Relay Protection Basics rev 3

7/31/2019 Relay Protection Basics rev 3

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1GE Consumer & Industrial

Multilin

Protection BasicsPresented by

John S. Levine, P.E.

Levine Lectronics and Lectric, Inc.

770 565-1556

[email protected]


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Protection Fundamentals

By

John Levine


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Outline

Introductions Tools

Enervista Launchpad

On Line Store Demo Relays at Levine

ANSI number

Training CDs Protection Fundamentals


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Objective

We are here to help make your job easier.This is very informal and designed around

Applications. Please ask question. We

are not here to preach to you. The knowledge base in the room varies

greatly. If you have a question, there is a

good chance there are 3 or 4 other peoplethat have the same question. Please askit.


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Tools


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Demo Relays at L-3


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Relays at L-3


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GE Multilin Training CDs


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ANSI Symbols


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Conversion of Electro-Mechanical

to Electronic sheet

P P i t t ti t


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13GE Consumer & IndustrialMultilin

PowerPoint presentations at:

http://l-3.com/private/ieee/
http://l-3.com/private/ieee/http://l-3.com/private/ieee/


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Desirable Protection Attributes

Reliability: System operate properly Security: Dont trip when you shouldnt

Dependability: Trip when you should

Selectivity: Trip the minimal amount to clear thefault or abnormal operating condition

Speed: Usually the faster the better in terms ofminimizing equipment damage and maintaining

system integrity Simplicity: KISS

Economics: Dont break the bank


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Selection of protective relays requirescompromises:

Maximum and Reliable protection at minimumequipment cost

High Sensitivity to faults and insensitivity tomaximum load currents

High-speed fault clearance with correct

selectivity Selectivity in isolating small faulty area

Ability to operate correctly under allpredictable power system conditions

Art & Science of ProtectionArt & Science of Protection


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Cost of protective relays should bebalanced against risks involved ifprotection is not sufficient and notenough redundancy.

Primary objectives is to have faultedzones primary protection operate first,but if there are protective relays

failures, some form of backupprotection is provided.

Backup protection is local (if local

primary protection fails to clear fault)and remote if remote rotection fails

Art & Science of ProtectionArt & Science of Protection


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Primary Equipment & ComponentsPrimary Equipment & Components

Transformers - to step up or step down voltage level

Breakers - to energize equipment and interrupt fault current to

isolate faulted equipment

Insulators - to insulate equipment from ground and otherphases

Isolators (switches) - to create a visible and permanent

isolation of primary equipment for maintenance purposes and

route power flow over certain buses.

Bus - to allow multiple connections (feeders) to the same

source of power (transformer).


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Primary Equipment & ComponentsPrimary Equipment & Components Grounding - to operate and maintain equipment safely

Arrester - to protect primary equipment of sudden overvoltage

(lightning strike).

Switchgear integrated components to switch, protect, meter

and control power flow

Reactors - to limit fault current (series) or compensate for

charge current (shunt)

VT and CT - to measure primary current and voltage and

supply scaled down values to P&C, metering, SCADA, etc.

Regulators - voltage, current, VAR, phase angle, etc.


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Types of Protection

Overcurrent Uses current to determine magnitude of fault Simple

May employ definite time or inverse time curves

May be slow Selectivity at the cost of speed (coordination stacks)

Inexpensive

May use various polarizing voltages or ground current

for directionality Communication aided schemes make more selective

I O P i (IOC) &


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Instantaneous Overcurrent Protection (IOC) &

Definite Time Overcurrent

t

I

CTI

55

+5

55

+5

CTI Relay closest to fault operates

first Relays closer to source

operate slower Time between operating for

same current is called CTI(Clearing Time Interval)

Distribution

Substation


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(TOC) Coordination

t

I

CTI

Relay closest to fault operates

first Relays closer to source

operate slower

Time between operating forsame current is called CTI

Distribution

Substation


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Selection of the curvesuses what is termedas a timemultiplier or timedial to effectively

shift the curve up ordown on the time axis

Operate region liesabove selected curve,

while no-operateregion lies below it

Inverse curves canapproximate fusecurve shapes

Time Overcurrent Protection (TOC)


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Multiples of pick-up

Time Overcurrent Protection

(51, 51N, 51G)

