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Energoterra

35kV. Directions for Development

Января 17, 2014

Lately, much more interest has been sparked in 35 kV substations. Due to growing capacity of electric power networks, they replace 10 kV substations more and more often. Our experts have described the most popular types of package transformer substations, basic equipment selection principles, and other market trends.

Package substation
Package transformer substations rated at 35 kV class are used in electric power supply systems of industrial and urban facilities to step down the mains voltage. 
The prefix “package” was added to transformer substations back in the Soviet times when a challenge was set to reduce the construction period and to improve reliability of substations at the expense of using a pre-fabricated set of equipment.  Previously, equipment was supplied to the energy site by separate deliveries and often had to be adjusted at the installation site, which was very inconvenient. 
As of today, PTSs are certainly not always delivered to the customer as highly prefabricated modules. They are often composed of equipment produced by a few different manufacturers. However, the prevailing trend is to make one and the same company responsible for both supply and installation of PTS. The equipment supplied by such company may include own-produced items or equipment purchased from other vendors. 
 
Outdoor or indoor?
Depending on the equipment used, package transformer substations may be either indoor or outdoor type. It means that they may be equipped with an outdoor switchgear (OSG) or indoor switchgear (ISG) respectively. Main part of the equipment of the latter one is located inside the substation building.
“Talking about 35 kV voltage, transformer substations with outdoor switchgears are the most popular”, says Mikhail Yegorov, the First Vice-President of Electroshield -TM Samara Group CJSC. “They have been marked with trouble-free performance and reliable operation for decades, are well-known by the maintenance personnel, and customers are used to them as well. The obvious advantage of indoor distribution substations (ISG 35 kV) is that all the equipment items such as electrical devices, transformers, surge arresters, etc. are arranged inside a modular building and protected against environmental impact. However, ISGs have not been that widely used yet due to poor operating experience and a high price compared to OSGs”.
 
“As of today, PTS with OSG is the most common type of package transformer substation 35 kV used in the Russian networks”, agrees Andrey Trubkin, Technical Director of Elektronmash CJSC, Saint-Petersburg. “Such transformer substations may benefit from a lower price, easy installation, and reduced construction period compared to PTSs arranged inside permanently installed buildings. Among disadvantages, there are severe operating conditions of OSG equipment, especially during winter season, and too much space required for the OSG layout.
 
Package modular transformer substation 35 kV (photo by Alstom Grid JSC)
 
The advantage of an indoor PTS is that all its components are arranged inside permanently installed buildings equipped with centralized environmental control and life support systems. The rooms where the equipment is laid out are marked with comfortable service environment and equipped with convenient cable structures. The disadvantage of an indoor PTS is its high price and extended construction period. [If we talk about a capital construction building - Editor's note]. Using such PTS is primarily justified in the regions with a high level or air pollution and severe weather conditions”.
“Today, the scope of service of indoor PTSs with the high voltage 35 kV is restricted to substations installed in populated areas, especially in the regions with restrained urban conditions, substations generating electric power for oil and gas production and transportation facilities, or substations located in the premises or in the vicinity of oil refineries”, says Victor Ivanov, Chief Designer of MC KTP-Ural CJSC (a member of EnTerra Group CJSC, Ekaterinburg). 
However, according to some experts, the situation will be changing little by little. Sooner or later, most PTSs 35 kV will move indoors. Taking into account the costs of construction, installation and further operation, it would be quite fair to assume that the price difference between OSG and ISG may not be that big, at least over the long term. The question is, to what types of buildings will PTSs move?
 
From capital construction to modular buildings
“The material for a PTS 35 kV building must be selected depending on the facility service life, appearance requirements, and construction technology available at the facility”, explains Victor Ivanov. “Capital construction buildings made of brick and concrete are the most durable ones. If a substation is scheduled to be in service for at least 50 years, those materials must be used. Having said that, all the substation equipment will have to be replaced at least once during the substation service life since the lifetime of equipment rarely exceeds 25-30 years. Construction of such substations takes more time and requires more materials but it may be justified by the requirement to preserve the architectural look of the community. Also, those substations are maintenance-friendly. The equipment is not exposed to weather conditions. Although, there are some inconveniences related to the surplus heat removal, especially from powerful transformers. Modular PTSs are essentially different. In my opinion, it is the most requested type of indoor substations”.
 
