Large power transformers and shunt reactors are largest and costliest equipment in the transmission network. Transformers being a long delivery item, reliable performance of this equipment are very important to maintain the availability of power system.
The specification forms the basis on which the manufacturer designs, manufactures and tests a transformer. With the growth in size of power system and complexities of modern system, the adequacy of transformer specification plays a major role on its reliability. A good specification must properly consider the effect of application, system operating conditions, environmental considerations, site details, service conditions, termination details etc so that the transformer is designed for the conditions under which it has to operate. Thus, the adequacy of specification of transformers, particularly, that of high voltage and larger rating assumes tremendous importance and an inadequate specification can lead to a deficient design and poor reliability.
The various parts of transformers viz magnetic circuit, insulation system, termination, cooling equipment etc are designed as per technical specification details. A technical specification has three objectives:
I. To provide the tendered, or manufacturer, with all that technical information necessary to carry out his design and which will vary for each design, for example rating, voltage ratio, type of cooling, losses, applicable tolerances on performance parameters, applicable standards, tests requirements etc.
II. To provide the tendered, or manufacturer, with an indication of the strategic importance of the transformer and the value to be placed on reliability, maintainability and long service life.
III. To provide the tendered, or manufacturer with information which will ensure that the transformer will satisfactorily interface with its associated plant and equipment and that installation and commissioning will proceed smoothly and without undue delays.
Clearly, the first two objectives will have a significant bearing on the cost of the transformer and must be met by the enquiry document to enable the manufacturer to prepare his tender. The third will include many items which will have relatively little bearing on the overall cost and which could possibly be resolved during the engineering of the contract. However, it is good discipline to identify in the technical specification all those aspects which should be known at the time of initially depicting this up, since not only does this minimize the use of engineers’ time during the contract stage and ensure that there are no unnecessary delays during the contract, but it also avoids the risk that these items might be overlooked during the detail engineering of the contract.
The specification should be a crystal clear information to designer without leaving any chance of ambiguity, particularly, each parameters defined in clear terms of unit in the technical specification with or without any tolerance shown in Table 1.
Design reviews have since long been a formal activity accompanying critical orders, mainly because of their impact over the cost of a project and, or expected operation costs. A design review is a planned exercise to ensure that both parties understand the application and the requirements of the technical specifications and applicable standards. Hence, its purpose is to create a common understanding of purchaser’s specifications and requirements, and it also gives purchasers an opportunity to check in detail the design and requisition fulfillment process of the manufacturer.
Design review is the opportunity for both purchaser and manufacturer to scrutinize the proposed design, to ensure that the supplies will be met not only technical requirements but also those relating to other aspects of the contract, like quality, testing, inspections, site installations etc. Therefore, the design review is a good opportunity to interchange experiences that can be used to propose enhancements or betterments. For these reasons, it is strongly advisable for the purchaser to have expert transformer engineer(s) with them during the meeting.
The design review cannot be taken as a second stage of the specification of the transformer, because of the potential costs involved with the modification of design, but is an excellent opportunity to correct minor deviations, when the design does not match all desired purchaser’s needs. Transformer manufacturers always benefit from interaction with purchasers, as they learn from first hand of their changing needs to improve their operation. The purchaser benefits from a closer understanding of transformer technology, as they can improve their specifications and take better operations decisions.
The process of the design review involves formally one or more sessions between the purchaser (purchaser, applications engineer and consultants) and the manufacturer (contract manager and designers) with the purpose of making sure that the product is designed according to the purchaser’s needs. As power transformers are very costly equipment, and field failures are undesirable. The followings are the pre-requisites for carrying out the design review:
i. Design review should be part of the tender inquiry and it becomes obligatory on the part of the customer and manufacturer to ensure compliances with the contract specifications. ii. It is to be ensured that all stakeholders are involved in the design review.
iii. Review may include certain information which is of proprietary nature. It is, therefore, desirable to have mutual agreement between the purchaser and the manufacturer for the confidentiality of information.
iv. It is important for the success of design review that both the purchaser and manufacturer are clear on the requirements and well prepared to have open and frank sharing of information.
The main objective of design review is to verify that the product is designed according to the purchaser’s needs. Following are the main objectives of the design review:
I. To ensure that there is a clear and mutual understanding of the transformer technical requirements according to purchaser specification & applicable industry standards.
II. To understand the application and verify the system and project requirements and to indicate areas where special attention may be required.
III. To verify that the design complies with the technical requirements.
IV. To identify any prototype features and evaluate their reliability and risks.
V. To interchange experiences that can be used to identify eventual betterments in the design and / or improvements and changes in the specification.
