Power System Protection

Fundamentals

Dr. Youssef A. Mobarak

y.a.Mobarak@Gmail.com

2014

1

AGENDA

Why protection is needed

Principles and elements of the protection system

Basic protection schemes

Digital relay advantages and enhancements

2

Topic_1

DISTURBANCES: LIGHT OR SEVERE

The power system must maintain acceptable operation 24 hours a day

Voltage and frequency must stay within certain limits

Small disturbances

The control system can handle these

Example: variation in transformer or generator load

Severe disturbances require a protection system

They can jeopardize the entire power system

They cannot be overcome by a control system

3

POWER SYSTEM PROTECTION

Operation during severe disturbances:

System element protection

System protection

Automatic reclosing

Automatic transfer to alternate power supplies

Automatic synchronization

4

Generation-typically at 4-35kV

Transmission-typically at 230-765kV

Subtransmission-typically at 69-161kV

Receives power from transmission system and transforms

into subtransmission level

Receives power from subtransmission system and

transforms into primary feeder voltage

Distribution network-typically 2.4-69kV

Low voltage (service)-typically 120-600V

TYPICAL BULK POWER SYSTEM

5

PROTECTION ZONES

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 zoneTransformer zone

Bus zone

Generator

~

XFMR Bus Line Bus XFMR Bus Motor

Motor zone

6

WHAT INFO IS REQUIRED TO APPLY PROTECTION

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 maximum allowed 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. 11

C37.2: DEVICENUMBERS

Partial listing

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

13

LINE SYMBOLS [1]

14

LINE SYMBOLS [2]

15

LINE SYMBOLS [3]

16

LINE SYMBOLS [4]

17

1-LINE [1]

18

PROTECTION SYSTEM

A series of devices whose main purpose is to

protect persons and primary electric power

equipment from the effects of faults

19

BLACKOUTS

Loss of service in a large area or population region

Hazard to human life

May result in enormous economic losses

Overreaction of the protection system

Bad design of the protection system

Characteristics Main Causes

SHORT CIRCUITS PRODUCE HIGH CURRENTS

FaultSubstation

a

b

c

I

IWire

Three-Phase Line

Thousands of Amps

20

ELECTRICAL EQUIPMENT THERMAL DAMAGE

I

t

In Imd

Damage Curve

Short-Circuit

Current

Damage

Time

Rated Value

21

MECHANICAL DAMAGE DURING SHORT CIRCUITS

Very destructive in busbars, isolators, supports, transformers, and machines

Damage is instantaneous

i1

i2

f1 f2

Rigid Conductors f1(t) = k i1(t) i2(t)

Mechanical

Forces

22

ELECTRIC POWER SYSTEM EXPOSURE TO EXTERNAL AGENTS

23

DAMAGE TO MAIN EQUIPMENT

24

THE FUSE

Fuse

Transformer

25

PROTECTION SYSTEM ELEMENTS

Protective relays

Circuit breakers

Current and voltage transducers

Communications channels

DC supply system

Control cables

26

THREE-PHASE DIAGRAM OF THE PROTECTION TEAM

CTs

VTs

Relay

CB

Control

Protected

Equipment

27

DC TRIPPING CIRCUIT

SI

52

TC

DC Station

Battery SI

Relay

Contact

Relay

Circuit

Breaker

52a

+

–

Red

Lamp

28

CIRCUIT BREAKERS

29

CURRENT TRANSFORMERS

Very High Voltage CTMedium-Voltage CT

30

VOLTAGE TRANSFORMERS

Medium Voltage

High Voltage

Note: Voltage transformers

are also known as potential

transformers

31

TYPICAL CT/VT CIRCUITS

Courtesy of Blackburn, Protective Relay: Principles and Applications

32

CT/VT CIRCUIT VS. CASING GROUND

Case ground made at IT location

Secondary circuit ground made at first point of use

Case

Secondary Circuit

Prevents shock exposure of personnel

Provides current carrying capability for the ground-fault current

Grounding includes design and construction of substation ground mat and CT and VT safety grounding

SUBSTATION TYPES

• Single Supply

• Multiple Supply

• Mobile Substations for emergencies

• Types are defined by number of transformers, buses,

breakers to provide adequate service for application

34

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 outdoor switchyards

The term "switchgear" is plural, even when referring to a single switchgear assembly (never say, "switchgears")

May be a described in terms of use:

"the generator switchgear"

"the stamping line switchgear"

35

PROTECTIVE RELAYS

38

EXAMPLES OF RELAY PANELS

Old Electromechanical

Microprocessor-

Based Relay

39

HOW DO RELAYS DETECT FAULTS?

