Statistical Analysis of Power System Faults

Project Plan

Senior Design

May99-15

“Statistical Analysis of Power System Faults”

Prof. Jim McCalley

Team Members:

Corey K. Proctor

Chris Dennison

Alvina Hendradi

Greg Bahl

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December 8,1998
Group Members: Team MAY 99-15

Corey K. Proctor

321 South 5th Apt. 245

Ames, IA 50010

(515) 233-8452

Chris Dennison

Friley 4525 Meeker

Ames, IA 50012

(515) 296-3783

Alvina Hendradi

221 N Sheldon Ave Apt. 4

Ames, IA 50010

(515) 292-2443

Greg Bahl

Friley 3391 Knapp

Ames, IA 50012

(515) 296-3090

Client/ Advising Professor

Dr. James McCalley

1113 Coover

Ames, IA 50011

(515) 294-4844

Table of Contents

1Abstract......

2Problem Statement......

3What is risk?......

3.1Events......

3.2Consequences......

4Proposed Technical Solution......

5Objectives......

6Progress to Date......

6.1Research......

6.1.1Weather......

6.1.2Transmission Line......

6.1.3Load......

6.2Risk Calculation......

6.2.1Weather analyses......

6.2.2Conductor characteristics......

6.2.3Determining the conductor’s temperature......

6.2.4Calculate the reduction in tensile strength......

6.2.5Calculate the risk of sagging......

6.3Power Flow......

6.4MATLAB......

7Future Work......

7.1JAVA interface for MATLAB Code......

8Appendix......

8.1Design Flow Chart......

8.2Budget Chart......

9References......

Figures

Figure 1. One line diagram......

Figure 2. Design flow chart......

Tables

Table 1. Transmission line characteristics......

Equations

Equation 1. Heat balance equation......

Equation 2. Reduction of tensile strength......

Equation 3. Risk calculation formula......

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1Abstract

The purpose of this project is to study a power line to determine the risk of an outage. In order to make this decision two kinds of analysis will be conducted using statistical data including wind velocity, temperature, and outage probability. The first analysis to be done will be the study of the overloading risk of the current power line without any changes. The second analysis is the benefit assessment, which will analyze the cost and benefits associated with reinforcement to the given power line. These results will then be used to make an assessment of risk of the power line and compare that to an upgraded system.

2Problem Statement

A power line transports electricity for most of its life, barring outages or normal switching operations. This means that the power line is subject to all seasons, all types of weather, and all outage cases.

The effects of weather in a statistical setting have not been fully explored in the past operation of the power system. A conservative estimate has been used to determine the safe operating point of the power line. Because of deregulation in the power system industry, construction has slowed or stopped while the load is increasing and slowly approaching the lower limit of the estimate.

This project will look at weather and outage statistics to try to determine a new operating limit of a power line. It will also determine the amount of risk associated with running the power line over the current limits and study reinforcements that will reduce this risk.

3What is risk?

In order to understand the problems of running a power delivery system, a definition of risk is needed.

Risk is the product of an event’s probability and its resultant impact or consequence.

3.1Events

Load growth, weather, and/or outages are events that may cause overloading of a transmission line. Outages can be forced or scheduled. Forced outages are faults, miss-operation, or miss-coordination. The utility and the consumer plan scheduled outages. Scheduled outages have minimal impact or are financially rewarding to the consumer, allowing the utility to do scheduled maintenance or reduce loading during peak load conditions.

3.2Consequences

Consequences of these events affect the consumer’s ability to live and do business. Depending on the type of consumer, there could also be financial losses. There will also be a cost to the utility due to these consequences, these costs include damage to the system and loss of profits.

4Proposed Technical Solution

The proposed solution takes into account how risk it affects the planning of the power delivery system in order to develop better loading strategies for a power line. The flow chart in appendix 8.1 will outline the steps that need to be taken in order to reach the final conclusion.

To start this project, research was done by each of our team members to get the data that will be used in the analysis process. The data was obtained from weather stations, library, Internet, and information given by the utility company. The data obtained from our research includes weather data, line characteristics, actual power flow data, and types of reinforcements that will be suitable for the study.

In order to make our data useful for the analysis of the power line; the raw data needs to be processed. By processing, the format of our data should fit into our software-input format. Temperature data was obtained from many weather stations and the data needs to be interpolated about the power line.

