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The Secret Cost of Implementing High Efficiency Equipment

With the introduction of energy efficient equipment and lighting technologies in recent years, hydro users everywhere, whether they be commercial, industrial, or residential have been able to shave their utility costs. With the rate of hydro increasing and the cost of technology going there are greater and greater incentives to upgrade aging equipment and lighting with new high efficiency alternatives. However most of these companies do not take into consideration the harmonics and the power quality issues that come with the use of this energy efficient equipment. Through this report I will detail the possible additional costs that could be accrued with the implementation of these high efficiency equipment.

Harmonics where do they come from?

As more high efficiency equipment and lighting is now being implemented in building systems there are some problems are beginning to arise. Most loads on the grid prior to the introduction of energy efficient equipment were considered linear loads meaning the relationship between voltage and current is linear and proportional (Figure 1). Today’s technology’s such as computers, variable-speed drives, and electronic fluorescent lighting ballasts utilize semiconductor technologies. Semiconductor materials operate non-linearly in their response to voltage (Figure 2). Generally semiconductors are used in energy efficient devices to chop or cut the pure sinusoidal waveform (Figure 3) either to reduce power consumption or to control frequency, such as with VFD`s. This fast switching or change in voltage causes harmonics. Harmonics have a direct effect on power quality producing a highly distorted waveforms (Figure 4).

Voltage Current Relationship Voltage Response
Figure 1:  Voltage Current Relationship for Linear Components  Figure 2: Voltage Response of a Diode Semiconductor
Figure 3 Figure 4
Figure 3 Figure 4
(note diagram displays 50 Hz fundamental frequency not the North American 60 Hz)


Why are we concerned?

Equations 1
Equations 1

Harmonics occur at higher frequency than the pure fundamental frequency of 60 Hz. For instance 2nd harmonic occurs at 120 Hz, 3rd occurs at 180 Hz, 4th occurs at 240 Hz, and so on. Consider the reactance formulas (Equations 1) for an inductor and capacitor, by equation an increase in frequency causes an increase in inductance and a decrease in capacitance. Due to this relationship harmonics can wreak havoc on power systems and cause many issues.

 

 

 

Power Factor

As power factor can be considered a ratio of real power produced from purely resistive loads and the apparent power which is the geometric sum of real and reactive power in a system. An increase in inductance due to the higher harmonic frequencies causes an increase in reactive power causing the power factor to lag. There are many ways to combat lagging power factor including power factor correction capacitor banks as well as synchronous condensers. All will come at some additional cost and should be considered when installing a new high efficiency system.

Effect on power System Components

Conductors

The reactance formulas rely on a phenomena known as the skin effect where at higher frequencies electrons are forced to the outside of conductors and thus are provided with less cross sectional area of the conductor to travel through. This increases the resistance in any conductor and thus there are additional conductor losses that also should be considered. This effect also increases the heat produced by the conductors. To ensure conductors do not overheat derating of cables must be considered when installing a new high efficiency system.

Transformers

Figure 5 Eqv. Transformer Circuit
Figure 5 Eqv. Transformer Circuit

The harmful effects of Harmonics on transformers are often not noticed until an actual failure occurs. In many instances transformers will be operational for extremely long period of time until a building overhaul utilizing high efficiency components causes them to fail. Consider the equivalent circuit for a transformer below notice the large amount of inductors. The reactance formulas once again come into play, the inductors in the circuit represent the magnetic core of the transformer as well as transformer windings. Remembering that an increase in frequency results in an increase of impedance resulting in power losses that must be considered. This increase of impedance also causes the transformer to overheat and thus diminishes the life of a transformer. It is important that transformers also be properly derated when installing any high efficiency equipment.

Motors

Figure 6 Eqv. Motor Circuit
Figure 6 Eqv. Motor Circuit

Variable Frequency drives are more and more often used to drive motors, chopping the pure sine wave to slow the rotation

of the motor. This is an issue as VFD’s produce a large amount of harmonics, consider the equivalent circuit for a motor. A similar effect occurs of that of the transformer as the high frequency of the harmonics causes an increase in resistance and thus overheating of the motor resulting in a shorter life span. It is important for any large VFD motor installations that a harmonic analyses be performed prior to operation to determine the levels of harmonic distortions and determine their impact on the motor.

Capacitor Banks

Cap Banks as mentioned earlier are often used to correct power factor for a system that lags. Once again consider the reactance formula for a capacitor where an increase in frequency results in a decrease of impedance. Because of this, capacitor banks act like a sink for harmonic currents as the impedance of the bank will drop significantly by overloading the bank. This overloading results in an increase of thermal and dielectric stress to the cap banks thus reducing their life span. The more costly effect of harmonics on a capacitor bank is the possibility of a resonance condition. This is where the inductive and capacitive components in a system are equal. This causes the system to resonant and can cause catastrophic failure of capacitors.

Conclusion

VFD’s and high efficiency lighting are a great way to reduce utility costs for any building. However when implementing these systems the power systems they are connected to must be taken into careful consideration. Without proper derating of power system components and power factor correction these high efficiency equipment can wreak havoc on any power system.

References

Images
Figure 1
Figure 2 https://learn.sparkfun.com/tutorials/diodes
Figure 3 http://gnu.ets.kth.se/~nt/elecpow/le_lamps/
Figure 4 http://www.selectricity.com/harmonicdistortionanalysis.html
Figure 5 https://en.wikipedia.org/wiki/Transformer
Figure 6 https://en.wikipedia.org/wiki/Motor