**High Frequency Harmonics Emission in Smart Grids**

**High Frequency Harmonics Emission in Smart Grids**

Jaroslaw Luszcz

Jaroslaw Luszcz

[17] W. Xu, X. Liu y Y. Liu. *An Investigation on the Validity of Power-Direction Method for Harmonic Source Determination.* IEEE Transations on Power Delivery, Vol. 18, No. 1,

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[20] Cataliotti and V. Consetino, *A New Measurement Method for the Detection of Harmonic Sources in Power Systems Base on the Approach of the IEEE Std. 1459-2000,* IEEE Transac‐

[21] Reyes S. Herrera, Patricio Salmerón. *Harmonic disturbance identification in electrical sys‐ tems with capacitor banks*. Electric Power Systems Research, Volume 82, Issue 1, Janu‐

[22] J.E. Farach, W.M. Grady y A. Arapostathis. *An Optimal Procedure for Placing Sensors and Estimating the Locations of Harmonic Sources in Power Systems.* IEEE Transations on

[23] L. Critaldi, A. Ferrero y S. Salicone. *A Distributed System for Electric Power Quality Measurement.* IEEE Transaction on instrumentation and Measurement, Vol. 51, No. 4,

[24] C. Muscas, L. Peretto, S. Sulis, R. Tinarelli, *Investigation on Multipoint Measurement Techniques for PQ Monitoring*, IEEE Trans. Instrum. Meas. Vol. 51, pp. 1684, oct. 2006.

[25] N. Locci, C. Muscas, S Sulis, *On the Measurement of Power-Quality Indexes for Harmonic Distortion in the Presence of Capacitors,* IEEE Transactions on Instrumentation and

[26] *Definitions for the measurement of electric power quantities under sinusoidal, nonsinusoidal,*

January 2003.

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ary 2012, Pages 18-26.

August 2002.

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Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/52874

**1. Introduction**

The term 'smart grid', is nowadays very often used in many publications and so far has not been explicitly defined, however it refers mainly to such an operation of electricity delivery process that allows to optimize energy efficiency by flexible interconnection of central and distributed generators through transmission and distribution system to industrial and consumer end-users [1], [3], [11], [13], [15], [17]. This functionality of power delivery system requires the use of power electronic converters at generation, consumer and grid operation levels. Harmonic pollution generated by power electronics converters is one of the key problems of integrating them compatibly with the power grid, especially when its rated power is high with relation to the grid's short-circuit power at connection point [18], [19], [20].

Contemporary power electronics converters has already reached rated power of several MW and are integrated even at the distribution level directly to medium voltage (MV) grid. Power electronics technologies used nowadays in high power and MV static converter increase the switching frequency significantly due to the availability of faster power electronic switches which allows to increase power conversion efficiency and decrease harmonic and inter-harmonic current distortion in frequency range up to 2 *kHz*. This trend significantly increases harmonic emission spectrum towards higher frequencies correlated with modulation frequency of switching conversion of power. Therefore typical harmonic analysis up to 2 *kHz* in many power electronics application requires to be extended up to frequency of 9 *kHz* which is the lowest frequency of typical electromagnetic interference analysis interest. Numerous problems related to current and voltage harmonic effects on contemporary power systems are commonly observed nowadays, also in frequency range 2 − 9 *kHz*. Levels and spectral content of current distortions injected into electric power grids are tending to increase despite the fact that the acceptable levels are determined by numerous regulations [2], [3], [7], [9], [12], [14], [16].

In recent years many of grid-side PWM boost converters of relatively high rated power have been introduced into power grid because of many advantages, like for example:

Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Luszcz; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Luszcz; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

©2012 Luszcz, licensee InTech. This is an open access chapter distributed under the terms of the Creative

current harmonics limitation, reactive power compensation and bidirectional power flow. Implementation of smart grids idea will conceivably increase this tendency because of the need for bidirectional flow control of high power in many places of distribution and transmission power grid.

filter because of relatively high frequency which results with easiness of propagation by

Harmonics Conducted EMI Sub‐

Frequency [Hz]

PQ, THD CISPR A CISPR B

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DC 50/60 2k 9k 150k 30M

On the other hand, typical harmonic distortion components which are usually recommended for analysing and solving PQ problems by standards are within frequency range up to 2*kHz*. In this frequency range integer multiples of the fundamental power frequency (50*Hz* or

Consideration of conducted emissions and harmonic distortions in these frequency bands: one up to 2*kHz* and second 9*kHz* − 30*MHz*, were sufficient enough in last years in applications with classic line-commutated rectifiers and switch mode DC power supplies which are also fed by this type of rectifiers. During the last decades, with the increase of the rated power of single power supplies and increasing number of power supplies used the increased difficulties with acceptable current harmonic emission levels arise and other technologies like PWM boost rectifiers have been intensively introduced. The PWM modulation carrier frequency used in such applications is often within the range of 2 − 9*kHz* or adjacent ranges, which results with the significant increase of harmonic emission in this

60*Hz*) harmonic components are defined usually up to 40*th* order.

frequency range what will be discussed in the next sections of this chapter.

electronics converters topologies and technologies. These are:

**3. Harmonic distortions emission of grid-connected power electronics**

Harmonic distortion emission is commonly understood as harmonics produced by non-linear loads, usually power electronics converters in the frequency range up to 2*kHz* which are strongly related to some of the power quality indices. From this point of view (PQ) harmonic distortion emission in the frequency range above 2*kHz* can be named as high frequency harmonics emission. On the other hand, from the EMC point of view, the conducted EMI emission below 9*kHz* is usually defined as low frequency EMC conducted emission.

