21, rue d’Artois, F-75008 PARIS http : //www.cigre.org
A2-102
CIGRE 2006
ON-LOAD TAP CHANGER RELIABILITY AND MAINTENANCE STRATEGY M. FOATA*, C. RAJOTTE and A. JOLICOEUR
HYDRO-QUÉBEC CANADA
SUMMARY Hydro-Québec operates over 1,200 transformer units with On-Load Tap Changers (OLTC), covering a wide variety of makes, models, ratings and technologies. Transformers with OLTCs are known to have a lower reliability than transformers without. Statistics are provided to demonstrate the importance of OLTCs in transformer reliability. Considering that around two-thirds of Hydro-Québec transformers equipped with OLTCs are 25 years old or more, it becomes clear that OLTC performance is a major concern and that maintenance strategies have to be developed to deal with this reliability issue. Regarding preventive maintenance, the vibro-acoustic diagnostic technique will be the cornerstone of Hydro-Québec’s strategy for the coming years as it offers the widest detection spectrum and can detect most degradation at the earliest stages. As for the corrective maintenance strategy, the approach is to deal with OLTCs by family. Hence, in the first part of the analysis, a lower reliability OLTC family must be identified for further examination. A number of technical aspects must then be assessed for that particular family in a so-called OLTC Reliability Analysis process. Are the ageing mechanisms identified and quantifiable? Are the weaknesses known? Are they predictable? At what cost can they be fixed? Can the original design be improved? What are the retrofit possibilities? At what cost? The information above is then integrated into a second process called OLTC Life Decision Making, in which a techno-economic model takes into consideration book value, remaining life, life extension, overhaul costs, losses and future load requirements. The common difficulty here is to express all these quantities in terms of money units in order to compare different scenarios. This corrective maintenance strategy has already been applied successfully to a number of families selected according to their current incidence on transformer reliability; a complete example is illustrated in the present paper.
KEYWORDS Tap Changer – OLTC – Reliability – Maintenance – Diagnostic – Vibro-Acoustic – Transformer
___________________________________________________________________________ *
[email protected]
1. INTRODUCTION In operating transmission systems, major electric utilities must take into account their large inventory of power equipment as well as the complexities of maintaining high-voltage apparatus. In the present context, where transformers are increasingly solicited and continuously ageing, strategies must be put forward to optimize maintenance efforts with a view to reliability. This paper describes HydroQuébec’s particular situation for which both preventive and corrective maintenance strategies have been developed to specifically tackle the issue of On-Load Tap Changer (OLTC) reliability.
2. OLTC RELIABILITY – BACKGROUND 2.1 Hydro-Québec’s Context Hydro-Québec operates more than 2,300 high-voltage transformers ranging from 49 kV to 735 kV. More than half of these transformers are equipped with On-Load Tap Changers (OLTCs); the majority of these OLTC (more than 50 models) can be grouped into 15 families from six main manufacturers. Almost all OLTC technologies are represented: selector or diverter type, in-tank or bolt-on, resistive or reactive, oil or vacuum technology.
2.2 Reliability and Maintenance Figures A transformer with an OLTC requires more maintenance, of course, because of this additional mechanical component. Hydro-Québec’s preventive maintenance standards specify a man-hour effort that is generally 3 to 5 times higher for a transformer with an OLTC compared with an identical transformer without an OLTC. Moreover, a transformer with an OLTC will have a lower average reliability than a transformer without an OLTC. For corrective maintenance, similar ratios may be observed. The well-known international survey on large power transformers in service published in Electra in 1983 [1] revealed that for substation transformers equipped with an OLTC, about 41% of all failures were due to the OLTC. Similar proportions have also been observed at Hydro-Québec since then. Most recent data reveals that between 1998 and 2004, around 50% of the transformers (with OLTCs) that had had major failures requiring replacement or transportation to a repair shop were caused by OLTC failures. This high proportion of OLTC failure is without a doubt a concern, but it can be partly explained by a relatively low number of failures of other components. Indeed, Hydro-Québec’s transformer major failure rate, all voltage levels included, remains below 0.5%. However, considering that around twothirds of Hydro-Québec transformers equipped with OLTCs are 25 years old or more, it becomes clear that OLTC reliability is a major concern. Typical OLTC defects are from three different sources: mechanical, electrical or dielectric [2]. Tap changer problems are often initiated by mechanical problems involving components such as springs, bearings, shafts and drive mechanisms. These mechanical problems may cause only minor inconvenience to transformer operation, but could degenerate to a problem of electrical or dielectric nature and damage not only the OLTC itself, but also the regulation winding. Another important fact about OLTC reliability: almost 25% of Hydro-Québec transformers with OLTC have an acetylene concentration of more than 10 PPM in the main tank. The great majority of these transformers do not have any problems, and these gases come from a leaking OLTC compartment, or from an oil conservator design problem (oil or gas migration from OLTC to the main tank). This situation has to be taken into account in DGA interpretation for the transformer main tank [3], and in the worst case, a latent fault in the main tank could go undetected for some time.
