Conducted EMI and Power Quality in DC Microgrids – the ‘What’, ‘Where’ and ‘How’

Part 1 of 3 Blog posts – the ‘What’ 

With the recent economic feasibility in small scale renewable power generation, electricity now can not only be generated locally but also be shared within a close-knit community. It could mean that an entire street would share a common local grid that generates & consumes electric power using solar Photovoltaic Systems (PV) rooftop systems and be connected to the main AC grid as well. This revolutionary concept of democratised electric generation, supply and consumption is known as a Microgrid.

A Microgrid, by definition, means a singular, small scale, independent power generating and consumption grid. This independent power grid could be a household, or an interconnection of households capable of generating and sharing power and be connected to the main AC network. Microgrid technology is relatively new and is being extensively researched with multiple areas- grid tied & untied, for short circuits, for efficient load sharing being some of the active topics. Though sporadically implemented throughout the world, microgrid technology is still largely lab based and there are many active research areas that being pursued to develop the technology.

One of these main questions is what technology should the microgrid use? Theoretically, a microgrid can be applied using either AC or DC technology and can be connected to the mains AC utility network. Solar PV rooftops use switched mode power supply devices to convert the DC power generated by the PV systems from higher to lower voltages, as deemed suitable for household appliances. As compared to AC technology, there are many merits of using DC technology for microgrids, one of them being the intermediate steps of conversion. For Solar PV rooftops, the power generated is DC and will need to be converted to AC to be compatible with existing household appliances. However, a lot of research is being motivated to shift from AC to DC for low voltage applications for household appliances.

From a consumer’s perspective, the most important question is ‘will the microgrid be as good as the AC mains utility?’. In other words, ‘will the bulbs and the kettles work well?’. From an engineering perspective, ‘is the level of conducted emissions in the voltage & current low enough for acceptable power quality? To express specifically, it would mean how well is the Electromagnetic Compatibility (EMC) between these devices and the Solar PV? And how low are Conducted Emissions (CE) that affect the Power Quality (PQ)?

According to IEC 61000, PQ is defined as ‘the ability of a device, equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment’. In terms of DC technology, much of PQ remains to be understood since unlike AC, DC has no fundamental period of a frequency of operation. Hence, to quantify the harmonics (i.e CE), in DC cannot be fundamentally calculated. This poses new challenges as unlike AC, the number of sources for CE in DC could be greater as the grid could be tied or untied with the AC mains.

To provide further understanding, in AC the fundamental frequency is 50Hz (60Hz in USA and Latin America) and the harmonics in the grid are calculated using a term called percentage Total Harmonic Distortion. This percentage level decides the permissible CE that can be allowed in the grid to maintain the PQ that will ensure proper EMC within the connected equipment.

\[\text{%THD} =  \sqrt{ \sum_{h=1}^{H}[Q_{H}/Q_{1}]^2}\]

As can be seen from the formula above, the %THD expresses the greater frequency peaks (Q_H) captured in AC signals divided by the steady state value (Q_1) of the signal. The AC PQ can be sufficiently described within 10 (or 12) cycle of the fundamental frequency that covers the range of about 2000 Hertz. As can be seen in the figure below, the AC harmonics amplitude gradually reduce from the peak at 50Hz through to 500Hz. 


However, since DC has no fundamental frequency, the calculation is not as straightforward as AC. To overcome this, a new mathematical technique called ‘percentage Low Frequency Sinusoidal Disturbance’ (%LFSD) has been discussed. This quantity describes the CE present in a DC grid that could be connected to the mains AC grid. 

\[\text{%LFSD} = \sqrt{ \sum_{k=1}^{K}[Q_{K}/Q_{0}]^2}\]

As can be seen in the figure below, a typical DC microgrid frequency response to about 1MHz does show peaks related to the switched mode power converters present in system. The amplitude peaks tend to remain constant throughout the frequency range and would mean that unlike AC, the CE in DC tends to remain constant throughout for longer frequency range. This could be problematic in cases of tied DC microgrids as they can become potential sources of CE in AC grid as well.  


It is not yet understood if the %LFSD can be used as a given for DC PQ and studies about it are being conducted. As DC PQ develops, it could be that a combination of both time and frequency domain analysis could be used. Further, the use of power electronics consisting of different types of switch mode power supplies with different topologies would make the DC signal even more complicated. Hence, along with %LFSD, there could be different frequency bandwidths specifically quantifying the DC PQ for deeper analysis.

It could be that DC PQ in microgrids would further depend on where and how its measured. The next blog post will discuss the ‘where’.

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