Capacitors Selection for Power Factor Improvement

This article is for selection and application of capacitors for power factor correction. Main keywords for this article are Capacitors Selection for Power Factor improvement, Lagging Power Factor, Sources of Reactive Power, Economics of Power Factor Correction, Switching of Capacitors.

Capacitors Selection for Power Factor, Lagging Power Factor, Sources of Reactive Power, Economics of Power Factor Correction, Switching of Capacitors

Capacitors Selection References

Institute of Electrical and Electronic Engineers (IEEE)
18 Standard for Shunt Power Capacitors
141 Recommended Practice for Electric Power Distribution for Industrial Plants
National Electrical Manufacturers Association (NEMA)
CP-1 Shunt Capacitors
National Fire Protection Association (NFPA)
70 National Electrical Code

Capacitors Selection for Power Factor Improvement

For the purpose of understanding this standard, the following definitions apply.
Apparent Power (kVA).
The product of the current times the voltage or kVA, which at unity power factor is equal to real power.
Power Factor (pf).
The ratio of the real power to the apparent power, and is the cosine of the angle between the voltage and the current.
Real Power (kW).
The actual mechanical work or heat produced by the electric current, which is the product of the voltage and the ‘in phase’ component of the current.
Vars.
The product of the voltage and that component of the current at right angles to the voltage. If the current is leading the voltage, the vars are leading as produced by capacitors. If the current is lagging the voltage, the vars are lagging as produced by inductive loads.

Lagging Power Factor

  • The voltage drop caused by load current flowing over a circuit or through a transformer is a function of the current magnitude and the phase relation between the current and voltage. The voltage drop in large power circuits and transformers is mainly due to the inductance of the circuit. Induction motors constitute a large part of the plant load, and operate at a lagging power factor, because they require a constant magnetizing current that flows regardless of the load driven by the motor.
  • The inductance component of the voltage drop is a direct function of the sine of the phase angle. Power factor correction reduces the magnitude of the current and the phase angle and thus reduces the voltage drop.
  • A lagging power factor indicates that the power system is supplying current to the load for lagging vars. While the lagging vars are not real power, the current to create these vars flowing through the power system does cause real power to be consumed in the form of losses.
  • The reactive current for a lagging power factor load increases the overall current in the circuit and thus uses more of the circuit capacity than would be required for a unity power factor load. Power factor correction frees up system capacity in conductors, transformers and generators to supply more real power which can do mechanical work.

Sources of Reactive Power

It is highly desirable to maintain the system power factor in the range of 0.9 lagging to unity. Power factor improvement is necessary to accomplish this. The power factor can be improved by supplying leading vars from capacitor banks or synchronous motors.

Shunt Capacitors

  • Capacitors for power factor correction shall be applied as shunt capacitors and shall comply with IEEE 18 and NEMA CP-1.
  • Addition of capacitors is normally the most economical way to improve plant power factor. Capacitors  produce leading vars which cancel out the lagging inductive vars required by motors.
  • Capacitors can be applied as a few large banks connected to the substation bus, or as smaller units connected to individual motors, see section 8. A single capacitor bank is usually not feasible, due to
    different correction being required under various plant operating requirements.
  • Larger capacitor banks are more economical than smaller units connected to individual loads. However, if the capacitors are being applied to free up system capacity to the maximum extent, they should
    be connected near the terminals of the individual loads. If the purpose is to correct the overall plant power factor, they can be installed as one or more large capacitor banks.
  • Addition of capacitors to an electrical system can cause certain operating problems. The most common problem is over voltage transients. With proper care in selecting the size of the capacitors and the
    connection point on the power system, these problems can be avoided.
  • Application of capacitors on a power system supplying adjustable speed drives or other harmonic producing loads requires special attention. Harmonic filters may be required on the capacitor bank to
    prevent serious overvoltage problems.
  • All capacitors shall have drain resistors as required by NFPA 70, to ensure the charge is drained off quickly once de-energized.

