MAINTENANCE AND REFURBISHMENT STRATEGIES FOR M.V.

SUBSTATIONS

G. Balzer; O. Schmitt M. Halfmann A. Hößle

Darmstadt University ABB Calor Emag ABB Calor Emag Pfalzwerke AG,
of Technology Schaltanlagen AG Schaltanlagen AG Ludwigshafen
(Germany) Mannheim Mannheim (Germany)
(Germany) (Germany)

SUMMARY

This paper shows the principles of the developed reliability-centered maintenance strategy (RCM), which takes into
account the condition of the equipment and the importance of the equipment in the network.

The condition of the equipment can be evaluated on the basis of different criteria. The importance describes the con-
sequence of the equipment failures on the network and the economic. As a result the RCM-strategy leads to the
conclusion which equipment has to be maintained first.

Besides the theoretical principles of this RCM-strategy the authors describe the experience in Germany.

It makes sense to embed the described procedure in a financial management system and to develop a software tool. The
general data and measurements of the process (e. g. substation) should fed into the program to assess the condition and
the importance of the equipment. Whereas the main data of the equipment (e. g. type, manufacturer, year) are already
listed in network information systems. These values have to be directly imported to the software tool. After the overall
assessment a ranking of the equipment will be calculated and the responsible engineer has to handle the order via a
commercial software tool.

Keywords: Maintenance Strategies - Condition - Importance - Life Cycle

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 MAINTENANCE AND REFURBISHMENT STRATEGIES FOR M.V. SUBSTATIONS

G. Balzer; O. Schmitt M. Halfmann A. Hößle

Darmstadt University ABB Calor Emag ABB Calor Emag Pfalzwerke AG,
of Technology Schaltanlagen AG Schaltanlagen AG Ludwigshafen
(Germany) Mannheim Mannheim (Germany)
(Germany) (Germany)

1 INTRODUCTION - corrective
- time-based
Facing the competitive environment the utilities are - condition-based
forced to rethink their maintenance strategies. Economic - reliability-centered
considerations point out the need for the calculations
which are capable to extend the lifetime of equipment In a corrective maintenance (CM) strategy, replacement
and substations and allows an economic and reliable life or repair is performed only if a failure occurred. In the
cycle management. On this way one of the important case of equipment where investment costs are low and a
tasks of the utilities are: fault will have only a minor effect, this procedure may
result in the lowest overall costs. This strategy will be
- to select the equipment, which should be mainly used in systems with lower voltages. Only severe
maintained failure on certain type of equipment will influence the
- to make a ranking of the equipment. procedure.

The ranking of the equipment gives the opportunity to A time-based maintenance (TBM) strategy featuring
select those which should be maintained first. If the predefined intervals rooted in empirical feedback, where
ranking is derived from a reliability-centered mainte- components are replaced after a specified period of use,
nance strategy an optimum solution can be stated. In this has been practised as the usual maintenance strategy in
paper the term maintenance also covers the replacement electrical power systems for many years. This approach
of equipment as e.g. necessary for renovation. Whereas generally produces satisfactory results. It will not,
paper /1/ describes the principal approach to develop a however, be the most cost-effective option in all cases,
maintenance strategy for m.v. substations, this report is since the equipment will not usually remain in operation
dealing with the presentation of an assessment, which up to the end of the lifetime which is possible.
was performed in the past.
Since some years, however, there has been a developing
2 MAINTENANCE STRATEGIES shift away from time-based maintenance and towards
condition-based maintenance(CBM). Condition-based
The statistics of German utilities give a rough overview maintenance is driven by the technical condition of the
of the age of the equipment, which are installed in the equipment. Under this approach, all major parameters
system. For example assuming that the useful lifetime of are considered in order to determine the technical
a circuit-breaker will be about 35 years the new in- condition with maximized accuracy. For this reason
stallations of each year can be divided in two parts: detailed information via diagnostic methods or
monitoring systems should be available.
- extension of the system
- replacement of old circuit-breakers. A fourth strategy, which additionally include a reli-
ability-based component, has been under discussion
If a calculation is performed for the main voltage levels recently, and some applications are already in use /7/.
in Germany, the average age of the circuit-breakers are The aim of this approach is to include the influence on
as follows: the importance of the equipment in the network and the
actual condition of the equipment. A maintenance
10 kV: 17.0 years; 20 kV: 16.1 years; strategy is referred to in this paper as reliability-cen-
110 kV: 18.2 years; 220 kV: 22.4 years; tered maintenance (RCM) and it has to be noticed that
380 kV: 15.8 years. this RCM-method is different considering other RCM-
applications which consider equipment only.
The higher age of the 220 kV c.b. demonstrates the less
importance of this level in the transmission system of Figure 1 demonstrates the main conclusion of the dif-
Germany ferent maintenance strategies mentioned above, de-
pending on the two criteria: importance of the equip-
Several maintenance strategies are mainly used or in ment considering the network and the condition. The
discussion in electrical systems (figure 1) /1/ - /6/: presented evoluation of maintenance strategies including

