By Dave Woodside and John Clemens
There is no doubt that capacitors that are properly sized, located, controlled and monitored arevery beneficial to a facility's operation and maintenance. In any electrical installation whose loadvaries significantly from full to light load, there should be proper control and monitoring of powerfactor to avoid any detrimental effects to the facility's electrical equipment.
Regardless of the specific reasons for correcting power factor, (avoidance of utility power factorpenalty, to reduce demand charges, to release transformer capacity, etc.), power factor correctioncapacitors are an excellent and economical means to improve the efficiency of the electricalsystem.
Benefits Of A High Power Factor
A power factor between 90 per cent lagging and unity is considered good. A power factor below90 per cent lagging, depending on the utility, will result in a power factor penalty or increaseddemand charges to the customer.
The addition of properly sized and controlled capacitors will improve the power factor andeliminate the extra charges. The pay back period for capacitors is between six months and twoyears.
There are other advantages in adding capacitors such as reduced I2R losses in the facility,increased system capacity, the reduction of heating losses in transformers and distributionequipment and increased voltage stability.
To the utility, there are advantages for having a customer base with a high power factor because itreduces I2R and I2X losses, releases more system capacity to expand the customer base andincreases voltage stability.
The Importance Of Proper Control And Monitoring Of Power Factor
If the load profile changes significantly during normal operations or the load is reduced duringperiods of lower productivity or maintenance, the amount of capacitance should be properlymonitored and controlled to decrease or increase the amount of capacitance as required tomaintain the voltage within acceptable limits.
It has been our experience that this problem is not uncommon in many commercial and industrialfacilities whose power factor is over-corrected during periods of light load and the voltage ishigher than the acceptable limit. A voltage higher than 110 per cent of rated is undesirable.
In one case, we encountered a voltage 120.8 per cent higher than rated (580 volts on a 480 voltsystem) which is intolerable and will affect any devices still connected such as lightning loads,control and power transformers, capacitors and any motors still operating. The over-voltagecondition will decrease the life of the equipment and increase the maintenance costs. The utility isalso affected by customers with low leading power factor because of increased I2R and I2X lossesand the potential for voltage increase in neighboring customers facilities.
Power Quality
There are two main power quality problems that can arise from capacitors in an electrical system:harmonic resonance and transients.
Harmonics are caused by non-linear loads such as variable frequency drives, UPS systems,fluorescent lights, etc. A power system has a natural resonant frequency and the addition ofparallel capacitors will lower the resonant frequency. If this new resonant frequency coincideswith the characteristic harmonics of a harmonic-generating load, the current can excite theresonant circuit which will cause an oscillating current to be exchanged between the systems twostorage elements (the system capacitance and inductance).
The high harmonic currents can produce high harmonic voltages which can effect the operatingcharacteristics of electrical/electronic equipment and can force harmonic currents to flow inadjacent circuits. Harmonics can cause protective devices to malfunction. Capacitors can attract harmonics from neighboring customers into the facility.
Resonance problems can be avoided by having a harmonic study performed to determine ifharmonic filters are required to correct power factor.
If 20 per cent or more of a facility's loads are harmonic generating, a full power quality studyshould be performed by an experienced consultant.
Transients can be caused by capacitor switching which can seriously affect electrical/electronicequipment. There are various products on the market to suppress these disturbances.
Capacitor Testing
Capacitors should have a functional test upon installation to determine if the capacitor is operatingat its rated kvar. Once properly installed capacitors are, for the most part, maintenance free.
The only concern regarding capacitors is that in the unlikely event that one phase of a three-phasecapacitor bank failed, the voltage would be unbalanced. This voltage imbalance will significantlyreduce motor life. There is a simple test to check for capacitor failure if no capacitor stateindication system is in place. The test involves taking current readings of each phase to see if thecurrents are balanced.
Dave Woodside and John Clemens are with C.J. Power Limited located in Maple Ridge, B.C.
