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Power transformer

Power transformer

In a real transformer, some power is dissipated in the form of heat. A portion of these power losses occur in the conductor windings due to electrical resistance and are referred to as copper losses. However, so-called iron losses from the transformer core are also important. The latter result from the rapid change of direction of the magnetic field, which means that the microscopic iron particles must continually realign themselves technically, their magnetic moment—in the direction of the field (or flux). Just as with the flow of charge, this realignment encounters friction on the microscopic level and therefore dissipates energy, which becomes tangible as heating of the material.
Taking account of both iron and copper losses, the efficiency (or ratio of electrical power out to electrical power in) of real transformers can be in the high 90% range. Still, even a small percentage of losses in a large transformer corresponds to a sig- nificant amount of heat that must be dealt with. In the case of small transformers inside typical household adaptors for low-voltage d.c. appliances, we know that they are warm to the touch. Yet they transfer such small quantities of power that the heat is easily dissipated into the ambient air . By contrast, suppose a 10MVA transformer at a distribution substation operates at an efficiency of 99%: A 1% loss here corresponds to a staggering 100 kW.
In general, smaller transformers like those on distribution poles are passively cooled by simply radiating heat away to their surroundings, sometimes assisted by radiator vanes that maximize the available surface area for removing the heat.

Large transformers like those at substations or power plants require the heat to be removed from the core and windings by active cooling, generally through circulat- ing oil that simultaneously functions as an electrical insulator.

The capacity limit of a transformer is dictated by the rate of heat dissipation. Thus, as is true for power lines, the ability to load a transformer depends in part on ambient conditions including temperature, wind, and rain. For example, if a transformer appears to be reaching its thermal limit on a hot day, one way to salvage the situation is to hose down its exterior with cold water—a procedure that is not “by the book,” but has been reported to work in emergencies. When transformers are operated near their capacity limit, the key variable to monitor is the internal or oil temperature. This task is complicated by the problem that the temperature may not be uniform throughout the inside of the transformer, and damage can be done by just a local hot spot. Under extreme heat, the oil can break down, sustain an electric arc, or even burn, and a transformer may explode.
A cooling and insulating fluid for transformers has to meet criteria similar to those for other high-voltage equipment, such as circuit breakers and capacitors: it must conduct heat but not electricity; it must not be chemically reactive; and it must not be easily ionized, which would allow arcs to form. Mineral oil meets these criteria fairly well, since the long, nonpolar molecules do not readily break apart under an electric field.

Another class of compounds that performs very well and has been in widespread use for transformers and other equipment is polychlorinated biphenyls, commonly known as PCBs. Because PCBs and the dioxins that contaminate them were found to be carcinogenic and ecologically toxic and persistent, they are no longer manufactured in the United States; the installation of new PCB-containing utility equipment has been banned since 1977.11 However, much of the extant hardware predates this phase-out and is therefore subject to careful maintenance and disposal procedures (somewhat analogous to asbestos in buildings).

Introduced in the 1960s, sulfur hexafluoride (SF6) is another very effective arc-extinguishing fluid for high-voltage equipment. SF6 has the advantage of being reasonably nontoxic as well as chemically inert, and it has a superior ability to with- stand electric fields without ionizing. While the size of transformers and capacitors is constrained by other factors, circuit breakers can be made much smaller with SF6 than traditional oil-filled breakers. However, it turns out that SF6 absorbs thermal infrared radiation and thus acts as a greenhouse gas when it escapes into the atmos- phere; it is included among regulated substances in the Kyoto Protocol on global climate change. SF6 in the atmosphere also appears to form another compound by the name of trifluoromethyl sulfur pentafluoride (SF5CF3), an even more potent greenhouse gas whose atmospheric concentration is rapidly increasing.