Cl i Di ti l O t


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Classic Directional Overcurrent

Scheme for Looped System

Protection

5 5

5 5

5

A

B

E

D

C

c

e

d

a

b

L L

L L

Bus X Bus Y

E D C B A

a b c d e


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Types of Protection

Differential current in = current out

Simple

Very fast

Very defined clearing area

Expensive

Practical distance limitations Line differential systems overcome this using

digital communications


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Differential

Note CT polarity

dots

This is athrough-current

representation

Perfect

waveforms, nosaturation

IP

IS

IR-X

IP

IS

IR-Y

Relay

CT-X CT-Y

+ (- ) =5 5 5

+5

-5

5

Cu

rrent,pu

DIFF CURRENT

pu5


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Differential

Note CT

polarity dots

This is aninternal fault

representation

Perfectwaveforms, no

saturation

FaultIP

IS

IR-X

IP

IS

IR-Y

Relay

+ (+ ) =5 5 5

+5

-5

5

Current,pu

X

pu5 pu5

CT-X CT-Y

DIFF CURRENT


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Types of Protection

Voltage Uses voltage to infer fault or abnormal

condition

May employ definite time or inverse time

curves May also be used for undervoltage load

shedding Simple

May be slow

Selectivity at the cost of speed (coordination stacks)

Inexpensive


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Types of Protection

Frequency Uses frequency of voltage to detect power

balance condition

May employ definite time or inverse time

curves Used for load shedding & machinery

under/overspeed protection Simple

May be slow

Selectivity at the cost of speed can be expensive


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Types of Protection

Power Uses voltage and current to determine

power flow magnitude and direction

Typically definite time Complex

May be slow

Accuracy important for many applications

Can be expensive


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Types of Protection

Distance (Impedance) Uses voltage and current to determine impedance of

fault

Set on impedance [R-X] plane

Uses definite time Impedance related to distance from relay

Complicated

Fast

Somewhat defined clearing area with reasonableaccuracy

Expensive

Communication aided schemes make more selective


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Impedance

Relay in Zone 1 operates first

Time between Zones is calledCTI

Source

A B

21 21

T1

T2

ZA

ZB

R

X ZL


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Impedance: POTT Scheme

POTT will trip only faulted line section

RO elements are 21; 21G or 67N


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Powervs. Protection Engineer:

Views of the World

180 Opposites!

MVA = KV5

Z( R

c)

Rv

+Q

+P

-Q

-P

MorePo

wer

MorePower

SS

SS


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Generation-typically at 4-

20kV

Transmission-typically at 230-765kV

Subtransmission-typically at 69-161kV

Receives power from transmissionsystem and transforms into

subtransmission level

Receives power from subtransmissionsystem and transforms into primaryfeeder voltage

Distribution network-typically 2.4-69kV

Low voltage (service)-typically120-600V

Typical

Bulk

PowerSystem

Typical

Bulk

PowerSystem



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1. Generator or Generator-Transformer Units

2. Transformers

3. Buses

4. Lines (transmission and distribution)5. Utilization equipment (motors, static loads, etc.)

6. Capacitor or reactor (when separately protected)

Unit Generator-Tx zone

Bus zone

Line zone

Bus zone

Transformer zone Transformer zone

Bus zone

Generator

~

XFMR Bus Line Bus XFMR Bus Motor

Motor zone

ProtectionZonesProtectionZones


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1. Overlap is accomplished by the locations of CTs, the keysource for protective relays.

2. In some cases a fault might involve a CT or a circuit breakeritself, which means it can not be cleared until adjacentbreakers (local or remote) are opened.

Zone A Zone B

Relay Zone A

Relay Zone B

CTs are located at both sides ofCB-fault between CTs is cleared fromboth remote sides

Zone A Zone B

Relay Zone A

Relay Zone B

CTs are located at one sideof CB-fault between CTs is sensedby both relays, remote right side

operate only.