The frequency map of different harmonic emissions, usually considered as conducted type emissions which are mainly propagating by conduction process along power lines, is presented in Figure 2. From this prospective we can distinguish three primary types of harmonic distortion emission of typical sources which can be associated to particular power

means of omnipresent parasitic capacitive couplings.

Magnitudes

uV

**converters**

mV

V

kV

harmonics

**Figure 1.** Harmonic distortions frequency sub-ranges.

Typical carrier frequencies used in AC-DC PWM boost converters are within a range from single kHz for high power application up to several tens of kHz for small converters. Important part of conducted emission spectrum generated by those types of converters is located in frequency range below 2 kHz normalized by power quality regulations and above 9 kHz normalized by low frequency EMC regulation (especially CISPR A band 9kHz-150kHz). In between those two frequency ranges typically associated with power quality (PQ) and electromagnetic compatibility (EMC) respectively, where a characteristic gap of standard regulations still exists, the conducted emission of grid-connected PWM converters can be highly disturbing for other systems. Current and voltage ripples produced by grid-connected PWM converters can propagate through LV grids and even MV grids, where converters of power of few MW are usually connected. Filtering of this kind of conducted emission will require a new category of EMI filters with innovative spectral attenuation characteristic which is difficult to achieve by just adaptation of solutions that are already in use for current harmonics filtering for PQ improvement and radio frequency interference (RFI) filters used for EMC assurance.

### **2. Harmonic emissions of non-linear loads into power grid**

Harmonics content defined for currents and voltages is an effect of its non sinusoidal wave-shape. Power electronics switching devices used in power conversion process like diodes, thyristors and transistors change its impedance rapidly according to line or PWM commutation pattern and produce non sinusoidal voltages and currents which are required to perform the power conversion process properly. Unfortunately, these non sinusoidal currents, as a results of internal commutation process in a converter, are also partly injected into the power grid as an uninvited current harmonic emission. Non sinusoidal load currents charged from power grid produce voltage harmonic distortions in power grid which can influence all other equipment connected to that grid because of the existence of grid impedance. This mechanism results that non-linear current of one equipment can be harmful for other equipment supplied from the same grid and also for the grid itself, like e.g. transformers, transmission lines.

A frequency spectrum range of harmonic distortions introduced into power grid can be exceedingly wide, nevertheless the maximum frequency range which is usually analysed is defined by CISPR standard as 30*MHz*. Between 9*kHz* and 30*MHz* two frequency sub-bands are defined as CISPR A up to 150*kHz* and CISPR B above 150*kHz* (Figure 1). These two frequency rages are well known as conducted electromagnetic interference (EMI) ranges, where harmonic components of common mode voltages or currents are limited to levels defined by a number of standards.

In general, despite some specific cases, amplitudes of harmonic distortions observed in typical applications decrease with the increase of frequency, stating from several or tens percent in frequencies close to the power frequency and reach levels of only microvolts or microamps for the end frequency of conducted frequency band 30*MHz*. Unfortunately, even so small voltage and current amplitudes can be really harmful, disturbing, and difficult to

filter because of relatively high frequency which results with easiness of propagation by means of omnipresent parasitic capacitive couplings.

**Figure 1.** Harmonic distortions frequency sub-ranges.

2 Power Quality

transmission power grid.

for EMC assurance.

e.g. transformers, transmission lines.

defined by a number of standards.

current harmonics limitation, reactive power compensation and bidirectional power flow. Implementation of smart grids idea will conceivably increase this tendency because of the need for bidirectional flow control of high power in many places of distribution and

Typical carrier frequencies used in AC-DC PWM boost converters are within a range from single kHz for high power application up to several tens of kHz for small converters. Important part of conducted emission spectrum generated by those types of converters is located in frequency range below 2 kHz normalized by power quality regulations and above 9 kHz normalized by low frequency EMC regulation (especially CISPR A band 9kHz-150kHz). In between those two frequency ranges typically associated with power quality (PQ) and electromagnetic compatibility (EMC) respectively, where a characteristic gap of standard regulations still exists, the conducted emission of grid-connected PWM converters can be highly disturbing for other systems. Current and voltage ripples produced by grid-connected PWM converters can propagate through LV grids and even MV grids, where converters of power of few MW are usually connected. Filtering of this kind of conducted emission will require a new category of EMI filters with innovative spectral attenuation characteristic which is difficult to achieve by just adaptation of solutions that are already in use for current harmonics filtering for PQ improvement and radio frequency interference (RFI) filters used

Harmonics content defined for currents and voltages is an effect of its non sinusoidal wave-shape. Power electronics switching devices used in power conversion process like diodes, thyristors and transistors change its impedance rapidly according to line or PWM commutation pattern and produce non sinusoidal voltages and currents which are required to perform the power conversion process properly. Unfortunately, these non sinusoidal currents, as a results of internal commutation process in a converter, are also partly injected into the power grid as an uninvited current harmonic emission. Non sinusoidal load currents charged from power grid produce voltage harmonic distortions in power grid which can influence all other equipment connected to that grid because of the existence of grid impedance. This mechanism results that non-linear current of one equipment can be harmful for other equipment supplied from the same grid and also for the grid itself, like

A frequency spectrum range of harmonic distortions introduced into power grid can be exceedingly wide, nevertheless the maximum frequency range which is usually analysed is defined by CISPR standard as 30*MHz*. Between 9*kHz* and 30*MHz* two frequency sub-bands are defined as CISPR A up to 150*kHz* and CISPR B above 150*kHz* (Figure 1). These two frequency rages are well known as conducted electromagnetic interference (EMI) ranges, where harmonic components of common mode voltages or currents are limited to levels

In general, despite some specific cases, amplitudes of harmonic distortions observed in typical applications decrease with the increase of frequency, stating from several or tens percent in frequencies close to the power frequency and reach levels of only microvolts or microamps for the end frequency of conducted frequency band 30*MHz*. Unfortunately, even so small voltage and current amplitudes can be really harmful, disturbing, and difficult to

**2. Harmonic emissions of non-linear loads into power grid**

On the other hand, typical harmonic distortion components which are usually recommended for analysing and solving PQ problems by standards are within frequency range up to 2*kHz*. In this frequency range integer multiples of the fundamental power frequency (50*Hz* or 60*Hz*) harmonic components are defined usually up to 40*th* order.

