FAQ

POWER FACTOR EDUCATION

Power factor (PF) is a measure of how efficiently the current supplied by the utility company is converted into useful work. The current supplied by the utility company is TOTAL current. The current converted to useful work is ACTIVE current which is a component of total current. PF is determined by dividing active current by the total current. PF = active current / total current.

Since active current relates to active power (kW) and total current relates to total power (kVA), PF can also be expressed by dividing active power by total power. PF = kW / kVA.

It is important to understand that the total current consists of two components: active current and reactive current. Together they sum to equal total current. The same is true for kVA. kVA consists of two components: active power (kW) and reactive power (kVAr). Together they sum to equal total power.

Now the tricky part. Since active current and reactive current are 90° out of phase, their sum is a vector sum not a simple arithmetic sum.  With this in mind the equation for kVA is not kVA = kW + kVAr but rather (kVA)2  =  (kW)2 + (kVAr)2.

When kW is equal to kVA, the PF is 1. This can be expressed as 100% or unity. If the kW is less than kVA, the PF will be less than 1.

When the PF is unity the voltage and current wave forms are in phase with each other. When the PF is less than 1 the voltage and current wave forms are out of phase. PF is also the cosine (COS) of the phase angle difference between voltage and current. The phase angle difference is typically expressed by the Greek letter Փ. Therefore, PF = COS Փ.

Note: PF defined and discussed above is DISPLACEMENT PF.

Correcting power factor (PF) is the same as improving PF or increasing PF. They all mean the same and are used interchangeably. Correcting PF is accomplished by reducing the total current supplied by the utility company. Understanding that total current consists of two components, REAL current and REACTIVE current, a reduction in reactive current results in a reduction in total current.

Reactive current present in any facility is typically INDUCTIVE current. Inductive current is due to the coils in electrical motors used to create and sustain the magnetic fields. Power factor correction is accomplished by reducing the inductive current which in turn reduces the total current.

To reduce inductive current, power factor correction capacitors (PFCCs) are added to the electrical system. PFCCs provide CAPACITIVE current to the electrical system.

Note that inductive current and capacitive current are both reactive currents. This is because the reactive current wave form is 90° out of phase from the voltage wave form. The difference being the voltage wave form LEADS the inductive current wave form and the voltage wave form LAGS the capacitive wave form. This places the inductive current wave form 180° out of phase with the capacitive current wave form.

Since reactive current relates to reactive power, they are typically quantified as kilo volt-amp reactive (kVAr). And with the understanding that there are capacitive and inductive reactive power we have kVAr CAP and kVAr IND. Taking the 180° phase difference into consideration any addition of kVAr CAP will reduce the kVAr IND by an equal amount, thus reducing the reactive current and total current resulting in improved PF. The addition of kVAr CAP is achieved by adding PFCC to the electrical system.

Inductive loads result in low power factor because the current to an inductive load is reactive current. The most common load having a reactive current component is an induction motor. Other inductive loads include induction heating and magnetic ballast fluorescent lights. The power factor of induction motors is further reduced when motors are lightly or intermittently loaded.

Energy Charge:
The amount of energy consumed or registered during the billing period. This is indicated as kilowatt-hours (kW-h) on the utility bill and represents active energy. Some utility companies may indicate energy charge as kilovolt amperes-hours (kVA-h) consumed during the billing period. In this case, kVA-h represents total energy consumed and includes the reactive energy (kVAr-h) component. Correcting for power factor will not reduce kW-h but will reduce kVA-h.

 

Demand Charge:

This charge compensates the utility company for the capital investment for the infrastructure required to transmit and distribute electrical power. Depending on the utility company demand can be expressed as kilo watts (kW) or in kilovolt-amperes (kVA). Demand charges are calculated by determining the average kW or kVA load during a defined time period where the greatest amount of energy was consumed. The time period used in the calculation is typically 15 minutes or 30 minutes. Demand charges, if expressed in kW, can be reduced by reducing energy peaks. Demand charges, if expressed in kVA, can be reduced by reducing energy peaks or increasing power factor.

