ARE HARMONICS
STILL A PROBLEM
IN DATA CENTERS ?
by Mohammad Al – Rawashdeh,
Lead Consultant, Data Center Engineering Services
edarat group Are Harmonics Still A Problem In Data Centers?
INTRODUCTION
Harmonics are a mathematical way of describing distortion to a voltage or current waveform. The term
harmonic refers to a component of a wave form that occurs at an integer multiple of the fundamental
frequency. Fourier theory tells us that any repetitive waveform can be defined in terms of summing sinusoidal waveforms which are integer multiples (or harmonics) of the fundamental frequency. For the
purpose of a steady state waveform with equal positive and negative half-cycles, the Fourier series can be
expressed as follows:
∞
F (t) =
An . Sin (nPt / T)
S
n=1
where
f(t) is the time domain function
n is the harmonic number (only odd values of n are required)
An is the amplitude of the nth harmonic component
T is the length of one cycle in seconds
A common term that is used in relation to harmonics is THD or Total Harmonic distortion. THD can be
used to describe voltage or current distortion and is calculated as follows:
THD (%) = √(ID1² + ID2² + ... + IDn²)
where
IDn: is the magnitude of the nth harmonic as a percentage of the fundamental (individual distortion).
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Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power
frequency. If the fundamental power frequency is 60 Hz, then the 2nd harmonic is 120 Hz, the 3rd is 180 Hz,
etc. (see Figure 1). When harmonic frequencies are prevalent, electrical power panels and transformers
become mechanically resonant to the magnetic fields generated by higher frequency harmonics. When
this happens, the power panel or transformer vibrates and emits a buzzing sound for the different harmonic frequencies. Harmonic frequencies from the 3rd to the 25th are the most common range of frequencies measured in electrical distribution systems.
Distorted wave form (60 Hz - Fundamental + 3rd Harmonic)
Fundamental (60 Hz)
3rd Harmonic (180 Hz)
Figure 1
When all harmonic currents are added to the fundamental a waveform known as complex wave is formed.
An example of complex wave consisting of the fundamental (1st harmonic), 3rd harmonic and 5th harmonic
is illustrated in Figure 2:
1
50 Hz Waveform
0.8
150 Hz Waveform
0.6
200 Hz Waveform
0.4
0.2
0
-45
-0.2
0
45
90
135
180
225
270
315
Symmetrical
Complex
Waveform
-0.4
-0.6
-0.8
-1
Figure 2
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The total harmonic distortion (THD) of a signal is a measurement of the harmonic distortion present and is
defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental. It provides an indication of the degree to which a voltage or current signal is distorted (see Figure 3).
100
%-Fund.
80
THD = 98%
60
40
20
0
0
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
Harmonic
Figure 3
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NON-LINEAR LOADS
A linear load draws current that is instantaneously proportional to the voltage, such as in Figure 4 (Linear
time-aligned (unity or 100% power factor) voltage and current waveforms) and Figure 5 (Linear shifted
alignment (at 80% leading power factor) between voltage and current). One example of a linear load is a
resistive load, such as an incandescent light bulb.
1
0.8
0.6
0.4
0.2
220
270
310
360
225
270
315
360
180
130
90
-0.2
40
0
0
-0.4
-0.6
-0.8
-1
Figure 4
1
0.8
0.6
0.4
0.2
180
1.35
90
-0.2
45
0
0
-0.4
-0.6
-0.8
-1
Figure 5
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A nonlinear load either draws a current waveform that is not instantaneously proportional to the voltage
or is a load that causes current to distort its sinusoidal shape. The current waveform also leads or lags
voltage or phase.
Harmonics are distortions of the normal electrical current waveform, generally transmitted by nonlinear
loads -Non-linear loads occur when the resistance is not a constant and changes during each sine wave
of the applied voltage waveform, resulting in a series of positive and negative pulses. Switch-mode power
supplies (SMPS), variable speed motors and drives, photocopiers, personal computers, laser printers, fax
machines, battery chargers, and UPSs are examples of nonlinear loads. Single-phase nonlinear loads are
prevalent in modern office buildings- Data Center, while 3-phase nonlinear loads are common in factories and industrial plants.
A large portion of the nonlinear electrical loads in most electrical distribution systems comes from SMPS
equipment. For example, all computer systems use SMPS that convert utility AC voltage to regulated low-voltage DC for internal
electronics. These nonlinear
power supplies draw current
in high-amplitude short pulses
that create significant distortion
in the electrical current and voltage wave shape (see Figure 6).