Zone OverlapZone Overlap

Electrical Mechanical


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Parameter Comparisons




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Parameter Comparisons

Effects of Capacitive & Inductive Loads


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Effects of Capacitive & Inductive Loads

on Current


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Motor Model and Starting Curves


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1. One-line diagram of the system or area involved

2. Impedances and connections of power equipment,system frequency, voltage level and phase sequence

3. Existing schemes

4. Operating procedures and practices affecting

protection

5. Importance of protection required and maximumallowed clearance times

6. System fault studies

7. Maximum load and system swing limits

8. CTs and VTs locations, connections and ratios

9. Future expansion expectance

10.Any special considerations for application.

What Info is Required to Apply ProtectionWhat Info is Required to Apply Protection

C37 2


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C37.2:

Device

Numbers

Partial listing



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One Line Diagram

Non-dimensioned diagram showing how

pieces of electrical equipment are

connected

Simplification of actual system

Equipment is shown as boxes, circles and

other simple graphic symbols

Symbols should follow ANSI or IEC

conventions


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1-Line Symbols [1]


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1-Line Symbols [2]


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1-Line Symbols [3]


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1-Line Symbols [4]


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1-Line [1]


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1-Line [2]


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3-Line


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Diagram Comparison


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C37.2: Standard Reference Position

1) These may be speed, voltage, current,load, or similar adjusting devicescomprising rheostats, springs, levers, orother components for the purpose.

2) These electrically operated devices areof the nonlatched-in type, whose contactposition is dependent only upon thedegree of energization of the operating,restraining, or holding coil or coils thatmay or may not be suitable forcontinuous energization. The de-energized position of the device is thatwith all coils de-energized

3) The energizing influences for thesedevices are considered to be,respectively, rising temperature, risinglevel, increasing flow, rising speed,increasing vibration, and increasingpressure.

4.5.3) In the case of latched-in or hand-reset relays, which operate fromprotective devices to perform theshutdown of a piece of equipment andhold it out of service, the contacts shouldpreferably be shown in the normal,nonlockout position


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CB Trip Circuit (Simplified)

Showing Contacts NOT in


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Standard Reference Condition

Some people show the contactstate changed like this



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Standard Reference Condition

Better practice, do not change thecontact style, but rather use

marks like these to indicate non-

standard reference position


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Lock Out Relay

b55

PR

b55

Shown in RESET position

55

TCa55

CB Coil Circuit Monitoring:


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T with CB Closed; C with CB Opened

Coil Monitor

Input

Trip/Close

Contact

+

T/C

Coil

-

/a55

or

/b55

Breaker

Relay

/a for trip circuit55

/b for close circuit55



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Both T&C Regardless of CB state

Breaker

Relay

Breaker

Relay

Current Transformers


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Current transformers are used to step primary systemcurrents to values usable by relays, meters, SCADA,

transducers, etc.

CT ratios are expressed as primary to secondary;2000:5, 1200:5, 600:5, 300:5

A 2000:5 CT has a CTR of 400

Current Transformers

Standard IEEE CT Relay


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IEEE relay class is defined in terms of the voltage a CT

can deliver at 20 times the nominal current rating without

exceeding a 10% composite ratio error.

For example, a relay class of C100 on a 1200:5 CT means that

the CT can develop 100 volts at 24,000 primary amps

(1200*20) without exceeding a 10% ratio error. Maximum

burden = 1 ohm.

100 V = 20 * 5 * (1ohm)

200 V = 20 * 5 * (2 ohms)

400 V = 20 * 5 * (4 ohms)

800 V = 20 * 5 * (8 ohms)

Standard IEEE CT Relay

Accuracy

Excitation Curve


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Excitation Curve

Standard IEEE CT Burdens (5 Amp


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Application BurdenDesignation

Impedance(Ohms)

VA @amps5

PowerFactor

Metering B .55 .55 .55 .00

B .55 .55 5 .55

B .55 .55 .555 .55B .55 .55 .555 .55

B .00 .55 55 .55

Relaying B5 5 55 .55

B5 5 55 .55B5 5 555 .55

B5 5 555 .55

Standard IEEE CT Burdens (5 Amp(Per IEEE Std. C57.13-199

Current into the Dot, Out of the Dot


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,

Current out of the dot, in to the dot

Forward Power

IP

IS

IR

Relay

or Meter

Forward Power

IP

IS

IR

Relay

or Meter

Voltage Transformers


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VVPP

VVSS

Relay

Voltage (potential) transformers are used to isolateand step down and accurately reproduce the scaledvoltage for the protective device or relay