The processed data will then be inputted into our software, which includes IPFLOW and MATLAB risk simulator[1]. The result of the calculations done by these two software packages will be combined and produce the risk associated with a condition. Feasibility will also be considered by considering the company’s goals.

In order to determine the optimal solution reinforcements will be considered. The reinforcements possible are:

• Leave power line as is
• Re-conducting
• Re-building or adding new transmission lines
• Increasing power line voltage
• Adding micro-generation

This is not a complete list as more ideas surface in the process of researching. The annualized reinforcement cost and the risk will be added together. The feasibility and the total cost (reinforcement cost plus risk) will be analyzed and compared against other reinforcements with their risk. These changes will be compared using the following criteria:

• Cost of reinforcement

• Risk
• Feasibility

A decision will be made with this information regarding the best course of action in order to reduce the risk to the power line. This study will determine the lowest cost of a feasible alternative for the power line.

5Objectives

Develop a report that will outline the following items

• Overview of the research including:

-Description of the data

-Description of the statistical method

-Description of the power line

• Description of the process used to determine the associated risk
• Determine a recommendation for the power line
• Identify areas of further study

6Progress to Date

6.1Research

6.1.1Weather

Historically utilities have used the IEEE standard of 40º C and 2.0 ft/sec wind speed. This is a very conservative scenario in the life of most power lines. Consider the Des Moines mean temperature of 24.8º C for the month of July, this is about 2 whole standard deviations away from the IEEE calculated mean.[2]

Finding rigorous statistical weather data has been a real challenge. Most of the data available through standard methods (i.e. library and free web-sites) only provide monthly averages and standard deviation. The statistical method used in determining this data does not provide a rigorous statistical model.

Another problem experienced in researching data has been determining how location affects the usefulness of the data. The location of the data station to the power-line also affects the usefulness of the data (See section 6.2.1 for details).

Our group has found a data source. The source of data is from the High Plains Climate Center based in Nebraska. This site has collected data over the high plains area for about ten years. The source is not as rigorous as we would like, but due to the lack of time this source will be used.

6.1.2Transmission Line

Alliant Gas and Electric has provided system, power flow, and transmission line data and suggested a specific transmission line to study.

The transmission lines that Alliant suggested feeds Oakridge substation with two 69 kV transmission lines. A portion of these particular transmission lines is double circuited with a 34.5 kV transmission line that is build to 69 kV standards. These transmission lines are made up of two types of conductors thatTable1 shows.

Table 1. Transmission line characteristics

Conductor type / Structure type / Sheild wire / miles / ampacity @75 deg C
North Section A / T2 (2-336) ACSR / double circuit / 3/8" EHS / 0.6 / 917
South Sections B / T2 (2-336) ACSR / double circuit / 3/8" EHS / 2.6 / 917
T2 (2-336) ACSR / single circuit / 3/8 EHS / 1.3 / 917
636 ACSR / double circuit / 5/16" EHS / 4.2 / 789

The transmission line to the south, is restricted by the 4.2 miles of 636 ACSR conductor because of it lower ampacity. This section will be a good section to focus our study on.

6.1.3Load

Oakridge substation supplies power to Cedar River Paper Company in Cedar Rapids Iowa. Cedar River Paper Company is the largest recycling paper mill in the United States. They recycle between 500 and 600 tons of paper waste every day into a large blender called a pulper. There the paper is made into a liquid form and then spread onto a mesh screen conveyer where a series of presses remove excess water with vacuums and rollers. There are two machines that perform this process around the clock. The Cedar River Paper Company is a non-interruptible customer and their load fluctuates little throughout the year.

The south line also feed Kirkwood Community College via a tap. This load is interruptible during peak load conditions.

6.2Risk Calculation

According to the definition of risk above, risk is the product of the event’s probability and its resultant impact. The risk calculation will be described below.

