Basics of condition evaluation in distribution networks

Abstract

In this paper the development of a tool for a fast and simple evaluation of the technical condition of electrical networks
will be presented. With this tool it is possible to compare different networks and network sections of different power suppliers
as well as utilities of the same voltage level or network sections of the same utilities. Whole networks, but also individual
components such as transformers and switchgears can be evaluated. An exact condition evaluation of networks represents a disproportionately
large expenditure of time. To get a fast overview over the technical condition of networks and electrical components the knowledge
of experts is necessary. The presented method takes into consideration both: expert knowledge as well as the consideration
of age, maintenance and operating loads of networks and their components. These aspects are included in reliability computation
mathematical functions which will be used in the developed technical condition evaluation tool. Beside the evaluation of the
technical condition the tool is based on the maintenance expenditure and the priority of the examined networks, too.
In dieser Arbeit wird ein Weg für eine rasche und einfache Evaluierung des technischen Zustands von elektrischen Verteilnetzen
aufgezeigt. Mit dem erstellten Werkzeug ist es möglich, verschiedene Netzwerke und Netzwerkabschnitte der gleichen Spannungsebenen
auch von unterschiedlichen Netzbetreibern miteinander zu vergleichen. Dabei können sowohl einzelne Komponenten wie zum Beispiel
Transformatoren, Kabel oder Schaltanlagen als auch ganze Netze evaluiert werden. Eine exakte Beurteilung des technischen Zustands
eines ganzen Netzwerks würde einen sehr großen Aufwand bedeuten. Um jedoch eine einfache und rasche Übersicht über den Zustand
eines Netzes und dessen Komponenten zu erhalten, wird auf Expertenwissen zurückgegriffen. Für die entwickelte und vorgestellte
Methode wurden Fachwissen des Betriebs- und Instandhaltungspersonals als auch Daten aus dem Netz wie zum Beispiel Betriebsmittelalter,
Betriebsbelastungen, Wartungs- und Instandhaltungstältigkeiten usw. zusammengeführt und entsprechend mathematisch aufgearbeitet.
Dies wurde in Folge in einem Bewertungswerkzeug für die Evaluierung des Zustands von elektrischen Netzen und Betriebsmitteln
eingesetzt.

Full text

Page 1

C C I I R R E E D D

17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003
TUG_Koerbler_A1 Session 6 Paper No 66 - 1 -
BASICS OF CONDITION EVALUATION IN DISTRIBUTION NETWORKS

M. MUHR, B. KOERBLER, C. SUMEREDER
Graz University of Technology - Austria
muhr@hspt.tu-graz.ac.at
H. SANDER, M. MARKETZ, J. POLSTER
Kärntner Elektrizitäts AG - Austria
herbert.sander@kelag.at


INTRODUCTION


In this paper the development of a tool for a fast and
simple evaluation of the technical condition of electrical
networks will be presented. With this tool it is possible to
compare different networks and network sections of
different power suppliers and utilities of the same
voltage level or network sections of the same utilities
among themselves. Whole Networks, but also individual
components such as transformers and switchgears can
be evaluated.
An exact condition evaluation of networks represents a
disproportionately large expenditure of time. To get a
fast overview over the technical condition of networks
and electrical components the knowledge of experts is
necessary.
The developed and presented method is working on
expert knowledge as well with the consideration of age,
maintenance and operating loads of networks and their
components. These aspects are included in reliability
computation mathematical functions, which will be used
in the developed technical condition evaluation tool.
Beside the evaluation of the technical condition the tool
is based on the maintenance expenditure and the
priority of the examined networks too.


EVALUATION OF THE TECHNICAL CONDITION OF
ELECTRICAL NETS


To develop in the context of a project with 20 Austrian
power supply companies and the Austrian Association
of Electricity Utilities the department for high voltage
technology assigns a tool, which makes an evaluation
for the maintenance practice possible. Thereby the
technical condition, maintenance expenditure and the
priority of the examined networks will be evaluated.
Furthermore the possibility of a comparison between
different network sections and different utilities with
unrecognisable made data should be given.
It is problematic to judge different energy enterprises in
their total structure. For this reason the networks were
divided into the network levels according to the Austrian
energy liberalization law.
Only a few of the energy companies, which were
involved in the project, possess a portion on the extra-
high voltage level network in Austria.
Therefore only the network levels 3 to 7 became
consulted for the condition evaluation tool.


