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Transformer Ratings

Transformer Ratings

Transformer size or capacity is most often expressed in kVA. “We require 30 kVA of power for this system” is one example, or “The facility has a 480 VAC feed rated for 112.5 kVA”.

However, reliance upon only kVA rating can result insafety and performance problems when sizing transformers to feed modern electronic equipment.

Use of off-the-shelf, general purpose transformers for electronics loads can lead to power quality and siting problems:

  • Single phase electronic loads can cause excessive transformer heating.
  • Electronic loads draw non-linear currents, resulting in low voltage and output voltage distortion.
  • Oversizing for impedance and thermal performance can result in a transformer with a significantly larger footprint.

It is vital for the systems designer to understand all of the factors that affect transformer effectiveness and performance.

Thermal Performance

Historically, transformers have been developed to supply 60 Hz, linear loads such as lights, motors, and heaters. Electronic loads were a small part of the total connected load. A system designer could be assured that if transformer voltage and current ratings were not exceeded, the transformer would not overheat, and would perform as expected. A standard transformer is designed and specified with three main parameters: kVA Rating, Impedance, and Temperature Rise.

KVA Rating

The transformer voltage and current specification. KVA is simply the load voltage times the load current. A single phase transformer rated for 120 VAC and 20 Amperes would be rated for 120 x 20 = 2400 VA, or 2.4 KVA (thousand VA).


Transformer Impedance and Voltage Regulation are closely related: a measure of the transformer voltage drop when supplying full load current. A transformer with a nominal output voltage of 120 VAC and a Voltage Regulation of 5% has an output voltage of 120 VAC at no-load and (120 VAC – 5%) at full load – the transformer output voltage will be 114 VAC at full load. Impedance is related to the transformer thermal performance because any voltage drop in the transformer is converted to heat in the windings.

Temperature Rise

Steel selection, winding capacity, impedance, leakage current, overall steel and winding design contribute to total transformer heat loss. The transformer heat loss causes the transformer temperature to rise. Manufacturers design the transformer cooling, and select materials, to accommodate this temperature rise.

Transformer Heat Loss

Transformer Heat Loss

Use of less expensive material with a lower temperature rating will require the manufacturer to design the transformer for higher airflow and cooling, often resulting in a larger transformer. Use of higher quality materials with a higher temperature rating permits a more compact transformer design.

Transformer Insulation Systems

Transformer Insulation Systems


“K” Factor Transformer Rating

In the 1980′s, power quality engineers began encountering a new phenomenon: non-linear loads, such as computers and peripherals, began to exceed linear loads on some distribution panels. This resulted in large harmonic currents being drawn, causing excessive transformer heating due to eddy-current losses, skin effect, and core flux density increases.

Standard transformers, not designed for nonlinear harmonic currents were overheating and failing even though RMS currents were well within transformer ratings.

In response to this problem, IEEE C57.110-1986 developed a method of quantifying harmonic currents. A “k” factor was the result, calculated from the individual harmonic components and the effective heating such a harmonic would cause in a transformer. Transformer manufacturers began designing transformers that could supply harmonic currents, rated with a “k” factor. Typical “K” factor applications include:

  • K-4: Electric discharge lighting, UPS with input filtering, Programmable logic controllers and solid state controls
  • K-13: Telecommunications equipment, UPS systems, multi-wire receptacle circuits in schools, health-care, and production areas
  • K-20: Main-frame computer loads, solid state motor drives, critical care areas of hospitals

“K” factor is a good way to assure that transformers will not overheat and fail. However, “K” factor is primarily concerned with thermal issues. Selection of a “K” factor transformer may result in power quality improvement, but this depends upon manufacturer and design.

Transformer Impedance

Transformer impedance is the best measure of the transformer’s ability to supply an electronic load with optimum power quality. Many power problems do not come from the utility but are internally generated from the current requirements of other loads.

While a “K” factor transformer can feed these loads and not overheat, a low impedance transformer will provide the best quality power. As an example, consider a 5% impedance transformer. When an electronic load with a 200% inrush current is turned on, a voltage sag of 10% will result. A low impedance transformer (1%) would provide only a 2% voltage sag – a substantial improvement. Transformer impedance may be specified as a percentage, or alternately, in Ohms (Ω) from Phase- Phase or Phase-Neutral.

High Frequency Transformer Impedance

Most transformer impedance discussions involve the 60 Hz transformer impedance. This is the power frequency, and is the main concern for voltage drops, fault calculations, and power delivery. However, nonlinear loads draw current at higher harmonics. Voltage drops occur at both 60 Hz and higher frequencies. It is common to model transformer impedance as a resistor, often expressed in ohms. In fact, a transformer behaves more like a series resistor and inductor.

