Electrical energy is generated and the voltage level is raised to the transmission level and transmitted to further away from the generation point, and then the voltage is reduced from the transmission level to the distribution level and is offered to the people for various purposes. We call these facilities as transformer susbtations where voltage is increased and reduced. The voltage level between 1 and 52 KV is used to be called the medium voltage (MV) network, and the high voltage (HV) for the networks greater than 52 KV , and the low voltage (LV) for the networks less than 1 kV. Today, all over 1kV networks are called high voltage networks. however, it is still known as MV-HV-LV networks. As electrical energy is generated in rural and virgin areas, fixed substations are usually established to reduce to distribution level after being transferred to transmission level in these places. In these areas, fixed substations are usually established to reduce to distribution level after being transferred to transmission level. Temporary or urgent substations are established as mobile and used to serve energy distribution or consumption activities in other regions when necessary.

What is Mobile Substations?

Since mobile transformer substations are generally used in remote locations, all equipment, tools and hardware that will be required in the application should be kept in the content and no tools, tools and equipment should be required during the application. It is one of the advantages of the application that it provides the opportunity to be completed and commissioned as soon as possible. Voltage levels and power determine the type of the mobile substations such as 132/33kV, 25 MVA, 33/11kV, 31,5/MVA or 2500 kvA mobile substations. Mobile substations are designed and manufactured on one or more trailers according to the qualities of primary and seconder circuits and presented to end users.

Mobile substations can be manufactured up to 62.5 MVA power in transmission level systems up to 245 KV voltage level. Mobile substations can be manufactured at transmission level up to 245 KV voltage level up to 62.5 MVA power value.

Primary Components of Mobile Substations

Primary circuit elements are systems formed by high voltage or medium voltage switching and protection elements that provide connection conditions to the primary voltage level of the transformer. Mobile substations are usually requested as two trailers and one of these trailers has primary circuit elements and power transformers. In mobile substations, which are only from MV to LV voltage level, the design is requested on single trailer, and the substations which are rarelyfrom MV to MV are also requested on single trailer.

Primary trailers can be requested as GIS (gas insulated system) or AIS (air insulated system ) at HV level. In the mobile Transformers requested as GIS, design studies are carried out with “hybrid system” in which HV circuit breaker, HV disconnector and grounding separator, voltage transformers and current transformers are found in the tank filled with SF6 gas. By removing the current transformers from the system, bushings of the transformer are designed as current transformers and alternative price and size advantage solutions are offered. HV surge arrester should be connected between the transformer and Hybrid GIS system. The power transformer is also designed on the primary. In the mobile substations requested as AIS*, in accordance with the specifications on trailers, air insulated HV separator and grounding separator, oil insulated porcelain voltage rack, SF6 gas breaker, HV surge arrester, and current transformer or bushing current transformer designs are made and power transformer is applied on the rear of the trailer. Some MV/MV mobile substations also use two trailers. In the primary trailers of mobile substation projects, MV entrance, measurement, transformer protection and internal need transformer supply cells are found in the two-room hair kiosks and in the other room of this kiosk, there is a transformer control panel.

It is designed in HV/MV mobile transformer projects two-room hair kiosk with independent inputs on secondary trailer. The steel kiosk has MV switchgears in one room , and in the other room there is a control, measurement and protection panel, a redressor, LV, AC and DC distribution panel and a desk and chair for the personnel. On the secondary trailer, except the steel kiosk, there are auxiliary transformer and grounding transformer, mounted batteries in external type of battery cabinet, and cable drum.

In the secondary trey of MV/MV mobile transformer applications, there is a IP 54 protection class two-room steel kiosk with independent gates in which the control and measurement panel , LV AC-DC distribution panel, redressor are located with the secondary circuit MV switchgears in one chamber. Outside the steel kiosk, there is the cable drum where the cables for the primary and secondary connections are wrapped, the external type battery cabinet with the batteries and the auxiliary transformer.

Some MV/MV mobile substations are requested on single trailer. In such cases , there is a control and measurement panel in one room and an AC-DC distribution panel in the other room, with the power transformer on the single trailer and the IP 54 Protection class steel kiosk with two independent input gates with redresor. On the other side of the kiosk, there is the cable drum, where the cables for the primary and secondary connections are wrapped, and the external type battery cabinet with the batteries and the auxiliary transformer.