Synchronous Motors

  • Synchronous motors can also be used to provide leading vars but they are an expensive source of vars.
  • Synchronous motors are more expensive than induction motors, and also require a pilot field supply and control system. However, in certain applications synchronous motors are preferred due to mechanical characteristics of the driven load. In general, synchronous motors should not be specified to solve power factor problems. If they are specified for other reasons, then the engineer should take advantage of them to correct the system power factor.
  • Synchronous motors can be specified with unity power factor or various leading power factors, with 0.8 leading being a common rating. When 0.8 leading power factor is specified, the machine has to have  the capacity to handle the increased kVA, and thus is more expensive than the same kW rating at unity power factor.
  • If the synchronous machine(s) represents a large percentage of the plant load, unity power factor machines may be adequate to increase the overall plant power factor above 0.9.

Economics of Power Factor Correction

  • The utility company usually bills for power with a rate schedule which charges for energy plus demand. The energy charge is for kilowatthours as measured by a watthourmeter. The billing demand takes into account the maximum demand made on the power company’s facilities to support the customer’s load. A load with a poor power factor utilizes more of the utility system capacity than the same real power load at a higher power factor.
  • Correction of the power factor above 0.95 is not recommended. A point of diminishing return is reached where more and more kvar has to be added to obtain the same reduction in power factor. For a given load, approximately twice as many kvar would be required to improve the power factor from 0.95 to unity as would be required to correct the power factor from 0.8 to 0.85. The power factor should be corrected to 0.9.
  • When using capacitors for power factor correction, the cost of the switching device(s) shall be considered. Even with the cost of the circuit breaker or switch, capacitors are usually more economical than synchronous motors.
  • Installation of capacitors on individual motors avoids the cost of an extra switching device.However, the higher per unit cost of kvar for smaller capacitors makes this more expensive than a few large capacitor banks.

Capacitors Installed on Individual Motors

Addition of capacitors to individual motors offers the advantages of correcting the power factor problem at its source, and automatic control of the capacitors.

  • Capacitors can be connected directly at the motor terminals and switched with the motor as a unit. Such an arrangement is often desirable, as the capacitors are in service only when the motor is operating. The addition of capacitors for power-factor improvement does not change the motor performance characteristics, as the operating speed and shaft output depend upon the motor load and applied voltage, if the frequency is constant.
  • Capacitors shall not be installed on motors being supplied by variable frequency drives. The capacitor impedance will vary too widely due to the varying frequency, and thus cause problems with the drive. Capacitors may be successfully applied at the input to the variable frequency drive.
  • There are two important considerations which limit the kilovar rating of a capacitor which can be connected to the motor terminals and switched with the motor.
  • The first consideration is transient torques. If the motor and capacitor combination is reconnected to the line before the motor field collapses, it is possible to obtain transient torques as great as 20 times
    full-load motor torque (peak transient mechanical torque should not be permitted to exceed about 6 times full-load torque if damage to shaft and couplings is to be prevented). The actual transient torque developed depends upon the magnitude of the motor voltage, the angle between the motor voltage and the line voltage when reconnection takes place, and the ratio of load inertia (wk²) to total inertia. The higher transient torques occur when the load inertia is a major portion of the total inertia, and the motor voltage is above 50 percent of rated voltage.
  • The second consideration is overvoltage due to self-excitation. When a motor and capacitor combination is disconnected from the line, the motor continues to rotate for some period of time dependent
    upon the total inertia of the drive. During this period, the capacitor supplies magnetizing kilovars to excite the motor, which then acts as a generator. The voltage which can be produced by this generator action is dependent upon the capacitor kilovars and the motor speed. It is possible to produce 140-160 percent of  rated voltage.
  • The rule of thumb provided for capacitor sizing by IEEE 141 is that the capacitor connected on the load side of the motor controller should not exceed the value of capacitance required to correct the no-load power factor to unity. The use of the specific motor manufacturer’s recommendations for capacitors shall be the preferred method of sizing capacitors. IEEE 141 provides typical tables as a reference.
  • Capacitors shall not be connected directly to the motor circuit to correct for power factor in the following instances:
    a. Motors subject to reversing or plugging
    b. Motors that are restarted while still rotating, for example critical duty motors with 1 to 2 s time delay relay for restarting after voltage dip
    c. Multispeed motors
    d. Motors started by open-transition reduced voltage starters
  • Motor overload protection can be affected by capacitors applied to individual motors.
  • When capacitors are connected to the motor leads on the load side of the starter, the current through the overload relays will be less than the motor current. In order to properly protect the motor, overcurrent devices shall be selected on the basis of the reduced current, in accordance with NFPA 70.
  • When capacitors are applied on the line side of overload relays, no change is required in the overload settings because the current through the overload relay is the motor current. The reduction in
    current due to the capacitors is seen from their electrical effects back through the power system.
  • The capacitor circuit conductors and disconnecting means shall comply with NFPA 70 and have a  minimum size defined as following:
    a. The ampacity of capacitor-circuit conductors shall not be less than 135 percent of the rated current of the capacitor. The ampacity of conductors that connect a capacitor to the terminals of a
    motor, or to motor circuit conductors, shall not be less than one-third the ampacity of the motor circuit conductors.
    b. The ampere rating of the capacitor disconnecting device shall not be less than 135 percent of the rated current of the capacitor, to allow for overcurrent due to overvoltage at fundamental frequency
    and harmonic currents.

