ELECTRICAL REQUIREMENT AND PERFORMANCE

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5.0 ELECTRICAL REQUIREMENT AND PERFORMANCE

5.1 VOLTAGE RATIOS
The standard voltage ratios at no-load shall be as specified in  the Bill Of Qualities and/or Drawings and shall in generalbe as
follows :-
(a) 11000/433-250 V, so as to deliver load at the declared voltage of 415/240 volts, 3 phase, 4 wire system with the neutral solidly earthed.
(b) 11000/3300 V
(c) 6600/433-250 V

5.2 WINDING CONNECTIONS AND VECTOR GROUP
Unless otherwise specified, the winding of all 3 phase step-down transformers shall be connected as follows :-

Higher Voltage – Delta
Lower Voltage – Star
Vector Group – Dyn 11

5.3 IMPULSE WITHSTAND VOLTAGE

Completed transformers arranged for service but with arcing horns removed shall be capable of withstanding the following negative polarity 1.2/50 microseconds impulse voltage on the higher voltage windings :-

Nominal System Voltage Impulse Withstand Voltage
( Volt rms) ( Kv peak )
———————- ————————-
3300                  45
6600                  60
11000                75

All impulse shall be carried out in accordance with the provisions of BS 923 BS 171 or IEC 76. Type test certificates may be accepted in lieu of separateimpulse tests at the discretion of the S.O.

5.4 IMPEDANCE VOLTAGE

For the purpose of protection against short circuit stresses,the minimum value of impedance voltage at 75 degrees Celsius rated current shall be as follows :-

4.75% for transformer up to 1000 KVA
5.5% for transformer between 1000 – 1500 KVA
6.0% for transformer between 1500 – 2000 KVA

5.5 FLUX DENSITY

The maximum flux density at any point in the magnetic circuit when the transformer is connected on the principal tapping and operating at normal voltage and frequency shall normally be inthe region of 1.55 to 1.65 Tesla, but at the time of tendering alternative designs employing higher flux densities may bemay be submitted. However with such higher flux densities for consideration the magnetic circuit shall not unduly saturate during system over voltage conditions.

5.6 LOSSES

The no-load and load losses shall be separately quoted in the Schedule of Particulars And Guarantees at the time of tendering. The losses shall be low commensurate with the economic use of materials such that the total capitalised cost is a minimum. The no- load and full load losses shall not exceed the values as follows :-

Size No Load Loss Full Load Loss
(KVA)              (Watt)                (Watt)
—– ———— ————–
100                     220                     1950
250                     650                     2100
315                      700                    3800
500                     970                     5400
800                    1450                    9500
1000                   1650                  10500
1250                    1900                 13000
1600                    2450                17000

5.7 TAPPING RANGE AND METHOD

Tappings shall be provided on the higher voltage winding for a variation of the no-load voltage, as specified in clause 5.0 of + 5.0%, + 2.5%, 0%, – 2.5% and – 5.0%. All tap changing shall be carried out with the transformers off circuit by means of an externally operated tapping switch with tap
position indications. The operation handle shall be mounted on the tank side and provision shall be made for padlocking in any tap position. Padlocks of any approved typeshallbe supplied by the Electrical Contractor.

5.8 LIMITS OF TEMPERATURE RISE

The transformers shall be designed for continuous operation at their rated power without exceeding the temperature rise limits as follows :-
In Top Oil : 50 degrees Celsius In winding : 55 degrees Celsius

5.9 CLASS OF WINDING INSULATION

The class of winding insulation shall be Class A to BS 2757.

5.10 NOISE AND VIBRATION

Due care shall be taken to ensure that the design and manufacture of all transformers shall be such as to reduce noise and vibration
to a level normally obtained with good modern practice.


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