Consideration of conducted emissions and harmonic distortions in these frequency bands: one up to 2*kHz* and second 9*kHz* − 30*MHz*, were sufficient enough in last years in applications with classic line-commutated rectifiers and switch mode DC power supplies which are also fed by this type of rectifiers. During the last decades, with the increase of the rated power of single power supplies and increasing number of power supplies used the increased difficulties with acceptable current harmonic emission levels arise and other technologies like PWM boost rectifiers have been intensively introduced. The PWM modulation carrier frequency used in such applications is often within the range of 2 − 9*kHz* or adjacent ranges, which results with the significant increase of harmonic emission in this frequency range what will be discussed in the next sections of this chapter.

### **3. Harmonic distortions emission of grid-connected power electronics converters**

Harmonic distortion emission is commonly understood as harmonics produced by non-linear loads, usually power electronics converters in the frequency range up to 2*kHz* which are strongly related to some of the power quality indices. From this point of view (PQ) harmonic distortion emission in the frequency range above 2*kHz* can be named as high frequency harmonics emission. On the other hand, from the EMC point of view, the conducted EMI emission below 9*kHz* is usually defined as low frequency EMC conducted emission.

The frequency map of different harmonic emissions, usually considered as conducted type emissions which are mainly propagating by conduction process along power lines, is presented in Figure 2. From this prospective we can distinguish three primary types of harmonic distortion emission of typical sources which can be associated to particular power electronics converters topologies and technologies. These are:


**Figure 3.** Six pulse diode rectifier with DC link capacitor.

**Figure 4.** Six pulse diode rectifier - typical input current and voltage waveforms.

**Figure 5.** Six pulse diode rectifier - typical input current harmonics spectrum up to 2kHz.

frequency

The frequency domain representation of input current, calculated for 10 cycle period with rectangular widowing as a discrete Fourier transform (DFT) product [8] is presented as harmonic amplitudes *Ik* with 5*Hz* resolution in frequency range up to 2*kH*z in Figure 5 and up to 25*kHz* in Figure 6 . The characteristic harmonics for six-pulse rectifier are non nontriplen odd harmonics (5th, 7th, 11th, 13th etc.) and its amplitudes decrease with

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**Figure 2.** Characteristic distribution of harmonic emission spectra for different types of power electronics converters.

### **3.1. Low frequency harmonic emission of classic AC-DC converters**

Classic, diode-based AC-DC converters (rectifiers) were successfully used for many years in multiple applications. Nowadays, because of extremely increasing number of such devices used in power system and significantly increasing its rated power, AC-DC converters for power of hundreds of kW are quite often used, its harmonic emission levels cannot be accepted by power grid operator demands. Significantly increasing problem of harmonic distortions in power grid led to legislation numerous of grid regulations which are set-up by grid operators and international standards. A typical configuration of six pulse three phase diode rectifier with DC link capacitor commonly used in medium power applications is presented in Figure 3 .

The exemplary input current waveform for this type of rectifier is presented in Figure 4 with correlation to input AC voltage. The maximum value of line current and its flow duration which is in six pulse rectifier always shorter than half cycle are accountable for the level of distortion. These parameters of current wave-shape are dependent of grid impedance and DC link capacitor parameters, especialy size, equivalent serial resistance (ESR) and equivalent serial inductance (ESL) In the evaluated case significant distortion of input current *IAC* make a distortion effects slightly visible also at voltage waveform, where voltage deformations are correlated in time with the current pulses.

**Figure 3.** Six pulse diode rectifier with DC link capacitor.

4 Power Quality

tens of *kHz*,

is presented in Figure 3 .

correlated in time with the current pulses.

Harmonic distortions

as power switches diodes or thyristors,

easily propagate also towards AC power lines.

 

Power Quality PQ

• classic PQ frequency range up to 2*kHz*, where the main sources of harmonic distortions are usually line commutated rectifiers used in single- and multi-phase topologies using

• high frequency harmonic distortion emission in the frequency range 2 − 9*kHz*, where mainly PWM boost rectifiers, as a relatively new topology, are generating harmonic components correlated to the used PWM carrier frequency which depending on the topology and rated power of the converter is usually located between a few *kHz* and

• conducted EMI emission in frequency range (9*kHz* − 30*MHz*), which is primarily an effect of DC voltage conversion by switching mode methods where power transistor switching processes are key sources of high frequency conducted emission which can

> EMI Conducted Emission EMC

> >

Frequency [Hz]

 

DC 50/60 2k 9k 150k 30M

Classic, diode-based AC-DC converters (rectifiers) were successfully used for many years in multiple applications. Nowadays, because of extremely increasing number of such devices used in power system and significantly increasing its rated power, AC-DC converters for power of hundreds of kW are quite often used, its harmonic emission levels cannot be accepted by power grid operator demands. Significantly increasing problem of harmonic distortions in power grid led to legislation numerous of grid regulations which are set-up by grid operators and international standards. A typical configuration of six pulse three phase diode rectifier with DC link capacitor commonly used in medium power applications

The exemplary input current waveform for this type of rectifier is presented in Figure 4 with correlation to input AC voltage. The maximum value of line current and its flow duration which is in six pulse rectifier always shorter than half cycle are accountable for the level of distortion. These parameters of current wave-shape are dependent of grid impedance and DC link capacitor parameters, especialy size, equivalent serial resistance (ESR) and equivalent serial inductance (ESL) In the evaluated case significant distortion of input current *IAC* make a distortion effects slightly visible also at voltage waveform, where voltage deformations are

**Figure 2.** Characteristic distribution of harmonic emission spectra for different types of power electronics converters.