1
The OLTC reliability issue is being tackled by two complementary strategies: a revision of the preventive maintenance program, and specific corrective maintenance programs applied to lower reliability OLTC types.
3. PREVENTIVE MAINTENANCE STRATEGY 3.1 Periodic Internal Inspection Periodic internal inspection is specified in terms of both time and the number of operations. Based on the manufacturer’s recommendations and Hydro-Québec’s experience, this task interval is 3 to 6 years, or 40,000 to 100,000 operations, depending on OLCT technology and type. Internal inspection requires draining the oil from the OLTC compartment in order to examine the critical parts. Basically, it consists of a visual inspection of the moving parts, a contact erosion measurement, a functional test, and lubrication. For bolt-on OLTCs, where all components are in the same tank, selector and inverter switches can also be easily inspected. To improve OLTC reliability at Hydro-Québec, it was decided not only to maintain the internal inspection, but even to improve it. This improvement will not necessarily take the form of a task interval reduction but rather the use of good maintenance procedures conducted by well-trained workers. Intensive efforts have thus been invested to review and improve maintenance procedures, to ensure easy access to these procedures, and to enhance the training program based on these improved procedures.
3.2 Off-Line Non-intrusive Electrical Tests Non-intrusive electrical tests performed on each OLTC tap, such as magnetization current and winding resistance, can be used for OLTC diagnostic. A magnetization current test on each tap is not easy to interpret because its pattern is design dependent [4], and because measurements are affected by residual magnetism that may be present in the transformer core. Hydro-Québec is using this test only at transformer commissioning. 1
1,1
Phase A
1
Phase B
Resistance (ohm)
Resistance (ohm)
1,2
Phase C
0,9 0,8 0,7
Phase A
0,9
Phase B Phase C
0,8 0,7 0,6
0,6 1
3
5
7 9 11 13 OLTC Tap position
a) Inverter problem
15
17
1
3
5
7 9 11 13 OLTC Tap position
15
17
b) Selector problem
Figure 1: Examples of winding resistance measurements
Figure 1 shows a typical winding resistance variation with OLTC tap position on a plus-minus OLTC that normally exhibits a "V" pattern. This measurement may be useful to detect poor contacts on inverter switches (Figure 1a) or on selector switches (Figure 1b). Phase comparisons may also reveal
2
other abnormalities such as bad contacts on the diverter switches. Nevertheless, this test has a limited sensitivity, especially for an OLTC located on the HV side of the transformer, where the relatively high winding resistance may mask bad contacts. In addition to detecting contact problems, the winding resistance test verifies that electrical continuity is maintained during all tap movements. At HydroQuébec, this test is performed during the same outage as the internal inspection described above. It is fair to say that in general, this non-intrusive electrical test is effective as a routine test, but it can detect only a limited number of OLTC failure modes.
3.3 OLTC DGA Analysis Like the winding resistance test, DGA aims to detect bad contacts (coking, overheating) inside the OLTC compartment. Theoretically, typical gases produced by a bad contact are different from those produced during normal OLTC switching operation, but practical interpretation is rarely simple. Nevertheless, even though this technique would permit to monitor some specific types of OLTCs with a particular problem, it is not currently part of the Hydro-Québec routine maintenance program.
