Power Factor:PF Correction Techniques

PF Correction Techniques

The term compensation is used to describe the intentional insertion of reactive power devices, either capacitive or inductive, to achieve one or more desired effects in an electric power system. These effects include improved voltage profiles, enhanced stability, and increased transmission capacity. The devices are either in series or in shunt (parallel) with the load(s) at one or more points in the power circuit.

To keep the power factor as close as possible to 100%, utility companies place capacitor banks in par- allel with the load at various locations in the distribution system, offsetting the inductive loading (lagging power factor) of most user equipment. The goal is to create an equal amount of leading PF in the system to match the lagging PF of the load. When balanced, the power factor is 100%. In practice, this is seldom attainable because loads are switched on and off at random times, but utilities routinely maintain an overall power factor of approximately 99%. To accomplish this, capacitor banks are switched automati- cally to compensate for changing load conditions. In addition, static capacitors are used for power factor correction. These devices are similar to conventional oil-filled, high-voltage capacitors. Operating volt- ages range from 230 V to 13.8 kV and higher for pole-mounted applications.

The PF correction capacitors are connected in parallel, with the utility lines as close as practical to the low-PF loads. The primary disadvantage of static PF correction capacitors is that they cannot be adjusted for changing power factor conditions. Remotely operated relays can be used, however, to switch capacitor banks in and out of the circuit as required. Synchronous capacitors, on the other hand, can be adjusted to provide varying capacitance to correct for varying PF loads. The capacitive effect of a syn- chronous capacitor is changed by varying the dc excitation voltage applied to the rotor of the device.

Utilities usually pass on to customers the costs of operating low-PF loads. Power factor can be billed as one, or a combination, of the following:

• A penalty for PF below a predetermined value or a credit for PF above a predetermined value

• An increasing penalty for decreasing PF

• A charge on monthly kVAR hours

• A straight charge for the maximum value of kVA used during the month

PF loads. These conditions result in the need for over- sized cables, transformers, switchgear, and protection circuits.

The copper losses in a system are proportional to the square of the load current. The installation of PF factor correction capacitors near offending low PF loads decreases the reactive power and load current drawn from the utility [17]. This reduced load current translates into lower conductor losses throughout the electrical system. As copper losses are reduced, voltage levels increase throughout the system. This rise can be described by

When individual capacitors are installed at large motors or at a motor control center with several smaller motors attached, the individual capacitor reactive power ratings can be arithmetically summed and the resulting value inserted into Equation 3.2.

On-Site Power Factor Correction

The first step in correcting for low PF is to determine the existing situation at a given facility. Clamp-on power factor meters are available for this purpose. Power factor can be improved in two ways:

power factor and percentage losses in sys- tem feeder and branch circuits.

• Reduce the amount of reactive energy by eliminating low PF loads, such as unloaded motors and transformers

• Apply external compensation capacitors or other devices to correct the low-PF condition

PF correction capacitors perform the function of an energy-storage device. Instead of transferring reactive energy back and forth between the load and the power source, the magnetizing current reactive energy is stored in a capacitor at the load. Capacitors are rated in kVARs, and are available for single- and multiphase loads. Usually, more than one capacitor is required to yield the desired degree of PF correction. The capacitor rating required in a given application can be determined by using lookup tables provided by PF capacitor manufacturers. Installation options include:

• Individual capacitors placed at each machine

• A group or bank installation for an entire area of the plant

• A combination of the two approaches Figure 3.8a shows a simple circuit with shunt capacitor compensation applied at the load. The line current IL is the sum of the motor load current IM and the capacitor current IC. From the current phasor diagram of Figure 3.8b, it can be seen that the line current is reduced with the insertion of the shunt capacitor. Figure 3.8c displays the corresponding voltage phasors. The effect of the shunt capacitor is to increase the voltage source to VS1 from VS0.

