ARE HARMONICS STILL A PROBLEM IN DATA CENTERS? by Mohammad Al Rawashdeh, Lead Consultant, Data Center Engineering Services

Transcription

1 ARE HARMONICS STILL A PROBLEM IN DATA CENTERS? by Mohammad Al Rawashdeh, Lead Consultant, Data Center Engineering Services

2 edarat group 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). info@edaratgroup.com 2

3 Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency. If the fundamental power frequency is 6 Hz, then the 2 nd harmonic is 12 Hz, the 3 rd is 18 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 3 rd to the 25 th are the most common range of frequencies measured in electrical distribution systems. Distorted wave form (6 Hz - Fundamental + 3rd Harmonic) Fundamental (6 Hz) 3rd Harmonic (18 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 (1 st harmonic), 3 rd harmonic and 5 th harmonic is illustrated in Figure 2: Hz Waveform 15 Hz Waveform 2 Hz Waveform Symmetrical Complex Waveform Figure 2 info@edaratgroup.com 3

4 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). 1 %-Fund Harmonic THD = 98% Figure 3 info@edaratgroup.com 4

5 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 1% power factor) voltage and current waveforms) and Figure 5 (Linear shifted alignment (at 8% leading power factor) between voltage and current). One example of a linear load is a resistive load, such as an incandescent light bulb Figure Figure 5 info@edaratgroup.com 5

6 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: z Overheating of electrical distribution equipment, cables, transformers, standby generators, etc. z High voltages and circulating currents caused by harmonic resonance z Equipment malfunctions due to excessive voltage distortion z Increased internal energy losses in connected equipment, causing component failure and shortened life span z False tripping of branch circuit breakers z Metering errors z Fires in wiring and distribution systems z Generator failures z Crest factors and related problems z Lower system power factor, resulting in penalties on monthly utility bills info@edaratgroup.com 6

7 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. 2% neutral conductors One option is to specify an 2% 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 2% 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 198), 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 5. 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% K 1 Harmonic producing equipment representing less than 35 % K4 Harmonic producing equipment representing less than 5 % K 7 Harmonic producing equipment representing less than 75 % K 13 Harmonic producing equipment representing less than 1 % K 2 Table 1 info@edaratgroup.com 7

8 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 MAINS Connection i MAINS MAINS H LOAD H MAINS AHF i LOAD i AHF i MAINS info@edaratgroup.com 8

9 Advantages z Guarantee compliance with IEEE if sized correctly z Harmonic cancellation from the 2nd to 51st harmonic z No series connection provides easy installation with no major systema rework z Provide VAR currents, improving system power factor Disadvantages z Can be more expensive than other methods due to the high performance control and power sections z 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 25 th order, but in critical applications, those are measured up to 5 th or 1 th 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.,.5 pf = 5%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. info@edaratgroup.com 9

10 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 2% 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, 1% 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. info@edaratgroup.com 1

11 References Mitigating data center harmonics Neutral Ratings for Power Distribution Systems in the Data Center Harmonics in your electrical system Training/HarmonicsInYourElecSystem.pdf. Harmonic reduction methods