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5.3 Tanks and Enclosures
5.3.1 Oil Preservation
The transformer oil acts as an insulation and heat transfer medium. To keep good insulating properties the oil must be dry and free from contaminants.
The transformer tank has to be strong enough to take the mass of oil (plus the core and windings) and to allow lifting and haulage into position.
The simplest way of keeping the oil in good condition is to seal the oil inside the tank and not permit any contact with the atmosphere. However, the oil volume changes as the transformer heats up and it is necessary to allow for this expansion in the tank design. The following methods may be used depending on the rating of the transformer, its location and the particular policy of the manufacturer:
1. Sealed rigid tank -- Oil expansion is catered for by not completely filling the tank with oil. The space above the oil is filled with a dry inert gas, such as dry nitrogen, which has no chemical reaction with the oil.
Pressure changes within the tank are relatively large and require a stronger tank than normal. This type of tank construction is common in the USA up to quite large ratings of, say, 50 MVA, but rather uncommon in Europe above about 10 MVA at 33 kV.
2. Sealed expandable tank -- There is a limit to the size of transformer that can successfully use this technique. Ratings up to 2 MVA at 11 kV are normally satisfactory. The tank is completely filled with oil but the surfaces are flexible (usually corrugated) in order to allow for changes in the effective volume of oil with temperature. Distribution transformers can be specified as hermetically sealed units with a semi-flexible tank of corrugated appearance to take up the forces of oil expansion on heating.
3. Positive pressure nitrogen -- This method applies to very large transformers and has the advantage of a sealed rigid tank system but with pres sure changes minimized by a venting and topping-up method using an external nitrogen supply. The nitrogen is kept at a small pressure above atmospheric by gas supply cylinders attached to the tank and operating through pressure sensing valves. Careful maintenance of the gas supply and supply valves is necessary.
4. Conservator (with breather) -- This method may be used for virtually any size of transformer. The main tank is completely filled with oil and changes in volume are allowed by an expansion tank (conservator) mounted above the main tank. The conservator has a vent to the atmosphere. To avoid excessive moisture intake, an air drying device is used in the vent. Such drying is either by silica gel crystals, or more effectively, by a refrigerant drier unit. Careful oil maintenance is necessary especially at transmission voltages. The refrigerant drier is widely used at 275 kV and 400 kV in UK.
5. Conservator (with diaphragm seal) -- This method is used by some manufacturers to give the advantages of an expansion tank system but without contact with the atmosphere. The expansion tank contains a flexible synthetic rubber diaphragm which allows for oil expansion, but seals the oil from the atmosphere (see FIG. 23). In theory, oil maintenance is less than with the breather systems but a disadvantage is that any moisture and contaminants trapped in the oil during manufacture may be sealed in for life.
The quality of tank welding, gasketing and painting must be carefully specified and inspected prior to release from the manufacturer's works. Such precautions will avoid oil leakage due to poor welds or gasket assembly and ensure suitable paint thickness, finish and application procedures for harsh environments.
5.3.2 Dry Type Transformer Enclosures
Dry type transformers have some physical protection around them to keep personnel away from live parts, and to protect the core end windings from dust, water ingress, condensation, etc. A sheet steel enclosure or, more simply, an open steel mesh surround may be specified for indoor applications depending upon the IP classification required.
FIG. 23 Arrangements using expansion bag in the conservator.
5.4 Cooling Plant
Oil cooling is normally achieved by heat exchange to the surrounding air.
Sometimes a water jacket acts as the secondary cooling medium. Fans may be mounted directly onto the radiators and it is customary to use a number of separate fans rather than one or two large fans.
Oil pumps for OFAF cooling are mounted in the return pipe at the bottom of the radiators. The motors driving the pumps often use the transformer oil as their cooling medium.
With ODAF cooling, the oil-to-air coolers tend to be compact and use relatively large fan blowers. With this arrangement the cooling effectiveness is very dependent upon proper operation of the fans and oil pumps since the small amount of cooling surface area gives relatively poor cooling by natural convection alone.
