Voltage unbalancing problem due to overhead line

Baljinder,

You said that sometimes the 11 kV motor malfunctions (overheated?). I assume that most of the time the motor runs OK. This means that sometimes the line voltages feeding your motor are not equal (unbalanced), but most of the time the voltages are equal (the motor runs well).

The overcurrent is the result of voltage unbalance, which in turns creates a corresponding current unbalance.

Normally the currente unbalance (in % of the nominal current)is much greater than the corresponding voltage unbalance. In general the ratio of voltage unbalance to current unbalance is close to 8 to 1. This means that a mere 1% voltage unbalance can produce 8% of current unbalance in 3-phase motors. This will heat up the motor operating at the rated load, even when there is a very small voltage unbalcne in the line voltages. As a consequence the motor life will be shortened and the power consumption will increase. This is the reason you should take care of the problem. If you cannot control the line voltages (the power authorities should do this), then you must control the amount of load you apply to the motor.

My advice:

• The first thing you should do is to calculate or measure the percentace of voltage unbalance. If you do not have the right instruments to make the measurements, then contact the manufacturer of the motor.
• If this number exceed 1% or 2% (rule of thumb, but some people accept 5%) then you should reduce the load to the motor. There are many tables available from the manufacturers of motors that show by what percentage the load should be reduced as a function of the percentage of voltage unbalance. This information is normally supplied by the manufacturer of the motor.

Hope this will help.

Abe Michelen

First of all, You got to find out where is the actual problem causes the voltage unbalance. Is the unbalance voltage is comming from your local power station?. If it is so, there is nothing you can do about it. Your local power station authorities has to solve this problem. But if the unbalance voltage is generated by your load, then you have to find someway to solve it by install some filters to improve the power factor. Try to measure or recorded down the incoming voltage from your local power station by measuring the 11KV voltage from the incoming breaker P.T. (Potential Transformer). You should check your P.T. ratio before do the measurement. Usually, for 11KV incoming supply, the PT is rated at 11KV/110V. Which means, at full 11KV voltage, you should get 110V at the output of the P.T. If any voltage unbalance, your P.T. will also show the voltage drop. Since I do not know what is your intake voltage to your plant, I presume that your intake is 11KV from your local power station. If your intake is 11KV directly from your local power station and you drive a 11KV load, you will have this unbalance voltage problem. My sugesstion is that, if possible, try to apply for higher voltage for example, 33KV from your local power station authority and install a step down transformer from 33KV/11KV to drive your 11KV load. This will give you more stable voltage. In my plant, my intake voltage is 132KV where we step down to 33KV and 11KV. Further stepdown to lower voltage like 3.3KV and 415Volts. There is a lot of power meter avaliable in the market to measure voltage dip, surge and unbalance voltage.

If I understand correctly you are receiving power supply from the electric supplier through their 40 KM long 33KV overhead line . You are having step down transformer at your load centre either your own or that of supplier? Another information required is weather there are other power consumers on the same 40 KM feeder. You have to conform first that the unbalancing of voltage happens at Power dispatching centre of supplier or at your receiving end . The solution of your problem depends on these points. Is the unbalancing is at your place ?Is there any other consumer on your feeder ? If no check for dry joints or cracked joints on the 40 KM OH line . So give more details to find solution. The problem can be solved with proper investigation of the transmission system . also check up the tap changing switches of the transformers for improper or loose contacts or loose connections specially Cable lugs of cable terminations.

Dear friend,

Requirment of Ring Main Distribution is to maintain the power supply uninterrupted, which is nothing but alternate feeding.

Your voltage unbalance may be due to provision of unequal no. of single phase 11/ 230 V transformer with local grounding in the three phases. The impact of laod in the LV side these transformers will cause un balance in voltage.

Out of balance in voltage will be protected in the case of EHT Transformer. Norammly No MV Transformers will be equipped with this protection.

Please check the load distribution all the three phases. To inform the local power authorities to check this.

Regards,

N.Suresh