Interview with Srdjan Srdic

Picture1Srdjan Srdic (M'09 -SM'17) received the B.S., M.S and Ph.D. degrees in electrical engineering from University of Belgrade, Serbia, in 2004, 2010, and 2013, respectively. From 2005 to 2015, he was with the Department of Power Engineering, School of Electrical Engineering, University of Belgrade, where he served as a Teaching Assistant and an Assistant Professor. From 2015 to 2019, he was with North Carolina State University, Raleigh, NC, as a Research Assistant Professor. Currently, he is with EGSTON Power Electronics GmbH, where he serves as Director of R&D. His research interests include power hardware-in-the-loop systems and SiC-based medium-voltage and low-voltage power converters for automotive industry and renewable energy applications.


In this interview, Dr. Srdic answers questions from his webinar, Toward Extreme Fast Charging: Challenges and Opportunities in Directly Connecting to MV Line, originally presented on April 30, 2020. For more info on this webinar and to stream the sessions, please click here.

Q: Are the increased power losses just from the increased load?

A: If you are referring to increased grid power losses mentioned in slide 12, as one of the negative impacts of EVs to power grid, then the answer is yes. The losses are increased due to the increased load.

 

Q: How does EV increase the grid power losses?

A: The grid power losses are increased because the load power is increased. The EVs are high-power electric loads.

 

Q: Isn't the efficiency of a HF SST less than a standard MV to LV transformer?

A: Yes, it still is, but the difference is not that big (up to 1 %). However, the overall efficiency of the SOA DC fast charger system, which includes a MV/LV transformer + conventional DC fast charger is significantly lower (approx. 92%) than that of a SST-based MV Fast Charger (97.5%).

 

Q: What is the cost difference between SOA and MV SST systems?

A: It depends on the size of the system and on the actual installation cost. For example, the cost of the 675 kW SOA system shown in slide 15 was $250,000. Our rough estimate at the time (2018) was that the 900 kW SST-based system would cost around $200,000.

 

Q: Who are the leading solid-state circuit breaker research groups or companies?

A: ABB has developed a solid-state circuit breaker that will be available on the market this year.  One research group within FREEDM Systems Center has developed a 15kV/40A super-cascode solid-state switch.

 

Q: Is the comparison in slide 15 based on estimated sizes, or has it been proven by developing and packaging a prototype system?

A: The size of the SST-based system is estimated. The MV metering is AZZ SPMG-315. The MV switchgear is GW Trident. The size of the “3-ph SST”, “DC busbar” and “DC/DC Converters” enclosures is estimated based on the size of the power modules that would be housed in those enclosures.

 

Q: Are there certified revenue meters in the US for DC metering for utilities to procure?

A: We do not have that information.

 

Q: How large will the electric utility have to be to supply both an AC and an DC bus for the neighborhood?

A: It depends on the size of the neighbourhood. The SST-based system does not have to be bigger than the typical distribution transformer.

 

Q: What’s the downside of the SST based fast chargers? What stops it from mass adoption at the moment? What are the reliability concerns?

A: The following are major non-technical challenges:

  • Reliability concerns: passive transformer versus power electronic equivalent
  • Limited ability to monetize the excellent power quality supplied by the SST
  • Large inertia still present in the distribution system

 

A: The following are major technical challenges:

  • Lack of fast-acting protection systems and lack of circuit breakers that can be used in these protection systems
  • Standardization and certification of the EV charging equipment that connects directly to MV line
  • Integration of the SSTs in the existing power systems

 

 

Q: What causes the insulation cost to decrease with MV charging station?

A: If you meant the installation cost, the answer is smaller system size for the same system power.

 

Q: 2.4kV is typically phase to ground.  Is this system grounded or isolated?

A: The 2.4 kV system was only a proof-of-concept. The 2.4-kV side was grounded and the charger output was isolated.

 

Q: What protection from faults are you proposing for the converters?

A: For the input side, a fast solid-state circuit breaker must be used to be able to disconnect the system in case of SC fault and overvoltage fault.

 

Q: What is the purpose of high frequency transformer?

