Voltage Problem

You're stuck with either up-sizing the cable or transforming the source voltage to a higher value and reducing it at the load end.

Cable insulation rating will limit the amount you may raise the voltage.

When you say there are 4 joints in the way do you mean splices in the cable? Each splice will cause resistance.

I agree with BILL about starting with a higher voltage.But remember that if you raise the source voltage you will need to ensure that your cable is rated for that higher voltage. You may need to run a new cable. You can then use a transformer to step down the voltage at the gate office and guest house.

Good luck.

Your voltage is being dropped along the long length of the cable due to the conductor's resistance. You need to do the voltage drop calculation before selecting the cable. Find the resistance of the conductors from the cable manufacturer and calculate the voltage drop, the voltage drop shall be limited to a specified range (generally less than 5%). You already installed the cable – the solution now is to replace the cable with higher size or run more parallel cables to reduce the voltage drop.

Simple voltage drop calculation formula:

For Single phase: 2 x I x R x L

For three phase: 1.732 x I x R x L

Where, I = current passing through per conductor, R = resistance per unit length (meter or ft), L = length of the cable (in meter or ft)

- MS

How much load do you have at the site?

Dear All, Thanks for ur response.

We have a step up transformer if I can install that in the one third of the way from the source or from the destination. The load at the guest house and the gate office is total about 125 A. Its tappings on the secondary side are 400V, 415V and 430V. Kindly suggest me is this the right way.

Best REGARDS

WOW. You are trying to push a 125 amps through 0.932056788 miles of wire.

The utility would use 11 kv distribution line to do this. What is the name plate voltage output of the step up transformer? Need the voltage ratio to guess at the resultant voltage. You would connect the 300 volts to the 400 volt tap. Place the transformer at the guest house and not at the 1/3 point. The voltage will probably still be low.

You should lay a length of cable without any joint in between. There is always a waiting potential problem with those cables joint. If the cable joint is not done properly, it will caused heat generated at the cable joint point and increase your cable resistance in thus your voltage will drop. Heat generated at the cable joint will also able to damage your cable and most of the cable failure is at the joint portion.

Increasing the voltage in the network able to help up to a certain stage but increasing the voltage too much will also effect other electrical apparatus that is connected to the main distribution board.

What I can recommend is as follows

a) Check and measure your final output voltage at the end of the cable (Gate Office)

b) Conformed the final voltage at the end of the cable at your gate office.

c) Then calculate and installed a step up transformer at your gate office with protection such as MCCB for the primary side and ELCB or ZPCT (Zero Phase Current Transformer) for the secondary side of the transformer

Since your requirement of the power is about 380V at 125A, the step up transformer will be as follows;

a) Primary Voltage 300V (voltage at your gate office)

b) Secondary voltage will be 380V/400V/420V with 3 tapping

c) Step up Transformer power (KVA) = about 100 KVA (base on 420V calculation)

Since your cable is 150mm2, the current carrying capacity is about is 300 Ampere. Therefore, your primary step up transformer will be about 192 Amp (full load consumption) and your secondary side of your step up transformer, you can have the maximum current about 151A for 380V, 144.3A for 420V and 137A for 420V

I read your last post wrong. What is the rating of the primary voltage? Do you have taps on the primary?

Before using the transformer, first try this:

1. Measure the voltage at your Gate office without any load. I am expecting it is almost 380 V.
2. Apply around 50% of load (60 A) and measure the voltage (assume it is 340 V)
3. Apply as much as load up to full load (125 A) and measure the voltage (assume it is 300 V).

The voltage regulation of your Guest house to Gate office system is (380-300) x 100 / 380 = 21% (very poor)

Now if you use a transformer of 300V/380V at the Gate office, the output voltage will be ok (380 V) when there is full load. But if the load decreases (say 20% of full load, 25 A), then the drop across the cable will decrease to 20% ((80 x 25)/125 = 16 V only) causing the transformer input voltage increase to 364 V which eventually increases the transformer output voltage to 364 x (380/300) = 461 V. This voltage will be supplied to the 20% loads of the Gate office and you can think what will happen to these poor loads. If your load is guaranteed to be 100% all the times, then it may be ok, otherwise the voltage keeps increase with the decrease of loads.

So, think before proceed.

- MS

Dear All, The tapping on the primary side is 380v

GA especially this bit:-

you should not use a step-up transformer as has been pointed out, this will lead to excessive over-voltage at light loads.

Regards.

You have not mntioned the IP supply voltage & whether a single or 3 phase.

dear All,

Thanx for your valuable discussion. I have checked today the voltage at the distribution board from where the guest house and the gate office will be fed. To my surprise with 70 percent load ON the voltages were LINE TO LINE 362V ac. Kindly suggest what can be the possible reason of this. Two days before at the same distribution board I was getting only 300V ac.May be the capacitor bank of MCC or source feeding to that distribution board put ON today by any supervisor without informing to anyone.

Best

If you want to boost up some voltage at your gate office, you can add some capacitor bank at your office DB Board. It will help to boost up about 15 to 20 volts.

If you added capacitors, that will definitely give you a voltage rise. See if you can get the size of the caps in KVAR. We can calculate the rise.

Simon

The voltage rise depends on the capacitor size. How did you get the 15v or 20 volts?

Dear All, We are using the capacitor bank of 25 KVAR at the source. It might happen that the power factor improvement panel did not work and the voltages in the field were less. But now the voltages have been improved. Best Regards,

Check the voltage at the source and see if it has increased.

egards.

Power Factor correction is not that simple to achieve a positive result unless you have the indicating of + & - power factor.

Or with Auto-Correction system installed

Dear All,

we are going to install at the guest house the step up transformer that will help us to take the voltage from 362 V to 380V . The step up transformer has on its primry side the tapping of 380V and on the secondary side the tappings are 400V,415V and 430V.whts ur suggestions?

Check out buck-boost transformers. Here is an example.

Dear ,

We have already a one transformer of type step up and its tappings I have already mentioned. I will use it at Guest House.

Best Regards,

Be careful!

Is the load at the guest house constant? If not what do you think will happen to the voltage when the load is very small?

Dear Randall, Sometimes the load is very light when there is only 1 or 2 guests so I think the voltages will rise n may make a danger for the appliances what do u suggest now

I'm by no means an expert in this area, but:-

I'm afraid you only have two options, and both are expensive.

1.) Lay much thicker cable.

2.) Install high voltage cable; with a step up transformer at the source end, and, a step down transformer at the far end.

If you do a google search for "voltage drop calculator", and try some of the online calculators you will get a feel for the volts drop. I believe you should be trying to restrict it to less than 3%.