Water cooling (ODWF) has similar characteristics to the ODAF cooling described above and is sometimes found in power station situations where ample and well-maintained supplies of cooling water are available. Cooling effectiveness is dependent upon the flow of cooling water and therefore on proper operation of the water pumps. Natural cooling with the out-of-service water pumps is very limited. Operational experience has not always been good, with corrosion and leakage problems, and the complexity of water pumps, pipes, valves and flow monitoring equipment. The ODAF arrangement is probably favorable as a replacement for the ODWF designs.
Double wall cooler pipes give added protection against water leakage. The inner tube carries the water and any leakage into the outer tube is detected and causes an alarm. This more secure arrangement is at the expense of slightly reduced heat transfer for a given pipe size.
Normal practice with cooling plant is to duplicate systems so that a failure of one need not directly affect operation of the transformer. Two separate radiators or radiator banks and duplicate oil pumps may be specified. In the larger ODAF cooling designs there may be four independent unit coolers giving a degree of redundancy. The transformer may be rated for full output with three out of the four coolers in service.
Dry type transformers will normally be naturally air-cooled (classification AN) or incorporate fans (classification AF).
5.5 Low Fire Risk Types
5.5.1 General
Mineral oil-immersed transformers present a potential fire hazard. The spreading of a fire resulting from a transformer fault is limited by including a bund and oil-catch pit arrangement in the civil installation works. In sensitive locations such as inside buildings, power station basements, offshore oil rigs, underground railways, etc. dry type construction or non-flammable oil-cooled transformers may be specified. The possible instances where a transformer may be involved in a fire may be considered to fall into three categories:
1. An internal fault leads to ignition and subsequent burning of the materials within the transformer. However, note that transformers are normally protected by overcurrent devices which should clear arcing faults in 0.5 seconds or less and even in poorly protected cases in less than approximately 4 seconds. Under such conditions, a cast resin transformer should not allow any small flames produced to be sustained for, say, longer than 45 seconds.
2. The transformer is housed in an enclosed space involving traditional building materials (wood, etc.) which could ignite and engulf the transformer in flames with temperatures of 800 degrees C to 900 degrees C. The contribution of the transformer to the fire should be severely limited and should not emit toxic smoke or fumes, and visibility should not be greatly impaired due to the transformer smoke contribution.
3. The transformer is housed in an enclosure in which a fire involving hydrocarbon fuels or plastic materials (oil, polythene, etc.) occurs engulfing the transformer in flames with temperatures in excess of 1,000 degrees C.
5.5.2 Dry Type Transformers
Dry types are available up to 10 MVA and 36 kV with cast resin or conventional dry type Class 180 (previous categorization H) insulation temperature limits. The fully cast resin-encapsulated transformer units have the following advantages:
- Unaffected by humidity, dust, etc.
- Relatively simple assemblies using few insulating materials and less prone to electrostatic stress.
- High thermal time constant and superior short circuit withstand giving good overload performance often better than conventional air-cooled types.
- Avoids non-biodegradable problems associated with polychlorinated biphenyls (PCBs) or Askarels which are now banned from use.
A cast resin 1 MVA- 21/0.4 kV transformer with a sheet metal IP 21 enclosure is shown in FIG. 24. The interconnections for the delta configuration are clearly visible together with 'off-circuit' tap connections and links.
FIG. 24 1 MVA 21/0.4 kV cast resin transformer.
5.5.3 Non-flammable Liquids
There are a number of non-flammable liquids available, with little to choose between them. Some types are not as good as mineral oil in heat conduction or in lubrication properties, so there may be some minor design differences between a mineral oil-immersed unit and a non-flammable liquid-immersed unit. Some older transformers may be filled with Askarel or similar non flammable fluid. This fluid is not permitted now for new installations in most countries due to its high level of toxicity.
Both dry type and non-flammable liquid-immersed types will cost more than an equivalent mineral oil-immersed unit. Possible reduced civil works must be taken into account when assessing overall dry type transformer installation costs.
5.5.4 Fire Protection
Transmission system transformers are generally only protected to the extent that oil spillage from a burst tank is contained. The oil drains through adjacent stonework and is held in a bund with a wall surround approximately 300 mm high. The draining and cooling of the oil through the stone chip pings into the bund is intended to extinguish flame and the wall prevents pollution to natural drainage. Additional protection for outdoor installations may be offered by temperature sensors located above the transformer which initiate a water spray or foam system to extinguish the fire. Indoor installations may use CO2 or a modern replacement for halon gas.