The voltage drop of the resistive portion is independent of frequency, the voltage drop of the inductor is frequency dependent.

Standard Transformer impedances rise rapidly with frequency. However, devices designed specifically for use with nonlinear loads use special winding and steel lamination designs to minimize impedance at both 60 Hz and higher frequencies. As a result, the output voltage of such designs is far better quality than for standard transformers.

Recommendations for Transformer Sizing

System design engineers who must specify and apply transformers have several options when selecting transformers.

Do It Yourself Approach

With this approach, a larger than required standard transformer is specified in order to supply harmonic currents and minimize voltage drop. Transformer oversizing was considered prudent design in the days before transformer manufacturers understood harmonic loads, and remains an attractive option from a pure cost standpoint. However, such a practice today has several problems:

  • A larger footprint and volume than low impedance devices specifically designed for non-linear loads
  • Poor high frequency impedance
  • Future loads may lead to thermal and power quality problems
Standard Isolation Transformer

Standard Isolation Transformer


“K”-factor Rated Transformers

Selecting and using “K”-factor rated transformers is a prudent way to ensure that transformer overheating will not occur. Unfortunately, lack of standardization makes the “K” factor rating a measure only of thermal performance, not impedance or power quality.

Percent Impedance

Percent Impedance

Some manufacturers achieve a good “K” factor using design techniques that lower impedance and enhance power quality, others simply derate components and temperature ratings. Only experience with a particular transformer manufacturer can determine if a “K” factor transformer addresses both thermal and power quality concerns.

Transformers Designed for Non-Linear Loads

Transformers designed specifically for non-linear loads incorporate substantial design improvements that address both thermal and power quality concerns. Such devices are low impedance, compact, and have better high frequency performance than standard or “K” factor designs. As a result, this type of transformer is the optimum design solution.

This type of transformer may be more expensive than standard transformers, due to higher amounts of iron and copper, higher quality materials, and more expensive winding and stacking techniques. However, the benefits of such a design in power quality and smaller size justify the extra cost, and make the low impedance transformer the most cost effective design overall.


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Caterpillar C175 Diesel Generator 2-4MW

Caterpillar C175 Diesel Generator 2-4MW

The C175 family of diesel generator sets offers the most power you can get in any single high-speed package: 2-4MW.

One of the most significant components in the development of the C175 was the integration of ACERT™ Technology into the engine platform.

ACERT Technology is a synergistic approach utilizing a suite of complementary building block technologies that can be individually adapted to accommodate a specific application. In recent years, Caterpillar has spent more than $1 billion on the development of clean diesel technologies. Today, more than 330,000 engines are currently in operation with ACERT Technology, accumulating more than 2 million hours of use each day.

With the C175, the building blocks of ACERT Technology have been tailored to meet the current and pending emissions requirements of stationary diesel generator sets in a variety of applications.


Thousands of hours of customer research created the foundation for the C175 design concept.

Some of the major advantages of the C175 family include:

  • Proven Reliability with platform based on industry standard Cat® 3500 series, and supported by thousands of hours of lab and field-testing.
  • Wider Power Range including 2000kW to 4000kW @ 1500 and 1800 rpm.
  • Power Generation at Higher Speed than traditional medium speed products in the same power range.
  • Higher Power Density equals more output from a given engine displacement / footprint, resulting in lower installed cost.
  • Complete Package including SR5 generators, EMCP3 package controls and package/remote radiator with flexible controls packaging options simplifies installation.
  • Lower Emissions meet U.S. EPA Tier 2 standards with a line of sight to meet U.S. EPA Tier 4 and EU Stage IIIB emissions levels.
  • Lower Maintenance Costs due to increased oil change intervals, longer life (durability) of components and longer top end as well as full overhaul periods.
  • Lower Operating Costs due to lower brake specific fuel consumption than competitive products.
  • Systems Integration. The C175 electrical system components are engineered to work together with a wide range of products such as Uninterruptible Power Supplies (UPS), Automatic Transfer Switches (ATS), switchgear, remote monitoring services and customer building SCADA systems.
  • Extensive Product Support from a worldwide dealer network with 24/7/365 parts and service availability.

Design Features

1. Fuel System

C175 - Fuel System

C175 - Fuel System

The new C175 engine features a Cat® Common Rail Fuel System designed specifically for this engine platform.

Full Control of Both Fuel Delivery and Fuel Pressure At Any Load or Speed results in superior transient response and block load acceptance, as well as shorter recovery time.