Design of Mobile Substations

They are designed as air insulated or gas insulated systems according to the specifications up to 245 kV insulation level.

The Design of Air Insulated HV busbar

HV voltage transformers which is the first element of the primary circuit in mobile substation applications are manufactured in capacitive or inductive types with oil insulated and proselen shielded. We use capacitive type voltage transformers in mobile substation applications. In order to carry the dynamic and static loads of the conductors that provide the connection to the powerline line, first the connection to the post insulator is made and then the input transformer is used as the first circuit element. The purpose is to ensure that whether there is a tension in the transmission line where the disconnector switch and circuit breaker are open, and they are also used to lock the operations of grounding blades on the transmission line side of the circuit breakers. Voltage transformers should consist of at least two seconders in the measurement and protection class. The information from the measurement sequence gives voltage information to the measurement instruments and voltage counters based on the billing. The protection sequence provides voltage information to the protection relays with locking circuits.

The second element of the primary circuit is the HV separators and earthing switches. Non-invasive types are used to provide electrical values which are stated in the specifications. The disconnector switches can be manufactured with an optional manuel or single motor driven or earthing switch. The disconnector switches, which is a single earthing switch, is applied with earthing switches on the power transformer side and the dual earthing switches on both the power transformer and the transmission line side. Disconnectors are installed by locking to prevent work under load. In mobile transformer centers, the disconnectors and earthing switches are processed from 110 VDC voltage source. Manual operations are left at the request of personnel with a selective key. The main contacts of the disconnector provide absolute resistance to the short circuit current due to the increase in contact pressures in cases of short circuit currents.

The disconnectors can not separate under load when the circuit breaker is closed but with the circuit breaker, they can do the separation process. If the earthing switches in transmission line traffic are requested in the specification and there is tension due to the voltage information obtained from the voltage transformers at the entrance, the closing can be prevented, if there is no voltage, the closing operation can be done. Generally, in mobile substation applications, earthing switches are not required on the transmission line side.

The circuit element used after the HV disconnector on the primary circuit is SF6 gas cutters. They do cut and close current for protection and operation under load, and should be selected in the capacity to cut short-circuit current and nominal current calculated to provide the electrical values specified in the specifications. The engine of the HV circuit breaker is applied as 110 VDC to feed the engine, opening and grabbing coil. HV circuit breakers come to the mobile substations disassembled and assembled on site. SF6 filling of HV circuit breaker is done after all these assembly works are completed, so all equipment and necessary gas must be located at the mobile substations.

In the primary circuit, the current transformers are found after the breaker, in some applications, current transformers are used in the bushing of the transformer. The current transformers are also manufactured as oil insulated and proselene protected. The secondary number depends on the number of measurement and protection devices to be used. Generally, transformer differential protection relays and spare protection relays are applied with a secondary for measuring instruments, meters. Sensitivity classes are determined to serve these purposes in accordance with the specifications.

In front of the transformer, porcelain or silicone coated surge arresters which provide electrical properties in the specifications are used. Surge arresters are applied with surge counter. The surge arresters manufactured in metal oxide type are generally applied between the current transformer or circuit breaker in CL:2 or 3 class to discharge the voltage from the current to withstand the current at 10 Ka levels. Also, surge arresters, which is selected according to the same criteria, is used on the circuit where the star point of the transformer is grounded.

The last circuit element of the primary train is the power transformer. Power transformers are required to have at least +/-8 number 1.25% with oil cooling system. Neutral point and HV bushes are used as bushing type current transformers to save space in the design.

Onan cooling: oil cooling system with natural air circulation, ONAF cooling is the cooling form where the oil is cooled by fans and the transformer temperature is kept under control. Fan cooling system the power of the transformer provides a power increase between 20% and 25% compared to Onan cooling.

OFAF cooling is the way that the oil circulation provided by the natural flow of the fan-cooled oil to the heated core in the ONAF cooling system is done by a circulation pump.

