High Frequency Harmonics Emission in Smart Grids

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].


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: 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.
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.

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 30MHz. Between 9kHz and 30MHz two frequency sub-bands are defined as CISPR A up to 150kHz and CISPR B above 150kHz ( 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 30MHz. 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.  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 2kHz. In this frequency range integer multiples of the fundamental power frequency (50Hz or 60Hz) harmonic components are defined usually up to 40 th order.
Consideration of conducted emissions and harmonic distortions in these frequency bands: one up to 2kHz and second 9kHz − 30MHz, 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 − 9kHz 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.

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 2kHz 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 2kHz can be named as high frequency harmonics emission. On the other hand, from the EMC point of view, the conducted EMI emission below 9kHz 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: • classic PQ frequency range up to 2kHz, where the main sources of harmonic distortions are usually line commutated rectifiers used in single-and multi-phase topologies using as power switches diodes or thyristors, • high frequency harmonic distortion emission in the frequency range 2 − 9kHz, 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 tens of kHz, • conducted EMI emission in frequency range (9kHz − 30MHz), 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 easily propagate also towards AC power lines.

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 I AC make a distortion effects slightly visible also at voltage waveform, where voltage deformations are correlated in time with the current pulses. 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 I k with 5Hz resolution in frequency range up to 2kHz in Figure  5 and up to 25kHz 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

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 2kHz have a number of other advantages [4], [5], [10], like for example: • ability to transform energy bidirectionally, which significantly increases the range of applications especially in energy saving purpose and renewable and distributed energy systems, • 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 correction system, • 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 L AC 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 2kHz. 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 L AC (Figure 12). 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 2kHz (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

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  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.

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 50Hz system it is up to 2kHz). 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  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%. 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.

AC-DC Converter topology Harmonics Inter-harmonics THD PWHD
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 − 9kHz is presented in Figures 17 and 18.  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 200Hz 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).

PWM carrier frequency range
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 15kHz 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 13kHz and 15kHz ( Figure 20). For analysing DFT product in accordance to CISPR 16 standard within CISPR A frequency band, 200Hz 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.

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 2kHz and 9kHz 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 − 9kHz 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.