3.4 New On-Line Vibro-Acoustic Diagnostic The vibro-acoustic diagnostic for OLTCs is a relatively new technology. Early development started about 15 years ago [5,[6], and the method is currently being implemented at Hydro-Québec [7]. This technology was developed to address the following needs: ability to detect a wide range of OLTC failure modes (mechanical problems as well as electrical malfunctions), capability to be used on-line to verify an OLTC’s condition between internal inspections, simplicity of testing, low cost, and compatibility with all OLTC technologies, old or new. An acoustic sensor, actually an accelerometer, is simply applied to the OLTC tank wall so that it picks up waves transmitted from the internal parts in the same way that a stethoscope would. A current sensor is also installed on the OLTC motor supply. Figure 2 shows a typical signature of a motor current and of an acoustic signal that can be interpreted and associated with different stages of OLTC operation in the tap changing sequence. Diverter switch
Startup
Selector switch
Acoustic High Frequencies Acoustique Basses-Fréquences Acoustic Low Frequencies
Contactor
Change-over switch
Courant Moteur Motor Current Acoustique Hautes-Fréquences
30.0 25.0
Braking
20.0
Diverter switch
15.0 10.0
Post-operation
5.0 0.0 0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
11.00
Temps Time (s) (s)
Figure 2: Decomposition of an OLTC signature during a change-over operation With the capabilities of this new instrument, acoustic measurements are performed on-line yearly. During this measurement, the OLTC is operated six times around its operating position (up and down 1 tap). In addition, the measurements are performed before an internal inspection, which may help to
3
focus on suspected problems. The measurement is repeated after the internal inspection to confirm the OLTC’s condition before the transformer is returned to service. Results from partial implementation have confirmed the potential of this new method. This diagnostic will therefore be introduced into the standard preventive maintenance program in the near future.
4. CORRECTIVE MAINTENANCE STATEGY The OLTC corrective maintenance strategy implemented at Hydro-Québec consists in applying two complementary processes: 1. The OLTC Reliability Analysis Process, which is applied to families of OLTCs (Figure 3) 2. The OLTC Life Decision Making Process, which is applied to individual units (Figure 4)
4.1 OLTC Reliability Analysis Process This process is an extensive investigation of the performance of a type of OLTC; it has been conducted at Hydro-Québec on a number of cases and has matured to the stages described below.
Failure data & Maintenance costs statistics
Continuous Reliability Monitoring
Lowest reliability OLTC type Failure Investigation Reports
OLTC Failure Modes Analysis
Revised maintenance
OLTC weaknesses OLTC Experts Manufacturer New Suppliers
OLTC Corrective Solutions Analysis
Minor work solutions
Major work solutions Solutions Cost Analysis
Major Work Solutions for OLTC Family
Figure 3: OLTC Reliability Analysis Process
4
Continuous Reliability Monitoring As part of Hydro-Québec’s standard practices, OLTC reliability is regularly monitored mainly through the maintenance database. Usual indicators include maintenance costs, outage time and failure rate. At this stage, data is compiled according to families of OLTCs. The information is then tentatively correlated with age, number of operations and field experience, to identify any anomaly. Continuous reliability monitoring would, under normal circumstances, occasionally identify an emerging OLTC reliability issue that would, in turn, trigger an in-depth analysis. However, the result of the first application was actually a backlog of OLTC types, which were then ranked by severity.
OLTC Failure Modes Analysis At this stage of the analysis, failure data is examined from the causal relationship perspective. While the immediate cause of the problem may seem obvious (broken parts, for example), the basic reasons are often underlying and must be determined in order to solve the problem rather than only fixing the consequences. Therefore, the primary output of this failure modes analysis is a complete portrait of OLTC family weaknesses; in other words, a ranking of all types of failures by family. Another important result is a review of the maintenance procedures for this family.
OLTC Corrective Solutions Analysis Starting from the main results of the failure modes analysis, more specifically the weak points identified, corrective solutions are sought. This stage takes the form of workshops for OLTC experts from all fields. Maintenance specialists (workers, technicians and engineers) from throughout the company are invited to exchange their ideas and experience. Manufacturers are also involved as early as possible, and their participation may take on a preponderant role as the analysis progresses. However, this expertise is not always available as some manufacturers have been out of the market for a number of years. In this event, new suppliers may be called upon. More specifically, the following questions are addressed: • • • • •
Is the OLTC technology under examination obsolete? What solutions have been tested in the field? Are they palliative or do they solve the root problem? What modifications should be required from the manufacturer? What are the possibilities of retrofitting with a new OLTC technology?