Transformer fan

Transformer fan

Heat from core losses and copper losses must be dissipated to the environment. In dry type transformers, cooling is accomplished simply by circulating air around and through the coil and core assembly, either by natural convection or by forced air flow from fans. This cooling method is usually limited to low-voltage indoor transformers (5 kV and below) having a three-phase rating below 1500 KVA. At higher voltages, oil is required to insulate the windings, which prevents the use of air for cooling the core and coils directly. At higher KVA ratings, the losses are just too high for direct air cooling to be effective. In outdoor environments, direct air cooling would introduce unacceptable amounts of dirt and moisture into the windings.
Transformers come in various cooling classes, as defined by the industry standards. In recent years, there have been attempts to align the designa- tions that apply to transformers manufactured in North America with the IEC cooling-class designations. Table below gives the IEC designations and the earlier designations that are used in this book. All of the IEC designations use four letters. In some respects, the IEC designations are more descriptive than the North American designations because IEC makes a distinction between forced-oil/air cooled (OFAF) and directed-flow-air cooled (ODAF). Some people find using the four-letter designations somewhat awkward, and this book uses the earlier designations throughout.
In small oil-filled distribution transformers, the surface of the tank is sufficient for transferring heat from the oil to the air. Ribs are added to the tanks of some distribution transformers to increase the surface area of the tank and to improve heat transfer. Large distribution transformers and small power transformers generally require radiator banks to provide cooling. Regardless of whether the tank surface, ribs, or radiators are used, transformers that trans-fer heat from oil to air through natural convection are all cooling class OA transformers.

Radiators used on OA transformers generally have round cooling tubes or flat fins with large cross section areas in order to allow oil to flow by natural convection with minimal resistance. Hot oil from the core and coils rises to the top of the tank above the inlet to the radiator. Cool oil from the radiator sinks to the bottom of the radiator through the outlet and into the bottom of the core and coils. This process is called thermo-siphoning and the oil velocity is relatively slow throughout the transformer and radiators. For this reason, OA transformers have relatively large temperature gradients between the bot- tom oil and the top oil, and relatively large temperature gradients between the winding temperatures and the top oil temperature. Likewise, the air circulates through the radiator through natural convection, or is aided by the wind.

Designations and descriptions of the cooling classes used in power transformers
Previous designationIEC designationDescription
Oil-air cooled (self-cooled)
Forced-air cooled
Oil-air cooled (self-cooled), followed by two stages of forced-air cooling (fans)
.OA/FA/FOA.ONAN/ONAF/OFAFOil-air cooled (self-cooled), followed by one stage of forced-air cooling (fans), followed by 1 stage of forced oil (oil pumps)
Oil-air cooled (self-cooled), followed by one stage of directed oil flow pumps (with fans)
. OA/FOA/FOA.ONAF/ODAF/ODAFOil-air cooled (self-cooled), followed by two stages of directed oil flow pumps (with fans)
Forced oil/air cooled (with fans) rating only—no self-cooled rating
Forced oil / water cooled rating only (oil / water heat exchanger with oil and wa- ter pumps)—no self-cooled rating
Forced oil / air cooled rating    only    with    di- rected oil flow pumps and fans—no self-cooled rating
Forced oil / water cooled rating only (oil / water heat exchanger with directed oil flow pumps and water pumps)— no self-cooled rating

As the transformer losses increase, the number and size of the radiators that are required to cool the oil must increase. Eventually, a point is reached where wind and natural convection are not adequate to remove the heat and air must be forced through the radiators by motor-driven fans. Transformers that have forced air cooling are cooling class FA transformers. FA transform- ers require auxiliary power to run the fan motors, however, and one of the advantages of OA transformers is that they require no auxiliary power for cooling equipment. Since additional cooling is not usually needed until the transformer is heavily loaded, the fans on most FA transformers are turned off until temperatures exceed some threshold value, so under light load the transformer is cooled by natural convection only. These transformers are cool- ing class OA/FA transformers.

Some transformers are cooled by natural convection below temperature T1, turn on one stage of fans at a higher temperature T2 and turn on a second stage of fans at an even higher temperature T3. These transformers are cooling class OA/FA/FA transformers. The direction of air flow in forced-air units is either horizontally outward or vertically upward. The vertical flow pattern has the advantage of being in the same direction as the natural air convection, so the two air flows will reinforce each other.

Although the cooling capacity is greatly increased by the use of forced air, increasing the loading to take advantage of the increased capacity will increase the temperature gradients within the transformer. A point is reached where the internal temperature gradients limit the ability to increase load any further. The solution is to increase the oil velocity by pumping oil as well as forcing air through the radiators. The usual pump placement is at the bottom of the radiators, forcing oil from the radiator outlets into the bottom of he transformer tank in the same direction as natural circulation but at a much higher velocity. Such transformers are cooling class FOA transformers. By directing the flow of oil within the transformer windings, greater cooling effi- ciency can be achieved. In recognition of this fact, the calculation of hot-spot temperatures is modified slightly for directed-flow cooling class transformers.

