Continued from first part: Electric Power Quality and Lighting (part 1)
What is power factor?
Power factor is a measure of how effectively a device converts input current and voltage into useful electric power. Mathematically it is defined as follows:
Where P is active power and S is the apparent power.
It is often confused with:
Where P1 is active power od the fundamental and S1 is apparent power of it.
cosφ concerns exclusively with the fundamental frequency and therefore differs from power factor when harmonics are present in the installation. It describes the combined effects of current THD and reactive power from phase displacement. A device with a power factor of unity (1.0) has 0% current THD and a current draw that is synchronized with the voltage. Resistive loads such as incandescent lamps have power factors of unity.
A device is said to have high power factor (HPF) if the power is 0.9 or greater. Power factor between 0.5 and 0.9 is called normal power factor (NPF). Magnetic and electronic ballasts for fluorescent lamps may be either HPF or NPF. HPF ballasts usually have filters to reduce harmonics and capacitors to reduce phase displacement.
On average these additional components add about 16% to the retail costs of ballasts (Dorret al. 1994).
NLPIP measured power factor for several types of lighting loads, and for common office equipment; these data are shown in Table 2 below.
Sample power quality characteristics for different electric loads
What problems result from poor power quality?
Poor power quality can damage the distribution system and devices operating on the system. In rare instances, poor power quality can cause a dangerous overload of the neutral conductor in a three phase circuit. In this type of circuit, three power supply wires share one grounded circuit conductor (the neutral conductor). In a system with no THD, the neutral wire carries no current. High current THD devices can send odd triple harmonics (third order, ninth order, fifteen order, etc.) onto the voltage supply, which do not cancel each other out. They add up on the neutral wire, and if the current exceeds the wire’s rating, the neutral conductor can overheat and pose a fire hazard.
Voltage distortion can also shorten the life of utilities’ transformers and cause capacitor banks to fail. Many utilities impose penalty charges on their customers if power factor, measured at the point where the utility service couples to the customer’s site, fall below a certain value:
Reactive power uses capacity on the distribution system, which limits the amount of active power that a utility can deliver. This may be a problem during periods of peak demand.
When voltage THD is below the IEEE limit of 5%, most devices do not experience problems. Resistive loads such as incandescent lamps actually reduce voltage harmonics. Motor loads also reduce harmonics, but the motors are subject to overheating as voltage distortion increases. Fifth order harmonics produce particularly negative effects: they rapidly degrade the motor’s effieciency by producing torque in opposition to normal for part of the cycle.
High frequency electronic ballasts operate at frequencies ranging from 20 to 60 kilo-hertz (kHz). The harmonics produced by these ballasts are at correspondingly high frequencies and can interfere with some communication equipment including radios, intercoms, and cordless phones. Devices that use power line carrier signals, such as synchronized clocks and control modules for building energy management systems may also experience problems if harmonics exist at frequencies close to carrier signal.
What limits for current THD and power factor are used in the lighting industry?
Standards organizations have not set power factor limits for lighting products, except for the requirement that power factor must meet or exceed 0.90 for manufacturers to claim that a product has high power factor. Lighting designers, architects, and other lighting specifiers often specify HPF ballasts for buildings with sensitive equipment, such as hospitals.
According to ANSI, maximum current THD limit of 32% for electronic ballasts for full size fluorescent lamps are established. It also limits the amplitude of the third order harmonic to 30% of the fundamental amplitude, and limits the amplitude of all high order harmonics (greater than eleven order) to 7% of the fundamental. CSA, IEC and IEEE set a 20 % current THD limit for electronic ballasts. Almost all electronic ballasts currently available for 4-foot T12 and T8 lamps are high power factor with current THD less than 20%.
Some compact fluorescent lamps (CFLs) have current THD greater than 100%, but they have low active power compared with other high THD products such as personal computers, so standards organizations have not set power quality requirements for CFLs.
Some utilities set current THD requirements for products in their lighting incentive programs. For example, the Duke Power Co. in North Carolina and New England Electric Systems limit current THD for electronic ballasts for full size fluorescent lamps to 20%.
Additionally, New England Electric Systems limits current THD for CFLs to 25%.
• National Lighting Product Information Program;
• American National Standards Institute;
• Schneider Electric – Electrical Installations Guide