ELECTRIC POWER ENGINEERING 2007 Title of paper:

Maintenance optimisation of transformers 110 kV/MV in distribution networks

Author(s):

Břetislav Stacho, Stanislav Rusek, Radomír Goňo, Vladimír Král, Tomáš Raška

MAINTENANCE OPTIMISATION OF TRANSFORMERS 110 KV/MV IN DISTRIBUTION NETWORKS Ing. Břetislav Stacho
VŠB – TU Ostrava, 17. listopadu 15, 708 33 Ostrava - Poruba, CZ, http://www.vsb.cz
tel.: +420 597 325 240, e-mail: [email protected] Prof. Ing. Stanislav Rusek, CSc.
VŠB – TU Ostrava, 17. listopadu 15, 708 33 Ostrava - Poruba, CZ, http://www.vsb.cz
tel.: +420 597 324 377, e-mail: [email protected] Ing. Radomír Goňo, Ph.D.
VŠB – TU Ostrava, 17. listopadu 15, 708 33 Ostrava - Poruba, CZ, http://www.vsb.cz
tel.: +420 597 324 554, e-mail: [email protected] Ing. Vladimír Král
VŠB – TU Ostrava, 17. listopadu 15, 708 33 Ostrava - Poruba, CZ, http://www.vsb.cz
tel.: +420 597 324 421, e-mail: [email protected] Ing. Tomáš Raška, Ph.D.
ČEZ Distribuční služby, s.r.o., 28. října 3123/153, 709 02 Ostrava, CZ, http://www.cez.cz
tel.: +420 596 673 591, e-mail: [email protected]

Abstract This paper deals with application of reliability centred maintenance (RCM) to distribution network elements, namely to 110 kV/MV transformers. There are presented and classified the input data needed to determine the optimal order of transformers for maintenance. Concerning these input data, this paper describes a procedure of determining resulting values of operational importance and technical condition. Interrelationship between operational importance and technical condition is essential to determine the optimal order for maintenance mentioned above. An example of possible evaluation of RCM program output data is presented in the last chapter. This program was developed at the Department of Electrical Power Engineering VŠB-TU Ostrava in order to apply RCM to the selected elements of distribution network.

1. Introduction From the principle point of view, we can use two approaches of optimisation for application of RCM. The first one is periodical RCM. The result of this approach is the optimal maintenance rate, where the total operational costs are minimal. The second approach is RCM according to condition – where we look for optimal order of equipments to maintenance. We chose the second approach for the 110 kV/MV transformers. It means that inspections and diagnostic measurements, incorporated in the Preventive Maintenance Code, are performed in regular intervals on the equipment. The maintenance activity depends on the results of these inspections, diagnostic measurements and operational importance of appropriate equipment. The data determining operational importance of transformers are other input data for the optimization algorithm. It is the data included in control systems or other parts of technical information system of the company. Evaluation is performed by RCM computer application which was developed at the Department of Electrical Power Engineering VŠB-TU Ostrava. 2. Determination of optimal order of 110 kV/MV transformers for maintenance Relation between importance and technical condition is essential to determine the optimal order for maintenance. So it is necessary to specify importance and technical condition of an element by the help of appropriate coefficients. We insert these two indicators in the two dimensional graph, where the position of the particular element determines the mentioned optimal order for maintenance. In addition to the importance and technical condition, it is also necessary to identify each element accurately. Identification is the third group of information about an element. A general example of a two-dimensional graph is in the Fig 1.

Fig. 1

3. Input data for application of RCM to 110 kV/MV transformers Input data can be divided into three groups: • • •

equipment identification data, data determining technical condition of equipment, data determining equipment importance.

3.1. Identification data Following data is used for accurate identification of a particular 110 kV/MV transformer in electricity network. • • • • • • • • •

substation, operational label, transformer age, type of a transformer, serial number of a transformer, tap changer age, type of a tap changer, tap changer serial number, nominal capacity.

3.2. Technical condition Four criteria were chosen for evaluation of technical condition of a 110 kV/MV transformer: • • • • • • • • • • • • • • • • •

climatic conditions, diagnostic tests evaluation, date of the last regular maintenance or repair, transformer age, tank condition (corrosion, paint), tank, pipeline and armature seal, condition of a control box and its equipment, cabling, type of a tap changer, tap changer age, sort of drive, number of switch-overs, bushing seal, bushing age, date of internal inspection, number of hours in operation, average load per hour.

Weights and ranges of evaluating of these criteria were determined by qualified estimations of operating and maintenance workers at the beginning. After having performed evaluation of all machines, the appropriate weight values were specified so that the resulting evaluation of technical condition was as real as possible. Thus the set weights of criteria and ranges of their evaluations were not changed at all. Weights of the particular criteria are in Tab. 1.

