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1.1 Background
Visual inspection of the transformer exterior reveals important condition information. For example, valves positioned incorrectly, plugged radiators, stuck temperature indicators and level gauges, and noisy oil pumps or fans. Oil leaks can often be seen which indicate a potential for oil contamination, loss of insulation, or environmental problems. Physical inspection requires staff experienced in these techniques.
1.2 Temperature Indicators Online
Winding temperature indicator
Check all temperature indicators while the transformer is online. The winding temperature indicator should be reading approximately 15 degrees above the top oil temperature. If this is not the case, one or both temperature indicators are malfunctioning. Check the top oil temperature next to the top oil indicator’s thermowell with an infrared camera.
Compare the readings with the top oil indicator. Reset all maximum indicator hands on the temperatures indicating devices after recording the old maximum temperature readings.
High temperature may mean overloading, cooling problems, or problems with windings, core, or connections.
1.3 Temperature Indicators Offline
When the transformer is offline and has cooled to ambient temperature, check the top oil and winding temperature indicators; both should be reading the same. If not, one or both temperature indicators are malfunctioning. Check the calibration according to the proper procedure. Also compare these readings with the indicated temperature on the conservator oil level indicator; all three should agree.
1.4 Conservator
Figure 1.—Conservator Oil Level
Check the oil level gauge on the conservator. See figure 1 at right. This gauge indicates oil level by displaying a temperature. Compare the indicated temperature on the conservator level gauge with the top oil temperature indicator. They should be approximately the same.
Calibrate or replace the conservator oil level indicator if needed, but only after checking the top oil temperature indicator as shown in the above section. Reference also IEEE 62-1995™ [11], section 6.6.2. If atmospheric gases (nitrogen, oxygen, carbon dioxide) and perhaps moisture increase suddenly in the DGA, a leak may have developed in the conservator diaphragm or bladder. With the transformer offline and under clearance, open the inspection port on top of the conservator and look inside with a flashlight. If there is a leak, oil will be visible on top of the diaphragm or inside the bladder. Reclose the conservator and replace the bladder or diaphragm at the first opportunity by scheduling an outage. If there is no gas inside the Buchholz Relay, the transformer may be re-energized after bleeding the air out of the bladder failure relay.
A DGA should be taken immediately to check for O2, N2, and moisture. However, the transformer may be operated until a new bladder is installed, keeping a close eye on the DGAs. It is recommended that DGAs be performed every 3 months until the new bladder is installed. After the bladder installation, the oil may need to be de-gassed if O2 exceeds 10,000 ppm. Also, carefully check the moisture level in the DGAs to ensure it is below recommended levels for the particular transformer voltage. Check the desiccant in the breather often; never let more than two-thirds become discolored before renewing the desiccant. All efforts should be made to keep the oxygen level below 2,000 ppm and moisture as low as possible.
1.5 Conservator Breather
Figure 2.—Conservator Breather
Check the dehydrating (desiccant) breather for proper oil level if it is an oil type unit. Check the color of the desiccant and replace it when approximately one-third remains with the proper color. See figure 2 for a modern oil type desiccant breather. Notice the pink desiccant at the bottom of the blue indicating that this portion is water saturated. Notice also that oil is visible in the very bottom 1-inch or so of the unit.
Many times, the oil is clear, and the oil level will not be readily apparent. Normally, there is a thin line around the breather near the bottom of the glass; this indicates where the oil level should be.
Compare the oil level with the level indicator line and refill, if necessary. Note the 1¼-inch pipe going from the breather to the conservator. Small tubing (½ inch or so) is not large enough to admit air quickly when the transformer is de-energized in winter. A transformer can cool so quickly that a vacuum can be created from oil shrinkage with enough force to puncture a bladder. When this happens, the bladder is destroyed; and air is pulled into the conservator making a large bubble.
1.6 Nitrogen
If the transformer has a nitrogen blanket, check the pressure gauge for proper pressure. Look at the operators recording of pressures from the pressure gauge. If this does not change, the gauge is probably defective. Check the nitrogen bottle to insure the nitrogen is the proper quality (see PEB No. 5 [20]). Check for any increased usage of nitrogen which indicates a leak. Smaller transformers such as station service or smaller generator-step-up transformers may not have nitrogen bottles attached to replace lost nitrogen. Be especially watchful of the pressure gauge and the operator’s records of pressures with these.
The pressure gauge can be defective for years, and no one will notice. The gauge will read nearly the same and will not vary much over winter and summer or night and day. Meanwhile, a nitrogen leak can develop; and all the N2 will be lost. This allows air with oxygen and moisture to enter and deteriorate the oil and insulation. Watch for increased oxygen and moisture in the DGA. An ultrasonic and sonic leak detection instrument (P-2000) is used for locating N2 leaks.
1.7 Oil Leaks
Oil Leaks
Check the entire transformer for oil leaks. Leaks develop due to gaskets wearing out, ultraviolet exposure, taking a “set,” or from expansion and contraction, especially after transformers have cooled, due to thermal shrinkage of gaskets and flanges. Many leaks can be repaired by applying an epoxy or other patch.
