Transformers, unlike a motor, have no mechanical output (expressed in kW), instead, they are characterized by the apparent nominal power kVA that they can supply continuously. However, they are also designed to supply significantly higher power. A three-phase transformer is a combination of three interconnected single-phase transformers with three primary and three secondary windings mounted on a core with three legs.

The primary and secondary windings of three single-phase transformers can be connected in the following ways:

  • Primary in delta – secondary in delta (D/D)
  • primary in delta – secondary in star (D/Y)
Three-phase delta-star 3 transformer
  • primary in star – secondary in star (Y/Y)
  • primary in star – secondary in delta (Y/D)

Low Voltage Power Network

Low Voltage (LV) power plants supply
3 x 380 V/50 Hz, 3 x 440 V/60 Hz, etc, to the power-distribution network. Smaller electrical motors require 3 x 220 V to function. Voltages are reduced from 3x 380V to 3x 220V. This distributes a reduced voltage to the consumer network.

As there is a lesser voltage in the load side
(3 x 220 V) then on the power supply side (3 x 380 V), the current on the load side is greater than the current provided from the source side.

A regular LV power plant has four step-down transformers, shown in Figure 14.3 asT1 toT4.

T1 and T2 are located near the main switchboard .T3 and T4 are located in the vicinity of the emergency switchboard. One transformer from each side of the busbar (MSB and ESB) is kept in continuous operation while another is available as a standby unit. It is normal that the primary winding of the transformer is permanently connected to the power supply. The switching of the load is only carried out in the secondary circuit. Therefore, the standby transformer can be kept separate from the power network until required.

Transformer Parallel Operation

All transformers are hardwired to the power network during the construction process. The power plant manufacturer will have followed the requirements for parallel operation, ie:

  • The supply and load voltages are equal for both transformers
  • short circuit voltages for both transformers are equal
  • the load carried by both transformers is equal
  • the type of connection to both primary and secondary windings is equal.

Figure 14.4 illustrates features of 3 x 380 V/3 x 220 V distribution, where T1 and T2 are step-down transformers, Q7 and Q8 are automatic circuit-breakers for primary windings (supply) and Q9 and Q10 are automatic circuit-breakers for secondary windings (load).

If T1 is in operation and T2 is intended to be paralleled with T1, and T1 is scheduled to be put on standby, the following sequence for Q7 – Q10 circuit-breakers should be followed:

For bringing -T2 in parallel:

  • Primary winding circuit-breaker Q8 must be closed first
  • secondary winding circuit-breaker Q10 must be closed last, connecting -T2 under the load.

For switching off-T1:

  • Secondary winding circuit-breaker Q9 must disconnect the load from T1 first
  • primary winding circuit-breaker Q7 must then disconnect T1 from the supply voltage.

A similar sequence should be exercised for emergency switchboard transformers.

Figure 14.4 – Step-down transformers’ network outline

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