Interview with David Mueller on Microgrids: Use Cases and Power Quality Considerations

d mueller

David Mueller is the Director of Power System Studies with EnerNex, an electric power research, engineering and consulting firm based in Knoxville, Tennessee. Since 1990 he has worked on a wide variety of power system projects to study and solve power quality problems. Dave has worked with companies such as Union Carbide, Exide Batteries, Public Service of New Mexico, Delmarva Power, Georgia Power, Wisconsin Electric Power Company, Con Edison of New York, the Electric Power Research Institute, Intel, General Motors, Honda of America, Motorola, American Airlines, East Midlands Electricity (UK), and the Electricity Supply Board (Ireland) to solve power quality problems.

He worked from 1993-1995 in Nottingham, England starting the Power Quality Services group for East Midlands Electricity. At that time he developed the 10-volume set, “Power Quality Training Manuals” for East Midlands Electricity. Dave has also written many technical papers and articles on power quality, and has given over 100 presentations on these topics.

He has presented in over 50 different cities, and has done work in over 25 different states in the U.S. Overseas, he has presented power quality in England, Ireland, Spain, Taiwan, Holland, Columbia, the Czech Republic, Sweden, Bulgaria, Indonesia, Singapore, Israel, and Brazil. Prior to joining EnerNex, Dave was at Electrotek Concepts for 21 years, and before that he was employed for eight years by General Motors. While working at GM Dave won a Corporate Energy Conservation Award. Mr. Mueller is a registered P.E. He received a B.S.E.E. from University of Cincinnati, and a Masters of Engineering from the Electric Power Engineering Department at Rensselaer Polytechnic Institute.

In this interview, Mueller answers questions as a follow up to his IEEE Smart Grid webinar, “Microgrids: Use Cases and Power Quality Considerations.” To view this webinar on-demand, visit the IEEE Smart Grid Resource Center here.

QUESTION: Which are the modern inverters?

ANSWER: Generally these are voltage source inverters, that have a high number of switching operations per cycle.

QUESTION: What are the different load curves for microgrids?

ANSWER: Generally this varies for the type of load, and the amount of time resolution.

QUESTION: What are the standards to define the tolerance of voltage, frequency deviation limits in microgrids?

ANSWER: ANSI/IEEE C84.1 defines voltage limits, frequency limits for microgrids need development.

QUESTION: Do we foresee DC microgrids coming up in near future?

ANSWER: There is a lot of speculation on dc power distribution these days. But do we have fuses and other protective devices that are ready for this?

QUESTION: Thanks for the very interesting presentation. How do you define a "weak system"?

ANSWER: A low short circuit ratio of available fault current to load current (generally less than 3).

QUESTION: Any current funded projects do you have?

ANSWER: Commercially sensitive question.

QUESTION: What are the different load curves for microgrids?

ANSWER: Generally this varies for the type of load, and the amount of time resolution.

QUESTION: What is LVRT mode, can the speaker shed more light on it?

ANSWER: Low voltage ride through – the ability of a generating source to maintain operation during a voltage sag or momentary outage.

QUESTION: What about ‘virtual inertia market’ in microgrids?

ANSWER: These are three words that don’t belong in the same phrase.

QUESTION: Which microgrid components are you seeing the largest cost movement in (either up or down)?

ANSWER: Solar panels, no question about it.

QUESTION: Do you believe the problem of motor inrush currents in microgrids has vanished due to modern power electronic controlled motor loads or will we still face it in many regions in the world, e.g. for remote microgrids?

ANSWER: Definitely agree that it is vanishing.

QUESTION: Do static switches not have on-load losses? For fault current condution, won't the fault current be limited to a limited value (based on thermal rating of the switches.

ANSWER: Load losses of static switches is low.

QUESTION: Have you come across any systems that conform to the USGBC's PEER certification process - PEER stands for Performance Excellence in Electricity Renewal and it is the nation's first comprehensive, consumer-centric, data-driven system for evaluating power. Fould you mention some KPIs regarding resilience? Thank you in advance.

ANSWER: Sorry, I’m not familiar with this, but thanks for letting us know about it.

QUESTION: How do you compare the importance of PQ vs Reliability vs Resiliency? Where do you spend the money?

ANSWER: Depends on the type of end user. Residential value resiliency, light industrial value reliability, while highly automated manufacturing values PQ. An interesting question!

QUESTION: You talked about the need to maintain coordination when interconnecting with the host utility. Does this also include directional relay protection for distribution equipment, such as line reclosers?

ANSWER: Yes, protection considerations (and thermal limits) are the most important coordination aspects.

QUESTION: Would you please review how Protection Coordination at low fault levels is addressed?

ANSWER: This is the $64,000 question, I suspect adaptive relaying may be necessary at the main distribution buses.

QUESTION: Do microgrids helps to improve voltage regulation? If yes, how?

ANSWER: No, and under islanded conditions this is more of a concern.

QUESTION: Is there a specific minimum Generation KW threshold that a source must meet to be called as a DER ? What kind of protocol will microgrid-to-microgrid or microgrid-to-grid control communication use?

ANSWER: To me, what makes sense is that a DER must be larger than the native load, so for residential this may suggest 5-10 kW. I suspect many distribution utilities define an even higher threshold. IEEE P2030 standards are important for DER.

QUESTION: What kind of protocol will microgrid-to-microgrid or microgrid-to-grid control communication use?

ANSWER: IEEE P2030 standards are important for DER.

