Bringing harmony to power

Understanding the causes and effects of harmonics can lead service providers to more reliable power systems

AC power quality has always been a concern, and we tend to put the onus for delivering clean power on the AC power provider. In fact, power quality is a product of a clean AC power source and low pollutant loads that are powered from that source. The telecommunications industry, in its attempts to maintain a reliable network, is questioning the effects that large power systems have on the AC power source.

Harmonic currents can affect power quality, and it's important to understand the issues.

Three basic technologies comprise today's large DC power plants: silicon-controlled rectifier (SCR) phase-controlled rectifiers, controlled ferroresonant rectifiers and high-frequency switch-mode rectifiers. The SCR phase control and controlled ferroresonant technologies have been around for almost 30 years and are widely accepted. Single-phase switch-mode rectifiers have been around since the 1980s and have not been a problem, but large three-phase switchers are relatively new to the industry.

Harmonic distortion is a key phrase used today when talking about power quality. Most power engineers know they want to use equipment with low harmonic distortion characteristics but do not fully understand what that means nor what constitutes low.

Harmonic distortion is measured in terms of percent of the fundamental. Without going into the deep mathematical derivations, the AC input current drawn by a piece of equipment is made up of a fundamental sine wave plus a number of harmonic sine waves (multiples of the fundamental frequency). The fundamental, when in phase with the AC voltage waveform, produces power equal to that current times the voltage. But the harmonics do not contribute to do any work - they merely heat the wiring and pollute the AC waveform.

Total harmonic distortion, the ultimate characteristic, is the ratio of the root mean square (RMS) sum of the harmonics to the fundamental. Total harmonic distortion can refer to the quality of the input current drawn by a piece of equipment or the quality of the AC power source feeding that equipment. Equipment with an input current rich in harmonics is referred to as a non-linear load.

Ithd is the abbreviation for the total harmonic distortion of the current, while Vthd represents the total harmonic distortion in the AC voltage waveform. It is important to understand the cause and effect principle. The non-linear load (cause) draws harmonic currents from the AC power source. Depending upon the impedance of the AC source feeding the non-linear load, the result may be distortion of that AC voltage (effect).

The AC power source in a telecom power room feeds a variety of loads such as computers, printers, power equipment, lighting and motors, as well as heating and air-conditioning equipment. These represent a mixture of linear and non-linear loads, all powered from the same AC source. If the content of non-linear loads becomes too large, it could cause significant distortion to the AC voltage. When this distortion is taken to extremes, it can result in malfunction or damage to other equipment sharing that source.

Examples of detrimental effects would be overheating of transformers and motors being powered or disruption of equipment that uses the AC line frequency for timing purposes. Extreme notching of the AC line voltage caused by motor controllers can cause additional zero crossings to appear on the AC waveform, causing inaccuracies in the zero crossing count.

Single-phase harmonics Most large power installations are fed from a three-phase AC source. Loads on that source will be a mixture of single-phase and three-phase equipment. Single-phase voltage is derived from a three-phase source by connecting line to line or line to neutral. A 208 VAC, three-phase power source would feed 208 VAC single-phase equipment from line to line, while feeding 120 VAC single-phase equipment from line to neutral.

A characteristic of single-phase non-linear loads is that the triplen harmonics (third, sixth, ninth, etc.) are predominant. When the non-linear load is connected from line to neutral, these triplen harmonics add together in the common neutral cabling back to the source. Large quantities of 120 VAC non-linear loads will cause the current in the neutral lead to exceed that of the other phase conductors. To prevent overheating of the neutral lead, it is recommended that it be oversized with respect to the phase conductors.

Single-phase non-linear loads when connected from line to line will cancel out the triplen harmonics and do not result in overheating of the neutral wire. When three like non-linear loads are connected to the three-phase line in a balance fashion (one load each in a delta connection), the resulting current will be similar to that of a three-phase non-linear load.

Three-phase loads Unlike the single-phase line to neutral non-linear loads, the three-phase loads do not include the triplen harmonics. The triplens are naturally cancelled by the balanced three-phase load. Instead, the predominant harmonics are the Kn x triplens +/- 1, where K represents the number of pulses in the conversion and n represents the triplen. The predominant harmonics for a six-pulse rectifier would be the fifth, seventh, 11th, 13th, etc.

During normal operation of a three-phase non-linear load there occurs an overlap or a momentary cross-connect of the line voltages created by the inductance in the circuit. It is this momentary cross-connect that distorts the AC waveform. Under mild to moderate conditions, the distortion will appear as small notches on the line at the point where the voltages cross. In extreme cases, such as large motor controllers, these notches can be deep enough to represent an additional zero crossing as mentioned earlier.

The distortion of the voltage waveform does not create any hardship for the non-linear load causing the distortion but can affect other loads that draw power from that same source. It is the sharing of a common line voltage source that makes it critical that the magnitude of non-linear loads be managed.

Three-phase rectifiers When single-phase switch-mode rectifiers first hit the market, they were not power factor corrected. This means their input current was rich in harmonic content. These power supplies were small with respect to the overall loading of the AC source, so they presented little threat to other equipment sharing the line. As harmonic consciousness became broader, demand for low harmonic power supplies became the norm.

Europe already had set standards for maximum harmonic currents in appliances with input currents below 16 A years before the U.S. saw it as a problem. Today, almost all new single-phase power supply designs have a power factor correction front end to extensively reduce the total harmonic content of the input current.

Single-phase power factor correction circuits dissipate power when they shape the input current into a near pure sine wave, but this is the only way to reduce the harmonics. This means that a power factor corrected power supply will have a lower overall efficiency than a non-power factor corrected power supply.

