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Troubleshooting Guide

This trouble shooting section is designed to help evaluate the performance of a transformer in the field. It is emphasized that only authorized personnel should be permitted to examine installed transformers.

Power must be shut off before any work is conducted on a transformer. It is also recommended that all terminals be grounded.

Symptoms/Issues:

  • Transformer is overheating

      Transformer insulation is generally rated for 220°C but may be lower for some designs including control or encapsulated. Standards permit the temperature of the transformer enclosure cover to be 65°C over ambient. When temperatures exceed the rating for the insulation system or enclosure, overheating occurs.

      Burned, darkened or damaged insulation may be apparent along with a burnt smell. The hottest part of a transformer is the coil near the top of the core. Energized transformers should not be touched. If the insulation is damaged or smoke is visible, the unit may need to be returned for testing and replaced or repaired.

      Check: Solution:
      Verify total load doesn’t exceed transformer kVA rating. Reduce size or load or replace with larger transformer. In some cases fans can be added to increase cooling and
      maximum load.
      Verify ambient temperature does not exceed transformer ratings. Relocate to area with lower ambient temperature, reduce load, reduce ambient temperature at primary location or
      replace with a low temperature rise transformer. Transformers installed in small rooms will need proper room
      ventilation.
      Verify tap connections are set up identically on all coils.
      Verify transformer is correctly rated for harmonic load, check for high neutral currents. Reduce or remove harmonic loads or replace transformer with a larger unit or unit with the proper k-rating.
      Verify that the transformer’s ventilation openings are not blocked. Transformers purchased as core and coil
      units and placed in enclosures not supplied by HPS require that the integrator properly size the enclosure and cooling
      requirements.
      Relocate the transformer to an area of better ventilation. Move the transformer away from walls, equipment or
      overhead projections that may impede airflow. Do not install fans to cool a transformer. Improperly installed fans may
      actually impede airflow and could result in transformer damage.
      Improper Input Voltages Verify taps are correctly set for the input voltage. Depending on the load and transformer type, continuous
      overvoltages or undervoltages as low as 5-10% may cause overheating.
      Check no load current. If no load current is high (varies with transformer efficiency but no load current is typically less than 2-3% of
      total kVA), inspect the core and coils for damage. In most cases you will not be able to inspect the insulation between
      the core and coil without returning to the factory for testing and disassembly. If there is a short between the core
      and coil, the unit will have to be replaced or repaired.
      Excessive and sustained airflow caused by exterior winds or fans generally moving horizontally to the ground can
      disrupt convection cooling and cause overheating at high loads.
      Relocate the transformer to an area with less wind or block the wind.
      Fan cooled transformers have broken or misaligned fans. Fans need to be replaced or realigned.
      Low Power Factor Low power factor can cause excessive current and higher overall loads. Current meters need to be able to register
      total current. Some digital meters may not be accurate.
      Unbalanced loads may cause excessive heating. Loads should be balanced to within 20 % of maximum kVA. No individual load should exceed the load specific load for
      each phase (1/3 of total kVA for three phase units).
      Transformer is installed above a heat source such as another transformer. Move either the transformer or the heat source. Redirect the hot airflow from the lower object away from the
      cooling entrances and surfaces of the higher object. Replace the top unit with a low temperature rise transformer.
      Check if output voltage is distorted. A highly distorted output voltage may be a sign that there is a turns to turns fault and the transformer is in
      danged of immediate failure. The transformer needs to be denergized and meggered. The damaged coil may need to be
      replaced or the transformer scrapped.
      Check the output circuits to make sure each leg of the transformer is functioning and overcurrent protection is
      ok.
      If a fuse on one or more of the legs has opened, determine and clear the fault and replace the fuse. This is more
      commong on delta transformer outputs, especially if three single phase units are used in a Delta bank.
      If a Drive Isolation Tranformer (DIT) is being used, verify the DIT kVA has been derated per the HP sizing charts in the catalog. If motor HP is unknown, use .746 kW/HP to determine the equivalent HP of the load. DIT’s are not current rated
      devices, the HP selection charts must be used to properly size a DIT. Extrusion applications tend to be the worst.
      Check if two or more transformers are operating in parallel to power one load. Transformers operating in parallel are rare. Large circulating currents and uneven load can result from
      transformers wired in parallel. The transformer s may have to replaced with one unit capable of power the entire
      load.
      Cable connections are discolored by heating. Cables should be periodically tightened. The surface should be cleaned of any insulation applied during the vacuum
      pressure impregnation process. Rough edges must be smoothed.
      Sparks or smoke is visible from the base of the transformer but the transformer has not failed and there isn’t any sound of arcing. During the VPI process, icicles of insulation can form under the tarnsformer and occassionally act as a ground. If
      discovered early enough the icicle can be removed and the transformer will not be damaged.
      Excessive dust could block air vents Dust needs to be blown out while transformer is denergized.
      Discolored Insulation The transformer’s insulation may have been damaged and may need to be repaired or replaced.
      Visible Flames or Smoke The transformer’s insulation may have been damaged and may need to be repaired or replaced.

