Analysis of Harmonic Problems in Power Systems

Harmonics are a distortion of the normal current waveform, usually emitted by non-linear loads. Non-linear loads include switch mode power supplies (SMPS), speed-controlled motors and drives, copiers, personal computers, laser printers, fax machines, battery chargers, and UPS. Single-phase non-linear loads are more common in modern office buildings, while three-phase non-linear loads are common in factories and industrial workshops.

The majority of non-linear power loads on most power distribution systems come from SMPS devices. For example, all computer systems use SMPS to convert the mains AC voltage into a stable, low-voltage DC power source for the internal electronic equipment. These non-linear power supplies generate short, high-amplitude pulses of current, which cause severe distortion of the current and voltage waveforms - harmonic distortion, usually measured as total harmonic distortion (THD). This distortion propagates back into the power system and will affect other devices connected to the same power source.

Most power systems can tolerate some level of harmonic currents, but problems arise when harmonics become a significant percentage of the total load. As these higher frequency currents flow through the power system, they can cause communication errors, overheating, and hardware damage, such as:

Overheating of power distribution equipment, cables, transformers, backup generators, etc.

High voltage and circulating current caused by harmonic impedance

High neutral lines that generate heat and waste power

Equipment failure due to severe voltage distortion

Increased internal energy consumption in connected devices, causing component failure and shortening service life

Branch circuit breaker false trip

Metering error

Fire in wiring and distribution systems

Generator failure

High amplitude coefficient and related issues

Reduced system power factor, resulting in reduced available power (kW vs. kVA) and monthly electricity bill penalties

Harmonic Technology Overview

Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental frequency. If the fundamental frequency is 60 Hz, then the 2nd harmonic is 120 Hz, the 3rd harmonic is 180 Hz, and so on (see Figure 1). When the harmonic frequencies dominate, the switchboard and transformer will mechanically resonate with the magnetic field generated by the high-frequency harmonics. When this happens, the switchboard or transformer will vibrate and buzz for different harmonic frequencies. The 3rd to 25th harmonic frequencies are the most common frequency range in the power distribution system.

Figure 1 Harmonic distortion of current waveform

All periodic waves are generated by sine waves of various frequencies. Fourier's law decomposes a periodic wave into its component frequencies.

Harmonic components: Larger 1st harmonic (fundamental frequency), smaller 5th harmonic, slightly larger 7th harmonic

Figure 2 Distortion waveform consisting of the fundamental frequency, 5th and 7th harmonics

The total harmonic distortion of a signal is a measure of harmonic distortion, which is defined as the ratio of the sum of the power of all harmonic components to the power of the fundamental frequency. It describes the degree of distortion of a voltage or current signal (see Figure 3).

Figure 3 Total harmonic distortion

Solutions to compensate and reduce harmonics

Although standards to limit the generation of harmonic currents are under consideration, today's harmonic control relies mainly on remediation methods. Compensating or reducing power system harmonics can be done in a variety of ways, with varying effectiveness and efficiency.

Increase the neutral wire specification

In modern facilities, neutral wiring is often required to be the same or greater than the power wiring—although power codes allow for a reduction. Designs that support multiple PCs (such as call centers) should specify that the neutral wiring exceed the phase wiring by 1.73 times. Wiring in office cubicles should be particularly careful. It is important to note that this approach protects the building wiring, but not the transformer.

Use a separate neutral conductor

In a three-phase branch circuit, a neutral conductor should be laid separately for each phase conductor, instead of sharing a neutral conductor for multiple branch circuits. This can increase the capacity and ability of the branch circuit to handle harmonic loads. This method can effectively suppress the increase of harmonics on the neutral line of the branch circuit, but the neutral busbar of the distribution board and the feeder neutral conductor must still be considered.

Use a DC power supply that is not affected by harmonics

In a typical data center, the power distribution system converts 480V AC mains power through a transformer to 208V AC power that feeds the server racks. One or more power supplies in each server then convert the AC power to a DC voltage used by the server's internal components.

These internal power supplies are not very energy efficient, and they generate a lot of heat, which increases the workload and operating costs of the room's air conditioning system. Heat dissipation also limits the number of servers that can be accommodated in a data center. It is worth choosing to use DC power to eliminate this step.

According to an article in the Energy and Power Management magazine, “Computers and servers equipped with DC power supplies instead of AC power supplies generate 20% to 40% less heat, consume 30% less power, and improve server reliability and installation flexibility, as well as reduce maintenance requirements.”