The three-phase switch-mode rectifier, because of the overlapping of AC voltages, lends itself to other, simpler methods of reducing harmonics. There are two approaches to reducing the harmonics in a three-phase switch-mode rectifier. It can be done using active circuits, similar to the single-phase power supplies, or it can be done using a smoothing inductor.

The active three-phase power factor correction circuit, like the single-phase counterpart, dissipates power during operation, reducing its overall efficiency. If extremely low harmonics are necessary, then this less-efficient, more expensive approach must be taken. If the application can tolerate a moderate amount of harmonic content, the smoothing inductor method can provide a more efficient, more cost-effective alternative.

Typical Ithd Non-linear loads have been in existence for a long time. Many of the products we use every day have high Ithd. Typical single-phase examples are shown in Table 1.

The high Ithd equipment used every day does not represent a high enough percentage of our total usage to significantly pollute the AC line. Table 2 shows examples of three-phase non-linear loads.

As you can see, the three-phase equipment has an overall lower Ithd than the single phase. In addition, the three-phase equipment does not generate the triplens that cause overheating in neutral conductors. On the other hand, the application for three-phase equipment is in larger systems, meaning that it will most likely represent a larger percentage of the total load.

Ultimate effects As mentioned above, the effects of harmonic currents can vary. Single-phase non-linear loads generate harmonic currents in the neutral that are additive, resulting in the need to oversize the neutral cabling. If planned for, this is not really a problem. When it is not planned for, it can present a hazardous potential for fire. This type of non-linear load with high triplens can distort the AC waveform by flattening its peak.

Unlike the single-phase non-linear loads, there is no additive current in the neutral lead naturally eliminating the potential for fire. An ill effect that can be generated by the three-phase non-linear load is notching of the voltage waveform. In extreme cases, the notches can represent additional zero crossings.

In either case, the bottom line is degradation of the AC source's quality. It should be noted that the harmonic current drawn by a non-linear load does not directly interfere with other equipment connected to the AC source. It is actually the magnitude of the resulting distortion of the common AC line voltage that can affect other equipment being powered by that source.

IEEE standard 519-1992 (IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems) recommends limiting the total harmonic distortion of the line voltage to less than 5%. Vthd greater than 5% can cause overheating in transformers and motors that are operating off this common AC source. This 5% Vthd should not be confused with specifications for Ithd. Often, confusing these parameters results in specifying an unnecessarily low Ithd of less than 5% when it is the Vthd that needs to be limited to less than 5%.

The extent to which one or more of the non-linear loads can affect the quality of the AC waveform depends upon the impedance of the AC source and the percentage of the total load that the non-linear loads comprise. A stiff, low impedance AC line can support a higher percentage of harmonics than a weak, high impedance source. Ultimately, the objective is maintaining a Vthd of less than 5%.

With this in mind, IEEE standard 519-1992 does not provide recommendations for limiting the harmonic content of individual harmonic contributors, but limiting the harmonic content for the aggregate harmonics of all the equipment powered by the common AC source. Table 10.3 of the IEEE standard (Table 3) provides recommendations for harmonic content based upon the relative capacity of the AC source, that is, the ratio of the short circuit current capability of the source to actual load current. It can be seen from the table that AC sources with large ratios can support a higher harmonic current content.

Typical CO The DC power systems in a typical central office (CO) consume about 50% of the total AC provided to the facility along with the lighting, air-conditioning, air handling equipment, PCs and other equipment. The harmonic currents supplied by this AC source will be the aggregate of the harmonics of this equipment.

To better understand the impact the new three-phase switch-mode technology can have on the AC voltage source, harmonic data was taken at a typical CO. The site chosen was a new installation. The power equipment was installed and tested, but the actual telecom load was not yet connected. This provided the freedom to connect a variable resistive load to the output of the DC power system and take data from no load to full load.

An analyzer was used to capture the data, allowing for storage of the current and voltage harmonic measurements and the current and voltage waveforms. Because the loads on the AC service continually change, it is important to have an instrument that can capture a snapshot of the voltage and current harmonics.

The office had a large power system equipped with four power conversion units for a total of 43.2 kW. The PCUs were fed from a 400 A supplemental AC distribution panel that was 150 feet from the main AC panel. It is important to note this because the additional impedance between main and supplemental panels will increase the voltage distortion seen at the supplemental panel.

As mentioned, a resistive load was used, allowing data to be taken at no load and full load, 800 A. At no load, the PCUs do not contribute any measurable distortion to the AC voltage waveform. With this in mind, the no load measurements will represent the harmonic voltage distortion present in the AC source, which is caused by other non-linear loads. The difference in the distortion level from no load to full load will represent the contribution made by the PCUs.

l Input current. Figure 1 is an oscillograph of the AC input current being drawn by the four PCUs. It has the typical waveform of a six-pulse rectifier with a smoothing inductor.

The magnitude of these harmonic currents and the momentary overlapping conduction of the phase voltages will cause a distortion in the AC line voltage. The magnitude of this voltage distortion will be a function of the impedance of the line.

l Input voltage. Figures 2 and 3 show a comparison of the AC line voltage waveform at both no load and full load for the DC power system. The second waveform shows the notches caused by the momentary overlapping caused by the inductance in the circuits.

Non-linear loads are not new to the AC power grid. As was shown in the examples at the beginning of this article, single-phase, non-power factor corrected switch-mode rectifiers and other loads with total harmonic current distortion of greater than 100% have been connected to the network for years without causing disruptions.

Balanced three-phase currents drawn by the three-phase switch-mode rectifiers do not generate neutral currents and provide safe operation. The use of a smoothing inductor, instead of an active power factor correction circuit, provides a more efficient rectifier and an improved reliability.

These new three-phase switch-mode rectifiers do not distort the AC line beyond the 5% limit, making it compliant with the IEEE 519 standard. With this compliance, they should not have adverse effects on the AC distribution network.