1. introduccion jirutitijaroen

EE5712 Power System Reliability:: Introduction

Panida Jirutitijaroen

1EE5712 Power System Reliability 12/27/2010

Outline

• About this class

• Introduction to reliability

• Basic steps in system reliability analysis

• Introduction to power system reliability

• Power system reliability indexes and criterion


ABOUT THIS CLASS

Overview

Assessment

Scope

Objective


Scope

• Power System Reliability

– Electric power

– System

– Reliability

• Reliability theory applied to power systems

• Analytical and simulation tools to conduct reliability analysis


Road Map

• Introduction to power system reliability (1 lec)

• Probability theory and Reliability theory (2 lec)

• Stochastic process (1 lec)

• Analytical methods for reliability analysis (1 lec)

• Frequency balance technique (1 lec)

• Simulation methods (1 lec)

• Single-area reliability analysis (2 lec)

• Composite system and multi-area reliability (2 lec)

• Distribution system reliability analysis (1 lec)

Theory

Application


Objectives

• What you will learn from this class…

1. Understand basic reliability concepts and reliability measures

2. Be able to perform reliability analysis of a small system using analytical tools.

3. Be able to perform reliability analysis of a large system using simulation tools.


INTRODUCTION TO RELIABILITY

What is reliability?

What causes a system to fail?

How to model uncertainties?


Example 1: You Are The Weakest Link!?!?

"A chain is only as strong as its weakest link”

Does this mean that system reliability is determined by the least reliable component in the system?


Example 2: Identical Transmission Lines

• Which system is more reliable?

• Which system is likely to fail more than another ?

• Next question is how much?

Line 1

Line 2

G Load

System A

Line 1

G Load

System B


Example 3: Non-Identical Transmission Lines


– Depend on t-line capability to deliver load, generation capacity, load level, how each line perform

– How to quantify line performance?

Line 1

Line 2

G Load

System A

Line 1

G Load

System B


Example 4: Identical Generators


• Which system is more cost-effective?

100 100 100

System A

Load 100 MW

100 100

System B

Load 100 MW

100

System C

Load 100 MW


Example 5: Non-Identical Generators


– Depend on how each generator perform

– How to quantify generator performance?

100 100 100

System A

Load 150 MW

150 150

System B

Load 150 MW

300

System C

Load 150 MW


What Is Reliability?

• Ability of a component/system to perform its intended function

• Within a specified period of time

• Under stated condition

→ Qualitative sense in terms of performance function, time, and surrounding conditions

How to quantify ‘reliability’?


Reliability

• Relate to the absence of failures, that due to random phenomenon

• Define numerically as ‘average’ or mean value

• Can be treated as a parameter

• Can be traded off with other parameters such as cost


What causes a system to fail?

• Human factors

• System design

• Operation condition

• Maintenance procedure

• Deterioration (function of time)

• Random failures

• Uncertainties…


How to model ‘Uncertainty’?

• Probability of failure– Chance that a component will fail

– Probabilistic value with no unit

– May be difficult to interpret

• Frequency of failure– In terms of number of failure within specified time

– Easier to predict from history

– Express in per hour, per day, per year

• We will discover later on in this course how to relate frequency of failure to probability of failure


Example 5: Transmission lines

• Given that each system has the following level of reliability

• Which system is more reliable? • Which system is more cost-effective?

100 MW

100 MW

G Load100 MW

System A

100 MW

G Load 100 MW

System B

System Failure Probability Cost (million SGD)

A 0.009 70

B 0.01 25


Example 5: Identical Generators

• If each system has the following level of reliability and cost


• Which system is more cost-effective?

100 100 100

System A

Load 100 MW

100 100

System B

Load 100 MW

100

System C

Load 100 MW

System Failure Probability

Cost (million SGD)

A 0.001 300

B 0.01 200

C 0.1 100


Motivations for Quantitative Reliability

• To evaluate system performance

• System design purpose

• Trade-off reliability with cost

• Increasing complexity of systems

• Competitiveness

• Establish standard in operation procedure


BASIC STEPS IN SYSTEM RELIABILITY ANALYSIS

Objective of the analysis

Component modeling

System modeling

Performance function

Reliability Evaluation


Objective

• Interest to know the likelihood that a component or a system will fail.

– Time-to-failure distribution of a component/system.

• Helps to predict the failure probability at any point in time

• For a complex system, need to estimate reliability index for design and operation purposes.

• Need to start with the component modeling


Component Modeling

• Identify components in the system

• Describe state of each component

– For example, a generator has two states, up or down.

• In terms of probability distribution

– For example, a generator fails with probability of failure = 0.01.

• Stochastic process model


Observation of A Component

12/27/2010 EE5712 Power System Reliability 27

Time

Z(t)

0

1

2

Time

Z(t)

0

1

2

System Modeling

• System configuration/topology

• How each component interact

C1 C2

C1

C2

C1 C2 C8

C6 C3 C9

C5 C11 C7

C10 C4 C12


Need to know how each device causes a system to fail!