A: The purpose is to provide the galvanic isolation of the system output from the ac grid.

 

Q: Have you looked at soft switching transformer (SS SST)? Lower EMI, better efficiency .....

A: Yes, we have done some research there as well. We plan to publish our findings soon.

 

Q: How large will utility transformer have to be?

A: For the SST-based systems, the MV/LV utility transformer is not used. Insted, a HF isolation transformer is used which is significantly smaller than the LF transformer of the same rated power. We are not sure if this answers your question.

 

Q: Do you foresee a parallel MV grid adjacent to the LV grid, catering to both generation (solar+wind) and consumption (fast chargers, Data centers, high profile loads) etc. How such a business model can be promoted? what could be the pros and cons?

A: We are not sure if we understand the question, but we will try to answer it. we do not believe there would be a parallel dedicated MV grid, since an existing MV grid can be used for all those purposes.

 

Q: How's the ramp rate controlled when such huge amout of power is taken from grid, it may cause huge voltage drop when EV gets connected to the station? I assume that the EV charging ramp rate would be much higher as compared to maximum allowable ramp rate from the grid?

A: That depends on the strength of the grid. The battery storage is typically used on site to mitigate the power peaks during high demand hours. The added energy storage needs to be sized properly to be capable of buffering the power and energy demands of the charging station during the peak demand period.

 

Q: Our example of a 17kW fast charger, with efficiency and cost comparisons, utilized a medium voltage supply of 2.4 kV.  Is it not true that most of the US distribution voltage levels are presently 15kV, 25kV, and 35kV.  Many decades ago 2.4kV systems were replaced or upgraded.

A: The presented 50 KW MVFC was a case study and served as a proof of concept. No one has claimed that “most of the US distribution voltage levels are presently 15kV, 25kV, and 35kV”. Typically, a 12.47 kV or 13.8 kV systems are used in the US. We agree that the vast majority of the 2.4 KV systems were upgraded to higher voltage levels.

 

Q: What about the cost? Does it same for same state as compare in slides?

A: The cost comparison is given for the 50-kW proof-of-concept system. For higher-power systems, the cost needs to be estimated on a case-to case basis, but we expect it to be comparable to the cost of the SOA systems.

 

Q: ADDRESS SAFETY ISSUES FOR THE PUBLIC

A: If the system is properly designed, we do not expect to have any additional safety issues. The isolation is achieved using HF transformer inside the charger.

 

Q: Amy Heidner to all panelists: So how many EVs which are on the road now could actually use fast charging or super-fast charging?

A: All battery electric vehicles are using fast charging.

 

Q: How many vehicles would have to be on the road to make these SST-MV stations feasible?

A: This would depend on the business model used, and there are still other challenges as outlined below:

A: The following are major non-technical challenges:

  • Reliability concerns: passive transformer versus power electronic equivalent
  • Limited ability to monetize the excellent power quality supplied by the SST
  • Large inertia still present in the distribution system

 

A: The following are major technical challenges:

  • Lack of fast-acting protection systems and lack of circuit breakers that can be used in these protection systems
  • Standardization and certification of the EV charging equipment that connects directly to MV line
  • Integration of the SSTs in the existing power systems

 

Q: What would be effect of wireless extreme fast charging from the perspective of SST system? Would there be any considerations to think about?

A: The SST-based system is proposed here for the conductive fast charging systems. The input stage of the SST could potentially be used in the wireless charging systems.

 

Q: How plausible is achieving a DC breaker that can achieve isolation in 200 micro-seconds (if I heard you correctly)? That speed sounds almost physically impossible.

This is probably impossible for a mechanical breaker. However, this is relatively easy to achieve with solid-state breakers.

 

Q: Have you looked at soft switching transformer (SS SST)? Lower EMI, better efficiency ....

A: Yes, we have done some research there as well. We plan to publish our findings soon.

 

Q: You quickly ran through the benefits of using high frequencies to transform power. Can you quickly explain that in a little more detail?

A: Sorry about that. If the high frequencies are used, all magnetic components in the system (inductors and transformers) will have significantly smaller size. That would result in the systems having significantly smaller size and weight.