High Frequency Harmonics Emission in Smart Grids

High Frequency Harmonics Emission in Smart Grids

Jaroslaw Luszcz

Additional information is available at the end of the chapter


Provisional chapter

High Frequency Harmonics Emission in Smart Grids

Jaroslaw Luszcz

Additional information is available at the end of the chapter

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 29 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:

2 Power Quality

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.

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 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.


READ FULL PAPER AT:  http://cdn.intechopen.com/pdfs-wm/44202.pdf


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