As in forced-air designs, forced-oil cooling can be combined with OA cooling (OA/FOA) or in two stages (OA/FOA/FOA). A transformer having a stage of fans and a stage of oil pumps that are switched on at different temperatures would be a cooling class OA/FA/FOA transformer.
The radiator design on FOA transformers can differ substantially with the radiator design on FA transformers. Since the oil is pumped under consid- erable pressure, the resistance to oil flow is of secondary importance so the radiator tubes can be designed to maximize surface area at the expense of cross section area. FOA radiators are sometimes called coolers instead, and tend to resemble automotive radiators with very narrow spaces between the cooling tubes and flat fins in the spaces between the cooling tubes to provide additional surface area. The comparison of the two types is illustrated in picture left (OA/FA type) and right (FOA type).

OA/FA radiator construction

OA/FA radiator construction. The large radiator tubes minimize restric- tion of oil flow under natural convection. The fan is shown mounted at the bottom with air flow directed upward.

FOA cooler construction

FOA cooler construction. The oil is forced through narrow tubes from top to bottom by means of oil pumps. The cooling fans direct air horizontally outward.

Cooling equipment requires maintenance in order to run efficiently and provide for a long transformer life. There is the obvious need to main- tain the fans, pumps, and electrical supply equipment. The oil coolers them- selves must be kept clean as well, especially FOA-type coolers. Many transformers have overheated under moderate loads because the cooling fins were clogged with insect and bird nests, dust, pollen, and other debris. For generator step-up transformers, where the load is nearly at nameplate rating continuously, steam-cleaning the coolers once every year is a good mainte- nance practice.


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ANSI CODE : 50BF Circuit Breaker Failure

ANSI CODE : 50BF Circuit Breaker Failure

This function is designed to detect the failure of breakers that do not open when a tripping order is sent. The “breaker failure” protection function is activated by an O1 output tripping order received from the overcurrent protection functions (50/51, 50N/51N, 46, 67N, 67). It checks for the disappearance of current during the time interval specified by the time delay T.

It may also take into account the position of the circuit breaker read on the logic inputs to determine the actual opening of the breaker. Wiring a volt-free closed circuit breaker position contact on the “breaker closed” equation editor input can ensure that the protection is effective in the following situations:

  • When 50BF is activated by protection function 50N/51N (set point Is0 < 0.2 In), detection of the 50BF current set point can possibly be not operational.
  • When trip circuit supervision (TCS) is used, the closed circuit breaker contact is short-circuited. Logic input I102 is therefore no longer operational.

Automatic activation of this protection function requires the use of the program logic circuit breaker control function. A specific input may also be used to activate the protection from the equation editor. That option is useful for adding special cases of activation (e.g. tripping by an external protection unit).
The time-delayed output of the protection unit should be assigned to a logic output via the control matrix.
The starting and stopping of the time delay T counter are conditioned by the presence of a current above the set point (I > Is).

Block diagram
Block diagram – 50BF

Block diagram – 50BF

Example of setting

The example below shows how to determine the time delay setting for the 50BF function Overcurrent protection setting: T = inst. Circuit breaker operating time: 60 ms.
Auxiliary relay operating time to open the upstream breaker or breakers: 10 ms.a

Example of setting

Example of setting using SEPAM relay

The time delay for the 50BF function is the sum of the following times: Sepam O1 output relay pick-up time = 10 ms Circuit breaker opening time = 60 ms Overshoot time for the breaker failure function = 20 ms.

To avoid unwanted tripping of the upstream breakers, choose a margin of approximately 20 ms. This gives us a time delay T = 110 ms.


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Energy management control center
Energy management control center

Energy management is the process of monitoring, coordinating, and controlling the generation, transmission, and distribution of electrical energy. The physical plant to be managed includes generating plants that produce energy fed through transformers to the high-voltage transmission network, interconnecting generating plants, and load centers. Transmission lines terminate at substations that perform switching, voltage transformation, measurement, and control. Substations at load centers transform to subtransmission and distribution levels. These lower-voltage circuits typically operate radially, i.e., no normally closed paths between substations through subtransmission or distribution circuits. (Underground cable networks in large cities are an exception.)

