voltage drop

[aid elect]

in my company one new blasting unit(300 kw )installed 200 m away from substation. they laid one 4 x 300 sq mm cable. now there is voltage drop. with out load 415 v, full load 380 v. have any other option instead of laying additional cable ? like capacitor bank in the receiving end will work ?

 

[dpc]

How about changing the transformer tap?

 

[IRstuff]

Sounds like you have some other problem; 200 m of 1200 sq. mm cable should only have 2.8 mohm of resistance.

https://www.omnicalculator.com/physics/wire-resistance;

you've got more than 10x that resistance from the IR drop, so something else isn't right.

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[che12345]

Dear Mr. aid elect

1. " changing the transformer tap" [to a higher tapping] if (available/accessible) as suggested by learned Mr dpc cost nothing but a very good solution.
2. Mr. aid elect: "they laid one 4 x 300 sq mm cable ..."
Mr. IRstuff: " Sounds like you have some other problem; 200 m of 1200 sq. mm cable should only have ...."
They laid [one] 4C x 300mmsq i.e. [1 cable] having 4 conductors. It is NOT [4 cables] each having 4 conductors i.e. 16 conductors. It is NOT having 4 conductors [in parallel] i.e. 4 x 300mmsq = 1200mmsq.
3. Adding capacitor [at the motor end] would be a good proposal. It would help but the capacitor can only be sized to a certain limit. Take note not to [over-sized] the capacitor.
4. If and (only if) running at 380V but the (full-load current) is [lower than the motor name-plate rating], than may be have to live with it. It is very common that the motor is over-sized for the load; therefore, the running full-load current [is lower] than the motor name-plate current rating.
[It is ok to run at 380V with full-load current lower than the motor name-plate current rating].

Che Kuan Yau (Singapore)

 

[IRstuff]

OK, so total area of 300 mm^2 --> 11.2 mohm, which is less than 1/3rd of the measured IR drop; that's still a disconnect

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[edison123]

Why do you need 4 core cable for a 3 phase motor?
Poor quality of copper/aluminum in cables
Poor terminations (improper cable lugs fixing)
Loose connections (improper tightening of terminals)

Muthu

http://www.edison.co.in

 

[IRstuff]

oooh... not copper, not good

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[che12345]

Dear All,

1. IRstuff : "...total area of 300 mm^2 --> 11.2 mohm, which is less than 1/3rd of the measured IR drop; that's still..."
Refer to BS7671 4C x 300mmsq current rating : non-armoured PVC 464A , armoured XLPE 628A.
A 300kW 415V motor full-load would be around 495A approx.
Depending on whether armoured or non-armoured and the type of insulation PVC or XLPE , the voltage-drop reported is of concern.
2. edison123 :
2.1. " Why do you need 4 core cable for a 3 phase motor?"
May be [3 conductors one for each line and 1 conductor (full-size) for earthing (CPC) ?.
2.2 "Poor quality of copper/aluminum in cables"
[Quality] of copper used for cable by reputable manufacturer is (never) a concern.
It is [unlikely an aluminium cable] as the (current rating would be far too low) for a 300kW motor..
2.3 " Poor terminations (improper cable lugs fixing)
Loose connections (improper tightening of terminals)"
Agreed.
If the terminations/contacts are property done, the contact resistance would be in the micro-ohm range. The voltage-drop would be minimal.

Che Kuan Yau (Singapore)

 

[7anoter4]

In my opinion, the remaining voltage will be 394 V [5.1% VD?]
What is your actual load? 300 kW [580 A?]
cosfi=0.85 eff=85%
BS7671 for 90oC insulation 4*300 mm^2 copper r=0.140 x=0.12 ohm/1000 m [multiplied by √3 already]. So VD=(0.140*200/1000*0.85+0.12*0.527*200/1000)*580=21.14 V [ 21.14/415*100=
5.09%]

 

[Padlock01]

Hi Aid Elect
I have attached an excel calc yes you do have a problem even a soft start won't solve it, unusual reticulation but hey not my cup of tea.

 

[7anoter4]

Hi Padlock01
Your excel file is very interesting. However, I have some questions.
How do you know the 300 MW load is of a single induction motor ?
How do you know if the cable is copper conductor PVC insulated made? It may be XLPE?
Why 40 oC ambient [ in IEC 60364-5-52 it is only 30oC].It could be underground also.
The resistance and reactance of the cable is larger than per BS7671[ or calculated following IEC 60228 and IEC 60502-1] even for 90oC.
What standard [if any] do you follow for this calculation way?

 

[VTer]

You must not only consider voltage drop in the conductors but also the drop due to source impedance which impacts the total voltage drop. I am assuming that the voltage measurements are made at the load terminals, therefore you can split your thevenin impedance into two components consisting of source impedance and conductor impedance. This will give you a good idea of the impact on voltage drop due to source stiffness and conductor impedance. From there you can consider a few options such as changing taps of upstream transformer if present. However that may affect other loads, but you may have some flexibility there and could be good cost effective solution. Other options will most likely require purchase of additional hardware such as buck-boost transformers, power electronic solutions, capacitors...

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[ScottyUK]

IRstuff, you need to account for cable reactance as well as DC resistance. In the larger sizes reactance becomes dominant. On really large stuff the AC resistance is higher than the DC resistance due to skin and proximity effects, in addition to the reactance.