Power generating equipment, such as large DC rectifiers, uninterruptible power supply systems, and generator sets, has also led to a great diversification of equipment designed for current needs.
At the same time, companies have increased their reliance on these electronic devices, which are becoming increasingly sophisticated and, at times, more vulnerable to poor power quality, largely caused by harmonics in the electrical distribution network, primarily generated within our own facilities.
However, all these previously described loads are particularly concentrated and amplified in data centers, which are named differently depending on their size.
Facilities with one to five racks are called technical rooms or data rooms; small data centers have between five and twenty racks; medium-sized data centers have between twenty and one hundred racks; and large data centers have more than one hundred racks.
To gain a better understanding of what can occur in a data center, we should analyze the single-line diagram of the facility in Figure 1.
We can see that the electronic loads that generate current harmonics are generally located downstream of the installation; at the end of the line, these harmonics always flow towards the source that generates the supply voltage.
The consequence of these current harmonics is the contamination of the voltage sources, starting with the closest, which is usually the uninterruptible power supply (UPS); upstream are the medium-voltage/low-voltage (MV/LV) transformer and the emergency generator. These currents will also be affected when they are in operation, supporting the general non-critical loads, the UPS systems, lighting, cooling loads, and the rest of the building's services, such as elevators, access control, security, surveillance, etc.
One of the fundamental characteristics of these harmonic-generating equipment is the deterioration of the voltage quality circulating through the power supply networks. The worse the voltage quality, the greater the contamination, which increases the total current distortion (THDI).
These loads significantly distort the voltage generated by the MV/LV transformer substation, potentially reaching a threshold where the voltage quality may be unacceptable for certain equipment or power supplies. The IEEE-519-1992 standard specifies that the overall voltage distortion threshold must be below 5% and the individual harmonic distortion rate of the highest amplitude must be below 3%.
This standard is specific to electronic equipment due to its sensitivity to power wave distortion. Excessive distortion leads to irregular breakdowns, premature failures, and accelerated aging, negatively impacting the average lifespan of the products as estimated by the equipment manufacturers themselves, who always base this calculation on a high-quality power supply.
Installations are generally designed for normal operation at the transformer substation. Sometimes, the correct sizing of the generator set is not sufficiently considered, or the installation's own evolution may result in it falling outside acceptable parameters for any expansion or remodeling of the data center. To
appreciate the oversizing required when installing an emergency generator set, let's examine Table I, which shows the necessary generator power depending on whether we use a PFC rectifier or one with electronic harmonic correction (THM), a rectifier with a classic harmonic filter (FAH), or a standard rectifier.
A UPS with a standard thyristor rectifier requires a generator with twice the UPS's power rating. If harmonic filtering (FAH) is implemented, the generator must be oversized by 160%. If the rectifier incorporates active harmonic compensation (THM) or uses an IGBT transistor rectifier with PFC (power factor correction), the minimal distortion results in a sufficient oversize of 118%.
It's easy to see that, once harmonics are controlled, a smaller generator can be used, resulting in savings on the initial purchase price, maintenance, and fuel consumption during operation.
What are harmonics?
Introduction
It is problematic to design an installation knowing that it will supply power to an environment rich in nonlinear or distorting loads, with a high presence of single-phase loads, which will generate multiple problems.
A linear load is one whose current consumption waveform is sinusoidal, like the voltage from which it is supplied, and a nonlinear or distorting load is one whose current waveform has nothing to do with the voltage waveform.
The graph in Figure 2 represents a classic waveform of a single-phase switched-mode power supply (like that of a PC). It is possible to decompose it into its fundamental and the other harmonic currents that compose it, all of which are sinusoidal.
Thanks to this decomposition, we can determine the range and amplitude of each harmonic, allowing us to use this information to find the most appropriate solution.
Various electrotechnical solutions exist, but they don't resolve the presence of harmonics in the installation, nor do they address the real problem they cause: voltage quality.
If the installation allows, the third and ninth harmonics can be confined to a specific area using a galvanic isolation transformer designed for this purpose. This prevents them from circulating upstream and thus relieves the load on the neutral conductor, keeping these harmonics under control in a restricted zone.
Alternatively, a filter tuned to the desired frequency can be used. This involves taking the necessary measurements and designing a filter appropriate for the harmonic to be eliminated. These types of filters are called passive filters. Full effectiveness is achieved at nominal power; below this value, their effectiveness decreases significantly.
A series inductor can be used, whose purpose is not to eliminate harmonics but to attenuate them, achieving an attenuation of approximately 50% of the overall current distortion rate. This application is primarily used with rectifiers and variable frequency drives. As in the previous case, its effectiveness decreases with increasing load.
To help solve the problems caused by harmonics effectively and definitively, active harmonic compensators have been available on the market for over five years.
How to decongest: active harmonic compensation.
The active harmonic compensator continuously analyzes each phase, taking into account the waveform of the load current (Ich). From this analysis, the harmonic spectrum is extracted, which consists of the sum of the fundamental current and the currents of each individual harmonic.
The compensator generates a current signal equal to the difference between the load current and the fundamental current. This difference, which is the sum of the harmonic currents shifted by 180°, is injected into the load so that the resulting sinusoidal current equals the fundamental current of the source.
The compensator is connected in parallel between the source and the load and must be sized exclusively for the harmonic currents, not for the total current.
To ensure high reliability and optimal system performance, IGBT transistors are used in the power stage.
The active harmonic compensator continuously analyzes each phase, taking into account the load current waveform (I. load), extracting the harmonic spectrum, and generating a signal that mirrors the load current waveform without considering the fundamental component, focusing solely on the harmonics present in the installation (IH. harmonics).
It also helps reduce upstream THDU voltage distortion, resulting in a sinusoidal waveform that allows sensitive electronic equipment to be powered with a high-quality, distortion-free voltage.
Harmonic distortion correction can reduce the apparent power of the installation by up to 30%, and the greater the overall harmonic distortion rate that needs to be corrected, the more effective the active harmonic compensator will be.
Furthermore, the excess power not used to reduce harmonics can be dedicated to improving the cos j, thus decreasing the reactive power consumed by the installation.
Conclusions:
In facilities like data centers, we can't wait for problems to arise; we must anticipate them.
First, we need to assess the facility and conduct a comprehensive electrical audit, starting with the single-line diagram, to determine how the most polluting loads are installed, what they are, their function, the current distortion at each point, and the voltage distortion to assess power quality.
These analyses should be repeated annually to analyze trends in the facility and identify when changes, improvements, or actions are needed to mitigate the effects.
Audits will allow us to improve many vital aspects of the facility, not just address the threat of harmonics.
Author: Ángel Pérez, Technical Director of APC by Schneider Electric
