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Should Buck-Boost transformers be used to develop 3-phase 4 wire Wye circuits from 3-phase 3 wire Delta circuits?

No – a three-phase “Wye” buck-boost transformer connection should be used only on a 4-wire source of supply. A delta to Wye connection does not provide adequate current capacity to accommodate unbalanced currents fl owing in the neutral wire of the 4-wire circuit.

  • Can a transformer convert single-phase power to three-phase power?

      Single-phase power can be derived from a three-phase source. Transformers cannot convert a single-phase source to a three-phase source. The typical method to convert single-phase power to three-phase power is to utilize devices generally termed as rotary or static phase converters.

  • Explain Balance Loading on Single and Three Phase Transformers?

      A single-phase transformer with a series/parallel 120/240V secondary winding has two separate 120V secondary windings and is usually connected into a 3-wire system. When the winders are wired in series for 240 VAC, 120 VAC can be obtained at either between the neutral and centerpoint or between the centerpoint and 240VAC. If both 240 VAC and 120 VAC are going to be used, care must be exercised in distributing the load on the two 120V windings evenly, so each winding is carrying about half of the total 120VAC load if the 120 VAC load exceeds 5% of the total tranformer rating.

      Similarly for a three-phase transformer, each phase should be considered as a single-phase transformer. When distributing single-phase loads between the three phases, each of the three windings should be evenly loaded with single phase loads.

      Failure to balance loads can cause secondary voltage imbalances, additional transformer losses and high neutral currents. Significantly unbalanced loads can reduce the life of a transformer.

  • What is Parallel Operation?

      Single and three phase transformers may be operated in parallel by connecting similarly marked terminals, provided their ratios, voltages, resistances, reactance and ground connections are designed to permit parallel operation. Current and voltage angular displacements are also required to be the same in the case of three phase transformers.

  • Can Buck-Boost transformers be used on 3 phase systems?
  • Should Buck-Boost transformers be used to develop 3-phase 4 wire Wye circuits from 3-phase 3 wire Delta circuits?

      No – a three-phase “Wye” buck-boost transformer connection should be used only on a 4-wire source of supply. A delta to Wye connection does not provide adequate current capacity to accommodate unbalanced currents fl owing in the neutral wire of the 4-wire circuit.

  • What is a Buck Boost transformer?
  • What is an Open Delta transformer?

      An open delta transformer is a three phase transformer that only has two primary and secondary windings, with one side of the delta phase diagram “open”. Open delta transformers are rare and are typically only used for small loads where cost is important. More common is critical loads being wired with three single phase transformers in a banked configuration. Should one of these transformers fail, the three phase circuit can remain active although the two remaining transformers are limited to about 57% of the total load. This allows a circuit to remain powered during a failure of a transformer, albeit at a lower overall load factor.

      Open Delta

  • What is a triplex transformer?

      A triplex transformer is composed of three separate single phase transformers which are banked and connected directly together to form a single three phase unit in a common enclosure. While slightly larger than a dedicated three phase unit. a triplex design can be broken down into three significantly smaller and lighter components when there are size and/or weight restrictions for transporting the transformer. These are often used in mining or high rise building installations where the unit must be transported in an elevator.

  • Explain the term Phase?
  • Do different types of non-linear loads generate different harmonics?

      By far the majority of today’s non-linear loads are rectifiers with DC smoothing capacitors. These rectifiers typically come in 3 types:
      (i) single phase, line-to-neutral
      (ii) single phase, phase-to-phase
      (iii) three-phase

      Single-phase line-to-neutral rectifier loads, such as switch-mode power supplies in computer equipment, generate current harmonics 3rd, 5th, 7th, 9th and higher. The 3rd will be the most predominant and typically the most troublesome. 3rd, 9th and other odd multiples of the 3rd harmonic are often referred to as triplen harmonics and because they add arithmetically in the neutral are also considered zero sequence currents. Line-to-neutral non-linear loads can be found in computer data centers, telecom rooms, broadcasting studios, schools, financial institutions, etc.

      208V single-phase rectifier loads can also produce 3rd, 5th, 7th, 9th and higher harmonic currents but if they are reasonably balanced across the 3 phases, the amplitude of 3rd and 9th will be small. Because they are connected line-line, these loads cannot contribute to the neutral current. The largest current and voltage harmonics will generally be the 5th followed by the 7th. Typical single phase, 208V rectifier loads include the switch-mode power supplies in computer equipment and peripherals.

      Three-phase rectifier loads are inherently balanced and therefore generally produce very little 3rd and 9th harmonic currents unless their voltage supply is unbalanced. Their principle harmonics are the 5th and 7th with 11th and 13th also present. They cannot produce neutral current because they are not connected to the neutral conductor. The rectifiers of variable speed drives and Uninterruptible Power Supplies (UPS) are typical examples of three-phase rectifier loads.

      More Harmonic Mitigating Transformer Frequently Asked Questions

  • Can a Scott-T connected transformer be fed to go from two phases to three phases?

      In theory, you can feed a Scott-T connected transformer with two phase power and derive three phases. However, the two phase feed would need to have the two phases offset by 90 degrees. In practicality, there are no two phase generators on the market that can provide this feed.

      In some cases, this is discussed with an open delta system. Since an open delta system does not have the two phases offset by 90 degrees, a Scott T connected transformers can not be used in this application.

  • Can a Scott-T connected transformer be fed from two phases of an open delta system to create a three phase system?

      No. The two phase input of a Scott-T connected transformer requires the two phases to be offset by 90 degrees. Since an open delta system does not have the two phases offset by 90 degrees, a Scott-T connected transformer can not be used in this application. There are currently no practical magnetics solutions to this application.

  • What is the duty cycle of a transformer?