**3.1. Low frequency harmonic emission of classic AC-DC converters**

The frequency domain representation of input current, calculated for 10 cycle period with rectangular widowing as a discrete Fourier transform (DFT) product [8] is presented as harmonic amplitudes *Ik* with 5*Hz* resolution in frequency range up to 2*kH*z in Figure 5 and up to 25*kHz* in Figure 6 . The characteristic harmonics for six-pulse rectifier are non nontriplen odd harmonics (5th, 7th, 11th, 13th etc.) and its amplitudes decrease with frequency

**Figure 4.** Six pulse diode rectifier - typical input current and voltage waveforms.

**Figure 5.** Six pulse diode rectifier - typical input current harmonics spectrum up to 2kHz.

**Figure 6.** Six pulse diode rectifier - typical input current harmonics spectrum up to 25kHz.

The total harmonic distortion (THD) content of input current can be calculated using formula (1) where each harmonic group *In* is determined according to formula (2) . In the analysed example presented in Figure 4 the obtained THD was over 95%. To reduce so high harmonic emission number of passive filtering techniques can been introduced. AC reactors (*LAC*) and DC chokes (*LDC*) (Figure 7) are typically used and allow to decrease input current THD below 30%. Adequate input current waveform and its frequency domain representation for diode rectifier with passive filtering are presented in Figure 8, 9 and 10.

$$THD(I) = \frac{\sqrt{\sum\_{n=2}^{40} I\_n^2}}{I\_1} \tag{1}$$

**Figure 8.** Six pulse diode rectifier with passive filtering - input current and voltage waveforms.

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**Figure 9.** Six pulse diode rectifier with passive filtering - input current harmonics spectrum up to 2kHz.

**Figure 10.** Six pulse diode rectifier with passive filtering - input current harmonics spectrum up to 25kHz.

$$I\_n = \sqrt{\frac{I\_{k-5}}{2}^2 + \sum\_{i=k-4}^{k+4} I\_i^2 + \frac{I\_{k+5}}{2}^2} \tag{2}$$

**Figure 7.** Six pulse diode rectifier with passive filtering of line current harmonic distortions.

**Figure 8.** Six pulse diode rectifier with passive filtering - input current and voltage waveforms.

6 Power Quality

**Figure 6.** Six pulse diode rectifier - typical input current harmonics spectrum up to 25kHz.

diode rectifier with passive filtering are presented in Figure 8, 9 and 10.

*In* = *Ik*<sup>−</sup><sup>5</sup> 2 <sup>2</sup> <sup>+</sup>

**Figure 7.** Six pulse diode rectifier with passive filtering of line current harmonic distortions.

*THD*(*I*) =

The total harmonic distortion (THD) content of input current can be calculated using formula (1) where each harmonic group *In* is determined according to formula (2) . In the analysed example presented in Figure 4 the obtained THD was over 95%. To reduce so high harmonic emission number of passive filtering techniques can been introduced. AC reactors (*LAC*) and DC chokes (*LDC*) (Figure 7) are typically used and allow to decrease input current THD below 30%. Adequate input current waveform and its frequency domain representation for

> <sup>40</sup> ∑ *n*=2 *I*2 *n*

*k*+4 ∑ *i*=*k*−4

*I*1

*I*2 *<sup>i</sup>* + *Ik*<sup>+</sup><sup>5</sup> 2

<sup>2</sup> (2)

(1)

**Figure 10.** Six pulse diode rectifier with passive filtering - input current harmonics spectrum up to 25kHz.

### **3.2. High frequency harmonic emission of modern AC-DC converters**

Severe limitations of the line current harmonic performance improvement possibilities of classic diode rectifiers stimulate introducing fully controlled switches in AC-DC converters. Accompanying significant increase of IGBT transistor performance during the last decade allows to obtain successful implementation of PWM boost AC-DC three phase converter topology in many applications where harmonic distortion emission has to be limited. PWM boost type AC-DC converters besides line current harmonic distortion significant reduction in frequency range up to 2*kHz* have a number of other advantages [4], [5], [10], like for example:


**Figure 12.** Line current and voltage ripples generated by PWM boost converter

boost converter are presented in Figure 13, 14 and 15.

**Figure 13.** PWM boost converter – input current and voltage waveforms.

**rectifiers**

maximum emission is observed around modulation frequency which in the tested converter was set to 15*kHz*. In higher frequency range, close to integer multiples of PWM carrier frequency harmonic products of modulation are usually also observed. Perfect elimination of this PWM-related emission is not possible and became more difficult to realize by using passive filters with the increase of frequency. An example of input current and voltage waveforms and its harmonic content in frequency domain representation recorded in PWM

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**3.3. Comparison of line current harmonic distortion of diode and PWM boost**

Detailed comparison of current harmonic distortion emission has been done for three phase six pulse diode rectifier and PWM boost converter with the three phase IGBT transistor bridge. Both converters has been tested in similar supply condition and using similar load, which allows to minimize the influence of line impedance and DC load level on the obtained results. Comparison of input current harmonic distortion emission should be carried out

• autonomous operation as a harmonic distortion compensator for other non-linear loads working in the power grid.