Potential Problems after Power Factor Correction

Switching of Capacitors

  • When large capacitor banks are used to correct the overall plant power factor, it is frequently necessary to switch the capacitors in two or more groups to avoid overcorrecting during light load periods. Switching a capacitor bank when there is another capacitor bank in close proximity (electrically) can cause severe switching transients.
  • When switching a single capacitor bank, the current transients are a function of the system impedance as seen by the capacitor bank, and seldom exceed the fault current previously present at the bus. The switching of an isolated capacitor bank does not usually present a problem.
  • When switching a capacitor bank in parallel with and in close proximity to another energized capacitor bank, the transient current is limited only by the impedance between the banks. This impedance is quite low and results in very high transient current values.
  • In most cases, it should be possible to place adequate impedance between capacitor banks to avoid the back-to-back switching problem. Capacitor banks can be applied at the 480 or 4160 V level with no more than one capacitor bank installed per bus. With the capacitors installed in this manner there will always be the impedance of two transformers between the banks.

Capacitor Installations With Harmonics

  • The inductance and capacitance in a power distribution system form a resonance circuit. This is usually not a problem, as the typical values of circuit inductance and capacitance cause the resonant
    frequency to be well above the 60 Hz operating frequency. The addition of capacitors lowers the resonant frequency to a value within the range of harmonics found in the power system.
  • Harmonics near or at the resonant frequency of the power system can cause sever voltage spikes and current surges. The surges can not only damage the capacitors but also the motors, transformers and other components of the power system.
  • To ensure safe operation of capacitor banks on power systems with harmonic producing loads, a filter shall be installed with the capacitor bank. The filter consists of an inductor installed in series with the capacitor bank which is chosen to tune the capacitor bank below the first characteristic harmonic of the harmonic source.
  • With the capacitor bank tuned below the first harmonic, it actually de-amplifies the harmonics, and the capacitor bank – inductor combination acts as a harmonic filter while improving the power factor.
    However, additional capacitor vars are required to offset the inductive vars of the tuning reactor.

 Overvoltage Due to Leading Power Factor

  • Just as a lagging current flowing through the inductance of a circuit creates a voltage drop, a leading  current flowing through an inductive circuit will cause a voltage rise. If the system power factor goes leading, the bus voltages may go above acceptable tolerances.
  • If fixed capacitors are installed to correct the power factor at maximum load, the power factor will go leading under light load conditions. The load for most industrial plants is constant, but may vary for unit turn arounds or batch process plants. If capacitors are installed on individual motors, they will automatically be disconnected with the motors. If capacitor banks are installed on the bus, automatic switching will be required to control the power factor under light loading.

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