Power System Protection and Transient Stability

1
ECE 476POWER SYSTEM ANALYSIS

• Lecture 23
• Power System Protection and Transient Stability
• Professor Tom Overbye
• Department of Electrical andComputer Engineering

2
Announcements

• Design Project 2 and HW 10 are due today.
• Be reading Chapter 10 and 13
• Including CH 10 article about zone 3 relays, CH
  13 DSA and blackout articles
• HW 11 is not turned in but should be done before
  final. HW 11 is 13.1, 13.7, 13.8, 13.18, and SP1
• Final is Wednesday Dec 12 from 130 to 430pm in
  EL 269 (note room change). Final is
  comprehensive. One new note sheet, and your two
  old note sheets are allowed

3
Other Types of Relays

• In addition to providing fault protection, relays
  are used to protect the system against
  operational problems as well
• Being automatic devices, relays can respond much
  quicker than a human operator and therefore have
  an advantage when time is of the essence
• Other common types of relays include
• under-frequency for load e.g., 10 of system
  load must be shed if system frequency falls to
  59.3 Hz
• over-frequency on generators
• under-voltage on loads (less common)

4
Sequence of Events Recording

• During major system disturbances numerous relays
  at a number of substations may operate
• Event reconstruction requires time
  synchronization between substations to figure out
  the sequence of events
• Most utilities now have sequence of events
  recording that provide time synchronization of at
  least 1 microsecond

5
Use of GPS for Fault Location

• Since power system lines may span hundreds of
  miles, a key difficulty in power system
  restoration is determining the location of the
  fault
• One newer technique is the use of the global
  positioning system (GPS).
• GPS can provide time synchronization of about 1
  microsecond
• Since the traveling electromagnetic waves
  propagate at about the speed of light (300m per
  microsecond), the fault location can be found by
  comparing arrival times of the waves at each
  substation

6
SEL 351S Fault Demonstration

• SEL standards for Schweitzer Engineering
  Laboratories, a major relay manufacturer located
  in Pullman, Washington. Other manufacturers
  include GE, ABB, Cooper Power Systems, Basler
  Electric, Siemens
• Relays are designed to act quickly to make or
  break contact with another circuit when activated
  
• Protection relays detect and isolate faults on
  transmission and distribution lines by
  calculating operating conditions and tripping
  circuit breakers when a fault is found

7
Relay Overview

• Current transformers (CTs) are used to step down
  and monitor very large AC currents.
• Potential transformers (PTs) do the same for AC
  voltages.
• The relay analyzes the signal once it is in the
  0-5A range.

8
Relay Overview

• Historically, relays were electromagnetic devices
  
• Now relays are becoming more microprocessor based
  
• Increased ability to communicate with other
  relays before operating
• Capable of having more than one function

9
Relay Functions

• There are many functions categorized by ANSI
  standard
• Instantaneous overcurrent relays operate
  immediately when a specific current level is
  exceeded
• Time-overcurrent relays operate at a time which
  is inversely proportional to the operating current

10
Time-Overcurrent Settings
11
U3 (Very Inverse) Curve
12
Demonstration Overview

• SEL AMS
• Receives voltage and current values from the
  SEL-5401 via serial port
• Outputs low level analog signals are sent to the
  SEL 351S
• Acts as a circuit breaker
• Sense inputs detect if relay has tripped the
  breaker
• Output contacts tell relay the breaker status

13
Demonstration Overview SEL 351S

• Bypass relays input transformers
• Receives voltages and currents from AMS
• Time-overcurrent element picks up for the
  three-phase fault
• Relay tells breaker to trip
• Breaker trips
• Breaker stays open unless reclosing is enabled or
  until it is told to close

14
Demonstration

• Prefault State (Breaker Closed)

15
Demonstration

• Fault State

16
Demonstration

• Post-Fault (Breaker Open)

17
Questions?

• References
• 1 Instruction Manual, SEL-351S Relay, Meter,
  Control, Fault Locator, Schweitzer Engineering
  Laboratories, Inc., 2007.
• 2 Instruction Manual, SEL-RTS Relay Test
  System, Schweitzer Engineering Laboratories,
  Inc., 1997,

18
Power System Transient Stability

• In order to operate as an interconnected system
  all of the generators (and other synchronous
  machines) must remain in synchronism with one
  another
• synchronism requires that (for two pole machines)
  the rotors turn at exactly the same speed
• Loss of synchronism results in a condition in
  which no net power can be transferred between the
  machines
• A system is said to be transiently unstable if
  following a disturbance one or more of the
  generators lose synchronism

19
Generator Transient Stability Models

• In order to study the transient response of a
  power system we need to develop models for the
  generator valid during the transient time frame
  of several seconds following a system disturbance
• We need to develop both electrical and mechanical
  models for the generators

20
Example of Transient Behavior
21
Generator Electrical Model

• The simplest generator model, known as the
  classical model, treats the generator as a
  voltage source behind the direct-axis transient
  reactance the voltage magnitude is fixed, but
  its angle changes according to the mechanical
  dynamics

22
Generator Mechanical Model
Generator Mechanical Block Diagram
23
Generator Mechanical Model, contd
24
Generator Mechanical Model, contd
25
Generator Mechanical Model, contd
26
Generator Swing Equation
27
Single Machine Infinite Bus (SMIB)

• To understand the transient stability problem
  well first consider the case of a single machine
  (generator) connected to a power system bus with
  a fixed voltage magnitude and angle (known as an
  infinite bus) through a transmission line with
  impedance jXL

28
SMIB, contd
29
SMIB Equilibrium Points
30
Transient Stability Analysis

• For transient stability analysis we need to
  consider three systems
• Prefault - before the fault occurs the system is
  assumed to be at an equilibrium point
• Faulted - the fault changes the system equations,
  moving the system away from its equilibrium point
• Postfault - after fault is cleared the system
  hopefully returns to a new operating point

31
Transient Stability Solution Methods

• There are two methods for solving the transient
  stability problem
• Numerical integration
• this is by far the most common technique,
  particularly for large systems during the fault
  and after the fault the power system differential
  equations are solved using numerical methods
• Direct or energy methods for a two bus system
  this method is known as the equal area criteria
• mostly used to provide an intuitive insight into
  the transient stability problem

32
SMIB Example

• Assume a generator is supplying power to an
  infinite bus through two parallel transmission
  lines. Then a balanced three phase fault occurs
  at the terminal of one of the lines. The fault
  is cleared by the opening of this lines circuit
  breakers.

33
SMIB Example, contd
Simplified prefault system
34
SMIB Example, Faulted System
During the fault the system changes
The equivalent system during the fault is then
During this fault no power can be
transferred from the generator to the system
35
SMIB Example, Post Fault System
After the fault the system again changes
The equivalent system after the fault is then
36
SMIB Example, Dynamics
37
Transient Stability Solution Methods

• There are two methods for solving the transient
  stability problem
• Numerical integration
• this is by far the most common technique,
  particularly for large systems during the fault
  and after the fault the power system differential
  equations are solved using numerical methods
• Direct or energy methods for a two bus system
  this method is known as the equal area criteria
• mostly used to provide an intuitive insight into
  the transient stability problem