Climatic conditions Date of the last common maintenance or repair Type of transformer transformer age tank condition Tank seal Condition of control box Cabling Type of a tap changer

5

Tap changer age

10

30

Sort of drive

20

50 50 10 40 20 10 50

Number of switch-over Bushing seal Bushing age Date of internal revision Number of hours in operation average load per hour Diagnostic tests evaluation

30 40 20 50 80 80 100

Tab. 1

3.3. Operational importance Technical condition is actually determined by the transformer itself. Considering operational importance of a transformer, we are interested in its position in the network. Three criteria were chosen for evaluation of operational importance of a 110 kV/MV transformer: • • •

possibility of a connected point of supply backup, importance of supplied distribution transformers (number and type of connected customers), transferred energy in a year.

These three criteria were considered coessential, so the value of weight of each criterion equals one third. The range of importance evaluation of MV lines was determined by the help of socalled credit evaluation according to the number and type of customers, which are connected to the transformer. The following methodology to evaluate a criterion was designed - possibility of a connected point of supply backup 3.3.1. Evaluation of a criterion – possibility of a connected point of supply backup The following criteria were chosen for evaluation of this operational importance criterion: • • • •

reserve by the transformer (%), manipulating time of reserve by the transformer (min), reserve by the MV line (%), manipulating time of reserve by the MV line (min).

The most important criterion for evaluation is the reserve by the transformer. Namely, if the reserve is 100%, there is no reserve, or the reserve is between these two extreme values. Design of methodology for evaluation of the criterion - possibility of a connected point of supply backup - is in Tab. 2.

No back up T0/- MV(0-100)/120 T0/- MV100/120 T0/- MV(0-100)/60 T0/- MV100/60 T0/- MV(0-100)/20 T0/- MV100/20 T(0-100)/15 MV(0-100)/120 T(0-100)/15 MV100/120 T(0-100)/15 MV(0-100)/60 T(0-100)/15 MV100/60 T(0-100)/15 MV(0-100)/20 T(0-100)/15 MV100/20 T100/15 MV0 T100/15 MV(0-100)/120 T100/15 MV100/120 T100/15 MV(0-100)/60 T100/15 MV100/60 T100/15 MV(0-100)/20 T100/15 MV100/20

100 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

T0

- reserve by the transformer is not possible, T(0-100) - reserve by the transformer exists, but it is not 100%, T100 - reserve by the transformer is 100%, MV0 - reserve by the MV line is not possible, MV(0-100) - reserve by the MV line exists, but it is not 100%, MV100 - reserve by the MV line is 100%, 15,20,60,120 - activation up to (min).

Tab. 2

In the coming time new methodology for determination of the criterion value range possibility of a connected point of supply backup - shall be completed. It will contain determination of weights for the particular criteria mentioned above. Consequently the possibility of a connected point of supply backup will be determined more accurately by the multicriteria analysis (MCA), probably by concordance and discordance analysis. 4. Resulting evaluation of RCM application to 110 kV/MV transformers As it was mentioned in chapter 2, the relation between operational importance and technical condition is essential to determine optimal order of technical evidence elements for maintenance. In our case, the 110 kV/MV transformers are the above mentioned elements. Ratio of the resulting values of operational importance and technical condition is respected and may be changed deliberately in RCM computer application, which was developed at the Department of Electrical Power Engineering VŠB-TU Ostrava. For the primary theoretical consideration this ratio is 1:1. Fig. 2 describes one of the possible approaches of evaluation of application of RCM for the ratio mentioned above. The three-level decision model is presented in Fig. 2. These three levels determine the contingent maintenance activity for individual transformers. It is possible to determine a multilevel model, however, this approach seems to be quite adequate. At present it is necessary to determine more accurately limits between individual levels so that they correspond to reality as best as they can.

Fig. 2

4. Conclusion This paper deals with determination of optimal order for maintenance of 110 kV/MV transformers in network of the distribution company. Input data were divided into three groups: identification data, technical condition and operational importance. Value ranges and weights of criteria for technical condition mentioned in chapter 3.2. were already determined with adequate accuracy. These values will not change any further. However, value ranges and weights of criteria for operational importance were determined recently, and their values will be specified more accurately in the coming time. Technical condition and operational importance ratio (we use 1:1 ratio in this paper) will also be specified more accurately. We may expect the resulting ratio between the operational importance and technical condition about 2:3. 5. Acknowledgements This work is supported by The Ministry of Education, Youth and Sports of the Czech Republic - project No: CEZ MSM6198910007.

6. References

[1] Rusek S. : Spolehlivost elektrických sítí, VŠB - TU Ostrava, ISBN 80-7078-847-X, 2001 [2] Raška T., Rusek S.: Využití počítačové aplikace RCM pro optimalizaci údržby transformátorů 110 kV/vn, 11. ročník konference Poděbrady, 2006 [3] Stacho B., Rusek S., Goňo R.: Application of RCM to the transformers 110 kV/MV, Transcom Žilina, 2007 [4] Goňo, R., Rusek, S., Král, V.: Application of Reliability Centered Maintenance in Electricity Distribution Company. In Proceedings of the IASTED International Conference PEA 2006. Ed. G. O. Anderson, Calgary:ACTA Press, 2006, p. 96 - 101, ISBN 0-88986-616-3