Power Factor Correction Equipment: advantages and disadvantages

January 28th, 2013 | Posted in Power quality
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Normally, the of the whole load on a large generating station is in the region of 0•8 to 0•9. However, sometimes it is lower and in such cases it is generally desirable to take special steps to improve the . This can be achieved by the following equipment:

Static capacitor

The power factor can be improved by connecting capacitors in parallel with the equipment operating at lagging power factor. The capacitor (generally known as static capacitor) draws a leading current and partly or completely neutralizes the lagging reactive component of load current. This raises the power factor of the load. For three-phase loads, the capacitors can be connected in delta or star.

Advantages

  1. They have low losses
  2. They require little maintenance as there are no rotating parts
  3. They can be easily installed as they are light and require no foundation
  4. They can work under ordinary atmospheric conditions

Disadvantages

  1. They have short service life ranging from 8 to 10 years
  2. They are easily damaged if the voltage exceeds the rated value
  3. Once the capacitors are damaged, their repair is uneconomical

Synchronous condenser

A takes a leading current when over-excited and, therefore, behaves as a capacitor. An over-excited running on no load is known as synchronous condenser. When such a machine is connected in parallel with the supply, it takes a leading current which partly neutralizes the lagging reactive component of the load. Thus the power factor is improved.

Advantages

  1. By varying the field excitation, the magnitude of current drawn by the motor can be changed
    by any amount. This helps in achieving step less †control of power factor
  2. The motor windings have high thermal stability to short circuit currents
  3. The faults can be removed easily

Disadvantages

  1. There are considerable losses in the motor
  2. The maintenance cost is high
  3. It produces noise
  4. Except in sizes above 500 kVA, the cost is greater than that of static capacitors of the same
    rating
  5. As a synchronous motor has no self-starting torque, therefore, an auxiliary equipment has to
    be provided for this purpose

Phase advancers

Phase advancers are used to improve the power factor of induction motors. The low power factor of an induction motor is due to the fact that its stator winding draws exciting current which lags be-hind the supply voltage by 90 degrees. If the exciting can be provided from some other a.c. source, then the stator winding will be relieved of exciting current and the power factor of the motor can be improved. This job is accomplished by the phase advancer which is simply an a.c. exciter. The phase advancer is mounted on the same shaft as the main motor and is connected in the rotor circuit of the motor. It provides exciting to the rotor circuit at slip frequency. By providing more than required, the induction motor can be made to operate on leading power factor like an over-excited synchronous motor.

Advantages

  1. As the exciting ampere turns are sup-plied at slip frequency, therefore, lagging kVAR drawn by the motor are considerably reduced
  2. The phase advancer can be conveniently used where the use of synchronous motors is inadmissible

However, the major disadvantage of phase advancers is that they are not economical for motors below 200 H.P.


What do you think about it? Do you have others advantages or disadvantages in mind? If yes, just add a comment so that we can discuss about it.

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Comment(s) to “Power Factor Correction Equipment: advantages and disadvantages”

  1. Al-Firasah says:

    Before installs of capacitor
    kVAri = kW*tan(thetai) ……… (1)

    After installs of p.f.correction
    kVArf = kW*tan(thetaf) …….. (2)

    The value of capacitor, kVAr = kVAri – kVArf

    equation (1) – (2)
    kVAri – KVArf = kW*tan(thetai) – kW*tan(thetaf)
    kVAr = kW*(tan(thetai) – tan(thetaf)) …… (3)

    i = initial
    f = final
    theta = angle of power factor
    kVAr = reactive power = 117.6
    kW = real power = 100

    assume that,
    cos(thetai) = 0.85, therefore tan(thetai) = 0.6197
    cos(thetaf) = 1, therefore tan(thetaf) = 0.0000

    where
    thetai = initial angle
    thetaf = final angle
    kVAri = initial reactive power
    kVArf – final reactive power
    kW = real power of electrical load

    The reduction in current after installs of capacitor, Ir is:

    Ir = [kVAi – kW*cos(thetai)]/(sqrt(3)*V) ……. (4)

    where V in kV = nominal voltage supply from supply authority = 0.415kV

    The reduction of power in cable = Ir^2*resistance of cable ………… (5)
    assume that resistance of cable 0.005 ohm

    From equation (3): kVAr = 100*(0.6197 – 0)
    = 61.97

    From equation (4): Ir = [117.6 – 100*0.85]/(1.732*0.415)
    = 32.6/0.7188
    = 45.4 Amperes

    From equation (5): The reduction in power = 45.5^2*0.005
    = 10.4 Watts

  2. sinnadurai sripadmanaban says:

    Please give a chart for selecting capacitors for PFC of individual motors of different hp,speed,design code,starting kVa/kW etc
    Also another chart for selecting capacitors for shunt capacitor type starting of induction motors.

