POWER TRANSFORMERS AND ON-LOAD-TAP CHANGERS: IS YOUR SPECIFICATION UP TO DATE?

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1 POWER TRANSFORMERS AND ON-LOAD-TAP CHANGERS: IS YOUR SPECIFICATION UP TO DATE? ABSTRACT Tim Farrell - Reinhausen Asia-Pacific Sdn Bhd Level 11 Chulan Tower, No 3 Jalan Conlay, Kuala Lumpur, Malaysia T.Farrell@my.reinhausen.com Dhanvalarp Sukkaraks - TIRATHAI Public Company Limited 516/1 M.4 Bangpoo Industrial Estate T, Samutprakarn 10280, Thailand dhanvalarp@tirathai.co.th As power transformers are the workhorses of the power grid, the utilization of detailed specifications is key for matching the end users need and the manufacturer s capabilities. Based on the long lasting experience of Senior transformer and transformer component design and application engineers, this paper tries to summarize some key findings of the interpretation of the user s specifications for power transformers. In the recent years the interaction between manufacturers and users showed some common trends that affect the industry at present. Arguably, different interpretations of standards and specifications may result in situations where the end-user receives a transformer that does not meet his needs in each regard. Therefore this paper shall address particular areas that have space for improvement, as well as provide recommendations and clarifications to some common questions related to the specification and to illustrate the implications of the specification on the product performance. The general base is the utilization of international and national technical standards and transformer specifications. The interaction of these different requirements should be taken into consideration, as well as the operational requirements in the relevant grid application. The paper is focussing the discussion on the electrical design, the mechanical design, the secondary controls and implications to the life cycle costs. KEYWORDS: Transformer, Specification, OLTC, Vacuum Switching Technology, life cycle costs Introduction: Power transformers are key assets in energy transmission and distribution networks. Life time expectations vary from 20 years up to 60 years and more, depending on the operating and environmental conditions. Although the basic principle of regulated power transformers is unchanged since many decades, there are various developments in the last 10 to 20 years that are worth being considered in the specification of a power transformer. The term specification means stating explicitly and in detail, how a product shall be. In case of regulated power transformers the specification covers various aspects, such as international, national and user-specific standards, as well as aspects related to the integration in the existing network. In addition specifications cover also commercial, environmental and maintenance requirements. Specifications of power transformers are typically a textual transcription of the technical data of power transformers in general, as well as detailed descriptions of key components along with the reference to applicable standards. The purpose of a specification of power transformers is to ensure that different bidders will offer transformers that fully meet the user s expectation and allow a price comparison between alternative offers. For deviations in the transformer efficiency in most cases an established loss evaluation procedure is applied to make different offers comparable. The specification for power transformers is the fundamental basis for transformer design and application engineers of the OEM and the major component suppliers, such as on-load tap-changer (OLTC) and bushings, etc. Based on long years of experience it can be stated, that for some aspects of the transformer specifications there is no room for interpretation, whereas there are aspects where different options or different interpretations are possible. This paper shall highlight some aspects of transformer specifications that can be considered by users in further optimization of their specifications based on the joint experience of manufacturers of transformers and OLTCs. Topics that will be discussed in this paper are related to electrical design, mechanical design, control circuits and finally - aspects that consider an improvement of the life cycle costs (LCC) of a transformer. 1

