Current Problems

With the massive decrease in cost of acquisition of renewable energy technology, implementation of the technology is accelerating. Often installed in micro-grid configurations for benefits which include improved operational stability, protection of critical infrastructure, reducing peak loads on transmission networks and more. In case no transmission network is available, a micro-grid installation can act as a self-sufficient energy bubble. This enables many to move away from the distribution network and become energy independent. The micro-grid does not however come without challenges. Another technology advancement has been responsible for some problems, sometimes very difficult to diagnose. 

If you have a look around your house today most likely you will only have devices containing power electronics converters in sight. Cellphone\laptop chargers, Television, LED lighting. This change has been sneaking up on us for the last two decades or so. The cost and quality of LED lighting for example now makes most other types of lighting redundant as seen by adoption rates in Figure 1. 

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Fig 1. Figure 1 Micro-grid example layout [2]

Now this is not specifically a bad thing. Power electronics converters offer great efficiency, lower cost and weight. All good. But what are the negative effects of these devices? Not many people know and it does not directly effect most of us. These devices are so small and use so little power, how can they cause harm. As is the case with many examples, a small and seemingly insignificant impact multiplied by large quantities stack up. One plastic straw doesn’t cause pollution. But billions can. For industrial and commercial cases with large amounts of LED lighting, or a neighborhood with high adoptions rates of solar PV and inverters. Offices with many computers and other devices. These power electronic converters generally draw harmonic currents which can reduce the quality of power in the local electricity grid. This can deform the voltage waveform away from a pure sinusoidal and transformers and other equipment to heat up as well as causing electromagnetic interference with other local devices. Heating of equipment in general equites to a reduction in lifespan. This is one problem. 

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Fig 2. Figure 2 LED lighting adoption rate by market share [1].

The other problem is arguably more serious and the point of this article. When a power electronics devices are switched on. This usually involves a capacitor charging up and inevitable causing an inrush current. This large current spike with switch on can cause a power supply source to fail. Or in electromagnetic compatibility terms, interfere with the ability of the power supply to perform its purpose. This is especially harmful for weak grids, for example a remote installation relying solely on a generator or inverter power. In Figure 3 a 1800W linear load (red) is energized with nearly no inrush current compared to a set of two ±200W non-linear loads (Blue and green) with a large inrush. 

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Figure 1. Steps of model-based design of generic system.

This is the basis of the problem. Some isolated microgrids experience severe reliability problems due to this effect. Hospitals, which by law require a backup generators for critical life support, especially have been experiencing increasing issues. During monthly controlled tests all power is disconnected. These non-linear loads discharge during this switch over time and when the generator is activated, faults occur. This may stop the backup system from functioning as it should. If these problems exist during backup system tests. Imagine what would happen during an actual emergency. Similar problems occur in other types isolated micro-grid systems.

The effect of non-linear loads are not always fully understood or underestimated which in turn results in reliability issues. Due to the phase firing angle having a large impact on inrush current intensity, the interference cause by these inrush currents are often intermittent. Making the problem difficult to diagnose. Some solutions exists to control these effects such as adding filters. But these may not always be practical, add cost and the load profile connected to a micro-grid may change as different technologies are connected to a system in time.

Research is being performed on new measurement techniques to gauge the severity of the problem as well as possible solutions. A good start is spread awareness and educate those who are involved with design and installation of micro-grid systems. That is in part what the ETOPIA research group has been tasked to do.

References

[1] M. Shams, "Hyperikon.com," Hyperikon, 1st June 2018. [Online]. Available: https://www.hyperikon.com/blog/why-led-lighting-is-better/. [Accessed 20 1 2021]. 


[2] "www.pngegg.com," PNGEGG, [Online]. Available: https://www.pngegg.com/en/png-mwkql. [Accessed 21 01 2021].


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