Harmonics: Overview and correcting harmonics

20/06/2009 by David Slade.

Introduction
Nonlinear loads cause harmonics to flow in the power lines. Harmonics are unwanted currents that are multiples of the fundamental line frequency (50 or 60 Hz). Harmonic currents can overload wiring and transformers, creating heat and, in extreme cases, fire. In information technology power systems it is important to know when and how to address this issue. Recently, the problem has been widely eliminated by international regulations.

Nonlinear loads
Many desktop personal computers present a nonlinear load to the AC supply. This is because they have a power supply design known as a “capacitor input switch mode power supply”.

Information Technology equipment including servers, routers, hubs, and storage systems almost universally use a different power supply design known as “Power Factor Corrected”. These devices present a very linear load to the AC supply and do not generate harmonic currents. In fact they are one of the cleanest loads on the power grid and generate less harmonic current than many other devices such as fluorescent lighting or variable speed motors. Ten years ago, these devices were nonlinear loads like Personal Computers, but today all of these loads are subject to international regulations which require them to be made with the “Power Factor Corrected” design.

Regulations
There is a significant interest on the part of society to reduce the amount of nonlinear loading on AC power systems. This loading reduces the distribution capacity of the public power system, and it can degrade the quality of the power by distorting the AC power waveform delivered to nearby customers. It can also cause a risk of fire on a customer’s premises.

In the 1980s, public utilities and international regulatory authorities including the IEC (International Electrotechnical Commission) took notice of the trend that an increasing percentage of electrical power consumption was caused by electrical equipment, and that an increasing percentage of this equipment used a “capacitor input switch mode power supply”. Fluorescent lighting, high performance air conditioning systems, and personal computers were key product categories driving this change. In response the IEC created in 1982 the international standard IEC 555-2 “Harmonic injection into the AC Mains”. This standard specifically limited harmonic current injection of “non-professional” equipment.

Switzerland, Japan, and other countries adopted the IEC 555 standard soon after release. Global suppliers of computing products first began to see a restriction on the ability to sell computers into countries that had adopted IEC 555-2 in the mid 1980’s. This situation precipitated the development of Power Factor Corrected power supply technology.

In 1995, the IEC introduced a update of the IEC 555-2 standard, called IEC 1000-3-2. In IEC 1000-3-2 the scope of applicability was greatly expanded over IEC 555-2 to cover all equipment drawing up to 16Amps per phase. The standard added additional limitations on both the absolute and percentage values of harmonics for products with nonlinear switch mode power supplies. Many countries outside of the US and the EC adopted this standard. The EC adopted its own version of this standard later in 1995 as EN61000- 3-2 and required equipment manufacturers to comply with the standard under an EC directive called “The EMC Directive”.

By 1995, almost all new computer equipment introduced for networks and communication was in compliance with IEC 1000-3-2. Even though not all countries had adopted the standard immediately, the standard represented a formidable trade barrier for companies that delayed compliance. Computer OEMs were almost universally specifying IEC 1000-3-2 compliance for OEM equipment intended for system integration. This caused virtually 100% of the IT industry to come into compliance.

Consequences of the standards on actual systems
A system comprising equipment meeting the IEC 1000-3-2 standard will have the following characteristics:

1. The harmonic current in the neutral circuit will have the currents resulting from the higher harmonics reduced to the point where less than 2% per unit of the current will be due to harmonics greater than the third harmonic, the consequence being that all harmonics other than the third can be neglected for neutral current contribution.
2. The “K” factor of the system has a theoretical maximum value of 9, but only if no loads are above 675W. If there are larger loads, then the theoretical maximum “K” factor is reduced: For example, with 2kW loads the maximum “K” factor is 3.
3. The theoretical maximum neutral current will be 1.7 of the rated phase current value, if all circuits are loaded to max rating, no loads are above 675W, and all loads are generating third harmonic at the compliance limit. If there are larger loads, then the theoretical maximum neutral current is reduced: For example, with 2kW loads the theoretical maximum neutral current is less than the phase current.