Since transmission systems provide negligible energy storage, supply and demand must be balanced by either generation or load. Production is controlled by turbine governors at generating plants, and automatic generation control is performed by control center computers remote from generating plants. Load management, sometimes called demand-side management, extends remote supervision and control to subtransmission and distribution circuits, including control of residential, commercial, and industrial loads.

Events such as lightning strikes, short circuits, equipment failure, or accidents may cause a system fault. Protective relays actuate rapid, local control through operation of circuit breakers before operators can respond. The goal is to maximize safety, minimize damage, and continue to supply load with the least inconvenience to customers. Data acquisition provides operators and computer control systems with status and measurement information needed to supervise overall operations. Security control analyzes the consequences of faults to establish operating conditions that are both robust and economical.

Energy management is performed at control centers (see picture below), typically called system control centers, by computer systems called energy management systems (EMS). Data acquisition and remote control is performed by computer systems called supervisory control and data acquisition (SCADA) systems. These latter systems may be installed at a variety of sites including system control centers. An EMS typically includes a SCADA ‘‘front-end’’ through which it communicates with generating plants, substations, and other remote devices.

Picture below illustrates the applications layer of modern EMS as well as the underlying layers on which it is built: the operating system, a database manager, and a utilities=services layer.

Layers of a modern EMS

Layers of a modern EMS (Energy management systems)

SCADA – Supervisory control and data acquisition

A SCADA system consists of a master station that communicates with remote terminal units (RTUs) for the purpose of allowing operators to observe and control physical plants.

Generating plants and transmission substations certainly justify RTUs, and their installation is becoming more common in distribution substations as costs decrease. RTUs transmit device status and measurements to, and receive control commands and setpoint data from, the master station.

Communication is generally via dedicated circuits operating in the range of 600 to 4800 bits=s with the RTU responding to periodic requests initiated from the master station (polling) every 2 to 10 s, depending on the criticality of the data.
The traditional functions of SCADA systems are summarized:

  • Data acquisition: Provides telemetered measurements and status information to operator.
  • Supervisory control: Allows operator to remotely control devices, e.g., open and close circuit breakers. A ‘‘select before operate’’ procedure is used for greater safety.
  • Tagging: Identifies a device as subject to specific operating restrictions and prevents unauthorized operation.
  • Alarms: Inform operator of unplanned events and undesirable operating conditions. Alarms are sorted by criticality, area of responsibility, and chronology. Acknowledgment may be required.
  • Logging: Logs all operator entry, all alarms, and selected information.
  • Load shed: Provides both automatic and operator-initiated tripping of load in response to system emergencies.
  • Trending: Plots measurements on selected time scales.

Since the master station is critical to power system operations, its functions are generally distributed among several computer systems depending on specific design. A dual computer system configured in primary and standby modes is most common. SCADA functions are listed below without stating which computer has specific responsibility.

  • Manage communication circuit configuration
  • Downline load RTU files
  • Maintain scan tables and perform polling
  • Check and correct message errors
  • Convert to engineering units
  • Detect status and measurement changes
  • Monitor abnormal and out-of-limit conditions
  • Log and time-tag sequence of events
  • Detect and annunciate alarms
  • Respond to operator requests to:
    • Display information
    • Enter data
    • Execute control action
    • Acknowledge alarms
  • Transmit control action to RTUs
  • Inhibit unauthorized actions
  • Maintain historical files
  • Log events and prepare reports
  • Perform load shedding


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Transformator shot with thermovision camera

Transformator shot with thermovision camera

Substation ventilation is generally required to dissipate the heat produced by transformers and to allow drying after particularly wet or humid periods. However, a number of studies have shown that excessive ventilation can drastically increase condensation. Ventilation should therefore be kept to the minimum level required. Furthermore, ventilation should never generate sudden temperature variations that can cause the dew point to be reached. For this reason: Natural ventilation should be used whenever possible. If forced ventilation is necessary, the fans should operate continuously to avoid temperature fluctuations. Guidelines for sizing the air entry and exit openings of substations are presented hereafter.

