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Power System Protection. Dr. Ibrahim El-Amin. Protective Device Coordination. Definition. Overcurrent Coordination A systematic study of current responsive devices in an electrical power system. Objective. To determine the ratings and settings of fuses, breakers, relay, etc.

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power system protection

Power System Protection

Dr. Ibrahim El-Amin

definition
Definition
  • Overcurrent Coordination
    • A systematic study of current responsive devices in an electrical power system.
objective
Objective
  • To determine the ratings and settings of fuses, breakers, relay, etc.
  • To isolate the fault or overloads.
criteria
Criteria
  • Economics
  • Available Measures of Fault
  • Operating Practices
  • Previous Experience
design
Design
  • Open only PD upstream of the fault or overload
  • Provide satisfactory protection for overloads
  • Interrupt SC as rapidly (instantaneously) as possible
  • Comply with all applicable standards and codes
  • Plot the Time Current Characteristics of different PDs
analysis
Analysis

When:

  • New electrical systems
  • Plant electrical system expansion/retrofits
  • Coordination failure in an existing plant
protection vs coordination
Protection vs. Coordination
  • Coordination is not an exact science
  • Compromise between protection and coordination
    • Reliability
    • Speed
    • Performance
    • Economics
    • Simplicity
protection
Protection
  • Prevent injury to personnel
  • Minimize damage to components
    • Quickly isolate the affected portion of the system
    • Minimize the magnitude of available short-circuit
spectrum of currents
Spectrum Of Currents
  • Load Current
    • Up to 100% of full-load
    • 115-125% (mild overload)
  • Overcurrent
    • Abnormal loading condition (Locked-Rotor)
  • Fault Current
    • Fault condition
    • Ten times the full-load current and higher
coordination
Coordination
  • Limit the extend and duration of service interruption
  • Selective fault isolation
  • Provide alternate circuits
coordination12

C

D

A

B

t

A

C

D

B

I

Coordination
equipment
Equipment
  • Motor
  • Transformer
  • Generator
  • Cable
  • Busway
capability damage curves

I2t

I2t

I2t

t

I22t

Motor

Xfmr

Cable

Gen

I

Capability / Damage Curves
transformer

FLA

200

Thermal

(D-D LL) 0.87

Infrequent Fault

(D-R LG)0.58

Frequent Fault

Mechanical

2

Inrush

Isc

2.5

25

Transformer

t

(sec)

I2t = 1250

K=(1/Z)2t

I (pu)

protective devices
Protective Devices
  • Fuse
  • Relay (50/51 P, N, G, SG, 51V, 67, 46, 79, 21, …)
  • Thermal Magnetic
  • Low Voltage Solid State Trip
  • Electro-Mechanical
  • MCP
  • Overload Heater
slide21
Fuse
  • Non Adjustable Device
  • Continuous and Interrupting Rating
  • Voltage Levels
  • Characteristic Curves
    • Min. Melting
    • Total Clearing
  • Application
slide22

Total Clearing

Time Curve

Minimum Melting

Time Curve

current limiting fuse clf
Current Limiting Fuse(CLF)
  • Limits the peak current of short-circuit
  • Reduces magnetic stresses (mechanical damage)
  • Reduces thermal energy
let through chart

100,000

Let-Through Chart

15% PF (X/R = 6.6)

230,000

300 A

100 A

12,500

Peak Let-Through Amperes

60 A

5,200

Symmetrical RMS Amperes

slide26
Fuse

Generally:

  • CLF is a better short-circuit protection
  • Non-CLF (expulsion fuse) is a better Overload protection
selectivity criteria
Typically:

Non-CLF: 140% of full load

CLF: 150% of full load

Selectivity Criteria
molder case cb
Thermal-Magnetic

Magnetic Only

Integrally Fused

Current Limiting

High Interrupting Capacity

Types

Frame Size

Trip Rating

Interrupting Capability

Voltage

Molder Case CB
slide29

Thermal Maximum

Thermal Minimum

Magnetic

(instantaneous)

lvpcb
LVPCB
  • Voltage and Frequency Ratings
  • Continuous Current / Frame Size
    • Override (12 times cont. current)
  • Interrupting Rating
  • Short-Time Rating (30 cycle)
  • Fairly Simple to Coordinate
slide31

