents can be limited by grounding the neutralof the source and choosing suitable grounding method

• Phase fault currents can not be controlled

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Transient and permanent faults• Transient faults - do not damage insulation permanently

(eg. Tree branches on O/H line), re-closing will besuccessful

• Permanent - the insulation has broken down permanentlyrequiring repair to restore insulation levels (re-closing willfail)



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Types of faults• Phase Faults (limited only by positive

sequence impedance of system)– High Fault Currents.– Only limited by inherent impedance of Power System.• Earth Faults– Solid grounding means high earth fault currents– Only limited by inherent zero sequence impedance of Power system.

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Consequences• Heavy currents damage equipment extensively.– Danger of fire hazard.• This leads to long outage times.– Lost production and lost revenue.• Heavy currents in earth bonding gives rise to high

touch potentials - dangerous to human life.

• Large Fault currents are more hazardous in ignitinggases.– Explosion Hazard.



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Solutions• Phase Segregation (separating phases far apart)– Eliminates phase-to-phase faults.• Resistance grounding– Means lower earth fault currents– Value can be chosen during design stage to limit current todesired value - say 400Amps

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Earth faults• Most faults in systems are due to insulation failures

• The current that will flow depends on the type of systemearthing adopted and the effectiveness of protection earthing

• The current flow will influence– The touch voltage (in the protective earthing)– The time of protection operation



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Types of System earthing and Earth Fault Magnitude

• Unearthed: No current except through thesystem capacitance

• Solidly earthed: High, only limited by earthcircuit impedance

• Impedance earthed: Mainly dependant onneutral impedance

• Tuned earthed: Extremely low (< 10 amps)

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Effects of electricity on humans

Four main factors determining the seriousness of

shock:

• Path of current flow through body

• Magnitude of current

• Time that current flows for

• The body’s electrical resistance



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Role of Earth Fault Protection• Useful for Indirect Contact only

• Danger is solely decided by touch/stepvoltage and time for fault isolation

• Sensitivity of protection is important wherefault loop resistance is likely to be high

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Effects of a current flow through the body

Perception -

tingle - 1 mA

Let go -

10mASpasm - 16

mA

Constriction -

70 - 100 mA -

DEATH



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Resistance of the human body

For design purposes, a resistance of 1000 Ohms is considered

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Important: Earth fault loop resistance

• The impedance of the earth fault current loop starting andending at the point of earth fault. The earth fault loop impedance comprises the following starting at the point of fault.– The circuit protective conductor– The consumers earthing terminal and earthing conductor– The metallic or earth return path as applicable– The path through the earthed neutral point of the transformer– The transformer winding and– The phase conductor from the transformer to the point of fault



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Earth Loop Resistance and Protection

• Earth Loop Resistance governs flow of earth fault current

• In LV systems, flow through earth return path can cause lowfault currents due to high loop resistance and therebyprotection failure

• Avoid problems by using TN type of connections (TN-S or TN-C-S/MEN systems where permissible)

• Verify by measurement of loop resistance in branch circuits

• Match fault current to protective device sensitivity

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Neutral CT (Standby Earth Fault) scheme



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Core balance (zero sequence) CT Scheme

A Residual Current device (RCD) uses this principle for obtaining sensitive protection for earth leakage currents

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Summation CT scheme (4-wire feeders)



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Important• 4 Wire systems can cause problems due

to:– Unbalance (zero sequence currents)– TripleN harmonics

• Ensure correct connections of CTs toavoid false trips

• With multiple sources: Incorrect relay pickup-Neutral isolation for avoidance ofparallel return paths

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Protective devices• Used for sensing and isolating faulty

circuits• Basic types:

– Direct acting devices: Fuse– Mounted integrally with breakers: Releases– External protective devices (relays)



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Fuse-The Basic Protection Device

• A fuse is the most basic of all protectivedevices and performs all the protectionfunctions normally obtained by severaldevices

• A fuse protects against short circuits andsometimes earth faults

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Fuses - types• ReReReRe----wireable Typewireable Typewireable Typewireable Type

