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Microgrid Safety: Four electrical challenges a monkey can’t solve

June 24, 2019

By Dan Jones, P.E.
POWER Engineers

Dan Jones is an electrical engineer in POWER’s SCADA and Analytical Studies group who has worked on numerous microgrid projects leading electrical protection and system studies in facilities, utility distribution, and transmission systems. His expertise includes challenging overcurrent coordination and protection of feeder, buses, transformers, and other major equipment as well as protection of medium-voltage and low-voltage motors. Dan’s assertions about monkey behavior are purely theoretical; no actual monkeys attempted microgrid safety design during the development of this article.

Given enough time, an infinite number of monkeys banging away at an infinite number of computers would eventually make every possible microgrid safety design mistake. Fortunately, engineers are smarter than your average monkey and can head off those safety mistakes ahead of time. That said, not much writing exists specifically on microgrid safety design considerations. In this post, I will cover a few of the safety concerns to be examined when designing and analyzing a proposed microgrid.

Effective ground sources limit transient over-voltages and provide fault current during ground fault events. It is important for equipment and personnel safety to carefully consider the location and strength of ground sources in a microgrid system. Ground current is typically sourced from the grounded neutral connection of a delta-wye transformer, synchronous generators, or grounding banks. Depending on whether the microgrid is grid-connected and the connection (or, conversely, the isolation) of the ground sources, the microgrid may not achieve effective grounding. Barring resistive grounded industrial systems, effective grounding is required for all portions of the system under all system configurations, both grid-connected and islanded. Without sufficient ground fault current, overcurrent-based protective relays can’t detect and clear electrical system faults. Reliable and fast detection and clearing of faults is of utmost importance for electrical safety. Careful attention should be paid to both ground source location as well as strength when analyzing the microgrid design.

Like the ground source discussion, the difference in available phase fault current between grid-connected and islanded can differ by orders of magnitude. This can greatly affect the sensitivity and speed of protective relays. Reduced fault current can result in either slow fault clearing or possibly faults being undetected altogether. Faults that remain undetected or clear slowly pose a greater risk to equipment and personnel. Equipment damage ranges from insulation damage resulting in the increased risk of future faults to the more immediate risk of fire. The risk to personnel is electrical shock and increased arc flash hazards. Electrical system protection should be designed to be fast and reliable for all fault locations and system conditions.

The amount of energy released in an electrical arc depends on the fault current magnitude, the duration of the arc, and the enclosure in which the arc occurs. Typically, the enclosure in which a fault may occur in the system would not change depending on whether the system is grid-connected or islanded. As previously discussed, fault magnitude and subsequent clearing time may vary greatly. Intuitively, it follows that if fault magnitude decreases and clearing time remains constant, the energy released in the arc would decrease. Counterintuitively, reduced fault current can result in an increased arc flash hazard, due to the characteristic of inverse time overcurrent elements. Inverse time overcurrent elements trip increasingly faster with higher fault currents and increasingly slower with lower fault currents. Under certain conditions, the fault clearing time gets so long that the total amount of energy released in the low magnitude arc is greater than higher magnitude faults that clear faster. The primary recommendation is to model and calculate the worst-case arc flash incident energy to quantify the hazard. Ideally, equipment should be repaired and maintained while deenergized. If de-energization is not an option, fast and sensitive arc flash maintenance mode settings should be added to protective relaying to reduce the arc flash incident energy to acceptable levels. As a last resort, higher rated personal protective equipment is needed to protect personnel against arc flash hazards.

Up to now, we’ve focused on the dangerous situations presented by the faulted power system. What about day-to-day operation and maintenance? Being able to isolate equipment such that it can be maintained while de-energized greatly increases personnel and equipment safety. Additionally, being able to de-energize the minimum amount of the system to isolate faulted equipment or de-energize equipment for maintenance results in greater system uptime. If the microgrid supports a hospital or emergency services, the increased uptime may result in increased public safety. So, make sure you’ve considered both what equipment needs to be isolated and when, so you can develop an adequate isolation scheme for your microgrid project. Downtime costs money, and in the case of sensitive facilities like hospitals, can even cost a life.

A little forethought in the microgrid design stage will alleviate a lot of headaches in the long run and free up the monkeys to work on recreating the complete works of Shakespeare.