When a fault takes place, the current, voltage,frequency, and other electrical variables behave in apeculiar way. For example:

Current suddenly increases

Voltage suddenly decreases

Relays can measure the currents and the voltages anddetect that there is an overcurrent, or an undervoltage, ora combination of both

Many other detection principles determine the design ofprotective relays

40

MAIN PROTECTION REQUIREMENTS

Reliability

Dependability

Security

Selectivity

Speed

System stability

Equipment damage

Power quality

Sensitivity

High-impedance faults

Dispersed generation

41

PRIMARY PROTECTION

42

PRIMARY PROTECTION ZONE OVERLAPPING

Protection

Zone B

Protection

Zone A

To Zone B

Relays

To Zone A

Relays

52 Protection

Zone B

Protection

Zone A

To Zone B

Relays

To Zone A

Relays

52

43

BACKUP PROTECTION

A

C D

E

Breaker 5

Fails

1 2 5 6 11 12

T

3 4 7 8 9 10

B F

44

BALANCED VS. UNBALANCED CONDITIONS

Balanced System Unbalanced System

cI

aI

bI

aI

cI

bI

45

Typical Short-Circuit Type Distribution

Single-Phase-Ground: 70–80%

Phase-Phase-Ground: 17–10%

Phase-Phase: 10–8%

Three-Phase: 3–2%

DECOMPOSITION OF AN UNBALANCED SYSTEM

Positive-Sequence

Balanced Balanced

Negative-Sequence

1bI

1cI

1aI

2bI

2aI

2cI

0aI

0bI

0cI

aI

cI

bI

Zero-Sequence

Single-Phase46

CONTRIBUTION TO FAULTS

47

FAULT TYPES (SHUNT)

48

X

X

Z

Z

Z

G

BC

A

Short Circuit Calculation

Fault Types – Single Phase to Ground

X

X

Z

Z

Z

G

BC

A

Short Circuit Calculations

Fault Types – Line to Line

Z

Z

Z

G

BC

AX

X

X

Short Circuit Calculations

Fault Types – Three Phase

AC & DC CURRENT COMPONENTS OF FAULT CURRENT

49VARIATION OF CURRENT WITH TIME DURING A FAULT

VARIATION OF GENERATOR REACTANCEDURING A FAULT

USEFUL CONVERSIONS

50

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 power equations

51

SHORT CIRCUIT CALCULATIONSPER UNIT SYSTEM

Per Unit Value = Actual QuantityBase Quantity

Vpu = Vactual

Vbase

Ipu = Iactual

Ibase

Zpu = Zactual

Zbase

52

3 x kV L-L base

I base =x 1000MVAbase

Z base =kV2

L-L base

MVAbase

Zpu2 =Zpu1 x kV 2base1 x MVAbase2

kV 2base2 MVAbase1

FAULT INTERRUPTION AND ARCING

57

POWER LINE PROTECTION PRINCIPLES

Overcurrent (50, 51, 50N, 51N)

Directional Overcurrent (67, 67N)

Distance (21, 21N)

Differential (87)

58

tRelay

Operation

Time

I

Fault Load

Radial Line

APPLICATION OF INVERSE -TYPE RELAYS

INVERSE-TIME RELAY COORDINATION

59

tRelay

Operation

Time

I

Fault Load

Radial Line

Distance

Distance

t

I

T T T

DIRECTIONAL OVERCURRENT PROTECTIONBASIC PRINCIPLE

F2

Relay

F

1

Forward Fault (F1)Reverse Fault (F2)

V

IV

I

IV

60

11 )8.0( LS

SETTINGZZ

EI

11

)()8.0( LS

LIMITFAULTZZ

EI

Relay operates when the following condition holds:

SETTINGaFAULT III

As changes, the relay’s “reach” will change, since

setting is fixed

DISTANCE RELAY PRINCIPLE

L

Three-Phase

Solid Fault

d

Radial

Line2

1

Suppose Relay Is Designed to Operate When:

||||)8.0(|| 1 aLa IZV

cba III ,,

cba VVV ,,

61

21

22rZXR

R

X Plain Impedance Relay

Operation Zone

Zr1

Radius Zr11rZZ

NEED FOR DIRECTIONALITYF1

1 2 3 4 5 6

F2

R

XRELAY 3

Operation Zone

F1

F2

Nonselective Relay

Operation

62

1 2 3 4 5 6

F1F2

R

XRELAY 3

Operation Zone

F1

F2

The Relay Will

Not Operate for

This Fault

Directional Impedance

Relay Characteristic

MTMZZ cos

ZM

Z

R

X

MT

MTMZIV cosOperates when:

THREE-ZONE DISTANCE PROTECTION

1 2 3 4 5 6

Zone 1

Zone 2

Zone 3

Time

Time

Zone 1 Is Instantaneous

63

X

RA

B

C

D

DISTANCE PROTECTION SUMMARYCurrent and voltage information

Phase elements: more sensitive than 67 elements

Ground elements: less sensitive than 67N elements

Application: looped and parallel lines

64

Communications

Channel

Exchange of logic information

on relay status

RL

Relays Relays

T

R

R

T

LI RI

PERMISSIVE OVERREACHING TRANSFER TRIP

1 2 3 4 5 6

FWD

FWD

Bus A Bus B

65

1 2 3 4 5 6

FWD

FWD

RVS

RVS

Bus A Bus B

DIFFERENTIAL PROTECTION PRINCIPLE

No Relay Operation if CTs Are Considered Ideal

External

Fault

IDIF = 0

CT CT

50

Balanced CT Ratio

Protected

Equipment

66

Internal

Fault

IDIF > ISETTING

CTR CTR

50

Relay Operates

Protected

Equipment

PROBLEM OF UNEQUAL CT PERFORMANCE

False differential current can occur if a CT saturates during a through-fault

Use some measure of through-current to desensitize the relay when high currents are present

External

Fault

Protected

Equipment

IDIF 0

CT CT

50

67

POSSIBLE SCHEME – PERCENTAGE DIFFERENTIAL PROTECTION PRINCIPLE

Protected

Equipment

ĪRĪS

CTR CTR

Compares:

Relay

(87)

OP S RI I I

| | | |

2

S RRT

I Ik I k

ĪRPĪSP

68

DIFFERENTIAL PROTECTION APPLICATIONS

Bus protection

Transformer protection

Generator protection

Line protection

Large motor protection

Reactor protection

Capacitor bank protection

Compound equipment protection

69

DIFFERENTIAL PROTECTIONSUMMARY

The overcurrent differential scheme is simple and economical, but it does not respond well to unequal current transformer performance

The percentage differential scheme responds better to CT saturation

Percentage differential protection can be analyzed in the relay and the alpha plane

Differential protection is the best alternative selectivity/speed with present technology

70

MULTIPLE INPUT DIFFERENTIAL SCHEMESEXAMPLES

Differential Protection Zone

Bus Differential: Several Inputs

ĪRPĪSP

OP

ĪT

I1 I2 I3 I4

Three-Winding Transformer

Differential: Three Inputs

71

ADVANTAGES OF DIGITAL RELAYS

Multifunctional

Compatibility with

digital integrated

systems

Low maintenance

(self-supervision)

Highly sensitive,

secure, and

selective

AdaptiveHighly reliable

(self-supervision)

Reduced burden

on

CTs and VTs

Programmable

VersatileLow Cost

72

A GOOD DAY IN SYSTEM PROTECTION……

CTs and VTs bring electrical info to relays

Relays sense current and voltage and declare fault

Relays send signals through control circuits to circuit breakers

Circuit breaker(s) correctly trip

73

A BAD DAY IN SYSTEM 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 is sent from relay to circuit breaker

Circuit breakers do not have power, burnt trip coil or otherwise fail to trip

PROTECTION PERFORMANCE STATISTICSCorrect and desired: 92.2%

Correct but undesired: 5.3%

Incorrect: 2.1%

Fail to trip: 0.4%

74

THE FUTURE

Improvements in computer-based protection

Highly reliable and viable communication systems (satellite, optical fiber, etc.)

Integration of control, command, protection, and communication

Improvements to human-machine interface

Much more