VT ratios are typically expressed as primary tosecondary; 14400:120, 7200:120

A 4160:120 VT has a VTR of 34.66

Voltage Transformers

Typical CT/VT Circuits


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Typical CT/VT Circuits

Courtesy of Blackburn, Protective Relay: Principles and Applications

CT/VT Circuit vs Casing Ground


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CT/VT Circuit vs. Casing Ground

Case ground made at IT location

Secondary circuit ground made at first point of

use

Case

Secondary Circuit

E i t G di


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Equipment Grounding

Prevents shock exposure of personnel

Provides current carrying capability for the

ground-fault current

Grounding includes design and construction ofsubstation ground mat and CT and VT safety

grounding


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System Grounding

Limits overvoltages

Limits difference in electric potential through local

area conducting objects

Several methods

Ungrounded

Reactance Coil Grounded

High Z Grounded Low Z Grounded

Solidly Grounded


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1. Ungrounded: There is nointentional ground applied tothe system-however itsgrounded through naturalcapacitance. Found in 2.4-

15kV systems.

2. Reactance Grounded: Totalsystem capacitance is cancelled

by equal inductance. Thisdecreases the current at thefault and limits voltage acrossthe arc at the fault to decreasedamage.

System Grounding


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3. High Resistance Grounded:Limits ground fault current to10A-20A. Used to limittransient overvoltages due toarcing ground faults.

R0 = 2X0

System Grounding

S G di


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5. Solidly Grounded: There is aconnection of transformer orgenerator neutral directly tostation ground.

Effectively Grounded: R0


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Grounding Differences.Why?

Solidly Grounded

Much ground current (damage)

No neutral voltage shift Line-ground insulation

Limits step potential issues

Faulted area will clear

Inexpensive relaying


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Somewhat Grounded

Manage ground current (manage damage)

Some neutral voltage shift

Faulted area will clear

More expensive than solid, less expensive then

ungrounded


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Ungrounded

Very little ground current (less damage)

Big neutral voltage shift

Must insulate line-to-line voltage

May run system while trying to find ground fault

Relay more difficult/costly to detect and locate ground

faults

If you get a second ground fault on adjacent phase,

watch out!

System Grounding Influences


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Ground Fault Detection MethodsSource

55

00

N55

N55

55

55

N55

N55

55

55

N55

N55

Low/No Z



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Syste G ou d g ue ces

Ground Fault Detection MethodsSource

55

00

G55

G55

55

55

G55

G55

55

55

G55

G55

Med/High Z

Basic Current Connections:


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Basic Current Connections:How System is Grounded

Determines How Ground Fault is Detected

Medium/HighResistance

Ground

Low/NoResistance

Ground


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Substation TypesSubstation Types

Single Supply

Multiple Supply

Mobile Substations for emergencies

Types are defined by number of

transformers, buses, breakers to

provide adequate service for

application

Industrial Substation Arrangements


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Industrial Substation Arrangements(Typical)

Industrial Substation Arrangements


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Industrial Substation Arrangements(Typical)

Utility Substation ArrangementsUtility Substation Arrangements


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Single Bus, 1 Tx, Dual supply Single Bus, 2 Tx, Dual Supply 2-sections Bus with HS Tie-Breaker,

2 Tx, Dual Supply

(Typical)



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Breaker-and-a-halfallows reduction of

equipment cost by using 3 breakers for

each 2 circuits. For load transfer and

operation is simple, but relaying is

complex as middle breaker is responsible

to both circuits


Bus

1

Bus 2

Ring bus advantage that one

breaker per circuit. Also each

outgoing circuit (Tx) has 2 sources

of supply. Any breaker can be taken

from service without disrupting

others.