6.2.1Weather analyses

For determining the risk of conductor’s sag, air temperature and wind speed level will be the two most significant factors that will influence the conductor. To get this data, the location of the stations that will monitor the transmission line should be determined. The sites should include shielded and heavily forested locations where there would be little or no beneficial effects from meteorological conditions. These sites should be reviewed carefully to insure that the sample data would not be biased or skewed. Some sites should be in areas of moderate to high-sustained wind, and some sites should be on open farmlands and on exposed hilltops[3]. In summary, the site that will be selected should represent the weather around the transmission line accurately. The transmission line should be monitored continuously to provide precise data. By determining the mean and the standard deviation, a plot of the wind velocity vs. time and a plot of temperature vs. time can be obtained. As a reminder, the longer the scope of time of the data, the better the sample data is. After the data is obtained from the station monitor, then we shall proceed to the next step.

6.2.2Conductor characteristics

The conductor’s characteristics necessary to calculate the risk are:

• Factor considering skin effect
• Current through the conductor / ampacity
• The dc resistance of the conductor
• Temperature coefficient of resistance
• Solar absorptivity of surface
• External diameter of the conductor
• Thermal conductivity

6.2.3Determining the conductor’s temperature

The conductor’s temperature during a specified amount of time is determined from several different heat transfer equations. The heating effect on the transmission line is canceled out by the cooling effect. The heat balance equation used in calculating conductor temperature is [4]:

Pj + PM + PS + Pi = Pc + Pr + Pw

Equation 1. Heat balance equation

Pj : Joule heating

PM: magnetic heating

PS : solar heating

PI: corona heating

Pc: convective cooling

Pr: radiative cooling

Pw: evaporative cooling

Knowing all the values of parameter, conductor temperature can be calculated by solving the above equation.

6.2.4Calculate the reduction in tensile strength

The reduction of tensile strength of the conductor was used as the index for the present study that is expressed by:

W = exp(C(ln t – A – BT))

Equation 2. Reduction of tensile strength

W: Reduction of tensile strength in %

T: Conductor Temperature (C)

t: Time when the conductor is in the condition of temperature T

A,B,C : Constants characteristics of the conductor material

6.2.5Calculate the risk of sagging

Regarding the thermal overload, the thermal risk is the expected impact when the conductor is running on a particular current level. Mathematically, it is the product of:

1. The probability that the conductor temperature may exceed the limit.
2. The resultant impact whenever the conductor is running beyond its temperature limit.

For this case, the event is thermal overload of a conductor and the risk is the expectation of costs that may result. Given the current I, we may compute thermal overload risk as the probability of the temperature being greater than MDT, times its related impacts:

Equation 3. Risk calculation formula

:conductor temperature which is influenced by current I together with the ambient

conditions

P[/I] :the probability density function (pdf) of  given current I

R[I] :the risk regarding the line loading

6.3Power Flow

Alliant's planning department has given us a their power flow case in PSSE version 23. IPFLOW will be used to do the analysis of the power flow through the line in question under different conditions. The transmission lines that we are studying residein the one-line diagram shown in Figure1.

Figure 1. One line diagram

Currently we are experiencing difficulties with loading the raw data file obtained from Alliant. Initially they gave us version 24 that IPFLOW will not read. Alliant used PSSE to convert to version 23 and also has problems loading because it exceeds 12 thousand buses.

6.4MATLAB

A Matlab risk simulator that was written at Iowa State University will be used to perform the risk calculation [1]. The data obtained from our research will be processed and inputted into the Matlab program.

7Future Work

7.1JAVA interface for MATLAB Code

Our group is also considering a JAVA interface that would allow an interface to the MATLAB code via the Internet. This would allow utilities that do not have MATLAB the chance to use the risk simulation software in their power system planning.

8Appendix

8.1Design Flow Chart

Figure 2. Design flow chart

8.2Budget Chart

9References

[1]Wan, Hua.“Thermal Risk Simulator”,Graduate paper.

[2]WeatherPost. Washington Post (1993). [One-line]. Available;

[3]D. Douglas, A. Edris, G. Pritchard. “Field Application of a Dyanamic Thermal Circuit Rating Method”, IEEE Trans. on Power Delivery, vol. 12, No. 2, pp. 823-828, 1997.

[4]Y. Mizuno, H. Nakamura, K. Adomah, K. Naito. “Assesment of Thermal Deterioration of Transmission Line Conductor by Probabilistic Method”, IEEE Trans. on Power Delivery, vol. 13, No. 1, pp.266-271, 1998.

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