  • Corrosion Damage

      Category: | Troubleshooting Guide  

      Tags: FAQ

      Corrosion can damaged the transformers enclosure, core and coils. This can be a consistent problem in corrosive areas such as marine and waste water treatment.

      Check: Solution:
      Check for enclosure and mounting hardware corrosion. Replace corroded parts. In some cases they may need to be replaced with stainless steel or other types of protected parts. Enclosures can be ordered in various grades of stainless steel or with plated base mounting brackets.
      Core steel is corroded. In some cases, light corrosion may not severely damage the core function although a more pressing concern may be conductive rust settling on the coils and causing dielectric problems. In severe cases the core may need to be replaced. Epoxy can be applied at the factory to protect the core from corrosion. Transformers that overheat and damage the insulation are more susceptible to corrosion.
      Coil and conductor corrosion. Protectants such as No-LOX grease may be applied to exposed parts. Only remove insulation from the taps that need to be used. Keep all connections tight. Utilize copper coils when possible. Epoxy coatings (VPE) or encapsulated (potted) transformers can be used.

  • Excessive Conductor Heating

      Category: | Troubleshooting Guide  

      Tags: FAQ

      Cabling attached to the transformer can have excessive heating that causes conductor damage.

      Check: Solution:
      Check conductor connections to make sure they are. Tighten conductor connections. Replace conductors if they are damaged.
      Verify conductors are properly sized. Replace conductors if they need to be sized larger.
      Check conductor insulation rating to make sure it can withstand the high tempertures within the enclosure. Replace with cabling with higher ambient rated conductors.
      Verify conductors aren’t placed above the coils where the warmest air will rise. Reposition conductors in enclosure to avoid the hottest air existing from the top of the coils.

  • Onsite Staff do not know electrical codes or common electrical terms

      Category: | Troubleshooting Guide  

      Tags: FAQ

      HPS greatly appreciates your use of our magnetics in your applications. Our products are meant to be installed by qualified electricians familiar with the local electric codes and the overall system requirements. HPS is the leader in the design and manufacturing of transformers and other magnetics. We are not experts on electrical codes which can vary across the US. Our units are fully compatible with the common installation requirements of the NEC and other local codes. Due to the complexity of electrical systems, there are many questions and problems which require a broad systems solution and local knowledge of the system that we can not offer. Installation by unqualified people can cause equipment failures and dangerous situations.

      Transformer insulation is generally rated for 220°C but may be lower for some designs including control or encapsulated. Standards permit the temperature of the transformer enclosure cover to be 65°C over ambient. When temperatures exceed the rating for the insulation system or enclosure, overheating occurs.

      Burned, darkened or damaged insulation may be apparent along with a burnt smell. The hottest part of a transformer is the coil near the top of the core. Energized transformers should not be touched. If the insulation is damaged or smoke is visible, the unit may need to be returned for testing and replaced or repaired.

  • High Neutral Currents

      Category: | Troubleshooting Guide  

      Tags: FAQ

      High neutral currents can be caused by unbalanced and/or non-linear loads with high harmonics. K-rated transformers have a 200% rated neutral. Amperage should be measured with a meter capable of measuring true RMS currents.