When rectifier loads that generate harmonic load current are the cause of a low-PF condition, the addition of PF correcting capacitors will not necessarily provide the desired improvement. The capacitors, in some cases, may actually raise the line current and fail to improve the power factor. Harmonic currents generally are most apparent in the neutral of three-phase circuits. Conductors supplying three- phase rectifiers using a neutral conductor require a neutral conductor that is as large as the phase conduc- tors. A reduced neutral should not be permitted. When adding capacitors for PF correction, be careful to avoid any unwanted voltage resonances that might be excited by harmonic load currents.

If a delta/wye-connected power transformer is installed between the power source and the load, the power factor at the transformer input generally will reflect the average PF of the load on the secondary. This conclusion works on the assumption that the low PF is caused by inductive and capacitive reactances in the loads. However, if the load current is rich in harmonics from rectifiers and switching regulators, some of the harmonic currents will flow no farther toward the power source than the transformer delta winding. The third harmonic and multiples of three will flow in the delta winding and will be significantly reduced in amplitude. By this means, the transformer will provide some improvement in the PF of the total load.

An economic evaluation of the cost versus benefits, plus a review of any mandatory utility company limits that must be observed for PF correction, will determine how much power factor correction, if any, may be advisable at a given facility. Figure 3.9 shows a “before” and “after” comparison of a hypothetical facility. Correction to 85% will satisfy many requirements. No economic advantage is likely to result from correcting to 95% or greater. Overcorrecting a load by placing too many PF correction capacitors can reduce the power factor after reaching unity and cause uncontrollable overvoltages in low-kVA-capacity power sources.

PF correcting capacitors usually offer some benefits in absorbing line-voltage impulse-type noise spikes. However, if the capacitors are switched on and off, they will create significant impulses of their own. Switching can be accomplished with acceptably low disturbance using soft-start or preinsertion resistors. Such resistors are connected momentarily in series with the capacitors. After a brief delay (0.5 s or less), the resistors are short-circuited, connecting the capacitors directly across the line.

Installation of PF correction capacitors at a facility is a complicated process that requires a knowledgeable consultant and licensed electrician. The local utility company should be contacted before any effort is made to improve the PF of a facility.

Shunt Reactors

Shunt reactor compensation is typically required under conditions that are the opposite of those requiring shunt capacitor compensation [1]. Such a case is illusinstalled to remedy utility company power-generation and transmission issues, including the following:

• Overvoltages that occur during low load periods at utility substations served by long lines as a IR result of the inherent capacitance of the line

• Leading power factors at generating plants result- ing in lower transient and steady-state stability

It should also be noted that coupling from nearby energized lines can cause severe resonant over- voltages across the shunt reactors of unenergized compensated lines.

Unwanted Resonance Conditions

At a specific frequency, the inductive reactance of an electrical distribution system will equal the capacitive reactance; this is the definition of resonance. Resonance takes on one of two forms, parallel or series, depending upon the circuit configurations. The application of PF correction capacitors at a facility must be carefully planned to avoid unwanted harmonic resonances in the ac distribution system [2].

When a system is in parallel resonance, the impedance of the transformer and capacitor is maxi- mized. Harmonic currents at or near the resonant frequency can create high harmonic voltages across the high parallel impedance. When a system is in series resonance, the impedance of the transformer and capacitor is minimized. During series resonance, the only impedance to current flow is the pure resis- tance of the distribution circuit, which is normally quite low.

Nonlinear loads often behave as harmonic current generators, operating on the 60 Hz source voltage and producing harmonic-rich disturbances on the distribution system. By spreading the load current across the harmonic spectrum, nonlinear loads significantly increase the likelihood of resonances within a distribution system that includes PF correction capacitors. High harmonic voltages and currents can result, therefore, from nonlinear load operation that excites a resonant condition.

Harmonic-related problems include overheating of equipment, blown fuses, and equipment failure. Excessive harmonic voltages and current in capacitors results in increased losses in iron, insulation, and conductors, with a corresponding increase in temperature [3].

Simple equations can be used to determine whether the installation of capacitors on an electrical distribution system might lead to a resonant condition. The short-circuit kVA available from the utility must be determined first, and is given by

A resonant harmonic on the order of 50 or greater does not usually represent a potential resonant condition; because the distribution-system inductive reactance increases proportionally with frequency, higher order harmonic currents are significantly attenuated. Additionally, harmonic analysis may reveal negligible harmonic current magnitudes at or near the resonant frequency. In either case, harmonic mitigation techniques may not be required to prevent resonance within the distribution system.