PWM boost rectifier basic topology is based on the six pulse power transistors bridge which is connected to power grid through AC line reactor (Figure 11). AC line reactor *LAC* allows to control line current freely using suitable PWM strategies, which results in a possibility to considerably decrease the line current harmonic emission level in frequency range below 2*kHz*. Essential problem, tightly related to the current harmonic distortion emission in the frequency range close and above PWM modulation carrier frequency are input current ripples which are an effect of line and DC bus voltage commutations over the AC line reactor inductance *LAC* (Figure 12).

**Figure 11.** Three-phase grid connected PWM boost converter topology.

The exemplary line current waveform obtained using this method is presented in Figure 13, where nearly sinusoidal current can be seen with low harmonic content in frequency band below 2*kHz* (Figure 14), however with some noticeable distortions in higher frequency range which are an effect of existing limitations of the used PWM control method. To minimize the PWM carrier frequency related harmonic emission low pass filtering methods are used,usually based on the LCL filter topology. Nevertheless, the harmonic emission effect correlated with PWM carried frequency is observable in most of applications (Figure 15). The

**Figure 12.** Line current and voltage ripples generated by PWM boost converter

8 Power Quality

example:

systems,

correction system,

working in the power grid.

inductance *LAC* (Figure 12).

**Figure 11.** Three-phase grid connected PWM boost converter topology.

**3.2. High frequency harmonic emission of modern AC-DC converters**

Severe limitations of the line current harmonic performance improvement possibilities of classic diode rectifiers stimulate introducing fully controlled switches in AC-DC converters. Accompanying significant increase of IGBT transistor performance during the last decade allows to obtain successful implementation of PWM boost AC-DC three phase converter topology in many applications where harmonic distortion emission has to be limited. PWM boost type AC-DC converters besides line current harmonic distortion significant reduction in frequency range up to 2*kHz* have a number of other advantages [4], [5], [10], like for

• ability to transform energy bidirectionally, which significantly increases the range of applications especially in energy saving purpose and renewable and distributed energy

• possibility to control line current phase, which allows to maintain reactive power consumption within required limits and also stand-alone operations as a power factor

• autonomous operation as a harmonic distortion compensator for other non-linear loads

PWM boost rectifier basic topology is based on the six pulse power transistors bridge which is connected to power grid through AC line reactor (Figure 11). AC line reactor *LAC* allows to control line current freely using suitable PWM strategies, which results in a possibility to considerably decrease the line current harmonic emission level in frequency range below 2*kHz*. Essential problem, tightly related to the current harmonic distortion emission in the frequency range close and above PWM modulation carrier frequency are input current ripples which are an effect of line and DC bus voltage commutations over the AC line reactor

The exemplary line current waveform obtained using this method is presented in Figure 13, where nearly sinusoidal current can be seen with low harmonic content in frequency band below 2*kHz* (Figure 14), however with some noticeable distortions in higher frequency range which are an effect of existing limitations of the used PWM control method. To minimize the PWM carrier frequency related harmonic emission low pass filtering methods are used,usually based on the LCL filter topology. Nevertheless, the harmonic emission effect correlated with PWM carried frequency is observable in most of applications (Figure 15). The maximum emission is observed around modulation frequency which in the tested converter was set to 15*kHz*. In higher frequency range, close to integer multiples of PWM carrier frequency harmonic products of modulation are usually also observed. Perfect elimination of this PWM-related emission is not possible and became more difficult to realize by using passive filters with the increase of frequency. An example of input current and voltage waveforms and its harmonic content in frequency domain representation recorded in PWM boost converter are presented in Figure 13, 14 and 15.

**Figure 13.** PWM boost converter – input current and voltage waveforms.

### **3.3. Comparison of line current harmonic distortion of diode and PWM boost rectifiers**

Detailed comparison of current harmonic distortion emission has been done for three phase six pulse diode rectifier and PWM boost converter with the three phase IGBT transistor bridge. Both converters has been tested in similar supply condition and using similar load, which allows to minimize the influence of line impedance and DC load level on the obtained results. Comparison of input current harmonic distortion emission should be carried out

**Figure 16.** Harmonic emission of three phase six pulse diode rectifier with comparison to PWM boost converter emission

whereas inter-harmonic content is considerable: around 10%.

index is substantially higher: 43% and 48% respectively.

*3.3.2. Frequency range* 2 − 9*kHz*

of power frequency: inter-harmonics (Figure 16 grey area). Detailed calculation results are presented in Table 1, where the decrease of harmonics content from 97% down to 3.5% and increase of inter-harmonics from 2% up to 10% are listed. The final effect of obtained harmonic reduction using PWM boost technology is the decrease of THD from 97% to 11%

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**AC-DC Converter topology Harmonics Inter-harmonics THD PWHD** Diode rectifier 97% 2% 97% 48% Diode rectifier with passive filtering 29% 0.5% 29% 28% PWM boost converter 3.5% 10% 11% 34%

Higher order harmonic distortion of line current is particularly important in several applications because of its disturbing potency in power grid. To asses the certain limitation levels in standard [6] partial weighted harmonic distortion (PWHD) is extra defined using formula (3). According to this rule, harmonics above 14*th* order up to 40*th* order are considered with the weighting factor increasing with harmonic order. The best performance in terms of PWHD index, have been observed for diode rectifier with passive filters: only 28% (Table1). For the PWM boost converter and diode rectifier without passive filter PWHD

> <sup>40</sup> ∑ *n*=14 *nI*<sup>2</sup> *n*

Harmonic distortions in frequency range up to 9*kHz* are characterized in standard [8] as a result of grouping of harmonics DFT product obtained for 5 cycles of observation within 200*Hz* sub-bands using rectangular window. Proposed grouping method results with 35

*I*1

(3)

**Table 1.** Comparison of current harmonic distortion emission spectra of different AC-DC converters topologies

*PWHD*(*I*) =

**Figure 14.** PWM boost converter – input current harmonics spectrum up to 2kHz.

**Figure 15.** PWM boost converter – input current harmonics spectrum up to 25kHz.

separately for different frequency sub-ranges presented in Figure 1 and 2, because of different evaluation methods which have to be used in each particular sub-range.

