Webcast presenters Ben Langstraat and Sam Fopma from Interstates answered additional questions about topics such as natural gas as a fuel source, battery back-up systems, and variable frequency drives (VFDs).
By Ben Langstraat and Sam Fopma, InterstatesApril 13, 2018
The "Power Distribution" webcast was presented live on Apr. 10, 2018, by Ben Langstraat, division manager, Interstates; and Sam Fopma, electrical engineer, Interstates. The webcast can be found here. They supplied written answers to some of those questions that weren’t addressed from the webcast attendees:
Question: Is there a variable frequency drive (VFD) available with integral line and load reactors that keep harmonics of the load and bus?
Answer: There are some manufacturers that make combination units and drives with harmonic mitigating features. Most often, line and load reactors are installed as a separate component than the VFD.
Q: Please explain the difference between prime, continuous, and standby units for cat units? Many units were used for extended hours not knowing the difference and causing many units to fail.
A: Backup is for emergency use only on systems where power is normally provided by a utility source.
Prime rated generators are designed to be the primary power source for the facility.
Continuous rated generators can supply a specific load for a long duration, but is not designed to handle variable or overload conditions like would be the case on a prime generator.
Q: How did the size of local diesel storage came into play? Were any lessons learned?
A: Fuel storage capacity should be considered as part of the backup power/recovery plan. Having on-site fuel storage will provide some flexibility in the recovery effort, but fuel management remains an important part of recovery planning.
Q: Would you say natural gas is a reliable fuel source for emergency generators?
A: Natural gas can certainly be a viable fuel source, but the reliability of the fuel delivery system must be considered in the overall design. For example, if system pressure requires an on-site gas compressor to power the generator then an auxiliary power system will be required to power the compressor so the main generator can start.
Q: Any thoughts about the feasibility of constructing underground distribution systems after disasters?
A: Underground distribution should be considered for system design. In the cases of hurricanes or tornadoes it certainly provides an additional level of protection against flying debris.
Q: Any thoughts about a greater use of battery back-up systems?
A: Battery systems can aid in ride-through of minor power failures or power quality issues, but the duration of the outage should be considered before relying on battery systems as the only backup power source.
Q: How did you manage to handle with peak load demands (if any)? If load demand exceed the generators capacity at some point would that cause a trip of the genset?
A: The facility operation was managed to stay within the rating of the generator. Overload condition would result in an overcurrent trip of the protection at the generator.
Q: Any thoughts about more usage of renewable energy source for more resiliency to storm events?
A: Renewable sources are options for power during recovery. However, the resiliency and capacity of the renewable sources should be considered within the overall backup plan. In a hurricane, for example, both wind and solar sources could have sustained damage that resulted in them being unusable during the recovery.
Q: Is it possible the generator tips due to inrush current from transformer down the line?
A: It is possible to cause tripping at the generator due to inrush and demand current from loads. The specific application should be reviewed with your generator vendor to determine if a larger unit is required to handle the inrush better.
Q: What are VFD cables? We have 300 hp VFD with line reactors built-in. We have hundred of VFDs from 50 to 400 hp.
A: VFD cables are shielded multi-conductor cables that are designed for use on circuits with drives. They are built to better withstand the harmonics present on those systems.
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