This harmonic distortion, measured as total harmonic distortion (THD), travels back into the
power source and can affect
other equipment connected to
the same source.
Figure 6
Most power systems can accommodate a certain level of harmonic currents but will experience problems when harmonics become a significant component of the overall load. As these higher frequencies
harmonic currents flow through the power system, they can cause communication errors, overheating
and hardware damage, such as:
zz
zz
zz
zz
zz
zz
zz
zz
zz
zz
Overheating of electrical distribution equipment, cables, transformers, standby generators, etc.
High voltages and circulating currents caused by harmonic resonance
Equipment malfunctions due to excessive voltage distortion
Increased internal energy losses in connected equipment, causing component failure and shortened
life span
False tripping of branch circuit breakers
Metering errors
Fires in wiring and distribution systems
Generator failures
Crest factors and related problems
Lower system power factor, resulting in penalties on monthly utility bills
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HARMONICS REDUCTION SOLUTIONS
To determine if harmonic mitigation is necessary, should conduct an assessment to precisely measure
the harmonics affecting the data center and identify their origin. Options for harmonic mitigation vary
in complexity and cost and can be deployed individually or in combination. The strategy that makes the
most sense for a facility will vary based on the loads it supports, its budget, and the nature of the harmonic-related problems it is experiencing.
There are several approaches that can be taken to compensate for or reduce harmonics in the power
system, with varying degrees of effectiveness and efficiency.
200% neutral conductors
One option is to specify an 200% neutral conductor or to use separate neutral conductors. Triple n harmonics (3, 9, 15, etc.) can produce neutral currents that can be up to the theoretical maximum of 173% of
the phase current, which is why a 200% neutral conductor is usually specified.
K-rated transformers:
A standard transformer is not designed for high harmonic currents produced by nonlinear loads.
It will overheat and fail prematurely when connected to these loads. Therefore, when harmonics were first introduced into electrical systems at levels that showed detrimental effects (circa 1980), the industry responded by developing the K-rated transformer. K-rated transformers are not used to eliminate harmonics, but to manage the heat generated by harmonic currents.
K factor ratings range between 1 and 50. A standard transformer designed for linear loads is designated
with a K-factor of 1. The higher the K-factor, the more heat from harmonic currents the transformer is
able to withstand. When selecting a K rating, managers should consider the trade-offs between size, efficiency, and heat tolerance. For example, transformers with higher K factors are typically larger than those
with lower K factors. The table 1 below shows appropriate K ratings to use for different percentages of
nonlinear current in the electrical system.
Non-linear load
K- Rating
Incidental electronic equipment representing less than 5%
K1
Harmonic producing equipment representing less than 35 %
K4
Harmonic producing equipment representing less than 50 %
K7
Harmonic producing equipment representing less than 75 %
K 13
Harmonic producing equipment representing less than 100 %
K 20
Table 1
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Uninterruptable Power Supply- UPS
Since UPSs and the loads they serve have historically been considered some of the most significant harmonic loads on a power system, companies can also use harmonic-mitigating UPSs to reduce line harmonics. Much like active filters, harmonic-mitigating UPSs eliminate harmonic distortion by inserting
equal and opposite current into the line. They also compensate for reactive power from low power factor
loads and balance loads across three phases to avoid stranded capacity, while providing clean and continuous power during utility outages or in response to electrical disturbances.
Harmonics filters
Harmonic filters remove harmonics and correct the phase of the fundamental current, thus converting
non-linear loads into linear loads. Passive, and Active filters, it can be either used as a standalone part
integral to a large nonlinear load or can be used for a multiple small single phase nonlinear loads by connecting it to a switch board.
Passive filter
A Passive Filter uses inductors capacitors and resistors in combination to create the filter. Passive implementations of linear filters are based on combinations of resistors (R), inductors (L) and capacitors (C).
These types are collectively known as passive filters, because they do not depend upon an external power
supply and/or they do not contain active components such as transistors.
Active filter
This method uses sophisticated electronics and power section IGBTs to inject equal and opposite harmonics onto the power system to cancel those generated by other equipment. These filters monitor the
nonlinear currents demanded from nonlinear loads and electronically generate currents that match and
cancel the destructive harmonic currents. Active filters are inherently non-resonating and are easily connected in parallel with system loads.