Fusible wire– Disadvantages: Open to abuse, incorrect rating used to keep circuitin-rating drops as time goes by– Advantage: Fail safe• Cartridge TypeCartridge TypeCartridge TypeCartridge Type– Silver element enclosed in a barrelof insulating material (sometimes filled with quartz sand)



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Fuses - typesCARTRIDGE TYPECARTRIDGE TYPECARTRIDGE TYPECARTRIDGE TYPEAdvantages :– fault energy contained by insulating tube– Sealed hence does not deteriorate as fast as open type – Better grading possible– Quartz sand absorbs energy and melts across ionized metalpath to quench arc– Faster and can handle very high currents up to 100 kA– Normal currents are closer to fusing currents today due to improved materials and design

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Fuses characteristics



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Fusing factorThis British Standard lays down:

• limits for Temperature rise• Fusing factor - Minimum fusing current = 1.4

Rated cont. current• Breaking capacity

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Energy “ Let Through”Energy let through basically refers to the energy let into the circuit till fusing and its value is proportional to I 2 x t

• Fuses can limit this energey by fusing very quickly -usually under ¼ cycle

• Circuit breakers can take up to 10 cycles (10 x 20ms =200 ms) to open i.e., 40 times more energy is released into the fault !! (compared to a fuse which breaks the current flow in ¼ cycle)



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Energy “ Let Through”

Fault current one full

cycle (0.02 second)

Energy (I t) letthrough by faultof one cycle duration

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Energy (I t) letthrough by H.R.C.Fuse-link

2

Peak

Time

H.R.C Fuse-link duration

H.R.C

Fuse-link

cut-off

I (rms)

I (rms)

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Fuse applicationsSteady loads - Normally protect against over load as well as short circuit.

Fluctuating loads eg. DOL motors with high inrush compared to normal rating, cranes, etc - Here fuses generally protect against short circuit only.



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General benefits and limitationsBenefits:• Serves two purposes: fault detector and interrupter• Main virtue – SPEED and limiting fault energyLimitations:• Can only detect overcurrent faults (not overloads)– In solidly earthed systems fuses can serve as earth faultprotection also• Fixed current/time characteristic• Needs replacing after operating• Use for LV and MV applications (up to 66kV)

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Circuit breakers and protective devices

• Fuses act as fault current sensors and interrupters

• Other approaches use a circuit breaker to interrupt shortcircuits/earth faults

• Protection can be– External to the circuit breaker (ex: relays)– Integrated within the breaker (Trip unit/release)

• External relays need auxiliary power-Expensive,maintenance-intensive



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Circuit breaker with releases• Integrated protection-independent, self-

actuating• Electro-mechanical

– Themo-magnetic and electromagnetic typesfor IDMTL+Instantaneous current protection

– Ground fault through an external relay• Current trend: Integral digital protection-

versatile

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Typical device highlights• Built-in current sensors (output 0.5 amps)• Rating plug to decide primary current settings (from p.u.

values)• A/D converters for signal processing• Breakers designed to trip with minimum mechanical effort• Minimum of 20% rated current necessary for operation• PTs for voltage-dependent protection• Separate power supply



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Digital protection features• Compact size-the same basic device used for an entire

range of breaker ratings• Widely adjustable characteristics• Built-in ground fault protection by default• Switching memory for obtaining exact thermal behaviour

of protected equipment• Other protections (voltage, frequency and reverse power)• True RMS current sensing• Panel indication of load current and cause of tripping• Remote alarms and commands• Communication capabilities for power management

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Digital protection-ACB



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Digital protection-MCCB

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Features• Long time delay (IDMT), Short delay

(DMT) and instantaneous settings• Selectable I2.t feature • Optional ground fault protection with fixed

or selectable I2.t feature • Ground fault with summation (internal

input)• Alternatively with system neutral CT input



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Typical protection curves

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Typical ground fault protection curves

a) Ground fault delay with

fixed time settingb) Time setting-I2.t type



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Extended protection• Default current protections are overload,

short circuit and ground faults• Special protections also available

– Current/Voltage unbalance• Motor loads against single-phasing conditions

– Over/Under voltage• Motor applications to prevent restarting after power

interruption– Reverse Power and Over/Under frequency

• Useful in cogeneration applications

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Other common features• True RMS current sensing

