Cut Off Current And Peak Current Energy Though Curves Fuse And MCB – Parameters Of The Supply Circuit And Transformer – Engineering Assignment Help

Assignment Task -

Q1. (a) One 50m-long circuit for a water pump is made by one 3/c PVC/SWA/PVC cable, and is protected by DPX125 or DPX-E125 MCCB with a current rating in the range of 16A-125A  as seen in Appendix Q1. The cable is fed from a main LV switchboard, and installed on a  tray with one touched similar cable. SWA of the cable is used as the circuit protective  conductor. The earth fault loop impedance external to the installation at the main LV  switchboard Ze is equal to 0.3 ohm. The fault level at the main LV switchboard is 16 kA.  The ambient temperature is 35 degree. 

With the information of the motor given below 

Motor rating 30 kW, Power factor 0.87, Efficiency 0.87, Starting current 4 x full load current, Starting time 4 sec 

i) Select the model, rating (In) and setting (Ir and Im) of MCCB using the datasheet in  Appendix Q1. 

ii) Determine the cable size for this circuit by following the cable sizing procedure. 

iii) Check whether the requirements of overcurrent protection are fulfilled or not. If not,  provide a measure for achieving the protection. You need to explain, with the  calculation, the requirements are fulfilled. 

State all your assumptions clearly. 

(b) Describe physical differences between a cut-off current and a peak current, and sketch  energy let-though curves of a fuse and a MCB.

Q2. Shown in Fig. Q2 is part of the electrical schematic for a building, with relevant parameters of  the supply circuit and transformer. The main LV switchboard is protected by a 1600A MICB  (main incoming circuit breaker). One 100m-long outgoing circuit is wired with two 400mm2  PVC/SWA/PVC cables on tray, and is protected by a 630A ACB (air circuit breaker). The fault  level at the main LV switchboard is assumed to be 32kA, and the earth fault loop impedance at  the main LV switchboard is 20 mohm. Both the 1600A and 630A circuit breakers are  controlled with IDMT relays for overcurrent and earth fault protections. Electronic relays with  SI and EI curves are available for selection.  

(a) Determine the MAX and MIN fault currents at the remote end of the outgoing circuit.   

(b) Select relay types, and determine PS and TM of both overcurrent and earth-fault relays for  MICB so that both the supply company’s requirement & the requirement of protection against indirect contact can be achieved.  

Q3.(a) A capacitor bank was installed at a main LV switchboard for the improvement of power  factor. An engineer measured the current into the capacitor bank with a harmonic analyzer  and found the harmonic current distortion was very high (especially the 11th order harmonic). Do capacitors generate a harmonic current? With the aid of appropriate diagrams and formulas, explain why the harmonic distortion was so high, and provide a mitigation measure  with explanation. 

(b) A 70m-long 120mm2 XLPE/SWA/PVC cable was installed on tray to deliver power to the balanced loads in a computer centre. The cable is protected by a 250A fuse, and is fed from a  main switchboard on the ground. The total power consumed by computers is 120 kW with the  displacement power factor of 0.9. The ambient temperature in the duct is 40oC.

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