Substation 35/6 kV. Supply for LUKOIL-AIL CJSC (photo by Electroshield -TM Samara Group CJSC)
 
“Today, modular building (MB) substations are the most popular ones among customers since they do not require any additional construction works”, agrees Andrey Sasin, Chief Designer of ALSTOM-GRID JSC (Ekaterinburg branch). “The substation that is delivered to the installation site is basically ready for use. Thanks to that, its assembly and installation is usually performed much faster which allows reducing the substation commissioning period”.
“The cost of modular building PTSs is high but it may also be adjusted by means of reduction of construction and operating costs”, says Andrey Trubkin.
“Among advantages of modular package transformer substations, it may be noted that they are usually installed in the beginning of construction. So, they may generate power for the construction site first, and then for the facility itself. Therefore, customers save money on installation of temporary PTSs”, adds Danil Borodkin, Commercial Director of Modul, Ekaterinburg.
A substation installed in a modular building allows for floor space saving. Compactness is guaranteed since such type of PTS is composed of modules small enough to be easily transported by rail or by road.
However, compactness has its reverse site. The modular construction method imposes some restrictions in terms of outline dimensions of the equipment to be used at the substation. Unfortunately, it may happen that the equipment specified in the design project does not fit into the modular building.
In this case, there is an alternative solution which in a certain sense is a trade-off type of PTS design. We refer to pre-fabricated frame buildings. Usually, sandwich panels are used as enclosing structures for such type of buildings. As a result, construction period and construction costs are significantly reduced compared to capital construction buildings. At the same time, there are no severe restrictions in terms of shipping clearance: a frame building is assembled on site so it may be designed with account of cubicles dimensions.
 
Substations GS-1 and GBS 35 kV for Lukoil JSC in the workshop of MC KTP-Ural CJSC (photo by EnTerra Group CJSC)
 
Material for the building
Modular buildings where the PTS 35 kV equipment is laid out may be fabricated with the use of sandwich panels, corrugated steel plates, or cast-in-place concrete.
“The main advantage of modular buildings made of corrugated steel plates is the increased module resistance to mechanical stress which is especially required under strict transportation and operation conditions”, says Andrey Trubkin. “However, such buildings weigh much more than buildings made of sandwich panels. The appearance of a modular building fabricated of cast-in-place concrete blocks easily fits in the general architectural style of most cities. Due to historically poor development of 35 kV networks within the boundaries of large cities and their lack in small towns, the cast-in-place concrete design of PTS 35 kV is rarely used. The major downsides of concrete buildings are their heavy weight and a higher cost of installation works compared to metal MB”.
However, concrete modular buildings have their bright side as well. Thus, according to Danil Borodkin, today there are some production methods that allow modifying outline dimensions of construction parts (“variable-size formwork”). For example, the building length may be increased by one meter to fit some additional equipment.
 
What do customers want?
The review of PTS design options already reveals customers’ requirements in many respects: compactness, speed of construction, the lowest possible cost, and for the cities – architectural appearance. For example, they told us at the PR service of Lenenergo OJSC that as of today, it is scheduled to install nine 35 kV substations in the historic center of Saint-Petersburg which will be made of concrete, and their dimensions will correspond to those of 6-10 kV facilities. On the one hand, such solution would bring power supply centers closer to consumers, and on the other hand, it would ease up the process of facilities approval by the city administration authorities.
What other challenges do customers set for manufacturers of PTSs 35 kV?
“At any competitive tender, one of the customer’s requirements to the product sounds as follows: it’s better be “already supplied”, or the delivery time has to be really tight; moreover, they want the lowest possible price”, shares Sergey Buriakov, Deputy Commercial Director of High Voltage Union corporate group, Ekaterinburg. “That is why the manufacturer really faces a challenge to meet the cost requirements, to supply the product on time, and to provide high quality.
It takes 30 to 120 days to produce some of the components so the procurement department has to put out its own tender for purchasing this or that product. And to make sure the goods are delivered sound and within the established deadlines, PTS manufacturers track down the production progress of the most complex equipment that requires maximum production time, has lots of component parts, or requires large amounts of raw materials. Often, step-by-step operation control is carried out”.
 
Inside a modular package transformer substation 35/0.4 kV manufactured by Modul LLC (author’s photo)
 
Main equipment
Some key requirements of the customers are related to selection of the main equipment for the substation. Having said that, experts agree that today there are no common preferences in this matter.
“The equipment selection trends depend on a specific industry field where the customer performs its activities”, explains Andrey Sasin. “For example, in the Tyumen region, customers prefer to have oil transformers for their PTSs. It may be explained by special characteristics of the networks where dry transformers get out of order more often. On the other hand, some other customers prefer dry transformers. Generally, one could say there are no perfect transformers, all of them are characterized by ferrimagnetic resonance, and networks parameters affect the transformer selection”.
According to Andrey Trubkin, selection of dry transformers is more typical for urban distribution networks. Although, power engineers of oil and gas production facilities sometimes are quite skeptical about it since overload capacity of dry transformers is low, there are no outdoor dry transformers,  and issues arise when they are operated  under low temperatures.
“Similarly, there is no unique trend in selection of circuit breakers type. It also depends on the industry sector”, assumes Andrey Sasin. “Some companies prefer SF6 CBs because they think that a vacuum circuit breaker generates overvoltage when being switched too often. Conversely, others prefer using equipment with vacuum CBs. SF6 equipment has not been used in Russia for a long time yet, and even now, many operators fear a probability of gas leak, a need to constantly control pressure, and complicated disposal conditions, which leads to additional costs associated with environmental project development”.
Based on his experience of the most recent supplies by Elektronmash CJSC, Andrey Trubkin says that vacuum circuit breakers are generally used in 35kV SGs of package transformer substations. More and more often, electrically driven load-break switches and earthing switches are used. 
 
