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What does loop-feed mean?

A loop-feed transformer has two separate fully rated connection points per phase. This allows the parallel connection of two or more transformers on one circuit.

  • 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 radial-feed mean?
  • What does loop-feed mean?
  • 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.

  • 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.

  • 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