Performance Function

• To evaluate system reliability

• Recall,

• Need to define “intended function”

Ability of a system to perform its intended function


Reliability Evaluation

• Each component described by random variables

– For example, a generator has 3 capacity output, 100 MW with 0.85 probability, 50 MW with 0.14 probability, 0 MW with 0.01 probability

• System states constructed from possible combinations of component states

• Evaluate performance function of each system state

• Calculate reliability index


INTRODUCTION TO POWER SYSTEMS

Functional Zones in Power Systems

Objective of Reliability Analysis

Levels of Reliability Analysis

Power System Reliability Indexes


Functional Zones of Power Systems

• Generation system

– Generators

– Load

• Transmission system

– High voltage transmission lines

• Distribution system

– Low voltage transmission lines

– End users


Main Components of a Power System

• Generation (11 – 36 KV)

• Transmission and distribution (110 – 765 KV)

• Load (0.12 – 138 KV)

– Industrial customer (23 – 138 KV)

– Commercial customer (4.16 – 34.5 KV)

– Residential customer (120 – 240 V)

33

Generation Capacity in Singapore

34

Transmission and Distribution

• Interconnected network• Transformers used to step up voltages from generation

units to transmission-line.• High voltage used when transmitting power to lower I2R

loss for better efficiency.• Distribution systems can provide power at different voltage

levels for different loads.

Transmission network Distribution network

35

36

North American Electric Power Connections

http://www.nerc.com/regional/NERC_Interconnections_color.jpg

Singapore Power Grid

Zone Maximum Import capacity (MW)

A 1275

B 1275

C 1275

D 1275

• 400 kV, 230 kV, and 66 kV

• Full underground cable

• Four 230 kV zones connected by meshed 400 kV

37

Load

• Varies with time

– Moment-to-moment fluctuations

– Hour-to-hour changes

– Daily

– Weekly

– Seasonal

• Base load counts for less than a half of peak load.

• Typical weekly load curve, data from ERCOT

38

Sun. Mon. Tue. Wed.0

1

2

3

4

5

6

x 104

Day

Load (

MW

)

ERCOT Weekly Load Curve from Aug. 19th to Aug. 22, 2006

Singapore Electric Demand

• Peak demand in 2007 is 5946 MW.• Electricity demand in 2007 is 41134 GWh. 39

Operational Conditions

Economic operationCost of operation differs by type of fuel.How to operate the system with least cost?

Secure operationComponent physical limit. How to operate the system securely?

Reliable operationPower Quality, interruptions, brownout and blackoutHow to operate the system reliably?

40

Uncertainties in Power Systems

• Generation

– Generating units with failure and repair rates

– Generating capacity associated with probability

• Transmission line capacity

– Transmission line with failure and repair rates

– Transmission line capacity associated with probability

• System load

– Vary with time

– Construct load distribution from history


Objective of Reliability Analysis

• The function of power system is to serve load.

• We want to have,

• For most of the time,

• With least cost.

Generation > Load


Three Areas of Reliability Analysis

1. Generating capacity reliability

– Concern with generation adequacy

2. Composite system reliability

– Consider both generation and transmission lines

3. Distribution system reliability

– Local network connected to end-users


Generating Capacity Reliability

1. Single-area reliability analysis– All generators and loads are connected to a

single bus

2. Multi-area reliability analysis– Generators and loads within area are connected

to a single bus

– Consider tie-lines between areas

– Limitation of intra-area transmission are included when determining inter-area transmission capability


Single Area Reliability Analysis

• Interest to find out the ability of existing generation to serve load

• Single bus analysis

• Generators and loads are within the same bus

• Each generators has their own performance indexes


Composite System Reliability

• Concern with generation and transmission capability adequacy

• High-voltage transmission lines

• May include high-voltage transformers, circuit breakers http://www.powerworld.com/images/7-bus%20Oneline.jpg


Multi-Area Reliability Analysis

• Interest to find out if area generation or tie-line capability are adequate to serve load

• Consider thousands of nodes then simplify the system to small workable nodes (areas)

• Generator and load from different nodes within the same area are grouped into one.

• Tie-line capability between areas


Distribution System Reliability

• Interest to find out the reliability level at load point

• Network configuration/ topology

• Analysis takes into account reliability of the following low-voltage components,– Transformers

– Circuit breakers

http://www.tpub.com/content/construction/14027/css/14027_63.htm


POWER SYSTEMS RELIABILITY INDEXES AND CRITERION

Reliability indexes

Reliability criterion


Power Systems Reliability Indexes

• Deterministic indexes

– Do not take into account the uncertainties that affect reliability

– Simple calculation

– Require less data

• Probabilistic indexes

– Reflect uncertainties in the system

– Require failure statistics of the devices


Deterministic Indexes

• Operating reserve margin

– Excess generation capacity in case of emergency

• Percentage reserve

– Amount of reserve capacity as a percentage of the total peak load

• Reserve margin as the largest unit online

– Amount of reserve equals to the capacity of the largest unit online


Probabilistic Indexes

• Loss of load probability– Probability that generation will not meet demand

in a year

– Commonly shown as number of hours/days. (by multiply LOLP by number of hours/days in a year)

• Loss of load frequency– How often does the system fail in a year

• Expected Energy Not Supplied (EENS) or Expected Unserved Energy (EUE)


Why need reliability criterion?

• Develop standardized quantity

• Planning and Operation purposes

– To avoid catastrophic failures

• Design problems


Power Systems Reliability Criterion

• Deterministic criteria

– N-m contingency analysis

– System with ‘N’ components should be able to serve peak load when loss ‘m’ components

– Sometimes called security analysis

• Probabilistic criteria

– Loss of load expectation, for example, 1 day in 10 years


Cost-Benefit Analysis

• High reliability achieved with high cost

• Is it worthwhile to have high reliability?

http://www.eppo.go.th/power/ERI-study-E/ERI-EOCS-1-E.html


Summary

• We know what reliability is

• We know a bit about power system reliability

• We need to know

– Probability theory

– Reliability theory

– Random processes


Reading Materials

• Review basic probability theory

– Random variables

– Probability rules (addition, multiplication,

– Conditional probability

– Probability distribution functions


Documents

1. introduccion jirutitijaroen