(Typical)

Utility Substation ArrangementsUtility Substation ArrangementsUtility Substation ArrangementsUtility Substation Arrangements


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Double Bus: Upper Main and

Transfer, bottom Double Main bus

Main bus

Aux. bus

Bus 1

Bus 2

Tie

breaker

Utility Substation ArrangementsUtility Substation ArrangementsUtility Substation ArrangementsUtility Substation Arrangements

Main

ReserveTransfer

Main-Reserved and Transfer

Bus: Allows maintenance of any

bus and any breaker

(Typical)

Switchgear Defined


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Switchgear Defined Assemblies containing electrical switching,

protection, metering and management devices Used in three-phase, high-power industrial,commercial and utility applications

Covers a variety of actual uses, including motor

control, distribution panels and outdoorswitchyards

The term "switchgear" is plural, even whenreferring to a single switchgear assembly (never

say, "switchgears") May be a described in terms of use:

"the generator switchgear"

"the stamping line switchgear"

Switchgear Examples


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Switchgear Examples

Switchgear:M t lCl d M t l E l d


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MetalClad vs. Metal-Enclosed

Metal-clad switchgear (C37.20.2) Breakers or switches must be draw-out design Breakers must be electrically operated, with anti-pump

featureAll bus must be insulated

Completely enclosed on all side and top with groundedmetal Breaker, bus and cable compartments isolated by

metal barriers, with no intentional openingsAutomatic shutters over primary breaker stabs.

Metal-enclosed switchgear Bus not insulated Breakers or switches not required to be draw-out No compartment barriering required

Switchgear Basics [1]


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Switchgear Basics [1] All Switchgear has a metal

enclosure

Metalclad constructionrequires 11 gauge steelbetween sections and maincompartments

Prevents contact with livecircuits and propagation ofionized gases in the unlikelyevent of an internal fault.

Enclosures are also rated asweather-tight for outdoor use

Metalclad gear will includeshutters to ensure thatpowered buses are covered atall times, even when a circuitbreaker is removed.

S it h B i [2]


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Devices such as circuit breakers or fused switchesprovide protection against short circuits and groundfaults

Interrupting devices (other than fuses) are non-automatic. They require control signals instructing

them to open or close. Monitoring and control circuitry work together withthe switching and interrupting devices to turn circuitson and off, and guard circuits from degradation orfluctuations in power supply that could affect ordamage equipment

Routine metering functions include operatingamperes and voltage, watts, kilowatt hours,frequency, power factor.



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91


Switchgear Basics [3] Power to switchgear is

connected via Cables or BusDuct

The main internal bus carriespower between elements withinthe switchgear

Power within the switchgearmoves from compartment tocompartment on horizontal bus,and within compartments onvertical bus

Instrument Transformers (CTs& PTs) are used to step downcurrent and voltage from theprimary circuits or use in lower-energy monitoring and controlcircuitry.

Air Magnetic Breakers


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92


Air Magnetic Breakers

SF6 and Vacuum Breakers


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93


SF6 and Vacuum Breakers

A Good Day in System


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94


y y

Protection

CTs and VTs bring electrical info to relays

Relays sense current and voltage and declare

fault

Relays send signals through control circuits tocircuit breakers

Circuit breaker(s) correctly trip

What Could Go WrongHere????

A Bad Day in System


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95


Protection

CTs or VTs are shorted, opened, or their wiring is Relays do not declare fault due to setting errors,

faulty relay, CT saturation

Control wires cut or batteries dead so no signal issent from relay to circuit breaker

Circuit breakers do not have power, burnt trip coil

or otherwise fail to trip

Protection Systems Typicallyare Designed for N-1


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96


Protection Performance Statistics

Correct and desired: 92.2%

Correct but undesired: 5.3% Incorrect: 2.1%

Fail to trip: 0.4%

Contribution to Faults


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97


Fault Types (Shunt)


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98


Fault Types (Shunt)

Sh t Ci it C l l ti


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99


X

X

Short Circuit Calculation

Fault Types Single Phase to Ground

Sh t Ci it C l l ti


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100


X

X

Short Circuit Calculations

Fault Types Line to Line

Short Circ it Calc lations


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101



Fault Types Three Phase

X

X

X

AC & DC Current Components


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102


of Fault Current

Variation of current with time


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103


Variation of current with time

during a fault

Variation of generator reactance


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104


Variation of generator reactance

during a fault

Useful Conversions


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105



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106


Per Unit System

Establish two base quantities:

Standard practice is to define Base power 3 phase

Base voltage line to line

Other quantities derived with basic powerequations

Per Unit Basics


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107




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108


Per Unit System

Per Unit Value = Actual QuantityBase Quantity

Vpu = VactualVbase

Ipu = Iactual

Ibase

Zpu = ZactualZbase



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109



Per Unit System

5x kVL-L baseI

base =

x5555MVAbase

Zbase =

kV5L-L base

MVAbase



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110


Per Unit System Base Conversion

Zpu = ZactualZbase

Zbase = kV2base

MVAbase

Zpu1 = MVAbase1 kV2base1

X Zactual

Zpu2 = MVAbase2 kV2base2

XZactual

0Zpu2 =Zpu1 x kV2base1 xMVAbase2

kV2base2 MVAbase1

Information for Short Circuit,


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111


Load Flow and Voltage Studies

To perform the above studies, information

is needed on the electrical apparatus andsources to the system under consideration


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112


Utility Information


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113


Utility Information

kV

MVA short circuit

Voltage and voltage variation Harmonic and flicker requirements

Generator Information


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114


Generator Information

Rated kV

Rate MVA, MW

Xs; synchronous reactance Xd; transient reactance

Xd; subtransient reactance

Motor Drive


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115


Motor Drive

kV Rated HP or KW

Type

Sync or Induction Subtransient or

locked rotor current

Is it regenerative Harmonic spectrum

Transformers


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Rated primary and secondary kV

Rated MVA (OA, FA, FOA) Winding connections (Wye, Delta)

Impedance and MVA base of impedance

Reactors

Rated kV Ohms

Cables and Transmission Lines


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Cables and Transmission Lines

For rough calculations, some can be neglected

Length of conductor

Impedance at given length Size of conductor

Spacing of overhead conductors

Rated voltage

Type of conduit

Number of conductors or number per phase

ANSI 1-Line


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ANSI 1 Line

IEC 1-Line


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IEC 1 Line

Short Circuit Study [1]


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A Study of a Fault.


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A Study of a Fault.

Fault Interruption and Arcing


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Arc Flash Hazard


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Arc Flash Hazard

Arc Flash Mitigation:

Problem Description


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Problem Description

An electric arc flash can occur if a conductive object getstoo close to a high-amp current source or by equipment

failure (ex., while opening or closing disconnects, racking

out)

The arc can heat the air to temperatures as high as 35,000 F, and

vaporize metal in equipment

The arc flash can cause severe skin burns by direct heat exposure

and by igniting clothing

The heating of the air and vaporization of metal creates a pressure

wave (arc blast) that can damage hearing and cause memory loss(from concussion) and other injuries.

Flying metal parts are also a hazard.

Methods to Reduce

Arc Flash Hazard


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Arc Flash Hazard

Arc flash energy may be expressed in I2t terms,so you can decrease the I or decrease the t tolessen the energy

Protective relays can help lessen the t byoptimizing sensitivity and decreasing clearing time

Protective Relay Techniques

Other means can lessen the I by limiting fault

current Non-Protective Relay Techniques

Non-Protective Relaying Methods

of Reducing Arc Flash Hazard


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System designmodifications increase

power transformer

impedance

Addition of phase reactors

Faster operating breakers

Splitting of buses

Current limiting fuses

(provides partial protection

only for a limited currentrange)

Electronic current limiters

(these devices sense

overcurrent and interrupt very

high currents with replaceable

conductor links (explosive

charge)

Arc-resistant switchgear (this

really doesn't reduce arc flash

energy; it deflects the energy

away from personnel)

Optical arc flash protection via

fiber sensors

Optical arc flash protection via

lens sensors

Protective Relaying Methods



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Bus differential protection (this

reduces the arc flash energy by

reducing the clearing time

Zone interlock schemes where

bus relay selectively is allowed

to trip or block depending on

location of faults as identified

from feeder relays

Temporary setting changes to

reduce clearing time during

maintenance

Sacrifices coordination

FlexCurve for improved

coordination opportunities

Employ 51VC/VR on feeders

fed from small generation to

improve sensitivity and

coordination

Employ UV light detectors with

current disturbance detectors

for selective gear tripping

Fuses vs. Relayed Breakers


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Arc Flash Hazards


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Arc Pressure Wave


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Arc Flash Warning Example [1]


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Copy of this presentation are at:

www.L-3.com\private\IEEE


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QUESTIONS?

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Relay Protection Basics rev 3