EnTerra Group CJSC has marked increased customers’ interest in ISGs with SF6 insulation based on analysis of technical inquiries made for the most recent years. It may be explained by the fact that compared to air insulation, SF6 allows using minimum outline dimensions for PTSs, reducing total substation construction costs, and making it easier to build the facility under restrained urban conditions or in hard-to-reach areas.
 
Higher than high
Finally, it is common practice for the operating companies to express their interest in reduction of power networks losses and capacity improvement. In this regard, bringing the higher voltage facilities closer to the customer has become a popular solution lately. As we mentioned above, the same decision was taken by Lenenergo OJSC. It is not by coincidence that in the cities, 10/0.4 kV PTSs are being replaced with 35/0.4 kV substations. Many manufacturers now produce such type of PTSs. “Transformer substations rated at 35/0.4 kV class are of current concern for remote areas and extended lines as well as for fast growing regions”, thinks Danil Borodkin. “In the future, 35 kV PTSs will possible be used in distribution networks of urban electric power supply. However, even now there are some technical solutions that allow for construction of two-transformer PTSs 35/0.4 kV in a reinforced concrete shell which will be able to be used in the urban networks on equal terms with traditional 2-transformer modular PTSs 6(10,20)/0.4 kV”.
The major advantages of the voltage increase are: reduction of load on connection points and improvement of power losses during transmission process. If power consumption increases in the course of operation under standard voltage 10 kV, the network loads go up as well since capacity of such line is not really high. The problem of capacity increase or transmission of power for long distances may be solved by increasing the voltage level. According to D. Borodkin, production of modular PTSs 35/0.4 kV has become possible thanks to 35/0.4 kV power transformers with the same dimensions as more traditional ones rated for 10/0.4 kV class, and thanks to other state-of-the-art compact multi-purpose equipment.
 
PTS 35kV at Poyma substation (photo by High Voltage Union CJSC) 
 
Counting money
There is an idea that a modern PTS is not just a facility that steps the voltage level up or down but a whole complex that ensures reliable performance of the network section and performs a set of additional tasks such as electric power metering. But do substations really need those additional functions, or is it more of an undue sophistication? 
“Today, everybody starts to count money, and we face a requirement to integrate electric power metering systems into PTSs in almost every order accepted by ALSTOM Grid”, says Andrey Sasin. “Also, customers often want to have remote control of the substation. Using remote control leads to reduction of costs related to maintenance personnel and provides a possibility to monitor equipment operation and status in the online mode”.
“Certainly, integration of automated systems of electric power metering (ASEPM) in 35 kV PTSs is of a current concern especially if it is the main substation of the power supply system”, agrees Andrey Trubkin. “Improving the automation level is the basic trend of power networks development”.
“Integration of an automated information and measurement system of power fiscal metering makes it possible to obtain data required for calculation of technical and economic parameters, issuance of electric power and capacity balances”, explains Sergey Buriakov. “The information provided by AIMS PFM is used to control PTS operation modes and to settle payments with other network operators. So, such information is of high priority as well as the PTS remote control system since many of substations lack full-time maintenance staff”.
Mikhail Yegorov also agrees with the opinions expressed above. It is common practice for Electroshield -TM Samara Group CJSC to supply maintenance-free 35 kV substations equipped with automation devices, fiscal and technical metering systems, equipment remote control systems, and devices remote command systems. There were some solutions developed on the basis of items manufactured by the R&D center Mechanotronika LLC which enable to establish a monitor and control system of remote power facilities (e.g., oil field substations), to provide access to the information about the equipment status, and to improve efficiency of electric power and other resources supply and metering.
 
Forging ahead
It is worth to mention another operating requirement that is often imposed by oil and gas production companies. According to EnTerra Group, customers operating in that industry field need mobile PTSs 35 kV. It is assumed that while being in service, the substation will be moving from one field area to another (let’s say if production is interrupted in the first area). So, a requirement is imposed to design PTS modules in such a way to make sure that the substation may be moved up to seven times over its 30-year lifetime. 
When completing such order, it is required to fit maximum number of equipment items into the building of shipping dimensions, and at the same time, to elaborate the simplest way of connecting the substation modules to each other. In fact, this is quite feasible. For example, in Europe, mobile PTSs 110 kV are already in use. 
The long and the short of it is that modern PTSs must combine compactness and affordable cost, be user-friendly and cheap in operation, have additional functions without sacrificing their reliability, be easily and quickly constructed, and on top of that, be in service for decades. And the leading manufacturers try to comply with all those requirements that may seem conflicting as one would think. Otherwise, they would not be able to fight off the competition.
 
Ekaterina Zubkova

Translated by Vladimir Alekseev
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