### *3.3.1. Frequency range up to* 40*th harmonic order*

Typical analysis, important from the total harmonic distortion (THD) limitations point of view, consider frequency range up 40*th* harmonic order of power frequency (in 50*Hz* system it is up to 2*kHz*). In this frequency range classic line commutated rectifiers generate dominating characteristic harmonics orders *n* ∗ (*p* ± 1) correlated with number of pulses *p* depending on rectifier topology. According to this rule, for six pulse rectifiers harmonics of order H5, H7 and H11, H12 and H17, and 19 etc. are dominating (Figure 16 blue line).

The use of PWM boost conversion technology allows to decrease harmonic emission for this specific orders significantly (about tens of times for H5 and H7, about ten times for H11, H13 and H17, H19). Unfortunately, use of PWM boost conversion technology introduces extra harmonic components emission for frequencies values in between integer multiplies

**Figure 16.** Harmonic emission of three phase six pulse diode rectifier with comparison to PWM boost converter emission

of power frequency: inter-harmonics (Figure 16 grey area). Detailed calculation results are presented in Table 1, where the decrease of harmonics content from 97% down to 3.5% and increase of inter-harmonics from 2% up to 10% are listed. The final effect of obtained harmonic reduction using PWM boost technology is the decrease of THD from 97% to 11% whereas inter-harmonic content is considerable: around 10%.


**Table 1.** Comparison of current harmonic distortion emission spectra of different AC-DC converters topologies

Higher order harmonic distortion of line current is particularly important in several applications because of its disturbing potency in power grid. To asses the certain limitation levels in standard [6] partial weighted harmonic distortion (PWHD) is extra defined using formula (3). According to this rule, harmonics above 14*th* order up to 40*th* order are considered with the weighting factor increasing with harmonic order. The best performance in terms of PWHD index, have been observed for diode rectifier with passive filters: only 28% (Table1). For the PWM boost converter and diode rectifier without passive filter PWHD index is substantially higher: 43% and 48% respectively.

$$PWHD(I) = \frac{\sqrt{\sum\_{n=14}^{40} nI\_n^2}}{I\_1} \tag{3}$$

### *3.3.2. Frequency range* 2 − 9*kHz*

10 Power Quality

**Figure 14.** PWM boost converter – input current harmonics spectrum up to 2kHz.

**Figure 15.** PWM boost converter – input current harmonics spectrum up to 25kHz.

*3.3.1. Frequency range up to* 40*th harmonic order*

evaluation methods which have to be used in each particular sub-range.

and H11, H12 and H17, and 19 etc. are dominating (Figure 16 blue line).

separately for different frequency sub-ranges presented in Figure 1 and 2, because of different

Typical analysis, important from the total harmonic distortion (THD) limitations point of view, consider frequency range up 40*th* harmonic order of power frequency (in 50*Hz* system it is up to 2*kHz*). In this frequency range classic line commutated rectifiers generate dominating characteristic harmonics orders *n* ∗ (*p* ± 1) correlated with number of pulses *p* depending on rectifier topology. According to this rule, for six pulse rectifiers harmonics of order H5, H7

The use of PWM boost conversion technology allows to decrease harmonic emission for this specific orders significantly (about tens of times for H5 and H7, about ten times for H11, H13 and H17, H19). Unfortunately, use of PWM boost conversion technology introduces extra harmonic components emission for frequencies values in between integer multiplies

Harmonic distortions in frequency range up to 9*kHz* are characterized in standard [8] as a result of grouping of harmonics DFT product obtained for 5 cycles of observation within 200*Hz* sub-bands using rectangular window. Proposed grouping method results with 35 sub-bands (groups) *Hg*, 200*Hz* wide each with the center frequencies of the band starting from 2.1*kHz* up to 8.9*kHz*. Grouping algorithm is represented by formula 4.

$$H\_{\mathcal{g}(200Hz)} = \sqrt{\sum\_{k=\mathcal{g}-90Hz}^{\mathcal{g}+100Hz} I\_k^2} \tag{4}$$

fundamental power frequency with relation to diode rectifier results, they are roughly at similar level. However, inter-harmonic emission for PWM boost converter is more or less at the same level as harmonics (Figure 18), whereas for diode rectifier inter-harmonic levels were in average at least more than ten times lower, which results with increase of power spectrum density in the whole frequency band. By employing the grouping method of harmonic content proposed in standard [8] the total power of harmonic emission within each of 200*Hz* wide frequency sub-range can be calculated using formula 4 . This standardized analysis shows a significant increase of total spectral power emission of PWM boost rectifier in relation to diode rectifier (Figure 19), whereas the maximum individual amplitudes of

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DFT product for both converters are at similar level (Figures 17 and 18).