Flange leaks may be stopped with these methods using rubberized epoxy forced into the flange under pressure. Very small leaks in welds and tanks may be stopped by peening with a ball-peen hammer, cleaning with the proper solvent, and applying a “patch” of the correct epoxy.
Experienced leak mitigation contractors whose work is guaranteed may also be employed. Some leaks may have to be welded. Welding may be done with oil in the transformer if an experienced, qualified, and knowledgeable welder is available. If welding with oil in the tank is the method chosen, oil samples must be taken for DGA both before and after welding. Welding may cause gases to appear in the DGA and it must be determined what gases are attributed to welding and which ones to transformer operation.
1.8 Pressure Relief Device
With the transformer under clearance, check the pressure relief device indicating arm on top of the Figure 3.— Pressure Relief Device. transformer to see if it has operated. If it has operated, the arm will be in the up (vertical) position, and alarm and shutdown relays should have activated.
Figure 3.— Pressure Relief Device
CAUTION:
Do not re-energize a transformer after this device has operated and relays have de-energized the transformer, until extensive testing to determine and correct the cause has been undertaken. Explosive, catastrophic failure could be the result of energization after this device has operated.
1.9 Oil Pumps
If the transformer has oil pumps, check flow indicators and pump isolation valves to ensure oil is circulating properly. Pump motor(s) may also have reversed rotation, and flow indicators may still show that oil is flowing. To ensure motors are turning in the proper direction, use an ammeter to check the motor current. Compare results with the full-load-current indicated on the motor nameplate. If the motor is reversed, the current will be much less than the nameplate full-load-current. Check oil pumps with a vibration analyzer if they develop unusual noises.
Have the DGA lab check for dissolved metals in the oil and run a metal particle count for metals if the bearings are suspect. This should be done immediately, as soon as a bearing becomes suspect; bad oil-pump bearings can put enough metal particles into the oil to threaten transformer insulation and cause flashover inside the tank. An explosive catastrophic failure of the transformer tank could be the result.
1.10 Fans and Radiators
Inspect all isolation valves at the tops and bottoms of radiators to ensure they are open. Inspect cooling fans and radiators for cleanliness and fans for proper rotation. Check for dirty or damaged fan blades or partially blocked radiators. Fans are much more efficient if the blades are clean and rotating in cool air. Normally, fans blow cool air through the radiators; they should not be pulling air through. Check to see if fans are reversed electrically (i.e., pulling air first through the radiators and then through the fan blades). This means the blades are rotating in warm air after it passes through the radiator which is much less efficient. Place a hand on the radiator opposite the fans; air should be coming out of the radiator against your hand.
Watch the blades as they rotate slowly when they are starting or stopping to determine which way they should be rotating and correct the rotation if necessary.
1.11 Buchholz Relay
Figure 4.—Buchholz Relay
Inspect the isolation valve on the Buchholz relay to ensure it is open. With the transformer offline and under clearance, examine the Buchholz relay by lifting the window cover (center in figure 4 at right) and looking inside. If there is gas inside, the oil will be displaced, and the gas will be evident as a space on top the oil. If sufficient gas is found to displace the upper float, the alarm should be activated. The small valve at the top left is to bleed the gas off and reset the relay. If a small amount of gas is found in this relay when the transformer is new (a few months after startup), it is probably just air that has been trapped in the transformer structure and is now escaping; there is little cause for concern.
If the transformer has been on line for some time (service aged), and gas is found in the Buchholz, oil samples must be sent to the lab for DGA and extensive testing. Consult with the manufacturer and other transformer experts. A definite cause of the gas bubbles must be determined and corrected before re-energization of the transformer.
1.12 Sudden Pressure Relay
Figure 5.—Sudden Pressure Relay
An example relay is shown in figure 5 at the left. The purpose of this relay is to alarm if there is a sudden pressure rise inside the tank. This relay is very sensitive and will operate if the pressure rises only a little. If a very small pressure change occurs caused by a small electrical fault inside the tank, this relay will alarm. In contrast, the pressure relief device (shown above in figure 5) operates if a large pressure builds inside the tank caused by heavy arcing and heating causing the oil to boil and bubble. Inspect the isolation valve to ensure it is open.
With the transformer offline and under clearance, functionally test the sudden pressure relay by slowly closing the isolating valve. Leave it closed for a few seconds and reopen the valve very suddenly; this should activate the alarm. If the alarm does not activate, test the relay, and replace it with a new one if it fails to function.
1.13 Bladder Failure Relay
On newer transformers, a bladder failure relay may be found on or near the conservator top on the oil side of the bladder. This relay is near the highest point of the transformer. Its purpose is to alarm if the bladder fails and admits air bubbles into the oil.
The relay will also serve as a backup to the Buchholz relay. If the Buchholz relay overfills with gas and fails to activate an alarm or shutdown, gas will bypass the Buchholz and migrate up into the conservator, eventually to the bladder failure relay. See figure 6. Of course, these gases should also show up in the DGA. However, DGAs are normally taken only once per year, and a problem may not be discovered before these alarms are activated.