      Check: Solution:
      Check the load to make sure it is balanced. Any load unbalance results in neutral currents. Improve the load balance between the three phases. General purpose transformers have a 125% rated neutral.
      Check the load to determine if it has high amounts of harmonics from single phase sources. For non-linear loads, use k-rated tranformers with 200% rated neutrals. Select a k-rated transformer with a k-factor suitable for the specific level of harmonics.
      Check to make sure all three legs of the transformer or transformer bank are operating. Check coil continuity and blown fuses or tripped breakers. If a fuse is blown, clear the source of the fault and replace the fuse. If a breaker has tripped, clear the fault, check the settings and turn on the breaker.  If the transformer coil is damaged, the transformer will have to be repaired or replaced.  Please contact HPS Customer Service.

  • Noise & Vibration

      Sound levels for transformers vary from 40 dbA for smaller distribution units to 68 dbA for 3000 kVA power transformers and higher for larger units. All transformers vibrate at 120 Hz because the EMF vibrates the unit due to 60 Hz oscillations. The audible noise is measured at no-load and tested in a low ambient noise environment with walls or reflecting surfaces at least 10’ away from the transformer per NEMA ST-20. When installed in more confining electrical rooms and connected to a load, transformers will exhibit higher sound levels than these standards. Transformers can be ordered that produce less noise that the NEMA ST-20 standard, generally at -3 and -5 dB but lower levels are also available. Rubber pads or springs can also be installed between the enclosure and floor to further reduce vibrations and noise. Harmonics generally don’t increase noise too much.

      Check: Solution:
      High Input Voltage Verify taps are correctly set for the input voltage.
      High Frequency If power is coming from a local generator, adjust the generator. If the frequency can’t be adjusted, the transformer may have to be rebuilt.
      Unbalanced Loads Loads should be balanced to within 20 % of maximum kVA. No individual load should exceed the load specific load for each phase (1/3 of total kVA for three phase units).
      Damaged Core. Check any core welds or clamped brackets when fully denergized to make sure the welds are intact and the brackets haven’t slipped. Loose brackets can be tightened on-site but broken welds or cores that have shifted will need to be sent back to the factory for repair.
      Missing Core to Enclosure anti-vibration pads. All ventilated transformers should have rubber pads between the core and enclosure or mounting points. If these are missing they can either be field replaced or the unit can be sent back to the factory.
      Loose Enclosure Tighten enclosure screws and bolts were necessary.
      Transformer mounted on a suspended floor or wall may create and echo chamber and increase noise. Reinforce the floor/wall or move the transformer to a more solid mounting area.
      Multiple transformers installedin one room. Multiple transformers in one area can sometimes resonate with each other and increase the overall noise. The transformers either have to be moved further apart or isolation pads or springs can be mounted under the transformer enclosure.
      Transformer is running correctly but still producing too much ambient noise. A low audible dB transformer can be used to replace the existing unit and/or vibration isolation pads and springs can be installed between the transformer base and floor.

  • Transformer is Physically Damaged

      Category: | Troubleshooting Guide  

      Tags: FAQ

      Transformers can become physically damaged during installation or shipping.

      Check: Solution:
      Enclosure is damaged but the core and coil is intact. Minor damage can be repaired in the field with spray paint (scratches) or individual enclosure pieces. Enclosure components located in the base or replacing the entire enclosure will require facilities capable of lifting the entire core and coil Units can also be returned to the plant for enclosure repair and full testing. Encapsulated (potted) units that have enclosure damage may not be repairable.
      Jumpers are damaged. Replace jumpers.
      The coil has been damaged with arcing between the coil taps. Voltage spikes such as lightning or utility transients caused voltages to exceed the transformers BIL ratign and caused arcing. If a conductive dust or liquid (water) is present, this may also cause arcing. The transformer will need to be repaired or replaced. Voltage spikes can be mitigated with lightning arrestors or transformers with higher BIL rates.
      The coil has been damaged with arcing and physcical damage beteen the coils (turn to turn). The insulation between the coils has failed. This may be due to age, overheating, overvoltage or damaged insulation. Each of these failure modes will have to be checked and if found to be a problem fixed. The transformer will need to be repaired or replaced.
      The coil has been damaged with arcing and physcical damage between the coil layers or between the coil and shield. The insulation between the coils has failed. This may be due to age, overheating, overvoltage or damaged insulation. Each of these failure modes will have to be checked and if found to be a problem fixed. The transformer will need to be repaired or replaced.
      Connections points show signs of heat including damaged insulation or excessive corrosion. Replace jumpers or lugs if they are damaged. Verify the mounting surfaces are clean of insulation and smooth. Verify any mechanical components are tight. In some cases a braise joint at a lug or pad point may be damaged and cause excessive heat.
      Coil is damaged. Coil damage can include insulation that has been ripped, worn or is burnt or discolored from heat. Some units can beturned to HPS for repair or coil replacement. In some cases it may be more economical to replace the unit than to repair it.

