Edited by Robert Repas
Energy-Efficient Motion, tinyurl.com/mmoujx
Efficient AC, tinyurl.com/nkc79e
Most loads applied to modern electrical-distribution systems are inductive. Electric motors, transformers, and high-intensity discharge lighting make up a large portion of the power consumed in an industrial setting and all contribute to the inductive load. An inherent problem with inductive loads is that they lead to poor PFs that degrade overall system-operating efficiency. When applied correctly, power-factor-correction (PFC) systems can eliminate the negative effects of a poor PF.
Readers will probably recall that inductive loads need two types of power: Working or real power, measured in kilowatts (kW), performs the actual work of creating heat, light, motion, and so forth. But inductive loads also need a second type of power known as reactive power, measured in kilovolt-Amps reactive (kVAR), to sustain the magnetic field inherent to inductive loads.
When combined, working and reactive power form apparent power measured in kilovolt-Amps (kVA): the total power supplied to the system by the power source.
The power factor (PF) of a circuit is the measure of electrical distribution efficiency. In simple terms, it is the ratio between the amount of real power used to perform actual work (kW) and the apparent power (kVA) supplied by the power source. For example, if the apparent power of a circuit was 200 kVA, but only 190 kW of actual work is done, then the PF is said to be 0.95 or 95%.
Low PFs can lead to higher energy costs, larger wire needs (to carry the excess reactive current), inefficiencies in power transmission leading to heat losses, and premature equipment failure. Utilities commonly charge penalties when the PF drops below a predetermined level, typically less than 0.95.
|Contactors within banked or group PFC systems are typically in close proximity to low-impedance capacitors that are already charged. The short wire lengths add little impedance to the charge path letting charged, low-impedance capacitors combine with the power source to produce inrush currents that exceed the ratings of normal contactors.|
Fortunately, the condition that creates a poor PF is correctable through the use of capacitors that cancel inductive effects by acting as reactive power generators.
By providing reactive power to the circuit, capacitors help reduce total current drawn from the power source, raising the PF closer to the ideal 1.00 or 100% figure. However, too much capacitance can tip the scale in the other direction, creating a capacitive-reactive load that again lowers the PF. The trick is to supply just enough capacitive reactance to cancel the inductive reactance of the circuit.
There are two basic types of capacitor installations: individual capacitors on linear or sinusoidal loads, and banks or groups of fixed or automatically switched capacitors. Individual PFC uses a single capacitor of specific value that is assigned or switched to the load. This technique works well when the inductive load remains constant.
With banked or group PFC, multiple capacitors are automatically switched in or out of a circuit by a VAR controller. The controller continuously measures PF, then switches capacitors in and out of the circuit to compensate for changes in the inductive load reactances. Thus circuit PF stays above a preset level. Both installations improve the PF of the circuit, minimizing the apparent power needed from the utility.
While both types of PFC have been around for many years, new technologies used in the manufacture of capacitors impact the demands on each installation. New materials and manufacturing processes have reduced the internal impedance in capacitors. This makes the capacitors more effective at PFC, but also makes them more difficult to switch in and out. The low internal impedance leads to the creation of a high inrush current as the capacitor charges.
Capacitors in individual PFCs are typically installed as close as possible to the equipment so the power cables see the reduced reactive power. The current path from the medium-voltage transformer to the capacitor contains numerous individual impedances that generally help limit the peak inrush currents to less than 30 × In, where In is the capacitor steady-state operating current.
In individual PFC applications, the PFC capacitors are wired in parallel with the load, and both are switched with the same power contactor. Because peak inrush current is generally within the make capacity of a standard electromechanical contactor, and inrush current time is limited to a few milliseconds when power is first applied, individual PFC applications can typically use standard electromechanical contactors.
Banked or group PFC
With banked PFC, the physical location of the contactors and capacitors is typically in the area of the low-voltage transformer. This usually means the operating voltage and available short-circuit current are generally high. The power ratings of specialty contactors designed for switching capacitors relate to these voltages.
Capacitor charging current in a group PFC comes not only from the main supply power with its internal impedance, but also from other charged low-impedance capacitors within the bank. For this reason, peak inrush currents when switching banked capacitors can exceed 150 × In. That level of current far surpasses the make capacity of standard electromechanical contactors. A current of this magnitude strips the anti-weld material from the contact alloy, reducing the contacts to pure silver. The contacts would weld after just a few hundred switching operations. To handle current at this level requires special contactors and design considerations.
Contactors designed for banked or grouped PFC use special anti-weld contact material to prevent the higher inrush currents from welding the contacts. They also include current limiting resistors that work in conjunction with a special early-make auxiliary contact to precharge the capacitors so that the primary contacts never see the peak inrush current levels. The earlu make auxiliary set of contacts closes first, connecting the capacitors to the power source through current-limiting resistors. The resistors let the capacitors charge while limiting inrush current. After a short time delay, the main contacts close, and conduct only the continuous current. The XTCC capacitor contactors from Eaton are examples of PFC contactors that operate in this manner.
Another option equips PFC systems with upstream chokes. The growing use of nonlinear loads in electrical installations has led to a rising level of harmonics in the main supply lines. In some cases, the higher harmonic levels can lead to damage due to thermal overload of the PFC capacitors. Upstream chokes help eliminate this damage and offer other potential benefits such as: correcting reactive power, removing undesirable harmonics from the main power line, and avoiding resonance phenomena. Moreover, they work well with electrical power networks that have ripple control systems.
Beyond these advantages, upstream chokes also dampen inrush current peaks. Thus, when switching choked PFC stages, it may be possible to use standard electromechanical contactors. The same would also apply for nonchoked systems, when an inductance greater than 5 H is added between the contactor and capacitor.
The contactors must be sized correctly and must be capable of continuously conducting 1.5× the capacitor current to conform to EN 60931-1. Another consideration for contactors used in banked or group PFC is the distribution of switching operations among all of the contactors. Often, multiple contactors used in group PFC continuously remain on while only a few actually perform control tasks. To boost contactor lifespan, switching operations should be evenly distributed across all of the contactors within the PFC system. Various manufacturers of PFC capacitors offer VAR controllers that cyclically exchange the switching sequence of the contactors to distribute wear and lengthen contactor life.