2 Electrical Design of Transformers: 1. Rated load and overload capability Transformer specifications often require the suitability of components like OLTCs for overload conditions of 1.5 times rated load. In case of such requirements, however, in most cases the definition of the overload conditions is missing. Thus it is unknown whether it is a constant or cyclic load; nor does it indicate the overload duration minutes, hours or days. The only way to cope with such requirement is to consider a continuous overload of factor 1.5 which means to increase the basic rating by factor 1.5 for the components in question. Other specifications state a minimal rated current for components like OLTCs, for example the On-Load Tap Changer shall have a rating of 1200 amps, irrespective of the real requirement according to the technical data of the transformer. Both cases usually result in the application of an oversized OLTC, which increases the cost of a transformer without bringing an advantage in the performance, as the limiting factor might be the transformer itself or another component in the current path. Based on the operating experience, OLTCs and bushings designed and manufactured in accordance with the Loading guide for oil-immersed power transformer [1], are capable to cope with all relevant operational requirements. Therefore there is no need to add additional overload requirements to a specification, as this might increase the cost of components and transformers without any value added. In case of specific customer requirements exceeding the limits of [1], a detailed description of the overload factor and the required overload conditions (intervals and duration of intervals) should be given. 2. Transformer regulating range and step size: Transformers fitted with Off-Circuit Tap-Changers (OCTC) have a most common step size of 2.5% of the rated transformer voltage, typically realized with 5 positions +/-2 steps. Whereas, for the On-Load Tap-Changer (OLTC), the most common step size is 1.25% with a range from 17 to 33 tap positions for network transformers. Transformers with higher numbers of steps, for example in rectifier transformers used in aluminium electrolyses are equipped with special OLTCs with up to 107 tap positions. It is important to note that the size and number of steps should be chosen according to network conditions, the transformer will be applied to. When a very fine voltage regulation is required as for an industrial application, a small step size <1% and higher number of positions is recommended. For typical network applications, a step size of 1.25 to 1.6% is more common. If the step voltage is comparatively small and the number of tap positions is high, voltage fluctuations in the network can trigger many tap-change operations per day. A tap-changer with less tap positions along with a higher step voltage would allow the use of a more reasonable OLTC that carries out fewer operations and needs less maintenance without any reduction of the voltage quality. [2] In a concrete example there was a specification of a municipality utility in a developing country asking for 24 steps at 0.8% for their 33kV transformer. Each step is only 264Volts. As small voltage fluctuations in the network occurs the bandwidth of the voltage regulator was infringed and an immediate OLTC operation was triggered, without any useful impact to the voltage quality of the network. It would have been a better technical solution to utilize the same 24 steps with a larger step size to increase the effective regulation range. In result such on OLTC could adjust for a lower system voltage without an increase of the OLTC switching frequency. 3. Alternative insulation liquids: It is more and more common to use alternative insulation liquids in power transformers and OLTCs. The use of such alternative insulation liquids allows for a number of advantages, in reduced fire / explosion risk (and therefore insurance costs), as well as environmental benefits of green products. As the properties of each alternative insulation liquid differ from that of mineral oil, each liquid has to be approved for use in power transformers and OLTCs [3]. Main aspects to be considered generally for OLTCs are described in Fig.1. The qualification of alternative insulation liquids is fairly complex; therefore a joint specification should be worked out between grid operator, transformer manufacturer and component supplier. 2

3 Fig. 1: Comparison of technical requirements in a transformer using insulation liquids [4] Mechanical design of transformers 1. Transformers in indoor application in Underground or multilevel substations An increasing number of utilities are moving towards underground transformers or multilevel substations, where the main tank is located at a different level as the radiators and conservators (Fig. 2). Such arrangements enables many benefits for space reduction. Fig. 2: Example of an Multi-Level Substation 3

4 Nevertheless a few key points need to be considered in the specification: - It is recommended not to exceed a head height of 10 m, as this limits the differential pressure to a level where standard design accessories can be used. If a head height greater than 10m is used, a major number of components would need to be of custom design, greatly increasing engineering costs. This is valid for the OLTC as well. - Natural cooling is mostly limited in an indoor multilevel substation, so OFAN or OFAF is recommended. - For an underground substation, the requirement to ensure all transformer oil is contained in the event of a leakage has to be balanced with any issue of localised flooding. The transformer room needs to be designed to contain and to store the oil of the transformer in case of a leakage. Such a design also means that in case of localised flooding due to a natural event or flooding due to broken water mains, the transformer room can be flooded with water. For such designs it is recommended to locate the critical transformer control cabinets at a higher level on the transformer or into an upper room in the substation. 2. Provisions for easy maintenance of OLTCs in the transformer specifications There are transformer specifications that require for example a separate compartment for the tap selector. Others require drop down tanks associated with the tap changing apparatus to be fitted with a guide rod. Both requirements follow the purpose to make maintenance easier. However, the OLTC design has progressed significantly in the last 20 years. Field experience and design improvements therefore allow a re-consideration of such specification criteria or even offer the potential for improvement. a) Separate oil compartment for tap-selector A separate oil compartment for an OLTC tap-selector can fulfil two major purposes, the separation of the oil and thus the dissolved gases in the transformer main tank for individual and separate diagnosis, as well as the isolated access to the tank of the tap-selector in case of any maintenance or repair matter. With maintenance requirements typically beyond 1,000,000 operations, tap-selectors normally do not need maintenance in case of network application transformers. Routine maintenance therefore does not have to be considered. Any other reason that requires access to a tap-selector must be related to an irregularity. It is therefore an economical aspect to consider the cost increase of each transformer for the separation of the tap selector with the cost advantage in case of necessary repair measures under consideration of the probability of an irregularity. Such comparison also has to include the influence of the necessary barrier board as additional solid insulation material on the failure probability. According to the current experience from the manufacturer perspective, OLTC tap-selectors are quite reliable and minor formation of combustible gasses at the pre-selector can be estimated based on the individual transformer data. Thus a judgement within a transformer DGA is generally possible. Modern tap-selectors even are equipped with mechanisms to minimize the formation of gasses during pre-selector operations (Fig. 3). It is therefore recommended to re-consider the need of a strict separation of the oil of the tap-selector from the oil of the main transformer tank. Figure 3: Active Gas Inhibition System (AGIS) integrated into OLTC selector 4