VI. To strengthen the technical relationship between purchaser and manufacturer and to understand the technical capabilities of the manufacturer.
C. CIGRE Guide for Design Review
This document is a general purpose guide for purchasers that want to review the design of their transformers. The scope of the guide is 100 MVA, 123 kV and above, but it may well be applicable to transformers of a much smaller rating. The review is understood as an activity that must be planned well ahead within the contract acknowledgement process with the purpose to confirm a common understanding of purchaser’s specifications and requirements, and that gives purchasers an opportunity to check in detail the design and requisition fulfillment process of the manufacturer. The guide does not include limits for relevant design parameters, leaving the responsibility for an appropriate selection to the manufacturer. Purchasers are advised that they should be qualified with the technical expertise necessary to understand and evaluate the design.
One particular purpose of the design review is to identify any new developments or solutions that might be applied as a prototype and assess risks and reliability. Experience shows that many purchasers would prefer to use mature solutions, replicated invariant for decades. As purchasers and consultants will have access to manufacturer’s proprietary information, sometimes considered strategic and classified (e.g, new developments, competitive edge projects, design policies), good control of documents and a clear definition of responsibilities are essential matters.
Design reviews can be very productive exercises for purchaser and manufacturer as they foster a better understanding of purchaser’s needs, and have the main objective of verification that the product is designed according to the purchaser’s needs. The design review cannot be construed as a second stage of the specification of the transformer, because of the potential costs involved with the modification of design, but is an excellent opportunity to correct minor deviations, when the design does not match all desired purchaser’s needs.
The purchaser and his consultant must be aware of the limited access to copies of manufacturer’s proprietary information, as he must protect his competitive advantages. Sometimes, the perfect match of a transformer to the purchasers needs implies solution of problems that go beyond the state-of-the-art of design technology. Those would have to be assessed regarding their impact on the contract as development projects might bring uncertainty to the process.
As long as a contract is valid, the manufacturer remains accountable for the performance of the final product. Adequacy to the initial specification is fundamental unless further commitments are agreed between both parties. Also, design review does not absolve or substitute manufacturer’s ultimate responsibility for the adequacy of transformer design and construction, including design limits and margins, quality, performance on test and in service.
Life Management Concepts
Transformer Life Management concepts help customers on secure high reliability and gain in-depth knowledge of the condition of the transformer. Life management concepts of transformer is a very vast subject, which deals with many activities like environmental conditions maintained during manufacturing, storage, installation and commissioning. Also condition assessment by on-line monitoring and diagnostic testing to assess the condition of oil and insulation material used in the transformer. On the topic, we have covered the effect of environmental factors and insulation ageing factors on the life of the transformer. Also we have presented some related case studies for better understanding of subject.
A. Environmental Factors
First and foremost requirement of any product to sustain for a long life particularly, electrical equipment like transformers is starting from its initial processes of manufacturing to installation and commissioning at site. Care shall be taken at each step for environmental conditions i.e. dust, moisture, metal particles etc, which need to be individually dealt critically as these are the main constituents and putting direct consequences on equipment life. Effect of these factors is explained below:
a. Dust: The transformer of EHV class must be manufactured in dust proof chamber so that dust cannot enter into the job and reduce its service life, the presence of dust particles in the job gives higher partial discharge and one should not negotiate / avoid / overlook such for good health of transformer. People don’t allow manufacturing in such environment even for 132 KV Class of transformer and insisting for all voltage class.
b. Moisture: The content of moisture even in one part of 1000 is so dangerous that it can lead the transformer to fail in few months. Hence, IS & IEC is very particular to agree certain PPM for each voltage class and in general my views not to allow any job with above 5 PPM for at least EHV class of transformer. Moisture in combination with temperature accelerates the ageing of cellulosic insulation material used in transformers, which in turn reduces the life of transformer. Impact of temperature and moisture on the lifetime of transformer is explained in detail as under:
Let us consider the critical temperature which a manufacturer should take into consideration when planning the cooling system of a transformer. In the previous century, a general estimate placed the temperature limit, below which ageing of paper insulation does not occur, at 80°C. Montsinger established in 1930 that in the temperature area between 90°C and 110°C, the ageing of normal paper insulation is doubled with each increase of 8°C. Some other researchers found that the doubling of velocity occurs when temperature increases between 5°C and 10°C. Consequently, today the generally adopted estimate states that an increase in temperature of 6°C doubles the velocity of ageing. Based on experience and findings, the family of standards IEC 60076-x and the standard IEC 60354, which was recently renamed to IEC 60076-7, give guidance to the maximum allowed temperatures of oil and windings so that in given climatic conditions and with the full load of a transformer, a transformer ought to reach a normal life-time of at least 30 years. But are such expectations firmly grounded? If we analyze the results of the latest research, the question is more than justified.