More Compact single camshaft that is used only to open the intake and exhaust valves. It features a simpler injection design with no pumping function necessary.

Improved Cold Start Capability uses higher pressures at low speeds and produces less smoke.

“Fluid Containment” Design. The high-pressure lines and rails are designed to provide outer concentric low-pressure containment. If a leak occurs in the high-pressure section, it leaks back to the outer low-pressure section and drains back to the tank.

Integrated Manifold or Monoblock offers a single point of connection to the engine, which eliminates leak paths while improving reliability.

Fuel Cooler Eliminated. Unlike the unit injector system, the Common Rail System used on the C175 does not require excess bypass fuel to cool the injector since injection pressure load is taken off the injector. The result is a reduction of heat generation in the return fuel and reduction in fuel flow rate by a factor of 4 when compared to the unit injector system. This eliminates the need for a fuel cooler in most cases.

Improved Fuel Filters. The C175 uses an eco-friendly fuel filter system. Instead of throwing away the whole canister, only the disposable non-metallic element inside the canister is changed.

Electronic Fuel Priming Pump is Engine Control Module (ECM)-controlled and offered as standard equipment. No manual effort is required to pump the fuel, so it’s more convenient and requires less operator effort.

2. Cooling System

C175 - Cooling System

C175 - Cooling System

The design philosophy for the Cat® C175 cooling system is to minimize heat rejection by cooling only the parts that require cooling.

Inlet-Regulated System. The C175 features an innovative design unique to Caterpillar. The system senses the temperature at the inlet and controls the output providing more consistent temperatures and better control of oil viscosity than an outlet-regulated system.

Electronic Fluid Temperature Controller regulates inlet temperature of the coolant and allows for troubleshooting without removing the thermostat. Improved diagnostics enable the operator to pinpoint a problem quickly, which increases reliability and uptime.

Integral Water Supply and Return Manifold are built into the engine block to minimize connection points and bolted joints. This design helps contain fluids and improves overall engine reliability and serviceability.

Two-Stage After-Cooler. The first stage is cooled by a jacket water circuit and the second stage has a two-pass separate circuit. The after-cooler is constructed with tube fin cores that are more robust compared to the traditional bar plate fin design. The tubes of the core can be cleaned without removing the core from the engine, and the core can be remanufactured. The tubes also have more surface area per volume and less pressure drop, resulting in more efficient cooling. This after-cooler design also minimizes the size of the SCAC circuit, thereby reducing the size of the radiator. The location of the first stage jacket water core provides protection from high air temperatures. These features improve the durability and reliability of the cooling system.

3. Air Management

C175 - Air Management

C175 - Air Management

Air management is one of the ACERT™ Technology building blocks used on the Cat® C175 engine.

Crossflow Head provides separation between both the intake and exhaust ports and manifolds. The outboard air manifold location eliminates re-heating of intake air by preventing heat transfer from the exhaust to the intake. This results in reduced charge air temperature and increased charge air density, enabling higher power density as well as reducing SCAC cooling.

Taller Head accommodates larger ports and helps direct a large amount of cool air into the cylinder with the least resistance, resulting in the best port performance of any engine in the world. The taller head also accommodates increased valve lift of 22mm compared to 18mm on the Cat 3500, further improving breathing.

Improved Breathing. The tall crossflow head results in a greater amount of cooler air in and out of the engine, which helps produce higher power ratings and lower emissions. This, along with lower air pumping losses, results in lower fuel consumption.

New Generation of Turbochargers designed specifically for the C175. Four single-stage turbochargers provide a higher pressure ratio in a single stage. The turbocharger includes a cast titanium impeller and an improved bearing system that provides a higher load-bearing capacity and greater reliability, while increasing efficiency by 5% and extending the component life when compared to traditional cast aluminum impellers.

4. Lube System

C175 - Lube Management

C175 - Lube Management

The C175 lube system features two piston-cooling jets per piston.

A large capacity oil pump pressure regulation valve allows the engine to maintain optimum oil pressure at all speeds, loads and throughout the life of the engine, ultimately increasing durability.




5. Core Engine Components

C175 - Core Engine Components

C175 - Core Engine Components

The components of the new C175 centerline engine are designed for higher strength, durability and compactness.

Crankshaft has a larger diameter to handle bigger loads. It is made of steel forged material and features induction hardened fillets and journals. Thrust plates are located at the rear end of the crankshaft to reduce motion inside of the coupling between the engine and generator.

Block is made of cast iron and provides increased strength and stiffness, and is lighter weight.