ODAF cooling system is a type of cooling that is made by passing directly to a circulating pump of the oil in the closed circuit, which is cooled by the fans in an external radiator, which is designed apart from the transformer.

The most important features in transformers are the total power loss values on/off load. The transformers manufactured for mobile substations are manufactured with special design and do not exceed 3.500 mm in width and 3.500 mm in height. Automatic Control Panel under load is applied in the second trailer in the kiosk. In the specifications, the vector connection group is usually requested as YNd11.

The number of axles, length, and height of the lowbad treyler where the equipment form the primary circuit is calculated according to the design and requested to be manufactured according to the following criteria.

We need to determine the height of the highest equipment to be mounted on the trailer design and the maximum permissible height on the road. Apart from this, the weights of all the equipment to be mounted on the trailer should be determined one by one. The insulation gap between each element and the maximum permissible jump distance value of the trailers must also be applied according to the specifications. After all these data are obtained, the number of axles is determined depending on the total weight of the equipment, and the total weight of the trailer should be kept in the calculations. The size of the trailers is determined after being drawn by considering the minimalist with the necessary insulation gaps on the trailer.

The Design of Gas Insulation HV (GIS)

If a gas insulated (GIS) system with HV circuit elements is described in the specification of mobile substations, it is used because the voltage transformer, disconnector and earthing switches, circuit breaker, current transformer and optionally the HG systems in which the surge arresters are located in a tank filled with SF6 gas are less occupied. There is a power transformer behind the GIS systems on the primary trailer where GIS systems are used. On the front side, the post insulator should be used to carry the load of the conductor from the power transmission line. In terms of operating limits, it is not possible to find any element other than power transformer with HV equipment in primary trailer.

There are the steel kiosk with AC and DC power distribution board and battery charger rectifier, measurement panel, MV switchgears on one of the rooms of secondary trailer which forms HV/MV mobile substations, on the other room; there are control & protection panels.

Outside of the kiosk, there is an auxiliary transformer, an external type panel with the batteries in it, and a cable drum with reducer that provides motor and circuit control between the transformer and the MV bar input cell and the MV output cells to the distribution systems where the cables are wrapped. The overhead line conductors to be connected to the HV transmission line are also included in this trailer.

All the equipment to be assembled into the dimensions of the kiosk on the secondary trailer is determined according to the terms of minimalism. It consists of two rooms, the first room contains the MV cells and the air conditioner capable of cooling the heat energy to be released as the nominal current passes through these cells.

Secondary circuit MV cells in air or gas insulated cell classes are requested as determined electrical features in the specifications. Usually, air insulated metal clad-grade cells are used. Since the dimensions of the steel kiosk are limited in height and depth, these cells should be designed with the pressure reduce channel at high temperature and pressure resulting from the possible internal arc and this system is applied by the channel from one or two points out of the kiosk so that personnel safety is ensured at the optimum level. The important point is that the switchgears must pass the type tests.

The point to be considered is that the cells have passed the type tests as they are applied here. However, since the cells are mobile substation equipment, they must have passed the seismic type tests (GR-63-core zone 4) according to the most severe conditions.

The second room of the kiosk is the control room and takes its name from the control panel where all HV and MV switching elements are controlled and monitored. The control panel includes protection relays that perform diff. & back-up protection of the power transformer, switches required to make opening & closing of MV circuit breaker, disconnectors and earthing switches, position indicators and alarm warning indicators belonging to these equipment.

Operation personnel perform all maneuvers in HV circuit via the control panel, alternatively, they can perform manual operation by moving the selection key close to the operation location on the field. The personnel doesn’t perform manual operations unless he has to.

The measurement of the electrical energy passing through the MV input and output switchgears inside the measurement panel is carried out with noble and spare counters for each feeder. These panels can be sealed.

AC axuiliary panel receives the energy from the internal demand transformer via a suitable switch and distributes it to all devices and receptacles consumed AC energy in the field. In addition to this, the charger of the batteries providing the DC needs, MV cells and HV equipment and power transformer under load of the stage changer heater and other consumption points and kiosk rooms of the rooms are fed by this panel.