The corrective solutions analysis produces two sets of technical solutions depending on the amount of work required. The minor work solutions include simple design enhancements and palliative recommendations, while Overhaul, Replace and Retrofit are considered major work and will be examined further from a cost viewpoint.
Solutions Cost Analysis In this last stage of the process, all the technical solutions are evaluated from an economic point of view. One difficulty here is to evaluate field complications that may arise from the implementation of new solutions. In some cases, it may be decided to carry out a pilot experiment. The output is a set of major work corrective solutions that is agreed on with all participants and standardized in the
5
corrective maintenance program. These solutions are then available when individual OLTC units are examined under the complementary process: OLTC Life Decision Making.
4.2 OLTC Life Decision Making Process Contrary to the previous process, which is applied to a complete OLTC family, the OLTC Life Decision Making Process (Figure 4: OLTC Life Decision Making Process) is applied to units individually. The most important benefit from this process is that the entire transformer must be analyzed before any major work is performed on the OLTC. In many instances, a complete cost analysis will reveal that it is not economically sound to invest in the OLTC of a transformer that either has a short extendable life, or does not fit in with future plans for the network.
Individual OLTC Data : - Failure - # Operations - Age - Reliability - Preventive Maintenance
OLTC Unit Condition Appraisal
Major Work Solutions for OLTC Family
Major Work on OLTC required?
No
Minor OLTC repair Return to service
Yes
Transformer overall condition: - Insulation - Components - Etc. Network Planning
Xfo Life Extension Assessment
No
Replace/Discard Transformer
Yes
Apply Decision to OLTC Unit
Replace
Retrofit
Overhaul
Figure 4: OLTC Life Decision Making Process
OLTC Condition Appraisal The process can be triggered by a number of different events: failure, ageing, number of operations, tagging of the OLTC type with a low reliability, or advanced degradation revealed by a preventive maintenance inspection. At the first stage of this process, there is a necessary OLTC Condition Appraisal, which will usually consist of an internal examination with precise guidelines on anomalies and degradation signs to look for.
6
Decision: Major Work Required on OLTC? This decision has always been troublesome to substation managers. However, once a Reliability Analysis Process has been applied to a given OLTC family, a decision regarding an individual OLTC will be based not only on the condition of the unit in question, but also on due consideration of all the problems identified for that family. The listing of major and minor work solutions considerably facilitates and improves decision making.
Transformer Life Extension Assessment Considering that most OLTCs that need major work are installed on old transformers (more than 30 years old), the cost of repair/overhaul can be quite significant in relation to the remaining book value of the whole transformer. Therefore, a OLTC life decision should always trigger a complete assessment of the transformer. At this stage, it becomes necessary to extend the analysis to all components (windings, bushings, accessories etc.) and to produce a number of estimates, namely: • • • • •
Expected life extension Total cost of life extension Cost of losses Maintenance costs Other possible costs such as transportation or environmental.
All the above information is fed into a so-called techno-economic model so that all the different life extension scenarios, as well as a replacement scenario, can be compared on a common basis that is Net Present Value. This assessment can also be influenced by network planning considerations such as the load growth or any possible voltage changes.
Apply Decision to OLTC Unit At this final stage, the decision on the OLTC (Replace, Retrofit, or Overhaul) should be straightforward, but there may be exceptional circumstances for which it is not possible to estimate all the costs (such as emergencies), and those cases could require a judgment call by substation managers.
5. CASE EXAMPLE The reliability strategy described above will be illustrated with the example of a bolt-on, selector switch, resistor type family of OLTCs that was targeted following a marked decline in performance after about 30 years of duty. Priority was also given to this family due to the high number of units (close to 200) that Hydro-Québec operates. Failure modes analysis resulted in a review of the maintenance procedures, as summarized in Table I below. It can be seen that preventive maintenance actions can address a wide range of problems. However, the corrective solutions analysis also revealed a number of failure modes that required more extensive work. Minor work solutions included modification of the mechanical couplings of the drive mechanism and installation of a new mobile contact design, both solutions as per the manufacturer’s recommendations.