5.6 Neutral Earthing Transformers
The single zigzag-connected winding is all that is needed to provide the neutral earthing facility as shown in FIG. 16c. A secondary earthing transformer winding is often specified to provide a substation low-voltage auxiliary power source. An explanation of such earthing transformer selection is given in Section 4, Sec. 4.4.6.
5.7 Reactors
5.7.1 General
Reactors have single windings and are intended to provide inductive reactance. Shunt reactors are connected to the system to provide an inductive load for the purposes of compensating the capacitive loads of cables and lightly loaded overhead lines.
Series reactors are connected in series with a circuit in a system to reduce fault currents, or in some instances to balance the impedance between two parallel paths.
An air-cored coil will have a relatively low inductive reactance and above about 145 kV the impulse withstand requirements limit their use. Such reactors are often suitable for series reactors or reactive compensation schemes.
Air-cored construction offers the cheapest solution up to certain MVAr and voltage ratings. FIG. 25 shows a 60 MVAr air-cored reactor associated with the Channel Tunnel single phase 25 kV traction load-to-three phase 132 kV supply reactive compensation balancer scheme. Higher values of reactance can be achieved by introducing a magnetic core but with a deliberate air gap in series with the steel components. Shunt reactors will often use this method.
Steel tank reactors are normally oil-filled, with similar cooling requirements to oil-filled transformers. They are more expensive than equivalent air-cored units, but have much lower external magnetic fields (see SEC. 5.7.2), which may be important in urban or industrial areas.
5.7.2 Assessment of Acceptable Levels of Magnetic Fields
The World Health Organization published Environmental Health Criteria 69 in 1987 and considers that the magnetic field strength not considered to pro duce any biological effects is about 0.4 milli Tesla for 50 Hz or 60 Hz. The maximum rate of change of magnetic field for some heart pacemakers to remain synchronous is 40 milli Tesla/second. This has been calculated to correlate to a 50 Hz field strength of 0.12 and 0.1 milli Tesla at 60 Hz.
Warnings are also given concerning metallic implants but without any specific restriction levels. These figures are of the same magnitude as other guidelines concerning exposure to static and time varying electromagnetic fields and radiation. A conservatively safe value of exposure and reference level currently under consideration is 40 milli Tesla divided by the frequency. At 50 Hz this corresponds to 0.8 milli Tesla.
FIG. 25 60 MVAr air-cored reactor.
It is obviously necessary to take these values into account especially when dealing with large air-cored reactors. An estimation of the field strength should be made at the design stage and practical precautions taken to screen or fence off areas on site. Particular attention must be paid to the level of magnetic field strength likely to result in adjacent public areas and staff working areas. Adequately screened or oil-filled reactors may have to be specified if safe levels cannot otherwise be achieved.
The civil works must take into account the effects of induced currents in reinforcement and switchyard fencing. Reinforcement must be segregated into short sections and loops avoided by use of spacers made of insulating materials. The switchyard fence close to high field sources must also be divided into short sections and precautions taken with earthing arrangements in order to avoid circulating currents.
5.7.3 Underground Transformers
In dense urban areas where substation sites are difficult to obtain, distribution transformers may be directly buried or installed in underground chambers. Small units up to 1 MVA may directly buried, and are typically ON/AF types in Europe or solid insulation AN/AF in the USA, in each case with air ducts leading to a radiator on the surface. Particular care has to be taken to minimize the effect of soil drying; use of thermal backfill is one means to achieve this. Maintenance facilities are minimal, and problems have arisen in some cases with tank corrosion of directly buried units.
In exceptional cases special designs have been used for transformers up to 100 MVA in underground substations, but these are in fully accessible rooms, with carefully built arrangements for allowing future replacement.
6. ACCESSORIES
6.1 General
The basic transformer assembly of windings, core, tank and terminations is supplemented by a number of accessories for monitoring, protection, and safety purposes.
Some accessories are optional and will not necessarily be justified on every transformer; others are important to the safety or operation of the transformer and will therefore be of a mandatory nature.
The following sections briefly describe some accessories available.
6.2 Buchholz Relay
A Buchholz relay is connected in the oil feed pipe connecting the conservator to the main tank. The relay is designed to:
- Detect free gas being slowly produced in the main tank, possibly as a result of partial discharging. Under such conditions the relay may be set to give an alarm condition after a certain amount of gas has evolved.
