Precautionary Best Practices for Electrical Power Quality
Electrical power quality disturbances can disrupt production by causing equipment malfunction or failure and reduce the life of electrical equipment. Power quality problems can cause frustration, be difficult to diagnose and expensive to solve. A few precautionary steps can help you to avoid them.
Electrical power quality disturbances can disrupt production by causing equipment malfunction or failure and reduce the life of electrical equipment. Power quality problems can cause frustration, be difficult to diagnose and expensive to solve. A few precautionary steps can help you to avoid them.
Add Line Reactors to Each Variable Frequency Drive (VFD)
Nuisance tripping of VFDs
VFDs are popular for fan and pump applications due to energy savings available at reduced motor speed. VFDs include overvoltage protection that shuts down the drive when harmful transients are sensed by the protective circuitry. This is often referred to as nuisance tripping because it can cause process interruption or material scrap. A line reactor connected in series with the input terminals of a drive can absorb voltage transients and enable thedrive to ride through many of these events. It also protects the input rectifiers from damage due to voltage transients. For transient protection, use3% impedance reactors as a minimum.
Reduction of harmonic current
Line reactors are the first step towards reducing harmonic current for individual drives. Use 5% impedance line reactors to reduce harmonic current distortion for individual drives to about 35% THD-i or less at fullload (could be 50% to 100% without reactor). Line reactors are generally the lowest cost means of reducing harmonic distortion for VFD systems.
If further attenuation of harmonics is required, then various types of harmonic filters may be added. Using line reactor will generally reduce the cost of further harmonic filtering. Voltage Notch Reduction Line reactors are useful on DC motor drives or other SCR/Thyristor controllers.These devices cause voltage notching which can interfere with zero crosscircuitry due to extra zero crosses caused by the notches. Line reactors connected at the input terminals of the SCR rectifiers will provide impedance at the right place and reduce notch depth.
Improve PF to 0.95
Your electrical resources can be used more effectively byimproving power factor to 0.95. You will also reduce circuit conductor andequipment power losses upstream of the point of capacitor connection. If yourutility adjusts your demand charge based on power factor or bases your demandcharge on kVA (as opposed to kW), then you will be able to reduce your electricenergy cost by improving power factor.
Use Detuned PF Capacitors
Capacitors are a low impedance path to harmonic frequencies andtherefore will absorb significant harmonic current and can fail rapidly whennon-linear (harmonic producing) loads share the same power system. Non-linearloads include power electronics equipment such as adjustable speed drives,uninterruptible power supplies, computers, rectifiers, battery chargers,welders, LED lighting, etc. Since power electronics equipment is common withincommercial and industrial facilities, use detuned capacitors when ever applyingpower factor improvement to a power system consisting of these types of loads.
Measure the True rms CurrentThrough Transformers
Power electronics type loads produce harmonics at frequencies thatare (odd) multiples of the fundamental (60Hz) frequency. These frequencies arenot visible to non-trms metering equipment. Harmonics can cause excessive heatin transformers due to increased amperes and higher frequencies, thus reducingtransformer life. Measure both true rms current and peak current intransformers using true rms equipment. Loads drawing sinusoidal current have acrest factor (CF = pk/rms) of 1.414, however harmonic producing loads havecrest factors above this. Transformers serving significant harmonic producingloads should generally be K-factor transformers. Standard transformers that aremore than 50% loaded and serving harmonic producing loads should be deratedbased on the current waveform crest factor (usable kVA = rated kVA x 1.414/CF).An alternative is to reduce harmonic distortion by using harmonic filters.
Check Transformer Capacity vs. Motor Starting Current
Transformers are often overlooked parts of the electrical system.They are called upon to provide rms and peak voltage and current sufficient forall of the operating loads. While starting across-the line motors, the inrushcurrent may be 5-10 times the motor rated full load current. If the transformerkVA is not sufficient to deliver both the peak motor starting current to themotor in addition to the rms and peak current for all other operating loads,then voltage sags can be expected during motor starting. Check to be certainthat transformers are sufficiently rated to handle motor starting requirements.Consider improving power factor or reducing harmonics so that full transformercapacity is available.