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 the influencing factors gives a hint to the expense for were caused by the equipment. Statistical informations
applying a certain strategy. are a basis for this analysis comparing the different types
of c.b. according to figuire 2.
min-oil airblast vacuum total

0,12
0,1
0,08

failures
0,06
0,04
0,02
0
10 kV 20 kV

Figure 1: Evolution process of maintenance strategies

Figure 2: Failures of 100 c.b. per year (primary equip-
Without a doubt, the RCM strategy offers definite ad-
ment)
vantages, since besides the condition of an equipment or
a system, its importance in the network also crucially
In addition, rating and weighting factors have to be
influences the maintenance action required. It clearly
applied to take account of the influence of each criterion
emerges that network reliability can be upgraded by
on the overall value c (condition) representing the
correct selection of the equipment for maintenance.
condition of the equipment.
These equipment will be maintened which have the main
influences of the network reliability
Both reliability-centered and condition-based mainte-
nance strategies require the same data for assessing the
3 RELIABILITY-CENTERED MAINTENANCE
condition of the equipment. Of course, many equip-
A reliabilitiy-centered maintenance strategy, which ment/network-specific data can be collected for evalua-
combine the two aspects of condition and importance, tion. In practice, however, they must be restricted to the
requires the following procedure: major influencing variables, for which there is actually
utilizable information derived from the process, from
- The condition of the equipment has to be deter- relevant statistics, or from manufacturer’s docu-
mined. mentation. Results are crucially dependent on
- The importance of the equipment for the net- assessment of the individual criteria involved in evalu-
work as a whole must be determined, e.g. the ating a particular equipment, and the weightings as-
influence of equipment failure on the reliability signed to them.
of supply.
- Both information inputs must be combined and Defining an substations’s importance and assessing the
evaluated in order to specify the optimum se- consequences of a fault basically constitute a practical
quence of maintenance work for the indi-vidual but also subjectively influenced value. In this context,
devices (equipment, substation). there are numerous different parameters to be consid-
ered, e.g.:
The condition of the total m.v. substation has to be de-
rived combining the condition of the single items of the - non-availability of the substation
asset, for example bays, circuit-breakers, instrument - failure rate of the equipment
transformers, secondary equipment and so on. The - substation configuration
condition, e. g. of a circuit-breaker, can be evaluated on - interrupted active power
the basis of different criteria for example: - kind of customers, social impact
- financial payment in case of loss energy.
- age
- type of circuit-breaker Figure 3 shows the basic procedure for linking the two
- number of short-circuit interruptions assessment criteria:
- number of switching operations - condition of the equipment
- experience with this type of circuit-breaker - importance of the equipment in the network
- measurement results
- know-how of the service department and so on. Both these criteria are combined in an appropriate
manner enabling an overall assessment to be arrived at.
In general the condition of a circuit-breaker has to The overall sequence of maintenance according to figure
consider the different experiences and failures which 3 will lead to a ranking, which equipment has to be
maintened first, second, and so on. This will be

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 expressed using the index o combining the two aspects
which are mentioned above. 4 COMPARISON WITH PREVIOUS PRACTISE

The evaluation levels cM and cR are entered in figure 5

in accordance with empirical feedback from the network
engineers concerned, i.e. as user-specific data. The
parallels to abscissa i through cM and cR characteristics
limit the areas in which from a technical viewpoint a
servicing routine or replacement is required. Area
assignment can be defined as follows:

100 - cR: replacement

cR - cM: servicing, repair
< cM: no action required
Figure 3: Procedure for maintenance planning Inside a particular area, the priority for individual
measures is obtained by examining the distances d from
The procedure is described in the following. After the the straight line. With the aid of the classification
values for the c (condition) and i (importance) pa- achieved in figure 5 the necessary maintenance action
rameters have been calculated, the results (crosses) can can be specified. Basically, it is also possible to specify
be listed in an X, Y system of coordinates, as shown in an area of corrective maintenance (CM) in this scheme,
figure 4. The c and i axes are scaled in such a way that i.e. an area in which all the substation needed to be
the c and i values can at maximum assume the value maintained only after a malfunction. These elements are
100. characterized by being below a critical importance
threshold, so that a failure has no significant effect on
A large value for i signifies that the substation con- network reliability.
cerned is of high importance in the network. The vertical
axis represents the condition of the substations 5 EXAMPLE
concerned, while the horizontal axis reflects its im- 5.1 Circumstances
portance in the network. A cross in the top left-hand
corner corresponds to a substation which, although in a Some companies are complaining about the increasing
poor technical condition, would not cause any major pressure of cost savings in asset management which is
consequences if it fails. accompanied by the necessity to maintain a constantly
high level of availability. However, the approaches for
c
100 achieving these goals differ from each other consid-
erably.
2 3
It is obvious, that an one-sided reduction of the main-
cR 1 tenance budget, for example in the course of changing
4 5 from a Time Based Maintenance (TBM) to a Condition
d1 d2
d3 Based Maintenance strategy, is often not enough.
cM Especially in this context, it is much more necessary to
d4 optimise maintenance processes so that a strategy can
d5
achieve its full potential.

100 i A German utility (approximately 6,000 km2 service area,

1.6 million private and commercial clients, 1,150 staff
members) which had realised the importance of
optimising maintenance processes wanted in 1999 to
Figure 4: Interpretation of the assessment result audit, evaluate and optimise the existing assets as well
c condition of the substation as to analyse and optimise the maintenance processes in
i importance of the substation co-operation with an internationally active consultancy
company within the scope of a model project. The
By contrast, a cross in the bottom right-hand corner electric supply company followed a CBM strategy. The
designates an substation which is in very good condi- maintenance processes were in line with this strategy
tion. A failure of this asset would entail substantial and had been optimised. The aim of the project was to
consequences for network operation. The distances d1 to reduce the costs of the whole maintenance area by a
d5 illustrate the sequence in which the individual further 30% and the duration of the project was nine
substation must be serviced or replaced. weeks.

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 5.2 Analysis phase average value of 22.6 years of operation.

The total consult concept served as the strategic frame- type number age
work of analysis - a holistic approach for asset man- air blast 208 34.3
agement which does not only consider technical aspects min. oil 284 20.7
(for instance diagnosis, network analysis), but also takes vacuum 58 7.7
into account commercial factors (for example SF6 55 3.8
investment strategies, budget planning) in the analysis other 1 30.0
figure 5. total 606 22.6

Component Diagnosis Network Diagnosis Table 1: Main data of the assessed 20 kV circuit-
(Equipment Data; Visual (Load Scenarios, Standards; System
Inspections; Measurements; Supply Concept; Automation
Diagnosis
breaker
Operation History) Concept, etc.)
50

40
Development of Scenarios, Technical and Economical
Evaluation and Ranking

30

number
Maintenance Concept
(Maintenance, RETROFIT,
Network Concept
(Expansion,
System 20

Reinvestment, etc.) Reduction) Concept

10

Investment-/Budget-Planning
0
52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96
year