......transformer......transformer
...…overhead line...…overhead line...…cable...…cable
......switchgear......switchgear...…consumer ...…consumer
33
44
55
66
77
≤110 kV
≤110 kV
≤ 110 kV / ≥ 36 kV
≤ 110 kV / ≥ 36 kV
≤ 36 kV
≤ 36 kV
≤ 36 kV / ≥ 0,4 kV
≤ 36 kV / ≥ 0,4 kV
≤ 0,4 kV
≤ 0,4 kV

Fig. 1: examined network areas

As an result of this assignment (Fig. 1) arises the
advantage that also small utilities, which operate with
only local area networks, can also be served with the
provided tool as large utilities with networks in all
network levels.


Course of the Project


In the entrance phase of the project a questionnaire
according to the Austrian standard OENORM M 8100:
1985 04 01 was provided to the companies which were
involved in the project. Standard OENORM M 8100 was
already replaced by the standard OENORM EN 13306,
the terminology of OENORM M 8100 (TABLE 1) is still
in use [1] [2].

TABLE 1 - maintenance terminology acc. OENORM M 8100.
maintenance
inspection servicing overhaul, rebuilding
asses and
gauge the
component
condition
preserve
nominal
condition
restore the nominal condition:
condition / failure based
tech. diagnostic
e.g. clear
overhead lines
e.g. exchange of components

After a revision by the energy companies the
questionnaire was consulted as a starting point for the
determination of the input data of the evaluation tool.

Page 2

C C I I R R E E D D

17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003
TUG_Koerbler_A1 Session 6 Paper No 66 - 2 -
Arrangement of the Evaluation Tool


In the prepared program depending on the network level
between 80 and 170 input data are mathematically
linked and evaluated. Altogether in sum approximately
790 input data variables will be processed. The not
visible mathematical expenditure is corresponding in the
background of the program.
Each network level is arranged into an input form for
data acquisition as well as into two output forms for data
output and representation. With the output form an
individual evaluation of entered networks or network
sections and a summation evaluation of selected net
sections of the same network level can be done.
As previously mentioned, the point technical condition,
maintenance expenditure and priority of the regarded
networks or network sections became evaluated.


Fig 2: maintenance tool input and output surfaces


Evaluation of maintenance
evaluation of the maintenance expenditure takes place
over economic factors. Hereby replacement costs of
electrical systems play significant roles. In the
developed tool replacement costs are compared with
the annually resulting costs of the maintenance. To an
evaluated expenditure of 100% it applies that the annual
maintenance costs are equivalent high or higher than
the capital allowance of the concerned network or
network section.


Evaluation of priority. In order to be able to compare
the priority of the examined network or network sections
with the priority of other networks and network sections
of the same network level, possibilities, which can be
consulted, are contained in the input form of the
evaluation tool.
Thereby the redundancy of the network and electric
lines, as well as other characteristics like operation
loads will be taken for the evaluation of the network
priority. Highest priority is assigned to a value of 100%.

expenditure. An
Evaluation of technical condition. For the evaluation
of the technical condition of networks, network sections
and electrical components input data will be appointed
which it makes possible to estimate the technical
condition. For this the age of the electrical components
used in the concerned network level as well as
inspection intervals and maintenance activities will be
consulted. These aspects were accordingly weighted
and divided.


POSSIBILITIES OF THE TECHNICAL CONDITION
EVALUATION


An evaluation of the technical condition of networks,
network sections and electrical components can be
implemented differently. An exact evaluation of the
technical condition is only possible if measurements of
condition describing variables, which are describes the
condition of the individual components are with few
spends enforceable.
If all components of the network can be measured, the
received variables of the condition have to be weighted
and joined accordingly.
Thereby it has to be considered that the joined data
have to include the priority and the risk of a loss and it
results of the concerned network, network section or
component.
Such a detailed collection of the technical condition is
hardly possible, since an admission of the condition
variables would be time and cost-intensive. Further an
independent institution has to be established for
comparisons of networks of different utilities.
A further possibility for a simplification of the condition
collection is a reduction of the components to
components of high and increased priority as well as a
subjective evaluation of the nets by an inspection of
experts. The possible evaluation thereby corresponds
no more to an evaluation with all details but it
corresponds however still to actual conditions.
Nevertheless the driven
disproportionately high also due to this version of a
technical condition evaluation. For this reason another
way for the evaluation of the technical condition of
networks and electrical components was found.