  • No Secondary Voltage

      Category: | Troubleshooting Guide  

      Tags: FAQ

      Transformer secondary voltage can be too high, too low or there may be no voltage. Please note that transformer voltage is a ratio of the primary voltage. If the primary voltage is too high or too low, the secondary voltage will also be too high or too low.

      Check: Solution:
      No secondary voltage. Verify the transformer primary is energized.
      Perform a continuity check on the primary and secondary coils. If a continuity check fails on the secondary, also perform a continuity check on the tap cables. If the tap cables are bad, replace the tap cables. If the continuity check fails and it is not the result of the tap cables, the transformer will need to be either repaired or replaced.
      If one phase a delta secondary has no voltage check to make sure that one of the three legs are grounded. If two legs are grounded, remove the ground from one of the legs.
      Verify all connection points (lugs and pads) are tight, smooth and cleaned of any insulation. Replace jumpers or lugs if they are damaged. Verify the mounting surfaces are clean of insulation and smooth. Verify any mechanical components are tight. In some cases a braise joint at a lug or pad point may be damaged and cause excessive heat.
      Inspect transformer for visual signs of a short circuit including damaged or burned insulation or smoke. If the transformer has been damaged, the transformer may need to be either repaired or replaced.
      Verify all connection points (lugs and pads) are tight, smooth and cleaned of any insulation. Smooth or replace pads and lugs if damaged or rough. Tighten any loose mechanical connections. Clean off any insulation remaining on electrical connection points.

  • Secondary Voltage is Too High

      Category: | Troubleshooting Guide  

      Tags: FAQ

      Transformer secondary voltage can be too high, too low or there may be no voltage. Please note that transformer voltage is a ratio of the primary voltage. If the primary voltage is too high or too low, the secondary voltage will also be too high or too low.

      Check: Solution:
      Verify the taps are set up the same or set up incorrectly for the incoming voltage. Set up taps at the same level and for the correct primary voltage.
      The voltage exceeds the ability of the taps to tune the voltage. A different transformer with a larger tap range or higher input voltage may be needed. Adjust the input voltage to be within the range of the transformer taps.
      The transformer is wired correctly per the input voltage. Verify the transformer being installed is the correct voltage for the application. If the nameplate matches the application, the transformer may be incorrectly nameplated.
      Verify the neutral connection is properly wired and grounded per the applicable codes. All three coils should be attached to a common neutral point. If the transformer neutral is damaged, it will have to repaired. Wire and ground the neutral per applicable codes. A coil may have been inverted during manufacturing and would have to be replaced.
      The transformer is wired correctly per the input voltage. The transformer may be wired in reverse.
      Check the power factor. If the power factor is above 1.0 (leading), high voltages will result. This must be corrected at a systems level.
      Verify the coils are look similar and have similar resistance values. The turns ratio may have been incorrectly wound or one or more of the coils may be incorrect in comparison to the other coils.

  • Secondary Voltage is Too Low

      Category: | Troubleshooting Guide  

      Tags: FAQ

      Transformer secondary voltage can be too high, too low or there may be no voltage. Please note that transformer voltage is a ratio of the primary voltage. If the primary voltage is too high or too low, the secondary voltage will also be too high or too low.

      Check: Solution:
      Verify the taps are set up the same or set up incorrectly for the incoming voltage. Set up taps at the same level and for the correct primary voltage.
      The voltage exceeds the ability of the taps to tune the voltage. A different transformer with a larger tap range or higher input voltage may be needed. Adjust the input voltage to be within the range of the transformer taps.
      The transformer is wired correctly per the input voltage. Verify the transformer being installed is the correct voltage for the application. If the nameplate matches the application, the transformer may be incorrectly nameplated.
      Verify the neutral connection is properly wired and grounded per the applicable codes. All three coils should be attached to a common neutral point. If the transformer neutral is damaged, it will have to repaired. Wire and ground the neutral per applicable codes. A coil may have been inverted during manufacturing and would have to be replaced.
      The transformer is wired correctly per the input voltage. The transformer may be wired in reverse.
      Check the power factor. If the power factor is above 1.0 (leading), high voltages will result. This must be corrected at a systems level.
      Verify the coils are look similar and have similar resistance values. The turns ratio may have been incorrectly wound or one or more of the coils may be incorrect in comparison to the other coils.