– Useful to avoid faulty operation of protection in circuitswith high level of harmonic content

• Panel indications– Uses a built in display on the protection device

• Remote trip alarm and remote commands– Alarm of trips and remote command for CB on/off

• Communication features– Ability to store and communicate circuit analog/digital

parameters



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Basic types of relays• Electromagnetic relay

– Attracted armature instantaneous– Induction disc type inverse time

• Static relay– Analog (discrete components)– Digital (Microprocessor-based)

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Timeline of relay developmentDevelopment of

electrical distribution

networks

Development and

wide use of Electro-

mechanical relays

Development of Static

relays - discrete

components

1930 1960

1970Wide use of Static relays

- discrete components -

1990

Development and manufacture of microprocessor based relays



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Attracted armature type relay

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Attracted armature type relay• Simple construction• Instantaneous operation

– No intentional time delay– Operation time less than 100 m. sec and usually

around 0.05 sec– High speed relay operation measured in cycles– Applications in several protection schemes• High set over current protection, differential protection– Time delay obtained using a timer relay in cascade (if

required)



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Induction disc type inverse time relay

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Characteristic of IDMTL relayCharacteristic of IDMTL relayCharacteristic of IDMTL relayCharacteristic of IDMTL relay



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Current Setting IDMT Relay• Plug setting : This adjusts the setting current by means of

a plug bridge which varies the number of turns on theupper magnet

• This setting determines the level of current at which therelay will start or pick-up

• BS142 says - relay must definitely operate at 130%setting and definitely reset at 70% setting

• Normally the relay picks up at about 105% to 130% of itsplug setting

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Time Multiplier Setting�TM setting : This rotates the tripping bar attached to the disc closer to or further away from the tripping contacts� Effectively moves the curve DOWN the axis � This curve shows the relay will operate in 3 seconds at 10 times the plug setting (with the time multiplier =1)



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Digital Relay advantages� Cost� Flexibility� Functionality that is not given by electro-mechanical

relays� Size� CT burden low� DC power drain low� Improved sensitivity and speed� With microprocessor relays any characteristic is

possible

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IEC 60255 Inverse Curves� t = k x β /( (I/I>)α - 1) where:� t = operate time in secs.� K = time multiplier� I = measured current� I> = set starting current � α & β are constants for curve selection

-Normal Inverse, Very Inverse and Extremely

Inverse (and any other user defined curves)



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Typical digital relay Schematic

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Static relay features• High set overcurrent with time delay– Closer settings due to absence of transient over-reach• Breaker fail protection built-in– Impulse to a second trip coil or a back-up breaker• Digital display of relay parameters and operating values• Memorized information available after tripping and

cumulative operational values• Low auxiliary power requirement and burden on

current/voltage transformers• High input AC variation acceptable



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Information from static relays• Measurement data of current and voltage• Information stored by the relay after a fault

situation• Relay setting values• Status information on the circuit breakers

and isolators• Event information

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Integrated protection and control



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What is an IED?• An electronic device that possesses some

kind of local intelligence• IED in protection applications should have:

– Versatile electrical protection functions– Advanced local control intelligence– Monitoring abilities– Capability of extensive communications

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Main functions of IED• Protection• Control• Monitoring• Metering• Communications



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Communication• Communicating all data in previous slides to/from

Control center

• Facilitate remote control, monitoring andmeasurement

• Facilitate remote protection settings• IED forms the basis of modern substation

automation systems

Thank You For Your InterestIf you are interested in further training, please visit:

The Engineering Institute of TechnologiesOnline Certificate and

Advanced Diploma programs:

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IDC Technologies

1, 2 & 3 day practical workshops, technical manuals,

onsite training & International conferences:

www.idc-online.com



Fundamentals of Power System Protection - EIT · 2020. 10. 22. · • The impedance of the earth...

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