The distribution companies in Poland operate several thousand of medium powers transformers, which connect high voltage power lines with medium voltage distribution network. Most of transformers in this group are units with powers 10, 16, 25 and 40 MVA and many of them are older than 30 years. Although they are operated for so long, their technical condition allows them to continue running, which results from the fact that at the time of their design and construction there was tendency to oversize insulation system.
The modern approach to operation of power transformers is based on the complex assessment of transformer's technical condition and its meaning in a power system. By combining these two factors it is possible to thoroughly determine possibilities of further operation and economical profitability assessment of necessary repairs or unit replacement.
Basing on mentioned reasons Energo-Complex company, cooperating with specialists from West Pomeranian University of Technology and Opole University of Technology (Poland), have introduced complex system of transformers management called TrafoGrade. It is based on the point scale for assessment of transformer's technical condition and its importance in power system and combines technical aspects of diagnostics with economical and financial issues to plan ongoing operation, repairs management and investment policy.
In this brochure, there is given basic information on TrafoGrade system, as well as exemplary results of 44 transformers assessment operated in one of Polish distribution companies. There is also presented economical analysis of system introduction referred to the costs of transformer's failure.
TrafoGrade SYSTEM CHARAKTERISTIC
Transformer management system consists of four components shown on Fig. 1.
In TrafoGrade system there are points given for each property of parameter used both for technical condition assessment and transformer's importance in power system. There is three point scale introduced: good, fair, poor. The simple point-based form of technical condition assessment allows to conduct analysis of measurements results not only by engineering staff but also by economical services usually not having sufficient technical knowledge.
ASSESSMENT OF TRANSFORMER'S TECHNICAL CONDITION
The main part of TrafoGrade system is thorough assessment of transformer's technical condition performed by modern diagnostic methods. The assessment of technical condition is based on 14 diagnostic parameters:
dissolved gases analysis (DGA)
windings deformations analysis (FRA)
history of operation
oil dielectric strength
visual examination
assessment of on-load tap changer (OLTC)
assessment of cooling system
assessment of transformer's accessories
insulation resistance
windings resistance
assessment of cellulose degradation level
age of transformer
moisture contents in solid insulation
ageing level of oil
All diagnostic parameters are divided into three groups:
Group I - basic diagnostics of transformer
Group II - technical condition of the active part
Group III - development of ageing phenomena
Group I contains transformer's basic properties. Even their poor results have no significant influence on transformer's ability to be operated and powered. Only extreme cases of very poor technical condition of these properties may lead to switching off the unit. It has been assumed that repairs in this group are technically easy and cheap. For instance this group contains OLTC accessories assessment.
Group II contains elements having significant impact on transformer's functionality. Bad technical condition of any of these elements eliminates the unit from operation and restoring it to proper condition needs considerable financial and organizational efforts. This group contains e.g. DGA analysis, as its negative result leads to switching off the transformer and at least need of internal inspection.
In Group III there are properties which are not directly connected with current operation of transformer, but take into consideration expected "lifetime" of the transformer and planning of repairs and modernization. Therefore even negative assessment of any property from this group does not eliminate the unit from operation, however it needs proper repairs to be planned. In this group there are such parameters as age of the unit or physicochemical properties of transformer oil.
For the requirement of TrafoGrade system there has been prepared detailed instruction, which contains definitions and values of all parameters and also methodology of preparing the group and final grading scale.
ASSESSMENT OF TRANSFORMER'S IMPORTANCE IN A POWER SYSTEM
There are two important factors influencing the decision-making process of investment and repair policy: technical condition of transformer and its importance in the power system. This issue has significant meaning for improving reliability of electric energy distribution and energetic safety of the given region. TrafoGrade system has importance of a unit also given in the point scale and introduced by the analysis of:
configuration of power network in which transformer is working, with detailed analysis of reliability parameters
the type of powered consumers
backup possibilities of transformers etc.
The combination of two issues, very important for the distribution company activity, allows to match the transformer to proper operation group. Fig. 3 shows the assessment results of 44 transformers performed in one of distribution companies.
In the TrafoGrade system transformers are divided into three basic operational groups (Fig. 3):
transformers with simplified operation (U)
transformers with normal operation (N)
transformers with special care operation (S)
The first group (U) contains units having a very good technical condition or with insignificant meaning for the power system. In these cases costs for current diagnostics can be limited just to periodical visual inspections and DGA analysis of insulating oil. On the other hand, in the group (S) there are included all units with strategic meaning to the system or transformers with poor technical condition. Of course each of operation group needs separate instruction to be prepared, but such system, based on real technical condition and importance of the transformer leads to effective reduction of operation costs of electric energy distributor.
OPERATION OUTLOOK ANALYSIS
The final result of transformer's complex assessment of its technical condition is preparing its operation outlook, individually for each unit. This information is especially helpful for staff managing transformer's operation, as it allows preparing thorough plan of diagnostics and repairs for next years. The Table 1 presents two examples of operation outlooks prepared for transformers having different technical condition.
Examples of operation outlooks for transformers operation:
TDR3b 25000/110, manufactured year 1980, 10,4 pts.
Technical condition:
Many leaks
OLTC: loud drive movement
DGA: low temperature overheating
Moisture X = 3,3%
Furane: 3,24 ppm
Phenol: 1,94 ppm
Physicochemical properties of oil: fair
Operation outlook:
The transformer has high level of ageing processes development in oil and solid insulation. High moisture contents and existing thermal defect accelerate insulation deterioration.