Figure 5: Consult concept

Figure 6: Distribution and type of the m.v. circuit-
Auditing the available equipment and comparing them breakers according to the age of the installation
to the existing data bases was the first necessary step for
achieving the goals of the project - firstly because until Figure 6 illustates the distribution of the installation of
then the maintenance and archiving of data depended the c.b.’s. The oldest types are installed in 1952, and if
largely on the person who conducted it, and secondly an maximum life time of 40 years is assumed about 90
because certain data (such as condition and degree of c.b.’s has to be replaced (about 15 % of the whole in-
maintenance) was not available for all equipment. stallation). A further reduction of the life time to 35
years will increase this value to about 160 c.b.’s.
In order to conduct a cost-effective and quick data ac-
quisition of the condition of equipment on a specific 5.3 Development of strategies
day, the e-Pat solution was used. This solution is a
software tool for handheld computers which is used to In the next step, the information acquired in the analysis
document the evaluation of condition efficiently on the phase was used for developing alternative maintenance
spot. The required data was acquired with the help of the strategies. In TBM strategies, the equipment are
staff members of the client who had been trained replaced after a hypothetical period of service life and
thoroughly before that. The data which was acquired in maintenance is carried out continuously after specific
this manner was sent via Internet to central computer on time intervals. A maintenance budget based on this
a daily basis. In this way it was possible to evaluate approach corresponds to an index value of 100.
more than 8,000 equipment (substation bays, circuit-
breakers, power and instrument transformers, discon- CBM strategies, on the other hand, prescribe replace-
nectors and so on) within four weeks, compare the data ment or maintenance measures only after an equipment
to the existing data bank of the utility and generate an has achieved a certain technical condition. This
updated version simultaneously. In addition to that, all condition is specified with the help of a so called con-
critical equipment were identified in this step of the dition index. The condition index is defined differently
project (critical in the sense that the security of supply for different equipment
depends on them to a large extent in case of a power
breakdown). The scenario for a CBM strategy showed of the com-
plete asset a saving potential of 40% compared to that of
As an example which detailed informations of the a TBM strategy. In this connection, it might be
complete asset are stored the following description interesting to note that the model calculations for the
comprises the main data of the m.v. circuit-breakers, application of a CBM strategy coincided exactly with
table1. In total 606 circuit-breakers are assessed with an the amount of the maintenance budget of the client.

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 However, this was not enough for achieving the goal of vaccum cav = 28
the project. Assessing the possibilities of approaches SF6 cav = 37
using modern Reliability Centered Maintenance (RCM) other cav = 70
was an even more important task. Such approaches
distinguish themselves through the fact that they not If the values of cM = 40 and cR = 60 are defined, as
only take the technical condition of the assets into discussed above, the air blast c.b.’s should be replaced,
consideration but also their importance in the overall whereas the minimum oil circuit-breakers should be
network. serviced. And no actions are necessary for the new types
(SF6 and vaccum). According to the assessmane the
circuit-breakers can be divided into several categories
according to table 2.

If the CBM strategy is applied 202 c.b.’s should be

replaced in the next time, whereas 271 are in service. An
important reduction of the replaced number of c.b’s is
only possible, if the importance of the circuit-breakers
for the network is taken into consideration and the
maintenance strategy is RCM – based.

type replace service no action

air blast 169 39 -
min. oil 32 217 33
vacuum - 2 54
SF6 - 13 42
other 1 - -
total 202 271 129
Figure 7: Assessment of 20 kV substations
Table 2: Condition of the circuit-breakers
The result of the total assessment of the 20 kV substa-
tion is listed in figure 7. In addition to that, customer 7 CONCLUSION
specific critical values for the technical condition of the
equipment are defined. In the example shown in figure It makes sense to embed the described procedure in a
7, the critical value for a maintenance measure (cM) of financial management system and to develop a software
the complete substation is fixed at c = 40 whereas the tool. The general data and measurements of the process
critical value for a replacement or retrofitting measure (e. g. substation) should fed into the program to assess
(cR) is fixed at c = 60. the condition and the importance of the equipment.
Whereas the main data of the equipment (e. g. type,
A further step is to investigate which item (for example manufacturer, year) are already listed in network
circuit-breaker, instrument transformer, civil works ans information systems. These values have to be directly
so on) has the most impact on the result assessing the imported to the software tool. After the overall as-
condition of the complete substation. And the benefit sessment a ranking of the equipment will be calculated
may be, that a retrofit solution of one component would and the responsible engineer has to handle the order via
increase the condition and further actions are not a commercial software tool.
necessary in accordance with the importance of the
substation. When looking at this optimized process design concept,
the time needed to achieve this goal should not be
Summarizing the RCM strategy influences the main- underestimated. Since, however, the procedure for the
tenance processes significantly. The optimisation of basic and crucial core process of network component
these maintenance processes offers an additional cost- evaluation has already been defined, and an appropriate
cutting potential. In addition to that many companies PC program for this purpose are already available for
conduct processes that are too complicated and have high voltage equipments on the market. Further devel-
interfaces that require manual inputs, making them opment is targeting only the automatic incorporation of
prone to errors. time-dependent evaluation parameters. It is decided to
It may be of interest to consider the condition of the integrate this program into maintenance management
m.v. circuit-breakers in detail. The average value of the system. In the final analysis, computerized maintenance
condition cav of the c.b.’s is as follows: planning in a company necessitates a shared database, to
serve as a foundation for both maintenance planning and
air blast cav = 67 a higher-order information system for the network /8/.
minimum oil cav = 50

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