  • Secondary Voltage Varies

      Category: | Troubleshooting Guide  

      Tags: FAQ

      Transformer secondary voltage can be too high, too low or there may be no voltage. Please note that transformer voltage is a ratio of the primary voltage. If the primary voltage is too high or too low, the secondary voltage will also be too high or too low.

      Check: Solution:
      Incoming line voltage varies. A transformer is a voltage ratio device so the secondary voltage will generally vary as the primary voltage varies. This is a systems issue a transformer can not fix.
      Verify all connection points (lugs and pads) are tight, smooth and cleaned of any insulation. Smooth or replace pads and lugs if damaged or rough. Tighten any loose mechanical connections. Clean off any insulation remaining on electrical connection points.
      Verify the neutral connection is properly wired and grounded per the applicable codes. All three coils should be attached to a common neutral point. If the transformer neutral is damaged, it will have to repaired. Wire and ground the neutral per applicable codes. A coil may have been inverted during manufacturing and would have to be replaced.
      Are there any large loads that occur where total standard or momentary inrush load exceeds the kVA rating of the transformer. During high current, such as when a large motor is started across the line or during a short circuit, the output voltage will be lowered. The total load will either have to be decreased, limited by devices such as reduced motor starting devices or a larger transformer will have to be installed.

  • Smoke is visible or smelled

      Category: | Troubleshooting Guide  

      Tags: FAQ

      Smoke normally means the tranformer’s insulation has been damaged or weakened and failure could have already occurred. The transformer must be immediately deenergized to prevent further damage.

      Check: Solution:
      Visible signs of damaged or failed insulation. The transformer will have to be sent back to the factory for testing and possible repair or replacement. In many cases, it may not be economically possible to repair a damaged tranformer and the unit will have to be replaced.
      No visible signs of damage. The transform should be meggered. If the insulation values are lower than required (consult the maintenance manual), the transformer will have to be sent back to the factory for testing and possible repair or replacement. In many cases, it may not be economically possible to repair a damaged transformer and the unit will have to be replaced.
      No visible signs of damage. Check vertical air ducts in the transformer. If any metallic ferrous metal (screws, nuts, washers, etc.) have fallen between the transformer coils, these will heat through eddy currents and cause insulation damage and eventual failure.
      No visible signs of damage. The transform should be meggered. If the insulation values are within the required range and if the transformer was just manufactured (consult the maintenance manual), the transformer may have some insulation that was not cured during the bake cycle. Please call HPS for further instructions.

  • Transformer has been backfed

      Category: | Troubleshooting Guide  

      Tags: FAQ

      HPS does not recommend that transformers be backfed due to safety issues. The nameplate no longer matches the application and may violate electrical codes and common wiring practices. Backfed transformer are susceptible to high inrush currents, voltage unbalances and lower than expected output voltages.

      Check: Solution:
      Inrush current is causing nuisance tripping. Size the fuse or breaker to the largest allowed by the local electrical code and use time delay fuses or high short circuit trip points on breakers. Replace with a step up transformer is problem persists.
      Output voltage is lower than expected. Windings are turns compensated to take into account the natural voltage drop which occurs in the wires. When a transformer is backfed, the voltage drop still occurs but the winding compensation works in reverse. If possible, use taps to adjust. Replace with a step up transformer if the problem persists.
      Is the transformer a delta-wye transformer that is being backfed such that the wye becomes the primary. This application is not suggested and involves the removal of ground straps which would violate the UL listing of the transformer. This must be replaced with a custom step up transformer.

  • Transformer has been stored in humid area or exposed to water

      Category: | Troubleshooting Guide  

      Tags: FAQ

      If a transformer has been stored deenergized in a humid area, the windings and insulation may absorb water decreasing the dielectric strength of the insulation. In worst cases the windings have been exposed directly to water or submerged in it. If a transformer has been exposed to any liquid, immediately denergize and remove from service.