It sounds good, but when cost, compatibility, reliability and efficiency are considered, abandoning AC power in favor of DC power is not feasible for most data centers. AC power - although slightly less efficient - is generally acceptable for existing equipment.

Additionally, there are no Underwriters Laboratory (UL) safety standards for high-voltage points in data centers, while standards are well established for AC systems. This means that the safety risks outweigh the potential benefits of DC power, at least for now.

Use of K-class transformers in power distribution components

Standard transformers are not designed for the high harmonic currents generated by non-linear loads. They will overheat and fail prematurely when these loads are connected. When harmonics began to be introduced into electrical systems at levels that had a deleterious effect (circa 1980), the industry responded by developing the K-class transformer. K-class transformers are not designed to eliminate harmonics, but to handle the heat generated by harmonic currents.

K-factor ratings range from 1 to 50. The standard transformer designed for linear loads has a K-factor of 1. The higher the K-factor, the more heat the transformer can handle from harmonic currents. Selecting the correct K-factor is critical because it affects cost, efficiency, and safety.

Transformers with higher K factors are generally larger than transformers with lower K factors. Therefore, a reasonable K factor should be selected based on the harmonic curve of the data center to achieve the best balance between size, efficiency and thermal resistance.

Power distribution units (PDUs) with K-13 rated transformers (and oversized neutrals) can effectively handle harmonic power. PDUs with K20 rated transformers are common but are oversized for most modern data centers.

Use harmonic mitigation transformers

K-class dry-type transformers are widely used in electrical environments—including in PDUs or as backup units. But recent advances in transformer design can provide better performance in terms of reducing harmonic voltage distortion and power losses.

Harmonic Mitigation Transformer (HMT) is used to handle non-linear loads in electrical systems. This transformer specifically handles triple order (3rd, 9th, 15th…) harmonics using electromagnetic mitigation technology. The secondary winding of the transformer is used to cancel zero sequence flux and eliminate primary winding circulating current. This transformer also handles the 5th and 7th harmonics by using phase shifting.

Utilizing these two electromagnetic technologies, HMTs allow loads to operate in the manner for which they were designed by the manufacturer, while minimizing the effects of harmonics on energy consumption and distortion. Most HMTs exceed NEMA TP-1 efficiency standards, even when tested with 100% non-linear loads. As long as a K-class transformer is specified, an equivalent HMT can be used as a direct replacement.

Key benefits of using HMT

Prevent voltage flattening caused by non-linear loads

Reduce upstream harmonic currents

Eliminate transformer overheating and excessive operating temperatures

Eliminate primary winding circulating current

Energy saving by reducing harmonic losses

Maintains high energy efficiency even under severe non-linear loads

Dealing with power quality harmonics that K-class transformers cannot solve

Suitable for loads with higher K factors without increasing inrush current

Improve power factor

Other harmonic mitigation methods

When a data center is first designed, HMT is the transformer of choice. However, if harmonics are a problem in an existing data center, a sawtooth autotransformer can be used to limit the effects of triple order harmonics and the 5th and 7th harmonics.

The zigzag autotransformer is a neutral forming transformer with only a primary winding and no secondary winding. Each core has two primary windings, which are wound in opposite directions, providing a higher impedance to the normal phase current.

When placed close to the load, a sawtooth autotransformer can capture triple order harmonics. Such an autotransformer must be sized large enough to handle the harmonics. The triple order harmonics will be confined to the autotransformer and the load, thus preventing upstream distribution equipment from encountering the harmonics. However, an autotransformer cannot be used to change the voltage to a level different from the mains (voltage).

The triple sequence harmonics, 5th and 7th harmonics can be eliminated by connecting the above autotransformer in parallel with a secondary feeder. This feeder is usually powered by a different source. The autotransformer and the secondary phase transfer source work together to capture the triple sequence harmonics, 5th and 7th harmonics. This application is quite tricky because both sources must carry a balanced load to effectively capture the triple sequence harmonics, 5th and 7th harmonics.

Both applications are very effective in eliminating harmful harmonics. However, installing a single harmonic mitigation transformer is the most cost-effective way to prevent harmful harmonics from affecting distribution equipment.

summary

Harmonic currents have a significant impact on distribution systems and the facilities they feed. The effects of harmonics must be considered when planning system expansions or modifications. In addition, sizing and locating non-linear loads is an important part of any maintenance, troubleshooting, and repair program.