Examples of incipient faults include broken down core bolt insulation on older transformers, shorted laminations, bad contacts and overheating in part of the windings.
- Detect a sudden surge movement of oil due to an internal transformer fault. Under such conditions the relay is normally set to trip the high and low voltage transformer circuit breakers. Examples of such oil surge faults include earth faults, winding short circuits, puncture of bushings and short circuits between phases.
- Provide a chamber for collection and later analysis of evolved gas.
Chemical analysis of the gas and transformer oil can give maintenance staff an indication as to the cause of the fault.
Buchholz relays are considered mandatory for conservator type transformers since they are protective devices. They should be installed in accordance with the manufacturers' instructions, since a certain length of straight oil piping is required either side of the relay to ensure correct operation.
From time to time maloperation of Buchholz relays is reported. This is often due to vibration effects and a relay designed for seismic conditions may overcome the problem.
6.3 Sudden Pressure Relay and Gas Analyzer Relay
On non-conservator type transformers, the useful protective and gas analysis feature of the Buchholz relay cannot be provided. In its place, a sudden pres sure relay detects internal pressure rises due to faults, and gas devices can be used to detect an accumulation of gases.
Sudden pressure relays are normal accessories for sealed transformers.
Gas analyzer devices tend only to be used on large important transformers.
6.4 Pressure Relief Devices
A pressure relief device should be regarded as an essential accessory for any oil-immersed transformer. Very large transformers may require two devices to adequately protect the tank. Violent pressures built up in the transformer tank during an internal fault could split the tank and result in the hazardous expulsion of hot oil. In order to avoid tank rupture resulting from the high pressures involved in an internal transformer fault a quick acting pressure relief device is specified and used to give a controlled release of internal pressure.
Older transformers may have been fitted with a rupturing diaphragm type device where the excess pressures breaks a fragile diaphragm and allows oil to be discharged. Not only does this not reseal but the overall operating time may be too slow to protect the tank against splitting.
6.5 Temperature Monitoring
A correctly specified and loaded transformer should not develop excessive temperatures in operation. Oil and winding temperature is monitored in all but small (say, less than 200 kVA) distribution transformers.
Apart from the facility to monitor temperature (useful during controlled overloading), an important feature of the winding temperature indicator is to initiate automatic switch-on and switch-off of cooling fans and oil circulation pumps. In this way a dual rated transformer with a cooling classification of, for example, ONAN/ONAF will automatically switch from ONAN to ONAF (and back) according to the transformer loading conditions. The winding temperature monitor on an oil-immersed transformer simulates the tempera ture by using an oil temperature sensor and injecting additional heat into the sensor from a current transformer connected to one of the transformer terminals. In this way the winding temperature monitor registers a temperature above that of the oil by an amount that is dependent on the load current of the transformer. This arrangement is usually calibrated on site and is used to indicate the hot spot winding temperature.
The oil temperature monitor is usually a capillary type thermometer with the sensor placed in the vicinity of the hottest oil in the tank (i.e. at the top of the tank just prior to entering the radiators).
Both oil and winding temperature monitors are fitted with contacts which can be set to operate at a desired temperature. Such contacts are used for alarm (and possibly trip) purposes and also to operate auxiliaries as noted above.
Alarm and trip temperature settings are usually advised by the manufacturer. Note that it will usually be necessary to modify the settings if the transformer is used for controlled overloading since winding and oil tempera ture are allowed to reach higher temperatures during overloading than during normal operation.
Dry type transformers incorporate thermistor probes through the resin, usually by the low voltage winding hot spot area. Negative temperature coefficient thermistors are available with a resistance range from some 2,000 O at 200 degrees Cto1MO at ambient temperature with an accuracy of some 63%.
Settings for alarm and trip conditions may be made using an electronic control device.
6.6 Breathers
As noted in SEC. 5.3, breathers are placed in the vent pipes of conservators to dry the air entering the conservator as the volume of oil contracts on transformer cooling.
Traditional breathers use the moisture absorbing properties of silica gel crystals. These crystals need replacement when they become saturated with moisture. Replacement is indicated by a change in colour of the crystals from blue to pink.