LT PU

CB 2

CB 1

CB 2

LT Band

ST PU

CB 1

480 kV

IT

If =30 kA

ST Band

motor protection
Motor Protection
  • Motor Starting Curve
  • Thermal Protection
  • Locked Rotor Protection
  • Fault Protection
motor overload protection nec art 430 32
Motor Overload Protection (NEC Art 430-32)
  • Thermal O/L (Device 49)
  • Motors with SF not less than 1.15
    • 125% of FLA
  • Motors with temp. rise not over 40
    • 125% of FLA
  • All other motors
    • 115% of FLA
locked rotor protection
Locked Rotor Protection
  • Thermal Locked Rotor (Device 51)
  • Starting Time (TS < TLR)
  • LRA
    • LRA sym
    • LRA asym (1.5-1.6 x LRA sym) + 10% margin
fault protection nec art 430 52
Fault Protection (NEC Art 430-52)
  • Non-Time Delay Fuses
    • 300% of FLA
  • Dual Element (Time-Delay Fuses)
    • 175% of FLA
  • Instantaneous Trip Breaker
    • 800% of FLA*
  • Inverse Time Breakers
    • 250% of FLA

*MCPs can be set higher

slide36

(49)

I2T

tLR

O/L

MCP

(51)

ts

200 HP

Starting Curve

MCP (50)

LRAs

LRAasym

overcurrent relay
Overcurrent Relay
  • Time-Delay (51 – I>)
  • Short-Time Instantaneous ( I>>)
  • Instantaneous (50 – I>>>)
  • Electromagnetic (induction Disc)
  • Solid State (Multi Function / Multi Level)
  • Application
time overcurrent unit

IL

CT

IR

51

Time-Overcurrent Unit
  • Ampere Tap Calculation
    • Ampere Pickup (P.U.) = CT Ratio x A.T. Setting
    • Relay Current (IR) = Actual Line Current (IL) / CT Ratio
    • Multiples of A.T. = IR/A.T. Setting

= IL/(CT Ratio x A.T. Setting)

instantaneous unit

IL

CT

IR

50

Instantaneous Unit
  • Instantaneous Calculation
    • Ampere Pickup (P.U.) = CT Ratio x IT Setting
    • Relay Current (IR) = Actual Line Current (IL) / CT Ratio
    • Multiples of IT = IR/IT Setting

= IL/(CT Ratio x IT Setting)

relay coordination
Relay Coordination
  • Time margins should be maintained between T/C curves
  • Adjustment should be made for CB opening time
  • Shorter time intervals may be used for solid state relays
  • Upstream relay should have the same inverse T/C characteristic as the downstream relay (CO-8 to CO-8) or be less inverse (CO-8 upstream to CO-6 downstream)
  • Extremely inverse relays coordinates very well with CLFs
fixed points
Fixed Points

Points or curves which do not change regardless of protective device settings:

  • Motor starting curves
  • Transformer damage curves & inrush points
  • Cable damage curves
  • SC maximum fault points
  • Cable ampacities
situation

4.16 kV

Relay: IFC 53

CT 800:5

50/51

CB

Cable

CU - EPR

1-3/C 500 kcmil

Isc = 30,000 A

5 MVA

DS

6 %

Situation

Calculate Relay Setting (Tap, Inst. Tap & Time Dial)

For This System

solution

Transformer:

IL

IR

Set Relay:

CT

R

Solution
answer
Answer
  • For delta-delta connected transformers, with line-to-line faults on the secondary side, the curve must be reduced to 87% (shift to the left by a factor of 0.87)
  • For delta-wye connection, with single line-to-ground faults on the secondary side, the curve values must be reduced to 58% (shift to the left by a factor of 0.58)
question49
Question

What T/C Coordination interval should be maintained between relays?

answer50

A

B

t

CB Opening Time

+

Induction Disc Overtravel (0.1 sec)

+

Safety margin (0.2 sec w/o Inst. & 0.1 sec w/ Inst.)

I

Answer
answer52
Answer
  • Class 10 for fast trip, 10 seconds or less
  • Class 20 for, 20 seconds or less
  • There is also a Class 30 for long trip time