Figure 6.—Bladder Failure Relay
If the bladder failure alarm is activated, place the transformer under clearance and check the Buchholz for gas as mentioned in section 1.10. Open the conservator inspection port and look inside with a flashlight to check for oil inside the bladder. Bleed the air/gas from the conservator using the bleed valve on top of the conservator. If the transformer is new and has been in service for only a few months, the problem most likely is air escaping from the structure as mentioned in section 1.11.
With the transformer under clearance, open the inspection port on top of the conservator and look inside the bladder with a flashlight. If oil is found inside the bladder, it has developed a leak; a new one must be ordered and installed.
SOURCE: Transformer Diagnostics, Fascilities Instructions, Standards And Techniques
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Siemens TechTopic | Bus Joint Fundamentals
Proper design of bus bar joints is a necessity for long equipment life. The objectives that a good bolted bus bar joint must fulfill include:
• It must provide good conductivity, so that the bus system will meet the temperature rise requirements in the ANSI standards.
• It must withstand thermal cycling, so that the low resistance joint will be maintained for the life of the equipment.
• The joint pressure should be high (for good conductivity), but not so high that cold flow of the bus material occurs, which would cause the joint to deteriorate with time.
•The joint should have good resistance to corrosion in normal installation environments.
• It must be able to withstand the mechanical forces and thermal stresses associated with short-circuit conditions.
Figure 1: Anatomy of a bolted bus bar joint
Figure 1 shows a bolted bus bar joint, simplified to show two bus bars connected using a single bolt. Except in rare situations, the bus bars are silver plated (standard) or tin plated (optional), to improve the resistance to corrosion. The bolt is a high strength grade 5 cap screw, while the nut is a grade 2 (heavy wall) nut. The joint includes a large diameter, thick flat washer on both sides of the joint, adjacent to the bus bars. A split lock washer is installed under the nut to assure that the joint stays tight over the life of the equipment.
Why do we use a grade 2 nut with a grade 5 bolt? The grade 2 nut is more ductile than the grade 5 bolt, so that when the nut is torqued in place, the threads in the nut will tend to be swaged down and burnished to a degree, which results in a more equal distribution of load on all threads. This spreads the force more evenly and avoids unacceptable stress levels in the bolt and the nut.
Some users request that special non-magnetic hardware be used in bus joints. Historically, particularly in open bus systems exposed to the weather, difficulties were encountered with corrosion, and this may be one reason that some still ask for non-magnetic hardware. Others prefer non-magnetic hardware because of the perception that it results in a lower temperature rise. While these reasons may have had merit decades ago, we feel they are unnecessary today. Non-magnetic hardware (usually stainless steel or silicon bronze) is expensive and difficult to obtain. In addition, the tensile strength and yield strength of non-magnetic hardware is lower than that of high strength steel, so that tightening torques will generally be lower with the special hardware. The net effect of lower torque and pressure may very well counterbalance any slight temperature rise benefit associated with non-magnetic hardware.
We also specify that the flat washers are to have larger diameter and greater thickness than standard washers. The purpose of the washers is to distribute the clamping force of the bolts over a wider area. To accomplish this, we need a washer that is relatively rigid, with a larger diameter than would be normal for the size bolt used. If a normal small diameter, thin washer (or worse, none at all) is used, the joint will deteriorate over time because of cold flow of copper from the high pressure region directly under the bolt head (or the nut).
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Figure 2: Distribution of forces in a bolted bus bar joint
Figure 2 shows the distribution of forces in a bolted bus bar joint. To obtain a low resistance bus bar joint, we must establish and maintain sufficient pressure, and distribute the pressure over a large area. Initially, the two bus bars mate at only a few peaks or high spots. As the bolt is tightened, the bus conductors begin to deform, bringing more of these peaks into contact. At the design pressure, there is a relatively larger contact area, so that there are a multitude of parallel electrical connections between the bars.
As shown in figure 2, the force is concentrated more heavily around the bolt hole. Since the pressure is highest in the vicinity of the bolt hole, the surface irregularities in this area are flattened out as the mating surfaces are forced into more intimate contact. The joint resistance in this area will be lower than elsewhere in the joint. As distance from the bolt hole increases, pressure decreases and joint resistance increases. Beyond the area defined by the washer, pressure decreases rapidly and little effective current carrying capacity results.
From figure 2, we can see how the large diameter washers serve to distribute the clamping force more uniformly over a wider area than would be the case with a smaller washer, or none at all.
A properly designed bolted bus bar joint will allow the bus system to meet the temperature rise limits imposed by the ANSI standards, and will also have the thermal and mechanical capability to withstand the heat generated and forces imposed under the worst case short-circuit conditions.
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SOURCE: T. W. (Ted) Olsen – Manager, Technology | Siemens Power Transmission & Distribution, Inc.
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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
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:
where:
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]
Note:
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.
Example:
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.
Calculation:
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. The MV cubicle is subjected to sudden temperature variations. |  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.
Source:
Instruction: Medium Voltage equipment on sites exposed to high humidity and/or heavy pollution by Schneider Electric
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