1. Low Voltage Ride Through (LVRT)
Performance and Reliability of PV Inverter Component and Systems due to Advanced Inverter Functionality
Jack Flicker and Sigifredo Gonzalez– Sandia National Laboratories
In order to identify reliability issues associated with advanced inverter operation and array states (e.g. volt-VAR control, high DC/AC ratios), we have collected system and component-level electro-thermal information in a controlled laboratory environment under both nominal and advanced functionality operating conditions. The results of advanced functionality operation indicated increased thermal and electrical stress on components, which will have a negative effect on inverter reliability as these functionalities are exercised more frequently in the future.
Introduction
For deviations from unity, changes in system efficiency can become quite large.
96% efficiency at unity PF  91.3% efficiency at 0.55 PF
Doubling of energy loss will have significant effect on component operating temperature
Substantially reduce component lifetime
Inverter is challenging reliability target  complicated switching/monitoring system with numerous responsibilities
Trends in power electronics systems and devices have placed increasing demands on thermal management and control systems for MOSFET and IGBT modules
switching frequencies
voltage ratings
non-unity power factor (PF) operations
 higher heat dissipation and temperature-related degradation directly impacts reliability
Low Voltage Ride Through (LVRT)
Of special concern are additional inverter functionalities and PV array states (e.g. high DC/AC ratios)  not envisioned for inverters made as few as five years ago
Functionalities are pushing operational limits of inverters  increasing stress on components
Expected to have a significant effect on the reliability and lifetime of inverters
Currently unknown what extent these functionalities will have on inverter reliability
Large-scale penetration of variable and intermittent distributed resources into the electrical grid can have significant effects on frequency and voltage stability of the grid
 number and severity of grid deviations will increase (esp. far from distribution feeders)
Voltage and current waveforms of MOSFET switches were recorded simultaneously along with system-level information in a 3 kW residential inverter during LVRT events
tsag=0-1.6 s, Vsag=0-220 V
Energy loss normalized by tsag to determine relative severity of LVRT event
Rate of energy loss during LVRT is independent of duration
approximately 682±40 J/s
inverter attempts to export energy for all LVRT events
does not adhere to Rule 21 (would be ~ 0 J/s)
Rate of energy loss during the event is linearly related to Vsag (increased current on bus)
Maximum normalized energy loss equal to 879 J/s for Vsag = 0V
Energy loss decreases by a rate of approximately -3.5 J/V as Vsag increases
If followed Rule 21 would be ~ 0 J/s for Vsag < 120 V
Non-Unity Power Factor Operation
Future markets will incentivize operators to utilize the inverter up to its full power-rating for all daytime hours at various PFs
subjecting inverter to higher losses over operational life and shorter lifetimes
Unity and non-unity PF operation monitored in a 3 kW residential inverter
PF from (current lags voltage) to (current leads voltage)
Voltage and current waveforms of MOSFET switches recorded with system information
MOSFET voltage and current measured over an entire 16 ms AC cycle with resolution 2-4 ns
System-level efficiency and unit temperature (8 different positions) monitored and logged
Cumulative energy loss is function of inverter operation conditions
Higher input power levels yield higher losses in the MOSFETs
Losses are not symmetric around unity power factor operation (even though the operational voltages and currents handled by the MOSFET are)
Switching energy losses per cycle increasing by 168% from PF=1 to PF=
Non-switching loss mechanisms show the same trend with regard to PF operation
Possibly be due to filtering choices made by the manufacturer
Conclusion
From emergence over the past 5 years to eventual large-scale implementation, advanced inverter functionalities look to enable higher penetration of renewable energy generation through the stabilization of grid voltage and frequency
However, these modes increase system losses and will lead to shorter unit lifetimes
It is necessary to consider the balance of grid stability with the shorter lifetimes of the units that will result
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
This work was funded by the US Department of Energy Solar Energy Technologies Program.