 

Power Factor Penalty Charge:

This is a rate structure charge imposed to encourage industrial, commercial and institutional users to decrease the reactive current required by their loads. The power factor (PF) penalty charge is not always easy to recognize on the utility bill. Methods of indicating the PF penalty charge vary from utility company to utility company. The charge is obvious if shown as a PF penalty or PF adjustment. The charge is less obvious when demand is billed as adjusted kW, kVA or energy consumption is billed as kVA-h. With some utility companies, penalty billing is imposed when the PF drops below 95%. In most cases, the least expensive, most efficient and most reliable method to reduce this charge is to increase PF by installing power factor correction capacitors (PFCCs). PFCCs can be fixed or automatic and equipped with or without filter reactors.

 

There are four tangible benefits that result from power factor correction.

  1. Reduced utility charges

            Not to be confused with reduced energy

            Utility charges are reduced by eliminating power factor (PF) penalties, adjustments, etc

            Different utility companies use different methods to determine PF and penalties

 

  1. Release of capacity (less kVA for the same kW)

            Reduces transformer load

            Reduction in transformer size

Allows for additional load when transformers are at maximum capacity

Reduced total current in the plant distribution system (only realized if the power factor correction capacitors are connected at the load)

 

  1. Increased transformer secondary voltage

 

  1. Reduced facility distribution losses

Only realized if capacitors are connected at the load

Improved voltage regulation due to reduced voltage drop

Reduced I2R losses

Reduces real power kW resulting in lower kW demand and lower kW-h energy charges

Insignificant cost savings < 3%, typically only 1%

Every application and installation is different. The greatest savings result from reducing or eliminating power factor penalties/adjustments charged by utility companies. In most cases, the pay back is less than two years. Review the FAQ on the benefits of power factor correction and our Power Factor Correction Application Guide for details on how potential savings can be calculated.

 

 

UNDERSTANDING HARMONIC DISTORTION

AC power is characterized by sinusoidal voltage and current wave forms. These wave forms indicate the instantaneous values of voltage and current. Clean or non-distorted voltage and current wave forms are shown as a perfect sine wave. When these repeating wave forms are not a perfectly shaped sine wave, they are said to be distorted.

Harmonic distortion results from the presence of wave forms having frequencies different than the 60 Hz fundamental frequency. When wave forms of other frequencies are present, they are added to the fundamental 60 Hz. The result is a distorted wave form which is seen as a sine wave but with other peaks and valleys superimposed on it.

These other wave forms are referred to as harmonics because their frequencies are integer multiples of the fundamental 60 Hz wave. For example, a 3rd harmonic wave would have a frequency of 3 x 60 or 180 Hz and a 5thharmonic wave would have a frequency of 5 x 60 or 300 Hz, and so on.

Harmonics are created by the use of non-linear devices such as UPS systems, solid state variable speed motor drives, rectifiers, welders, arc furnaces, LED lighting, photo voltaic systems and personal computers. Individual harmonic frequencies will vary in amplitude and phase angle, depending on the harmonic source.

These non-linear loads draw distorted current wave forms. When distorted current waves are present, they interact with the system impedance causing the voltage wave forms to also become distorted. This is due to Ohm’s law (V=I x Z).

Some indications are overheating, frequent circuit breaker tripping, unexplained fuse clearing, capacitor failures, electronic equipment malfunction, flicking lights, telephone interference, melting of conductor insulation (skin effect).

Typically, power factor correction capacitors will be the first equipment to fail due to harmonic distortion.

Capacitors, particularly power factor correction capacitors, will be the first equipment to be affected by harmonic distortion. A distorted voltage wave consists of voltage wave forms at frequencies greater than the fundamental 60 Hz frequency. Since capacitor reactance is inversely proportional to frequency, as frequency increases so does the capacitor current. For example, a voltage wave form at the 5th harmonic (300 Hz) translates to a reactance, in ohms, to one fifth of the reactance of that of the voltage wave form at 60 Hz. This results in capacitor current at 300 Hz FIVE times that of the current at 60 Hz! Additionally, capacitor current will be amplified if resonance occurs.