PRINCIPLE DRAWING
LOAD
i LOAD
i MAINS
MAINS
Connection
MAINS
H MAINS
H LOAD
AHF
i LOAD
i AHF
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i MAINS
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Advantages
zz
zz
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Guarantee compliance with IEEE 519 1992 if sized correctly
Harmonic cancellation from the 2nd to 51st harmonic
No series connection provides easy installation with no major systema rework
Provide VAR currents, improving system power factor
Disadvantages
zz Can be more expensive than other methods due to the high performance control and power sections
zz The filter’s input semiconductors are exposed to line transients.
With increase in use of non-linear loads, the issues of power supply harmonics are more noticeable than
ever. Controlling and monitoring industrial system designs and their effects on utility distribution systems
are potential problems for the industrial consumer, who is responsible for complying with the IEEE 519,
recommended practices and procedures.
Industrial facilities should include a system evaluation, including a harmonic distortion analysis, while
planning facility construction or expansion. Vendors of non-linear loads, such as variable frequency
drives, can provide services and recommend equipment that will reduce harmonics in order to comply
with IEEE 519 guidelines.
Generally, at any point the measured value of total harmonic voltage distortion should not exceed 5%
and that of any individual harmonic voltage distortion should not exceeding 3% of the fundamental value
of the line voltage. Normally, in typical applications, the harmonics are measured up to 25th order, but in
critical applications, those are measured up to 50th or 100th order.
A better way to handle harmonics in the data center is to simply design better systems and devices, which
fortunately has already been accomplished for the most part. In order to offer products in the world-wide
market, nearly every computer equipment manufacture is in compliance.
To comply with the regulation, global computer equipment vendors developed Power Factor Corrected
(PFC) power supply technology. Power factor is the ratio of the real power to the load and the apparent
power and is a number between zero and one (e.g., 0.5 pf = 50%pf). Real power is the capacity of the
circuit. Apparent power is the product of the current and voltage in the circuit. If energy is stored in the
load and returned to the source, or if is distorted by a non-linear load, the apparent power will be greater
than the real power. Loads with a high power factor draw less current than loads with low power factors,
and these higher currents increase energy loss and require larger equipment.
Switched-mode power supplies have a low power factor. PFC power supplies control the harmonic current using either a filter or an electronic system that controls the amount of power drawn by the load. The
purpose of the PFC is to make the power factor as close to one as possible, where the current waveform
is proportional to the voltage waveform. When this is the case, the voltage and current are in phase and
the reactive power consumption is zero, enabling power companies to efficiently deliver power. In other
words, all of the energy supplied by the source is consumed by the load and none is returned to the source.
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CONCLUSION
Harmonics in the data center was a real concern in the past. Fortunately, it is one of those issues that are
close to being solved, at least for computing equipment, by regulatory agencies and computer equipment
manufactures. It’s not an issue that receives a lot of press or marketing, as companies tend to focus more
on the more tangible benefit of energy efficiency and reduced energy costs, which is why the subject of
harmonics is still occasionally raised as an issue when designing new power distribution systems. Facilities
managers or electrical architects who are unfamiliar with modern computer equipment might specify
costly solutions such as K-factor transformers or 200% neutral conductors, to handle non-linear loads
and harmonic neutral currents that are relatively rare in today’s IT environments. Data center managers
who are armed with the knowledge that computer vendors are compliant with harmonic elimination
standards can help drive more realistic and cost-effective power distribution designs in the data center
and be more proactive in replacing non-conforming, legacy equipment. Simply stated, 100% rated neutral
conductors are suitable for the vast majority of applications.
Circumstances might arise where oversize neutral conductors are necessary for the remaining non-linear
loads in the data center, such as lighting and cooling equipment, or legacy equipment that must be maintained for a particular application.
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References
Mitigating data center harmonics
http://pqlit.eaton.com/ll_download_bylitcode.asp?doc_id=26534
Neutral Ratings for Power Distribution Systems in the Data Center
http://www.starlinepower.com/busway/uploads/docs/en/UE_NeutralRatings.pdf
Harmonics in your electrical system
http://www.newark.com/pdfs/techarticles/eaton/Eaton_Technical_Articles/UPS_Training/Powerware_
Training/HarmonicsInYourElecSystem.pdf.
Harmonic reduction methods
http://www.eaton.com/ecm/idcplg?IdcService=GET_FILE&dID=687518.
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