  3. Jamal Kimolo says:

    everything has been explained by a fellow member, no comment on that.

  4. Al_Firasah says:

    By using equation: kVAr = kW*(tan(acos(thetai))-tan(acos(thetaf)))

    Assume kW = 1.00
    cosΦf 0.85 0.86 0.87 0.88 0.89 0.9 0.91
    cosΦi from 0.02 49.37 49.40 49.42 49.45 49.48 49.51 49.53
    0.03 32.70 32.72 32.75 32.78 32.81 32.83 32.86
    0.04 24.36 24.39 24.41 24.44 24.47 24.50 24.52
    0.05 19.36 19.38 19.41 19.44 19.46 19.49 19.52
    0.06 16.02 16.04 16.07 16.10 16.12 16.15 16.18
    0.07 13.63 13.66 13.68 13.71 13.74 13.77 13.80
    0.08 11.84 11.87 11.89 11.92 11.95 11.98 12.00
    0.09 10.45 10.47 10.50 10.53 10.55 10.58 10.61
    0.1 9.33 9.36 9.38 9.41 9.44 9.47 9.49
    0.11 8.42 8.44 8.47 8.50 8.52 8.55 8.58
    0.12 7.65 7.68 7.71 7.73 7.76 7.79 7.82
    0.13 7.01 7.03 7.06 7.09 7.11 7.14 7.17
    0.14 6.45 6.48 6.51 6.53 6.56 6.59 6.62
    0.15 5.97 6.00 6.02 6.05 6.08 6.11 6.14
    0.16 5.55 5.58 5.60 5.63 5.66 5.69 5.71
    0.17 5.18 5.20 5.23 5.26 5.28 5.31 5.34
    0.18 4.85 4.87 4.90 4.93 4.95 4.98 5.01
    0.19 4.55 4.57 4.60 4.63 4.65 4.68 4.71
    0.2 4.28 4.31 4.33 4.36 4.39 4.41 4.44
    0.21 4.04 4.06 4.09 4.12 4.14 4.17 4.20
    0.22 3.81 3.84 3.87 3.89 3.92 3.95 3.98
    0.23 3.61 3.64 3.66 3.69 3.72 3.75 3.78
    0.24 3.43 3.45 3.48 3.51 3.53 3.56 3.59
    0.25 3.25 3.28 3.31 3.33 3.36 3.39 3.42
    0.26 3.09 3.12 3.15 3.17 3.20 3.23 3.26
    0.27 2.95 2.97 3.00 3.03 3.05 3.08 3.11
    0.28 2.81 2.84 2.86 2.89 2.92 2.94 2.97
    0.29 2.68 2.71 2.73 2.76 2.79 2.82 2.84
    0.3 2.56 2.59 2.61 2.64 2.67 2.70 2.72
    0.31 2.45 2.47 2.50 2.53 2.55 2.58 2.61
    0.32 2.34 2.37 2.39 2.42 2.45 2.48 2.51
    0.33 2.24 2.27 2.29 2.32 2.35 2.38 2.40
    0.34 2.15 2.17 2.20 2.23 2.25 2.28 2.31
    0.35 2.06 2.08 2.11 2.14 2.16 2.19 2.22
    0.36 1.97 2.00 2.02 2.05 2.08 2.11 2.14
    0.37 1.89 1.92 1.94 1.97 2.00 2.03 2.06
    0.38 1.81 1.84 1.87 1.89 1.92 1.95 1.98
    0.39 1.74 1.77 1.79 1.82 1.85 1.88 1.91
    0.4 1.67 1.70 1.72 1.75 1.78 1.81 1.84
    0.41 1.60 1.63 1.66 1.68 1.71 1.74 1.77
    0.42 1.54 1.57 1.59 1.62 1.65 1.68 1.71
    0.43 1.48 1.51 1.53 1.56 1.59 1.62 1.64
    0.44 1.42 1.45 1.47 1.50 1.53 1.56 1.59
    0.45 1.36 1.39 1.42 1.44 1.47 1.50 1.53
    0.46 1.31 1.34 1.36 1.39 1.42 1.45 1.47
    0.47 1.26 1.28 1.31 1.34 1.37 1.39 1.42
    0.48 1.21 1.23 1.26 1.29 1.32 1.34 1.37
    0.49 1.16 1.19 1.21 1.24 1.27 1.29 1.32
    0.5 1.11 1.14 1.17 1.19 1.22 1.25 1.28
    0.51 1.07 1.09 1.12 1.15 1.17 1.20 1.23
    0.52 1.02 1.05 1.08 1.10 1.13 1.16 1.19
