What is in this article?:
Over the past 10 years, manufacturers have significantly improved solar photovoltaic (SPV) inverter technology. Previously, utilities were concerned that high penetrations of solar inverters would destabilize public electricity supply networks, but recent testing has revealed that inverters with advanced features may actually improve power quality. As we move toward scenarios where many SPV inverters are connected to the public electricity network, some advanced features appear necessary, including volt/VAR control, voltage ride-through (VRT)  capability, frequency ride through, real power/frequency control, ramp rate control, and communications.
A series of tests that were conducted a few years ago were used to characterize typical ”legacy” inverters, including an assessment of their contribution to distortion, anti-islanding issues, and potential aggravated disturbances. Next, test results are presented for an off-the-shelf advanced commercial SPV inverter designed to operate according to recent German grid codes. This inverter has features not presently available in inverters designed according to standards in the United States. This inverter was subjected to various typical voltage and frequency deviations to assess its dynamic performance on the grid. Various standards and technical reports—such as IEC61000-3-15  and the Italian CEI 0-21 —mention the possibility that SPV inverters may be used to improve power quality.
The initial tests on legacy inverters were aimed at verifying that the inverters function safely in accordance with US and international standards. Fig. 1 shows the test setup.
The grid simulator handles bidirectional power flow, just like the electric grid. The inverter is being fed with a DC power supply that simulates the PV panel’s I-V curve. The programmable load provides both linear and non-linear current loads, effectively simulating typical “household load” patterns, such as those produced by PCs, cooking appliances, TVs, air conditioners, etc. The power analyzer provides information on the current flow in the load, as well as into or out of the public supply and the inverter.
Fig. 2 shows a typical display from the power analyzer.
The top graph shows the voltage (green) and the current (black) flow of the grid simulator, i.e. the electric grid. The bottom graph shows the load current (red) and the inverter current (blue). The inverter delivers 1274.9 Watt to the household (load) and 1766.5 Watt to the grid. A few are lost in the system wiring etc.
The public supply rarely has such a nice sinusoidal voltage. Voltage distortion of 2 – 5 % V-THD is not uncommon. To evaluate the PV inverter response to a distorted voltage, the grid simulator was programmed in 1% steps to have from 3 - 9 % voltage distortion, at harmonic order 9 (VH9).
As the graph in Fig. 3 shows, the current distortion into the grid is about double the programmed voltage distortion.
This is due to the fact that the inverter “tracks” the supply voltage, i.e. adds about the same amount of current distortion that is already present. If the inverter were to be “permitted” to compensate, it could actually reduce the current distortion into the public supply. Modern inverters have this capability as we will see.
Another undesirable consequence of early requirements placed on PV inverters is their response to voltage dips and short interrupts to avoid “islanding operation.” Standards require that the inverter separate itself from the electric grid within 160 ms in the event that the supply voltage goes outside specified tolerances (usually about ± 10 % from V-nom).
Fig. 4 shows the inverter response to a short voltage dip.
The PV inverter disconnects within 10 ms and remains “off-line” – sometimes for as long as several minutes. Such a response will generally aggravate the “dip” as the power contribution from the inverter “goes away”. It is now recognized that a certain amount of low voltage ride through (LVRT) is much more desirable.