Input Variables


To keep the expenditure of input variables collection
small, the input variables, describing the technical
condition of electrical components, were strictly reduced
to a limit number of variables.
These input variables are reflecting the maintenance
activities as well as the operating loads of the respective
components. This justifies itself with the fact that the
condition of the net depends of
-
age,
expenditure remains

Page 3

C C I I R R E E D D

17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003
TUG_Koerbler_A1 Session 6 Paper No 66 - 3 -
-
-
-
maintenance activities
operating loads of the components as well as
the environmental and climate conditions.


Basic Functions for Condition Evaluation


It was one goal of the development of the evaluation tool
to make comparisons between different networks and
network sections of the same network level possible.
That means that the developed tool represents a basis
for such comparisons. With that it is not necessary to
define technical condition, priority or maintenance as
absolute values. It is only necessary to build a
comparable basis of these values. For this no
probabilistic methods for the technical condition
evaluation have to be used like in [3].
For a time-dependent function for a description of the
technical condition the basic Weibull-function of the
reliability computation was selected and adapted
accordingly (1). The selected functions were adapted for
each electrical component according to their usual
technical life span (Fig. 3).

n
d
n
n
n
d
nn
c
bct
n
etC












−⋅+
−
−=
1
) 2(ln
1)(

(1)
Cn(t).… component condition according to
standard environmental conditions
bn.....….
cn, dn… gradient



condition vs. time
0
0,2
0,4
0,6
0,8
1
1,2
1,4
0 100200 300400500 600700800900
time in month
condition in p.U.
condition 40 yrs lft
condition 50 yrs lft
condition 60 yrs lft
appr. hazard rate
appr. hazard rate
appr. hazard rate
hazard rate

Fig. 3: time depending condition and hazard rate

As previously mentioned not only the age deals with the
technical condition of
components, but also the maintenance activities and the
operational loads of the components. An example of the
influence of operational loads on components shows [4].
From these reasons the total condition of networks and
network components is built up from the mentioned
influences. The technical condition of individual
electrical components will be accumulated and weighted
with the technical condition of other components
regarding their priority and risk of loss (Fig.6).
networks and network
Influence of Maintenance on Electrical Networks


The influence of the maintenance on the technical
condition of networks can be shown at the best on
overhead lines of medium-voltage distribution networks
in the forest-rich areas of Austria.
The main part of the maintenance activities of these
overhead lines is to free them from vegetation.
If this activity is not processed, then it can be assumed
that the probability of failure will rise by snowfall and
thunderstorms according to the regeneration of the
vegetation. The experience of the utilities, which were
involved in the project, show that an inspection of these
lines every 2 to 3 years is meaningful. From this a
meaningful, common interval of approximately of 50-60
months results for clearing the overhead line from
vegetation.
With a maintenance interval over 80 months it can be
assumed that the condition of the overhead line will be
substantially worsened and a safe supply cannot be
ensured any longer. From these facts and with the help
of the failure data of energy suppliers a suitable function
for a description of the technical condition was found.


Receiving maintenance. In Fig. 4 the affects of a not
accomplished maintenance work like clearing overhead
line from vegetation, contrary to periodic maintenance is
shown. Thereby it is assumed that the technical
condition is getting down strongly worse if a
maintenance interval of 10 years is overdrawn.

condition vs. time
0
0,2
0,4
0,6
0,8
1
1,2
0100200300400500600700800900
time in months
condition in p.U.
periodic maintenance
without influences
no maintenance


Fig. 4: condition depending on receiving maintenance


Improving maintenance. In the evaluation tool also
maintenance activities are considered, which can
expand the lifetime of networks and network
components. As an example for this the exchange of
overhead conductors and the renewal of poles and
service entrances in the context of network extensions
should be mentioned (Fig. 5).