5 b) Provisions for maintenance of conventional type OLTCs There is a trend towards easy maintenance or even drastically reduced maintenance requirements. However, due to a split in conventional type, using arcing contacts under oil and vacuum type OLTC, some aspects still should be considered in transformer specifications. Conventional type OLTCs still require a routine maintenance, on average every 5 years or 70,000 operations. Hence, the specification should include points to make this procedure easy. If using an OLTC that is integrated in the main transformer tank (in-tank type) no pipe or wiring work should cross the OLTC cover and a suitable head height to remove the diverter insert for routine maintenance should be available. In cases where the roofs are low in height, a permanently mounted lifting device should be supplied. For the bolt-on design, the OLTC should be at a comfortable elevation, where the maintenance works can be conducted at ground level or provisions should be made for the supply of a suitable platform. The placing of the transformer is also vital to allow the OLTC control cabinet door to swing open completely. Due to the reduced maintenance requirements of Vacuum type OLTCs (in network application), less importance is placed on the access to the OLTC, though, it needs to be assured that it should not be to the point where access is totally blocked. c) Option of new drive concepts for a technical / economical optimization Latest developments in vacuum OLTC designs led to solutions, where the drive motor is mounted on the tap changer cover. Therefore drive shafts and bevel gears are not longer used. Beside a more compact OLTC design, the synchronisation of OLTC and motor drive can be done automatically to simplify the assembly of the transformer, its commissioning and any future maintenance activity. MR offers this technical solution with the TAPMOTION TD (Fig. 4). Figure 4: Drive motor mounted on OLTC Cover (TAPMOTION TD) d) Oil sampling: Taking oil samples and determining breakdown voltage and water content is a common procedure for transformer and tap changer oil. While in case of conventional type OLTCs the tap-changer oil usually is replaced during the maintenance interval of 5 to 7 years, topping up the oil level between maintenance activities is not necessary. As the vacuum type OLTC has service intervals of operations resulting in time intervals of 25 years or more in network applications, no frequent oil changes are done. Though, regular testing of the oil for moisture and breakdown is required. Beyond that, 3 decades ago, DGA was a new technology and limited to the main oil tank, where in present days, it is more common to test the OLTC oil as well. Doing so, it is recommended to specify oil sampling points for taking oil samples, as well as taking provisions for topping up the oil in case of reduced oil level in the conservator. 5

6 Secondary Controls 1. Control circuits Specifications usually describe the required functions textually. In case of OLTC motor drive units, for example, the standard diagram will be taken and any additional function will be added based on the interpretation of the written specification by the transformer design engineer and the OLTC application engineer. Since misinterpretations easily can happen and often flexible interpretations are possible, there are often cases where modifications are needed when transformers are commissioned at site. In spite of identical user specifications, individual transformer control schemes can differ considerably. This increases complexity at the OLTC manufacturer, the transformer OEM and the operations- and maintenance department of the end user. Because a drawing tells a thousand words To achieve standard control cabinets it is highly recommended to supply standard drawings or references in the transformer specification. Then all control cabinets can be custom made to one design, solely independent of the transformer manufacturer. Standardized circuit diagrams allow a quicker design review process and improved fault finding process on site. Beyond the functional requirements related to control circuits and wiring, specifications clearly should state country specific requirements that are not defined by international standards, such as wiring colours or cable designations. 2. Monitoring points / provisions for monitoring and communication: If a monitoring system is not installed at the time of manufacturing and considering a transformer lifetime of years, there may be a decision of retrofitting such a system in the future. Without any provisions, however there are handicaps that result in considerable additional costs. On the other hand, preparing a transformer for easy retrofitting of sensors at the time of its production has only minor influence on the costs and is therefore more reasonable. Examples for such provisions are spare thermometer pockets and an oil valve at an appropriate position that allows for example retrofitting online DGA sensor. A spare CT, is also beneficial, as the cost to retro fit or replace a damaged internal CT on the transformer is many times higher in comparison to have an additional CT installed during manufacturing. Comparing options with regard to life cycle costs Figure 5: Life Cycle Costs Trade-off 1. Vacuum or conventional type OLTC: With the increased acceptance of vacuum type OLTCs the price gap between conventional type OLTCs and vacuum OLTCs is getting smaller worldwide. In case of OLTCs for large transformers, prices for both options are almost equal. Without a clear requirement in the specifications typically the cheaper option will be chosen 6

