Power System Protection Fundamentals
What should we teach students about power system protection?
Agenda
Why protection is needed Principles and elements of the protection pr otection system
Basic protection schemes
Digital relay advantages and enhancements
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
Power System Protection Operation during severe disturbances:
System element protection
System protection
Automatic reclosing Automatic transfer to alternate power power supplies Automatic synchronization
Electric Power System Exposure to External Agents
Damage to Main Equipment
Protection System
A series of devices whose main purpose is to protect persons and primary electric power equipment from the effects of faults
The “Sentinels”
Blackouts Main Causes
Characteristics
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
Short Circuits Produce High Currents Three-Phase Line a b c
I Fault
Substation Thousands of Amps
I
Wire
Electrical Equipment Thermal Damage t
Damage Curve
Damage Time
Rated Value
I n
I md
I Short-Circuit Current
Mechanical Damage During Short Circuits
Very destructive in busbars, isolators, supports, transformers, and machines Damage is instantaneous Mechanical Forces
f 1
f 2
i1 i2 Rigid Conductors
f 1(t ) = k i1(t ) i2(t )
The Fuse
Fuse
Transformer
Protection System Elements
Protective relays
Circuit breakers
Current and voltage transducers
Communications channels
DC supply system
Control cables
Three-Phase Diagram of the Protection Team CB
CTs
Protected Equipment
Control
Relay
VTs
DC Tripping Circuit +
SI DC Station Battery
Relay Contact
SI
52a 52 TC
–
Relay
Circuit Breaker
Red Lamp
Circuit Breakers
Current Transformers
Very High High Voltage CT
Medium-Voltage CT
Voltage Transformers
Medium Voltage
High Voltage
Note: Voltage transformers are also known as potential transformers
Protective Relays
Examples of Relay Panels
MicroprocessorBased Relay Old Electromechanical
How Do Relays Detect Faults?
When a fault takes place, the current, voltage, frequency, and other electrical variables behave in a peculiar way. For example:
Current suddenly increases
Voltage suddenly decreases
Relays can measure the currents and the voltages and detect that there is an overcurrent , or an undervoltage, or a combination of both Many other detection principles determine the design of protective relays
Main Protection Requirements
Reliability
Dependability
Security
Selectivity
Speed
System stability
Equipment damage
Power quality
Sensitivity
High-impedance faults Dispersed generation
Primary Protection
Primary Protection Zone Overlapping Protection Zone A 52 To Zone A Relays
Protection Zone B To Zone B Relays
Protection Zone A 52 To Zone Zon e A Relays
Protection Zone B To Zone B Relays
Backup Protection Breaker 5 Fails C
D
A
E
1
2
5
6
11
12
T B
F
3
4
7
8
9
10
Typical Short-Circuit Type Distribution Single-Phase-Ground:
70–80%
Phase-Phase-Ground:
17–10%
Phase-Phase:
10–8%
Three-Phase:
3–2%
Balanced vs. Unbalanced Conditions
I a
I c I c
I a
I b I b Balanced System
Unbalanced System
Decomposition of an Unbalanced System I a I c I b I a1 I c1
I a 0 I b 0 I c 0
I b 2 I a 2
I b1
Zero-Sequence
Positive-Sequence
Single-Phase
Balanced
I c 2
Negative-Sequence
Balanced
Power Line Protection Principles
Overcurrent (50, 51, 50N, 51N)
Directional Overcurrent (67, 67N)
Distance (21, 21N)
Differential (87)
Application of Inverse-Type Relays Relay Operation Time
t
I Radial Line
Fault
Load
Inverse-Time Relay Coordination
I
Distance
t
T
T
T Distance
Addition of Instantaneous OC Element t
Relay Operation Time
I Radial Line
Fault
Load
50/51 Relay Coordination
I
Distance
t
T
T
T Distance
