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Connections

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  • How do I use a Grounding Transformer

      Three-phase grounding transformers provide an artificial neutral for grounding. The main requirement is a specific zero-sequence impedance from the Zig-Zag or the Wye/Delta transformer in addition to the fault current withstand rating. For grounding purpose, only the Zig-Zag or a Wye/Delta connected transformer can be used. Autotransformers will have a high zero-sequence impedance and hence, cannot be used for grounding. Air-core reactors can be normally connected between the artificial neutral and ground to provide some additional current-limiting impedance.

      Grounding transformers are often required by the utility to attach a load generator such as solar, wind or a generator to the power grid.

  • Can You Use a Control Transformer Connected in Reverse?
  • 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.

  • What is Common Mode Noise Attenuation (CMNA)?

      It is the attenuation of electrical noise or voltage disturbance, that occurs between all of the line leads and the common ground, or between the ground plane and either line or the neutral. A transformer’s impedance, isolation properties and electrostatic shield may all contribute to the total TMNA level. This is typically expressed in decibels.

  • What is a Zig-Zag Connection?
  • What is a Wye Connection?

      A standard 3-wire transformer connection with similar ends of the single-phase coils connected. This common point forms the electrical neutral point and may be grounded.

  • What is a Tap?
  • What is a T-Connection?
  • What is a Short Circuit?
  • What is a Scott T Connection?
  • What is a Grounded Conductor (Grounding)?
  • What is a Full Capacity Tap?

      A full capacity tap is one through which the transformer can deliver its rated kVA output without exceeding the specified temperature rise.

      Transformers may be designed with full capacity taps ABOVE or BELOW nominal (FCAN and FCBN, respectively). Full capacity taps allow for the input primary voltage to be increased or decreased from its nominal voltage while retaining its ability to deliver its rated kVA output without exceeding it’s specified temperature rise.

  • What is a Delta connection?

      The delta connection is a standard three phase connection with the ends of each phase winding connected in series to form a closed loop with each phase 120 degrees from the other.

  • 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.

  • What is a Center Tap?

      It is a center point of a two winding transformer or a true tap in the middle winding of a coil. The voltage from the neutral to the center point will equal the voltage from the center point to end of the coil.

      For three phase units, the center tap is often limited to 5% of the total transformer kVA.

  • 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?
  • What are transformer wire leads

      Some transformers, often smaller control or potted units, will use wire leads for their primary and secondary connections instead of copper pads or terminal blocks. Typically these are insulated multistrand wires which are connected to the rest of the circuit using a terminal block, lugs or wire nuts.

  • What are Primary Voltage Taps?

      In some cases, the actual supply voltage to the primary of the transformer is either slightly higher or lower than the nameplate rating. Taps are provided on most transformers on the primary winding to correct this condition and maintain full rated output voltage and capacity. Standard taps are usually in 2 1/2% or 5% increments.

      Example: The transformer has a 480V primary rating and the incoming voltage is at 504V. The primary connection should be made at the +5% tap in order to maintain the nominal secondary voltage.

  • 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 does the dot mean on a single phase transformer wiring schematic?

      Typically, a single phase transformer wiring schematic has a dot on both the primary and secondary windings.

      The placement of these dots next to the ends of the primary and secondary windings informs us that the instantaneous voltage polarity seen across the primary winding will be the same across the secondary winding. In other words, the phase shift from primary to secondary will be zero degrees, which is important for some types of circuits. If the wiring to the dots is reversed on one side, the primary and secondary will be 180 degrees out of phase.

  • Under what circumstance is D.C. Resistance Measurement needed?

      Current from a D.C. resistance bridge is applied to the transformers windings to determine the D.C. resistance voltage of the coils. This test is important for the calculation of transformer winding heat losses used for winding temperature testing, and as base data for future assessment in the field.

  • Can variable frequency drives be powered from an open delta system?

      There are several issues that occur when a Variable Frequency Drive (VFD) is powered from an open delta system:

      • Uneven voltages in an open delta circuit can cause the diode bridge to unevenly draw current which causes additional heating.
      • The current harmonic distortion caused by the diode bridge will also not be balanced line to line which will cause even more additional heating.
      • Some utilities may require a harmonic study to be performed anytime a large VFD load is to be supplied by an open delta system.

      Overall, an open delta system can result in shorted diodes or DC bus capacitor failures on a VFD. Using DC Link Chokes and/or Line Reactors will mitigate some of this additional distortion. The VFD may need to be de-rated for open-delta configurations.

  • 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.

  • Describe a Flexible Connection.
  • What is a Bonded Conductor (Bonding)?

      Bonding all metal parts together and then to the system winding (typically to the X0 terminal of a transformer) is done to provide a low-impedance path to the source (system) to facilitate the opening of the circuit-protection device to remove dangerous voltage on metal parts. In addition, bonding the system to metal parts (typically to the X0 terminal of a transformer) stabilizes the system voltage to the metal parts and it provides a zero system reference (to the metal parts).

  • Can a Control Transformer regulate the output voltage?

      A control transformer will not regulate the voltage. The output (or secondary voltage) of a control transformer is dependent on the magnitude of the primary voltage and the transformer turns ratio. Therefore, fluctuations in the secondary voltage are direct results of any fluctuations in the primary voltage.