This article reports on a new generation of photovoltaic synchronous generator (PVSG) plants developed at the University of Texas, which convert existing grid forming (GFM) plants into GFM PV plants. Three years of research and development (R&D), sponsored by Duke Energy, has demonstrated the PVSG power plant integrates naturally to a current electric system, resembling and even improving, the behavior of conventional generators.
Background
Today's ac power systems are designed based on synchronous generator (SG)-based generation resources, such as coal, gas, hydro, and nuclear. SG-based power plants operate as voltage sources that set the voltage and frequency of the power grid, which is 60 Hz in the United States. Usually any imbalance between the power generation and power demand causes small deviations from the standard frequency. In a SG, the kinetic energy stored in the rotor serves as inertia to fight against frequency deviation and provides time to balance the power demand and power generation. Additionally, SG-based power generation can start without the existence of a power grid (black start). For these reasons, SG-based power plants are called GFM power plants.
In the last decade, the penetration of PV in the electric power grid increased rapidly because of the falling price of PV panels, government incentives, and increased value of PV-based energy production in reducing carbon emission. However, safe operation of the power grid may become a challenge if the percentage of PV production and other inverter-based energy resources, such as wind and battery energy storage, keep increasing. This is because today's PV power plants are not GFM but grid following (GFL).
GFL PV power plants are controlled as a current source and follow the power grid frequency in generating ac power. They do not react to the frequency change or power imbalance in the grid. In other words, they do not provide any frequency support, in contrast to an SG-based power plant. The GFL PV inverter typically uses a phase locked loop (PLL) to estimate the phase angle and voltage magnitude at the point of common coupling (PCC). Therefore, without the grid, a GFL PV plant cannot start operation (no black-start capability). As a matter of fact, modern GFL PV inverters have sophisticated anti-islanding protection algorithm which will trip the power plant offline when the grid is not present, even if there is sufficient power generation to maintain the feeder or local loads. This common practice is clearly in contrast to the need for a more resilient grid when a natural disaster damages the main grid. Another major issue of today's GFL PV plant is the large power ramp rate caused by solar irradiation intermittence.
Grid Forming Photovoltaic Synchronous Generator
Several approaches are being studied to address the above-mentioned operational issues in today's PV power plant. One attractive solution is to make PV plants and inverters behave as GFM SGs. This will require changing the PV inverter control from a GFL current source to a GFM voltage source. Additional energy storage buffer will also be needed to provide frequency inertia, smoothing solar intermittence and black start.
Several methods have been proposed by researchers to build a completely new generation of GFM PV plant, with new hardware and software changes. But they do not work with existing GFL PV plants. Therefore, the question, how to transform existing PV systems from GFL into GFM operation with minimum cost is very meaningful.
The Semiconductor Power Electronic Center (SPEC) at the University of Texas in Austin has developed a novel GFM PVSG system that converts an existing GFL PV plant to a GFM PV plant. In the PVSG, an additional GFM energy storage system is connected at the ac side of a GFL PV inverter as shown in Fig. 1. The overall system then operates like a GFM SG with inertia support frequency response. It also smooths the PV irradiation intermittence. The operation principle is shown in Fig. 2.
The frequency inertia support can be done regardless of solar irradiation and is not related to PV power generation. The reactive power can also be provided by the PVSG. Moreover, this system does not need the grid to start so it can perform black start. The SPEC has successfully demonstrated this concept in a 40-kW PVSG testbed, connected to the 480-V, 60-Hz Austin energy grid.
The energy storage capacity needed in the PVSG depends on the functionality of the PVSG system. SPEC researchers estimated that only about 0.3xPPVx1 sec of usable energy is needed in a PVSG to provide 1 sec of inertia and to smooth PV intermittence, where the PPV is the PV plant rating. This is a small amount of energy storage and can be best done by an ultracapacitor energy storage unit, as shown in Fig. 1. With a battery energy storage system, the PVSG can provide many more services including secondary frequency response, fast frequency response (FFR), and frequency regulation, which have a much higher energy demand.
In Fig. 3 and Fig. 4, the simulated inertia and primary frequency response of the PVSG are shown when the grid frequency is changed at t= 4 sec. The PVSG has its own frequency (blue) that is always trying to be synchronized with the grid, similar to the situation of an SG. The difference in this synchronization results in the injection or absorption of the active power (Fig. 5). In another simulated case (Fig. 5) the PV irradiation has a sudden change from 50 kW to 70 kW at t= 8 sec, but this intermittence is substantially smoothed on the grid side by the PVSG.
PVSG Demonstration
A 40-kW PVSG system is developed and demonstrated with a configuration similar to Fig. 1. Only an ultracapacitor is used as the energy storage unit. The system is connected to the 480-V, 60-Hz Austin energy grid system. Fig. 6 shows a typical result demonstrating the frequency inertia support. As the grid frequency changes up and down, the PVSG provides the needed frequency support by injecting and absorbing active power. Fig. 7 shows a typical result when the PV power intermittence is emulated. Again, the grid power is smoothed by the PVSG because of the GFM control and the ultracapacitor energy buffer.
Conclusions
Modern power systems with higher level of PV penetration will have substantial operational challenges, including but not limited to the lack of inertia and frequency support. Therefore, a GFM PV plant is needed in the future. The proposed GFM PVSG concept can transform today's GFL PV plants into GFM PV plants, thereby providing a cost-effective approach forward. The required energy storage is very low and can be effectively provided by an ultracapacitor bank.