      Duty cycle is the amount of load over set periods of time. Transformers are designed to run continuously at full load without exceeding the insulation temperature limits provided that parameters such as ambient temperature, harmonic distortion, power factor, etc., are met. Transformers can also be designed to run for short term duty cycles which may result in a smaller unit. Short term duty cycles will need lower or no-load periods to aid in cooling. High load duty cycles can affect parameters such as impedance and voltage regulation.

  • Where is a Scott-T Transformer used?

      Typical applications for a Scott-T transformer include:

      • Used in an electric furnace installation where it is needed to operate two single-phase feeds and draw a balanced load from the three-phase supply.
      • Used to supply the single phase loads such as traction power. This helps to keep the load on the three-phase system as nearly balanced as possible.
      • Used to link a 3-phase system with a two–phase system with the flow of power in either direction.

      The Scott-T connection permits conversions of a 3-phase system to a two-phase system or vice versa. Since 2-phase generators are not available, the conversion from two phases to three phases is not a practical application.

  • What does radial-feed mean?
  • What does the term Banked describe?

      Two or three, single-phase transformers can be inter-connected to make a three-phase bank. The primary windings of the single-phase transformers can be connected in Delta or Wye. Likewise the secondary windings can be connected in either a Delta or Wye configuration.

      The equivalent capacity of the bank will be equal to three times the nameplate rating of each single-phase transformer. Usually, this type of installation is more expensive than using a single three-phase transformer. Advantages to banked transformers include:

      • For utility applications, the loss of one transformer in a delta configuration creates an open delta configuration using only two transformers. This will continue to supply power albeit at a reduced total kVA rating.
      • Banked transformers can be used to applications where the size or weight of a three phase transformer is too large. An example would be the ability to move a banked transformer in three lighter and smaller sections in a freight elevator.

  • What does loop-feed mean?
  • Why isn’t a ‘closed Delta’ Buck-Boost connection recommended?

      This connection requires more kVA power than a “Wye” or open delta connection, and phase shifting occurs on the output. The closed delta connection is more expensive and electrically inferior to other three-phase connections.

  • What is a solar grounding bank?
  • How do Harmonic Mitigating Transformers reduce voltage distortion?

      Delta-wye transformers, even those with a high K-factor rating, generally present high impedance to the flow of harmonic currents created by the non-linear loads. Non-linear loads are current sources that push the harmonic currents through the impedances of the system. Any voltage drop across the impedance of the transformer at other than the fundamental frequency (60 Hz) is a component of voltage distortion.

      Because of its higher impedance to harmonic currents, the voltage distortion at the output of a delta-wye transformer often reaches the 8% maximum voltage distortion limit recommended by IEEE Std. 519-2014 by the time that the secondary side load has reached just one-half of full-load RMS current. At closer to full-load, these transformers can produce critically high levels of voltage distortion and flat-topping at their outputs and at the downstream loads.

      To minimize the voltage distortion rise due to the transformer itself, Harmonic Mitigating Transformers (HMTs) are designed to reduce the impedance seen by the harmonic currents. This is accomplished through zero sequence flux cancellation and through phase shifting. The secondary winding configuration of the HMT cancels the zero sequence fluxes; those produced by the 3rd, 9th, 15th (triplen) current harmonics, without coupling them to the primary windings.

      This prevents the triplen current harmonics from circulating in the primary windings as they do in a delta-wye transformer. The flux cancellation also results in much lower impedance to the zero sequence currents and hence lower voltage distortion at these harmonics. In addition, the reduced primary winding circulating current will lower losses and allow the transformer to run cooler.

      The remaining major harmonics (5th, 7th, 11th, 13th, 17th & 19th) are treated to varying degrees through the introduction of phase shifts in the various HMT models.

      Single output HMTs are offered in 0° and -30° models to provide upstream cancellation of 5th, 7th, 17th and 19th harmonic currents on the primary feeder.

      More Harmonic Mitigating Transformer Frequently Asked Questions

  • What is a Harmonic Mitigating Transformer and how is it different than a K-Rated Transformer?

      Harmonic Mitigating Transformers, or HMTs, are specifically designed to minimize the voltage distortion and power losses that result from the harmonics generated by non-linear loads such as personal computers. The accomplish this through the use of a zig-zag winding.

      K-rated transformers, on the other hand, are simply designed to prevent their overheating when subjected to heavy non-linear loading, but do very little to reduce the harmonic losses themselves. And as for voltage distortion, K-rated transformers perform the same as conventional general purpose delta-wye transformers.

      More Harmonic Mitigating Transformer Frequently Asked Questions

  • When can you Reverse Connect a transformer?

      In general, distribution transformers can be reverse connected without de-rating the nameplates KVA capacity. However, this is rarely considered in modern applications due to NEC code changes. Several precautions need to be taken for reverse connection of some smaller transformers. These would include:
      Dealing with higher current inrush which can cause nuisance tripping.

      HPS transformers under 6kVA three-phase and 3kVA single-phase, there is a “turns ratio compensation” on the low voltage winding. When backfed the turns compensation actually reduces the output voltage. When a three-phase transformer is reverse connected thus resulting in a Wye-Delta configuration, the neutral terminal must be isolated. This modification may violate the warranty and agency listings such as U.L.

      Back-fed transformers increase the installer’s liability since a future user may not realize what is the primary while de-energizing the transformer.

      In general HPS suggest that a proper step up transformer which is designed with the low voltage terminals as the primary terminal be used.

  • What is preferred for a neutral grounding transformer?

      It is up to the user to specify this. Typically, The most common magnetic grounding device is a zig-zag autotransformer. This design offers greater flexibility at a cost and size smaller than a comparable Wye-Delta isolation transformer.