please give details of the load during full load and during Off working hours.

also write details of PF improvement panels.

As Sanjay mentioned, there is not enough information to determine the reason, something very common is when you leave the capacitor bank connected all the time, then when disconnecting the main load, your system may become capacitive and have a lower power factor, but there is not enough information to make sure what is going on.

Capacitor bank, could be increased, at low power factor.....

Right Prof. Desai, Power Factor Improvement can be performed by installing the additional Capacitor Bank in order to decrease the Reactive Power (Q, VAR) of the power plant.

I would like to share the following link:

http://www.eaton.fr/ecm/groups/public/@pub/@electrical/documents/content/sa02607001e.pdf

During working hours load is domestic (including inductive ) 120A and during off working hours only lighting load is ON which is 30A.

Capacitor banks are on at all time 

Hello Mr. Rana,

According to your mentioned data, you only apply the lighting load during the off-working hours. Therefore, the power factor must increase due to lack of inductive loads.

(1) Do you check the performance of the installed Capacity bank? I think there is something wrong in it. You should its performance by changing your loads. Turn off all your lighting loads and operate the other loads.  

(2) What types of lighting loads are you applying? Are you applying the Normal Power Factor Fluorescent lamps? Then, you should change the High Power Factor Fluorescent lamps.

(3) Investigate the electrical wiring systems and other switch gear of your plant.

Regards,

Induction motor (under leading power factor) can also be used.. But yeah please specify what is the load.

That's right, Mr. Sandeep.

However, due to Mr. Rana's question, the power factor problems at his plant is related with lighting loads, may be poor power factor ratings of fluorescent lamps. 

Try to inject leading  reactive power to the system near load side.

http://www.electrical-installation.org/enwiki/Why_improve_the_power_factor%3F

use  Automatic Power Factor Control . You will get it from Companies like Crompton Greaves ,ABB ,  Areva  etc who is havig power quality business.

If your capacitor bank is ON all the time and is about right for full load, then there is too much capacitance at light load. Your power factor is again low (but negative), the only think is that the phase voltage/current is reversed. A low power factor can be due not only but too much iductive load but also to too much capacitative load.

At low load you should remove capacitors.

Basic idea behind the power factor correction is to nullify the effect of inductive reactance by adding the equivalent effect of capacitive reactance.

In your case, capacitance contributed at the time of load (above 80%) is sufficient to nullify the effect of inductance (i.e, current and voltage is nearly in phase).

when your load goes down to 30% during off working hours, the capacitive reactance is higher than the inductive reactance. Hence the overall reactance is increases (i.e, current is leading the voltage with some angle).

Solution to this problem is to use automatic adjustment of capacitive reactance by controlling the fraction of total capacitor. 

Use capasitor banks, it's the simplest way to improve power factor.

Capacitor banks can be used to improve the power factor. When it comes to light load or full load correspondingly you can sense and connect or disconnect the capacitor banks using the relays automatically

The size of capacitor banks or any energy storage system may not be sufficient to sustain a high rate of power unbalances between source and loads. If your source is a wind generator, you can adjust the turbine pitch-angle to decrease the input power to the wind-turbine or if it is a PV system you can adjust the position of your PV-pannel to sustain only the 30% loads.

In order to improve the power factor, in general, it is necessary to follow the following steps.

- Identify the type of network (transmission, distrbution, commercial, industrial).

- Improved power factor is related to the voltage improvement.

- The most common way is to act on the transit of reactive power.

- In transmission networks, we can use shunt or serie capacitors. The use of Facts must be correlated with the other performance targets.

- For other types of networks, it is necessary to describe the type of load or the aggregate  loads (linear, non-linear, balanced, unbalanced, concentrated, dispersed) and their time behavior (stationary, variable, intermittent, etc. ). So you can begin your  study in the light of what follows. One can, therefore, use simple shunt (fixed or switched) capacitors or other sophisticated and more costly devices such as Facts.

- It is well known that the no loaded  or ligntly loaded  power transformers and asynchronous motors have a very low power factor.

In your case, check whether your power factor improvement panel  is equipped with an automatic power factor regulator (APFR). If it's not there, it can be installed as an additional module in your panel (with of course an additional cost).

-Of course, in the case of networks with distributed ressources (multi-directional power flow), the power factor improvement procedures need not, in general, the presence of capacitors as long as possible all states of 'power flow are well established and a more diverse qualitative estimate is also identified.

Power factor correction can be achieved by placing capacitor bank of distributed generation real load centers of suitable size.

It is commonly known that most power distribution systems are designed so that power flows in one direction from the substation to the customers. The installation of a distributed generator (DG) unit on a feeder changes this situation since it represents an additional source of power. A convenient way to characterize DG interaction on such feeders is called “zero point analysis” which focuses on the point on the feeder where power flow is zero. Such analysis is similar to that involving the impact of a shunt capacitor on reactive power flow.

 The best location for a DG unit in relieving feeder loading is at the end of the feeder. If the DG is installed at the beginning, it makes no impact on the loadings of the feeder. It is important to keep in mind that DG output and feeder load will not be correlated over time, hence the zero point will move, perhaps over a wide section of the feeder. Furthermore, multiple DG installed on the same feeder can results in multiple zero points.

 The impact of DG on feeder losses can be analyzed in a similar fashion as capacitor placement studies on distribution systems that optimize power loss. Capacitor placement to optimize power loss also applies to DG installation. Capacitors reduce losses by minimizing reactive power flow, while distributed generators reduce losses by minimizing active power flow.

 DG will provide voltage support, just like a capacitor installation. A sufficiently large DG installation, however, may result in an overvoltage in some parts of the feeder. The highest overvoltage point will be at the DG interconnection point. The closer the DG installation is to the beginning (substation), the higher the possibility of overvoltage occurrence. When a DG is installed towards the end of the feeder, it will raise the voltage thus providing voltage support.

•  In the case of networks with distributed resources (multi-directional power flow), the power factor improvement procedures need not, IN GENERAL, the presence of capacitors as long as possible all states of 'power flow are well established and a more diverse qualitative estimate is also identified.