When Equation 3.5 indicates a relatively low resonant harmonic, and spectrum analyses indicate that the magnitude of harmonic currents are significant at or near the resonant frequency, the most likely solution will be to install harmonic filters. In fact, low total power factor (TPF) can be principally the result of harmonic currents generated by the load. In such instances, the TPF may be improved by install- ing filters or traps alone [4]. In most cases, a combination of harmonic filters and capacitors designed to operate at the fundamental frequency are required to improve the TPF to acceptable levels in systems in which harmonic currents are present.

Active harmonic filters are available for facility applications that sense the load parameters and inject currents onto the distribution system that cancel the harmonics generated by nonlinear loads. In lieu of active filters, strategic placement of static filters to trap harmonics near the resonant frequency and to attenuate higher order harmonics can effectively protect the entire electrical system from damaging har- monic current and voltage magnitudes. Capacitor banks can be specified as integral components of a harmonic filter, tuned at or near the resonant frequency. Harmonic filters may provide a single package that will mitigate distribution-system resonant frequency problems and simultaneously improve the total power factor [4, 5].

Series Capacitor Compensation

Series capacitors are employed to neutralize part of the inductive reactance of a power circuit [1]. (See Figure 3.11.) From the phasor diagram of Figure 3.12, it can be seen that the load voltage is higher with the capacitor inserted than without the capacitor. Such application of a series capacitor facilitates an increase in the circuit transmission capacity and enhanced stability of the distribution network. Other useful by-products include:

• Improved load distribution

• Control of overall transmission losses

• Control over reactive power throughout the system

It should be noted, however, that the reduction in the circuit

inductive reactance gained through the use of series-capacitor- compensation also increases the short-circuit current levels over those for the noncompensated system.

Another consideration involves the interaction between a series-capacitor-compensated ac transmission system in electrical resonance and a turbine-generator mechanical system in torsional mechanical resonance. These resonances result in the phenomenon of subsynchronous resonance (SSR). In this mode, energy is exchanged between the electrical and mechanical systems at one or more natural frequencies of the combined system below the synchronous frequency of the system. The resulting mechanical oscillations can increase until mechanical failure occurs. A number of measures can be taken Advances in thyristor technology for power-system applications have led to the development of the static VAR compensator (SVC) [1]. This class of devices contains standard shunt elements (reactors and capaci- tors) that are controlled by thyristors. Static VAR devices are used to address two common problems encountered in practical power systems:

• Load compensation, where there is a need to reduce or cancel the reactive power demand of large and fluctuating industrial loads. Because heavy industrial loads are normally concentrated in one plant and served from one network terminal, they can usually be handled by a local compensator connected to the same terminal.

• Balancing the real power drawn from the ac supply lines. This type of compensation is related to the voltage support of transmission lines at a given terminal in response to disturbances of both the load and the supply. This voltage support is achieved by rapid control of the SVC reactance and, thus, its reactive power output.

The main objectives of such VAR compensation schemes are:

• To increase the stability limit of the ac power system

• To decrease terminal voltage fluctuations during load changes

• To limit overvoltages resulting from large system disturbances SVCs are essentially thyristor-controlled reactive power devices, usually designed around one of two basic configurations:

• Thyristor-switched shunt capacitor (TSC). As illustrated in Figure 3.13a, this configuration splits a capacitor bank into small steps and switches those steps on and off individually. This approach offers stepwise control, virtually no transients, and no harmonic generation. The average delay for executing a command from the regulator is one half-cycle.

• Thyristor-switched shunt reactor (TSR). Shown in Figure 3.13b, the fundamental frequency current component through the reactor is controlled by delaying the closing of the thyristor switch with respect to the natural zero crossing of the current. In this case, harmonic currents are generated from the phase-angle-controlled device.

In many applications, the arrangement of the SVC consists of a few large steps of thyristor-switched capacitors and one or two thyristor-controlled reactors, as shown in Figure 3.13c.