      Check: Solution:
      If the transformer has been stored in a humid area, megger the transformer. If the insulation ratings are lower than needed, consult the factory. Dry hot air may need to be directed over the transformer for 24 hours ore more. In some cases, the transformer may need to be baked in industrial ovens to sufficiently dry out.
      If the transformer coil has been exposed to rain or other dripping water, megger the transformer. Immediately remove from service. Since liquid water can shift, even a proper megger reading may not be sufficient. Consult the factory. Dry hot air may need to be directed over the transformer for 24 hours or more. In some cases, the transformer may need to be baked in industrial ovens to sufficiently dry out.
      If the transformer has been exposed to a liquid other than water, please consult the factory. Some chemicals may leave conductive or corrosive residue that weakens the insulation. In this case, the transformer may need to be repaired or replaced.
      If the transformer coil has been submerged in water, consult the factory. In this case, the transformer will usually need to be replaced or returned to the factory for evaluation and possible repair.

  • Transformer is covered in dust

      Category: | Troubleshooting Guide  

      Tags: FAQ

      During normal operation, transformers can become covered in dust which needs to be periodically cleaned off to prevent arcing and loss of cooling capacity.

      Check: Solution:
      Is dust simple non-conductive dust. Denergize and blow off with compressed air.
      Is dust corrosive such as salt. Simply blowing off the dust may not be sufficient to protect the transformer from corrosion. Megger checks may not diagnose some of these problems. Transformers may need to be placed in a location away from corrosive dust. The transformer may need to be professionally cleaned by manufacturer or replaced. Please consult factory. Certain coatings such as epoxy (VPE) or encapsulated (potted) transformers are available.
      Is dust conductive such as coal, carbon black or metallic. Simply blowing off the dust may not be sufficient to protect the transformer from coronal discharge or short circuiting. Megger checks may not diagnose some of these problems. Transformers may need to be placed in a location away from conductive dust.

  • Nuisance Tripping of Fuses or Breakers at Energization

      Category: | Troubleshooting Guide  

      Tags: FAQ

      When a transformer is energized, it draws a large current for a few cycles to establish the eletromagnetic field. During this time, the transformer load has little to no effect on the initial transformer inrush. Inrush can vary depending on where the sine wave is during initial energization and how long the transformer was denergized before being energized again. Inrush current can be up to 12 times rated FLA current. In some cases custom transformers can be built to have very low inrush values.

      Check: Solution:
      Are overcurrent device sized correctly per applicable codes. Size overcurrent devices correctly. NEC code should be reviewed. In general buck-boost overcurrent protection should be sized NEC 450-4 which is also found in the back of our buck boost section in the catalog. 600 volt class distribution overcurrent protection should be sized per NEC Table 450.3(A).
      Are overcurrent devices time delay? If overcurrent devices are quick trip, replace with time delay version.
      Overcurrent devices are sized for a load which is significantly lower than the rated transformer amperage. Overcurrent devices should be sized for the transformer amperage since inrush is dependent on transformer size and not load.
      Is the transformer being back fed? Backfeeding is generally done when taking a standard step down transformer and using it to step up the voltage which reverses the order of energization for the windings. Transformers being backfed will experience higher inrush currents since the coil being energized (secondary) is further from the core. In this case the overcurrent devices should be sized to the maximum allowed per the applicable code requirements. In some cases a custom transformer may need to be used instead of backfeeding a standard transformer.
      Is the input voltage higher than the input is set up for? Lower the input voltage, adjust the taps or use a transformer capable of adjusting for a higher input voltage.
      Check in the input frequency, lower input frequencies can cause higher inrush currents. Adjust the input frequency or replace transformer with a unit capable of using a lower frequency (us a 50/60 Hz core).
      Check the core to make sure it is properly aligned, tight and has not shifted. If possible tighten any core clamps. If the core has shifted or is damaged, it may have to be repaired or replaced. Bad core steel or a core built too small for the application would have similar problems.
      Check the coil insulation. Shorted turnscan cause nuisance tripping initially but this will quickly turn into a turn to turn fault which will destroy the coil.
      Check the primary power factor. Low power factor can cause higher inrush currents. This is a systems problem that will require power factor correction.