Results of laboratory analyses of the 5-year long project of paper in oil of artificial ageing brought new knowledge about the influence of temperature and other chemical factors on the velocity of ageing of transformer insulation in a wider temperature interval. It dismiss the old estimate that insulation does not age at temperatures below 80°C, which is evident from the plot of results in Fig 1.
Heat formed in a transformer because of losses, causes a thermal oxidative hydrolytic degradation of the paper-oil insulation system. The ageing of paper insulation in transformer oil is an autocatalytic (self-accelerating) process, whose velocity is largely determined by the temperature and the percentage of humidity in the insulation.
Figure 1 shows results of the laboratory analyses of artificial ageing, from which the expected life-time of a transformer (time of reduction from DP 1000 to DP 200), whose dependence on temperature and humidity of paper insulation, is evident.
Figure 1 shows that the presence of humidity substantially speeds up the ageing of paper insulation. Thus, with the temperature of a hotspot at 98°C, a shortened life-time of 20 years could be expected in a transformer with completely dry paper throughout its lifetime, however, in reality no insulation is completely dry. In an operating transformer which has on average at least 2% of humidity (but even can have more), for a life-time of 20 years the temperature of the hot-spot would have to be limited to 75°C. In order to reach a 40 years lifetime, however, the temperature of the hotspot would have to be limited to a maximum of 70°C. The majority of humidity in a transformer is formed as a degradation product of the ageing of paper-oil insulation (normally ca 0.1% annually). Humidity, however, can also enter a transformer from the air if a transformer which fails to be properly protected. We cannot prevent humidification of a transformer as majority of humidity is formed and retained in the paper insulation of windings due to ageing processes, which–again–are temperature conditioned (autocatalysis). Consequently temperature, or cooling, remains practically the only method which can influence the velocity of humidification and the ageing of an operating transformer. c. Metal Particles: Winding, insulation and active parts of the transformer should not be exposed to the forced air / gasses having metallic dust flying due to grinding or other operation like turning, milling and surfacing of conductive parts which are very dangerous and under influence of electric charges it forms a local cell and in course of time it deteriorates the insulation property and starts ageing and becomes more and more conductive in nature. Thus failure of Job occurs in short span of life. To avoid such situation to prevail, it is always suggested that the machine shop should be kept away from the manufacturing point of electrical appliances.
B. Insulation Ageing Characteristics & Related Case Studies
When a transformer is manufactured in the factory, the paper insulated windings are subjected to drying before they are oil impregnated. At this state, the transformer has a moisture content of <0.5% by weight in paper and 6 ppm in oil. As the transformer ages, the moistures content will increase progressively at a rate of ~0.1% to 0.2% / year. In severely deteriorated system, the moisture content could reach >4%.
Insulation aging is directly related to moisture content. During manufacturing process the drying out procedure is carried out as per manufacturing norms. There are possible areas of ingress of moisture in the insulation during the course of manufacturing of transformer i.e. excess hours taken when the CCA is taken out from oven or VPD for tanking, use of unprocessed insulation blocks during tanking. To avoid ingress of moisture in insulation, exposure time during tanking process to be minimized.
After tanking, testing is completed at works, the transformer (>50MVA) may be dispatched with nitrogen. In such cases dew point, relative humidity (RH) of the nitrogen / dry air and gas pressure inside the transformer shall be checked and monitored. On getting satisfactory results transformer shall be allowed for dispatch. The acceptable limit of dew point with corresponding insulation temperature is mentioned in Table 2.
It is needless to say that after dispatch and during long storage, the transformer shall be monitored as per manufacturer’s guidelines. In case of any damage noticed during transformer receipt at site, it shall be sealed immediately temporarily or permanently, as is possible and matter to be brought to the notice of the manufacturer.
Case Studies on the Incidences of Moisture Ingress
i. 50 MVA, 132/33kV transformer
After complete processing by end user the transformer was made ready for testing and charging. It has been informed by the customer that all other electrical test were in order except the IR values are not matching with factory test results. After thorough investigation, it has been observed that porcelain bushing was damaged during transportation and it could not be noticed at the time of receipt of transformer at site, same is shown in photo-1 below. Since the substation work was delayed due to delay in project, priority was not given to transformer erection and commissioning, and the transformer was without nitrogen for 2 years, resulting in ingress of moisture. Rusting on core lamination below bushing was noticed, same in shown in photo-2.