An alternative technique is to continuously extract moisture dissolved in the transformer oil by freezing the moisture out of the air by passing it over refrigerating elements and then evaporating it off to the atmosphere. This approach is used in the 'Drycol' device shown in FIG. 26. The oil is kept particularly dry, and researches have shown an improved life span of the transformer. The 'Drycol' device is a standard fitting on 275 kV and 400 kV transformers in UK.
FIG. 26 Principle of the Drycol breather _ showing direction of air flow
during a drying period (GEC Transformers Ltd).
6.7 Miscellaneous
6.7.1 Core Earth Link
The magnetic core is earthed in order to the transformer tank at one point only to prevent a metallic path for circulating currents. It is useful to specify that the earth is made through a removable link so that on-site tests can be made to check for other core earths that may have been produced by rough handling of the transformer or a fault in service.
Some manufacturers may have difficulty with this facility and it is worth ensuring during manufacture that a truly accessible link is being provided.
6.7.2 Oil Level Gauge
The simplest arrangement is a sight glass, but on many transformers a dial type indicator will be used. The correct oil level should be shown on the indicator for a range of ambient temperatures appropriate to the site. Low oil level contacts can be used to provide an alarm.
Transformers with expandable tanks, or with diaphragm seals, do not have a free oil level. In the case of the diaphragm seal it may be possible in some designs to attach the float of a conventional oil level indicator to the diaphragm seal in order to provide an alarm for seal breakage.
6.7.3 Tap Changer Accessories
An on-load tap changer mechanism should be equipped with an oil surge detection relay to indicate a fault within the tap change compartment if this is separate from the main tank. Gas evolution during tap changing is not abnormal so if a conventional Buchholz relay is applied the gas collection facility is not used. With the double-compartment tap changer arrangement the diverter and selector are located in different compartments and full Buchholz protection is possible.
6.7.4 Oil-Sampling Valve
Routine maintenance of transformers includes the testing of oil for moisture, contaminants, and possibly also for dissolved gas content. An oil-sampling valve is therefore a necessary accessory on most transformers. Oil sampling is not common on sealed distribution transformers which may be regarded as 'sealed for life.'
6.8 Transformer Ordering Details
6.8.1 Specifications
The primary objective of the purchaser's specification for a transformer should be to state precisely the duty the transformer is to perform and the conditions under which it will operate. Having specified these, the details of the design and construction should, as far as possible, be left up to the manufacturer. To specify intimate details in the design, such as current density or flux density, merely ties the hands of the designer and could well result in an inefficient or unnecessarily expensive design.
The purchaser's main concern is to ensure that as far as possible every new transformer purchased is capable of performing under the specified operating conditions for a service life in the order of 40 years. Key activities in ensuring this are the contract specification for the transformer, quality assurance during design and manufacture, effective testing before leaving the manufacturer's works and on site and appropriate maintenance and diagnostic testing in service. Most electrical supply utilities now rely on functional or technical specifications which cover the requirements of their own particular company. These are reinforced with an approval procedure which includes a design review to ensure product compatibility with their particular needs.
The best use should therefore be made of recognized specifications typically as described in SEC. 2.6.
6.8.2 Rated Power and Rated Voltage
A major item which is sometimes subject to confusion is the rated power of the transformer. Taking into account the anticipated loading conditions, IEC 60076-10 gives examples for determining the required rated power of a transformer for a given set of loading conditions.
Transformers are assigned a rated power for each winding which refers to a continuous loading. This is a reference value for guarantees and tests concerning the load losses and temperature rises. A two winding transformer has only one value of rated power (identical for both windings). For multi-winding transformers the rated power of each winding should be stated. In the case of a three-winding transformer, the rated power and voltage of each winding must be known in order to determine steady state operation.
The rated power for the three phase case given by IEC definitions is:
Rated power = ??? x p x rated winding voltage x rated winding current
The rated secondary voltage is given as the no-load voltage on the secondary side of the transformer with rated voltage applied to the primary winding. This differs from the full load secondary voltage by the amount of voltage drop through the short circuit impedance of the transformer.
According to this definition the rated apparent power is the power input, taken by the primary from the supply, and not the power output delivered to the load (which is smaller due to the secondary voltage drop on full load).
This differs from the ANSI definition which defines rated power as the out put supplied at rated secondary voltage.