**Figure 19.** Comparison of harmonic emission of diode rectifier and PWM boost converter.

Values of PWM carrier frequency used in typical applications are mainly correlated with converter's input voltage and its rated power. In contemporary applications of PWM boost rectifiers modulation frequency values are usually in a range of few kHz for high power converters (above hundreds of *kW*) up to tens of kHz low power converters (below *kW*). This frequency range is located just above power quality frequency range and includes significant part of CISPR A range (Figure 1). PWM carrier frequency and its integer multiples define frequency sub-rages, where increased current harmonic emissions usually appear. There are known different method of decreasing this emission, nevertheless it is difficult to eliminate

In the evaluated converter modulation carrier frequency was set to 15*kHz* and more than ten times higher current harmonic amplitudes in analysed DFT product has been observed for this frequency, and about few times higher for frequencies close to PWM carrier frequency, between 13*kHz* and 15*kHz* (Figure 20). For analysing DFT product in accordance to CISPR 16 standard within CISPR A frequency band, 200*Hz* resolution band width should be used. Power spectral density (PSD) calculated according to this rule is presented in Figure 21. The obtained results show that harmonic emission in this frequency range is significantly higher

*3.3.3. PWM carrier frequency range*

them entirely by for example passive filtering.

in relation to diode rectifier converter.

For the purpose of comparison of current harmonic emission of the diode rectifier and PWM boost converter the FFT analysis has been done according to [8]. To demonstrate more clearly the effect of harmonic emission character the raw DFT product in frequency range 2 − 9*kHz* is presented in Figures 17 and 18.

**Figure 17.** Comparison of harmonic emission of three phase six pulse diode rectifier and PWM boost converter in frequency range 2 − 9*kHz* .

**Figure 18.** Comparison of harmonic emission of three phase six pulse diode rectifier and PWM boost converter - more detailed view at some exemplary frequency sub-range.

The obtained results show that the use of PWM boost converter do not change significantly current harmonic amplitudes for the frequencies close to integer multiples of the fundamental power frequency with relation to diode rectifier results, they are roughly at similar level. However, inter-harmonic emission for PWM boost converter is more or less at the same level as harmonics (Figure 18), whereas for diode rectifier inter-harmonic levels were in average at least more than ten times lower, which results with increase of power spectrum density in the whole frequency band. By employing the grouping method of harmonic content proposed in standard [8] the total power of harmonic emission within each of 200*Hz* wide frequency sub-range can be calculated using formula 4 . This standardized analysis shows a significant increase of total spectral power emission of PWM boost rectifier in relation to diode rectifier (Figure 19), whereas the maximum individual amplitudes of DFT product for both converters are at similar level (Figures 17 and 18).

**Figure 19.** Comparison of harmonic emission of diode rectifier and PWM boost converter.

### *3.3.3. PWM carrier frequency range*

12 Power Quality

range 2 − 9*kHz* .

is presented in Figures 17 and 18.

view at some exemplary frequency sub-range.

sub-bands (groups) *Hg*, 200*Hz* wide each with the center frequencies of the band starting

For the purpose of comparison of current harmonic emission of the diode rectifier and PWM boost converter the FFT analysis has been done according to [8]. To demonstrate more clearly the effect of harmonic emission character the raw DFT product in frequency range 2 − 9*kHz*

**Figure 17.** Comparison of harmonic emission of three phase six pulse diode rectifier and PWM boost converter in frequency

**Figure 18.** Comparison of harmonic emission of three phase six pulse diode rectifier and PWM boost converter - more detailed

The obtained results show that the use of PWM boost converter do not change significantly current harmonic amplitudes for the frequencies close to integer multiples of the

*g*+100*Hz* ∑ *k*=*g*−90*Hz*

*Ik*

<sup>2</sup> (4)

from 2.1*kHz* up to 8.9*kHz*. Grouping algorithm is represented by formula 4.

*Hg*(200*Hz*) =

Values of PWM carrier frequency used in typical applications are mainly correlated with converter's input voltage and its rated power. In contemporary applications of PWM boost rectifiers modulation frequency values are usually in a range of few kHz for high power converters (above hundreds of *kW*) up to tens of kHz low power converters (below *kW*). This frequency range is located just above power quality frequency range and includes significant part of CISPR A range (Figure 1). PWM carrier frequency and its integer multiples define frequency sub-rages, where increased current harmonic emissions usually appear. There are known different method of decreasing this emission, nevertheless it is difficult to eliminate them entirely by for example passive filtering.

In the evaluated converter modulation carrier frequency was set to 15*kHz* and more than ten times higher current harmonic amplitudes in analysed DFT product has been observed for this frequency, and about few times higher for frequencies close to PWM carrier frequency, between 13*kHz* and 15*kHz* (Figure 20). For analysing DFT product in accordance to CISPR 16 standard within CISPR A frequency band, 200*Hz* resolution band width should be used. Power spectral density (PSD) calculated according to this rule is presented in Figure 21. The obtained results show that harmonic emission in this frequency range is significantly higher in relation to diode rectifier converter.

Results of investigations presented in this chapter demonstrate that line current and voltage ripples, as an effect of PWM modulation carrier frequency in PWM boost converters, can induce compatibility problems in numerous applications which usually cannot be easily solved by using conventional passive harmonic-filters or radio frequency interference (RFI) filters. Harmonic emission filtering in this frequency band require new specific types of

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Faculty of Electrical and Control Engineering, Gdansk University of Technology, Gdansk,

[1] Benysek, G. [2007]. *Improvement in the Quality of Delivery of Electrical Energy using Power*

[2] Bollen, M. & Hassan, F. [2011]. *Integration of Distributed Generation in the Power System*,

[3] Bollen, M., Yang, Y. & Hassan, F. [2008]. Integration of distributed generation in the power system - a power quality approach, *Harmonics and Quality of Power, 2008. ICHQP*

[4] Cichowlas, M., Malinowski, M., Kazmierkowski, M. & Blaabjerg, F. [2003]. Direct power control for three-phase pwm rectifier with active filtering function, *Applied Power Electronics Conference and Exposition, 2003. APEC '03. Eighteenth Annual IEEE*, Vol. 2,

[5] Cichowlas, M., Malinowski, M., Kazmierkowski, M., Sobczuk, D., Rodriguez, P. & Pou, J. [2005]. Active filtering function of three-phase pwm boost rectifier under different line voltage conditions, *Industrial Electronics, IEEE Transactions on* **52**(2): 410 – 419.