The solution can be accomplished by:

  1. Adding or subtracting capacitance from the system to move the parallel resonance frequency to one that is not harmful.
  2. Adding harmonic suppression reactors (de-tuned filter reactors) in series with the capacitor to prevent resonance.
  3. Altering the size of the non-linear devices.

The 5th harmonic is generally considered to be the most offending. The 5th harmonic is of the greatest amplitude of the harmonics associated with drives or any 6-pulse rectification.  It is important to size the filter reactors to the tuning point to address the 5th harmonic. Tuning points at the 4.2nd or the 3.78th are commonly used. Either of these will filter harmonic currents from the capacitors that have frequencies above the specific tuning point and prevent parallel resonance.

Voltage and current amplification results from parallel resonance between the power factor correction capacitors (PFCC) and the power transformer. Parallel resonance will occur if a harmonic frequency is present AND that frequency happens to be the resonant frequency of the PFCC and the power transformer. The resonant frequency of the PFCC and the power transformer is easily determined by calculating the frequency at which the reactance of the PFCC (Xc) equals the reactance of the power transformer (XL).

APPLICATION QUESTIONS

Power factor correction capacitors (PFCC) can be connected at the incoming service, on the bus or at the load. They can also be installed as a combination of the above-mentioned locations. In either of these cases power factor improvement will be realized at the utility meter.

Since not all operations or facilities are the same each application needs to be considered separately.

The location that provides most benefits is at-the-load. PFCCs supply reactive current resulting in reduced total current. Reduced total current occurs within the electrical system form the point of PFCC connection back to the source. This results in additional benefits. Refer to FAQ section “Power Factor Education,” Q5.

In some cases, it is more practical to connect larger PFCC banks on the distribution bus or at the incoming service. These PFCC banks can be either automatic or fixed depending on the amount of kVAr required relative to the power transformer kVA.

A capacitor is an energy storage device. The energy stored in a capacitor when power is disrupted will remain until dissipated by the load or by discharge resistors. This takes a few minutes. Problems can occur when power is disrupted and the capacitor is re-energized before the stored energy has time to dissipate. In this case, the applied voltage can be over twice the rated voltage. This extreme voltage and associated excessive current and heat results in capacitor cell damage and premature failure.

The solution is to provide for a two- to five-minute time delay prior to re-energizing the capacitor after power loss. This time delay is a function of the power factor controller on our automatic systems (Autocapacibank™, Autocapacitrap™). For fixed systems, the time delay can be accomplished by the addition of an auxiliary contactor and timer circuit. This option is available in the Calmount®, Traymount®, Caltrap™, Capacibank®, and Capacitrap® systems.

It is highly unlikely. Since most residential homes in the US are billed for consumed energy which is registered kilowatt-hours (kW-h), the benefit of installing power factor correction capacitors is minimal, if any. The only possible benefit you could achieve is the reduction of I2R line losses. I2R losses are typically achieved by either reducing the resistance of conductors (increasing the size of wires) or reducing the current flowing through them. Either one of these methods will reduce heat generated in the circuit.

Installing a power factor correction capacitor (PFCC) reduces total current, but only from the point where the capacitor is connected back to the source. With this in mind, to reduce the current throughout the house wiring, the PFCCs need to be connected at each load. This is not practical in residential applications. Even if it were, the savings related to line losses would only be 1 to 2% of the kW-h energy charge.

Also, it should be noted that residential PFCC systems presently being marketed are configured to be connected at the electrical service on the load side of the meter. This connection location has zero effect on reducing current throughout the house wiring and the related I2R losses.

If interested in learning more about residential power factor correction, please visit the following links:

Wire size and overcurrent protection supplying power to power factor correction capacitors (PFCC) are based on the full-load amps (FLA) of the particular PFCC and governed by the NEC tap rules and NEC 460.

NEC 460 specifies conductors and switches be sized to the next higher size of FLA x 135% but as low as practicable.

MYRON ZUCKER, INC.