The Dc distribution panel takes the DC feed from the battery and rectifier group and distributes it to all cells and DC controlled MV equipment through a two-pole switch with appropriate capacity.Emergency lighting systems in the kiosk rooms are also fed from this panel.

Battery charging rectifier usually provides the DC energy consumed in the system as 110 VDC as long as the energy is present. In AC energy sections, charging of the atteries that will provide DC voltage is also provided by the redressor.

In the selection of batteries, the total power of the system to be fed should be calculated and it should be able to feed this power value for at least 5 hours and 8 hours in some projects.

Batteries are applied outside of the kiosk so that the hydrogen gas released from electrostatic powder coated panel and the batteries do not affect the equipment in the kiosk.

The earthing and auxiliary transformer are chosen to ZNyn connection group and used as current limiting for detecting and switching phase-earthing short circuit faults of HV system as well as providing AC electricity energy to be consumed in the plant.

In some applications, they are applied with a resistance at the calculated values in order to avoid earthing transformers from falling into resonance. In applications from OG to OG, they are not used as grounding transformers and they only meet the 400V AC electrical power requirement of the plant by selecting in Dyn 11 Connection Group. In this case, neutral grounding resistors are used to limit phase earth fault currents of transformer and system.

In applications from MV to MV, they are not used as earthing transformers and they only meet the 400V AC electrical power requirement of the plant as Dyn 11 connection group. In this case, neutral grounding resistors are used to limit phase-earth fault currents of transformer and system.

Neutral grounding resistors are used to limit the current value that can flow from the neutral point of the power transformer in possible phase-earth faults. The value of the resistance is calculated based on the current limit value to be determined. In external type galvanized sheet metal which is resistant to atmospheric weather conditions. There are current, voltage transformer and surge arrester in addition to the resistance. In these sensors, the system is protected by opening the circuit breaker on the HV side of the transformer according to the current and voltage values.

All of the cables needed to connect the mobile substation to the energy transmission or distribution line and transfer the electrical energy from a voltage level to the energy distribution lines at another voltage level are available as ready to be installed heads on a cable drum l. When the trailer is positioned according to these lines, the primary line connection cables or busbars on the drum are removed and connected to the line by taking security measures. Then, the cable between the transformer and the secondary bus input cell is connected and connections are made to the lines to be fed to the secondary busbar. All these connections are complemented by the materials contained in the Mobile Substation without any material, tools and equipment.

Control and control cables are installed between two trailers. Both trailers have a plug-in plug socket junction box and all connections are completed by plugging the plugs on both ends of the cables into the sockets in these junction boxes. Since the plug sockets used for the connection used for each purpose are marked, there is no possibility of incorrect connection.

Portable (Mobile) substations to MV / MV can be requested on a single or double trailer with requiring design specifications. In the two-trailer applications, there are power transformer on the primary trailer, air or gas insulated MV cells in accordance with the specification up to 40,5 kV in one room and in the other room there is a steel kiosk with power transformer control and protection panel.

Outside this kiosk, there is a neutral grounding resistance with the auxiliary transformer according to the specification. On the secondary trailer, there are gas or air insulated input and output cells at a secondary voltage level in a room, the AC and DC distribution board in the other room, the steel kiosk with the battery charger rectifier and outside of the kiosk, there is an external panel with the batteries inside and a rectifier cable drum that allows the control of the primary circuit and the secondary circuit to be connected to the lines and the connection of the transformer to the primary and secondary circuit cells.

In mobile substations designed on a single trailer, the power transformer is applied to the extreme point on a longer trailer compared to normal applications. After the power transformer, there is a steel kiosk with power transformer control panel, AC and DC distribution panel and battery charge redressor in the other room. Outside of the steel kiosk, with neutral grounding resistance, auxiliary transformer, external type battery panel and reducer with MV input and output cables and motor driven cable drum which can rotate at constant speed is applied. Selection criteria of the equipment in MV/MV voltage level mobile substations on single or double trailers are designed the same as the secondary trailers of mobile transformer centers at HV/MV voltage levels.

