Throwing some light on power quality

According to Australian leading electronics solutions provider Benbro Electronics, electronic designers need to consider a number of factors to achieve power quality when designing a lighting system. This is more so with the advent of LEDS, compact fluorescent lighting (CFL), high intensity discharge (HID) lighting and other new technologies in lighting.

This is the view of Benbro Electronics that, together with Astrodyne, explores the issues involved in attaining power quality in lighting systems.

Many new lighting technologies, while certainly more efficient, can adversely affect power systems through harmonics, which decrease power quality. Inefficiencies in power quality may limit the number of devices that can be placed on the distribution network. It may also cause equipment to underperform or to behave erratically. In the most severe cases it can even harm the system and the devices along the network.

Poor power quality management inevitably leads to an increase in operational costs and places an unnecessary strain on already dwindling resources. Therefore, lighting systems should be driven by efficient and sustainable power sources that will not unnecessarily burden the power grid but still provide the perfect electrical environment for advanced lighting systems.

There are several standards that exist for lighting industry compliancy. It is incumbent on power supply manufacturers to produce supplies capable of meeting the appropriate lighting standards for safety and emissions. The standard which most directly affects power quality is the standard for limitations on harmonic currents, or EN61000-3-2 Class C limits. ANSI C82.77-2002 is the harmonic emissions specifications for luminaries in the US.

This standard regulates the harmonic currents drawn from the main’s supply connected to a low voltage distribution system. It is applicable to any electrical or electronic equipment with an input current of up to 16 A per phase and nominal 230 VAC, or 414 VAC for three-phase.

There are four classifications within the standard: Class A balanced three-phase equipment; Class B portable tools; Class C lighting equipment and dimming devices; Class D ITE.

The allowable power consumption is 75-1000 W and 25 W for Class C (a standard LED lamp or fluorescent strip consumes less than 25 W - though changes in the standard are still anticipated and this number may go lower).

The limits for Class A and B are static while C and D have dynamic limits determined by current drawn from the equipment (see Table 1, 2 and 3).

Class B (not shown) is 1.5 times the allowable limits of Class A.

Out of the four classes, the limits for Class C are the most stringent.

There are two major elements that affect power quality - phase displacement and wave distortion through harmonics.

Elements affecting power quality are distortion and displacement.

Harmonic currents are distorted waveforms caused by nonlinear loads that appear at multiples or ‘harmonics’ of the power frequency. Switching power supplies is a major source of nonlinear loads.

Figure 2 illustrates a power supply that draws its current in a non-sinusoidal burst. Note the difference between a linear (Figure 1) and nonlinear voltage and current relationship (Figure 2).

Figure 1.

Figure 2.

Harmonic currents cause distortions of the applied voltage. All distorted waveforms, current and voltage can be described as the fundamental waveform plus one or more harmonics (Figure 3). Even numbered harmonics tend to cancel each other out, but unfortunately the odd-numbered harmonics add in a way that quickly increases distortion because the peaks of these waveforms often coincide.

Figure 3.

Note that Table 2 for Class C lighting is primarily concerned with odd numbered harmonics up to the 40th harmonic.

The aggregate, or total, of all harmonics is measured as the total RMS value of all harmonics to the I fundamental or first harmonic expressed as a percentage.

Current distortion. Fortunately, harmonic currents are path-dependent and can only flow in the nonlinear loads that created them, so their effect on other loads in the system is negligible. However, they can, and do, impact on the distribution system.

Since real power can only be delivered at the fundamental frequency, harmonic currents reduce system capacity and limit the power for additional loads.

In three-phase systems, the odd triple harmonics (or triplens) are sent to the voltage supply. These currents may be larger than any of the phase currents. Currents may be as high as 200% of phase currents. If the wire is not properly sized, it may cause a fire.

An indirect effect takes place when harmonic currents excite resonant frequencies, which in turn cause high harmonic voltages capable of destroying a load.

Voltage distortion. Unlike current distortion, voltage distortion is not path-dependent; and will appear on common buses within a facility.

Harmonic voltages are higher the farther away from the source due to the increased impedances that harmonic currents must flow through. These harmonic voltages, in turn, cause voltage distortions that may cause significant harm, such as shortening the performance of capacitor banks or shortening the life of the utility’s transformers.

Voltage distortions also cause multiple zero crossings that may adversely affect the operation of equipment that rely on zero crossing for the sequencing of devices.

Nonlinear loads with phase-to-neutral or phase-to-phase connections are not generally affected. However, the fifth harmonic voltage distortion in a three-phase motor will cause a negative torque to be placed on the motor. This might cause the motor to overheat, fail or the motor will try to draw more current from the fundamental frequency in an attempt to regain its speed. The elimination of the fifth harmonic is crucial in industrial applications.

Harmonic currents may also be displaced or out of phase with the applied voltage. In Figure 4, the current lags the voltage by 90 degrees of displacement. During part of the cycle the voltage is positive and the current is negative. This means the voltage and current are working against each other in a reactive manner. This is called reactive power. Unless a power supply has a purely resistive impedance or power factor of 1 it will have some phase displacement.

Figure 4.

Many incandescent lighting systems do not reduce power quality because they have sinusoidal waveforms that are in phase with the voltage waveform. However, magnetic ballasts for fluorescent and HID lamps have lagging current and create reactive power. Switching power supplies also show phase displacement and create reactive power. Reactive power is not useful work - it is merely absorbed and returned to the load.

The equation for reactive power is V x A x sin (displacement angle (Figure 5)). Any power that does not perform useful work is detrimental to the system since it reduces power quality.

Figure 5.

Power quality in a system can be measured by its power factor. The power factor is the ratio of its true power to its apparent power. True power is the power delivered to the load, while apparent power is the product of its voltage and current.

If the true power equals the apparent power then the system is said to have a power factor of 1 or unity. This means that the system is drawing AC current directly proportional to the AC line voltage.

Once a waveform becomes distorted or displaced, as in the case of harmonics, the power factor is less than 1 and there is a reduction in power factor (and power quality).

Traditional power factor is generally thought of as displacement power factor (DPF) since it measures the displacement or phase angle or shift between voltage and current, measured as the cosine of that angle. However, true power factor takes into account the wave shaping or distortion as well as the displacement of the phase angle.

“Electronic engineers can clearly see how total harmonics distortion (THD) and displacement factor (DF) can greatly influence power quality. These need to be taken into consideration when designing effective and efficient lighting systems,” said John Bennett, director engineering at Benbro Electronics.