7
Table I : Summary of preventive maintenance review from failure modes analysis Problems Erroneous DGA interpretation due to contaminated oil leaks Wrong tap indication
Wrong or incomplete OLTC operation
Weakness Loss of clamping pressure on sealing gasket Oxidation of relay contact surfaces
Revision Retighten clamping bolts with specified torque Emphasize cleanup of relay contacts
Drive strap failure
Emphasize verification of drive strap condition
Insufficient braking
Remove built-up grease on brake flywheel Check for brake pad wear
Excessive wear or abnormal effort on the drive mechanism
Emphasize lubrication of critical parts
Mobile contact mechanism misadjusted
Issue more precise procedure for contact replacement Caution when handling fragile transition resistors
Dielectric failure or arcing
Overheating
Transition resistor failure
The two overhaul options examined necessitated what is considered major work: 1- changing the sealing gaskets between the OLTC and the main tank, and 2- changing the epoxy blocks supporting the stationary contacts. Following the process, these major work options had to be compared with replacing and retrofitting alternatives on a total cost basis. The retrofit solution was offered by the original manufacturer starting from a recent design modified to fit on the older tap lead connection panels. This retrofit was submitted to a field pilot installation where some minor retrofit problems were discovered but quickly solved, so that overall, the operation was considered a success. Since this specific OLTC family was no longer supported by the original manufacturer, the replacement solutions were developed with either a new parts supplier or using recycled parts. At the end of the cost estimate, the following general guidelines were decided: • • • •
The OLTC family must be declared obsolete When major work is required, the retrofit solution with the newer generation must be applied The replacement solution with new parts manufactured by a new supplier must be eliminated For emergency purposes only, a couple of old units are to be reconditioned and put into storage
The final decision for each individual unit was left to the techno-economical evaluation of the OLTC Life Decision Making process.
8
6. CONCLUSIONS With respect to the present situation, it is observed that: • • •
OLTC performance has a significant impact on transformer reliability. In the context of an ageing inventory, it is expected that OLTC reliability may take on increasing importance, and that utilities will have to keep adapting their preventive maintenance strategies. Utilities are in need of new on-line diagnostic tools or methods.
On the new reliability process, it is concluded that: • • • • •
It is important to identify low-reliability OLTC families at the very first stage. Corrective maintenance strategies must be then personalized for each family of OLTCs to take their particularities into account. The OLTC reliability process by family becomes very helpful to substation managers when decisions on individual units have to be made. High cost decisions on an OLTC unit should always trigger a question on the transformer’s life. Know-how and skilled personnel are keys to success. Accordingly, maintenance procedures should be updated regularly.
On future work, it is recommended to: • •
Finalize the on-going implementation of the vibro-acoustic diagnostic technique. Exchange experiences on OLTC reliability and diagnostic methods with other utilities.
7. BIBLIOGRAPHY [1] [2] [3] [4] [5] [6] [7]
CIGRÉ WG 12.05, “An International Survey on Failures in Large Transformers in Service”. Electra 88, 1983, page 21. CIGRÉ, “Guide for Life Management Techniques for Power Transformers”. Brochure WG A2.18, 2003. CIGRÉ, “Recent Developments in DGA Interpretation”. Brochure JTF D1-01/A2-11, 2004 M. F. Lachman, “The Influence of Transformer Load Tap Changers on Single-Phase ExcitingCurrent Test Results”. Doble User’s Conference, Boston, 1992. EPRI, “Study of Improved Load-Tap-Changing for Transformers and Voltage Regulators”. Report EL-6764, April 1990. T. Bengtsson, M. Foata, et al., “Acoustic Diagnosis of Tap Changers”. CIGRÉ 1996, paper 12101. M. Foata, B. Girard, C. Landry, A. Mow and C. Rajotte, “Field Experience with the Implementation of a New On-Line Vibro-Acoustic Diagnostic for On-Load Tap Changers”. Doble User’s Conference, Boston, 2005.
9