Mobile Substations With MV Input, Low Voltage Level Output

The mobile substations from MV to LV are more compact, and their total weight is lighter than other mobile substations, so they fit on the trailer. In relation to the level of LV 3200 kVA up to 4000 KVA in these applications, every equipment is designed in steel kiosk. The steel kiosk consists of 3 rooms, the first room is the MV cells insulated by air or gas, second room is distribution transformer, third room is equipped with an LV panel and a cable drum where all the connection cables are available, tool box and other equipments.

Effective Factors In Desinging The Mobile Substations

The most important factor in the design of mobile substation is the characteristics of the region. The highest altitude, relative humidity rate, seasonal average, minimum and maximum temperature values, the effects that disturb the conductivity of the air should be known. When designing Mobile Substation Center, this information is used when determining the energy leakage distance (creepage distance) between the equipments. The distance between the two trailers, primary traile and the transmission or distributions line to receive energy should be known.

In the field, calculating earthing area that will provide the step and touch voltage values, and in accordance with these calculations, the installation and supply of materials that will form the earthing area is up to the responsibility of the administration of the operation. The reason is that the specific earth resistance in different places to be applied to mobile substations differs and this earthing area is also required to be established regionally different.

All tools, tools and equipment necessary for the operation of mobile substation should be identified and kept on proper basis. Portable stairs and walking platform should also be considered in order to provide access to the kiosks of mobile substations.

The factory of the equipments forming the mobile substation is one of the most important issues. All functions of the control panel are tested and all of the maneuvers must be tested by applying the control voltage.

All protection functions must be tested when the set values are exceeded. The same procedures must be carried out for the measuring system and ensure that the measurements are carried out correctly, and calibration of the instruments which require calibration must be ensured. When the Mobile Substation reaches the application area, it must be ready to operate when the power circuit and control connections are completed. The mobile substation is now ready to be put in place by applying the following tests to MV cells and power transformers.

Routuine Test Of MV Switchgears: (IEC62271-200)

• Main frequency voltage tests in main circuit (70 kV, 1 minute)
• Network frequency voltage tests in auxiliary circuits
• Primary and secondary current injection and protection relays function tests and opening supervision of characteristics
• DC isolation tests-main busbar, main busbar-earth (5 kV DC, 1 minute)
• Determination of current transformer rotation rates
• Secondary load testing
• Mechanical work experiments
• Measurement of resistance of the main
• Testing of auxiliary electrical devices
• Tests of suitability of connections
• Manual examination
• Measuring paint thickness

Factory Test Of Power Transformers

• Temperature increase test
• Short circuit test (type test)
• Measurement of zero component impedance
• Vacuum Test on Mounted Transformer
• Pressure Test of Mounted Transformer
• Measurement of audible noise level (as per the procedure specified in IEC 60076-10)
• Determination of transient regime voltage switching characteristics (IEC 60076-3)
• Assessment of the tank for proper carriage (IEC 60076-1)
• Determination of the weight of the transformer prepared for transport (IEC 60076-1)
• Measurement of energy consumption of fan motors (IEC 60076-1)
• Measurement of Winding Resistance
• Measurement of Voltage Turn Rate and Control of Phase Shift
• Measurement of capacity and loss factor
• D.C. Measuring Insulation Resistance
• Measurement of short circuit impedance and loas losses . ( According to IEC-60076-1, article 11.4 e)
• Measurement of free loss and current ( according to IEC-60076-1, article 11.5 )
• Dielectric Strength Tests ( according to IEC 60076-3)
• Switching pulse test ( according to IEC 60076.3, article 15)
• Lightning impact test ( according to IEC60076-3)
• Tests of load level converter ( according to IEC 60076-1, article 11.7)
• Pressure Leakage Test ( according to IEC 60076-1 article 11.8 e)
• Paint Application and Quality Control
• Routine Tests in Welding Connections
• Measurement of Magnetic Flux
• FRA (Frequency Response Analysis) Test
• Oil Tests
  • Power Factor (Doble, 25 °C+100°C)
  • Dielectric Strength
  • Acidity
  • Internal Surface Stress
  • Density
  • Viscosity
  • Number of Colors
  • Amount of water
• Checking oil-soluble gases (according to the procedure specified in 60567)
• Control of all protection of fittings and transformers