Power Factor Improvement by Capacitive VAR Compensation - Power System Analysis Lab

Laboratory Title

Power Factor Improvement by Capacitive VAR Compensation

 

Objective

The purpose of this experiment is to demonstrate the power factor correction procedure at a customer bus bar for three-phase loads.

 

Apparatus

1.        Lab volt Setup

2.        3 Phase Resistive Load

3.        3 Phase Inductive Load

4.        Power Supply Unit

5.        Connection Wires

6.        Data Acquisition and Control Interface

7.        Host Computer

 

 

Procedure

 

  

 

1.        At the beginning, all the equipment were connected according to the figure. The power supply was used to implement the AC power source. And, the resistive and inductive load were used to implement the 3 phase load. The capacitive load was used to implement the 3 phase capacitor.

 

2.        Then, the Y- connected 3 phase load was set as 960Ω and, the reactance of the each inductor was set to 686Ω using switches.

 

3.        The reactance of the 3 phase capacitor was kept equal to infinite.

 

4.        The 240V (Line-to-Line) 50 Hz power supply was connected to the 3 phase load.

 

5.        Then, the system was powered up and follow measurements were taken using the host computer.

l  Line-to-line voltage (E1, E2) 

l  Line Current (l1, l2)

l  Total Active Power

l  Total Reactive Power

l  Total Apparent Power

l  Power Factor (El1, El2)

 

6.        Then, the capacitive load was set as Power Factor is as close as possible to unity using the switches.

 

7.        Again, all the measurements were taken for this case and the reactance of the each capacitor also.

 

8.        Then, the measurements of with capacitors and without capacitors were compared.

Observations

 

Discussion

 

3.      Power factor correction (PFC) aims to improve power factor, and therefore power quality. It reduces the load on the electrical distribution system, increases energy efficiency and reduces electricity costs. It also decreases the likelihood of instability and failure of equipment.

In power factor correction the procedure is to reduce the lagging power factor in inductive loads by fixing a high value capacitor across the phase and neutral close to the load. Power factor correction is obtained via the connection of capacitors which produce reactive energy in opposition to the energy absorbed by loads such as motors, locally close to the load. When the Voltage and Current are in phase with each other in an AC circuit, the energy from the source is fully converted into another form to drive the load and in this case power factor is in unity. This improves the power factor from the point where the reactive power source is connected, preventing the unnecessary circulation of current in the network. When the power factor drops, the system becomes less efficient.

4.      The major effect of poor power factor is higher value of line current. We know that power factor (pf) is an important parameter for calculation of power in an AC circuit. For a given power and voltage, the current flowing through the line is inversely proportional to the power factor. This means that a poor power factor i.e. low power factor will result in higher load current and hence higher losses.

The four detrimental effects of low power factor on the distribution system of the are the following.

·         Large Copper Losses

·         Requirement of large kVA Rating of Equipment

·         Greater Conductor Size

·         Poor Voltage Regulation

 

·         Requirement of large kVA Rating of Equipment

Electrical equipment / machines are generally rated in kVA. But as per the power triangle

∴ CosØ = P / S = kW / kVA

⇒ kVA = kW / CosØ 

Thus, lower value of CosØ will lead to higher value of kVA rating. This in turn will result in higher machine losses. Therefore the size of machine will increase and hence its cost will also increase.

• Greater Conductor Size:

Poor power factor is not only important for loads but it also plays an important role is power transmission. As discussed, low power factor causes higher line current. As the current carrying capacity of a conductor is directly proportional to its cross-sectional area, higher current will require greater conductor size. Thus the size of transmission conductor increase.  

• Large Copper Losses:

Ohmic loss (I²r loss) is also known as copper loss. Larger current due at low power factor causes more ohmic loss and hence reduced efficiency.

• Poor Voltage Regulation:

Percentage Voltage Regulation of Transmission Line is given as

% Voltage Regulation = [(V_S – V_R) x100/ V_R ] %

Since large line current causes larger voltage drop in the line impedance. Therefore the receiving end voltage (V_R) will be much lower than the sending end voltage (V_S). Hence voltage regulation will be poor.

3.      Because the Energy stored depends on the square of the voltage. So using DELTA ensures line-line voltage and more reactive energy stored than in STAR using phase voltages.

 

Typically line to phase voltage is √3 larger and since E=0.5CV2 it ensures 3 times the energy connected in DELTA than STAR for the same value of capacitance.