In a practical system, the harmonic currents will be lower than the theoretical values for the following reasons:

1. Manufacturers must meet the regulations over wide ranges of voltage, manufacturing tolerances, and load, the result being that actual products are well below the compliance limits at typical operating conditions.
2. Some loads are connected phase-to-phase (particularly in the USA), and therefore do not contribute to the neutral current

Harmonics overload building power transformers and cause them to wear out.

Power transformers are rated in KVA and are designed to carry currents at the power line frequency (50 or 60 Hz). The factor that limits the power handling capacity of a transformer is how hot it gets. The heat in a transformer is caused by the inherent resistance of the transformer and the current carried by it. When a power transformer carries harmonic currents, an effect known as the proximity effect (sometimes confused with the eddy current effect) causes the effective resistance of the transformer to increase with frequency.

The result is that the transformer rating must be decreased if the transformer carries significant harmonic currents, otherwise it will overheat and wear out due to insulation degradation. Transformer failures are often catastrophic and emit noxious fumes or fire; they can result in facility closure for days and a variety of health and safety consequences.

For this to be a problem, three things must happen together:

1. The transformer must be loaded nearly to capacity (unusual);
2. The transformer must have a poor “K” factor rating (bad proximity effect design); and
3. The load in the building must be mainly PCs. This is a real potential problem especially in situations where a large number of PCs have been deployed. Again, the location for concern is typically an office environment with high PC density such as a call center.

Abatement and mitigation of harmonic problems
There are a number of approaches to avoiding harmonic problems. These include:

1. Specifying equipment that does not create harmonics
2. Correcting harmonics
3. Oversizing neutral wiring
4. K-rated transformers

Specifying equipment that does not create harmonics
In the case of networking equipment, the problem is solved because of the IEC regulations. In the case of PCs, it is more difficult since a large amount of the harmonic contribution comes from the monitor. One approach is to use PCs and monitors with lower power draw overall, such as the use of LCD monitors or laptop PCs. This avoids both building wiring and transformer problems.

Correcting Harmonics
If a UPS is used in conjunction with the equipment, then in some cases the UPS can correct or eliminate the harmonics. Some single phase UPS eliminates neutral current entirely. If a power factor correcting UPS is used to power clusters of PCs, the harmonics problem cannot pass upstream to the building wiring or power transformers. This approach has the advantage that it can be retrofit to an existing building, and used with existing loads. It also corrects both the wiring and the transformer issues. For other types of loads, such as large industrial motor drives which are not covered by the harmonic regulations, specialized products are available that can absorb harmonics near the source.

Oversizing neutral wiring
In modern facilities the neutral wring should always be specified to be the same capacity as the power wiring (or larger). This is in contrast the electrical codes which may permit under sizing the neutral wire.

An appropriate design in the case of a large Personal Computer load like a call center is to specify the neutral wiring to exceed the phase wire capacity by about 50% (2 wire gauges in USA i.e. if the phase wiring is 8 gauge, the neutral wiring should be 6 gauge). Particular attention should be paid to wiring in office cubicles. This protects the building wiring, but does not help protect the transformers.

K-rated transformers
Modern office facilities with high densities of PCs should always be specified to include transformers with a “K” rating of at least 9. These transformers have been specially designed to withstand harmonic currents. For datacenters, a “K” rating of 9 would be sufficient to ensure harmonic carrying capability for the fraction of the datacenter consisting of old legacy loads, PC loads, or lighting loads.

Conclusion
International regulations have dramatically affected the power requirements for computing systems.

Networking equipment, once rightly accused of “power pollution” and of causing fires due to overheated transformers and wiring, have transformed into one of the “cleanest” loads to be found in a modern commercial or industrial establishment. Datacenter design standards specifying double neutrals or transformers with K=20 ratings are driving needless expense and should be updated.

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