Calculation methods
Natural ventilation

Natural ventilation

A number of calculation methods are available to estimate the required size of substation ventilation openings, either for the design of new substations or the adaptation of existing substations for which condensation problems have occurred.

The basic method is based on transformer dissipation. The required ventilation opening surface areas S and S’ can be estimated using the following formulas:


S = Lower (air entry) ventilation opening area [m2] (grid surface deducted)
S’= Upper (air exit) ventilation opening area [m2] (grid surface deducted)
P = Total dissipated power [W]
P is the sum of the power dissipated by:

  • The transformer (dissipation at no load and due to load)
  • The LV switchgear
  • The MV switchgear

H = Height between ventilation opening mid-points [m]

This formula is valid for a yearly average temperature of 20 °C and a maximum altitude of 1,000 m.
It must be noted that these formulas are able to determine only one order of magnitude of the sections S and S’, which are qualified as thermal section, i.e. fully open and just necessary to evacuate the thermal energy generated inside the MV/LV substation. The pratical sections are of course larger according ot the adopted technological solution.

Indeed, the real air flow is strongly dependant:

  • on the openings shape and solutions adopted to ensure the cubicle protection index (IP): metal grid, stamped holes, chevron louvers,…
  • on internal components size and their position compared to the openings: transformer and/or retention oil box position and dimensions, flow channel between the components, …
  • and on some physical and environmental parameters: outside ambient temperature, altitude, magnitude of the resulting temperature rise.

The understanding and the optimization of the attached physical phenomena are subject to precise flow studies, based on the fluid dynamics laws, and realized with specific analytic software.


Transformer dissipation = 7,970 W LV switchgear dissipation = 750 W MV switchgear dissipation = 300 W The height between ventilation opening mid-points is 1.5 m.


Dissipated Power P = 7,970 + 750 + 300 = 9,020 W

Ventilation opening locations

To favour evacuation of the heat produced by the transformer via natural convection, ventilation openings should be located at the top and bottom of the wall near the transformer. The heat dissipated by the MV switchboard is negligible. To avoid condensation problems, the substation ventilation openings should be located as far as possible from the switchboard.

«Over» ventilated MV/LV Substation

«Over» ventilated MV/LV Substation. The MV cubicle is subjected to sudden temperature variations.

Substation with adapted ventilation

Substation with adapted ventilation. The MV cubicle is no longer subjected to sudden temperature variations.

If the MV switchboard is separated from the transformer, the room containing the switchboard requires only minimal ventilation to allow drying of any humidity that may enter the room.

Type of ventilation openings

To reduce the entry of dust, pollution, mist, etc., the substation ventilation openings should be equipped with chevron-blade baffles. Always make sure the baffles are oriented in the right direction.

MV cubicle ventilation

Any need for natural ventilation is taken into account by the manufacturer in the design of MV cubicles. Ventilation openings should never be added to the original design.

Instruction: Medium Voltage equipment on sites exposed to high humidity and/or heavy pollution by Schneider Electric


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Na stranici Stručni programi su postavljeni linkovi za download programa za specificiranje srednjenaponskih distributivnih postrojenja 8DJ, 8DH i NXPLUS C proizvođača Siemens. Programi koriste jednostavan interfejs i imaju mogućnost kreiranja porudžbina direktno u SAP-u. Postoj predefinisane kombinacije vodnih, trafo, prekidačkih i merne ćelije, kao i export izveštaja, tj. detaljnog crteža sa prednjim izgledom i otvorima za kablove u MS Powerpoint. Grafički deo programa uključuje samo jednostavne izglede jednopolnih šema ćelija, kao i opcija koje te ćelije podržavaju.

Korisni link:

Profix – Concept Finding & Planning / Procurement & Erection


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Na stranici Projektna dokumentacija je postavljen download-link sa crtežom u AutoCAD-u sa prednjim izgledom (front view) srednjenaponskog METAL CLAD postrojenja MCset 1-2-3 proizvođača Schneider Electric-a. Crtež je kreiran u ePlusMenuCAD SCH, tj. brendiranoj verziji od strane Schneider Electric Srbija u kome se nalazi biblioteka simbola srednjenaponskih i niskonaponskih postrojenja i transformatora.

Korisni linkovi za MCset:

Electrical Distribution: MCset 1-2-3


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