Directional Overcurrent Protection Basic Applications
K
L
Directional Overcurrent Protection Basic Principle I
V
F1
F2 Relay Reverse Fault (F2)
Forward Fault (F1)
I
V V
I
Overcurrent Relay Problem I SETTING
E Z S 1 (0.8) Z L1
Relay operates when the following condition holds:
I FAULT I a
I SETTING
As Z s1 changes, the relay’s relay’s “reach” will change, since setting is fixed I FAULT ( LIMIT )
E Z S 1 (0.8) Z L1
Distance Relay Principle L d I a , I b , I c
V a ,V b ,V c
21
Three-Phase Solid Fault
Suppose Relay Is Designed to Operate When:
| V a | (0.8) | Z L1 || I a |
Radial Line
The Impedance Relay Characteristic R 2 X 2 Z r 21 X
Plain Impedance Relay
Operation Zone
Z Z r 1
Z r1
Radius Z r1 R
Need for Directionality F1
F2 1
2
RELAY 3 Operation Zone
3
4
5
6
X F1 F2
Nonselective Relay Operation
R
Directionality Improvement F1
F2 1
2
RELAY 3 Operation Zone
3
4
6
X F1 F2
The Relay Will Not Operate for This Fault
5
Directional Impedance Relay Characteristic
R
Mho Element Characteristic (Directional Impedance Relay) Operates when:
V I Z M cos MT Z Z M cos MT
X
Z M Z
MT R
Three-Zone Distance Protection Time Zone 3 Zone 2 Zone 1 1
2
3
4
5
6
Time Zone 1 Is Instantaneous
Line Protection With Mho Elements X C B
A
R D
E
Circular Distance Relay Characteristics X
PLAIN IMPEDANCE
X
OFFSET MHO (2)
R R X
X
LENS (RESTRICTED MHO 1)
MHO
R
R X
X
OFFSET MHO (1)
R
TOMATO (RESTRICTED MHO 2)
R
Semi-Plane Type Characteristics X
DIRECTIONAL
X
RESTRICTED DIRECTIONAL
R R X
X
RESTRICTED REACTANCE
REACTANCE
R X
R X
OHM QUADRILATERAL R
R
Distance Protection Summary
Current and voltage information Phase elements: more sensitive than 67 elements Ground elements: less sensitive than 67N elements Application: Applicatio n: looped and parallel parallel lines
Directional Comparison Pilot Protection Systems L
I L
I R
T Relays
R
Communications Channel
Exchange of logic information on relay status
R Relays
T
R
Permissive Overreaching Transfer Trip Bus A 1
2
Bus B
3
4
FWD
FWD
5
6
Basic POTT Logic
Key XMTR
Zone 2 Elements AND RCVR
Trip
Directional Comparison Blocking Scheme Bus A 1
RVS
2
Bus B
3
4
5
6
FWD
FWD
RVS
Basic DCB Logic Zone 3
Key XMTR
Carrier Coordination Time Delay
Zone 2 RCVR
CC 0
Trip
Differential Protection Principle Balanced CT Ratio CT
CT Protected Equipment
50
External Fault
I DIF = 0
No Relay Operation if CTs Are Considered Ideal
Differential Protection Principle
CTR
CTR Protected Equipment Internal Fault
50
I DIF > I SETTING
Relay Operates
Problem of Unequal CT Performance CT
Protected Equipment
50
CT
I DIF
External Fault
0
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
Possible Possib le Sch Scheme eme – Per Percen centag tage e Differential Protection Principle CTR
Ī SP
Ī RP Protected Equipment
Ī S
CTR
Ī R Relay (87)
Compares:
IOP
IS
I R
k I RT k
| I S | | I R | 2
Differential Protection Applications
Bus protection
Transformer protection
Generator protection
Line protection
Large motor protection
Reactor protection
Capacitor bank protection
Compound equipment protection
Differential Protection Summary
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
Multiple Input Differential Schemes Examples Differential Protection Zone Ī SP
Ī RP
Ī T I 1
I 2
I 3
I 4 OP
Bus Differential: Several Inputs Three-Winding Transform Transformer er Differential: Three Inputs
Advantages of Digital Relays
Multifunctional
Compatibility with digital integrated systems
Low maintenance (self-supervision)
Highly sensitive, secure, and selective
Adaptive
Highly reliable (self-supervision)
Reduced burden on CTs and VTs
Programmable Versatile
Low Cost
Synchrophasors Provide a “Snapshot” of the Power System
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