Page 4

C C I I R R E E D D

17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003
TUG_Koerbler_A1 Session 6 Paper No 66 - 4 -

condition vs. time
0
0,2
0,4
0,6
0,8
1
1,2
0100 200300 400500600 700800900
time in months
condition in p.U.
2 periodic maintenance activities
without influences

Fig. 5: condition depending on improving maintenance


Additional loads. If additional loads like special
environmental factors (environmental
topology…), high operational loads etc. can influence
the technical condition of the network or network
components more than in standard environmental
conditions, factors can be found with that the weighting
factors can be adapted to according lifetimes. These
factors can be found out at failure statistics of the
utilities.
If the maintenance activities and their effects are well
known and documented so the equations for technical
condition can be defined. These equations became
linked together according to Fig.6.

pollution,
network conditionnetwork conditionnetwork condition
network section conditionnetwork section conditionnetwork section condition
component conditioncomponent condition component condition
weighting factor
component conditioncomponent conditioncomponent condition
component condition factor component condition factorcomponent condition factor
weighting factor
component condition factorcomponent condition factorcomponent condition factor
weighting factor
network section conditionnetwork section conditionnetwork section condition
X111
X111
X111
X111
wX111
•••••
Z1
Z1
Z1
Z1
Z1
Y11
Y11
Y11
Y11
Y11
wY11
wY11
wY11
wY11
wY11
Σ, •
Σ, •
Σ, •
Σ, •
Σ, •
X11t
X11t
X11t
X11t
X11t
wX11t
•••••
... ............
...
•••••
......... ......
wZ1
•••••
•••••
Y1s
Y1s
Y1s
Y1s
Y1s
wY1s
•••••
X1s1
X1s1
X1s1
X1s1
X1s1
wX1s1
•••••
Σ, •
Σ, •
Σ, •
Σ, •
Σ, •
X1st
X1st
X1st
X1st
X1st
wX1st
wX1st
wX1st
wX1st
wX1st
............ ...
...
••••••••••
Zr
Zr
Zr
Zr
Zr
wZr
•••••
Yrs
Yrs
Yrs
Yrs
Yrs
wYrs
•••••
Xrs1
Xrs1
Xrs1
Xrs1
Xrs1
wXrs1
•••••
Σ, •
Σ, •
Σ, •
Σ, •
Σ, •
Xrst
Xrst
Xrst
Xrst
Xrst
wXrst
•••••
...... .........
...
•••••
... ............
Σ, •
Σ, •
Σ, •
Σ, •
Σ, •
Σ, •
Σ, •
Σ, •
Σ, •
Σ, •
Σ, •
Σ, •
Σ, •
Σ, •
Σ, •
...... ............... .........
...... ......... ...............
Z1
Z1
Z1
Z1
Z1
......... ......
CCCCC
weighting factorweighting factor
weighting factorweighting factor
weighting factorweighting factor
wX111
wX111
wX111
wX11t
wX11t
wX11t
wX11t
............
wZ1
wZ1
wZ1
wZ1
wY1s
wY1s
wY1s
wY1s
wX1s1
wX1s1
wX1s1
wX1s1
... ...... ...
wZr
wZr
wZr
wZr
wYrs
wYrs
wYrs
wYrs
wXrs1
wXrs1
wXrs1
wXrs1
wXrst
wXrst
wXrst
wXrst
... .........

Fig. 6: network condition factor C development


condition vs. time
0
0,2
0,4
0,6
0,8
1
1,2
0100200300400500600700800900
time in months
condition in p.U.
res. condition
lifetime app. 30 years

Fig 7: combined condition, receiving and improving maintenance

The result of this equation is a condition/time diagram.
With that the actually technical condition of the
examined network or network section can be evaluated
and compared with other network or network section of
the same network level (Fig. 7).


Future Works


In a further step a network characteristic value should
be created which can permit statements over risk and
maintenance expenditures. For this “risk”-value the
values of condition, maintenance expenditure and
priority should represent the basic foundation.
A further chapter of the evaluation tool is a reliable
forecast over the condition and the expenditure of the
evaluated networks and components depending on
different maintenance intervals.



REFERENCES


[1] ÖNORM M 8100: 1985-04: Instandhaltung,
Benennungen, Definitionen und Maßnahmen.

[2] ÖNORM EN 13306: 2001 08: Begriffe der
Instandhaltung.

[3] Endrendyi J., Anders G. J., Leite da Silva A.M.:
Probabilistic evaluation of the effect of
maintenance on reliability – an application; IEEE
Transactions on Power Systems, Vo. 13, No. 2,
May 1998.

[4] Sumereder C. Muhr M.: Life Time Management of
Power Transformers, Cired 2003 Barcelona.