Medium Voltage Indoor Dry Type Transformers Rating, Rated Voltage, Frequency and Ratio

By | August 4, 2017

The rated voltage ratio of the transformers at no load normally is specified at 50 Hz, this being related to the principal tapping of the transformer. Tapping shall be provided on the primary winding to provide a ratio variation of +7.5% and –2.5% in steps of 2.5%. The tap charge mechanism is of the off load type.

Rating: All components and construction of indoor dry type transformer  comply with IEC 60076, and IEC 60726. Transformer cooling status and operation shall be monitored from building management system.

Rated Power:                                                    1500kVA

Service Condition:

Maximum Ambient Temp.:                     50°C

Maximum Service Altitude:                     1000m

Temperature Rise:

Windings Average:                                90°K

Windings Hot Spots:                             110°K

Rated Voltages:

Primary:                                                11kV

Secondary at no load on principle tap:   0.415kV

Number of Phases:                                           3

Rated Current:

Primary at principal tap:                   78.73Amps.

Secondary:                                           2165.13Amps.

Thermal Class of Winding:                                 F1

Noise Level:                                                      56dBA

Cooling Method:                                               AN

Rated Impedance Voltage:                             6%

Rated Insulation Level:                                  12kV

Frequency:                                                        50Hz.

Full-wave lightning impulse voltage.

Primary :                                               75kV

LV cable termination details:

The LV cable termination on the LV side of the transformer shall be supported by a suitable wooden support, LV cables when terminated shall be provided with proper cable glands from the LV cable entry gland plate.

Ability To Withstand Short CircuitMedium Voltage Indoor Dry Type Transformers

The transformers should be capable of withstanding the mechanical and thermal stresses associated with short-circuit currents in accordance with the requirements of BS 171 or equivalent, when operating on any tap position including that corresponding to the minimum effective impedance.

It is assumed that full rated voltage is maintained across the MV winding when a short circuit is applied across the three phase and earth of the LV winding.

The following information shall be used:

The short circuit apparent power of 11 kV system is 346.50 MVA, taken from wiring regulations for 31.5kA (3sec) at 11kV, or as directed by authorities. Installation contractor to coordinate with regulatory body to verify the actual short circuit apparent power to be used for the overall power system protective coordination.

Transformers Fittings

All necessary fittings including the following are provided for each of the transformers:

    • Plain bi-directional rollers with blocking facilities.
    • Skid base, lifting and jacking lugs
    • Neutral and earthing terminals
    • Rating and terminal marking plates

Transformer Thermal Protection

Temperature protection shall be incorporated to prevent thermal overloading of the transformers.

BMS monitoring shall be provided for each signal accordingly:

  • A dial type thermometer shall be provided on the housing of each transformer for measurement of the winding temperature.
  • Two thermal sensors of the resistor type shall be moulded into each phase of the low voltage section of the spool, these sensors together with appropriate relays and timer shall be provided.
  • Temperature alarm and tripping signals to the switchgear
Earthing And Neutral Earthing

The neutral conductor of the LV winding shall be provided with a terminal which is brought out of the LV termination box for the main earth lead to be connected. The value of the earth resistance shall comply with the requirement of the BS7430 and authorities and shall not be higher than 1 ohm. All metal parts of the transformer shall be earthed.

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