[6] *IEC 61000-3-4 (1998): Electromagnetic compatibility (EMC) - Part 3-4: Limits - Limitation of emission of harmonic currents in low-voltage power supply systems for equipment with rated*

[7] *IEC 61000-3-6 (2008): Electromagnetic compatibility (EMC) - Part 3-6: Limits - Assessment of emission limits for the connection of distorting installations to MV, HV and EHV power systems*

[8] *IEC 61000-4-7 (2009): Electromagnetic compatibility (EMC) - Part 4-7: Testing and measurement techniques - General guide on harmonics and interharmonics measurements and instrumentation, for power supply systems and equipment connected thereto* [2009].

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*Electronics Systems*, Power Systems, Springer-Verlag.

*2008. 13th International Conference on*, pp. 1 –8.

[2008]. International Electrotechnical Commission Std.

International Electrotechnical Commission Std.

filters to be used.

**Author details**

Jaroslaw Luszcz

**References**

Wiley-IEEE Press.

pp. 913 –918 vol.2.

Poland

**Figure 20.** Current harmonic distortion of PWM boost converter in the frequency range close to PWM carrier frequency with comparison to classic diode rectifier distortions.

**Figure 21.** Power spectral density of current harmonic calculated for 200*Hz* resolution badnwidth.

### **4. Conclusions**

PWM boost AC-DC converters are increasingly used in contemporary application because of its considerable advantages, like bidirectional power transfer with unity power factor operation and low level of low order harmonic distortions emission. Systematic significant increase of the overall power quota converted from AC to DC and from DC to AC in the power system make these advantages more meaningful from the power quality point of view. This development trends also introduce some unfavourable effects, like increased emission in higher frequency ranges, which are presented in this chapter.

Increased harmonic emission in the frequency range between 2*kHz* and 9*kHz* as an effect of pulse width modulation method used for line current control in PWM boost converters becomes a fundamental problem to solve in such converters connected directly to the power grid. In recent years increased number of investigations focused on arising compatibility challenges in frequency band 2 − 9*kHz* has been reported and some new standardization methods has been initially proposed.

Results of investigations presented in this chapter demonstrate that line current and voltage ripples, as an effect of PWM modulation carrier frequency in PWM boost converters, can induce compatibility problems in numerous applications which usually cannot be easily solved by using conventional passive harmonic-filters or radio frequency interference (RFI) filters. Harmonic emission filtering in this frequency band require new specific types of filters to be used.

### **Author details**

Jaroslaw Luszcz

14 Power Quality

comparison to classic diode rectifier distortions.

**4. Conclusions**

**Figure 20.** Current harmonic distortion of PWM boost converter in the frequency range close to PWM carrier frequency with

PWM boost AC-DC converters are increasingly used in contemporary application because of its considerable advantages, like bidirectional power transfer with unity power factor operation and low level of low order harmonic distortions emission. Systematic significant increase of the overall power quota converted from AC to DC and from DC to AC in the power system make these advantages more meaningful from the power quality point of view. This development trends also introduce some unfavourable effects, like increased emission

Increased harmonic emission in the frequency range between 2*kHz* and 9*kHz* as an effect of pulse width modulation method used for line current control in PWM boost converters becomes a fundamental problem to solve in such converters connected directly to the power grid. In recent years increased number of investigations focused on arising compatibility challenges in frequency band 2 − 9*kHz* has been reported and some new standardization

**Figure 21.** Power spectral density of current harmonic calculated for 200*Hz* resolution badnwidth.

in higher frequency ranges, which are presented in this chapter.

methods has been initially proposed.

Faculty of Electrical and Control Engineering, Gdansk University of Technology, Gdansk, Poland

### **References**


16 Power Quality

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*Bologna*, Vol. 3, p. 8 pp. Vol.3.

Vol. 57(no 4): pp.383–393.

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URL: *http://bulletin.pan.pl/(57-4)297.pdf*

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pp. 297–309.

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[16] Strauss, P., Degner, T., Heckmann, W., Wasiak, I., Gburczyk, P., Hanzelka, Z., Hatziargyriou, N., Romanos, T., Zountouridou, E. & Dimeas, A. [2009]. International white book on the grid integration of static converters, *Electrical Power Quality and*

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## *Edited by Ahmed Zobaa*

Given our increasing dependence on technologies, security of the power supply is of the utmost importance. As such, we require cutting-edge and efficient methods of measuring its efficiency and reliability. This book offers chapters on the compression of power quality data, and methods of recognizing and classifying quality in distribution networks. Other chapters cover using robust symmetrical components' estimation to improve quality in weak grids, monitoring the quality of small-scale renewable energy and the use of active power conditioners to mitigate power-quality problems. Several chapters cover harmonic effects, such as case studies of bank harmonic filters, a parameter-estimation methodology and high-frequency harmonics emissions in smart grids. Finally, one contribution evaluates the distortion and imbalance of emission levels in electric networks.

Power Quality Issues

Power Quality Issues

*Edited by Ahmed Zobaa*

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