7 by the manufacturer although only minor additional initial investment may result in significant cost savings over the lifetime of the transformer. For smaller transformers, under 30MVA 66kV, the cost of Vacuum OLTC is usually still higher than the conventional type OLTC. In equivalence to the usual load loss calculation, the Life Cycle Cost evaluation should be considered for the comparison of both technologies (Fig. 5). 2. Dehydrating breathers: An OLTC including OLTC oil conservator contains between 200 l and 2000 l of oil, depending on the type and design. Often the size of the dehydrating breather is chosen solely based on the ratio of oil volume of OLTC and main tank resulting in an extremely small breather volume for the OLTC oil system (Fig. 6). Because of the energy released during OLTC operations, there is more breathing activity in the OLTC oil system than caused temperature expansion due to ambient temperature and OLTC operation. Field experience shows, the desiccant of small breathers for the OLTC reaches saturation much faster than that of the main tank. This needs either much more maintenance efforts from the user or it will result in an inefficiency of the dehydrating function. Specifications therefore should request either a breather of 2 kg or more in case of a conventional breather for the OLTC. Alternatively the specification could ask for maintenance free dehydrating breathers for OLTC and main tank. This allows automatic regeneration of the desiccant without the need for replacement, leading to a significant reduction of maintenance efforts along with a permanent high efficiency. Figure 6: Showcase of an insufficiently dimensioned dehydrating breather for OLTC in comparison with main tank breather Figure 7: Maintenance free dehydrating breather for OLTC and main tank 7

8 Summary Accurate specification is a key requirement for an equipment manufacturer to deliver a product that meets the user s expectation. As international standards adopt new technologies sometimes faster than the national or segment standards, it is economically advantageous to review and update specifications on a regular basis. This will prevent specifications that request technical features or combinations of technical features which are in contradiction to the specific technical requirements for the operation and maintenance of the equipment. Beside technical difficulties this might lead also to over-dimensioned and therefore more expensive products. Whereas the electrical design of transformer and OLTC should be guided by the final application in the grid, the mechanical design of multi-level substations should have a limit of less than 10 m height differences of transformer components. Doing so, the resulting pressure difference can be handled by conventional designs and components. As OLTCs are components which might require maintenance during the lifetime of the transformer, the mechanical design should guarantee the access to such components. Another critical area is the control circuits. In addition to a textual description of the specification, a wiring diagram should be agreed between manufacturer and customer. To extend the lifetime of critical transformers or even to have the option of a performance surveillance, sensors for monitoring devices should either be integrated in new transformer or sensor pockets for retrofit of monitoring sensors should be foreseen in the tank design. An example of a commercial evaluation is cited in the Life Cycle Cost Analysis, taking into account, the commercial benefits of employing the new technology of monitoring systems, vacuum OLTC or improved breather systems. References [1] IEC :2005, Loading guide for oil-immersed power transformer, IEC, 2005 Geneve, Switzerland [2] Krämer, Axel: On-load Tap-changers for Power Transformers: Operation, Principles, Applications and Selection, 2 nd edition, 2014 [3] Frotscher, Rainer: Tap-changer know-how: Insulating liquids Part II: Non-mineral insulating liquids, Transformers Magazine, Volume 3, Issue 3, 2016 [4] Frotscher, Rainer: Tap-changer know-how: Insulating liquids Part I: Mineral oils, Transformers Magazine, Volume 3, Issue 2,