A 20-storey and 60m-tall new office building is to be built in a city business d

Question

A 20-storey and 60m-tall new office building is to be built in a city business district. The total office area is 2500m^2. Assume that per-square-meter average power consumption by a similar building is 100VA/m^2. An allowance of 20% for future growth is reserved for the new building. Line-to-line voltage at a nearby point of three-phase utility power supply is 11kV. Three-phase line-to-line voltage of supply to the new building is 400V.

(a) Use "After Diversity Maximum Demand" (ADMD) method to estimate the power required by the new building.

(b) Calculate the maximum power that utility mains should be able to supply to the building with the consideration of allowance for future growth.

(c) Choose proper distribution transformers to form substation outside but close to the new building. Work out the rated voltages, the suitable numer of trasformers and power rating for each of them.

(d) List other devices and apparatus for the substation and describe how they work to form a reliable and safe power supply to the new building.

Explanation / Answer

Hello,

Here you mentioned the After Diversity Maximum Demand metod, but it is the mean of all custumer maximum demands. You only mentioned the building as single entity. So it is not applicable .

You said, 20 storeand 60m-tall building, and total office area is 2500 m^2. I am taking it as the total building office area.

a)   Per-square-meter average power consumption by a similar building is 100VA/m^2

For total 2500 m^2 area , average power consumption= 2500 * 100 VA

=25 KVA

b) The maximum power that utility mains should be able to supply to the building with the consideration of 20 percent allowance for future growth = 25 KVA + 0.2*25 KVA

= 30 KVA

c) Line-to-line voltage at a nearby point of three-phase utility power supply = 11 kV

  Three-phase line-to-line voltage of supply to the new building = 400V.

   By Observing the above, we have to use a 3-phase ,30 KVA, 11000/400 V ,step-down transformer near to the building and after the distribution of 11 KV.

Power rating of transformer should be greater than maximum demand of power.

therefore the power rating =30 KVA.

d) List of devices and apparatus for the substation

1. Transformer : It is used to step-down the voltage levels

2. Current transformer:It is basically used to take the readings of the currents entering the substation. This transformer steps down the current from 800 amps to 1 amp. This is done because we have no instrument for measuring of such a large current

3. The potential transformer : It is used a bus isolator to protect itself. The main use of this transformer is to measure the voltage through the bus. This is done so as to get the detail information of the voltage passing through the bus to the instrument.

4. The circuit breaker: It is used to break the circuit if any fault occurs in any of the instrument. These circuit breaker breaks for a fault which can damage other instrument in the station. For any unwanted fault over the station we need to break the line current. This is only done automatically by the circuit break. The spring type of circuit breakers is used for small kv stations. The spring here reduces the torque produced so that the breaker can function again. The spring type is used for step down side of 132kv to 33kv also in 33kv to 11kv and so on. They are only used in low distribution side.

5. Isolator: The use of this isolator is to protect the transformer and the other instrument in the line. The isolator isolates the extra voltage to the ground and thus any extra voltage cannot enter the line. Thus an isolator is used after the bus also for protection.

6. Protective relays are used to detect defective lines or apparatus and to initiate the operation of circuit interrupting devices to isolate the defective equipment. Relays are also used to detect abnormal or undesirable operating conditions other than those caused by defective equipment and either operate an alarm or initiate operation of circuit interrupting devices. Protective relays protect the electrical system by causing the defective apparatus or lines to be disconnected to minimize damage and maintain service continuity to the rest of the system.The over current relay responds to a magnitude of current above a specified value.

7.Lightening arrestor: It is the instrument that is used in the incoming feeders so that to prevent the high voltage entering the main station. This high voltage is very dangerous to the instruments used in the substation.

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