Mid-Support Liners provide stronger support to the liner and offer more efficient cooling by only cooling the top 25% of the cylinder liner. Mid-support liners allow for a smaller inside diameter of the combustion seal, as well as a higher position for the piston’s top ring. The result is reduced crevice volume, improved cooling and combustion efficiency, and reduced emissions. Mid-support liners allow the head bolts to be closer to the cylinder bore to minimize cylinder spacing and to create a more compact engine.

Cylinder Cuff is specially designed for improved durability. The ring of the “cuff” located at the top of the cylinder scrapes off carbon accumulation on the piston top end, preventing the carbon from polishing, scratching or seizing the liner. The cuff also helps reduce crevice volume beside the piston, resulting in lower emissions.

Cylinder Head is made of iron for added strength. The tall C175 head helps eliminate the external water manifold by returning the coolant to the cylinder block.

Head Gasket features a simplified two-piece (carrier seal and combustion seal) design, shortening service time, decreasing parts costs, and increasing reliability and durability.

Pistons and Rings feature increased oil flow to pistons for better cooling and higher power ratings. Rectangular piston rings provide a superior seal and less motion, resulting in less wear and longer life. Piston, rod and liner come out as one assembly, resulting in faster, easier service.

Connecting Rods. Large diameter fracture split connecting rods provide better alignment between the rod and cap, which eliminates the need for a special alignment procedure.

Bearings. Large main and rod bearings provide better seizure resistance and better tolerance of a wide range of oil temperatures. Larger rod bearing and main bearing are more scuff and seizure resistant.

6. Engine Management System

C175 - Engine Management System

C175 - Engine Management System

The Cat® C175 utilizes much of the ACERT™ Technology electronics experience gained on small-bore engines and employs many new improvements and technologies more useful on large bore engines. The C175 Engine Management System exploits the power of modern control technology to improve reliability, exceed customer expectations and accommodate future customer requirements.

Engine Control Module (ECM). C175 engine controls use the latest version of the ADEM A4 ECM to deliver 50 times the computing power of its predecessor. Specific benefits include monitoring over 30 points on the engine, driving up to 20 injectors, protecting the engine, communicating over 100 engine parameters to the customer, diagnosing and reporting on engine health. The ECM uses the latest advancements in ACERT Technology to improve engine performance while reducing emissions.

Engine Controls and Datalink. Three primary controllers are temperature control module, fuel high-pressure controller and ECM. These are connected to the engine J1939 datalink.

Rigid Wiring Harness. Metal enclosed rigid wiring harness system protects critical engine circuits from accidental damage, reducing service calls and increasing reliability.

Controls Packaging. The standard panel is a rear-mounted EMCP 3.1 with the option to upgrade to the EMCP 3.2 or EMCP 3.3.

7. Generator

C175 - Generator

C175 - Generator

Caterpillar is introducing the next evolution of generators, the SR5 Series, with the introduction of the C175 generator sets. The SR5 Series 1800 and 3000 frame generators have been designed specifically to work with the C175 engines. The structural design is matched to the C175 engine. Torsional and linear vibration analysis and testing have been performed to ensure durability.

The SR5 generator’s insulation system has been improved to meet insulation Class H. SR5 generators feature 2/3-pitch as standard on all low, medium and high voltage generators. SR5 generators have IP23 particle ingress protection.

Generator Set Packaging. The C175 uses a fusible coupling to connect the generator to the engine. All engines, generators and controls are tested individually prior to assembly. Once assembled, the entire generator set package is tested before shipping to dealers to ensure quality.


The versatility of the C175 makes it ideal for a variety of applications.

  • Continuous – A continuous rating has a typical load factor of 70% to 100% with no limit on the number of hours per year. Typical peak demand is 100% of continuous rated kW for 100% of operating hours. Typical applications include base load, utility or co-generation.
  • Prime – A prime rating has a typical load factor of 60% to 70% with no limit on the number of hours per year. Typical peak demand is 100% of prime rated kW with 10% overload available for emergency use for up to one hour in 12. Typical applications include industrial, pumping, construction, peak shaving or co-generation.
  • Standby – A standby rating has a typical load factor of 70% or less with variable load for about 200 hours per year,with a maximum expected usage of 500 hours per year. Typical peak demand is 80% of the standby rated kW with power available for the duration of an emergency outage. Typical applications include building service standby or emergency standby.
  • Load Management – A load management rating has a typical load factor of 100% of the prime rating for a maximum of 500 hours per year. Typical peak demand is 100% of the load management rating, with no overload available. Typical applications include base load or peak shaving.

SOURCE: Caterpillar C175


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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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