Understanding the Unintended Consequences of Renewable Energy
Distribution grid operators are concerned that high penetration of connected distributed energy resources (DERs) could lead to grid instability and create outages at end-user sites, such as commercial buildings or industrial plants. “Today, we don’t fully understand grid behavior under varying DER conditions, and are not sure how to address this properly,” said Fred Oshiro, Engineer from Maui Electric in Hawaii, during a DistrbuTECH panel discussion on January 2018.
Indeed, industrial consumers such as factories, labs with expensive equipment, and tool fabrication shops can also be impacted by unexplained malfunctions, resets, and shutdowns — all without any apparent power-related causes. Power supply units overheat and occasionally catch fire without circuit breakers ever tripping. Additional problems have been observed with street lighting control, interruptions in Power Line Communication (PLC), and even incorrect energy meter readings.
Power quality issues can stem from grid-connected power inverters, which are widely used in renewable generation to convert DC to AC and then connect solar PV, wind farms or batteries to the distribution grid. Inverters are designed and tested for compatibility individually, but the behavior of the control loops of multiple inverters connected as a system is unknown.
Inverters, as well as frequency converters and variable frequency drives (VFDs), are high-frequency switching devices, typically from 2 kHz to 200 kHz. Those devices can leak high-frequency conducted emissions into neighboring buildings and factories via the grid. As a result, commercial buildings and facilities located near renewable generation can be affected, though not necessarily in a predictable way. The intensity levels of emissions vary with distance from the source, inverter manufacturer design, and the presence of other inverters in the neighborhood.
Is this problem insurmountable? How can we efficiently troubleshoot issues that occur intermittently on the factory floor or in sensitive manufacturing areas, like semiconductor fabs or data centers?
As Lord Kelvin once said, “If you cannot measure it, you cannot improve it,” and this applies perfectly in this situation. The art of field troubleshooting consists of systematically closing doors to hypotheses to narrow down to the most probable cause (or root cause). To validate a hypothesis, you need to have measurements and the ability to precisely detect all disturbances, not just the most common ones.
The most common disturbances are voltage variations (rapid variations or sags, swells, interruptions), high levels of harmonics, system imbalance, high-frequency impulses, and in-rush currents.
Until recently, high-frequency conducted emissions in the 2-150 kHz range could remain under the radar of power quality investigations, due to lack of field measurement tools and data. Advancements in technology have led to new generations of ultra-precise instruments that continuously monitor the quality of AC and DC power, and detect and record all types of disturbances overtime at an affordable cost.
The image above shows a measurement graph automatically generated using an ultra-precise power analyzer. This is a time-based color map of high-frequency conducted emissions across a full day. Intermittent emissions at 130 kHz can be clearly identified. In this troubleshooting example, the periods of emissions activity coincided with observed failures on the shop floor of a factory. The minute granularity of the color map enabled the factory operator to conclusively confirm a strong correlation between those specific emissions and the equipment failures. Once the source was identified, the power maintenance engineer selected and installed a specific filter to protect the sensitive equipment. The equipment has since been operating reliably, without any issues.
The mitigation of high-frequency emissions must be coupled with the development of industry standards that will provide guidance for both grid operators (compatibility) and equipment manufacturers (immunity).
The IEC standard 61000-2-2 Ed. 2 AMD1 provides some guidance on limits in the range 2-30 kHz. IEC 61000-4-19 provides guidance on immunity levels for equipment connected to an electrical network. But there is, unfortunately, no real guidance as yet on emissions levels and their severity within factories (i.e., behind-the-meter locations)
In June 2017, California generated 27 percent of its electricity from renewable sources, on its way to a mandated 50 percent renewables by 2030. Maui Electric Company currently receives more than 37 percent of its energy from renewables, with a statewide goal of 100 percent renewables by 2050. In fact, more than 35 other states have renewable portfolio standards calling for greater use of clean energy sources.
As DER penetration increases, especially in islander grids and microgrids, power quality and the methods that we use to detect and analyze it will need to remain top of mind for energy users.
Stephane Do has 20 years of experience in the power quality business and is a global Product Manager at Powerside formerly Power Standards Lab.