    0.53 0.98 1.01 1.03 1.06 1.09 1.12 1.14
    0.54 0.94 0.97 0.99 1.02 1.05 1.07 1.10
    0.55 0.90 0.93 0.95 0.98 1.01 1.03 1.06
    0.56 0.86 0.89 0.91 0.94 0.97 1.00 1.02
    0.57 0.82 0.85 0.87 0.90 0.93 0.96 0.99
    0.58 0.78 0.81 0.84 0.86 0.89 0.92 0.95
    0.59 0.75 0.78 0.80 0.83 0.86 0.88 0.91
    0.6 0.71 0.74 0.77 0.79 0.82 0.85 0.88
    0.61 0.68 0.71 0.73 0.76 0.79 0.81 0.84
    0.62 0.65 0.67 0.70 0.73 0.75 0.78 0.81
    0.63 0.61 0.64 0.67 0.69 0.72 0.75 0.78
    0.64 0.58 0.61 0.63 0.66 0.69 0.72 0.74
    0.65 0.55 0.58 0.60 0.63 0.66 0.68 0.71
    0.66 0.52 0.54 0.57 0.60 0.63 0.65 0.68
    0.67 0.49 0.51 0.54 0.57 0.60 0.62 0.65
    0.68 0.46 0.48 0.51 0.54 0.57 0.59 0.62
    0.69 0.43 0.46 0.48 0.51 0.54 0.56 0.59
    0.7 0.40 0.43 0.45 0.48 0.51 0.54 0.56
    0.71 0.37 0.40 0.43 0.45 0.48 0.51 0.54
    0.72 0.34 0.37 0.40 0.42 0.45 0.48 0.51
    0.73 0.32 0.34 0.37 0.40 0.42 0.45 0.48
    0.74 0.29 0.32 0.34 0.37 0.40 0.42 0.45
    0.75 0.26 0.29 0.32 0.34 0.37 0.40 0.43
    0.76 0.24 0.26 0.29 0.32 0.34 0.37 0.40
    0.77 0.21 0.24 0.26 0.29 0.32 0.34 0.37
    0.78 0.18 0.21 0.24 0.26 0.29 0.32 0.35
    0.79 0.16 0.18 0.21 0.24 0.26 0.29 0.32
    0.8 0.13 0.16 0.18 0.21 0.24 0.27 0.29
    0.81 0.10 0.13 0.16 0.18 0.21 0.24 0.27
    0.82 0.08 0.10 0.13 0.16 0.19 0.21 0.24
    0.83 0.05 0.08 0.11 0.13 0.16 0.19 0.22
    0.84 0.03 0.05 0.08 0.11 0.13 0.16 0.19

  5. Bilal Aslam says:

    These steps are very useful , but but we can also better the power factor by replacing the induction motor (if being used ) with same rating of synchronous motor . it would draws same load with less power.

    • sinnadurai sripadmanaban says:

      What will be the impact of torque/speed curve on the load characteristics compared to induction motor?..

  6. Prakash Bedekar says:

    Power factor correction and harmonic mitigation go hand in hand and should be tackeled in totality

  7. Irvin says:

    Pretty! This was an extremely wonderful article. Thank you for supplying these details.

  8. How do you select harmonic filters ,by datalogger recording harmonics over a period or some other means?.
    What are the advantages/disadvantages of switching capacitors by contactors & thyristers?.

  9. Ibps says:

    Thanks for any other informative web site. Where else could I am getting that type
    of information written in such an ideal method?
    I have a venture that I’m simply now working on, and I’ve been on the look out for such info.

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