6.8.3 System Parameters
It is important that the transformer designer receives sufficient information on the system to determine the conditions under which the transformer will operate. The following information must be supplied together with any special conditions relating to the installed location, increased clearances and creepages due to atmospheric pollution, etc.:
- Range and variation of system voltage and frequency.
- Required insulation levels of line and neutral terminals.
- System fault level.
- Altitude if in excess of 1,000 m.
6.8.4 Technical Particulars and Guarantees
Tables 2 Parts 1 to 3 are intended to assist the reader in covering all the points raised in this Section when specifying oil-immersed power transformers. Another similar set of guidance -- particularly covering electrical aspects -- is given in annexes A and B of IEC60076-1. Details of test requirements are covered in more detail in Section 19. Environmental conditions together with general power system technical details (primary and secondary nominal voltages and tolerances, frequency, earthing arrangements, BIL, etc.) and a single line diagram showing the basic protection arrangements must also be included with the tender specifications.
[coming soon] TABLE 2 Part 1 -- Power Transformer Technical Particulars and Guarantees
TABLE 2 Part 1 covers the main electrical aspects. Remember to quote the no-load voltage ratio and to allow for regulation. The required percentage impedance must also be specified at a known transformer rating and tap.
This is especially important for dual rating transformers (ONAN/ONAF, etc.) in order to avoid confusion. Such items as flux density are best determined in conjunction with the manufacturer. Any loss capitalization formula must be included at the enquiry stage in order to allow the transformer manufacturer to optimize the design and for the purchaser to obtain competitive prices on an equal basis.
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[coming soon] TABLE 2 Part 2 -- Power Transformer Accessories and Physical Details, Particulars and Guarantees
Item -- Characteristics
Fittings and equipment (yes/no) On-load tap changer switch Lifting lugs Jacking lugs Holding down bolts Wheel flanges for rail mounting Skid underbase Conservator Drain valve Filter valve Oil-cooling system valve Oil-sampling device Thermometer pocket Oil level indicator Silica gel breather Pressure-relief device with alarm and trip contacts Earth terminals Oil level indicator with alarm contact Oil temperature indicator with alarm and trip contacts Oil temperature cooling system initiation contacts Gas/oil-operated relay alarm and trip (gas and surge) contacts Surge arresters on HV side as part of overall substation design Surge arrester counters on each phase Cooling system fault relay
Rating plate Bushing CTs to suit overall protection design Neutral CTs to suit overall protection design Auxiliaries supply voltage Single phase V Hz Three phase V Hz Transformer weights and dimensions Thickness of transformer tank Top mm Sides mm Bottom mm Radiator and/or cooling tubes mm No. coolers or cooling banks No.
Rating of each cooler or cooler bank Overall length mm Overall width mm Overall height mm Overall largest dimensions for transportation L mm3W mm3H mm Oil weights and capacities Total quantity of oil required l Filling medium of tank for shipment (note some transformers are shipped under dry nitrogen) Filling medium of coolers for shipment Oil quantity required to cover windings for shipment l
Total capacity of conservator l Quantity of oil in the conservator between the visible levels l highest and lowest Weight of core and windings kg Weight of each cooler complete with oil kg Total weight of transformer kg Weight of heaviest piece of transformer for shipment/transport kg Details of transport to site limitations (site location, lifting facilities at port and site, road and rail limitations, etc.) Impact recorders Yes/no or number required Additional information Brochure and technical detail references Note: Details of system parameters and environmental conditions (voltage and frequency variations, altitude if in excess of 1,000 m, temperature, etc.) also to be specified.
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[coming soon] TABLE 2 Part 3 _ Power Transformer Test Details, Particulars and Guarantees
[coming soon] TABLE 2 Part 2 covers accessories and physical details which are necessary in order to make arrangements for transport to site and also to allow civil design to proceed.
[coming soon] TABLE 2 Part 3 covers possible overload requirements and transformer tests. Type and special tests are expensive and may not be necessary if similar units made by the same manufacturer are already in satisfactory service with test certification. In any event, only one unit of each type being manufactured under a contract will normally require such special testing.
[coming soon] TABLE 3 details noise levels that leading manufacturers are able to con form to without serious cost penalties to the purchaser.
[coming soon] TABLE 3 Audible Sound Levels for Oil-Immersed Power Transformers