From site officials, it is understood that the transformer was idle for more than two years. No nitrogen pressure was monitored. Internal inspection was not carried out before erection of the transformer and directly bushing erection, vacuuming and oil filling and filtration was done. When the transformer is tested, it has been noticed that IR value is less as compared to factory test results. Hence, it was suggested to drain out the oil completely, carry out nitrogen purging cycles and hot oil circulation. After 3-4 cycles of drying out process, the IR values found satisfactory for charging the transformer. Thereafter, transformer was taken into service and now the said transformer is working satisfactory.
ii. 10 MVA, 66/33kV Single Phase Transformer
In this case also, the transformer was kept idle without charging for more than 1½ years and the area had huge humidity and no precaution was taken. When the transformer was taken for processing, it has been noticed that the HV-E IR value is less comparatively to other two combinations and w.r.t. factory test results. After thorough investigation of OLTC, oil view glass was found damaged which was not noticed by site officials, resulting ingress of huge moisture in OLTC chamber for longer duration. It was also noticed that though the transformer was oil filled condition, precaution was not taken for long storage as per the guidelines. Internal inspection revealed that there was huge quantity of water in the transformer oil and also some particles of water noticed on core resulting rusting of core as well as OLTC diverter chamber. Findings are shown in photo 3 & 4.
These phenomena caused the utility to return back the transformer to manufacturing works for complete reprocessing for removal of foreign particles and improvement of IR values with huge cost.
Photo – 3 & 4: Rusting of core as well as OLTC diverter chamber
iii. 100 MVA, 220/132 kV Transformer:
The transformer was supplied and was kept idle for more than 2 years. When it was taken into system, it was noticed that there is drastic deterioration of IR value and all other electrical test were in order.
When the matter came for discussion, it is revealed that the transformer was idle without monitoring of nitrogen for more than 2 years and after detailed discussions, it was decided to do the internal inspection to check the clearance of leads and if any non-conformity. After draining of oil it has been noticed that there is sludge over the windings. During further investigation, it has been noticed that the appearance of oil was found dark brown in new transformer; same is shown in photo- 5.
While investigating the reason of oil color change, it came to notice that customer has used oil from another 100MVA which was failed after service of more than 20 years. The idle unattended storage lead to ingress of moisture and recycled oil lead to ingress of other contaminants. For making transformer ready for service, it was decided to remove sludge/ contaminants by hot oil jet wash and replacement of oil by new. After site processing of transformer by hot oil filtration satisfactory IR values were achieved and transformer was taken into service.
iv. 63 MVA, 132/33kV Transformer
In other case, we found that the IR value measured at factory was not achieved during the commissioning of 63 MVA, 132/33kV. After detailed study and investigation, it has been noted that the transformer was supplied with nitrogen filled and oil was supplied separately in tankers. The oil samples drawn from oil tanker were found in order in all respect. The filtration machine which was used for this new transformer was used earlier for failed transformer oil processing and the same machine was used without cleaning of oil chamber for removal of contaminated oil. Thereafter, new high vacuum filter machine was used for achieving satisfactory results.
In many of the cases, we have found that moisture, dusts in oil are ingresses due to the improper care taken at the time of commissioning. Oil drums are kept vertical instead of horizontal, as captured in photo-6. Filter Machine, Suction & Delivery pipes and storage tanks are not used for a long period resulti8ng in heavy deposition of sedimentation and impurities which are not checked for its internal condition and cleaned before oil filling and filtration resulting ingress of moisture.
In view above it is recommended to check the following things to avoid ingress of moisture during the commissioning stage:
i. Monitor the gas pressure inside the transformer during storage at site. Storage in gas filled condition shall be limited to six months duration.
ii. For storage more than 6 months, transformer shall be stored in oil filled condition.
iii. Store oil drums in horizontal position with bangs at 45 degree.
iv. Check the oil parameters supplied in tanker / drums before filling in transformer.
v. Do the internal inspections of transformer and check thoroughly for presence of any foreign particles inside the transformer; if observed clean it thoroughly.
vi. Check the filtration machine chamber is cleaned before it is used for new transformer. vii. Check the oil storage tank used for filtration machine is having oil resistance paint on inside surfaces and is in good condition. Use the storage tank after proper cleaning.
viii. Breathers of transformers shall be maintained in healthy condition by periodical reconditioning