Due to decreasing the short-circuit strength and possible appearance of bubble effect further operation is unreliable.
Limitations of operation:
oil temperature max 75oC
do not switch off to cold reserve in low temperature
Repairs need large expenditures, e.g. rewinding and oil refilling.
TDR3b 16000/110, manufactured year 1985, 27,1 pts.
Stan techniczny:
OLTC: fair
DGA: early stage of PD (low hydrogen contents)
Moisture X = 2,8 %
Furane: none
Physicochemical properties of oil: good
Perspektywa eksploatacji:
Brak ograniczeń w eksploatacji.
W perspektywie 5-7 lat prawdopodobna potrzeba suszenia izolacji.
COMPUTER MANAGEMENT SYSTEM
Ongoing work on TrafoGrade system development is leading to elaborate the computer system of transformer's management. All data on technical condition, importance in the system and operation outlook analysis of given population of transformers will be gathered in the computer managing system. Its main aims are:
support of decision making process of repair-investment policy
creation of reliable budged prognosis based on the real condition of transformers
ergonomic data presentation in graphic form
easy access to archive results of measurements
TrafoGrade system computer managing tool will be a program fit to the individual needs of users adjusted to specific character of distribution company.
EXAMPLE OF TrafoGrade SYSTEM INTRODUCTION
TrafoGrade system has been introduced in one of distribution companies and performed complex assessment of 44 distribution transformers. Fig. 4 presents the pie-chart showing the age of tested transformers population.
Taking into consideration statistical data on Polish distribution companies it can be noticed that due to the period of operation this population is representative for all distribution transformers. Therefore it is expected that technical condition characteristic and operation outlook obtained with TrafoGrade method, as well as main technical problems concern also other companies distributing electric energy.
Among 44 tested transformers four units had poor technical condition, 33 - fair condition and seven had good condition. Thorough assessment of technical condition is based on modern high technology diagnostic methods. Fig. 5 shows generalized for the whole population exemplary results of DGA, moisture contents in solid insulation measurement and assessment of mechanical deformations with FRA method.
On the basis of results given on Fig. 5 it can be stated that in tested population only few percent of transformers have serious technical problems which may involve significant assets. The Fig. 6 presents operation outlook of analyzed population of 44 transformers.
The operation outlook determined on the base of technical condition indicators is the starting point for economical analysis.
ECONOMIC ISSUES OF TrafoGrade SYSTEM INTRODUCTION
The classical approach to measurement results working up needs performing by unit's owner the analysis of data gathered usually in several reports and drawing proper conclusions on this basis. Integration of all measurement data in a computer system and introducing point based scale for technical condition assessment makes the analysis of results easier and allows to make a decision for managing staff, not having detailed knowledge on diagnostics of transformers. Expenditures spent on repairs and modernization can be optimized due to introduction of complex managing system based on the multi-parameter assessment of transformer's technical condition. Such an approach to transformers population managing allows to easily catch the proper time, when technical condition of given unit can be significantly improved with relatively low costs involved. An example here is the transformer with high moisture contents in the solid insulation, poor parameters of oil, but having good mechanical condition of windings and solid insulation. For such unit following repairs to are to be performed:
drying and cleaning of windings
pressing the core and windings
oil refilling
radiators replacement
renovation of on-load tap changer
accessories replacement
The cost of presented repairs is estimated to be approx. 15-20% of a new unit value. Such repairs would allow to operate the unit for another 10-15 years. In the case of poor technical condition of transformer there must be decision taken on repairs or replacement. This decision should be based on following factors:
no-load losses analysis
analysis of demand for electric energy in the point of transformer's operation
change of transformers connection setup
The estimated cost of a new 25 MVA transformer is 1,4 - 2,0 million PLN, whereas performing the complex repairs costs 650 000 - 850 000 PLN. Additional costs related to disassembly and transport are approx. 170 000 PLN.
The main aim of performing the complex diagnostics of transformer is avoiding its breakdown, especially catastrophic. Such catastrophic breakdown covers usually the whole HV field and sometimes additionally MV field leading to considerable increase of damages. As an example the breakdown in one of distribution companies can be given, where fire of 25 MVA transformer destroyed MV and HV fields. Total cost of breakdown consequences elimination reached 6.5 million PLN and was 10 times higher than cost of complex repairs of such unit.
Taking into consideration typical distribution company, estimated cost of TrafoGrade system introduction returns when due to prevention just one transformer breakdown can be avoided.
Application of point based scale and adjusting transformers operation instructions to TrafoGrade method standards, makes it possible to reduce costs related to periodic diagnostics. The individual approach to given unit allows to limit the range of tests for transformers from U operation group and also rationalization of costs of necessary tests in group N and S.
SUMARY
The approach to management of transformers described in this paper allows to conduct rational repair management, which increase quality and reliability of operated units and significantly decrease the number of breakdowns, especially catastrophic ones. The assessment of transformers importance in a power system allows to take actions leading to increase power network reliability. In this assessment the most important issue is importance of energy receivers powered from analyzed transformer. This data together with technical condition assessment give estimation of a real breakdown consequences, including costs of undelivered energy. For distribution companies prevention of power delivery breaks is very important, not only because of economical losses, but also for improving company's brand reception. The complex assessment of technical condition gives many important information useful in logistic actions and financial policy of companies operating transformers of medium and high powers.
+48 32 7756 700
+48 32 7756 702
+48 601 260 808