QUESTION: For remote communities (similar to Himalayas and in Africa), we need flexibility, reconfigurable microgrid, nanogrid, minigrid. What's your thought on this and can you point to a resource/ use-cases?

ANSWER: The IEEE Smart Village program is exemplified in Jean Kumagai’s excellent article “Lights for the Enlightened” in IEEE Spectrum December 2016. Read it, it is one of the best articles I’ve read this year.

QUESTION: In microgrids, power sharing is an issue. What is the best way to plan power sharing in order to maintain system reliability?

ANSWER: Transactive energy based on blockchain technology is getting a lot of attention and investment.

QUESTION: How to microgrids deal with reverse power flow?

ANSWER: No differently than any other DER – issues with net metering and protection.

QUESTION: When designing a microgrid for a remote area, how do we take into account the development and, therefore, increase in demand of the remote area?

ANSWER: The ability of the population to sustain a higher standard of living, given the improvement of working conditions caused by the availability of electricity. It’s an interesting consideration!

QUESTION: What is your opinion on implementing a micro-grid solution for an actual island, in order to mitigate use of overloaded submarine cables, and use of diesel generators?

ANSWER: An islanded system is essentially a large microgrid. These two uses cases blend together.

QUESTION: Are the current and voltage harmonics resulted from microgrids connected at the LV still following the limits as stated in IEEE 591 Standards? If so, all calculations shall be done at PCC?

ANSWER: The IEEE 519 harmonic standards weren’t written for microgrid applications, but I predict they will be used. The new IEEE 1547 (2018) harmonic current limits are also interesting for these applications.

QUESTION: Are there particular sectors of our society that are installing microgrids like hospitals, security operating centers, disaster control centers etc.

ANSWER: Hospitals and the like have long had emergency backup power systems. The novelty of microgrids is to integrate renewable resources. Remote cities, mines, and military outposts have been some of the leaders.

QUESTION: What is a DC microgrid?

ANSWER: The idea is that electronic load converts ac to dc, so let’s skip a step and use the dc power straight from the solar panels. Not so simple to implement, however.

QUESTION: Do you think improving the power system interconnectivity as a whole should be taken into account in parallel with the microgrid implementation growth?

ANSWER: Of course everything matters. What is difficult in such analysis is sorting out what comes first, and what are the dependencies?

QUESTION: What software do you typically use to model your microgrids and perform analysis (and why so) ?

ANSWER: We like time domain software programs like EMTP-RV, PSCAD, or Digsilent. We’ve also used Homer, Power Analytics Design Base, and DER CAM.

QUESTION: Curtailable load can also mean curtailable by the utility during high demand periods. What mechanisms have been developed to ensure that the utility gets this benefit when required in return for the rate break to the owner of the microgrid?

ANSWER: Regulatory structures in general still need to be developed for microgrids. Some large customers do have interruptible power contracts.

QUESTION: How can we assure islanding detection during power quality issues in grid connected mode? in terms of temporary or permanent faults.

ANSWER: There are many anti-islanding schemes done by DER inverters, but in the future with LVRT requirements, this will be a challenge to balance the requirement to stay on line for remote faults, but trip for a local fault.

QUESTION: Is there a role for Demand Response in Micro Grids?

ANSWER: Demand response for a microgrid would have to be automatic and fast, such as with an energy storage units. Again, transactive energy is an enabling technology for the future.

QUESTION: Does resiliency imply also the recovering capability and capability to support the power delivery to consumers (even a lower quality service)?

ANSWER: I think resiliency is the ability to restore to the necessary PQ levels, so I reject that it should be lower. However, with a microgrid, there is more potential for voltage and frequency variation, so it must be properly designed and considered!

QUESTION: What is your opinion on implementing a microgrid solution for an actual island, in order to mitigate use of overloaded submarine cables, and use of diesel generators?

ANSWER: An islanded system is essentially a large microgrid. These two uses cases blend together.

QUESTION: What do you think about commercial quality? Is It possible to achieve a good interaction or commercial transaction between an electricity company and the customer? Don’t you think that is interesting to know if a client that provides higher quality energy can charge a little more for that energy supplied to the network than others?

ANSWER: Any energy supplier must maintain or improve the quality of the supply, otherwise they should be disconnected. So I believe in minimum standards, and not some “higher quality” beyond what is needed.

QUESTION: What is the voltage level of micogrid that can be a variable for dynamic loads?

ANSWER: IEEE ANSI C84.1 and also the IEEE 1453 flicker standards exist to manage voltage concerns.

QUESTION: Do you think that microgrid controllers that are being developed are moving towards resolution to the issues you mentioned? MIT Lincoln labs has a Hardware in the Loop test bed that has done simulation that is testing some of the controllers currently in the market that seems promising.

ANSWER: The microgrid controller is indeed an important component. System modeling is important, but time domain software like EMTP-RV and PSCAD are good alternatives to expensive HIL simulations for many analyses.

QUESTION: If we talk about the microgrid, what is the size of grid?

ANSWER: We can talk in terms of short circuit ratio to define the “size” of a grid, relative to the load it serves.

QUESTION: Why is it that microhydro systems are not really mentioned much when talking of microgrids. Usually fuel cells, solar and wind are the ones focused on despite that microhydro can provide reliable and better power quality to the microgrids?

ANSWER: Probably only because microhydro resources rely on an available natural resource at the site, and also these costs are not falling as drastically as they are for solar panels.